Reforming catalyst pattern for fuel cell operated with enhanced co2 utilization

Abstract

To provide a reforming element for a molten carbonate fuel cell stack and corresponding methods that can reduce or minimize temperature differences within the fuel cell stack when operating the fuel cell stack with enhanced CO2 utilization.SOLUTION: The reforming element can include at least one surface with a reforming catalyst deposited on the surface. A difference between the minimum reforming catalyst density and / or activity and maximum reforming catalyst density and / or activity on a first portion of the at least one surface can be 20% to 75%, with the highest catalyst density and / or activity being in proximity to the side of the fuel cell stack corresponding to at least one of the anode inlet and the cathode inlet.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] A system and method for operating a molten carbonate fuel cell stack are provided for improving the CO2 utilization rate when operating with a low CO2 content in the cathode input stream. The fuel cell assembly can include an internal reforming assembly having a reforming catalyst distribution that reduces or minimizes the temperature gradient within the fuel cell.

Background Art

[0002] This application discloses and claims subject matter that has been made as a result of activities within the scope of a joint research agreement between ExxonMobil Research and Engineering Company and FuelCell Energy, Inc. that was effective prior to the effective filing date of this application.

[0003] Molten carbonate fuel cells utilize hydrogen and / or other fuels to generate electricity. Hydrogen may be provided by reforming methane or other reformable fuels in a steam reformer, such as a steam reformer located upstream of the fuel cell or integrated within the fuel cell. The fuel can also be reformed in the anode cell of a molten carbonate fuel cell that is operated to create conditions suitable for reforming of the fuel in the anode. Yet another option can be to perform some reforming both external and internal to the fuel cell. Reformable fuels can include hydrocarbonaceous materials that can react with steam and / or oxygen upon heating and / or pressurization to produce a gaseous product containing hydrogen.

[0004] One attractive feature of molten carbonate fuel cells is the ability to transport CO2 from a low-concentration stream (such as the cathode input stream) to a higher-concentration stream (such as the anode output stream). During operation, CO2 and O2 in the MCFC cathode form carbonate ions (CO3 2-) is converted, which is then transported across the molten carbonate electrolyte as a charge carrier. The carbonate ions react with H2 in the fuel cell anode to form H2O and CO2. Thus, one of the final achievements of operating an MCFC is the transport of CO2 across the electrolyte. This transport of CO2 across the electrolyte can reduce or minimize the cost and / or difficulty of isolating carbon oxides from various CO x containing streams. Combining an MCFC with a combustion source such as a natural gas power plant can generate additional electricity while reducing or minimizing the overall CO2 emissions resulting from power generation.

[0005] Regarding fuel cell assemblies in which an internal reforming section is located within the fuel cell stack, various types of catalyst distributions are known. For example, U.S. Patent No. 8,822,090 describes a reforming catalyst pattern and a corresponding flow scheme. This catalyst pattern and flow scheme are described as providing an improved temperature distribution, in part, by causing additional reforming near the center of the fuel cell stack.

[0006] U.S. Patent Application Publication No. 2015 / 0093665 describes a method for operating a molten carbonate fuel cell that causes some combustion at the cathode to supply auxiliary heat for additional reforming (and / or other endothermic reactions) within the fuel cell anode. This published patent notes that when the CO2 concentration drops below about 1.0 mole%, the voltage and / or power generated by the carbonate fuel cell may begin to rapidly decline. This published patent further states that when the CO2 concentration drops further, for example, below about 0.3 volume%, at some point the voltage across the fuel cell becomes low enough that further transport of carbonate hardly occurs or ceases altogether, and the fuel cell ceases to function. SUMMARY OF THE INVENTION

[0007] In one aspect, a method for generating electricity is provided. The method may include passing a fuel stream including a reformable fuel through a fuel stack including a first surface. The first surface may include a first portion including a reforming catalyst. In some aspects, the reforming catalyst density on the first portion of the first surface may be such that the difference between the maximum catalyst density and the minimum catalyst density is from 20% to 75%. In some aspects, the reforming catalyst density on the first portion of the first surface may be such that the difference between the maximum catalytic activity and the minimum catalytic activity is from 20% to 75%. At least a portion of the reformable fuel may be reformed in the presence of the first surface to produce reformed hydrogen. At least a portion of the reformable fuel, at least a portion of the reformed hydrogen, or a combination thereof may be introduced into the anode of a molten carbonate fuel cell. The method may further include introducing a cathode input stream including O2, H2O, and CO2 into the cathode of the molten carbonate fuel cell. The direction of flow in the cathode of the molten carbonate fuel cell may be substantially orthogonal to the direction of flow in the anode of the molten carbonate fuel cell. The molten carbonate fuel cell operates at a utilization rate of 0.97 or less and an average current density of 60 mA / cm 2 2 or more to produce electricity, an anode exhaust including H2, CO, and CO2, and a cathode exhaust including 2.0 volume % or less of CO2, 1.0 volume % or more of O2, and 1.0 volume % or more of H2O.

[0008] In another aspect, a fuel cell stack is provided. The fuel cell stack may include a molten carbonate fuel cell having an anode and a cathode. The fuel cell stack may further include a reforming element associated with the anode. The reforming element can include a first surface, and the first surface can include a first portion including a reforming catalyst. The reforming catalyst density on the first portion of the first surface may correspond to a monotonically decreasing catalyst density. Additionally or alternatively, the reforming catalyst density on the first portion of the first surface may be such that the difference between the maximum catalyst density and the minimum catalyst density is from 20% to 75%. The fuel cell stack may further include a separator plate between the anode and the reforming element. BRIEF DESCRIPTION OF THE DRAWINGS

[0009]

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DETAILED DESCRIPTION OF THE INVENTION

[0010] Summary In various aspects, a reforming element for a molten carbonate fuel cell stack is provided that can reduce or minimize the temperature difference within the fuel cell stack when operating the fuel cell stack by reducing the CO₂ content of the cathode supply and improving the CO₂ utilization rate. The reforming element can include at least one surface on which a reforming catalyst is deposited. To reduce or minimize the temperature difference within the fuel cell stack, the difference between the minimum reforming catalyst density and the maximum reforming catalyst density on the first portion of the at least one surface can be 20% - 75%, or 20% - 70%, or 25% - 65%, and the highest catalyst density is proximate to the side of the fuel cell stack corresponding to at least one of the anode inlet and the cathode inlet. Additionally or alternatively, the difference between the minimum reforming catalyst activity and the maximum reforming catalyst activity on the first portion of the at least one surface can be 20% - 75%, or 20% - 70%, or 25% - 65%, and the highest catalyst activity is proximate to the side of the fuel cell stack corresponding to at least one of the anode inlet and the cathode inlet. For example, for a catalyst density pattern aligned with the anode flow, the maximum catalyst density and / or activity can be present at the end of the catalyst pattern closest to the side of the fuel cell stack corresponding to the anode inlet (i.e., proximate to the anode inlet). Optionally but preferably, the reforming catalyst density and / or activity can vary monotonically across the first portion of the at least one surface. Optionally, the second portion of the at least one surface in the reforming element can correspond to the portion of the surface proximate to the cathode inlet or the anode inlet side of the reforming element. Specifically, when the catalyst pattern of the first portion is aligned based on the anode flow pattern, the second portion can be proximate to the cathode inlet. Alternatively, when the catalyst pattern of the first portion is aligned based on the cathode flow pattern, the second portion can be proximate to the anode inlet. The second portion of the at least one surface can have a distinct catalyst profile, such as a relatively constant catalyst density profile and / or activity profile, to account for the increased fuel cell activity at the cathode inlet.This reforming catalyst pattern in the reforming element corresponding to the first part and the optional second part enables the fuel cell stack to be operated while increasing CO2 capture while making the temperature difference 70 °C or less, or 50 °C or less, or 45 °C or less, or 40 °C or less across the entire fuel cell stack. The temperature difference may be measured in the separator plate between the reforming element and the anode. By reducing or minimizing the temperature difference within the fuel cell stack, the presence of local "hot spots" within the fuel cell stack that could potentially damage the structure within the fuel cell stack can be reduced or minimized while using a higher average operating temperature.

[0011] In addition or alternatively, in various embodiments, the anode of the fuel cell can include at least one surface containing a reforming catalyst. In such embodiments, the difference between the minimum reforming catalyst density and the maximum reforming catalyst density and / or the difference between the minimum reforming catalyst activity and the maximum reforming catalyst activity on the first part of the at least one surface can be 20% - 75%, or 20% - 70%, or 25% - 65%, or 40% - 75%, or 40% - 70%, or 40% - 65%. In such embodiments, the highest catalyst density and / or activity can be close to the side of the fuel cell stack corresponding to the anode inlet. Optionally but preferably, the reforming catalyst density and / or activity can vary monotonically across the first part of the at least one surface. Optionally, the second part of the at least one surface in the reforming element can correspond to the cathode inlet side of the reforming element. The second part of the at least one surface can have a distinct catalyst profile, such as a relatively constant catalyst density profile and / or activity profile, to account for the increased fuel cell activity at the cathode inlet.

[0012] Regarding catalyst activity, it should be noted that the resulting catalyst activity can be modified by the nature of the catalyst pattern and the orientation of the gas flow relative to the catalyst pattern. For example, one option for providing a desired catalyst density on a surface could be to provide the catalyst as a series of (substantially) parallel lines of catalyst particles. The activity obtained from such a parallel-line catalyst pattern can vary depending on the orientation of the gas flow relative to the direction of the lines of catalyst particles. Specifically, the catalyst activity can be greater for a gas flow that is (substantially) aligned with (such as being substantially parallel to) the lines of catalyst particles, while a gas flow that is skewed or substantially perpendicular to the lines of catalyst particles can lead to various amounts of reduction in catalyst activity.

[0013] In contrast to conventional fuel cell operation, in various aspects, molten carbonate fuel cells can be operated to have increased CO2 capture, such as a transport rate of 0.97 or less, or 0.95 or less. Operating at a transport rate of 0.97 or less, or 0.95 or less, can lead to a different temperature distribution across the fuel cell compared to conventional operation. Specifically, it has been discovered that due to depletion of CO2 in the cathode, a portion of the current density in the fuel cell can be due to alternative ion transport. Such alternative ion transport typically involves greater waste heat generation. When a conventional reforming catalyst pattern is used, the waste heat due to alternative ion transport can cause significant unexpected temperature fluctuations.

[0014] Additional waste heat resulting from alternative ion transport has been found to accumulate near the cathode outlet. This is thought to be partly due to depletion of CO2 near the cathode outlet. This dependence of the temperature profile on the nature of the reaction at the cathode is in contrast to conventional fuel cell operation, where the temperature pattern depends largely on the flow patterns in the cathode and anode. As a result, using a conventional reforming catalyst pattern during an increase in CO2 consumption in the cathode can result in unexpected hot spots within the fuel cell. For example, when the flows within the cathode and anode are oriented such that they are approximately orthogonal, it has further been found that using a reforming catalyst pattern based on the expected anode reaction profile can result in substantial hot spots within the fuel cell near the corners corresponding to both the anode outlet and the cathode outlet. This is due to a combination of excess waste heat from alternative ion transport and reduced cooling from the conventional catalyst pattern near the corners corresponding to the anode outlet and the cathode outlet.

[0015] To overcome the difficulties associated with excess waste heat resulting from alternative ion transport, an alternative reforming catalyst pattern can be used such that more reforming occurs near the anode outlet. This can be achieved, for example, by using a reforming catalyst pattern with a reduced variation between the maximum and minimum catalyst densities. For example, the difference between the maximum and minimum densities in the reforming catalyst density can correspond to 20% - 75%, or 20% - 70%, or 25% - 65% of the maximum catalyst density. Additionally or alternatively, the reforming catalyst pattern can be used with a difference between the maximum reforming catalyst activity and the minimum reforming catalyst activity of 20% - 75%, or 20% - 70%, or 25% - 65% of the maximum catalyst activity. This results in increased reforming near the anode outlet, and as a result, additional cooling can occur that reduces the additional waste heat generated due to alternative ion transport.

[0016] Examples of suitable catalyst density profiles and / or catalyst activity profiles can be monotonically decreasing profiles. In such catalyst density / catalyst activity profiles, the catalyst density and / or catalyst activity is at a maximum at the anode inlet. Consequently, the catalyst density and / or activity is either substantially constant or decreases along the direction from the anode inlet to the anode outlet. Examples of monotonically decreasing profiles include catalyst density and / or activity profiles with a constant slope (i.e., a constant amount of decreasing catalyst density and / or activity), or modified catalyst density and / or activity corresponding to a series of steps, where the catalyst density / activity is substantially constant within each step. Still other catalyst patterns, such as combinations of steps and regions of decreasing catalyst density and / or activity, can also be used.

[0017] Using an improved modified catalyst pattern can be beneficial when operating an MCFC to improve or increase the CO2 utilization rate. One difficulty in operating an MCFC to increase the CO2 utilization rate is that when one or more of the reactants required for fuel cell operation are present in small amounts, the operation of the fuel cell may be kinetically limited. For example, when using a cathode input stream with a CO2 content of 4.0 volume% or less, achieving a CO2 utilization rate of 75% or more corresponds to a cathode outlet concentration of 1.0 volume% or less. However, a cathode outlet concentration of 1.0 volume% or less does not necessarily mean that CO2 is evenly distributed throughout the cathode. Instead, the concentration will typically vary within the cathode due to various factors such as the flow pattern in the anode and cathode. The variation in the CO2 concentration results in a portion of the cathode where the CO2 concentration is substantially less than 1.0 volume%.

[0018] Conventional operating conditions for molten carbonate fuel cells typically correspond to conditions where the amount of alternative ion transport is reduced, minimized, or non-existent. The amount of alternative ion transport can be quantified based on the transport rate for the fuel cell. The transport rate is defined as the fraction of ions that are transported across the molten carbonate electrolyte corresponding to carbonate ions, as contrasted with hydroxide ions and / or other ions. A convenient method for determining the transport rate can be based on comparing a) the measured change in CO2 concentration at the cathode inlet and the cathode outlet with b) the amount of carbonate ion transport required to achieve the current density generated by the fuel cell. It should be noted that this definition of the transport rate assumes that the back transport of CO2 from the anode to the cathode is minimal. Such back transport is considered to be minimal under the operating conditions described herein. For the CO2 concentration, the cathode input stream and / or the cathode output stream can be sampled, and the samples are converted to a gas chromatograph for determination of the CO2 content. The average current density of the fuel cell can be measured in any convenient manner.

[0019] Under conventional operating conditions, the transport rate can be relatively close to 1.0, such as 0.98 or greater, and / or substantially without alternative ion transport. A transport rate of 0.98 or greater means that more than 98% of the ionic charge transported across the electrolyte corresponds to carbonate ions. It should be noted that since hydroxide ions have a charge of -1 while carbonate ions have a charge of -2, in order to result in the same charge transfer as the transport of one carbonate ion, two hydroxide ions need to be transported through the electrolyte.

[0020] In contrast to conventional operating conditions, by operating a molten carbonate fuel cell at a transport rate of 0.95 or less (or 0.97 or less when operating with an increased open area and / or a decreased cross-section of unblocked flow), it is possible to increase the effective amount of carbonate ion transport achieved, even though a portion of the current density generated by the fuel cell is due to the transport of ions other than carbonate ions. To operate the fuel cell at a transport rate of 0.97 or less, or 0.95 or less, it is necessary to cause depletion of CO2 within the fuel cell cathode. It has been found that such depletion of CO2 within the cathode tends to be localized. As a result, many regions within the fuel cell cathode can still have sufficient CO2 for normal operation. These regions contain additional CO2 that is desirable to transport across the electrolyte, such as for carbon capture. However, CO2 in such regions typically does not transport across the electrolyte when operating under conventional conditions. By selecting an operating condition with a transport rate of 0.97 or less, or 0.95 or less, while additional CO2 can be transported using regions with sufficient CO2, the depleted regions can operate based on alternative ion transport. This can increase the practical limit of the amount of CO2 captured from the cathode input stream.

[0021] One of the advantages of transporting alternative ions across the electrolyte is that the fuel cell can continue to operate even though a sufficient number of CO2 molecules are not kinetically available. This can enable the transfer of additional CO2 from the cathode to the anode, even though the amount of CO2 present at the cathode might otherwise be considered insufficient for normal fuel cell operation. As a result, the calculated CO2 utilization rate (based on current density) can be at least 3% greater, or at least 5% greater, or at least 10% greater, or at least 20% greater than the measured CO2 utilization rate, while the fuel cell can operate at a measured CO2 utilization rate close to 100%. Note that alternative ion transport can enable the fuel cell to operate at a current density corresponding to a calculated CO2 utilization rate exceeding 100%.

[0022] Transport of replacement ions can enable a fuel cell to maintain a target current density, but it has further been discovered that transport of replacement ions across the electrolyte can reduce or minimize the life of a molten carbonate fuel cell. For this reason, it is desirable to reduce this loss in fuel cell life. It has unexpectedly been discovered that by controlling the temperature within the fuel cell by varying the reforming catalyst density and / or activity, such loss in fuel cell life can be reduced or minimized.

[0023] In some embodiments, enhanced CO2 capture can be defined based on an amount of a transport rate, such as a transport rate of 0.97 or less, 0.95 or less, or 0.93 or less, or 0.90 or less. Also, by maintaining an operating state at a transport rate of 0.97 or less, typically, the CO2 concentration in the cathode output stream can be 2.0 volume % or less, 1.5 volume % or less, or 1.0 volume % or less. When the CO2 concentration in the cathode output stream is higher, typically, local depletion of CO2 becomes insufficient to obtain lower transport rate values.

[0024] The presence of enhanced CO2 capture can also be indicated by other factors, but such other factors by themselves are typically not sufficient conditions to indicate enhanced CO2 capture. For example, when using a cathode input stream with a lower CO2 concentration, enhanced CO2 capture can, in some embodiments, correspond to a CO2 utilization rate of 70% or more, or 75% or more, or 80% or more, for example, up to 95%, or in some cases even higher. Examples of lower concentration CO2 sources can correspond to CO2 sources that result in a cathode input stream containing 5.0 volume % or less, or 4.0 volume % or less, for example, up to 1.5 volume %, or in some cases even lower, CO2. Exhaust from a natural gas turbine is an example of a CO2-containing stream that often has a CO2 content of 5.0 volume % or less or 4.0 volume % or less. Additionally or alternatively, enhanced CO2 capture can be achieved using a molten carbonate fuel cell at 60 mA / cm 2 or more, or 80 mA / cm 2 or more, or 100 mA / cm2 or more, or 120 mA / cm 2 or more, or 150 mA / cm 2 or more, or 200 mA / cm 2 or more, for example, up to 300 mA / cm 2 or may be even higher in some cases, etc., and can correspond to operating conditions that generate a significant amount of current density. It should be noted that the reaction pathway for alternative ion transport has a lower theoretical voltage than the reaction pathway using carbonate ions, so alternative ion transport can also be indicated by a reduction in the operating voltage of the fuel cell.

[0025] Conventionally, the CO2 concentration in the cathode exhaust of a molten carbonate fuel cell is maintained at a relatively high value, for example, 5 vol% or more of CO2, or 10 vol% or more of CO2, or may be even higher in some cases. In addition, a molten carbonate fuel cell typically operates at a CO2 utilization value of 70% or less. When any of these conditions exist, the dominant mechanism for transporting charge across the molten carbonate electrolyte is the transport of carbonate ions. It is possible to cause the transport of alternative ions (such as hydroxide ions) across the electrolyte under such conventional conditions, but the amount of alternative ion transport is negligible and corresponds to a current density of 2% or less (in other words, a transference number of 0.98 or more).

[0026] Instead of explaining the operating conditions from the perspective of the transference number, the operating conditions can be explained based on the measured CO2 utilization rate and the "calculated" CO2 utilization rate based on the average current density. In this consideration, the measured CO2 utilization rate corresponds to the amount of CO2 removed from the cathode input stream. This can be determined, for example, by using gas chromatography to determine the CO2 concentration in the cathode input stream and the cathode output stream. This can be referred to as the actual CO2 utilization rate, or simply the CO2 utilization rate. In this consideration, the calculated CO2 utilization rate means that all of the current density generated by the fuel cell is due to the transport of CO3 across the electrolyte. 2-It is defined as the CO2 utilization rate that may occur when generated based on ion transport (i.e., ion transport based on CO2). The difference between the measured CO2 utilization rate and the calculated CO2 utilization rate can be used individually to characterize the amount of alternative ion transport and / or these values can be used to calculate the transport rate as described above.

[0027] In some embodiments, any convenient type of electrolyte suitable for the operation of a molten carbonate fuel cell can be used. Many conventional MCFCs use a eutectic carbonate mixture such as a eutectic carbonate mixture of 62 mol% lithium carbonate and 38 mol% potassium carbonate (62% Li2CO3 / 38% K2CO3) or a eutectic mixture of 52 mol% lithium carbonate and 48 mol% sodium carbonate (52% Li2CO3 / 48% Na2CO3) as the carbonate electrolyte. Other eutectic mixtures, for example, a eutectic mixture of 40 mol% lithium carbonate and 60 mol% potassium carbonate (40% Li2CO3 / 60% K2CO3) are also available. Eutectic mixtures of carbonates can be convenient as electrolytes for various reasons, but non-eutectic mixtures of carbonates can also be suitable. Generally, such non-eutectic mixtures can include various combinations of lithium carbonate, sodium carbonate, and / or potassium carbonate. Optionally, smaller amounts of other metal carbonates, for example, other alkali carbonates (rubidium carbonate, cesium carbonate), or other types of metal carbonates such as barium carbonate, bismuth carbonate, lanthanum carbonate, or tantalum carbonate can be included as additives in the electrolyte.

[0028] In this discussion, the reforming element refers to the reforming stage located within the fuel cell stack. The reforming element can receive a fuel supply of a reformable fuel and convert at least a portion of the reformable fuel to hydrogen. After converting the reformable fuel to hydrogen, the hydrogen (optionally, in addition to any remaining reformable fuel) can be passed to one or more anodes associated with the reforming element. In this discussion, the temperature uniformity within the fuel cell stack can be determined based on the temperature of the separator plate located between the anode of the fuel cell and the adjacent associated reforming element.

[0029] In this consideration, the reforming catalyst density is defined as the average weight of the reforming catalyst per unit area on the surface. In a mode of discontinuously filling the reforming catalyst on the surface, such as filling catalyst grains or particles on the surface, the unit area for determining the reforming catalyst density at a certain position can be a square having a side length 25 times the representative length of the particle. The representative length of the particle can be the average diameter or average length along the longest axis of the particle. Next, a tessellation can be formed using such a square, and the catalyst density can be calculated in a separate manner for each unit square. For a catalyst pattern having no separate particles, or for a catalyst pattern where the representative length is not otherwise available, the square for defining the tessellation can have a side length of 1 mm. In this consideration, when determining the difference between the minimum reforming catalyst density and the maximum reforming catalyst density across the fuel cell (such as across the first portion of the surface within the reforming element), this difference can be determined based on the reforming catalyst density normalized such that the maximum density corresponds to a value of 100. The difference between the maximum density and the density at a given position can thus correspond to the percentage of the difference in the reforming catalyst density between positions.

[0030] Conditions for operating a molten carbonate fuel cell using alternative ion transport In various aspects, the operating conditions of a molten carbonate fuel cell (such as a cell as part of a fuel cell stack) are selected to correspond to a transport rate of 0.97 or less, whereby the cell can transport both carbonate ions and at least one type of alternative ion across the electrolyte. In addition to the transport rate, operating conditions that can indicate that a molten carbonate fuel cell is operating using alternative ion transport include, but are not limited to, the CO2 concentration of the cathode input stream, the CO2 utilization rate of the cathode, the current density of the fuel cell, the voltage drop across the cathode, the voltage drop across the anode, and the O2 concentration of the cathode inlet stream. In addition, the anode input stream and fuel utilization within the anode can generally be selected to provide a desired current density.

[0031] Generally, to cause alternative ion transport, the CO2 concentration in at least a portion of the cathode needs to be sufficiently low during the operation of the fuel cell to provide a sufficiently high current density. Sufficiently reducing the CO2 concentration in the cathode typically corresponds to some combination of a low CO2 concentration in the cathode input stream, a high CO2 utilization rate, and / or a high average current density. However, such conditions alone are not sufficient to exhibit a transport rate of 0.97 or less, or 0.95 or less.

[0032] For example, a molten carbonate fuel cell with a cathode open area of approximately 33% was operated at a CO2 cathode inlet concentration of 19 vol%, a CO2 utilization rate of 75%, and an average current density of 160 mA / cm 2 These conditions corresponded to a difference of less than 1% between the calculated CO2 utilization rate and the measured CO2 utilization rate. Therefore, the presence of substantial alternative ion transport / a transport rate of 0.97 or less or 0.95 or less cannot be simply inferred from the presence of a high CO2 utilization rate and a high average current density.

[0033] As another example, a molten carbonate fuel cell with a cathode open area of 50% - 60% was operated at a CO2 cathode inlet concentration of 4.0 vol%, a CO2 utilization rate of 89%, and a current density of 100 mA / cm 2 These conditions corresponded to a transport rate of at least 0.97. Therefore, the presence of a transport rate of 0.95 or less / substantial alternative ion transport cannot be simply inferred from the presence of a high CO2 utilization rate in combination with a low CO2 concentration in the cathode input stream.

[0034] As yet another example, a molten carbonate fuel cell with a cathode open area of 50% - 60% was operated at a CO2 cathode inlet concentration of 13 vol%, a CO2 utilization rate of 68%, and a current density of 100 mA / cm 2 These conditions corresponded to a transport rate of at least 0.98.

[0035] In this consideration, operating the MCFC to transport alternative ions across the electrolyte is defined as operating the MCFC such that more than a small amount of alternative ions are transported. It is possible to transport a small amount of alternative ions across the MCFC electrolyte under various conventional conditions. Such alternative ion transport under conventional conditions can correspond to a transport rate of 0.98 or higher, which corresponds to the transport of alternative ions corresponding to less than 2.0% of the current density of the fuel cell.

[0036] In this consideration, operating the MCFC to cause alternative ion transport is defined as operating the MCFC at a current density of 5.0% or higher (or 5.0% or higher of the calculated CO2 utilization rate), or 10% or higher, or 20% or higher, for example, up to 35% or higher, or in some cases even higher current density, such that the transport rate is 0.95 or lower to correspond to the current density based on the transport of alternative ions. It should be noted that in some aspects, by operating with an increased open area, the amount of alternative ion transport can be reduced or minimized under conditions that otherwise result in a transport rate of 0.95 or lower. For this reason, some operating conditions with enhanced CO2 capture / substantial alternative ion transport due to operation with an increased open area and / or a reduced cross-section of unblocked flow can correspond to a transport rate of 0.97 or lower.

[0037] In this consideration, operating the MCFC to cause substantial alternative ion transport (i.e., increasing the open area and operating at a transport rate of 0.95 or less or 0.97 or less) is further defined to correspond to operating the MCFC by dropping the voltage across the anode and cathode suitably for power generation. The total electrochemical potential difference of the reactions in the molten carbonate fuel cell is about 1.04V. For practical considerations, the MCFC is typically operated to generate current at a voltage of around 0.7V or about 0.8V. This corresponds to a combined voltage drop of approximately 0.34V across the cathode, electrolyte, and anode. To maintain stable operation, the combined voltage drop across the cathode, electrolyte, and anode can be less than about 0.5V such that the derived current generated by the fuel cell is at a voltage of 0.55V or more or 0.6V or more.

[0038] Regarding the anode, one condition for operating with substantial alternative ion transport can be to set the H2 concentration to 8.0 volume % or more, or 10 volume % or more, in the region where substantial alternative ion transport occurs. Depending on the embodiment, this can correspond to the region near the anode inlet, the region near the anode outlet, or a combination thereof. Generally, if the H2 concentration in the region of the anode is too low, the driving force for generating substantial alternative ion transport becomes insufficient.

[0039] Suitable conditions for the anode can include providing H2, a reformable fuel, or a combination thereof to the anode and operating at any convenient fuel utilization rate that produces a desired current density, including a fuel utilization rate in the range of 20% to 80%. In some embodiments, this can correspond to a conventional fuel utilization rate, for example, 60% or more, or 70% or more, for example, up to 85%, or optionally even higher. In other embodiments, this can correspond to a fuel utilization rate selected to provide an anode output stream with an increased H2 content and / or an increased total content of H2 and CO (i.e., syngas), for example, 55% or less, or 50% or less, or 40% or less, for example, down to 20%, or optionally even lower. The H2 content in the anode output stream, and / or the total content of H2 and CO in the anode output stream, can be sufficient to enable the production of the desired current density. In some embodiments, the H2 content in the anode output stream can be 3.0 volume % or more, or 5.0 volume % or more, or 8.0 volume % or more, for example, up to 15 volume %, or optionally even higher. Additionally or alternatively, the total amount of H2 and CO in the anode output stream can be 4.0 volume % or more, or 6.0 volume % or more, or 10 volume % or more, for example, up to 20 volume %, or optionally even higher. Optionally, when operating the fuel cell at a low fuel utilization rate, the H2 content in the anode output stream can be in a higher range, such as a H2 content of 10 volume % to 25 volume %. In such embodiments, the syngas content of the anode output stream can correspondingly be higher, such as a total content of H2 and CO of 15 volume % to 35 volume %. Depending on the embodiment, the anode can be operated to increase the amount of chemical energy produced (i.e., H2 produced by reforming available in the anode output stream) so as to increase the amount of electrical energy produced, or can be operated using any other convenient strategy compatible with causing alternative ion transport, such as operating the fuel cell.

[0040] In addition to the H2 concentration in the anode being sufficient, the CO2 concentration at one or more locations within the cathode needs to be low enough such that a more preferred pathway for carbonate ion transport is not readily available. In some embodiments, this may correspond to the CO2 concentration in the cathode exit stream (i.e., cathode exhaust) being 2.0 volume % or less, or 1.0 volume % or less, or 0.8 volume % or less. It should be noted that due to variations within the cathode, an average concentration of 2.0 volume % or less (or 1.0 volume % or less, or 0.8 volume % or less) in the cathode exhaust can correspond to even lower CO2 concentrations in local regions of the cathode. For example, in a crossflow configuration, at the corners of the fuel cell adjacent to the anode inlet and cathode outlet, the CO2 concentration can be lower than at the corners of the same fuel cell adjacent to the anode outlet and cathode outlet. Similar local variations in CO2 concentration can also occur in fuel cells having a coflow or counterflow configuration.

[0041] In addition to lowering the concentration of CO2, local regions of the cathode can also have 1.0 volume % or more or 2.0 volume % or more O2. In a fuel cell, O2 is used to form hydroxide ions that enable alternative ion transport. Since both the carbonate ion transport mechanism and the alternative ion transport mechanism depend on the availability of O2, the fuel cell will not operate without sufficient O2. With respect to O2 in the cathode input stream, in some embodiments, this can correspond to an oxygen content of 4.0 volume % to 15 volume %, or 6.0 volume % to 10 volume %.

[0042] To cause alternative ion transport, it has been observed that there should be a sufficient amount of water, such as 1.0 volume % or more or 2.0 volume % or more. Without being bound by any particular theory, when attempting to operate with substantial alternative ion transport, if water is not available within the cathode, the fuel cell appears to degrade at a much faster rate than the deactivation rate observed due to alternative ion transport when sufficient water is available. Since air is commonly used as the O2 source and H2O is one of the products generated during combustion, it should be noted that typically a sufficient amount of water is available within the cathode.

[0043] Due to non-uniform distribution of the cathode gas and / or anode gas during operation of a molten carbonate fuel cell to enhance CO2 capture, one or more of the corners and / or edges of the molten carbonate fuel cell are typically considered to have a substantially higher density of alternative ion transport. One or more corners may correspond to a location where the CO2 concentration in the cathode is lower than average, or the H2 concentration in the anode is higher than average, or a combination thereof.

[0044] In this discussion, a fuel cell can correspond to a single cell in which an anode and a cathode are separated by an electrolyte. The anode and cathode can receive an input gas flow to facilitate their respective anode and cathode reactions that transport charge across the electrolyte and generate electricity. A fuel cell stack can represent multiple cells within an integrated unit. A fuel cell stack can include multiple fuel cells, but the fuel cells can typically be connected in parallel and function (approximately) as if they collectively represent a single fuel cell of a larger size. When an input flow is delivered to the anode or cathode of a fuel cell stack, the fuel cell stack can include flow channels for dividing the input flow among each of the cells in the stack and flow channels for combining the output flows from the individual cells. In this discussion, a fuel cell array can be used to refer to multiple fuel cells (such as multiple fuel cell stacks) arranged in series, in parallel, or in any other convenient manner (e.g., a combination of series and parallel). A fuel cell array can include one or more stages of fuel cells and / or fuel cell stacks, where the anode / cathode output from the first stage can function as the anode / cathode input to the second stage. Note that it is not necessary to connect the anodes in a fuel cell array in the same way as the cathodes in the array. For convenience, the input to the first anode stage of a fuel cell array may be referred to as the anode input to the array, and the input to the first cathode stage of a fuel cell array may be referred to as the cathode input to the array. Similarly, the output from the final anode / cathode stage may be referred to as the anode / cathode output from the array. Note that in embodiments where a fuel cell stack includes a separate reforming element, the anode input flow may first pass through the reforming element before entering one or more anodes associated with the reforming element.

[0045] References herein to the use of a fuel cell typically refer to a "fuel cell stack" composed of individual fuel cells, and more generally, to the use of one or more fluidly connected fuel cell stacks. It should be understood that individual fuel cell elements (plates) can typically be "stacked" together in a rectangular array called a "fuel cell stack". The fuel cell stack can also include additional types of elements such as reforming elements. This fuel cell stack can typically take in a supply stream, disperse reactants between all of the individual fuel cell elements, and then collect products from each of these elements. Viewed as a unit, an operating fuel cell stack can be seen as a whole, even though it is composed of many (in many cases, dozens or hundreds) of individual fuel cell elements. These individual fuel cell elements can typically have similar voltages (due to similar reactant and product concentrations), and the total output can be obtained from the sum of the total currents of all the cell elements when the elements are electrically connected in series. The stack can also be arranged in series to generate a high voltage. A parallel arrangement can draw up the current. If a fuel cell stack of sufficient capacity is available to process a given exhaust stream, the systems and methods described herein can be used with a single molten carbonate fuel cell stack. In other aspects of the invention, multiple fuel cell stacks may be desirable or necessary for various reasons.

[0046] For the purposes of the present invention, unless otherwise specified, the term "fuel cell" should be understood to refer to a fuel cell stack composed of a set of one or more individual fuel cell elements having a single input and output, typically in the manner in which fuel cells are actually used, and / or defined as including reference thereto. Similarly, the term "fuel cell(s)" should be understood to refer to a plurality of separate fuel cell stacks, and / or defined as including the same, unless otherwise specified. In other words, all references within this document may, unless otherwise described, refer synonymously to the operation of a fuel cell stack as a "fuel cell". For example, the exhaust volume generated by a commercial-scale combustion generator may be too large to be processed by a conventionally sized fuel cell (i.e., a single stack). To process all of the exhaust, multiple fuel cells (i.e., two or more separate fuel cells or fuel cell stacks) can be arranged in parallel such that each fuel cell can process (substantially) equal amounts of combustion exhaust. Although multiple fuel cells can be used, each fuel cell can typically be operated in a generally similar manner given its (substantially) equal amount of combustion exhaust.

[0047] Example of molten carbonate fuel cell operation: Cathode and anode cross-flow orientation Figure 3 shows a general example of a part of a molten carbonate fuel cell stack. The part of the stack shown in Figure 3 corresponds to the fuel cell 301 and the associated reforming element 380. To isolate the fuel cell from adjacent fuel cells and / or other elements within the stack, the fuel cell includes separator plates 310 and 311. For example, the separator plate 310 separates the fuel cell 301 from the reforming element 380. An additional separator plate 390 is located on top of the reforming element 380. In Figure 3, the fuel cell 301 includes an anode 330 and a cathode 350 separated by an electrolyte matrix 340 containing an electrolyte 342. The anode collector 320 provides an electrical contact between the anode 330 and other anodes in the fuel cell stack, while the cathode collector 360 provides a similar electrical contact between the cathode 350 and other cathodes in the fuel cell stack. In addition, the anode collector 320 enables the introduction and exhaust of gas from the anode 330, while the cathode collector 360 enables the introduction and exhaust of gas from the cathode 350.

[0048] During operation, CO2 is passed through the cathode collector 360 together with O2. CO2 and O2 diffuse into the porous cathode 350 and move to the cathode interface region near the boundary of the cathode 350 and the electrolyte matrix 340. In the cathode interface region, a part of the electrolyte 342 may be present in the pores of the cathode 350. CO2 and O2 form carbonate ions (CO3 2-) can be converted to this, which can then be transported across the electrolyte 342 (and thus across the electrolyte matrix 340) to facilitate the generation of an electric current. In embodiments where alternative ion transport is occurring, a portion of the O2 can be converted to alternative ions such as hydroxide ions or peroxide ions for transport in the electrolyte 342. After being transported across the electrolyte 342, the carbonate ions (or alternative ions) can reach the anode interface region near the boundary of the electrolyte matrix 340 and the anode 330. The carbonate ions can be converted back to CO2 and H2O in the presence of H2 and release electrons that are used to form the electric current generated by the fuel cell. H2 and / or hydrocarbons suitable for the formation of H2 are introduced into the anode 330 via the anode collector 320.

[0049] In an exemplary portion of the fuel cell stack shown in FIG. 3, at least a portion of the H2 and / or hydrocarbons can be passed from the reforming element 380 to the anode collector 320. The reforming element 380 can include one or more surfaces that include a reforming catalyst. The reforming catalyst in the first portion of at least one surface in the reforming element can correspond to the reforming catalyst having the catalyst density pattern described herein. Optionally, the reforming catalyst can be present in the first portion of a surface (not shown) within the anode 330, and the reforming catalyst in the anode 330 has the catalyst density pattern described herein.

[0050] The direction of flow within the anode of a molten carbonate fuel cell can have any convenient orientation relative to the direction of flow within the cathode. One option can be to use a cross-flow configuration such that the direction of flow within the anode is at an angle of approximately 90° relative to the direction of flow within the cathode. This type of flow configuration can have practical benefits because, by using a cross-flow configuration, the manifolds and / or piping for the anode inlet / outlet can be located on a different side of the fuel cell stack than the manifolds and / or piping for the cathode inlet / outlet.

[0051] Figure 4 schematically shows an example of a top view of a fuel cell cathode, along with arrows indicating the direction of flow within the corresponding fuel cell anode. In Figure 4, arrow 405 indicates the direction of flow within cathode 450, while arrow 425 indicates the direction of flow within the anode (not shown).

[0052] Since the anode and cathode flows are oriented at approximately 90° to each other, the anode and cathode flow patterns can contribute to the reaction conditions being different at various parts of the cathode. The different conditions can be explained by considering the reaction conditions at the four corners of the cathode. In the description of Figure 4, the reaction conditions described herein are qualitatively similar to those of a fuel cell operating with a CO2 utilization rate of 75% or more (or 80% or more).

[0053] Corner 482 corresponds to the part of the fuel cell that is close to the entry points of both the cathode input flow and the anode input flow. As a result, the concentrations of both CO2 (in the cathode) and H2 (in the anode) are relatively high at corner 482. Based on this high concentration, it is expected that a portion of the fuel cell near corner 482 can operate under the expected conditions without substantial transport of ions other than carbonate ions across the electrolyte.

[0054] Corner 484 corresponds to the part of the fuel cell that is close to the entry point of the cathode input flow and close to the exit point of the anode output flow. At positions near corner 484, the amount of current density may be limited due to the low concentration of H2 in the anode, depending on the fuel utilization rate. However, sufficient CO2 should be present such that any ions transported across the electrolyte substantially correspond to carbonate ions.

[0055] Corner 486 corresponds to the part of the fuel cell that is close to the exit point of the anode output flow and close to the exit point of the cathode output flow. At positions near corner 486, it can be expected that little or no current occurs due to the low driving force for the fuel cell reaction, because the concentrations of both H2 (in the anode) and CO2 (in the cathode) are lower.

[0056] Corner 488 corresponds to a portion of the fuel cell that is near the entry point of the anode input stream and near the exit point of the cathode output stream. Due to the relatively high availability of hydrogen at locations near corner 488, it is expected that a substantial current density can be obtained. However, due to the relatively low concentration of CO2, a significant transport of hydroxide ions and / or other alternative ions can occur. Depending on the embodiment, the calculated CO2 utilization rate can increase by 5% or more, or 10% or more, or 15% or more, or 20% or more due to the significant transport of alternative ions. Additionally or alternatively, the transport rate can be 0.97 or less, or 0.95 or less, or 0.90 or less, or 0.85 or less, or 0.80 or less. The transport of a significant amount of alternative ions across the electrolyte can temporarily cause a higher current density to be maintained at locations near corner 488. However, the transport of alternative ions can also cause degradation of the cathode and / or anode structure, so the current density over time at locations near corner 488 will be lower (and possibly zero). Note that when the amount of alternative ion transport is less (such as a transport rate of 0.96 or more, or 0.98 or more), the amount of degradation over the lifetime is not as severe.

[0057] It has been found that when alternative ion transport becomes significant at one or more locations within the fuel cell, the fuel cell immediately begins to degrade. This is thought to be because one or more locations degrade and no longer provide additional current density. When the region(s) no longer contribute to the desired current density, the remaining locations within the fuel cell need to operate at a higher current density to maintain a certain overall (average) current density for the fuel cell. This can cause the region for alternative ion transport to increase and create an expansion portion of the fuel cell, which degrades and ultimately stops operating. Alternatively, due to the degradation of a part of the fuel cell, the total current density from the cell can be reduced, which is also undesirable. By operating the fuel cell with an improved reforming catalyst pattern, the temperature fluctuations caused by the non-uniform distribution of alternative ion transport can be reduced. This can reduce or minimize the amount of degradation caused by alternative ion transport that occurs while CO2 capture is increasing, enabling a longer fuel cell life.

[0058] Anode Input and Output In various aspects, the anode input stream for an MCFC can include hydrogen, hydrocarbons such as methane, hydrocarbonaceous or hydrocarbon-like compounds that may contain heteroatoms different from C and H, or combinations thereof. The source of hydrogen / hydrocarbon / hydrocarbon-like compounds can be referred to as a fuel source. In some aspects, most of the methane (or other hydrocarbon, hydrocarbonaceous, or hydrocarbon-like compound) supplied to the anode can typically be fresh methane. In this description, fresh fuel such as fresh methane refers to fuel that is not recycled from another fuel cell process. For example, methane recycled from the anode outlet stream back to the anode inlet may not be considered "fresh" methane and may instead be described as recycled methane.

[0059] The fuel source used can be shared with other components such as a turbine that uses a portion of the fuel source to provide a CO2-containing stream for the cathode input. The fuel source input can include water proportional to the fuel appropriate for reforming hydrocarbon (or hydrocarbon-like) compounds within the reforming section that generates hydrogen. For example, if methane is the fuel input for reforming to produce H2, the water-to-fuel molar ratio can be from about 1:1 to about 10:1, such as at least about 2:1. Ratios of 4:1 or more are typical for external reforming, while lower values can be typical for internal reforming. To the extent that H2 is part of the fuel source, in some optional embodiments, additional water may not be required in the fuel, as the oxidation of H2 at the anode can tend to produce H2O that can be used to reform the fuel. The fuel source can also optionally contain components associated with the fuel source (e.g., a natural gas supply can contain some amount of CO2 as an additional component). For example, a natural gas supply can contain CO2, N2, and / or other inert (dilute) gases as additional components. Optionally, in some embodiments, the fuel source may also contain CO, such as CO derived from a recycled portion of the anode exhaust. An additional or alternative possible source of CO in the fuel to the fuel cell assembly can be the CO produced by steam reforming of the hydrocarbon fuel performed on the fuel prior to entering the fuel cell assembly.

[0060] More generally, various types of fuel streams may be suitable for use as the anode input stream of the anode of a molten carbonate fuel cell. Some fuel streams may correspond to streams containing hydrocarbons and / or hydrocarbon-like compounds that may also contain heteroatoms different from both C and H. In this discussion, unless otherwise specified, reference to a hydrocarbon-containing fuel stream for an MCFC anode is defined to include fuel streams containing such hydrocarbon-like compounds. Examples of hydrocarbon (hydrocarbon-like) fuel streams include natural gas, streams containing C1-C4 carbon compounds (such as methane or ethane), and streams containing heavier hydrocarbons (including hydrocarbon-like compounds) of C5 and above, and combinations thereof. Still further additional or alternative examples of possible fuel streams for use in the anode input include biogas-type streams such as methane produced from the natural (biological) decomposition of organic materials.

[0061] In some embodiments, a molten carbonate fuel cell can be used to process an input fuel stream, such as natural gas and / or hydrocarbon streams, that have a low energy content due to the presence of diluent compounds. For example, some sources of methane and / or natural gas can be sources that contain significant amounts of CO2 or any of other inert molecules such as nitrogen, argon, or helium. Since the amount of CO2 and / or inert substances is increasing, the energy content of the fuel stream based on the source can decrease. Using a fuel with a low energy content for a combustion reaction (e.g., to power a combustion turbine) can present difficulties. However, a molten carbonate fuel cell can generate electricity based on a fuel source with a low energy content while reducing or minimizing the impact on the efficiency of the fuel cell. The additional gas volume may require additional heat to raise the temperature of the fuel to the temperature for reforming and / or the anode reaction. In addition, due to the equilibrium of the water gas shift reaction in the fuel cell anode, the presence of additional CO2 can affect the relative amounts of H2 and CO present in the anode output. However, otherwise, the direct effect of the inert compound on the reforming and anode reactions can be minimal. The amount of CO2 and / or inert compound in the fuel stream of the molten carbonate fuel cell, if present, can be at least about 1 volume %, for example, at least about 2 volume %, or at least about 5 volume %, or at least about 10 volume %, or at least about 15 volume %, or at least about 20 volume %, or at least about 25 volume %, or at least about 30 volume %, or at least about 35 volume %, or at least about 40 volume %, or at least about 45 volume %, or at least about 50 volume %, or at least about 75 volume %. Additionally or alternatively, the amount of CO2 and / or inert compound in the fuel stream of the molten carbonate fuel cell can be about 90 volume % or less, for example, about 75 volume % or less, or about 60 volume % or less, or about 50 volume % or less, or about 40 volume % or less, or about 35 volume % or less.

[0062] Still other examples of possible sources of the anode input stream may correspond to the output streams of refineries and / or other industrial processes. For example, coking is a common process in many refineries to convert heavier compounds to a lower boiling point range. Coking typically produces off-gas containing various compounds that are gases at room temperature, typically including CO and various C1-C4 hydrocarbons. This off-gas can be used as at least part of the anode input stream. Other refinery off-gas streams, such as light fractions (C1-C4) produced during cracking or other refining processes, may also be suitable for inclusion in the anode input stream, either in addition to or alternatively. Still other suitable refinery streams may include, either in addition to or alternatively, refinery streams containing CO or CO2 that also contain H2 and / or reformable fuel compounds.

[0063] Still other possible sources for the anode input may include, either in addition to or alternatively, streams with increased water content. For example, an ethanol output stream from an ethanol plant (or another type of fermentation process) may contain a significant portion of H2O prior to final distillation. The effect such H2O has on the operation of the fuel cell can typically be minimal. Thus, a fermentation mixture of alcohol (or other fermentation products) and water can be used as at least part of the anode input stream.

[0064] Biogas, or digester gas, is another possible additional or alternative source for the anode input. Biogas may consist mainly of methane and CO2 and is typically produced by the decomposition or digestion of organic matter. Anaerobic bacteria may be used to digest the organic matter and produce biogas. Impurities such as sulfur-containing compounds may be removed from the biogas before using it as the anode input.

[0065] The output stream from the MCFC anode can contain H2O, CO2, CO, and H2. Optionally, the anode output stream can also have unreacted fuel (such as H2 or CH4) or an inert compound in the feed as additional output components. Instead of using this output stream as a fuel source to provide heat for the reforming reaction or as a combustion fuel to heat the cell, one or more separations can be performed on the anode output stream to separate CO2 from components that may be valuable as inputs to another process, such as H2 or CO. H2 and / or CO can be used as synthesis gas for chemical synthesis, as a hydrogen source for chemical reactions, and / or as a fuel with reduced greenhouse gas emissions.

[0066] The anode exhaust can be subjected to various gas treatment options, including water gas shift and separation from each other of the components. Two general anode treatment schemes are shown in FIGS. 1 and 2.

[0067] Figure 1 schematically shows an example of a reaction system for operating a fuel cell array of a molten carbonate fuel cell in conjunction with a chemical synthesis process. In Figure 1, fuel stream 105 is provided to reforming stage(s) 110 associated with anode 127 of a fuel cell 120, such as a fuel cell that is part of a fuel cell stack in the fuel cell array. The reforming stage 110 associated with fuel cell 120 can be internal to the fuel cell assembly. In some optional embodiments, an external reforming stage (not shown) can be used to reform a portion of the reformable fuel in the input stream before passing the input stream to the fuel cell assembly. Fuel stream 105 preferably can include a reformable fuel, such as methane, other hydrocarbons, and / or other hydrocarbon-like compounds containing carbon-hydrogen bonds. Fuel stream 105 can also optionally contain H2 and / or CO, such as H2 and / or CO provided by an optional anode recirculation stream 185. Note that the anode recirculation stream 185 is optional and in many embodiments, a recirculation stream that returns directly or indirectly from anode exhaust 125 to anode 127, either directly or through combination with fuel stream 105 or the reformed fuel stream 115, is not provided. After reforming, the reformed fuel stream 115 can be passed to anode 127 of fuel cell 120. A CO2 and O2-containing stream 119 can also be passed to cathode 129. Carbonate ions 122, CO3 from the cathode portion 129 of the fuel cell 2-Due to the flow, the remaining reactants required for the anodic fuel cell reaction can be provided. Based on the reaction at the anode 127, the resulting anode exhaust 125 can include H2O, CO2, one or more components corresponding to unreacted fuel (H2, CO, CH4, or other components corresponding to reformable fuel), and optionally, one or more additional non-reactive components such as N2 and / or other contaminants that are part of the fuel stream 105. Next, the anode exhaust 125 can be passed through one or more separation stages. For example, the CO2 removal stage 140 can correspond to a cryogenic CO2 removal system, an amine scrubbing stage for removing acid gases such as CO2, or another suitable type of CO2 separation stage for separating the CO2 output stream 143 from the anode exhaust. Optionally, the anode exhaust can first be passed through a water gas shift reactor 130 to convert any CO present in the anode exhaust (along with some H2O) into CO2 and H2 in the optionally water gas shifted anode exhaust 135. Depending on the nature of the CO2 removal stage, a moisture condensation or removal stage 150 may be desirable to remove the water output stream 153 from the anode exhaust. Although shown after the CO2 separation stage 140 in FIG. 1, this can optionally be located before the CO2 separation stage 140 instead. Additionally, an optional membrane separation stage 160 for the separation of H2 can be used to produce a high-purity permeate stream 163 of H2. Next, the resulting retentate stream 166 can be used as an input to a chemical synthesis process. Additionally or alternatively, the stream 166 can be shifted in a second water gas shift reactor 131 to adjust the H2, CO, and CO2 contents to different ratios and produce an output stream 168 for further use in the chemical synthesis process. In FIG. 1, the anode exhaust recirculation stream 185 is shown as being drawn from the retentate stream 166, but the anode recirculation stream 185 can additionally or alternatively be drawn from other convenient locations within or between the various separation stages. The separation stages and the shift reactor(s) can additionally or alternatively be configured in a different order and / or in a parallel configuration. Finally, a stream 139 with a reduced CO2 content can be produced as the output from the cathode 129.For simplicity, various stages of compression and heat addition / removal, as well as steam addition or removal, which may be useful in the process, are not shown.

[0068] As described above, the various types of separation performed on the anode exhaust can be carried out in any convenient order. Figure 2 shows an example of an alternative order for performing separation on the anode exhaust. In Figure 2, the anode exhaust 125 can first be passed through a separation stage 260 for removing a portion 263 of the hydrogen content from the anode exhaust 125. This can, for example, reduce the H2 content of the anode exhaust and provide a hold-up 266 with an H2 to CO ratio close to 2:1. Next, the H2 to CO ratio can be further adjusted to achieve the desired value in the water gas shift stage 230. Thereafter, the water gas shifted output 235 can be passed through a CO2 separation stage 240 and a water removal stage 250 to produce an output stream 275 suitable for use as an input to the desired chemical synthesis process. Optionally, the output stream 275 can be exposed to an additional water gas shift stage (not shown). A portion of the output stream 275 can optionally be recycled (not shown) to the anode input. Needless to say, still other combinations and orderings of the separation stages may be used to produce a stream based on the anode output having the desired composition. For simplicity, various stages of compression and heat addition / removal, as well as steam addition or removal, which may be useful in the process, are not shown.

[0069] Cathode Input and Output Conventionally, a molten carbonate fuel cell can operate based on drawing a desired load while consuming a portion of the fuel in the fuel stream delivered to the anode. Next, the voltage of the fuel cell can be determined by the load, the fuel input to the anode, the air and CO2 provided to the cathode, and the internal resistance of the fuel cell. The CO2 to the cathode can conventionally be provided in part by using at least a portion of the anode exhaust as part of the cathode input stream. In contrast, the present invention can use separate / different sources for the anode input and the cathode input. By removing any direct connection between the composition of the anode input stream and the cathode input stream, additional options for operating the fuel cell become available, such as generating excess syngas, improving carbon dioxide capture, and / or improving the overall efficiency (electrical and chemical power) of the fuel cell.

[0070] In various aspects, the MCFC can be operated to cause alternative ion transport across the electrolyte for the fuel cell. To cause alternative ion transport, the CO2 content of the cathode input stream can be 5.0 volume % or less, or 4.0 volume % or less, for example, 1.5 volume % - 5.0 volume %, or 1.5 volume % - 4.0 volume %, or 2.0 volume % - 5.0 volume %, or 2.0 volume % - 4.0 volume %.

[0071] An example of a CO2-containing stream suitable for use as the cathode input stream can be an output or exhaust stream from a combustion source. Examples of combustion sources include, but are not limited to, sources based on the combustion of natural gas, coal, and / or other hydrocarbon-based fuels (including bio-derived fuels). Additional or alternative sources can include other types of boilers, combustion heaters, furnaces, and / or other types of devices that burn carbon-containing fuels to heat another substance (such as water or air).

[0072] Other possible sources of the cathode input stream may include, in addition to or alternatively, sources of biogenic CO2. This may include, for example, CO2 generated during the processing of biogenic compounds such as CO2 generated during ethanol production. Additional or alternative examples may include CO2 generated by the combustion of biogenic fuels such as the combustion of lignocellulose. Still other additional or alternative possible CO2 sources may correspond to output or exhaust streams from various industrial processes such as CO2-containing streams generated by plants for manufacturing steel, cement, and / or paper.

[0073] Yet another additional or alternative possible source of CO2 may be a CO2-containing stream from a fuel cell. The CO2-containing stream from a fuel cell may correspond to a cathode output stream from different fuel cells, an anode output stream from different fuel cells, a recycle stream from the cathode output to the cathode input of a fuel cell, and / or a recycle stream from the anode output to the cathode input of a fuel cell. For example, under conventional conditions, an MCFC operating in stand-alone mode can produce cathode exhaust with a CO2 concentration of at least about 5 volume %. Such a CO2-containing cathode exhaust can be used as the cathode input for an MCFC operating in accordance with aspects of the present invention. More generally, other types of fuel cells that produce a CO2 output from cathode exhaust, as well as other types of CO2-containing streams not generated by a "combustion" reaction and / or by a combustion power generator, can be used additionally or alternatively. Optionally, but preferably, a CO2-containing stream from another fuel cell may be from another molten carbonate fuel cell. For example, for molten carbonate fuel cells connected in series with respect to the cathode, the output from the cathode of the first molten carbonate fuel cell can be used as the input to the cathode of the second molten carbonate fuel cell.

[0074] In addition to CO2, the cathode input stream may include O2 to provide the components necessary for the cathode reaction. Some cathode input streams may be based on having air as a component. For example, a combustion exhaust stream may be formed by burning a hydrocarbon fuel in the presence of air. Such a combustion exhaust stream, or another type of cathode input stream having an oxygen content based on the inclusion of air, may have an oxygen content of about 20 volume % or less, for example, about 15 volume % or less, or about 10 volume % or less. Additionally or alternatively, the oxygen content of the cathode input stream may be at least about 4 volume %, for example, at least about 6 volume %, or at least about 8 volume %. More generally, the cathode input stream may have an oxygen content suitable for performing the cathode reaction. In some embodiments, this may correspond to an oxygen content of about 5 volume % to about 15 volume %, for example, about 7 volume % to about 9 volume %. For many types of cathode input streams, the combined amount of CO2 and O2 may correspond to less than about 21 volume % of the input stream, for example, less than about 15 volume % of the stream, or less than about 10 volume % of the stream. An air stream containing oxygen can be combined with a CO2 source having a low oxygen content. For example, an exhaust stream produced by burning coal may contain a low oxygen content, and this can be mixed with air to form the cathode inlet stream.

[0075] In addition to CO2 and O2, the cathode input stream can also be composed of inert / non-reactive species such as N2, H2O, and other typical oxidant (air) components. For example, for a cathode input derived from the exhaust of a combustion reaction, when air is used as part of the oxidant source for the combustion reaction, the exhaust gas can contain typical components of air such as N2, H2O, and small amounts of other compounds present in the air. Depending on the nature of the fuel source for the combustion reaction, additional species present after combustion based on the fuel source may include oxides of H2O, nitrogen (NOx) and / or sulfur (SOx), and one or more of other compounds that are partial or complete combustion products of compounds present in the fuel and / or present in the fuel such as CO. These species may be present in an amount that does not harm the cathode catalyst surface, but can reduce the overall cathode activity. Such a performance reduction may be acceptable, or species that interact with the cathode catalyst may be reduced to an acceptable level by known pollutant removal techniques.

[0076] The amount of O2 present in the cathode input stream (such as an input cathode stream based on combustion exhaust) can advantageously be sufficient to provide the oxygen required for the cathode reaction in the fuel cell. For this reason, the volume fraction of O2 can advantageously be at least 0.5 times the amount of CO2 in the exhaust. Optionally, additional air can be added to the cathode input as needed to provide sufficient oxidant for the cathode reaction. When using air in some form as the oxidant, the amount of N2 in the cathode exhaust can be at least about 78 volume %, for example, at least about 88 volume %, and / or 95 volume % or less. In some embodiments, the cathode input stream can contain, in addition or alternatively, compounds generally regarded as pollutants such as H2S or NH3. In other embodiments, the cathode input stream can be cleaned to reduce or minimize the content of such pollutants.

[0077] Temperatures suitable for the operation of MCFCs can be, for example, an inlet temperature of about 550°C, an outlet temperature of about 625°C, and at least about 500°C, such as from about 450°C to about 750°C. Before entering the cathode, heat can be added to or removed from the cathode input stream, if desired, to provide heat for other processes, such as reforming the fuel input for the anode. For example, if the source for the cathode input stream is a combustion exhaust stream, the combustion exhaust stream may have a temperature higher than the desired temperature at the cathode inlet. In such an embodiment, heat can be removed from the combustion exhaust before using it as the cathode input stream. Alternatively, the combustion exhaust can be at a very low temperature, for example, after a wet gas scrubber on a coal-fired boiler, in which case the combustion exhaust can be less than about 100°C. Alternatively, the combustion exhaust can be derived from the exhaust of a gas turbine operating in a combined cycle mode where the gas can be cooled by raising steam to operate an additional steam turbine for power generation. In this case, the gas can be less than about 50°C. Heat can be added to the combustion exhaust that has been cooled more than desired.

[0078] Operating Strategies for Additional Molten Carbonate Fuel Cells In some embodiments, when operating an MCFC to cause alternative ion transport, the anode of the fuel cell can be operated at a traditional fuel utilization value of approximately 60% to 80%. When attempting to generate power, operating the anode of the fuel cell at a relatively high fuel utilization can be beneficial for improving the electrical efficiency (i.e., the electrical energy generated per unit of chemical energy consumed by the fuel cell).

[0079] In some embodiments, it may be beneficial to reduce the electrical efficiency of a fuel cell in order to provide other benefits such as an increase in the amount of H2 provided to the anode output stream. This can be beneficial, for example, when it is desirable to consume excess heat generated in the fuel cell (or fuel cell stack) by performing additional reforming and / or another endothermic reaction. For example, a molten carbonate fuel cell can be operated to provide an increase in the production of synthesis gas and / or hydrogen. The heat required to perform the endothermic reforming reaction can be provided by an exothermic electrochemical reaction in the anode for power generation. Instead of attempting to transport the heat generated by the exothermic fuel cell reaction(s) away from the fuel cell, this excess heat can be used in situ as a heat source for reforming and / or another endothermic reaction. This can result in a more efficient use of thermal energy and / or a reduction in the need for additional external or internal heat exchange. Essentially, this efficient generation and use of thermal energy in situ can reduce the complexity and components of the system while maintaining advantageous operating conditions. In some embodiments, the amount of reforming or other endothermic reaction is not significantly less than the amount of heat required as typically described in the prior art, but rather is selected to correspond to or even be greater than the amount of excess heat generated by the exothermic reaction(s).

[0080] Additionally or alternatively, the fuel cell can be operated such that the temperature difference between the anode inlet and the anode outlet can be negative rather than positive. Thus, instead of increasing the temperature between the anode inlet and the anode outlet, a sufficient amount of reforming and / or other endothermic reactions can be carried out to make the output stream from the anode outlet cooler than the anode inlet temperature. Further additionally or alternatively, additional fuel can be supplied to the fuel cell and / or a heater for an internal reforming stage (or other internal endothermic reaction stage) such that the temperature difference between the anode input and the anode output can be less than the difference expected based on the relative demand for the endothermic reaction(s) and the combined heat generation of the cathode combustion reaction and the anode reaction for generating power. In embodiments where reforming is used as the endothermic reaction, operating the fuel cell to reform excess fuel can enable increased syngas and / or increased hydrogen production compared to conventional fuel cell operation while minimizing the complexity of the system for heat exchange and reforming. Thus, the additional syngas and / or additional hydrogen can be used in various applications, including chemical synthesis processes for using as a "clean" fuel and / or hydrogen collection / reuse.

[0081] The amount of heat generated per mole of hydrogen oxidized by the exothermic reaction at the anode can be substantially greater than the amount of heat consumed per mole of hydrogen generated by the reforming reaction. The net reaction of hydrogen in a molten carbonate fuel cell (H2 + 1 / 2 O2 => H2O) can have a reaction enthalpy of approximately -285 kJ / mol of hydrogen molecules. At least a portion of this energy can be converted into electrical energy within the fuel cell. However, the difference (approximately) between the reaction enthalpy and the electrical energy generated by the fuel cell can become heat within the fuel cell. This amount of energy can alternatively be expressed as the current density of the cell (current per unit area), or <current density> * (Vmax - Vactual), multiplied by the difference between the theoretical maximum voltage and the actual voltage of the fuel cell. This amount of energy is defined as the "waste heat" of the fuel cell. As an example of reforming, the enthalpy of reforming for methane (CH4 + 2H2O => 4H2 + CO2) can be approximately 250 kJ / mol of methane, or approximately 62 kJ / mol of hydrogen molecules. From a heat balance perspective, each electrochemically oxidized hydrogen molecule can generate enough heat to produce more than one hydrogen molecule by reforming. In a conventional configuration, this excess heat can cause a substantial temperature difference from the anode inlet to the anode outlet. Instead of allowing this excess heat to be used to increase the temperature within the fuel cell, the excess heat can be consumed by performing an equivalent amount of reforming reaction. The excess heat generated at the anode can be supplemented by the excess heat generated by the combustion reaction within the fuel cell. More generally, the excess heat can be consumed by performing endothermic reactions at the fuel cell anode and / or in an endothermic reaction stage integrated with the fuel cell.

[0082] Depending on the embodiment, the amount of reforming and / or other endothermic reactions can be selected relative to the amount of hydrogen reacted at the anode to achieve a desired temperature ratio of the fuel cell. As used herein, "temperature ratio" is defined as the heat generated by the exothermic reactions (including exothermic reactions at both the anode and the cathode) in the fuel cell assembly divided by the endothermic heat requirement of the reforming reactions occurring within the fuel cell assembly. Mathematically expressed, the temperature ratio (TH) = Q発熱 / Q 吸熱 where Q 発熱 is the total heat generated by the exothermic reaction, and Q 吸熱 is the total heat consumed by the endothermic reaction occurring in the fuel cell. Note that the heat generated by the exothermic reaction may correspond to the reforming reaction, water gas shift reaction, combustion reaction (i.e., oxidation of the fuel compound), and / or any heat resulting from the electrochemical reaction in the cathode of the fuel cell. The heat generated by the electrochemical reaction can be calculated based on subtracting the actual output voltage of the fuel cell from the ideal electrochemical potential of the fuel cell reaction across the electrolyte. For example, the ideal electrochemical potential of the reaction in an MCFC is considered to be about 1.04 V based on the net reaction occurring in the cell. During operation of the MCFC, the cell typically has an output voltage less than 1.04 V due to various losses. For example, a common output / operating voltage can be about 0.7 V. The heat generated may be equal to the difference between the electrochemical potential of the cell (i.e., about 1.04 V) and the operating voltage. For example, when an output voltage of about 0.7 V is achieved in the fuel cell, the heat generated by the electrochemical reaction in the cell can be about 0.34 V. Thus, in this situation, about 0.7 V of electricity and about 0.34 V of thermal energy can be generated by the electrochemical reaction. In such an example, the about 0.7 V of electrical energy is not included as part of Q 発熱 . In other words, thermal energy is not electrical energy.

[0083] In various aspects, the temperature ratio can be determined for any convenient fuel cell structure such as a fuel cell stack, an individual fuel cell within the fuel cell stack, a fuel cell stack in which a reforming stage is integrated, a fuel cell stack in which an endothermic reaction stage is integrated, or a combination thereof. The temperature ratio may also be calculated for different units within a fuel cell stack, such as an assembly of fuel cells or a fuel cell stack. For example, the temperature ratio may be calculated for a fuel cell (or fuel cells) within a fuel cell stack, along with an integrated reforming stage and / or an integrated endothermic reaction stage element that is sufficiently close to the integrated fuel cell(s) from a heat integration perspective.

[0084] From a heat integration perspective, the representative width in a fuel cell stack can be the height of an individual fuel cell stack element. It should be noted that a separate reforming stage and / or a separate endothermic reaction stage may have a height in the stack that is different from that of the fuel cell. In such a situation, the height of the fuel cell element can be used as the representative height. In this consideration, an integrated endothermic reaction stage can be defined as a stage that is heat integrated with one or more fuel cells, such that the integrated endothermic reaction stage can use the heat from the fuel cell as a heat source for reforming. Such an integrated endothermic reaction stage is positioned less than 10 times the height of the stack element from the fuel cell and can be defined as providing heat to the integrated stage. For example, an integrated endothermic reaction stage (such as a reforming stage) can be positioned less than 10 times the height of the stack element, or less than 8 times the height of the stack element, or less than 5 times the height of the stack element, or less than 3 times the height of the stack element, from any of the heat-integrated fuel cells. In this consideration, an integrated reforming stage and / or an integrated endothermic reaction stage representing a stack element adjacent to a fuel cell element is defined to have a height that is about 1 stack element or less from the adjacent fuel cell element.

[0085] A temperature ratio of about 1.3 or less, or about 1.15 or less, or about 1.0 or less, or about 0.95 or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 or less, or about 0.75 or less can be lower than the temperature ratio typically required in the use of MCFC fuel cells. In aspects of the present invention, the temperature ratio can be reduced to increase and / or optimize syngas production, hydrogen production, production of other products via endothermic reactions, or combinations thereof.

[0086] In various aspects of the present invention, the operation of the fuel cell can be characterized based on the temperature ratio. When the fuel cell operates to have a desired temperature ratio, the molten carbonate fuel cell can operate to have a temperature ratio of about 1.5 or less, for example, about 1.3 or less, or about 1.15 or less, or about 1.0 or less, or about 0.95 or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 or less, or about 0.75 or less. Additionally or alternatively, the temperature ratio can be at least about 0.25, or at least about 0.35, or at least about 0.45, or at least about 0.50. Further additionally or alternatively, in some aspects, the fuel cell can operate such that the temperature rise between the anode input and the anode output is about 40°C or less, for example, about 20°C or less, or about 10°C or less. Still further additionally or alternatively, the fuel cell can operate such that the anode outlet temperature is about 10°C lower to about 10°C higher than the temperature of the anode inlet. Also still further additionally or alternatively, the fuel cell can operate such that the anode inlet temperature is higher than the anode outlet temperature, for example, at least about 5°C higher, or at least about 10°C higher, or at least about 20°C higher, or at least about 25°C higher. Still further additionally or alternatively, the fuel cell can operate such that the anode inlet temperature is about 100°C or less, or about 80°C or less, or about 60°C or less, or about 50°C or less, or about 40°C or less, or about 30°C or less, or about 20°C or less higher than the anode outlet temperature.

[0087] Operating the fuel cell with a temperature ratio less than 1 can result in a temperature drop across the entire fuel cell. In some embodiments, the amount of reforming and / or other endothermic reactions may be limited such that the temperature drop from the anode inlet to the anode outlet can be about 100 °C or less, such as about 80 °C or less, or about 60 °C or less, or about 50 °C or less, or about 40 °C or less, or about 30 °C or less, or about 20 °C or less. Limiting the temperature drop from the anode inlet to the anode outlet can be beneficial, for example, to maintain a temperature sufficient to allow complete or substantially complete conversion (by reforming) of the fuel in the anode. In other embodiments, additional heat can be supplied to the fuel cell (such as by heat exchange or combustion of additional fuel) such that the anode inlet temperature is about 100 °C or less, such as about 80 °C or less, or about 60 °C or less, or about 50 °C or less, or about 40 °C or less, or about 30 °C or less, or about 20 °C or less higher than the anode outlet temperature, due to the balance between the heat consumed by the endothermic reaction and the additional external heat supplied to the fuel cell.

[0088] The amount of reforming may additionally or alternatively depend on the availability of reformable fuel. For example, if the fuel contains only H2, the H2 is already reformed and not further reformable, so no reforming occurs. The amount of "synthesis gas produced" by the fuel cell can be defined as the difference between the lower heating value (LHV) value of the synthesis gas at the anode input and the LHV value of the synthesis gas at the anode output. The net synthesis gas LHV produced = (LHV (exit sg) - LHV (inlet sg)), where LHV (inlet sg) and LHV (exit sg) refer to the LHV of the synthesis gas in the anode inlet stream or inlet flow and the LHV of the synthesis gas in the anode exit stream or exit flow, respectively. A fuel cell given a fuel containing a significant amount of H2 may have a limited amount of possible synthesis gas production because the fuel contains a significant amount of already reformed H2 rather than additional reformable fuel. The lower heating value is defined as the enthalpy of combustion of the fuel components to the completely oxidized products in the gas phase (i.e., gas phase CO2 and H2O products). For example, any CO2 present in the anode input stream does not contribute to the fuel content of the anode input because the CO2 is already completely oxidized. In this definition, the amount of oxidation occurring at the anode due to the anode fuel cell reaction is defined as the oxidation of H2 in the anode as part of the electrochemical reaction in the anode.

[0089] Examples of methods for operating a fuel cell with a reduced temperature ratio can be methods that balance heat generation and consumption in the fuel cell and / or perform excessive reforming of the fuel to consume more heat than is generated. Reforming a fuel that can be reformed to form H2 and / or CO can be an endothermic process, while the anodic electrochemical oxidation reaction and the cathodic combustion reaction(s) can be exothermic. During conventional fuel cell operation, the amount of reforming required to supply the feed components for fuel cell operation typically consumes less heat than the amount of heat generated by the anodic oxidation reaction. For example, conventional operation at a fuel utilization of about 70% or about 75% results in a temperature ratio greater than 1, e.g., at least about 1.4 or greater, or 1.5 or greater. As a result, the output stream of the fuel cell can be hotter than the input stream. Instead of this type of conventional operation, the amount of fuel reformed in the reforming stage associated with the anode can be increased. For example, additional fuel can be reformed so that the heat generated by the exothermic fuel cell reaction is (substantially) balanced by the heat consumed in reforming and / or more heat than is generated can be consumed. This can result in hydrogen being substantially in excess compared to the amount oxidized in the anode for power generation, and the temperature ratio can be about 1.0 or less, e.g., about 0.95 or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 or less, or about 0.75 or less.

[0090] Either hydrogen or synthesis gas can be withdrawn from the anode exhaust as a chemical energy output. Hydrogen can be used as a clean fuel without generating greenhouse gases when burned or combusted. Instead, for hydrogen produced by reforming hydrocarbons (or hydrocarbonaceous compounds), the CO2 would already be "captured" in the anode loop. Additionally, hydrogen can be a valuable input for various purification processes and / or other synthesis processes. Synthesis gas can also be a valuable input for various processes. In addition to having fuel value, synthesis gas can be used as a feedstock for producing other higher-value products, such as by using the synthesis gas as an input for Fischer-Tropsch synthesis and / or methanol synthesis processes.

[0091] In some embodiments, the content of reformable hydrogen in the reformable fuel in the input stream delivered to the anode and / or delivered to the reforming stage associated with the anode can be at least about 50% more, for example, at least about 75% more, or at least about 100% more than the net amount of hydrogen reacted at the anode. Additionally or alternatively, the content of reformable hydrogen in the fuel in the input stream delivered to the anode and / or delivered to the reforming stage associated with the anode can be at least about 50% more, for example, at least about 75% more, or at least about 100% more than the net amount of hydrogen reacted at the anode. In various embodiments, the ratio of the content of reformable hydrogen in the reformable fuel in the fuel stream to the amount of hydrogen reacted in the anode can be at least about 1.5:1, or at least about 2.0:1, or at least about 2.5:1, or at least about 3.0:1. Additionally or alternatively, the ratio of the content of reformable hydrogen in the reformable fuel in the fuel stream to the amount of hydrogen reacted in the anode can be about 20:1 or less, for example, about 15:1 or less, or about 10:1 or less. In one embodiment, it is contemplated that less than 100% of the content of reformable hydrogen in the anode inlet stream can be converted to hydrogen. For example, at least about 80%, for example, at least about 85%, or at least about 90% of the content of reformable hydrogen in the anode inlet stream can be converted to hydrogen in the anode and / or in the associated reforming stage(s). Additionally or alternatively, the amount of reformable fuel delivered to the anode can be characterized based on the LHV of the reformable fuel relative to the LHV of the oxidized hydrogen in the anode. This can be referred to as the excess ratio of the reformable fuel. In various embodiments, the excess ratio of the reformable fuel can be at least about 2.0, for example, at least about 2.5, or at least about 3.0, or at least about 4.0. Additionally or alternatively, the excess ratio of the reformable fuel can be about 25.0 or less, for example, about 20.0 or less, or about 15.0 or less, or about 10.0 or less.

[0092] Comparative Example - Conventional Catalyst Pattern During the conventional operation of a molten carbonate fuel cell, the cathode typically operates with a CO2 concentration of 8% by volume or more and / or a CO2 utilization rate of 70% or less in the cathode input supply. Such operation typically corresponds to a cathode exhaust containing 5.0% by volume or more of CO2. During such conventional operation, the anode operates using excess fuel with a fuel utilization rate of 60% or more. For operation under such conventional conditions, various reforming catalyst patterns have been developed to improve the operation of the fuel cell. These catalyst patterns are based on the expected temperature profile during conventional fuel cell operation. The expected temperature profile is based on the corresponding prediction of the location where the reaction occurs within the fuel cell. In particular, at a typical or conventional fuel utilization rate of 60% to 70%, some fuel depletion will occur as the fuel moves through the anode from the anode inlet to the anode outlet. Similarly, for a CO2 utilization rate of 40% to 60%, the amount of CO2 available for the reaction will be somewhat reduced because the CO2 is lost as it moves from the cathode inlet to the cathode outlet. Since the concentrations of the reaction components in both the anode and the cathode can contribute to the driving force of the reaction in the fuel cell, the amount of the reaction is expected to be highest near the cathode inlet and / or the anode inlet and to decrease in the direction of the anode outlet and the cathode outlet. Based on this, the conventional catalyst patterns are designed to provide additional reforming near the cathode inlet and the anode inlet while reducing or minimizing the reforming near the anode outlet and the cathode outlet. This balances the location of the exothermic reaction in the fuel cell with the endothermic reaction in the reforming element.

[0093] To illustrate a conventional reforming catalyst pattern and the relationship to the anode flow and cathode flow in a fuel cell, FIG. 5 shows an example of a flow pattern within a reforming element. In the flow pattern shown in FIG. 5, reformable fuel 521 first passes through a channel 530 that is alongside the reforming element 510. The channel 530 is in proximity to the cathode inlet of the fuel cell stack. The channel 530 is separated from the remainder or first portion of the reforming element 510 by a barrier 535. The catalyst density within the channel 530 can be relatively uniform, for example due to there being no reforming catalyst within the channel 530. More typically, the channel 530 can have a non-zero reforming catalyst density corresponding to, for example, an average reforming catalyst density in the first portion 540, a minimum catalyst density in the first portion 540, or any other convenient catalyst density. In the example shown in FIG. 5, after descending through the channel 530 (in the direction of arrow 507), the reformable fuel 521 is passed from the channel 530 into the first portion 540 of the reforming element 510, where it flows through the first portion 540 in the direction of arrow 508. Next, hydrogen produced by the remaining portion of the reformed fuel and / or un-reformed fuel exits the reforming element (not shown) and can be passed to one or more anodes by any convenient means, such as via one or more manifolds or conduits (via the anode collector). Arrow 550 indicates the direction of flow within the anode associated with the reforming element 510, while arrow 560 indicates the direction of flow within the cathode associated with the reforming element 510.

[0094] FIG. 6 shows an example of a reforming catalyst pattern suitable for use on a surface including a reforming catalyst. The catalyst pattern is formed based on parallel lines of catalyst particles, and the direction of flow is substantially parallel to the lines of catalyst particles for most of the flow path. The pattern shown in FIG. 6 corresponds to a pattern suitable for either the reforming catalyst of the entire reforming element or a portion such as the first portion 510 in the reforming element. In FIG. 6, the reforming catalyst density is highest at the left end and decreases such that there is no catalyst density at the right end. As indicated by arrow 550, the reforming catalyst pattern shown in FIG. 6 corresponds to an increase in the concentration of reforming catalyst near the anode inlet and little or no reforming catalyst near the anode outlet. For a flow pattern similar to that of FIG. 6, this means that the reformable fuel initially encounters little or no catalyst density, and the catalyst density is highest just before the reformable fuel exits the reforming element. Thereby, the amount of reforming generated can be maximized near the anode inlet and minimized near the anode outlet. For operation where excess fuel is present in the anode and excess CO2 is present in the cathode, the oxidation of the fuel in the anode can be maximized near the anode inlet. Thus, the maximum amount of endothermic reforming (within the reforming element) occurs near the maximum amount of exothermic oxidation in the anode. It should be noted that when an average catalyst density is used in an initial channel close to the cathode inlet, such as the initial channel 510 of FIG. 5, the catalyst pattern of FIG. 6 can provide substantial reforming close to both the anode inlet and the cathode inlet.

[0095] U.S. Patent No. 8,822,090 describes another type of reforming catalyst pattern. The catalyst pattern of U.S. Patent No. 8,822,090 has two separate catalyst density variations, one aligned with the cathode flow and the other aligned with the anode flow. In U.S. Patent No. 8,822,090, the fuel enters the reforming element on the side that is close to the cathode outlet side of the fuel cell stack. As the fuel moves from the cathode outlet side of the reforming element towards the cathode outlet side, the catalyst density increases. An example of the catalyst pattern is shown with variations in the reforming catalyst density based on the arrangement of the reforming catalyst particles. The maximum catalyst density corresponds to the arrangement of particles with a frequency of 1 particle per two possible arrangement sites and a minimum catalyst density of 1 particle per 64 possible arrangement sites. This corresponds to a variation from maximum to minimum of more than 90% under the definitions provided herein. It should be noted that in addition to the variation along the direction of the cathode flow, there is a second gradient along the direction of the anode flow in the anode associated with the reforming element.

[0096] Example 2 - Reforming of the Catalyst Pattern for Use with Increased CO2 Utilization Using a molten carbonate fuel cell model that includes aspects representing the fluid flow and heat transfer within the fuel cell, the effect of the catalyst pattern on the temperature difference within the fuel cell was determined. This model was constructed using a commercially available process modeling platform. For the model used in this example, the model showed a system having a reforming element, an associated fuel cell having an anode and a cathode, and a separator plate between the fuel cell associated with the reforming element. The flow pattern of the reforming element corresponds to the flow pattern shown in FIG. 5. Similar to FIG. 5, the reforming element included an initial channel close to the cathode inlet having a constant catalyst density (corresponding to the average of the first portion) and a first portion having a catalyst pattern aligned with the flow pattern in the anode.

[0097] In this example, modeling results are provided for a molten carbonate fuel cell having two different types of catalyst patterns for a first portion of a reforming element. One type of catalyst pattern corresponds to the catalyst pattern shown in FIG. 5. The second type of catalyst pattern corresponds to a catalyst pattern in which the difference between the maximum catalyst density and the minimum catalyst density is approximately 63%.

[0098] FIG. 7 shows three types of catalyst density profiles for a first portion of a reforming element. In FIG. 7, the x-axis corresponds to the axis of flow in the anode, 0 corresponds to the anode inlet, and 1 corresponds to the anode outlet. Line 910 corresponds to the conventional catalyst profile shown in FIG. 5. Line 920 corresponds to the catalyst profile used for the second fuel cell modeled. As shown in FIG. 7, for line 920, the ratio of the minimum catalyst density to the maximum catalyst density is 60 / 160, or 3 / 8. This corresponds to a difference of 62.5% between the maximum and the minimum. In contrast, the difference between the maximum and the minimum of line 910 is virtually 100% because the catalyst density drops to zero at the anode outlet of line 910. FIG. 7 also shows line 930, which is similar to the profile in line 920 but with a continuous monotonic decrease in catalyst density as opposed to the stepwise monotonic decrease in catalyst density shown for line 920.

[0099] In this model, a fuel cell using the catalyst profile explained by line 910 for a first portion of the surface within the reforming element was modeled under conditions that result in an increase in CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 3.8 volume %, a fuel utilization of 50%, an actual CO2 utilization of approximately 81%, an average cathode temperature of approximately 903.15°K (630° C.), and 90 mA / cm 2The current density was [value]. The input stream to the reformer contained 29 vol% H2, 9.0 vol% CO2, 41 vol% H2O, and 20 vol% CH4, with the balance corresponding to nitrogen. This resulted in an input stream to the anode input of each fuel cell containing 51 vol% H2, 9 vol% CO2, 24 vol% H2O, 9 vol% CH4, and 6 vol% CO. The cathode input stream of each fuel cell contained 3.8 vol% CO2, 11 vol% O2, and 10 vol% H2O.

[0100] In this model, a fuel cell using the catalyst profile described by line 920 for the first part of the surface within the reforming element was modeled under conditions that result in an increase in CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 4.5 vol%, a fuel utilization of 50%, an actual CO2 utilization of approximately 86%, an average cathode temperature of approximately 903.15°K (630°C), and a current density of 90 mA / cm 2 The current density was [value]. The input stream to the reformer contained 29 vol% H2, 9.0 vol% CO2, 41% H2O, and 20 vol% CH4, with the balance corresponding to nitrogen. This resulted in an input stream to the anode input of each fuel cell containing 53 vol% H2, 9 vol% CO2, 23 vol% H2O, 8 vol% CH4, and 8 vol% CO. The cathode input stream of each fuel cell contained 4.5 vol% CO2, 11 vol% O2, and 10 vol% H2O.

[0101] Figure 8 shows the temperature variations in the separator plate of a fuel cell modeled with a comparative reforming catalyst density corresponding to line 910 in Figure 7. In Figure 8, the x-axis corresponds to the direction of the anode flow, and the y-axis corresponds to the direction of the cathode flow. As shown in Figure 8, the temperature gradient across the entire fuel cell exceeds 80°K or 80°C. This is due to the excessive waste heat generated by alternative ion transport at the corners of the fuel cell corresponding to the cathode outlet and the anode outlet. Based on the catalyst pattern corresponding to line 910, the catalyst density is close to zero near the corners corresponding to the cathode outlet and the anode outlet. For this reason, there is only a minimal amount of endothermic reforming reaction available to balance the additional waste heat generated due to alternative ion transport.

[0102] Figure 9 shows the temperature variations in the separator plate of a fuel cell modeled with a reforming catalyst density corresponding to line 920 in Figure 7. By reducing the difference between the minimum and maximum reforming catalyst densities, the temperature variation across the entire fuel cell is reduced to approximately 37°K or nearly 37°C.

[0103] Example 3 - Reformation of the Catalyst Pattern for Use with Increased CO2 Utilization Similar to Example 2, a molten carbonate fuel cell model including aspects representing the flow and heat transfer of fluids within the fuel cell was used to determine the influence of the catalyst pattern on the temperature difference within the fuel cell. This model was constructed using a commercially available process modeling platform. For the model used in this example, the model showed a system having a reforming element, an associated fuel cell having an anode and a cathode, and a separator plate between the fuel cell associated with the reforming element. The flow pattern of the reforming element corresponds to the flow pattern shown in Figure 5. Similar to Figure 5, the reforming element included an initial channel near the cathode inlet having a constant catalyst density (corresponding to the average of the first portion) and a first portion having a catalyst pattern aligned with the flow pattern in the anode.

[0104] In this example, modeling results are provided for a molten carbonate fuel cell having three different types of catalyst patterns for a first portion of a reforming element. One type of catalyst pattern corresponds to the catalyst pattern shown in FIG. 5. The second type of catalyst pattern corresponds to a catalyst pattern in which the difference between the maximum catalyst density and the minimum catalyst density is approximately 100%, like the first type of catalyst pattern of this example. The third type of catalyst pattern corresponds to a catalyst pattern in which the difference between the maximum catalyst density and the minimum catalyst density is approximately 43%.

[0105] FIG. 10 shows four types of catalyst density profiles for the first portion of the reforming element. In FIG. 10, the x-axis corresponds to the axis of the flow in the anode, 0 corresponds to the anode inlet, and 1 corresponds to the anode outlet. Line 910 corresponds to the conventional catalyst profile shown in FIGS. 5 and 7 from Example 2 above. As described above, the difference between the maximum catalyst density and the minimum catalyst density of this catalyst profile is virtually 100% because the catalyst density drops to zero at the anode outlet of line 910. Similarly, line 1010 corresponds to another catalyst profile in which the difference between the maximum catalyst density and the minimum catalyst density is virtually 100%, again because the catalyst density drops to zero at the anode outlet of line 1010. As shown in FIG. 10, for line 1020, the ratio of the minimum catalyst density to the maximum catalyst density is about 80 / 160, or 1 / 2. This corresponds to a difference of about 50% between the maximum and the minimum. FIG. 10 also shows line 1030, which is similar to the profile at line 1020 but with a continuous monotonic decrease in catalyst density, in contrast to the stepwise monotonic decrease in catalyst density shown for line 1020.

[0106] In this model, a fuel cell using the catalyst profile explained by line 1010 for the first portion of the surface within the reforming element was modeled under conditions that result in an increase in CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 4.3 volume %, a fuel utilization of 50%, an actual CO2 utilization of approximately 85%, an average cathode temperature of approximately 903.15°K (630° C.), and 110 mA / cm 2The current density was [value]. The input stream to the reformer included 7 vol% H2, 2 vol% CO2, 60 vol% H2O, and 30 vol% CH4, with the remainder corresponding to nitrogen. This resulted in an input stream to the anode input of each fuel cell containing 50 vol% H2, 9 vol% CO2, 10 vol% H2O, 10 vol% CH4, and 5 vol% CO. The cathode input stream of each fuel cell included 4.3 vol% CO2, 11 vol% O2, and 10 vol% H2O.

[0107] In this model, a fuel cell using the catalyst profile described by line 1020 for the first portion of the surface within the reforming element was modeled under conditions that result in an increase in CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 4.4 vol%, a fuel utilization of 50%, an actual CO2 utilization of approximately 85%, an average cathode temperature of approximately 903.15°K (630°C), and a current density of 150 mA / cm 2 The current density was [value]. The input stream to the reformer included 10 vol% H2, 3 vol% CO2, 58% H2O, and 29 vol% CH4, with the remainder corresponding to nitrogen. This resulted in an input stream to the anode input of each fuel cell containing 51 vol% H2, 9 vol% CO2, 25 vol% H2O, 10 vol% CH4, and 6 vol% CO. The cathode input stream of each fuel cell included 4.3 vol% CO2, 11 vol% O2, and 10 vol% H2O.

[0108] In this model, a fuel cell using the catalyst profile described by line 1030 for the first portion of the surface within the reforming element was modeled under conditions that result in an increase in CO2 utilization. The model conditions were a cathode inlet CO2 concentration of 4.4 vol%, a fuel utilization of 50%, an actual CO2 utilization of approximately 85%, an average cathode temperature of approximately 903.15°K (630°C), and a current density of 150 mA / cm 2The current density was as follows. The input stream to the reformer contained 10 vol% H2, 3 vol% CO2, 58% H2O, and 29 vol% CH4, with the remainder corresponding to nitrogen. This resulted in an input stream to the anode input of each fuel cell containing 51 vol% H2, 9 vol% CO2, 25 vol% H2O, 10 vol% CH4, and 6 vol% CO. The cathode input stream of each fuel cell contained 4.3 vol% CO2, 11 vol% O2, and 10 vol% H2O.

[0109] FIG. 11 shows the temperature variations in the separator plate of a fuel cell modeled with a comparative reforming catalyst density corresponding to line 910 of FIG. 10. In FIG. 11, the x-axis corresponds to the direction of the anode flow and the y-axis corresponds to the direction of the cathode flow. As shown in FIG. 11, the temperature gradient across the entire fuel cell exceeds 100°K or 100°C. This is due to the excess waste heat generated by alternative ion transport at the corners of the fuel cell corresponding to the cathode and anode exits. Based on the catalyst pattern corresponding to line 910, the catalyst density is close to zero near the corners corresponding to the cathode and anode exits. Therefore, there is only a minimum amount of endothermic reforming reaction available to balance the additional waste heat generated by alternative ion transport.

[0110] Similar to FIG. 11, FIG. 12 shows the temperature variations in the separator plate of a fuel cell modeled with a reforming catalyst density corresponding to line 1010 of FIG. 10. As shown in FIG. 12, the temperature gradient across the entire fuel cell is approximately 70°K or 70°C, which is due to the excess waste heat generated by alternative ion transport at the corners of the fuel cell corresponding to the cathode and anode exits. Based on the catalyst pattern corresponding to line 1010, the catalyst density is close to zero near the corners corresponding to the cathode and anode exits. Therefore, there is only a minimum amount of endothermic reforming reaction available to balance the additional waste heat generated by alternative ion transport.

[0111] FIG. 13 shows the temperature fluctuations in the separator plate of a fuel cell modeled at a reforming catalyst density corresponding to line 1030 in FIG. 10. By reducing the difference between the minimum and maximum reforming catalyst densities, the temperature fluctuations across the entire fuel cell are reduced to about 38°K or approximately 38°C.

[0112] Additional Embodiments Embodiment 1. A method for generating electricity, the method comprising passing a fuel stream comprising a reformable fuel through a fuel stack comprising a first surface, the first surface comprising a first portion comprising a reforming catalyst, the reforming catalyst density on the first portion of the first surface being such that the difference between the maximum catalyst density and the minimum catalyst density is between 20% and 75%; reforming at least a portion of the reformable fuel in the presence of the first surface to produce reformed hydrogen; introducing at least a portion of the reformable fuel, at least a portion of the reformed hydrogen, or a combination thereof into the anode of a molten carbonate fuel cell; introducing a cathode input stream comprising O2 and CO2 into the cathode of the molten carbonate fuel cell, wherein the direction of flow in the cathode of the molten carbonate fuel cell is substantially orthogonal to the direction of flow in the anode of the molten carbonate fuel cell; operating the molten carbonate fuel cell at a utilization rate of 0.97 or less and an average current density of 60 mA / cm 2 or greater to produce electricity, an anode exhaust comprising H2, CO, and CO2, and a cathode exhaust comprising 2.0 volume % or less CO2, 1.0 volume % or more H2O, and 1.0 volume % or more O2.

[0113] Embodiment 2. The method of Embodiment 1, wherein the cathode input stream comprises 5.0 volume % or less CO2, or the cathode exhaust comprises 1.0 volume % or less CO2, or a combination thereof.

[0114] Embodiment 3. A method for generating electricity, the method comprising passing a fuel stream comprising a reformable fuel through a fuel stack comprising a first surface, the first surface comprising a first portion comprising a reforming catalyst, the reforming catalyst density on the first portion of the first surface being such that the difference between the maximum catalyst activity and the minimum catalyst activity is from 20% to 75%, passing at least a portion of the reformable fuel over the first surface to reform at least a portion of the reformable fuel to produce reformed hydrogen, introducing at least a portion of the reformable fuel, at least a portion of the reformed hydrogen, or a combination thereof into the anode of a molten carbonate fuel cell, introducing a cathode input stream comprising O2 and CO2 into the cathode of the molten carbonate fuel cell, wherein the direction of flow in the cathode of the molten carbonate fuel cell is substantially orthogonal to the direction of flow in the anode of the molten carbonate fuel cell, operating the molten carbonate fuel cell at a utilization rate of 0.97 or less and an average current density of 60 mA / cm 2 2 or greater to produce electricity, an anode exhaust comprising H2, CO, and CO2, and a cathode exhaust comprising 2.0 volume % or less CO2, 1.0 volume % or more H2O, and 1.0 volume % or more O2. A method comprising the steps of:

[0115] Embodiment 4. The method according to any one of Embodiments 1 to 3, wherein the utilization rate is 0.95 or less, or 0.90 or less.

[0116] Embodiment 5. The method according to any one of Embodiments 1 to 4, wherein the reforming catalyst on the first portion of the first surface comprises a monotonic catalyst density variation.

[0117] Embodiment 6. The method according to any one of Embodiments 1 to 5, wherein the maximum catalyst density is proximate to the anode inlet or the maximum catalyst density is proximate to the cathode inlet.

[0118] Embodiment 7. The method according to any one of Embodiments 1 to 6, wherein the minimum catalyst density is proximate to the anode outlet or the minimum catalyst density is proximate to the cathode outlet.

[0119] Embodiment 8. The method according to any one of Embodiments 1 to 7, wherein the fuel cell stack includes a reforming element associated with the anode, the first surface includes the inner surface of the reforming element, and the temperature variation in the fuel cell stack in the separator plate between the reforming element and the anode is optionally 70°C or less (or 40°C or less).

[0120] Embodiment 9. The method according to any one of Embodiments 1 to 7, wherein the first surface includes the inner surface of the anode, and the temperature variation in the fuel cell stack in the separator plate between the anode and another element is optionally 70°C or less (or 40°C or less).

[0121] Embodiment 10. The method according to any one of Embodiments 1 to 9, wherein the first surface further includes a second portion, the second portion optionally includes at least one of a constant catalyst density and a constant catalyst activity, and the second portion is close to the cathode inlet or the second portion is close to the anode inlet.

[0122] Embodiment 11. The method according to any one of Embodiments 1 to 10, wherein the reforming catalyst includes a plurality of catalyst particle lines, and reforming at least a part of the reformable fuel includes flowing at least a part of the reformable fuel over the catalyst particles in a direction substantially parallel to the catalyst particle lines.

[0123] Embodiment 12. A fuel cell stack, comprising a molten carbonate fuel cell including an anode and a cathode, and a reforming element associated with the anode, the reforming element including a first surface, the first surface including a first portion containing a reforming catalyst, the reforming catalyst density on the first portion of the first surface including a monotonically decreasing catalyst density, the reforming catalyst density on the first portion of the first surface having a difference between a maximum catalyst density and a minimum catalyst density of 20% to 75% (or 25% to 70%, or 25% to 65%), the reforming element, and a separator plate between the anode and the reforming element, the first surface optionally further including a second portion, the optional second portion being close to the cathode inlet or close to the anode inlet, the optional second portion including a constant catalyst density, the fuel cell stack.

[0124] Embodiment 13. The fuel cell stack according to Embodiment 12, wherein the maximum catalyst density is close to the anode inlet or the maximum catalyst density is close to the cathode inlet.

[0125] Embodiment 14. The fuel cell stack according to Embodiment 12 or 13, wherein the minimum catalyst density is close to the anode outlet or the minimum catalyst density is close to the cathode outlet.

[0126] Embodiment 15. The fuel cell stack according to any one of Embodiments 12 to 14, wherein the reforming catalyst includes substantially parallel catalyst particle lines.

[0127] All numerical values within the detailed description and claims of this specification are modified by the indicated value of "about" or "approximately" and are based on experimental errors and variations that can be expected by those skilled in the art.

[0128] Although the present invention has been described with respect to specific embodiments, it is not necessarily limited thereto. Modifications / changes suitable for operation under specific conditions will be apparent to those skilled in the art. Accordingly, the following claims are intended to be construed as encompassing all such modifications / changes that fall within the true spirit / scope of the present invention.

Claims

1. A method for generating electricity, the method comprising passing a fuel stream comprising a reformable fuel through a fuel stack comprising a first surface, the first surface comprising a first portion comprising a reforming catalyst, wherein the reforming catalyst density on the first portion of the first surface has a difference between a maximum catalyst density and a minimum catalyst density of 20% to 75%; reforming at least a portion of the reformable fuel in the presence of the first surface to produce reformed hydrogen; introducing at least a portion of the reformable fuel, at least a portion of the reformed hydrogen, or a combination thereof into the anode of a molten carbonate fuel cell; ２ and CO ２ introducing a cathode input stream comprising into the cathode of the molten carbonate fuel cell, wherein the direction of flow in the cathode of the molten carbonate fuel cell is substantially orthogonal to the direction of flow in the anode of the molten carbonate fuel cell; operating the molten carbonate fuel cell at a utilization rate of 0.97 or less and an average current density of 60 mA / cm ２ or greater to produce electricity, an anode exhaust comprising H ２ , CO, and CO ２ ; and a cathode exhaust comprising 2.0 volume % or less of CO ２ , 1.0 volume % or more of H ２ O, and 1.0 volume % or more of O ２ . A method comprising:

2. The cathode input stream contains 5.0% by volume or less of CO ２ or the cathode exhaust contains 1.0% by volume or less of CO ２ or a combination thereof, the method according to claim 1.

3. A method for generating electricity, the method comprising passing a fuel stream comprising a reformable fuel through a fuel stack comprising a first surface, the first surface comprising a first portion comprising a reforming catalyst, the reforming catalyst density on the first portion of the first surface being such that the difference between the maximum catalytic activity and the minimum catalytic activity is 20% to 75%, passing at least a portion of the reformable fuel over the first surface to reform at least a portion of the reformable fuel to produce reformed hydrogen, introducing at least a portion of the reformable fuel, at least a portion of the reformed hydrogen, or a combination thereof into the anode of a molten carbonate fuel cell, O ２ and CO ２ introducing a cathode input stream comprising into the cathode of the molten carbonate fuel cell, the direction of flow in the cathode of the molten carbonate fuel cell being substantially orthogonal to the direction of flow in the anode of the molten carbonate fuel cell, operating the molten carbonate fuel cell at a utilization rate of 0.97 or less and an average current density of 60 mA / cm ２ or greater to produce electricity, an anode exhaust comprising H ２ , CO, and CO ２ and a cathode exhaust comprising 2.0 volume % or less CO ２ , 1.0 volume % or more H ２ O, and 1.0 volume % or more O ２ comprising, a method.

4. The method according to any one of claims 1 to 3, wherein the conversion rate is 0.95 or less, or 0.90 or less.

5. The method according to any one of claims 1 to 4, wherein the reforming catalyst on the first portion of the first surface comprises a monotonic catalyst density variation.

6. The method according to any one of claims 1 to 5, wherein the maximum catalyst density is close to the anode inlet or the maximum catalyst density is close to the cathode inlet.

7. The method according to any one of claims 1 to 6, wherein the minimum catalyst density is close to the anode outlet or the minimum catalyst density is close to the cathode outlet.

8. The fuel cell stack includes a reforming element associated with the anode, the first surface includes the inner surface of the reforming element, and the temperature variation in the fuel cell stack in the separator plate between the reforming element and the anode is optionally 70 °C or less (or 40 °C or less). The method according to any one of claims 1 to 7.

9. The method according to any one of claims 1 to 7, wherein the first surface includes the inner surface of the anode, and the temperature variation in the fuel cell stack in the separator plate between the anode and another element is optionally 70 °C or less (or 40 °C or less).

10. The method according to any one of claims 1 to 9, wherein the first surface further includes a second portion, the second portion optionally includes at least one of a constant catalyst density and a constant catalyst activity, and the second portion is close to the cathode inlet or the second portion is close to the anode inlet.

11. The method according to any one of claims 1 to 10, wherein the reforming catalyst includes a plurality of catalyst particle lines, and reforming at least a part of the reformable fuel includes flowing at least a part of the reformable fuel over the catalyst particles in a direction substantially parallel to the catalyst particle lines.

12. A fuel cell stack, comprising a molten carbonate fuel cell including an anode and a cathode, a reforming element associated with the anode, the reforming element including a first surface, the first surface including a first portion including a reforming catalyst, the reforming catalyst density on the first portion of the first surface including a monotonically decreasing catalyst density, the reforming catalyst density on the first portion of the first surface having a difference between a maximum catalyst density and a minimum catalyst density of 20% to 75% (or 25% to 70%, or 25% to 65%), a reforming element, a separator plate between the anode and the reforming element, the first surface optionally further including a second portion, the optional second portion being close to the cathode inlet or close to the anode inlet, the optional second portion including a constant catalyst density, a fuel cell stack.

13. The fuel cell stack according to claim 12, wherein the maximum catalyst density is close to the anode inlet or the maximum catalyst density is close to the cathode inlet.

14. The fuel cell stack according to claim 12 or 13, wherein the minimum catalyst density is close to the anode outlet or the minimum catalyst density is close to the cathode outlet.

15. The fuel cell stack according to any one of claims 12 to 14, wherein the reforming catalyst includes substantially parallel catalyst particle lines.

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