Fuel cell staging for molten carbonate fuel cells

Abstract

Systems and methods are provided for reducing or minimizing current density variation using fuel cell staging when operating molten carbonate fuel cells with increased CO2 utilization. The fuel cell staging can reduce the amount of replacement ion transport that occurs when operating molten carbonate fuel cells with increased CO2 utilization.

Description

[0001] This application is a divisional application of the invention patent application with application number 201980090518.X, application date November 26, 2019, entitled "Fuel Cell Staging for Molten Carbonate Fuel Cells" (International Application No.: PCT / US2019 / 063337). Technical Field

[0002] A method for using fuel cell grading when operating molten carbonate fuel cells under conditions of increased CO2 utilization is provided. Background Technology

[0003] This application discloses and claims protection the subject matter of activities that are within the scope of a joint research agreement between ExxonMobil Research and Engineering Company and FuelCell Energy, Inc., which was effective on or before the effective filing date of this application.

[0004] Molten carbonate fuel cells utilize hydrogen and / or other fuels to generate electricity. Hydrogen can be supplied by reforming methane or other reformable fuels in a steam reformer (such as one located upstream of or integrated into the fuel cell). The fuel can also be reformed in the anode pool within the molten carbonate fuel cell, which can be operated to create conditions suitable for reforming the fuel at the anode. Alternatively, some reforming can be performed both externally and internally within the fuel cell. Reformable fuels can encompass hydrocarbon-containing materials that can react with steam and / or oxygen at elevated temperatures and / or pressures to produce gaseous products including hydrogen.

[0005] The basic structure of a molten carbonate fuel cell includes a cathode, an anode, and a matrix located between the cathode and the anode, the matrix comprising one or more molten carbonates used as an electrolyte. During normal operation of the molten carbonate fuel cell, the molten carbonate partially diffuses into the pores of the cathode. This diffusion of the molten carbonate into the cathode pores provides an interface region where CO2 can be converted into CO3. 2- It is transported across the electrolyte to the anode.

[0006] Conventionally, cathodes for molten carbonate fuel cells are typically constructed as monolayers. The properties of a monolayer cathode are selected based on several types of properties. First, the cathode needs to have sufficient structural stability to maintain the integrity of the cathode layer, while also providing sufficient conductivity to allow the fuel cell to operate. This typically leads to the use of materials such as nickel for the electrodes, which are oxidized and (optionally) lithium-ionized during the initial operation of the fuel cell. Regarding other properties, a sufficiently small pore size is desirable to provide adequate wetting of the cathode by the electrolyte, in order to again provide suitable conductivity.

[0007] One challenge with conventional electrode materials (such as (optionally lithiated) nickel oxide) is its susceptibility to polarization under fuel cell operating conditions. During fuel cell operation, cathode polarization leads to additional voltage losses across the cathode. The effects of polarization can be mitigated by increasing the fuel cell's operating temperature, but this results in a shorter operating life. Therefore, a molten carbonate cathode structure that reduces polarization is desirable, as this provides lower voltage losses and / or allows for lower operating temperatures.

[0008] U.S. Patent 9,077,007 describes a method for operating a molten carbonate fuel cell for integrated power generation and chemical production. The method includes operating the fuel cell at a reduced fuel utilization level. Configurations comprising operating the fuel cells in parallel or passing the anode and cathode flows through a series of fuel cells are also described. Summary of the Invention

[0009] In one aspect, a method for generating electricity is provided. The method includes introducing an anode input stream comprising H2, reformable fuel, or a combination thereof into a first anode stage of a plurality of molten carbonate fuel cell stages. The method further includes introducing a cathode input stream comprising O2 and CO2 into the first cathode stage of the plurality of molten carbonate fuel cell stages. The method further includes introducing a second anode input stream into a second anode stage of the plurality of molten carbonate fuel cell stages. The method further includes introducing an intermediate cathode output into the second cathode stage of the plurality of molten carbonate fuel cell stages. The method further includes operating the plurality of molten carbonate fuel cell stages to generate: i) an average current density of 60 mA / cm². 2 The first cathode stage or the second cathode stage operate at a transfer rate of 0.97 or higher. The first cathode stage is equipped with either: ii) anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H2, CO and CO2; and iii) cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising CO2 at a content of 2.0 vol% or lower, H2O at a content of 1.0 vol% or higher and O2 at a content of 1.0 vol% or higher. Attached Figure Description

[0010] Figure 1 An example of a fuel cell stage with a co-current cross-flow configuration is shown.

[0011] Figure 2 An example of a fuel cell stage with a counter-current cross-flow configuration is shown.

[0012] Figure 3 An example of a portion of a molten carbonate fuel cell stack is shown.

[0013] Figure 4 An example of a flow mode in a molten carbonate fuel cell is shown, in which the anode flow direction is aligned approximately perpendicular to the cathode flow direction.

[0014] Figure 5 The CO2 concentration pattern in the cathode of a fuel cell operating as a single stage under conditions of improved CO2 utilization is shown.

[0015] Figure 6 The CO2 concentration patterns in the cathodes of two fuel cell stages operating with improved CO2 utilization are shown. Detailed Implementation

[0016] SUMMARY

[0017] In various aspects, systems and methods are provided for using fuel cell stages to reduce or minimize current density variations when operating a molten carbonate fuel cell under conditions of increased CO2 utilization. In some aspects, operating a fuel cell or a series of fuel cell stages under conditions of increased CO2 utilization corresponds to operating at least one fuel cell stage at a transfer rate of 0.97 or less, or 0.95 or less, or 0.90 or less. Additionally or alternatively, during operation under conditions of increased CO2 utilization, the CO2 content of the output feed stream from the last cathode stage may contain 2.0 vol% or less, or 1.0 vol% or less, or 0.8 vol% or less CO2.

[0018] When molten carbonate fuel cell stacks are operated as a single stage to improve CO2 utilization, significant substitution ion transport has been observed. Substitution ion transport refers to the transport of ions other than carbonate ions (CO3-). 2- The transport of ions other than those in the molten carbonate electrolyte.

[0019] The typical operating conditions of molten carbonate fuel cells typically correspond to conditions of reduced, minimized, or absent substitutional ion transport. The amount of substitutional ion transport can be quantified based on the fuel cell's transfer rate. The transfer rate is defined as the fraction of ions transported across the molten carbonate electrolyte, corresponding to carbonate ions rather than hydroxide ions and / or other ions. A simple method for determining the transfer rate is to compare a) the change in CO2 concentration measured at the cathode inlet compared to the cathode outlet with b) the amount of carbonate ion transport required to achieve the current density produced by the fuel cell. It is worth noting that this definition of the transfer rate assumes that CO2 return transport from the anode to the cathode is minimal. It is believed that such return transport is minimal under the operating conditions described herein. For CO2 concentration, the cathode input and / or cathode output streams can be sampled, with the samples transferred to a gas chromatograph to determine the CO2 content. The average current density of the fuel cell can be measured in any convenient manner.

[0020] Under normal operating conditions, the transfer rate can be relatively close to 1.0, such as 0.98 or higher, and / or there is essentially no alternative ion transport. A transfer rate of 0.98 or higher means that 98% or more of the ion charge transported across the electrolyte corresponds to carbonate ions. It is worth noting that hydroxide ions have a charge of -1, while carbonate ions have a charge of -2, therefore two hydroxide ions need to be transported across the electrolyte to produce the same charge transfer as transporting one carbonate ion.

[0021] Operating a molten carbonate fuel cell at a transfer rate of 0.95 or lower (or 0.97 in at least one stage of a multi-stage configuration) compared to conventional operating conditions can increase the effective amount of carbonate ion transport achieved, even if 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 transfer rate of 0.97 or lower, or 0.95 or lower, CO2 must be consumed within the fuel cell cathode. It has been found that such CO2 consumption within the cathode tends to be localized. Therefore, many regions within the fuel cell cathode can still contain sufficient CO2 for normal operation. These regions contain additional CO2 that is desired to be transported across the electrolyte (e.g., for carbon capture). However, under conventional operating conditions, CO2 in such regions is typically not transported across the electrolyte. By selecting operating conditions with a transfer rate of 0.97 or lower, or 0.95 or lower, regions with sufficient CO2 can be used to transport additional CO2, while consumed regions can operate based on alternative ion transport. This can increase the practical limitation on the amount of CO2 captured from the cathode feed stream.

[0022] One challenge in using MCFCs to improve CO2 capture is that fuel cell operation can be kinetically limited if one or more reactants required for operation are present in small amounts. For example, achieving 75% or higher CO2 capture rates with a cathode feed stream containing 4.0 vol% or less CO2 corresponds to a cathode outlet concentration of 1.0 vol% or less. However, a cathode outlet concentration of 1.0 vol% or less does not necessarily mean that CO2 is uniformly distributed throughout the cathode. Instead, the concentration within the cathode typically varies due to various factors, such as flow patterns in the anode and cathode. Variations in CO2 concentration can cause some portions of the cathode to have CO2 concentrations significantly lower than 1.0 vol%.

[0023] Conventionally, CO2 consumption within the cathode is expected to lead to a decrease in voltage and current density. However, it has been found that due to CO3 removal... 2- Other ions can be transported across the electrolyte, and the current density can be maintained during CO2 consumption. For example, a portion of the ions transported across the electrolyte may correspond to hydroxide ions (OH-). - Alternative ion transport across the electrolyte can allow fuel cells to maintain target current density, even when the amount of CO2 transported across the electrolyte is insufficient.

[0024] One advantage of cross-electrolyte transport of alternative ions is that fuel cells can continue to operate even when there is not a sufficient number of CO2 molecules available kinetically. This allows additional CO2 to be transferred from the cathode to the anode, even when the amount of CO2 present at the cathode is typically considered insufficient for normal fuel cell operation. This allows fuel cells to operate with measured CO2 utilization close to 100%, while calculated CO2 utilization (based on current density) can be at least 3%, or at least 5%, or at least 10%, or at least 20% higher than the measured CO2 utilization. Notably, alternative ion transport can allow fuel cells to operate at current densities corresponding to calculated CO2 utilization exceeding 100%.

[0025] While the transport of alternative ions allows fuel cells to maintain the target current density, it has been further found that the transport of alternative ions across the electrolyte can also reduce or minimize the lifetime of molten carbonate fuel cells. Therefore, mitigating this loss in fuel cell lifetime is desirable. Unexpectedly, it has been found that using multiple fuel cell stages can allow for increased CO2 capture while reducing or minimizing the amount of alternative ion transport.

[0026] In some respects, improved CO2 capture can be defined based on the amount of transfer rate, such as a transfer rate of 0.97 or lower, or 0.95 or lower, or 0.93 or lower, or 0.91 or lower. Operating conditions maintaining a transfer rate of 0.97 or lower, or 0.95 or lower, typically also result in a CO2 concentration in the cathode output stream of 2.0 vol% or lower, or 1.5 vol% or lower, or 1.0 vol% or lower. At higher CO2 concentrations in the cathode output stream, there is generally not sufficient local CO2 consumption to result in lower transfer rate values.

[0027] An improved CO2 capture rate can also be indicated by other factors, but these other factors are generally not sufficient to indicate an improved CO2 capture rate on their own. For example, when using a lower CO2 concentration cathode feed stream, an improved CO2 capture rate can correspond in some respects to a CO2 utilization rate of 70% or higher, or 75% or higher, or 80% or higher, such as up to 95% or possibly higher. Examples of lower CO2 concentration sources can correspond to CO2 sources that result in the cathode feed stream containing 5.0 vol% or less, or 4.0 vol% or less (e.g., as low as 1.5 vol% or possibly lower). Exhaust gas from a natural gas turbine is an example of a CO2-containing feed stream, typically containing 5.0 vol% or less, or 4.0 vol% or less. Additionally or alternatively, an improved CO2 capture rate can correspond to operating conditions using a molten carbonate fuel cell that generate a considerable current density, such as 60 mA / cm². 2 Or larger, or 80 mA / cm 2 Or larger, or 100 mA / cm 2 Or larger, or 120 mA / cm 2 Or larger, or 150 mA / cm 2 Or larger, or 200 mA / cm 2 Or even higher, such as up to 300 mA / cm 2 It could be even higher. It's worth noting that alternative ion transport can also be indicated by a decrease in the operating voltage of the fuel cell, as alternative ion transport pathways have a lower theoretical voltage compared to reaction pathways using carbonate ions.

[0028] Typically, the CO2 concentration in the cathode exhaust of a molten carbonate fuel cell is maintained at relatively high values, such as 5 vol% CO2 or greater, or 10 vol% CO2 or greater, or possibly even higher. Furthermore, molten carbonate fuel cells typically operate at CO2 utilization rates of 70% or lower. Under any of these conditions, the primary mechanism for charge transport across the molten carbonate electrolyte is the transport of carbonate ions. While the transport of alternative ions (such as hydroxide ions) across the electrolyte may occur under these conventional conditions, the amount of alternative ion transport is negligible, corresponding to a current density of 2% or lower (or equivalently, a transfer rate of 0.98 or higher).

[0029] As an alternative to describing operating conditions based on transfer rate, the operating conditions can be described based on a measured CO2 utilization rate and a “calculated” CO2 utilization rate based on average current density. In this discussion, 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 concentrations in the cathode input and output streams. This can also be referred to as the actual CO2 utilization rate, or simply CO2 utilization rate. In this discussion, the calculated CO2 utilization rate is defined as the CO2 utilization rate at which all current densities generated by the fuel cell are based on CO32. 2- CO2 utilization rate is the rate of change that occurs when ions are transported across the electrolyte (i.e., CO2-based ion transport). The difference between the measured and calculated CO2 utilization rates can be used alone to characterize the amount of substituted ion transport, and / or these values ​​can be used to calculate the transfer rate, as described above.

[0030] It has been found that the amount of alternative ion transport generated when operating a molten carbonate fuel cell with improved CO2 utilization can be reduced or minimized by using more than one fuel cell stage to achieve the improved CO2 utilization. Even with the same (or preferably) smaller fuel cell area, operating multiple fuel cell stages to achieve improved CO2 utilization can reduce or minimize the amount of variation in CO2 concentration in the cathode flow pattern. This can correspondingly reduce the amount of alternative ion transport. For example, when operating a fuel cell stage with a cross-flow configuration of anode and cathode gas streams, the amount of alternative ion transport in the fuel cell corners corresponding to the anode inlet and cathode outlet can be reduced or minimized. This reduction in alternative ion transport can be beneficial, for example, by increasing the operating life of one or more fuel cells.

[0031] When multiple fuel cell stages are used in series, the anode and cathode flows through the cells can be arranged in various ways (e.g., with parallel or counter-current flow). In parallel flow, the fuel cell corresponding to the first stage of the anode flow is also the first stage of the cathode. In counter-current flow, the fuel cell corresponding to the first stage of the anode (or cathode) corresponds to the last stage and / or a stage different from the first stage of the cathode (or anode). Additionally, the flow within a single fuel cell can be characterized. For example, in a cross-flow configuration, the flow direction in the anode of a given fuel cell can be oriented approximately perpendicular to the flow direction in the anode. A cross-flow configuration is the opposite of an aligned flow configuration, in which the flow direction in the anode can be oriented along a flow axis approximately the same as the flow direction in the cathode. Depending on these aspects, various combinations of fuel cells (e.g., combinations of fuel cell stacks) can be arranged in series and / or parallel. In such aspects, for fuel cells arranged in series, any convenient combination of parallel, counter-current, cross-flow, and / or aligned flow can be used.

[0032] Figure 1 An example of a series of fuel cells (such as a series of fuel cell stacks) arranged in a parallel-flow, cross-flow configuration is shown. Figure 1 In the example shown, fuel cells 120, 130, and 140 are arranged in series. The cathode input stream 119 and the anode input stream 115 both enter fuel cell 120, which serves as the first fuel cell stage. The first-stage cathode intermediate output 129 and the first-stage anode intermediate output 125 then enter the second fuel cell 130. The second-stage cathode intermediate output 139 and the second-stage anode intermediate output 135 then enter the third fuel cell stage 140. Figure 1 In the example shown, the third fuel cell 140 corresponds to the last stage, therefore the output from the third fuel cell 140 corresponds to the cathode output 149 and the anode output 145. For example... Figure 1 As shown, in each of fuel cells 120, 130, and 140, the flow direction of the anode flow is approximately orthogonal to the flow direction of the cathode flow. Therefore, the flow in fuel cells 120, 130, and 140 corresponds to a cross-flow orientation.

[0033] Figure 2 An example of a series of fuel cells (such as a series of fuel cell stacks) arranged in a counter-current crossflow configuration is shown. Figure 2 In the example shown, fuel cells 220, 230, and 240 are arranged in series. The cathode input stream 219 enters fuel cell 220, which serves as the first cathode fuel cell stage. The first-stage cathode intermediate output 229 enters fuel cell 230, which serves as the second cathode fuel cell stage. The second-stage cathode intermediate output 239 enters fuel cell 240, which serves as the third cathode fuel cell stage. Figure 2In the example shown, fuel cell 240 corresponds to the last cathode fuel cell stage, therefore the output from the cathode of fuel cell 240 corresponds to cathode output 249. Compared to Figure 1 Anode input 215 enters fuel cell 240, which serves as the first anode fuel cell stage. This produces a first-stage anode intermediate output 255, which enters fuel cell 230, which serves as the second anode fuel cell stage. Second-stage anode intermediate output 265 enters fuel cell 220, which corresponds to the third (last) fuel cell stage of the anode flow. The output from the anode of fuel cell 220 corresponds to anode output 245. Figure 2 As shown, in each of fuel cells 220, 230, and 240, the flow direction of the anode flow is approximately orthogonal to the flow direction of the cathode flow. Therefore, the flow in fuel cells 220, 230, and 240 corresponds to a cross-flow orientation.

[0034] In some respects, any convenient type of electrolyte suitable for operating molten carbonate fuel cells can be used. Many conventional MCFCs use eutectic carbonate mixtures as carbonate electrolytes, such as a eutectic 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). Other eutectic mixtures can also be used, such as a eutectic mixture of 40 mol% lithium carbonate and 60 mol% potassium carbonate (40% Li₂CO₃ / 60% K₂CO₃). While eutectic carbonate mixtures can be conveniently used as electrolytes for various reasons, non-eutectic carbonate mixtures may also be suitable. Typically, such non-eutectic mixtures can contain various combinations of lithium carbonate, sodium carbonate, and / or potassium carbonate. Optionally, the electrolyte may contain a small amount of other metal carbonates as additives, such as other alkali metal carbonates (rubidium carbonate, cesium carbonate) or other types of metal carbonates, such as barium carbonate, bismuth carbonate, lanthanum carbonate or tantalum carbonate.

[0035] It is worth noting that the structure of a molten carbonate fuel cell can also affect the rate of degradation. For example, the open area of ​​the cathode surface available for receiving cathode gas can influence the rate of degradation. For electrical contact, at least a portion of the cathode current collector typically contacts the cathode surface in a molten carbonate fuel cell. The open area of ​​the cathode surface (adjacent to the cathode current collector) is defined as the percentage of the cathode surface not in contact with the cathode current collector. For conventional molten carbonate fuel cell designs, a typical value for the open area is approximately 33%. This is due to the nature of conventional cathode current collector configurations, which correspond to a plate-like structure resting on the cathode surface, a portion of which has openings allowing cathode gas to diffuse into the cathode. In various aspects, additional benefits can be obtained by using cathode current collectors that provide a larger open area at the cathode surface (e.g., 45% or more, or 50% or more, or 60% or more, such as up to 90% or possibly even higher).

[0036] Conditions for molten carbonate fuel cell operation with alternative ion transport

[0037] In various aspects, the operating conditions of a molten carbonate fuel cell (such as a cell as part of a fuel cell stack) can be selected to correspond to a transfer rate of 0.97 or lower, or 0.95 or lower, thereby enabling the cell to simultaneously transport carbonate ions and at least one type of substitution ion across the electrolyte. In addition to the transfer rate, operating conditions that may indicate the operation of a molten carbonate fuel cell in the case of substitution ion transport include, but are not limited to, the CO2 concentration of the cathode feed stream, the CO2 utilization rate in 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 in the cathode feed stream. Additionally, the anode feed stream and the fuel utilization rate in the anode can typically be selected to provide the desired current density.

[0038] Typically, in order to induce alternative ion transport, the CO2 concentration in at least a portion of the cathode needs to be sufficiently low while operating the fuel cell to provide a sufficiently high current density. A sufficiently low CO2 concentration in the cathode generally corresponds to some combination of low CO2 concentration in the cathode input stream, high CO2 utilization, and / or high average current density. However, such conditions alone are insufficient to indicate a transfer rate of 0.97 or lower, or 0.95 or lower.

[0039] For example, a molten carbonate fuel cell with a cathode open area of ​​approximately 33% operates at a CO2 cathode inlet concentration of 19 vol%, a CO2 utilization rate of 75%, and an average current density of 160 mA / cm². These conditions correspond to a difference of less than 1% between the calculated and measured CO2 utilization rates. Therefore, the presence of high CO2 utilization and high average current density cannot be simply inferred to indicate the existence of substantial substitutional ion transport (0.97 or lower, or 0.95 or lower transfer rates).

[0040] As another example, a molten carbonate fuel cell with a cathode open area between 50% and 60% achieves a CO2 cathode inlet concentration of 4.0 vol%, a CO2 utilization rate of 89%, and a current efficiency of 100 mA / cm². 2 The current density operation corresponds to a transfer rate of at least 0.97. Therefore, the presence of a transfer rate of 0.95 or lower / significant substitutional ion transport cannot be simply inferred from the combination of high CO2 utilization and low CO2 concentration in the cathode feed stream.

[0041] As another example, a molten carbonate fuel cell with a cathode open area between 50% and 60% operates at a CO2 cathode inlet concentration of 13 vol%, a CO2 utilization rate of 68%, and a current density of 100 mA / cm². These conditions correspond to a transfer rate of at least 0.98.

[0042] In this discussion, operating an MCFC to induce substitution ion transport across the electrolyte is defined as operating the MCFC such that the transport exceeds a minimum metric amount of substitution ions. Under various conventional conditions, a small amount of substitution ions may be transported across the MCFC electrolyte. Such substitution ion transport under conventional conditions can correspond to a transfer rate of 0.98 or higher, which corresponds to substitution ion transport at a fuel cell current density of less than 2.0%. In contrast, in this discussion, operating an MCFC to induce substitution ion transport is defined as operating the MCFC at a transfer rate of 0.95 or lower, such that a current density of 5.0% or higher (or a calculated CO2 utilization rate of 5.0% or higher) corresponds to a current density based on substitution ion transport, or 10% or higher, or 20% or higher, such as up to 35% or potentially higher. It is noteworthy that, in some respects, operating in multiple fuel cell stages can reduce the severity of the conditions required to achieve improved CO2 capture rates in any single fuel cell stage. Therefore, by operating in multiple stages, some operating conditions with improved CO2 capture rates can correspond to transfer rates of 0.97 or lower.

[0043] In this discussion, operating an MCFC to induce substantial substitutional ion transport (i.e., operating at a transfer rate of 0.95 or less, or 0.97 or less, within one or more stages in a multi-stage configuration) is further defined as corresponding to operating the MCFC with a voltage drop across the anode and cathode suitable for generating electricity. The total electrochemical potential difference for the reaction in a molten carbonate fuel cell is ~1.04 V. For practical reasons, MCFCs are typically operated at voltages close to 0.7 V or about 0.8 V to generate current. This corresponds to a combined voltage drop across the cathode, electrolyte, and anode of approximately 0.34 V. To maintain stable operation, the combined voltage drop across the cathode, electrolyte, and anode can be less than ~0.5 V, allowing the fuel cell to generate current at voltages of 0.55 V or higher, or 0.6 V or higher.

[0044] Regarding the anode, one condition for operation under conditions of high substitutional ion transport can be an H2 concentration of 8.0 vol% or higher, or 10 vol% or higher, in the region where high substitutional ion transport occurs. Depending on this aspect, this can correspond to the region near the anode inlet, the region near the cathode outlet, or a combination thereof. Typically, if the H2 concentration in the anode region is too low, there will not be sufficient driving force to generate high substitutional ion transport.

[0045] Suitable conditions for the anode may also include supplying the anode with H2, reformable fuel, or a combination thereof; and operating at any convenient fuel utilization rate to produce the desired current density, including fuel utilization rates in the range of 20% to 80%. In some aspects, this may correspond to conventional fuel utilization rates, such as 60% or higher, or 70% or higher, such as up to 85% or possibly even higher. In other aspects, this may correspond to fuel utilization rates selected to provide an anode output stream with increased H2 content and / or increased combined H2 and CO (i.e., syngas) content, such as 55% or lower, 50% or lower, or 40% or lower, such as as low as 20% or possibly even lower. The H2 content in the anode output stream and / or the combined H2 and CO content in the anode output stream may be sufficient to allow the production of the desired current density. In some aspects, the H2 content in the anode output stream can be 3.0 vol% or more, or 5.0 vol% or more, or 8.0 vol% or more, such as up to 15 vol% or possibly even higher. Alternatively or additionally, the combined H2 and CO content in the anode output stream can be 4.0 vol% or more, or 6.0 vol% or more, or 10 vol% or more, such as up to 20 vol% or possibly even higher. Optionally, when the fuel cell operates at low fuel utilization, the H2 content in the anode output stream can be in a higher range, such as 10 vol% to 25 vol% H2 content. In such aspects, the syngas content in the anode output stream can be correspondingly higher, such as 15 vol% to 35 vol% combined H2 and CO content. According to the aforementioned aspects, the anode can be operated to increase the amount of electrical energy produced, increase the amount of chemical energy produced (i.e., the amount of H2 available in the anode output stream through reforming), or the anode can be operated using any other convenient strategy compatible with operating the fuel cell to induce alternative ion transport.

[0046] In addition to a sufficient H2 concentration in the anode, one or more locations within the cathode need to have a sufficiently low CO2 concentration, making it difficult to obtain more favorable carbonate ion transport pathways. In some respects, this can correspond to a CO2 concentration of 2.0 vol% or less, or 1.0 vol% or less, or 0.8 vol% or less in the cathode outlet feed stream (i.e., cathode exhaust). It is noteworthy that due to variations within the cathode, an average concentration of 2.0 vol% or less (or 1.0 vol% or less, or 0.8 vol% or less) in the cathode exhaust can correspond to a still lower CO2 concentration in localized regions of the cathode. For example, in a cross-flow configuration, the CO2 concentration at the corner of the fuel cell adjacent to both the anode inlet and cathode outlet can be lower than the concentration at the corner adjacent to both the anode outlet and cathode outlet of the same fuel cell. Similar localized variations in CO2 concentration can also occur in fuel cells with co-current or counter-current configurations.

[0047] In addition to having a low concentration of CO2, localized areas of the cathode can also have 1.0 vol% or higher, or 2.0 vol% or higher, of O2. In fuel cells, O2 is used to form hydroxide ions, which allow for alternative ion transport. If sufficient O2 is not present, the fuel cell will not operate, as both carbonate ion transport and alternative ion transport mechanisms depend on O2 availability. Regarding O2 in the cathode feed stream, in some respects this can correspond to an oxygen content of 4.0 vol% to 15 vol% or 6.0 vol% to 10 vol%.

[0048] It has been observed that a sufficient amount of water (e.g., 1.0 vol% or more, or 2.0 vol% or more) should also be present for alternative ion transport. Without being bound by any particular theory, the rate of fuel cell degradation appears to be much faster in the cathode when no water is available when attempting to operate with a large amount of alternative ion transport than the rate of deactivation observed due to alternative ion transport when sufficient water is available. It is noteworthy that since air is typically used as the O2 source and since H2O is one of the products generated during combustion, a sufficient amount of water is usually available within the cathode.

[0049] Due to the non-uniform distribution of cathode and / or anode gases during molten carbonate fuel cell operation to enhance CO2 capture, it is believed that one or more of the corners and / or edges of the molten carbonate fuel cell will typically have significantly higher densities of alternative ion transport. One or more corners may correspond to locations where the CO2 concentration in the cathode is below average, or where the H2 concentration in the anode is above average, or a combination thereof.

[0050] In this discussion, a fuel cell can correspond to a single cell, where the anode and cathode are separated by an electrolyte. The anode and cathode can receive input gas flows to facilitate corresponding anode and cathode reactions, thereby transferring charge across the electrolyte and generating electricity. A fuel cell stack can represent multiple cells in an integrated unit. While a fuel cell stack can contain multiple fuel cells, fuel cells can typically be connected in parallel and can function (generally) as if they collectively represent a single fuel cell of a larger size. The fuel cell stack can include flow channels for distributing the input flow among each cell in the stack and flow channels for combining the output flow from the individual cells when the input flow is delivered to the anode or cathode. 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, parallel, or any other convenient manner (e.g., a combination of series and parallel). A fuel cell array can contain one or more stages of fuel cells and / or fuel cell stacks, where the anode / cathode output from the first stage can be used as the anode / cathode input for the second stage. It is worth noting that the anodes in a fuel cell array do not necessarily need to be connected in the same way as the cathodes in the array. For convenience, the input to the first anode stage of the fuel cell array can be referred to as the anode input of the array, and the input to the first cathode stage of the fuel cell array can be referred to as the cathode input of the array. Similarly, the output from the last anode / cathode stage can be referred to as the anode / cathode output from the array. In aspects where the fuel cell stack includes a separate reforming element, it is worth noting that the anode input stream may first pass through the reforming element before entering one or more anodes associated with the reforming element.

[0051] It should be understood that references to the use of fuel cells herein generally refer to a “fuel cell stack” consisting of individual fuel cells, and more generally to the use of one or more fuel cell stacks in fluid communication. Individual fuel cell elements (plates) can typically be “stacked” together in a rectangular array referred to as a “fuel cell stack.” Other types of elements, such as reforming elements, may also be included in a fuel cell stack. This fuel cell stack typically employs a feed stream and distributes reactants to all the individual fuel cell elements, and products can then be collected from each of these elements. An operating fuel cell stack, even if composed of many (typically dozens or hundreds) individual fuel cell elements, can be considered as a whole when viewed as a unit. These individual fuel cell elements can typically have similar voltages (because reactant and product concentrations are similar), and when the elements are electrically connected in series, the total power output can come from the sum of all currents in all the cells. Stacks can also be arranged in series to generate high voltage. Parallel arrangements can increase current. The systems and methods described herein can be used with a single molten carbonate fuel cell stack if the fuel cell stack is large enough to handle a given exhaust flow. In other aspects of the invention, multiple fuel cell stacks may be desired or required for a variety of reasons.

[0052] For the purposes of this invention, unless otherwise specified, the term "fuel cell" should be understood to also refer to and / or be defined to include references to a fuel cell stack consisting of a group of one or more individual fuel cell elements having a single input and output, which is how fuel cells are typically used in practice. Similarly, unless otherwise specified, the term "one or more fuel cells" should be understood to also refer to and / or be defined to include multiple individual fuel cell stacks. In other words, unless specifically indicated, all references in this document may interchangeably refer to "operation of a fuel cell stack" as "fuel cell". For example, the exhaust volume produced by a commercially scaled combustion generator may be too large to be handled by a conventionally sized fuel cell (i.e., a single stack). To handle all the exhaust, multiple fuel cells (i.e., two or more individual fuel cells or fuel cell stacks) can be arranged in parallel such that each fuel cell can handle (approximately) equal portions of the combustion exhaust. Although multiple fuel cells can be used, each fuel cell can generally operate in a substantially similar manner, given that the amount of combustion exhaust from each fuel cell is (approximately) equal.

[0053] Example of molten carbonate fuel cell operation: cross-flow orientation of cathode and anode

[0054] Figure 3 A general example of a portion of a molten carbonate fuel cell stack is shown. Figure 3The portion of the stack shown corresponds to fuel cell 301. To isolate the fuel cell from adjacent fuel cells and / or other components in the stack, the fuel cell includes partitions 310 and 311. Figure 3 In this fuel cell 301, an anode 330 and a cathode 350 are separated by an electrolyte matrix 340 containing an electrolyte 342. In various aspects, the cathode 350 may correspond to a double-layer (or multi-layer) cathode. An anode current collector 320 provides electrical contact between the anode 330 and other anodes in the fuel cell stack, while a cathode current collector 360 provides similar electrical contact between the cathode 350 and other cathodes in the fuel cell stack. Additionally, the anode current collector 320 allows for the introduction and exhaust of gas from the anode 330, while the cathode current collector 360 allows for the introduction and exhaust of gas from the cathode 350.

[0055] During operation, CO2 enters the cathode current collector 360 along with O2. CO2 and O2 diffuse into the porous cathode 350 and travel to the cathode interface region near the boundary between the cathode 350 and the electrolyte matrix 340. In the cathode interface region, a portion of the electrolyte 342 may be present in the pores of the cathode 350. CO2 and O2 can be converted into carbonate ions (CO3-) near / in the cathode interface region. 2- The carbonate ions can then be transported across electrolyte 342 (and thus across electrolyte matrix 340) to facilitate current generation. In the aspect where substitutional ion transport occurs, a portion of O2 can be converted into substitutional ions (such as hydroxide ions or peroxide ions) for transport in electrolyte 342. After transport across electrolyte 342, carbonate ions (or substitutional ions) can reach the anode interface region near the boundary between electrolyte matrix 340 and anode 330. In the presence of H2, carbonate ions can be converted back into CO2 and H2O, thereby releasing electrons for forming the current generated by the fuel cell. H2 and / or hydrocarbons suitable for H2 formation are introduced into anode 330 via anode current collector 320.

[0056] The flow direction within the anode of a molten carbonate fuel cell can have any convenient orientation relative to the flow direction within the cathode. One option is to use a cross-flow configuration, such that the flow direction within the anode is approximately at a 90° angle relative to the flow direction within the cathode. This type of flow configuration can be practically beneficial because it allows the anode inlet / outlet manifolds and / or conduits to be located on a different side than the cathode inlet / outlet manifolds and / or conduits of the fuel cell stack.

[0057] Figure 4 An example of a top view of a fuel cell cathode is schematically shown, with arrows indicating the flow direction within the fuel cell cathode and the corresponding fuel cell anode. Figure 4In the diagram, arrow 405 indicates the flow direction within the cathode 450, while arrow 425 indicates the flow direction within the anode (not shown).

[0058] Because the anolyte and cathode flows are oriented at approximately 90° relative to each other, the anolyte and cathode flow patterns can contribute to different reaction conditions in different parts of the cathode. These different conditions can be illustrated by considering the reaction conditions at the four corners of the cathode. Figure 4 In the description, the reaction conditions described herein are similar in nature to those of a fuel cell operating at 70% or higher (or 80% or higher) CO2 utilization.

[0059] Corner 482 corresponds to a portion of the fuel cell near the entry points of the cathode and anode input streams. Therefore, the concentrations of CO2 (in the cathode) and H2 (in the anode) are relatively high in corner 482. Based on these high concentrations, the portion of the fuel cell near corner 482 is expected to operate under anticipated conditions, where there is essentially no ion transport across the electrolyte except for carbonate ions.

[0060] Corner 484 corresponds to a portion of the fuel cell near the cathode input flow entry point and near the anode output flow exit point. At the location near corner 484, the amount of current density may be limited due to the reduced H2 concentration in the anode, depending on fuel utilization. However, sufficient CO2 should be present such that any ions transported across the electrolyte essentially correspond to carbonate ions.

[0061] Corner 486 corresponds to a portion of the fuel cell near the exit point of the anode output flow and near the exit point of the cathode output flow. At the location near corner 486, due to the low concentrations of both H2 (in the anode) and CO2 (in the cathode), the current is expected to be small or nonexistent due to the low driving force of the fuel cell reaction.

[0062] Corner 488 corresponds to a portion of the fuel cell near the anode input flow inlet and near the cathode output flow outlet. The relatively high availability of hydrogen at the location near corner 488 is expected to generate a considerable current density. However, due to the relatively low CO2 concentration, significant transport of hydroxide ions and / or other substitution ions may occur. Depending on the aspect described, significant substitution ion transport can increase the calculated CO2 utilization rate by 5% or more, or 10% or more, or 15% or more, or 20% or more. Alternatively or additionally, the transfer rate may be 0.95 or less, or 0.90 or less, or 0.85 or less, or 0.80 or less. Significant substitution ion transport across the electrolyte may temporarily allow a high current density to be maintained at the location near corner 488. However, substitution ion transport can also degrade the cathode and / or anode structure, resulting in a decrease (and possibly no decrease) in the current density at the location near corner 488 over time. It is worth noting that when lower amounts of substitute ions are transported (e.g., 0.96 or higher, or 0.98 or higher transfer rates), the amount of lifetime degradation is not as severe.

[0063] It has been found that fuel cells begin to degrade rapidly when alternative ion transport becomes significant at one or more locations within the fuel cell. This is thought to be due to the degradation of one or more locations and their failure to provide any further current density. Since one or more regions cease contributing to the required current density, the remaining locations in the fuel cell must operate at higher current densities to maintain a constant total (average) current density for the fuel cell. This can lead to the growth of regions used for alternative ion transport, resulting in the degradation of the expanded portion of the fuel cell and eventual cessation of operation. Alternatively, degradation of a portion of the fuel cell may result in a decrease in the total current density from the cell, which is also undesirable. Operating multiple fuel stages can reduce the amount of alternative ion transport occurring within a fuel cell stage during periods of increased CO2 capture, thereby extending the fuel cell's lifespan.

[0064] Anode input and output

[0065] In various aspects, the anode feed stream of an MCFC may contain hydrogen, hydrocarbons (such as methane), and may contain heteroatoms other than C and H, or combinations thereof. The source of hydrogen / hydrocarbons / hydrocarbon-like compounds may be referred to as the fuel source. In some aspects, the majority of the methane (or other hydrocarbons, hydrocarbon-containing compounds, or hydrocarbon-like compounds) fed to the anode may typically be fresh methane. In this description, fresh fuel (such as fresh methane) refers to fuel that has not been recycled from another fuel cell process. For example, methane recycled from the anode outlet feed stream back to the anode inlet may not be considered "fresh" methane but may be described as recovered methane.

[0066] The fuel source used can be shared with other components, such as turbines that use a portion of the fuel source to provide a CO2-containing feed to the cathode input. The fuel source input can contain water in proportion to the fuel, suitable for reforming hydrocarbon (or hydrocarbon-like) compounds in a reforming section that produces hydrogen. For example, if methane is the fuel input used for reforming to produce H2, the water-to-fuel molar ratio can be from about one to one to about ten to one, such as at least about two to one. Typical ratios for external reforming are four to one or greater, but typical values ​​for internal reforming may be lower. In some optional aspects, additional water may not be necessary in the fuel as long as H2 is part of the fuel source, since the oxidation of H2 at the anode often produces H2O, which can be used to reform the fuel. The fuel source can also optionally contain components incidental to the fuel source (e.g., a natural gas feed may contain a certain amount of CO2 as an additional component). For example, a natural gas feed may contain CO2, N2, and / or other inert (rare) gases as additional components. Optionally, in some respects, the fuel source may also contain CO, such as CO from the recirculation portion of the anode exhaust. An additional or alternative potential source of CO in the fuel entering the fuel cell assembly may be CO generated through hydrocarbon fuel steam reforming of the fuel prior to its entry into the fuel cell assembly.

[0067] More generally, various types of fuel feed streams can be suitable as anode input feed streams for molten carbonate fuel cells. Some fuel feed streams may correspond to feed streams containing hydrocarbons and / or may also contain heteroatoms other than C and H. In this discussion, unless otherwise stated, references to hydrocarbon-containing fuel feed streams for MCFC anodes are defined as including fuel feed streams containing such hydrocarbon compounds. Examples of hydrocarbon (including hydrocarbon-like) fuel feed streams include natural gas, feed streams containing C1-C4 carbon compounds (such as methane or ethane), and feed streams containing heavier C5+ hydrocarbons (including hydrocarbon-like compounds), and combinations thereof. Other additional or alternative examples of potential fuel feed streams for anode inputs may include biogas-type feed streams, such as methane produced from the natural (bio)decomposition of organic materials.

[0068] In some respects, molten carbonate fuel cells can be used to process input fuel streams such as natural gas and / or hydrocarbon feedstocks, which have low energy content due to the presence of diluent compounds. For example, some sources of methane and / or natural gas can contain significant amounts of CO2 or other inert molecules such as nitrogen, argon, or helium. The presence of increased amounts of CO2 and / or inert substances can reduce the energy content of fuel streams based on these sources. Using low-energy-content fuels for combustion reactions (such as powering combustion turbines) can present challenges. However, molten carbonate fuel cells can generate electricity based on low-energy-content fuel sources with minimal or no impact on fuel cell efficiency. The presence of additional gas volumes may require additional heat to raise the fuel temperature to the levels required for reforming and / or anode reactions. Additionally, the presence of additional CO2 can affect the relative amounts of H2 and CO present in the anode output due to the equilibrium nature of the water-gas shift reaction within the fuel cell anode. However, inert compounds may otherwise have only a minimal direct impact on reforming and anode reactions. The amount of CO2 and / or inert compounds in the fuel stream for a molten carbonate fuel cell (if present) may be at least about 1 vol%, such as at least about 2 vol%, or at least about 5 vol%, or at least about 10 vol%, or at least about 15 vol%, or at least about 20 vol%, or at least about 25 vol%, or at least about 30 vol%, or at least about 35 vol%, or at least about 40 vol%, or at least about 45 vol%, or at least about 50 vol%, or at least about 75 vol%. Alternatively or alternatively, the amount of CO2 and / or inert compounds in the fuel stream for a molten carbonate fuel cell may be about 90 vol% or less, such as about 75 vol% or less, or about 60 vol% or less, or about 50 vol% or less, or about 40 vol% or less, or about 35 vol% or less.

[0069] Other examples of potential sources for the anode input stream may correspond to refining and / or other industrial process output streams. For example, coking is a common process in many refineries used to convert heavier compounds to a lower boiling range. Coking typically produces exhaust gases containing a variety of compounds that are gases at room temperature, including CO and various C1–C4 hydrocarbons. This exhaust gas can be used as at least a portion of the anode input stream. Other refining exhaust gas streams may additionally or alternatively be suitable for inclusion in the anode input stream, such as light fractions (C1–C4) generated during cracking or other refining processes. Still other suitable refining streams may additionally or alternatively include refining streams containing CO or CO2, which also contain H2 and / or reformable fuel compounds.

[0070] Other potential sources for the anode input can additionally or alternatively include a feed stream with increased water content. For example, the ethanol output stream from an ethanol plant (or another type of fermentation process) may contain a significant portion of H2O before final distillation. This H2O typically has only a minimal impact on the operation of the fuel cell. Therefore, a fermentation mixture of alcohol (or other fermentation products) and water can be used as at least a portion of the anode input stream.

[0071] Biogas, or biogas, is another potential source of anode input. Biogas can primarily consist of methane and CO2 and is typically produced through the decomposition or digestion of organic matter. Anaerobic bacteria can be used to digest organic matter and produce biogas. Impurities, such as sulfur compounds, can be removed from biogas before it is used as an anode input.

[0072] The output stream from the MCFC anode can contain H2O, CO2, CO, and H2. Optionally, the anode output stream may also contain unreacted fuels (such as H2 or CH4) or inert compounds from the feed as additional output components. The anode output stream can be separated once or multiple times to separate CO2 from components (such as H2 or CO) that have potential value as inputs to another process, rather than using such output streams as a fuel source to provide heat for reforming reactions or as combustion fuel for heating the cell. H2 and / or CO can be used as syngas for chemical synthesis, hydrogen sources for chemical reactions, and / or fuels to reduce greenhouse gas emissions.

[0073] Cathode input and output

[0074] Conventionally, molten carbonate fuel cells can be operated based on drawing in a desired load while consuming a portion of the fuel in the fuel feed stream delivered to the anode. The voltage of the fuel cell can then be determined by the load, the fuel input to the anode, the air and CO2 supplied to the cathode, and the internal resistance of the fuel cell. CO2 can conventionally be supplied to the cathode, in part, by using the anode exhaust as at least a portion of the cathode input feed stream. In contrast, the present invention can use separate / different sources for the anode and cathode inputs. By eliminating any direct link between the composition of the anode and cathode input streams, additional options become available for operating the fuel cell, such as generating excess syngas, increasing CO2 capture, and / or increasing the overall efficiency of the fuel cell (electrical energy plus chemical energy), etc.

[0075] In various aspects, MCFCs can be operated to induce alternative ion transport across the electrolyte of the fuel cell. To induce alternative ion transport, the CO2 content of the cathode feed stream can be 5.0 vol% or less, or 4.0 vol% or less, such as 1.5 vol% to 5.0 vol%, or 1.5 vol% to 4.0 vol%, or 2.0 vol% to 5.0 vol%, or 2.0 vol% to 4.0 vol%.

[0076] An example of a suitable CO2-containing feed stream for use as a cathode input stream can be an output stream 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 fuels (including bio-derived fuels). Additional or alternative sources may include other types of boilers, flame heaters, furnaces, and / or other devices that burn carbon-containing fuels to heat another substance (such as water or air).

[0077] Other potential sources of CO2 in the cathode input stream may additionally or alternatively include sources of bio-generated CO2. This could include, for example, CO2 generated during the processing of bio-derived compounds, such as CO2 generated during ethanol production. Additional or alternative examples could include CO2 generated from the combustion of bio-generated fuels (such as the combustion of lignocellulose). Still other additional or alternative potential CO2 sources could correspond to output or exhaust streams from various industrial processes, such as CO2-containing streams generated by plants used in the manufacture of steel, cement, and / or paper.

[0078] Another potential source of CO2 could be a CO2-containing feed stream from a fuel cell. This CO2-containing feed stream could correspond to a cathode output feed stream from a different fuel cell, an anode output feed stream from a different fuel cell, a recirculated feed stream from the cathode output to the cathode input of a fuel cell, and / or a recirculated feed stream from the anode output to the cathode input of a fuel cell. For example, an MCFC operating in stand-alone mode under normal conditions can produce a cathode exhaust with a CO2 concentration of at least about 5 vol%. This CO2-containing cathode exhaust can be used as a cathode input of an MCFC operating according to one aspect of the invention. More generally, other types of fuel cells that generate CO2 output from cathode exhaust, and other types of CO2-containing feed streams not generated by a "combustion" reaction and / or a combustion-powered generator, can be used additionally or alternatively. Optionally, but preferably, a CO2-containing feed stream from another fuel cell can originate from another molten carbonate fuel cell. For example, for the cathodes of molten carbonate fuel cells connected in series, the output from the cathode of a first molten carbonate fuel cell can be used as an input to the cathode of a second molten carbonate fuel cell.

[0079] In addition to CO2, the cathode input stream may also contain O2 to provide the components required for the cathode reaction. Some cathode input streams may be based on the inclusion of air as a component. For example, a combustion exhaust stream can be formed by burning hydrocarbon fuels in the presence of air. This combustion exhaust stream, or another type of cathode input stream based on the oxygen content of the air-containing stream, may have an oxygen content of about 20 vol% or less, such as about 15 vol% or less, or about 10 vol% or less. Alternatively or additionally, the oxygen content of the cathode input stream may be at least about 4 vol%, such as at least about 6 vol% or at least about 8 vol%. More generally, the cathode input stream may have a suitable oxygen content for carrying out the cathode reaction. In some aspects, this may correspond to an oxygen content of about 5 vol% to about 15 vol%, such as about 7 vol% to about 9 vol%. For many types of cathode input streams, the combined amount of CO2 and O2 may correspond to an input stream of less than about 21 vol%, such as a stream of less than about 15 vol% or a stream of less than about 10 vol%. Oxygen-containing air streams can be combined with CO2 sources with low oxygen content. For example, exhaust streams produced by burning coal can contain low levels of oxygen, which can be mixed with air to form the cathode inlet stream.

[0080] In addition to CO2 and O2, the cathode input feed stream can also consist of inert / non-reactive substances such as N2, H2O, and other typical oxidant (air) components. For example, for a cathode input originating from combustion reaction exhaust gas, if air is used as part of the oxidant source for the combustion reaction, the exhaust gas may contain typical air components such as N2, H2O, and other small amounts of compounds present in air. Depending on the nature of the fuel source used for the combustion reaction, additional substances present after combustion of said fuel source may include H2O, oxides of nitrogen (NOx) and / or oxides of sulfur (SOx), and one or more other compounds present in the fuel and / or as partial or complete combustion products of compounds present in the fuel, such as CO. These substances may be present in amounts that do not poison the cathode catalyst surface but can reduce overall cathode activity. This reduction in performance is acceptable, or the substances interacting with the cathode catalyst can be reduced to an acceptable level using known contaminant removal techniques.

[0081] The amount of O2 present in the cathode feed stream (such as a cathode feed stream based on combustion exhaust) can be sufficient to advantageously provide the oxygen required for the cathode reaction in the fuel cell. Therefore, the volume percentage 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 feed stream as needed to provide sufficient oxidant for the cathode reaction. When some form of air is used as the oxidant, the amount of N2 in the cathode exhaust can be at least about 78 vol%, for example at least about 88 vol% and / or about 95 vol% or less. In some respects, the cathode feed stream can additionally or alternatively contain compounds typically considered contaminants, such as H2S or NH3. In other respects, the cathode feed stream can be purified to reduce or minimize the content of such contaminants.

[0082] Suitable operating temperatures for MCFCs can be between about 450°C and about 750°C, such as at least about 500°C, for example, an inlet temperature of about 550°C and an outlet temperature of about 625°C. Heat can be added to or removed from the cathode feed stream if needed before it enters the cathode, to provide heat, for example, for other processes such as reforming the fuel input to the anode. For example, if the source of the cathode feed stream is a combustion exhaust stream, the temperature of the combustion exhaust stream can be higher than the desired temperature at the cathode inlet. In such cases, heat can be removed from the combustion exhaust before it is used as the cathode feed stream. Alternatively, the combustion exhaust may 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 below about 100°C. Alternatively, the combustion exhaust can come from the exhaust of a gas turbine operating in combined cycle mode, where the gas can be cooled by generating steam to run a steam turbine for additional power generation. In this case, the gas can be below about 50°C. Heat can be added to the combustion exhaust at a temperature lower than desired.

[0083] Additional molten carbonate fuel cell operation strategies

[0084] In some respects, when operating an MCFC to induce alternative ion transport, the anode of the fuel cell can operate at approximately 60% to 80% of conventional fuel utilization values. When attempting to generate electricity, operating the anode of the fuel cell at a relatively high fuel utilization rate can be beneficial for improving electrical efficiency (i.e., the electrical energy produced per unit of chemical energy consumed by the fuel cell).

[0085] In some respects, reducing the electrical efficiency of a fuel cell to provide other benefits, such as increasing the amount of H2 supplied in the anode output stream, may be advantageous. For example, it may be beneficial if it is desired to dissipate excess heat generated in the fuel cell (or fuel cell stack) by performing additional reforming and / or another endothermic reaction. For instance, a molten carbonate fuel cell can be operated to increase the production of syngas and / or hydrogen. The heat required for the endothermic reforming reaction can be provided by the exothermic electrochemical reaction in the anode used to generate electricity. This excess heat can be used in situ as a heat source for reforming and / or another endothermic reaction, rather than attempting to remove the heat generated by one or more exothermic fuel cell reactions from the fuel cell. As a result, thermal energy can be used more efficiently and / or the need for additional external or internal heat exchange can be reduced. This efficient generation and use of thermal energy (essentially in situ) can reduce system complexity and the number of components while maintaining favorable operating conditions. In some respects, the amount of reforming or other endothermic reactions can be selected to have an endothermic heat requirement that is comparable to or even greater than the excess heat generated by one or more exothermic reactions, rather than significantly less than the heat requirement typically described in the prior art.

[0086] Alternatively or alternatively, the fuel cell can be operated such that the temperature difference between the anode inlet and anode outlet can be negative rather than positive. Therefore, sufficient reforming and / or other endothermic reactions can be carried out so that the output feed temperature from the anode outlet is lower than the anode inlet temperature, rather than raising the temperature between the anode inlet and outlet. Further, alternatively or alternatively, additional fuel can be supplied to the fuel cell's heater and / or internal reforming stage (or other internal endothermic reaction stage) such that the temperature difference between the anode input and anode output is less than a desired temperature difference based on the relative demand generated by the combined exothermic heat from one or more endothermic reactions and the cathode combustion reaction and anode reaction used to generate electricity. In the aspect where reforming is used as an endothermic reaction, operating the fuel cell to reform excess fuel can allow for the production of increased syngas and / or increased hydrogen relative to conventional fuel cell operation, while minimizing the system complexity of heat exchange and reforming. The additional syngas and / or additional hydrogen can then be used for various applications, including chemical synthesis processes and / or the collection / reuse of hydrogen as a "clean" fuel.

[0087] The heat generated per mole of hydrogen oxidized by the exothermic reaction at the anode can be significantly greater than the heat consumed per mole of hydrogen produced by the reforming reaction. The net enthalpy of the reaction of hydrogen in a molten carbonate fuel cell (H₂ + 1 / 2O₂ => H₂O) can be approximately -285 kJ / mol of hydrogen molecule. At least a portion of this energy can be converted into electrical energy within the fuel cell. However, the difference (approximately) between the enthalpy of reaction and the electrical energy produced by the fuel cell can be converted into heat within the fuel cell. This energy can also be alternatively expressed as the cell's current density (current per unit area) multiplied by the difference between the theoretical maximum voltage and the actual voltage of the fuel cell, or <current density> * (Vmax – Vact). This energy is defined as the "waste heat" of the fuel cell. As an example of reforming, the reforming enthalpy of methane (CH₄ + 2H₂O => 4H₂ + CO₂) can be approximately 250 kJ / mol of methane or approximately 62 kJ / mol of hydrogen molecule. From a thermal balance perspective, each electrochemically oxidized hydrogen molecule can generate enough heat to produce more than one hydrogen molecule through reforming. In a conventional configuration, this excess heat can result in a significant temperature difference from the anode inlet to the anode outlet. Instead of using this excess heat to increase the temperature within the fuel cell, it can be dissipated by performing a matched amount of reforming reactions. Excess heat generated at the anode can be supplemented by excess heat generated by combustion reactions within the fuel cell. More generally, excess heat can be dissipated by performing endothermic reactions at the fuel cell anode and / or in an endothermic reaction stage thermally integrated with the fuel cell.

[0088] Based on the aforementioned aspects, the amount of reforming and / or other endothermic reactions can be selected relative to the amount of hydrogen reacting in the anode to achieve the desired heat ratio of the fuel cell. As used herein, "heat ratio" is defined as the heat generated by the exothermic reactions in the fuel cell assembly (including the exothermic reactions in both the anode and cathode) divided by the endothermic demand of the reforming reactions occurring within the fuel cell assembly. Mathematically, heat ratio (TH) = Q EX / Q EN Q EX It is the total heat generated by the exothermic reaction, and Q is the sum of the heat produced. ENThis is the total heat consumed by the endothermic reactions occurring within the fuel cell. It should be noted that the heat generated by exothermic reactions can correspond to any heat generated due to reforming reactions in the cathode, water-gas shift reactions, combustion reactions (i.e., oxidation of fuel compounds), and / or electrochemical reactions within the cell. The heat generated by electrochemical reactions can be calculated based on the ideal electrochemical potential of the fuel cell reaction on the electrolyte minus the actual output voltage of the fuel cell. For example, based on the net reaction occurring in the cell, the ideal electrochemical potential of the reaction in an MCFC is considered to be approximately 1.04V. During MCFC operation, the cell's output voltage is typically below 1.04V due to various losses. For example, a common output / operating voltage can be approximately 0.7V. The generated heat can be equal to the cell's electrochemical potential (i.e., ~1.04V) minus the operating voltage. For example, when the output voltage in the fuel cell reaches ~0.7V, the heat generated by the electrochemical reactions in the cell can be ~0.34V. Therefore, in this case, the electrochemical reactions will produce ~0.7V of electrical energy and ~0.34V of thermal energy. In such instances, ~0.7V of electrical energy is not considered as Q. EX A portion of it is included. In other words, thermal energy is not electrical energy.

[0089] In various aspects, the thermal ratio can be determined for any convenient fuel cell structure, such as a fuel cell stack, a single fuel cell within a fuel cell stack, a fuel cell stack with an integrated reforming stage, a fuel cell stack with an integrated endothermic reaction stage, or a combination thereof. The thermal ratio can also be calculated for different units within a fuel cell stack (such as fuel cells or combinations of fuel cell stacks). For example, the thermal ratio can be calculated for the fuel cells (or multiple fuel cells) within a fuel cell stack, together with integrated reforming stages and / or integrated endothermic reaction stage elements that are sufficiently close to one or more fuel cells to be integrated from a thermal integration perspective.

[0090] From a thermal integration perspective, the characteristic width in a fuel cell stack can be the height of a single fuel cell stack element. It is worth noting that, compared to the fuel cell, a separate reforming stage and / or a separate endothermic reaction stage can have different heights in the stack. In such cases, the height of the fuel cell element can be used as the characteristic height. In this discussion, an integrated endothermic reaction stage can be defined as a stage thermally integrated with one or more fuel cells, such that the integrated endothermic reaction stage can use heat from the fuel cell as a heat source for reforming. Such an integrated endothermic reaction stage can be defined as having a location height 10 times smaller than the height of the stack element of the fuel cell that provides heat to the integrated stage. For example, the location height of an integrated endothermic reaction stage (such as a reforming stage) can be 10 times smaller than the height of the stack element from any thermally integrated fuel cell, or it can be 8 times smaller than the stack element height, or it can be 5 times smaller than the stack element height, or it can be 3 times smaller than the stack element height. In this discussion, an integrated reforming stage and / or an integrated endothermic reaction stage representing a stack element adjacent to a fuel cell element can be defined as having a height of approximately one stack element height or less from the adjacent fuel cell element.

[0091] A heat 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 heat ratio typically sought when using MCFC fuel cells. In aspects of the invention, the heat ratio can be reduced to increase and / or optimize the production of syngas, the production of hydrogen, the production of another product via an endothermic reaction, or a combination thereof.

[0092] In various aspects of the invention, the operation of the fuel cell can be characterized based on a heat ratio. When the fuel cell operates with a desired heat ratio, the molten carbonate fuel cell can operate with a heat 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 heat 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 be operated with a temperature rise between the anode input and anode output of about 40°C or less, such as about 20°C or less, or about 10°C or less. Still further, additionally or alternatively, the fuel cell can be operated with an anode outlet temperature that is about 10°C lower than the anode inlet temperature and about 10°C higher. Furthermore, or alternatively, the fuel cell can be operated such that its anode inlet temperature is higher than its anode outlet temperature, such as 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, or alternatively, the fuel cell can be operated such that its anode inlet temperature is about 100°C higher than its anode outlet temperature, or about 80°C higher, or about 60°C higher, or about 50°C higher, or about 40°C higher, or about 30°C higher, or about 20°C higher.

[0093] Operating a fuel cell at a heat ratio less than 1 can cause a temperature drop across the entire fuel cell. In some respects, the amount of reforming and / or other endothermic reactions can be limited such that the temperature drop from anode inlet to 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 anode inlet to anode outlet can be beneficial, for example, to maintain a sufficient temperature to allow the fuel to be fully or substantially fully converted in the anode (through reforming). In other respects, due to the balance between the heat consumed by the endothermic reaction and the additional external heat supplied to the fuel cell, additional heat can be supplied to the fuel cell (e.g., through heat exchange or combustion of additional fuel) such that the anode inlet temperature is about 100°C higher than the anode outlet temperature, such as about 80°C higher or less, or about 60°C higher or less, or about 50°C higher or less, or about 40°C higher or less, or about 30°C higher or less, or about 20°C higher or less.

[0094] The amount of reformed fuel can depend, either additionally or alternatively, on the availability of reformable fuel. For example, if the fuel consists only of H2, no reforming will occur because the H2 has already been reformed and cannot be further reformed. The amount of syngas "produced" by a fuel cell can be defined as the difference between the lower heating value (LHV) of the syngas at the anode input and the LHV of the syngas at the anode output. Produced syngas LHV(sg net) = (LHV(sg out) - LHV(sg in)), where LHV(sg in) and LHV(sg out) refer to the LHV of the syngas in the anode inlet feed stream or stream and the LHV of the syngas in the anode outlet feed stream or stream, respectively. Fuel cells supplied with fuel containing a large amount of H2 may be limited in potential syngas production because the fuel contains a large 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 into the gas phase, including the fully oxidized products (i.e., gas phase CO2 and H2O products). For example, any CO2 present in the anode feed stream does not contribute to the fuel content at the anode input because the CO2 has been completely oxidized. For this definition, the amount of oxidation that occurs in 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.

[0095] An example of a method for operating a fuel cell with a reduced heat ratio could be the excessive reforming of the fuel to balance the heat generated and consumed in the fuel cell and / or to consume more heat than generated. Reforming reformable fuel to form H2 and / or CO can be an endothermic process, while the anodic electrochemical oxidation reaction and one or more cathode combustion reactions can be exothermic. During conventional fuel cell operation, the heat consumed by the amount of reforming required to supply the feed components for fuel cell operation can typically be less than the heat generated by the anodic oxidation reaction. For example, conventional operation at about 70% or about 75% fuel utilization would produce a heat ratio substantially greater than 1, such as at least about 1.4 or greater, or 1.5 or greater. Therefore, the output feed stream of the fuel cell may be hotter than the input feed stream. In contrast to 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 such that the heat generated by the exothermic fuel cell reaction can be (substantially) balanced by the heat consumed in the reforming and / or to consume more heat than generated. This can result in a significant excess of hydrogen relative to the amount of hydrogen oxidized in the anode to generate electricity, and can result in a heat ratio of about 1.0 or lower, such as about 0.95 or lower, or about 0.90 or lower, or about 0.85 or lower, or about 0.80 or lower, or about 0.75 or lower.

[0096] Hydrogen or syngas can be extracted from the anode exhaust as a chemical energy output. Hydrogen can be used as a clean fuel, producing no greenhouse gases when burned or combusted. Conversely, for hydrogen produced through the reforming of hydrocarbons (or hydrocarbon-containing compounds), CO2 is already "captured" in the anode circuit. Additionally, hydrogen can be a valuable input to various refining and / or other synthetic processes. Syngas can also be a valuable input to various processes. Besides its fuel value, syngas can be used as a feedstock for producing other higher-value products, such as by using it as input to Fischer-Tropsch synthesis and / or methanol synthesis processes.

[0097] In some aspects, the reformable hydrogen content of the reformable fuel delivered to the anode and / or to the reforming stage associated with the anode may be at least about 50% greater than the net hydrogen content reacted at the anode, such as at least about 75% or at least about 100%. Additionally or alternatively, the reformable hydrogen content of the fuel delivered to the anode and / or to the reforming stage associated with the anode may be at least about 50% greater than the net hydrogen content reacted at the anode, such as at least about 75% or at least about 100%. In various aspects, the ratio of the reformable hydrogen content of the reformable fuel in the fuel stream to the amount of hydrogen reacted at the anode may 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. Alternatively or alternatively, the ratio of the reformable hydrogen content of the reformable fuel in the fuel stream to the amount of hydrogen reacted in the anode may be about 20:1 or less, such as about 15:1 or less, or about 10:1 or less. In one aspect, it is envisioned that less than 100% of the reformable hydrogen content in the anode inlet stream can be converted into hydrogen. For example, at least about 80% of the reformable hydrogen content in the anode inlet stream can be converted into hydrogen in the anode and / or in one or more associated reforming stages, such as at least about 85% or at least about 90%. Alternatively or alternatively, the amount of reformable fuel delivered to the anode may be characterized based on the lower heating value (LHV) of the reformable fuel relative to the LHV of the hydrogen oxidized in the anode. This may be referred to as the reformable fuel excess rate. In various aspects, the reformable fuel excess rate may be at least about 2.0, such as at least about 2.5, or at least about 3.0, or at least about 4.0. Alternatively or alternatively, the reformable fuel surplus rate may be about 25.0 or lower, such as about 20.0 or lower, or about 15.0 or lower, or about 10.0 or lower.

[0098] Example 1

[0099] A steady-state fuel cell model, created using a commercially available process modeling platform, was used to simulate the cathode flow modes for two fuel cell configurations. In the first configuration, a 250cm² fuel cell was used.2 Modeling of a single fuel cell. In the second configuration, the combined dimensions of the series connections in a parallel-crossflow manner are 166 cm. 2 Two fuel cells (approximately 2 / 3 the area of ​​a single fuel cell) are modeled. For each configuration, the anode input gas to the first stage is modeled as 72% H2, 18% CO2, and 10% H2O (molecular weight). The cathode input gas is modeled as 4% CO2, 10% O2, 10% H2O, and 76% N2 (molecular weight). In each configuration, the first (or sole) stage is modeled at 120 mA / cm². 2 The current density is [value missing], fuel utilization rate is 85%, and CO2 utilization rate is 72%. For the second configuration, the first stage has 120 mA / cm². 2 The first stage has a high current density and 85% fuel efficiency, while the second stage has 120 mA / cm². 2 The first configuration achieves a high current density and a 70% fuel utilization rate. The net CO2 capture or utilization rate across both stages in the second configuration is 72%, allowing for the use of the same net CO2 content in both configurations. However, due to CO2 consumption in the first configuration, a significant amount of additional substitution ion transport occurs. This results in a reduced transfer rate. This explains why the larger fuel cell region in the first configuration has the same current density as the smaller fuel cell region in the second configuration.

[0100] Based on the flow pattern modeled for the first configuration, Figure 5 The figure shows the CO2 concentration in the cathode at steady state. The direction of the anolyte and cathode flow is also indicated. Figure 5 As shown, although the concentration in the cathode output stream is approximately 1.0 mol% (based on a CO2 utilization rate of 72%), CO2 is distributed in a highly non-uniform manner within the cathode. Specifically, the CO2 concentration drops to almost zero at the corner corresponding to the anode inlet and cathode outlet, while at the corner corresponding to the cathode inlet and anode outlet, the CO2 concentration is almost 4.0 mol%. Due to the significant variation in CO2 concentration throughout the cathode, the amount of alternative ion transport in localized regions of low CO2 concentration within the cathode may be increased.

[0101] Figure 6 The cathode CO2 concentrations of the two stages in the second configuration under steady state are shown. Figure 6 As shown, the changes in CO2 concentration were reduced or minimized in both cathode stages. Furthermore, in Figure 5 The lowest concentration corresponds to a CO2 concentration of 0.08 mol% or lower. In contrast, Figure 6 The lowest concentration observed was 0.3 mol%.

[0102] Example 2

[0103] In this example, two configurations using two molten carbonate fuel cells are employed. In the first configuration, the two fuel cells are arranged in parallel, such that each fuel cell receives half of the anode feed stream and half of the cathode feed stream. In the parallel configuration, both fuel cells operate under identical operating conditions. The operating conditions are selected to achieve a current density of 90 mA / cm² for each fuel cell in the parallel configuration. 2 In the second configuration, the two fuel cells are arranged in series with respect to the cathode flow. Therefore, in this configuration, each fuel cell receives half of the anode input flow, while the first fuel cell receives the entire cathode input flow. The cathode exhaust from the first fuel cell is then used as the cathode feed flow for the second fuel cell. For the second configuration, the first fuel cell operates at 120 mA / cm². 2 Operation, while the second fuel cell operates at 60 mA / cm. 2 Operation. Both configurations have the same fuel efficiency.

[0104] As explained above, the net current density for operating parallel and series configurations is the same because the average current density of the two cells in both configurations is 90 mA / cm². 2 However, by using a series configuration, the transfer rate increased from 0.683 in the parallel configuration to 0.693 in the series configuration. This increase in transfer rate indicates a reduction in the amount of alternative ion transport in the series configuration. Therefore, even for the same current density, staged fuel cells can provide unexpected benefits by reducing or minimizing the amount of alternative ion transport when operating with a large amount of alternative ion transport. However, it is noteworthy that even with the reduced net amount of alternative ion transport, the operating voltage of the first cell in the series configuration is lower than that in the parallel configuration. This suggests that even with the reduction in net alternative ion transport, this reduction is based on the increase in the first stage of the series configuration and the larger reduction in the subsequent second stage.

[0105] Additional embodiments

[0106] Example 1. A method for generating electricity, the method comprising: introducing an anode input stream comprising H2, reformable fuel, or a combination thereof into a first anode stage of a plurality of molten carbonate fuel cell stages; introducing a cathode input stream comprising O2 and CO2 into a first cathode stage of the plurality of molten carbonate fuel cell stages; introducing a second anode input stream into a second anode stage of the plurality of molten carbonate fuel cell stages; introducing an intermediate cathode output into the second cathode stage of the plurality of molten carbonate fuel cell stages; and operating the plurality of molten carbonate fuel cell stages to generate: i) an average current density of 60 mA / cm². 2or greater power, ii) anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H2, CO and CO2, and iii) cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising CO2 at a content of 2.0 vol% or less, H2O at a content of 1.0 vol% or more and O2 at a content of 1.0 vol% or more, at least one of the first cathode stage and the second cathode stage operating at a transfer rate of 0.97 or less.

[0107] Example 2. According to the method of Example 1, wherein the first cathode stage operates at a transfer rate of 0.97 or lower, and the second cathode stage operates at a transfer rate of 0.97 or lower.

[0108] Example 3. The method according to any one of the foregoing embodiments, wherein at least one of the first cathode stage and the second cathode stage operates at a transfer rate of 0.95 or lower, or 0.90 or lower.

[0109] Example 4. The method according to any one of the preceding embodiments, wherein the second anode input stream includes at least a portion of the intermediate anode output stream, the intermediate anode output optionally including the output from the first anode stage.

[0110] Example 5. The method according to Example 4 further includes: allowing a second anode intermediate output to enter a third anode stage, the anode exhaust including an output from the third anode stage; and allowing a second cathode intermediate output to enter a third cathode stage, the cathode exhaust including an output from the third cathode stage, wherein the first anode stage, the second anode stage, and the third anode stage are arranged in parallel flow or in counterflow relative to the first cathode stage, the second cathode stage, and the third cathode stage.

[0111] Example 6. The method according to any one of the foregoing embodiments, wherein the transfer rate of the first cathode stage is different from the transfer rate of the second cathode stage.

[0112] Example 7. The method according to any one of the preceding embodiments, wherein at least the first anode stage, the second anode stage, the first cathode stage and the second cathode stage operate in a parallel flow cross-flow manner; or wherein at least the first anode stage, the second anode stage, the first cathode stage and the second cathode stage operate in a counter-flow cross-flow manner.

[0113] Example 8. The method according to any one of the preceding embodiments, wherein the anode exhaust includes an anode output from the second anode stage, or wherein the cathode exhaust includes a cathode output from the second cathode stage, or a combination thereof.

[0114] Example 9. The method according to any one of the foregoing embodiments, wherein the intermediate cathode output includes the output from the first cathode stage.

[0115] Example 10. The method according to any one of the preceding embodiments, wherein during operation of the plurality of molten carbonate fuel cells, the CO2 utilization rate measured in the combination of the first cathode stage and the second cathode stage is 75% or higher, or 80% or higher.

[0116] Example 11. The method according to any one of the foregoing embodiments, wherein the power is generated at an average current density of 100 mA / cm2 or higher, or 120 mA / cm2 or higher, or 150 mA / cm2 or higher.

[0117] Example 12. The method according to any one of the preceding embodiments, wherein the voltage drop across the cathode is 0.4V or less, or wherein power is generated at a voltage of 0.55V or greater, or a combination thereof.

[0118] Example 13. The method according to any one of the preceding embodiments, wherein the fuel utilization rate in the anode is 60% or higher, or the fuel utilization rate in the anode is 55% or lower, or the H2 concentration in the anode exhaust is 5.0 vol% or higher, or the combined concentration of H2 and CO in the anode exhaust is 6.0 vol% or higher, or a combination thereof.

[0119] Example 14. The method according to any one of the preceding embodiments, wherein the fuel cell operates at a heat ratio of 0.25:1.0; or wherein the amount of reformable fuel introduced into the first anode stage, the internal reforming element associated with the first anode stage, or a combination thereof, is at least about 75% greater than the amount of hydrogen reacted in the first anode stage to generate electricity; or a combination thereof.

[0120] Example 15. The method according to any one of the preceding embodiments, wherein the cathode input stream comprises 5.0 vol% or less CO2, or wherein the cathode exhaust comprises 1.0 vol% or less CO2, or a combination thereof.

[0121] Alternative embodiments

[0122] Alternative Example 1. A method for generating electricity, the method comprising: introducing an anode input stream comprising H2, reformable fuel, or a combination thereof into a first anode stage of a plurality of molten carbonate fuel cell stages; introducing a cathode input stream comprising O2 and CO2 into a first cathode stage of the plurality of molten carbonate fuel cell stages; introducing a second anode input stream into a second anode stage of the plurality of molten carbonate fuel cell stages; introducing an intermediate cathode output into the second cathode stage of the plurality of molten carbonate fuel cell stages; and operating the molten carbonate fuel cell stages to generate: i) an average current density of 80 mA / cm². 2 Or greater power, ii) anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H2, CO and CO2, and iii) cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising CO2, 1.0 vol% or more of O2 and 1.0 vol% or more of H2O, wherein the combined CO2 utilization rate measured across the plurality of molten carbonate fuel cell stages is 70% or higher (or 75% or higher, or 80% or higher), and wherein the calculated CO2 utilization rate in the plurality of molten carbonate fuel cell stages based on the current density is 5% or more greater (or 10% or more, or 20% or more greater) than the combined CO2 utilization rate.

[0123] Alternative Example 2. The method according to Alternative Example 1, wherein the cathode input stream comprises 5.0 vol% or less CO2 (or 4.0 vol% or less), or wherein the cathode exhaust comprises 1.0 vol% or less CO2, or a combination thereof.

[0124] Alternative Example 3. A method for generating electricity, the method comprising: introducing an anode input stream comprising H2, reformable fuel, or a combination thereof into a first anode stage of a plurality of molten carbonate fuel cell stages; introducing a cathode input stream comprising O2 and 5.0 vol% or less CO2 (or 4.0 vol% or less) into a first cathode stage of the plurality of molten carbonate fuel cell stages; introducing a second anode input stream into a second anode stage of the plurality of molten carbonate fuel cell stages; introducing an intermediate cathode output into the second cathode stage of the plurality of molten carbonate fuel cell stages; and operating the molten carbonate fuel cell stages to generate: i) an average current density of 80 mA / cm². 2Or greater power, ii) anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H2, CO and CO2, and iii) cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising 1.0 vol% or less of CO2, wherein the combined CO2 utilization rate measured across the plurality of molten carbonate fuel cell stages is 70% or higher (or 75% or higher, or 80% or higher), and wherein the calculated CO2 utilization rate in the plurality of molten carbonate fuel cell stages based on the current density is greater than the combined CO2 utilization rate, the calculated CO2 utilization rate optionally being 5.0% or more (or 10% or more, or 20% or more) greater than the combined CO2 utilization rate.

[0125] Alternative Embodiment 4. The method according to any one of the foregoing alternative embodiments, wherein the second anode input stream includes at least a portion of the intermediate anode output stream, the intermediate anode output optionally including the output from the first anode stage.

[0126] Alternative Example 5. The method according to Alternative Example 4, further comprising: directing a second anode intermediate output into a third anode stage, the anode exhaust including an output from the third anode stage; and directing a second cathode intermediate output into a third cathode stage, the cathode exhaust including an output from the third cathode stage, wherein the first anode stage, the second anode stage, and the third anode stage are arranged in parallel or countercurrent manner relative to the first cathode stage, the second cathode stage, and the third cathode stage.

[0127] Alternative Example 6. The method according to any one of the foregoing alternative examples, wherein the CO2 utilization rate measured in the first cathode stage is different from the CO2 utilization rate measured in the combination.

[0128] Alternative Embodiment 7. The method according to any one of the foregoing alternative embodiments, wherein at least the first anode stage, the second anode stage, the first cathode stage, and the second cathode stage operate in a parallel-flow cross-flow manner; or wherein at least the first anode stage, the second anode stage, the first cathode stage, and the second cathode stage operate in a counter-flow cross-flow manner.

[0129] Alternative Embodiment 8. The method according to any one of the foregoing alternative embodiments, wherein the anode exhaust includes an anode output from the second anode stage, or wherein the cathode exhaust includes a cathode output from the second cathode stage, or a combination thereof.

[0130] Alternative Embodiment 9. The method according to any one of the foregoing alternative embodiments, wherein the intermediate cathode output includes the output from the first cathode stage.

[0131] Alternative Embodiment 10. The method according to any one of the foregoing alternative embodiments, wherein power is generated at a current density of 120 mA / cm2 or higher (or 150 mA / cm2 or higher).

[0132] Alternative Embodiment 11. The method according to any one of the foregoing alternative embodiments, wherein the voltage drop across the cathode is 0.4V or less, or wherein power is generated at a voltage of 0.55V or greater, or a combination thereof.

[0133] Alternative Example 12. The method according to any one of the foregoing alternative examples, wherein the fuel utilization rate in the anode is 60% or higher, or wherein the fuel utilization rate in the anode is 55% or lower.

[0134] Alternative Example 13. The method according to any one of the foregoing alternative examples, wherein the H2 concentration in the anode exhaust is 5.0 vol% or higher, or wherein the combined concentration of H2 and CO in the anode exhaust is 6.0 vol% or higher, or a combination thereof.

[0135] Alternative Example 14. The method according to any one of the foregoing alternative examples, wherein the fuel cell operates at a heat ratio of about 0.25:about 1.0; or wherein the amount of reformable fuel introduced into the first anode stage, the internal reforming element associated with the first anode stage, or a combination thereof is at least about 75% greater than the amount of hydrogen reacted in the first anode stage to generate electricity; or a combination thereof.

[0136] All numerical values ​​in the specific embodiments and claims herein are modified by values ​​indicated by “about” or “approximately”, and take into account experimental errors and variations that can be expected by one of ordinary skill in the art.

[0137] Although the invention has been described with reference to specific embodiments, it is not necessarily limited thereto. Suitable changes / modifications to operation under particular conditions will be apparent to those skilled in the art. Therefore, the following claims are intended to be construed as covering all such changes / modifications falling within the true spirit / scope of the invention.

Claims

1. A method for generating electricity, the method comprising: An anode feed stream comprising H2, reformable fuel, or a combination thereof is introduced into the first anode stage of a plurality of molten carbonate fuel cell stages; A cathode feed stream including O2 and CO2 is introduced into the first cathode stage of the plurality of molten carbonate fuel cell stages; The second anode input feed stream is then introduced into the second anode stage of the plurality of molten carbonate fuel cell stages; The intermediate cathode output of the first cathode stage is fed into the second cathode stage of the plurality of molten carbonate fuel cell stages; as well as The plurality of molten carbonate fuel cell stages are operated to generate an average current density of 80 mA / cm². 2 Or even greater power, The CO2 utilization rate measured across the plurality of molten carbonate fuel cell stages is 70% or higher, and the calculated CO2 utilization rate of the plurality of molten carbonate fuel cell stages based on the average current density is 3.0% or more greater than the CO2 utilization rate measured across the plurality of molten carbonate fuel cell stages. The calculated CO2 utilization rate is defined as the CO2 utilization rate that occurs when all current densities generated by the plurality of molten carbonate fuel cell stages are based on the transport of carbonate ions across the electrolyte of the plurality of molten carbonate fuel cell stages.

2. The method of claim 1, wherein operating the plurality of molten carbonate fuel cell stages further comprises generating anode exhaust from the plurality of molten carbonate fuel cell stages, the anode exhaust comprising H2, CO and CO2.

3. The method of claim 2, wherein operating the plurality of molten carbonate fuel cell stages further comprises generating cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising CO2, O2 at a content of 1.0 vol% or higher, and H2O at a content of 1.0 vol% or higher.

4. The method of claim 3, wherein (a) the cathode input stream comprises 5.0 vol% or less CO2, (b) the cathode exhaust comprises 1.0 vol% or less CO2, or (c) both (a) and (b).

5. The method according to claim 2, wherein (a) the H2 concentration in the anode exhaust is 5.0 vol% or higher, (b) the combined concentration of H2 and CO in the anode exhaust is 6.0 vol% or higher, or (c) both (a) and (b).

6. The method according to any one of claims 1-5, wherein the calculated CO2 utilization rate in the plurality of molten carbonate fuel cell stages based on the average current density is 5.0% or more greater than the combined measured CO2 utilization rate.

7. The method according to any one of claims 1-5, wherein the second anode input stream comprises at least a portion of the intermediate anode output stream from the first anode stage.

8. The method of claim 7, wherein the first anode stage and the second anode stage are arranged in a countercurrent or parallel current manner relative to the first cathode stage and the second cathode stage.

9. The method according to any one of claims 1-5, wherein the measured CO2 utilization rate of the first cathode stage is different from the measured CO2 utilization rate of the combination.

10. The method according to any one of claims 1-5, wherein the average current density is 120 mA / cm². 2 Or larger.

11. The method according to any one of claims 1-5, wherein the combined voltage drop across the first cathode stage and the second cathode stage is 0.4 V or less, (b) the power is generated at a voltage of 0.55 V or greater, or (c) both (a) and (b).

12. The method according to any one of claims 1-5, wherein the plurality of molten carbonate fuel cell stages operate at a heat ratio of 0.25 to 1.

0.

13. A method for generating electricity, the method comprising: An anode feed stream comprising H2, reformable fuel, or a combination thereof is introduced into the first anode stage of a plurality of molten carbonate fuel cell stages; A cathode feed stream comprising O2 and CO2 at a content of 5.0 vol% or less is introduced into the first cathode stage of the plurality of molten carbonate fuel cell stages; The second anode input feed stream is then introduced into the second anode stage of the plurality of molten carbonate fuel cell stages; The intermediate cathode output from the first cathode stage is fed into the second cathode stage of the plurality of molten carbonate fuel cell stages; as well as The plurality of molten carbonate fuel cell stages are operated to generate an average current density of 80 mA / cm². 2 Or even greater power, The CO2 utilization rate measured across the plurality of molten carbonate fuel cell stages is 70% or higher, and the calculated CO2 utilization rate of the plurality of molten carbonate fuel cell stages based on the average current density is greater than the CO2 utilization rate measured across the plurality of molten carbonate fuel cell stages. The calculated CO2 utilization rate is defined as the CO2 utilization rate that occurs when all current densities generated by the plurality of molten carbonate fuel cell stages are based on the transport of carbonate ions across the electrolyte of the plurality of molten carbonate fuel cell stages.

14. The method of claim 13, wherein the calculated CO2 utilization rate is 3.0% or more greater than the CO2 utilization rate measured by the combination.

15. The method of claim 13, wherein the calculated CO2 utilization rate is 5.0% or more greater than the CO2 utilization rate measured by the combination.

16. The method according to any one of claims 13-15, wherein the second anode input stream comprises at least a portion of the intermediate anode output stream from the first anode stage.

17. The method according to any one of claims 13-15, wherein operating the plurality of molten carbonate fuel cell stages further comprises generating cathode exhaust from the plurality of molten carbonate fuel cell stages, the cathode exhaust comprising CO2 at a content of 1.0 vol% or less.

18. The method of claim 17, wherein operating the plurality of molten carbonate fuel cell stages further comprises generating an anode exhaust gas, wherein the anode exhaust gas has (a) an H2 concentration of 5.0 vol% or higher, (b) a combined H2 and CO concentration of 6.0 vol% or higher, or (c) both (a) and (b).

19. The method of claim 18, further comprising: This allows the intermediate output from the second anode stage to enter the third anode stage; And to allow the intermediate output of the second cathode from the second cathode stage to enter the third cathode stage, wherein the cathode exhaust includes the output from the third cathode stage, and the anode exhaust includes the output from the third anode stage.

20. The method of claim 13, wherein the plurality of molten carbonate fuel cell stages operate at a heat ratio of 0.25 to 1.0.

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