Ammonia flue gas treatment system

Abstract

To provide a treatment system for ammonia combustion exhaust gas capable of achieving both reduction of slip NH3 and suppression of by-production of N2O.SOLUTION: An ammonia combustion exhaust gas treatment system for treating a combustion exhaust gas of a marine engine using ammonia as a main fuel in which an SCR catalytic converter 3 for treating NOx in the combustion exhaust gas is disposed, a NH3 for treating a slip ASC5 is disposed downstream of the SCR catalytic converter 3, and a ratio of a volume of the SCR catalytic converter 3 in contact with the exhaust gas to a volume of the NH3 is adjusted so that a NH3 concentration after the slip NH3 treatment and a N2O concentration generated during the slip ASC5 treatment are equal to or less than a predetermined concentration.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a system for treating ammonia combustion exhaust gas, and more particularly to a system for treating ammonia combustion exhaust gas that can suppress the generation of N2O and reduce the volume of an ASC. [Background technology]

[0002] Patent Document 1 discloses that NH3 slip in exhaust gas can be suppressed by using an ammonia slip catalyst (ASC), and that NOx can be reduced by using a selective catalytic reduction (SCR) catalyst. Patent Document 1 further states as follows: It describes that the ASC is placed downstream of the SCR catalyst, and that in the SCR catalyst, NOx and NH3 react to remove NOx. It goes on to state that if NH3 is present after the SCR catalyst for some reason, it is oxidized by the ASC, which removes NH3, and that the ASC treats all gases, just like the SCR. Therefore, when an ASC is installed in a large two-stroke internal combustion engine, it must treat all exhaust gases, and so the size of the ASC will be similar to that of the SCR catalyst.

[0003] Therefore, Patent Document 1 points out that since the SCR catalyst is a device that is very voluminous, adding another ASC device that is extremely bulky is problematic.

[0004] Patent Document 1 also points out that another problem is that NO can be a by-product of NH oxidation in ASC. It discloses that a temperature exceeding 400°C is required to effectively remove NO, which is an exhaust gas temperature that cannot be easily achieved in highly efficient marine engines.

[0005] From this perspective, Patent Document 1 discloses a method for suppressing ammonia slip by highly adjusting the molar ratio of NH3 and NOx in exhaust gas treated by an SCR catalyst so that they undergo an equimolar reaction, thereby reducing the presence of excess or unreacted NH3 after the reaction. However, although the concept of this method seems to be excellent, in order to make it successful in actual equipment, there is an issue that the equipment configuration will be large in either case, for example, when NOx is loaded onto a ship or when it is produced at sea. Furthermore, there are various issues that need to be considered, such as improving the precision of various gas sensors and eliminating time lags in the control sequence. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2024-52583 [Patent Document 2] Japanese Patent Application Publication No. 2023-182938 Summary of the Invention [Problem to be solved by the invention]

[0007] Patent Document 2 focuses on the fact that a temperature of approximately 500°C is required for the activation of a decomposition catalyst for N2O, and confirms that by locating a first treatment section upstream of the exhaust receiver, the N2O decomposition rate can be improved to 60% to 73% at temperatures of 600°C to 800°C (see Table 1 in the Examples).

[0008] This technology is valuable in that it has enabled the treatment of N2O in the exhaust gas treatment of ammonia fuel.

[0009] The present inventors have investigated the reduction of slip ammonia (hereinafter also referred to as "slip NH3" as necessary) without performing the advanced control of the molar ratio of NH3 to NOx in exhaust gas as in Patent Document 1, and without setting high-temperature conditions for the decomposition of N2O. As a result, it was found that the N2O concentration in exhaust gas was not high enough to require treatment. However, since there is a trade-off between the reduction of slip ammonia and the by-production of N2O, the inventors investigated a method for reducing the ammonia emitted by reducing slip NH3, while suppressing the by-production of N2O and minimizing the amount of N2O emitted, and as a result, they arrived at the present invention.

[0010] Therefore, an object of the present invention is to provide a system for treating ammonia combustion exhaust gas that can simultaneously reduce slip NH3 and suppress the by-production of N2O.

[0011] Further objects of the present invention will become apparent from the following description. [Means for solving the problem]

[0012] The above problems are solved by the following inventions.

[0013] 1. This is an ammonia combustion exhaust gas treatment system that treats combustion exhaust gas from marine engines that use ammonia as their main fuel. An SCR catalyst for treating NOx in the exhaust gas of the combustion exhaust gas is disposed, An ASC that treats slip NH3 is disposed downstream of the SCR catalyst, A system for treating ammonia combustion exhaust gas, characterized in that the ratio of the volume of the SCR catalyst that comes into contact with the exhaust gas to the volume of the ASC is adjusted so that the NH3 concentration after slip NH3 treatment and the N2O concentration generated during the slip NH3 treatment are below predetermined concentrations. 2. 2. The ammonia combustion exhaust gas treatment system according to claim 1, wherein the ratio of the volume of the SCR catalyst to the volume of the ASC is such that, when the sum of the volume of the SCR catalyst and the volume of the ASC is 1, the volume of the ASC is 0.1 or more and less than 0.6. 3. 3. The system for treating ammonia combustion exhaust gas according to 1 or 2 above, wherein the space velocity SV (1 / hr) of the exhaust gas relative to the SCR catalyst and ASC is 5,000 or more. 4. 2. The ammonia combustion exhaust gas treatment system according to claim 1, wherein the ASC includes an SCR catalyst in a slip NH3 oxidation catalyst. 5. 2. The ammonia combustion exhaust gas treatment system according to claim 1, further comprising a configuration for adjusting the gas flow path when the gas composition of the exhaust gas changes so that a desired catalyst ratio is achieved for the gas composition. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a system for treating ammonia combustion exhaust gas that can simultaneously reduce slip NH3 and suppress the by-production of N2O. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1(A) is a block diagram showing an example of a system for treating combustion exhaust gas of ammonia, which treats combustion exhaust gas from a marine engine that uses ammonia as the main fuel. FIG. 1(B) is a block diagram showing another example. [Figure 2] Graph showing the experimental results DETAILED DESCRIPTION OF THE INVENTION

[0016] Preferred embodiments of the present invention will now be described. FIG. 1(A) is a block diagram showing an example of a system for treating ammonia combustion exhaust gas, which treats combustion exhaust gas from a marine engine that uses ammonia as its main fuel. 1, for example, a turbocharged large, low-speed, two-stroke diesel engine 1 (hereinafter simply referred to as "engine" or "engine") uses ammonia (NH3) as its main fuel. NH3 may be supplied to the engine in the form of liquid phase NH3 and / or gas phase NH3.

[0017] The engine 1 may have at least one ammonia mode, and may also have a conventional fuel mode in addition to the ammonia mode. In the ammonia mode, the engine 1 is operated using ammonia fuel, or in addition to an ammonia-based fuel, a pilot fuel to increase ignition and combustion temperatures. In the conventional fuel mode, the engine 1 is operated using conventional fuels, such as fuel oil (marine diesel fuel) or heavy fuel oil.

[0018] The engine in this embodiment is a two-stroke (2st) engine, and each cylinder of the engine may have a scavenging port provided in the lower region of the cylinder liner, and an exhaust valve provided in the center of the top of the cylinder liner, for example. In this embodiment, a four-stroke (4-stroke) engine can also be used, but a 2-stroke engine has a lower rotation speed and a longer combustion time than a 4-stroke engine, and because the air is compressed before the fuel is sprayed, ammonia slip and NO emission concentrations tend to be lower, so it is preferable to use a 2-stroke engine.

[0019] Exhaust gas from an engine 1 is sent to a selective catalytic reduction (SCR) catalyst 3 via a first exhaust pipe 2, then passes through a second exhaust pipe 4 and is sent to an ammonia slip catalyst (ASC) 5, and the treated exhaust gas is discharged outside the system through a third pipe 6. In this embodiment, as shown in FIG. 1(B), the second exhaust pipe 4 may be omitted and the SCR catalyst 3 and the ASC 5 may be housed in the same casing for processing, but the present invention is not limited to this.

[0020] In NH3 combustion, the exhaust gas from engine 1 may contain both NOx and NH3. Exhaust gas from the combustion of fossil fuels would not normally contain NH3. In contrast, in NH3 combustion, the exhaust gas may contain more NH3 than in the case of conventional fuels. In other words, the amount of slip NH3 may be greater.

[0021] The SCR catalyst 3 serves as a NOx removal catalyst that removes both NO and NO2, which are NOx components, from the exhaust gas. Moreover, the SCR catalyst 3 functions as an NOx removal catalyst and also as an NH3 removal catalyst. If NH3 is in excess, it must be taken into account that all NOx reacts with NH3 and the excess NH3 is discharged from the SCR catalyst as slip NH3. The amount of slipped NH3 allowed in the exhaust gas discharged to the outside air is low, and in the present invention, an upper limit of 25 ppm is considered to be the allowable value.

[0022] The SCR catalyst is not particularly limited, but it is preferable to use a catalyst that has a honeycomb structure in which an active component such as V, Cr, Mo, Mn, Fe, Ni, Cu, Ag, Au, Pd, Y, Ce, Nd, W, In, Ir, or Nb is supported on a carrier such as a binary composite oxide such as TiO2, SiO2-TiO2, WO3-TiO2, or Al2O3-SiO2, or a ternary composite oxide such as WO3-SiO2-TiO2, and that reduces NOx in the presence of NH3 (reducing agent) to convert it into nitrogen gas for purification. In the case of an ammonia slip catalyst (ASC) in the form of a catalyst in which an active component is supported on a honeycomb carrier, for example, metals such as Pt, Fe, Cu, or Pd can be used as the active component.

[0023] In this embodiment, the ASC may include a catalyst that functions as an SCR catalyst in addition to the NH3 oxidation catalyst that oxidizes and removes slipped NH3, which is the ASC's function, in order to ensure its role in suppressing secondary NOx formation. In this case, the ASC material preferably includes an SCR catalyst containing at least one of titania and vanadia, and one of Pt, Fe, and Cu, which are active materials with high ammonia oxidation ability.

[0024] When the ASC contains a precious metal, increasing the catalyst volume increases the environmental load, so it is preferable to make the volume of the ASC as small as possible. Therefore, by adjusting and optimizing the volume ratio of the catalyst in the present invention, the amount of precious metal used in the entire catalyst can be reduced.

[0025] In this embodiment, as shown in Fig. 1, the SCR catalyst 3 that treats NOx in the exhaust gas in the first exhaust pipe 2 is disposed upstream of the ASC 5. In other words, the ASC that treats slip NH3 is disposed downstream of the SCR catalyst.

[0026] In this embodiment, the ratio between the volume of the SCR catalyst that comes into contact with the exhaust gas and the volume of the ASC is adjusted so that the N2O concentration after suppressing N2O production becomes a predetermined concentration (for example, 25 ppm) or less. Here, the volumes of the SCR catalyst and ASC are estimated to be the volumes of a casing when the SCR catalyst and ASC are packed into a given casing. This is because even if the amount of catalyst itself varies depending on the catalyst shape and packing density, the volume of each catalyst is considered to be constant if they are packed into the same casing (space).

[0027] It has been confirmed through experiments that adjusting the volume ratio in this way contributes to both reducing the concentration of slip NH3 and suppressing the N2O generated during the treatment of slip NH3.

[0028] The experiment was conducted using the device shown in Figure 1, and the ratio of the volume of the SCR catalyst that comes into contact with the exhaust gas to the volume of the ASC was changed, assuming that the volume of the SCR catalyst 3 + the volume of the ASC 5 = the volume of the entire catalyst, with the volume of the entire catalyst being 1. In this experiment, the calculation formula for the volume ratio was ASC ratio=ASC / (SCR+ASC). The volume ratio was changed in the following five ways (1) to (5).

[0029] (1) Volume ratio at which the ASC rate becomes 0 When the volume of the entire catalyst is 1, assuming that the volume of the SCR catalyst 3 + the volume of the ASC 5 = the volume of the entire catalyst, the volume of the ASC is 0, and therefore the volume of the SCR is 1.0.

[0030] (2) Volume ratio for ASC ratio 0.4 The volume of the ASC is 0.4, so the volume of the SCR is 0.6.

[0031] (3) Volume ratio at which the ASC ratio becomes 0.5 Since the volume of the ASC is 0.5, the volume of the SCR is 0.5.

[0032] (4) Volume ratio for ASC ratio 0.6 The volume of the ASC is 0.6, so the volume of the SCR is 0.4.

[0033] (5) Volume ratio at which the ASC rate becomes 1.0 The volume of the ASC is 1.0, so the volume of the SCR is 0.

[0034] In a preferred embodiment of the present invention, the volume ratio of ASC5 is preferably 0.1 or more and less than 0.6 when the volume of the entire catalyst (volume of SCR catalyst 3 + volume of ASC5) is taken as 1. More preferably, the volume of ASC5 is 0.1 or more and 0.55 or less when the volume of the entire catalyst (volume of SCR catalyst 3 + volume of ASC5) is taken as 1. This makes it possible to reduce slip NH3 and suppress the generation of NO. Furthermore, the space velocity SV (1 / hr) of the exhaust gas relative to the SCR catalyst 3 and the ASC 5 is preferably 5,000 or more. This makes it possible to reduce slip NH3 while maintaining the space velocity, and also to suppress the generation of N2O that accompanies the reduction in slip NH3.

[0035] The experimental method was as follows: when the NH3 concentration in the exhaust gas was 1000 ppm and NO was 600 ppm, NH3 combustion exhaust gas was treated with a catalyst arrangement as shown in Figure 1, and the exhaust gas was allowed to flow at the space velocity shown in the table below, and the slip NH3 concentration at the outlet of ASC5 (SCR catalyst 3 in the table below when the ASC rate is 0) was determined, and the amount of N2O produced was also determined.

[0036] The ratio of the volume of the SCR catalyst to the volume of the ASC was varied. The results of the above experiments are shown in Table 1.

[0037] [Table 1]

[0038] Next, in Table 1, the maximum values ​​of the slip NH3 concentration and N2O production amount at catalyst temperatures of 250°C or higher relative to the space velocity were extracted, and the SV was determined so that the slip NH3 concentration and N2O production amount were both 25 ppm. The results are shown in Table 2.

[0039] [Table 2]

[0040] Furthermore, the curve in Figure 2 was graphed based on the data in Table 2. In Figure 2, one curve drawn with the ASC ratio ranging from 0 to 1 is the boundary line where NH3 can be reduced to 25 ppm or less. In other words, it is shown that if the space velocity SV is lower than this curve, NH3 can be reduced to 25 ppm or less. In addition, in Figure 2, the region surrounded by the parabola shaped like the other curve when the ASC ratio is in the range of 0 to 0.6 is the region where NO is 25 ppm or less. In other words, this shows that NO can be reduced to 25 ppm or less at the space velocity SV within this region.

[0041] The overlapping region of one curve and the other curve is where the volume of ASC5 is in the range of 0.1 or more and less than 0.6, where the volume of the entire catalyst (volume of SCR catalyst 3 + volume of ASC5) is 1. In this region, it is possible to achieve both a reduction in slip NH3 and suppression of the by-production of NO. Furthermore, in this embodiment, a space velocity SV (1 / hr) of 5000 or more is a preferred range.

[0042] In the present invention, it is preferable to provide a configuration for adjusting the gas flow passage so that, when the gas composition of the exhaust gas changes, a desirable catalyst ratio is achieved according to the gas composition.

[0043] Next, in the present invention, the relational expression for the amount of N2O produced after passing through the ASC can be expressed as follows:

[0044]

number

[0045] In Equation 2, [N2O] is the amount of N2O produced (ppm) after passing through the ASC, and α is a proportionality constant specific to the ASC, which is a parameter indicating the catalytic performance of the ASC. The amount of NH3 decomposed (ppm) when passing through the SC.

[0046] The relational expression for the amount of NH3 decomposition when passing through the ASC can be expressed by the following formula.

[0047]

number

[0048] In Equation 3, [NH3]in_asc is the NH3 concentration (ppm) before passing through the ASC, SV is the space velocity (1 / hour), EXP is an exponential function with Napier's number as the base, ka is a reaction rate constant specific to the ASC and is a parameter that indicates the catalytic performance of the ASC, and r is the volume ratio of the ASC when the sum of the SCR catalyst volume and the ASC volume is 1.

[0049] Next, the relational expression for the amount of NH3 decomposition when passing through the SCR catalyst can be expressed by the following formula.

[0050]

number

[0051] In Equation 4, Δ[NH3]_scr is the amount of NH3 decomposition when passing through the SCR, and [NH3]in_scr is the NH3 concentration (ppm) before passing through the SCR. ks is a reaction rate constant specific to the SCR catalyst, and depends on the NH3 concentration and NOx concentration before passing through the SCR. The catalyst-specific reaction rate constant ks for NH3 decomposition through the reaction of NH3 and NOx in the SCR catalyst is a parameter that indicates the performance specific to the SCR catalyst, and therefore differs for each catalyst. However, by determining the exhaust gas conditions ([NH3]in_scr, [NOx]in_scr, exhaust gas temperature), ks can be calculated according to those exhaust gas conditions.

[0052] From these equations, when an SCR catalyst is placed upstream and an ASC is placed downstream, the NH3 concentration and N2O concentration after slip NH3 treatment can be expressed as in the following equation 5.

[0053]

number

[0054] If the target value of the NH3 concentration after slip NH3 treatment is [NH3]_target and the target value of N2O is [N2O]_target, the volume ratio r of ASC that can achieve the target value can be expressed by the following equation 6.

[0055]

number

[0056] When the above equation 6 is rearranged with respect to r, the following equation is obtained.

[0057] [Number]

[0058] In Equation (7), LN is the natural logarithm function.

[0059] In this experiment, the data shown in Table 3 below were adopted, and the range of ASC was calculated based on Equation (7).

[0060] [Table 3]

[0061] When adopting the data shown in Table 3, in Equation (7), since ka >> ks in the derivation process, ks / (ka - ks) → 0 can be approximated, and (1 - EXP(-(ka·r) / SV)) → 1 can be approximated.

[0062] Also, although α, ka, and ks are values that depend on temperature, for example, it is preferable to adopt the values at the temperature with the largest N2O production amount within the assumed combustion exhaust gas temperature range. By adopting the temperature data with the largest production amount, higher safety can be ensured. Also, ks is a value that depends on [NH3]in_scr and the NOx concentration [NOx]in_scr before passing through SCR. For example, it is preferable to select the maximum NH3 concentration in the assumed combustion exhaust gas as [NH3]in_scr, and it is preferable to select the minimum NOx concentration in the assumed combustion exhaust gas as [NOx]in_scr.

[0063] For example, when adopting the data in Table 3 above, based on Equation (7), it can be calculated that the range of the volume ratio r of ASC is 0.19 < r < 0.52.

[0064] Next, the data shown in Table 4 below were adopted, and the range of ASC was calculated based on Equation (7).

[0065]

Table 4

[0066] When adopting the data shown in Table 4, in Equation (7), similar to the case of Table 3, since ka >> ks in the derivation process, it can be approximated that ks / (ka - ks) → 0, and (1 - EXP(-(ka·r) / SV)) → 1.

[0067] Also, similar to the case of Table 3, α, ka, and ks are values that depend on temperature. For example, it is preferable to adopt the values at the temperature with the largest N2O generation amount within the assumed combustion exhaust gas temperature range. By adopting the temperature data with the largest generation amount, higher safety can be ensured. Moreover, ks is a value that depends on [NH3]in_scr and the NOx concentration [NOx]in_scr before passing through the SCR. For example, it is preferable to select the maximum NH3 concentration in the assumed combustion exhaust gas as [NH3]in_scr, and it is preferable to select the minimum NOx concentration in the assumed combustion exhaust gas as [NOx]in_scr.

[0068] For example, when adopting the data in Table 4, based on Equation (7), it can be calculated that the range of the volume ratio r of the ASC is 0.04 < r < 0.27.

[0069] Based on the data in Table 3 and Table 4 above, it can be seen that when only the NOx concentration is different under the exhaust gas conditions, the reaction rate constant ks specific to the SCR catalyst that depends on the NOx concentration changes, and as a result, the range of the volume ratio r of the ASC also changes.

[0070] As a result of the experiment, the lower limit value of r can be calculated from the NH3 concentration of the combustion exhaust gas, the target value of the NH3 concentration after slip NH3 treatment, the catalyst performance of the ASC, the catalyst performance of the SCR catalyst, and the space velocity SV. Also, the upper limit value of r can be calculated from the NH3 concentration of the combustion exhaust gas, the target value of the N2O concentration after slip NH3 treatment, the catalyst performance of the ASC, the catalyst performance of the SCR catalyst, and the space velocity SV.

Explanation of Symbols

[0071] 1 Engine 2. First exhaust pipe 3 SCR catalyst 4 Second exhaust pipe 5 ASC 6. Third Pipe

Claims

1. This is an ammonia combustion exhaust gas treatment system that treats combustion exhaust gas from marine engines that use ammonia as their main fuel. an SCR catalyst for treating NOx in the exhaust gas of the combustion exhaust gas is disposed; A slip NH is provided downstream of the SCR catalyst. ３ An ASC is arranged to process Slip NH ３ NH after treatment ３ Concentration and the slip NH ３ N generated during processing ２ a ratio of the volume of the SCR catalyst in contact with the exhaust gas to the volume of the ASC is adjusted so that the O concentration is a predetermined concentration or less.

2. 2. The system for treating ammonia combustion exhaust gas according to claim 1, wherein a ratio of a volume of the SCR catalyst to a volume of the ASC is such that, when the sum of the volumes of the SCR catalyst and the ASC is taken as 1, the volume of the ASC is 0.1 or more and less than 0.

6.

3. 3. The system for treating ammonia combustion exhaust gas according to claim 1, wherein the space velocity SV (1 / hr) of the exhaust gas relative to the SCR catalyst and ASC is 5,000 or more.

4. The ASC is a slip NH ３ 2. The system for treating ammonia combustion exhaust gas according to claim 1, wherein the oxidation catalyst includes an SCR catalyst.

5. 2. The ammonia combustion exhaust gas treatment system according to claim 1, further comprising a configuration for adjusting the gas flow path when the gas composition of the exhaust gas changes so as to achieve a desired catalyst ratio for the gas composition.

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