JP7741540B2 - Ammonia engine exhaust gas treatment system and ammonia engine exhaust gas treatment method - Google Patents

Description

This disclosure relates to an exhaust gas treatment system for treating exhaust gas from an ammonia engine and an exhaust gas treatment method using the same.

Ammonia engines are known as marine diesel engines that use fuel oils such as heavy oil and light oil as their main fuel, and which co-combust carbon-free ammonia to reduce carbon dioxide, a greenhouse gas (see, for example, Patent Document 1).

Patent No. 6702475

When ammonia (NH ₃ ) is burned, NOx and N ₂ O are by-produced. NOx is a pollution-causing gas, and N ₂ O, while relatively harmless, has a global warming potential 310 times that of CO ₂ , so measures to deal with these nitrogen oxides are required. In addition, ammonia engine exhaust gas may contain ammonia that was not consumed in the combustion reaction, and measures to deal with this ammonia are also required. For ammonia engines to become more widespread, a new exhaust gas treatment system that can effectively treat the NOx, N ₂ O, and ammonia contained in the exhaust gas is needed.

The present disclosure aims to provide a new exhaust gas treatment system for an ammonia engine. It also aims to provide a new exhaust gas treatment method for an ammonia engine.

One aspect of the present disclosure relates to an exhaust gas treatment system for an ammonia engine. This exhaust gas treatment system for an ammonia engine includes an oxidation catalyst having a catalyst layer containing Pt and zeolite as a first catalyst, and a denitrification catalyst having a catalyst layer containing zeolite ion-exchanged with Cu, Co, or Fe ions as a second catalyst. Exhaust gas from the ammonia engine is treated by passing it sequentially through the first and second catalysts of this exhaust gas treatment system for the ammonia engine.

According to the above exhaust gas treatment system, exhaust gas from an ammonia engine can be suitably treated by burning ammonia in an upstream oxidation catalyst and reducing NOx and N ₂ O in a downstream denitration catalyst. This system utilizes the high temperature generated by the combustion of ammonia in the upstream oxidation catalyst to suitably treat nitrogen oxides in the downstream denitration catalyst.

One aspect of the present disclosure relates to a method for treating exhaust gas from an ammonia engine. This method for treating exhaust gas from an ammonia engine includes a first contacting step in which exhaust gas from the ammonia engine is brought into contact with a first catalyst of the above-described system, and a second contacting step in which product gas generated by the first contacting step is brought into contact with a second catalyst.

This disclosure provides a new exhaust gas treatment system for an ammonia engine. This disclosure also provides a new exhaust gas treatment method for an ammonia engine.

FIG. 1 is a diagram showing an outline of an exhaust gas treatment system for an ammonia engine according to one embodiment. FIG. 2 is a graph showing the results of the activity evaluation test in each example.

<Definition of terms>
The terms used in this specification have the following meanings unless otherwise specified.
Ammonia decomposition rate: This represents the ratio (%) of the ammonia concentration in the exhaust gas before and after contact with the catalyst.
_NOx production rate: This represents the ratio (%) of the concentration of _NOx produced in the exhaust gas after contact with the catalyst to the ammonia concentration in the exhaust gas before contact with the catalyst.
N ₂ O production rate: This represents the ratio (%) of N ₂ O produced in the exhaust gas after contact with the catalyst to the ammonia concentration in the exhaust gas before contact with the catalyst.
Nitrogen oxides: Refers to both _NOx and _N2O , and may be expressed as _NOx , etc.
_N2 selectivity: This is the value obtained by subtracting the production rate of _NOx and other substances in the exhaust gas after contact with the catalyst from the ammonia decomposition rate. In other words, it is the proportion of ammonia converted to _N2 before contact with the catalyst.

Embodiments of the present disclosure are described in detail below. Note that the present invention is not limited to the following embodiments.

<Ammonia engine exhaust gas treatment system>
Fig. 1 is a diagram showing an outline of an exhaust gas treatment system for an ammonia engine according to one embodiment. As shown in the figure, the exhaust gas treatment system 10 includes an oxidation catalyst 1 as a first catalyst and a denitration catalyst 2 as a second catalyst. Exhaust gas Gi is treated by the oxidation catalyst 1 and the denitration catalyst 2 in this order to become treated gas Gp. The exhaust gas treatment system can be said to be a composite catalyst for exhaust gas treatment that includes an oxidation catalyst and a denitration catalyst.

To ensure appropriate treatment capacity even when fluctuations in gas flow rate, gas concentration, or catalyst performance degradation occur, the exhaust gas treatment system may further include another oxidation catalyst having a catalyst layer containing Pt and zeolite as a third catalyst. Furthermore, the exhaust gas treatment system may further include another denitration catalyst having a catalyst layer containing zeolite ion-exchanged with Cu, Co, or Fe ions as a fourth catalyst. Furthermore, the exhaust gas treatment system may further include another oxidation catalyst having a catalyst layer containing Pt and zeolite as a fifth catalyst. In this way, the exhaust gas treatment system can alternately include oxidation catalysts and denitration catalysts. The oxidation catalysts or denitration catalysts may be the same or different.

[Oxidation catalyst]
The oxidation catalyst includes a catalyst layer containing Pt and zeolite. Specifically, the oxidation catalyst includes a support and an oxidation catalyst layer provided on the surface of the support. The oxidation catalyst can be referred to as an ammonia decomposition catalyst.

(Support)
The support S preferably has a shape that generates a small differential pressure during gas flow and has a large contact area with the gas. Specific examples of the support S include a honeycomb, a sheet, a mesh, a fiber, a pipe, a filter, and a sphere. The support material may be, for example, a known catalyst carrier such as cordierite or alumina, a carbon fiber, a metal fiber, a glass fiber, a ceramic fiber, or a metal such as titanium, aluminum, or stainless steel.

(Oxidation catalyst layer)
The oxidation catalyst layer can include a lower layer (first layer) and an upper layer (second layer) provided on the surface of the lower layer. The lower layer contains Pt, an inorganic oxide, and optionally phosphorus, and a first proton-type zeolite or a first ion-exchanged zeolite ion-exchanged with Cu, Co, or Fe ions. The lower layer is provided on the surface of the support S. The thickness of the lower layer is preferably 10 to 200 μm, more preferably 30 to 100 μm. In the lower layer, an oxidation reaction represented by the following formula (1) mainly proceeds, and a denitration reaction represented by the following formula (2) also proceeds.
xNH ₃ +O ₂ →yNO+zN ₂ O+2H ₂ O...(1)
2NO+ _2NH3 +1/ _2O2 → _2N2 + _3H2O ...(2)

The lower layer contains Pt. Pt has the effect of improving cracking activity and _N2 selectivity. By containing Pt, the generation of NOx and _N2O can be suppressed. The lower layer may further contain a noble metal other than Pt (Pd, Ir, Rh, a composite thereof, etc.).

The Pt content is preferably 0.3 to 10 mass%, more preferably 0.5 to 8 mass%, based on the total amount of Pt, inorganic oxide, optional phosphorus, and zeolite contained in the lower layer. The amount of Pt supported is preferably 0.03 to 1 g/L, more preferably 0.1 to 0.9 g/L, and even more preferably 0.15 to 0.8 g/L, relative to the catalyst volume. When the Pt amount is within the above ranges, better results can be obtained in terms of ammonia decomposition rate, _NOx production rate, and _N2O production rate.

The underlayer may contain an inorganic oxide.

Examples of the first inorganic oxide include titania (TiO ₂ ), zirconia (ZrO 2 ), silica (SiO ₂ ), alumina, and ceria-zirconia composite oxides and solid solutions (represented by CeO ₂ ·ZrO ₂ , with a CeO ₂ :ZrO ₂ molar ratio of 1: ₃ to 3:1), which improve the activity of Pt and also function as a carrier for Pt. One of these may be used alone, or two or more may be used in combination. The first inorganic oxide contributes to improving the activity of Pt, i.e., its cracking activity, particularly the sustainability of the cracking activity during long-term use. Among the above inorganic oxides, TiO ₂ , ZrO ₂ , and ceria-zirconia (composite oxide or solid solution) are particularly effective in sustaining cracking activity during long-term use.

The content of the first inorganic oxide is preferably 5 to 50 mass%, more preferably 10 to 35 mass%, based on the total amount of Pt, the first inorganic oxide, optionally phosphorus, and zeolite contained in the lower layer. The amount of the first inorganic oxide supported is preferably 1 to 50 g/L, more preferably 5 to 20 g/L, relative to the catalyst volume. When the amount of the first inorganic oxide is within the above ranges, better results can be obtained in terms of ammonia decomposition rate, _NOx production rate, and _N2O production rate.

The first inorganic oxide can be contained in the catalyst in a state where Pt is supported on it. For example, a catalyst composition containing Pt and the _first inorganic oxide can be prepared by preparing particles in which 0.1 to 10 mass % (based on the mass of _TiO2 ) of Pt is supported on TiO2 particles (referred to as Pt/ _TiO2 ) and mixing the particles with other components.

The first inorganic oxide is preferably particulate in order to more effectively utilize the function of Pt in the catalyst composition. The average particle size of the first inorganic oxide is, for example, 0.1 to 100 μm. Here, particle size refers to the size of secondary particles, and is the length of the major axis when observed with an SEM. The average particle size is the average value when the major axis of at least 10 particles is measured using an SEM.

When _TiO2 particles are used as the first inorganic oxide, the BET specific surface area of the _TiO2 particles is preferably 50 ^m2 /g or more, more preferably 200 ^m2 /g or more. When _ZrO2 particles are used as the first inorganic oxide, it is preferable to use porous _ZrO2 particles with a specific surface area of 10 ^m2 /g or more. _ZrO2 may be monoclinic, _tetragonal , or cubic. Composite _ZrO2 (e.g. _{, ZrO2.nCeO2} , _ZrO2.nSiO2 , _ZrO2.nTiO2 , where _n is generally 0.25 to 0.75) may also be used. When _SiO2 is used as the first inorganic oxide, it is preferable to use a high-silica zeolite having a zeolite structure (e.g., mordenite).

The second inorganic oxide may be copper oxide, which has the function of maintaining high decomposition activity and _N2 selectivity. Copper oxide refers to an oxide containing copper, including copper-containing complex oxides. Copper oxides include copper oxides represented by the general formula CuOx (0.45≦x≦1.1). These are typically CuO and _Cu2O , and include copper oxides present in copper-containing complex oxides such as hopcalite.

The copper oxide content is preferably 1 to 30 parts by mass, more preferably 5 to 10 parts by mass, based on 100 parts by mass of the total weight of the Pt, first inorganic oxide, optional phosphorus, and zeolite contained in the lower layer. If the copper oxide content is less than 1 part by mass, the generation of _NOx and other substances may increase, resulting in a decrease in _N2 selectivity. On the other hand, if the copper oxide content exceeds 30 parts by mass, the proportion of zeolite may decrease relatively, resulting in a decrease in decomposition rate. Furthermore, the amount of copper oxide supported is preferably 0.5 to 20 g/L, more preferably 5 to 10 g/L, relative to the catalyst volume. Within the above ranges, better results can be obtained with respect to _NOx generation rate, _N2 selectivity, and ammonia decomposition rate.

The copper oxide is uniformly mixed in the catalyst with the zeolite and the first inorganic oxide. Because the catalytic action is exerted in the presence of particles of other components, the average particle size is preferably 0.1 μm to 100 μm to ensure uniform dispersion with the other components.

Examples of the third inorganic oxide include colloidal silica, silica sol, alumina sol, silicic acid sol, titania sol, boehmite, clay, kaolin, and sepiolite, which function as inorganic binders.

When phosphorus is contained in the lower layer, examples of usable phosphorus-containing compounds include water-soluble phosphoric acids such as phosphoric acid (H ₃ PO ₄ ), metaphosphoric acid, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ), and diammonium phosphate ((NH ₄ ) ₂ HPO ₄ ), as well as inorganic salts thereof such as Na salts, K salts, and ammonium salts, or organic acid esters thereof.

The phosphorus content is preferably 0.1 to 10 mass% and more preferably 1 to 5 mass% based on the total weight of the Pt, first inorganic oxide, phosphorus, and zeolite contained in the lower layer. The amount of phosphorus supported is preferably 0.1 to 10 g/L and more preferably 0.5 to 5 g/L based on the catalyst volume.

If the phosphorus content and loading amount are lower than the above ranges, the durability improvement effect may be insufficient, and if the phosphorus content and loading amount are higher than the above ranges, the initial activity may decrease.

Zeolites may be natural or synthetic. Examples of proton-type zeolites include mordenite, erionite, ferrierite, chabazite, X-type zeolite, β-type zeolite, MFI-type zeolite, Y-type zeolite, and SAPO. Proton-type zeolites may be β-type zeolites, since the denitrification reaction proceeds at solid acid sites. These zeolites may be proton-type (H-type) or ion-exchanged zeolites ion-exchanged with Cu, Co, or Fe. Proton-type zeolites may have a CHA-type structure from the perspective of hydrothermal resistance. The CHA-type structure has a three-dimensional pore structure. Zeolites with a CHA-type structure may be proton-type (H-type) or ion-exchanged zeolites ion-exchanged with Cu, Co, or Fe. The ion-exchange amount is preferably 1.0 to 6.0% by mass, more preferably 1.5 to 5.0% by mass, of the zeolite mass. These zeolites may be used alone or in combination of two or more.

The zeolite is preferably in particulate form. The average particle size of the zeolite is, for example, 0.1 to 5.0 μm. Here, particle size refers to the size of the zeolite crystals, and is the length of the longest diameter when observed with an SEM. The average particle size is the average value when the longest diameter of at least 10 particles is measured using an SEM.

From the viewpoint of _NOx suppression performance and durability, Cu ion-exchanged zeolite is preferred as the zeolite. Furthermore, as the zeolite having a CHA structure, it is preferable to use Cu ion-exchanged CHA-type zeolite, and it is more preferable to use Cu ion-exchanged SSZ zeolite (SSZ-13) or Cu ion-exchanged SAPO-34 zeolite. The silica-alumina ratio (SAR, Si/Al) of the Cu ion-exchanged SSZ zeolite is preferably 10 to 50, more preferably 10 to 20. If the SAR is less than 10, the NOx reduction ability is enhanced but resistance under hydrothermal conditions tends to decrease. On the other hand, if it exceeds 50, resistance under hydrothermal conditions is enhanced but the NOx reduction ability tends to decrease. The amount of aluminum phosphate relative to silica in the Cu ion-exchanged SAPO zeolite (Al+P)/Si is preferably 1 to 30, more preferably 5 to 30. If the (Al+P)/Si ratio is less than 1, the durability under hydrothermal conditions tends to decrease, while if it exceeds 30, the NOx reduction performance tends to decrease.

The _NOx reduction performance (denitrification performance) of Cu ion-exchanged zeolite with ammonia tends to be better when the Cu dispersibility is higher, and also tends to be better when the zeolite has a higher solid acid content. Generally, the acid content of a zeolite increases as the SAR decreases, but zeolites with low SAR tend to be more hydrophilic and more susceptible to dealumination under high temperature and high moisture conditions, resulting in a decrease in acid content over time and a decrease in denitrification performance. Similarly, it can be inferred that SAPO-based zeolites also show a similar tendency, with a higher acid content when the (Al+P)/Si ratio is low. The Cu ion-exchanged SSZ zeolite (SSZ-13) or Cu ion-exchanged SAPO zeolite (SAPO-34) in this embodiment has high structural hydrothermal stability and can be catalyzed with a composition with a higher acid content than β zeolite (SAR: 35). Therefore, it is inferred that both _NOx suppression performance and durability can be achieved.

The content of zeolite in the lower layer is preferably 40 to 95 mass%, more preferably 50 to 90 mass%, based on the total of Pt, the first inorganic oxide, optionally phosphorus, and zeolite contained in the lower layer. The amount of zeolite supported in the lower layer is preferably 5 to 95 g/L, more preferably 10 to 90 g/L, relative to the catalyst volume. When the amount of zeolite in the lower layer is within the above ranges, better results can be obtained in terms of ammonia decomposition rate, _NOx production rate, and _N2O production rate.

When the lower layer has excellent hydrothermal resistance due to the CHA-type structure zeolite, the lower layer does not need to contain phosphorus. Even if the lower layer contains phosphorus, the content (based on the total of Pt, the first inorganic oxide, phosphorus, and zeolite contained in the lower layer) may be less than 0.05% by mass, or even less than 0.01% by mass.

The upper layer contains a second proton-type zeolite or a second ion-exchanged zeolite that has been ion-exchanged with Cu, Co, or Fe ions. The upper layer is provided on the surface of the lower layer. The thickness of the upper layer is preferably 10 to 200 μm, more preferably 30 to 100 μm. In the upper layer, a denitrification reaction represented by the following formula (2) mainly proceeds.
Denitrification reaction: 2NO + 2NH ₃ + 1/2O ₂ → 2N ₂ + 3H ₂ O... (2)

The zeolite contained in the upper layer can be any of the zeolites exemplified for the lower layer. The zeolite contained in the upper layer can be the same as or different from the zeolite contained in the lower layer.

The amount of zeolite supported in the upper layer is preferably 20 to 150 g/L, more preferably 30 to 130 g/L, relative to the catalyst volume. When the amount of zeolite in the upper layer is within the above range, better results can be obtained in terms of ammonia decomposition rate, _NOX production rate, and N ₂ O production rate.

The amount of zeolite contained in the upper layer (total amount of proton-type zeolite and ion-exchange type zeolite) is preferably 20 to 400 parts by mass, and more preferably 50 to 150 parts by mass, relative to 100 parts by mass of the total of Pt, the first inorganic oxide, optionally phosphorus, and zeolite contained in the lower layer. When this amount is 20 parts by mass or more, the _NOx production rate tends to be reduced by the denitrification reaction in the upper layer, while when it is 400 parts by mass or less, ammonia tends to efficiently come into contact with the lower layer and be decomposed.

The upper layer may contain an inorganic binder, if necessary. Examples of inorganic binders include colloidal silica, silica sol, alumina sol, silicic acid sol, titania sol, boehmite, clay, kaolin, and sepiolite.

An example of a method for producing an oxidation catalyst is shown below. However, the method for producing an oxidation catalyst is not limited to the following method.

First, a Pt-containing aqueous solution is placed in a container, and the first inorganic oxide is added to it. After the first inorganic oxide is thoroughly impregnated with the Pt-containing aqueous solution, it is heated while stirring to evaporate the water and dry. It is then further heated in a dryer, and the resulting powder is calcined in air to obtain first inorganic oxide particles carrying a predetermined amount of Pt (as metal).

This powder is mixed with deionized water, and then a predetermined amount of inorganic binder and the above-mentioned zeolite are mixed in to prepare a slurry for the lower layer. This slurry is applied to the support S, and the excess is blown off with an air blower. The slurry is then heated and dried, and then fired in a high-temperature furnace under air circulation, forming a lower layer on the surface of the support S.

Deionized water is mixed with a specified amount of inorganic binder and the above-mentioned zeolite to prepare a slurry for the upper layer. This slurry is applied to the surface of the lower layer, and the excess is blown off with an air blower. It is then dried and fired as described above. This results in an oxidation catalyst consisting of two layers: a lower layer and an upper layer.

To incorporate phosphorus into the lower layer, first, a solution of a phosphorus-containing compound is mixed with deionized water to prepare a phosphorus solution. This solution is then applied to a previously prepared layer containing Pt, a first inorganic oxide, and a first proton-type zeolite or a first ion-exchanged zeolite, and excess liquid is blown off with an air blower. The resulting layer is then dried and fired.

To incorporate copper oxide into the lower layer, it is preferable to use solid particles of copper oxide as the starting material. Another method is to mix an aqueous solution containing copper salts, such as copper sulfate or copper acetate, with other catalyst components, impregnate the lower layer, and then calcinate at 300-600°C in an air atmosphere to convert the copper salts into copper oxide.

In the above embodiment, the oxidation catalyst has a lower layer and an upper layer, but, for example, a sub-catalyst with the same configuration as the lower layer and a sub-catalyst with the same configuration as the upper layer may be prepared, and these sub-catalysts may be combined and used as the oxidation catalyst.

Commercially available oxidation catalysts may be used, such as IKOMAC-30, IKOMAC-Z3, and NHN-824E (manufactured by Nikki-Universal Co., Ltd.).

[Denitration catalyst]
The denitration catalyst comprises a catalyst layer containing zeolite ion-exchanged with Cu, Co, or Fe ions. Specifically, the denitration catalyst comprises a support and a denitration catalyst layer provided on the surface of the support.

(Support)
Examples of the support include the supports exemplified for the oxidation catalyst.

(Denitrification catalyst layer)
In the denitration catalyst layer, denitration reactions represented by the following formulas (2) to (4) mainly proceed.
2NO+ _2NH3 +1/ _2O2 → _2N2 + _3H2O ...(2)
3N ₂ O + 2NH ₃ → 4N ₂ + 3H ₂ O...(3)
2N ₂ O→2N ₂ +O ₂ ...(4)

From the viewpoint of the heat resistance and activity of the catalyst, the zeolite may be a high-silica zeolite having a silica-alumina ratio of 3 or more, and specifically, mordenite, chabazite, faujasite, β-type zeolite, MFI-type zeolite, Y-type zeolite, etc. are preferred. One of these zeolites may be used alone, or two or more may be used in combination. From the viewpoint of improving the _{N 2} O reduction performance, β-type zeolite may be used as the zeolite. From the same viewpoint, ion-exchanged zeolite ion-exchanged with Fe may be used as the zeolite. The amount of ion-exchanged Fe is preferably 1.0 to 3.0 mass% of the mass of the zeolite, and more preferably 1.5 to 2.5 mass%.

The zeolite is preferably in particulate form. The average particle size of the zeolite is, for example, 0.1 to 10 μm.

The amount of zeolite supported in the denitration catalyst layer is preferably 10 to 500 g/L, and more preferably 50 to 250 g/L, relative to the catalyst volume. When the amount of zeolite in the denitration catalyst layer is within this range, better results can be obtained with regard to the selective reduction of NOx by ammonia.

The denitration catalyst layer may contain an inorganic binder as needed. Examples of inorganic binders include colloidal silica, silica sol, alumina sol, silicic acid sol, titania sol, boehmite, clay, kaolin, and sepiolite.

An example of a method for producing a denitration catalyst is shown below. However, the method for producing a denitration catalyst is not limited to the method below.

First, zeolite is immersed in an aqueous solution containing a salt of the desired metal ion, and the alkali metal or alkaline earth metal ions in the zeolite are substituted with Cu, Co, or Fe ions. For example, for Fe ions, an aqueous solution of ferric chloride (FeCl ₃ ) or an aqueous solution of ferric nitrate (Fe(NO ₃ ) ₃ ) can be used; for Co ions, an aqueous solution of cobalt chloride (CoCl ₂ ) or cobalt nitrate (Co(NO ₃ ) ₂ ) can be used; and for Cu ions, an aqueous solution of copper nitrate (Cu(NO ₃ ) _{2 )} or copper chloride (CuCl ₂ ) can be used. The zeolite is then removed from the aqueous solution and calcined at about 400 to 900°C to obtain a denitration catalyst powder.

The denitration catalyst powder is mixed with deionized water, and then a predetermined amount of inorganic binder is added to prepare a denitration catalyst slurry. This slurry is applied to a support, and the excess is blown off with an air blower. The support is then heated and dried, and then fired in a high-temperature furnace under air circulation, forming a denitration catalyst layer on the surface of the support.

A commercially available denitration catalyst containing Feβ zeolite may be used as the denitration catalyst, such as NCDF-200 (manufactured by Nikki-Universal Co., Ltd.).

<Ammonia engine exhaust gas treatment method>
The method for treating exhaust gas from an ammonia engine includes a first contacting step in which exhaust gas from the ammonia engine is brought into contact with a first catalyst (oxidation catalyst) of the above-mentioned system, and a second contacting step in which product gas generated in the first contacting step is brought into contact with a second catalyst (denitrification catalyst). Ammonia engines are used as diesel engines, marine engines, gas turbines, etc.

The exhaust gas of an ammonia engine is a gas having the following characteristics, for example but not limited to:
・Gas temperature: Approximately 250 to 500°C.
・_NH3 concentration: 0.1 to 3% by volume.
・NO concentration: 50 to 1500 ppm by volume.
-N ₂ O concentration: 100 to 1000 ppm by volume.
- H ₂ O concentration: approximately 5 to 20% by volume.

The exhaust gas comes into contact with the oxidation catalyst of the exhaust gas treatment system, and then comes into contact with the denitration catalyst. The oxidation catalyst in the first stage is preferably a catalyst that generates heat through the oxidative decomposition of ammonia and is expected to experience a temperature rise of 50 to 300°C relative to the temperature of the exhaust gas. Furthermore, from the viewpoint of efficiently suppressing NOx and N ₂ O, the oxidation catalyst is preferably a catalyst that generates little of these gases. The temperature of the oxidation catalyst is preferably 260 to 500°C, more preferably 280 to 400°C, from the viewpoint of sending sufficiently high-temperature gas to the denitration catalyst and suppressing _catalyst deterioration. The temperature of the denitration catalyst in the second stage is preferably 350 to 700°C, more preferably 400 to 650°C, and even more preferably 450 to 650°C, from the viewpoint of fully exhibiting the reduction function of NOx and N 2 O. The SV (space velocity) of the oxidation catalyst is preferably 3,000 to 200,000 h ⁻¹ , more preferably 5,000 to 150,000 h −1 , and even more preferably 30,000 to 110,000 h ⁻¹ , from the viewpoint of obtaining an increase in exhaust gas temperature due to the decomposition of ammonia and easily retaining the ammonia concentration required as a reducing agent in the denitration catalyst. The SV of the denitration catalyst is preferably 1,000 to 50,000 h ⁻¹ , more preferably 5,000 to 25,000 h ⁻¹ , and even more preferably 5,000 to 20,000 h ⁻¹ , from the viewpoint of fully exhibiting the reduction function of NOx and N ₂ ^O.

The exhaust gas treatment method may further include a step of adding ammonia gas to the exhaust gas before the first contact step. Because the ammonia concentration in the exhaust gas can vary depending on the engine's combustion conditions, a sufficient amount of ammonia may not be supplied to the upstream oxidation catalyst. This prevents the temperature of the oxidation catalyst from rising sufficiently, preventing sufficiently hot gas from being sent to the denitration catalyst. Adding ammonia gas prior to exhaust gas treatment can prevent this from occurring.

The exhaust gas treatment method may further include a step of adding ammonia gas to the product gas produced by the first contact step after the first contact step. Ammonia functions as a reducing agent in the denitration catalyst, so adding ammonia gas as needed can optimally maintain the reduction reaction in the denitration catalyst.

The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples.

<Preparation of oxidation catalyst A-1>
As the oxidation catalyst A-1, a commercially available ammonia decomposition catalyst (IKOMAC-30, manufactured by Nikki-Universal Co., Ltd.) was used.
The ammonia decomposition catalyst is a honeycomb catalyst in which catalytic components are supported on a cordierite substrate in a two-layer structure consisting of a lower layer (thickness 100 to 200 μm) and an upper layer (thickness 80 to 150 μm). The lower layer contains Pt-supported TiO ₂ , Cu ion-exchanged β-zeolite (Cuβ), SiO ₂ (binder), and P, while the upper layer contains Cu ion-exchanged β-zeolite (Cuβ) and SiO ₂ (binder). The amounts of catalytic components supported per volume in each layer are as follows: lower layer: Pt 0.5 g/L, TiO ₂ 10 g/L, Cuβ 70 g/L, P 1 g/L, SiO ₂ 25 g/L; upper layer: Cuβ 64 g/L, SiO ₂ 16 g/L.
The Cu ion-exchanged β-zeolite used was a proton-type β-zeolite (HSZ-941HOA manufactured by Tosoh Corporation, _SiO2 / _Al2O3 ratio ₄₀ , average particle size 4.0 μm) with Cu supported thereon by an ion exchange method. The Cu ion-exchanged β-zeolite had a specific surface area of 560 ^m2 /g as measured by the ^N2 -BET method, and a Cu content (ion exchange amount) of 4.5 mass%.
As the TiO ₂ powder, ST-90D manufactured by Ishihara Sangyo Kaisha Ltd., with an average particle size of less than 1 μm and a specific surface area of 200 m ² /g or more, is used.
As _SiO2 , colloidal silica (Snowtex-CM manufactured by Nissan Chemical Industries, Ltd.) is used.

<Preparation of oxidation catalyst A-2>
As the oxidation catalyst A-2, a commercially available ammonia decomposition catalyst (IKOMAC-30Z3, manufactured by Nikki-Universal Co., Ltd.) was used.
The ammonia decomposition catalyst is a honeycomb catalyst in which catalytic components of a two-layer structure consisting of a lower layer (50-100 μm thick) and an upper layer (80-150 μm thick) are supported on a cordierite substrate. The lower layer contains Pt-supported TiO ₂ , Cu ion-exchanged chabazite-type zeolite (CuCHA), and SiO ₂ (binder), while the upper layer contains Cu ion-exchanged chabazite-type zeolite (CuCHA) and SiO ₂ (binder). The amounts of catalytic components supported per volume in each layer are as follows: lower layer: Pt 0.25 g/L, TiO ₂ 5 g/L, CuCHA 35 g/L, SiO ₂ 13 g/L; upper layer: CuCHA 64 g/L, SiO ₂ 16 g/L.
The Cu ion-exchanged chabazite-type zeolite used was CP-7208 manufactured by Zeolyst Co., Ltd., which has a SiO ₂ /Al ₂ O ₃ ratio of 14, an average particle size of 3.0 μm, a specific surface area of 480 m ² /g measured by the N ₂ -BET method, and a Cu content (ion exchange amount) of 4.0 mass%.
The _TiO2 powder and _SiO2 are the same as those in A-1.

<Preparation of Denitration Catalyst B-1>
As the denitration catalyst B-1, a commercially available denitration catalyst (NCDF-200, manufactured by Nikki-Universal Co., Ltd.) was used.
The denitration catalyst is a honeycomb catalyst in which Fe ion-exchanged β-zeolite (Feβ) is supported on a corrugated ceramic substrate. The denitration catalyst layer contains Feβ and alumina (binder). The supported amounts per volume of the catalyst components are 170 g/L of Feβ and 30 g/L of alumina.
The Fe ion-exchanged β-zeolite used was a proton-type β-zeolite (HSZ-931HOA manufactured by Tosoh Corporation, _SiO2 / _Al2O3 ratio ₂₈ , average particle size 4.0 μm) with Fe supported by an ion exchange method. The Fe ion-exchanged β-zeolite had a specific surface area of 510 ^m2 /g as measured by the _N2 -BET method, and an Fe content (ion exchange amount) of 2.1 mass%.
As the alumina, alumina sol (AS-520A manufactured by Nissan Chemical Co., Ltd.) is used.

<Preparation of Denitration Catalyst C-1>
As the denitration catalyst C-1, a commercially available honeycomb catalyst (NRU-5, manufactured by JGC Catalysts and Chemicals Co., Ltd.) containing V ₂ O ₅ , WO ₃ and TiO ₂ and having 45 cells/inch was prepared.

<Production of exhaust gas treatment system>
Cylindrical honeycomb catalysts (21 mm in diameter) were cut out to a predetermined length from each of the honeycomb catalysts prepared as described above. The cut-out honeycomb catalysts were packed into a flow reactor, in this order from the gas inlet side, with an oxidation catalyst as the first catalyst and a denitrification catalyst as the second catalyst, to prepare an exhaust gas treatment system.

<Activity evaluation test>
A gas simulating exhaust gas from an ammonia engine was passed through an exhaust gas treatment system. The flow rates of _NH3 gas, NOx gas, and _N2O gas were controlled using a mass flow controller, and the moisture content was controlled using a roller pump. The moisture concentration was set to 10% by volume. The catalyst was heated in an electric furnace to set the catalyst inlet temperature (inlet temperature) to a predetermined temperature, and the decomposition activity of each gas was evaluated. Table 1 shows the evaluation conditions. Figure 2 is a graph showing the results of the activity evaluation test for each example. The inlet gas _NH3 concentration is the concentration before contact with the oxidation catalyst, and the outlet gas concentration is the concentration after contact with the denitrification catalyst.

(Gas analysis method)
Ammonia: Gas chromatography (TCD detector) or gas detector tube NOx: Chemiluminescence analyzer N ₂ O: Gas chromatography (TCD detector)

(calculation)
・_NH3 decomposition rate (%): 100-{(outlet _NH3 concentration)/(inlet _NH3 concentration) x 100}
・NO decomposition rate (%): 100-{(outlet NOx concentration)/(inlet NOx concentration) x 100}
・N ₂ O decomposition rate (%): 100 - {(Outlet N ₂ O concentration) / (Inlet N ₂ O concentration) x 100}
・NO generation rate (%): (outlet NOx concentration) / (inlet _NH3 concentration) x 100
・N ₂ O production rate (%): {(Outlet N ₂ O concentration) / (Inlet NH ₃ concentration)} x 100

As shown in Table 1, an exhaust gas treatment system combining oxidation catalyst A-1 or A-2 and denitrification catalyst B-1 is believed to be able to effectively treat exhaust gas from ammonia engines.

1...oxidation catalyst, 2...denitrification catalyst, 10...exhaust gas treatment system

Claims (11)

an oxidation catalyst having a catalyst layer containing Pt and zeolite as a first catalyst;
a denitration catalyst having a catalyst layer containing zeolite ion-exchanged with Cu, Co, or Fe ions as a second catalyst ;
An exhaust gas treatment system for an ammonia engine, in which exhaust gas from an ammonia engine is treated by passing the exhaust gas through the first catalyst and the second catalyst in sequence . a catalytic layer of the oxidation catalyst comprising a first layer containing Pt, an inorganic oxide, and a first proton-type zeolite or a first ion-exchanged zeolite ion-exchanged with Cu, Co, or Fe ions;
a second layer provided on the surface of the first layer and containing a second proton-type zeolite or a second ion-exchanged zeolite ion-exchanged with Cu, Co, or Fe ions;
The system of claim 1 , comprising: The system described in claim 1 or 2, wherein the catalytic layer of the denitrification catalyst comprises a layer containing β-type zeolite ion-exchanged with Fe ions. The system described in any one of claims 1 to 3 further comprises another oxidation catalyst having a catalyst layer containing Pt and zeolite. The system described in claim 4 further comprises another denitration catalyst having a catalyst layer containing zeolite ion-exchanged with Cu, Co, or Fe ions. A method for treating exhaust gas from an ammonia engine, comprising: a first contacting step in which exhaust gas from the ammonia engine is brought into contact with the first catalyst of the system described in any one of claims 1 to 5; and a second contacting step in which product gas produced by the first contacting step is brought into contact with the second catalyst. The method according to claim 6, wherein the _NH3 concentration in the exhaust gas of the ammonia engine is 0.1 to 3% by volume. The method of claim 6 or 7, further comprising a step of adding ammonia gas to the exhaust gas before the first contact step. The method according to any one of claims 6 to 8, further comprising a step of adding ammonia gas to the product gas after the first contact step. The method according to any one of claims 6 to 9, wherein the temperature of the first catalyst is 260 to 500°C and the temperature of the second catalyst is 350 to 700°C. The method according to any one of claims 6 to 10, wherein the SV (space velocity) of the oxidation catalyst is 3,000 to 200,000 h ^-1 and the SV of the denitrification catalyst is 1,000 to 50,000 h ^-1 .