ALDEHYDE DECOMPOSITION CATALYST, EXHAUST GAS TREATMENT APPARATUS, AND EXHAUST GAS TREATMENT METHOD

Abstract

One object is to provide an aldehyde decomposition catalyst, and an exhaust gas treatment apparatus and an exhaust gas treatment method using the aldehyde decomposition catalyst that achieve low cost and sufficient aldehyde decomposition performance with a small amount of the catalyst. An aldehyde decomposition catalyst of the present invention is made of a zeolite in a cation form NH.sub.4 having a structure selected from MFI and BEA and carrying at least one metal selected from the group consisting of Cu, Mn, Ce, Zn, Fe, and Zr.

Claims

1.-3. (canceled)

4. A method for decomposing an aldehyde contained in a combustion exhaust gas, the method comprising a step of contacting the aldehyde with an aldehyde decomposition catalyst, wherein the aldehyde decomposition catalyst is made of a zeolite in a cation form NH.sub.4 having a structure selected from MFI and BEA and carrying at least one metal selected from the group consisting of Cu, Mn, Ce, Zn, Fe, and Zr.

5. An exhaust gas treating method in which a combustion exhaust gas is denitrated by contacting the combustion exhaust gas with a denitration catalyst, wherein an alcohol is fed as a reducing agent for the denitration to the combustion exhaust gas, and an aldehyde by-produced in the denitration is decomposed according to claim 4.

6. The method of claim 4, wherein the aldehyde decomposition catalyst contains no noble metal and contains the at least one metal selected from the group consisting of Mn and Ce.

7. A method for decomposing an aldehyde contained in a combustion exhaust gas, the method comprising a step of contacting the aldehyde with an aldehyde decomposition catalyst, wherein the aldehyde decomposition catalyst is made of a zeolite in a cation form NH.sub.4 having a structure selected from MFI and BEA and carrying at least one metal selected from the group consisting of Cu, Mn, Ce, Zn, Fe, and Zr, and excludes a noble metal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a flow chart showing a test apparatus used for catalyst performance tests.

[0029] FIG. 2 is a graph showing decomposition rates calculated from the catalyst tests for Reference Examples 1 and 2.

[0030] FIG. 3 is a graph showing decomposition rates calculated from the catalyst tests for Reference Example 3.

[0031] FIG. 4 is a block diagram showing an exhaust gas treatment apparatus of a marine diesel engine using an aldehyde decomposition catalyst.

[0032] FIG. 5 is a block diagram showing a variation of the exhaust gas treatment apparatus.

DESCRIPTION OF EXAMPLE EMBODIMENTS

[0033] The aldehyde decomposition catalyst of the present invention will be hereinafter described in detail.

Embodiment 1

[0034] Embodiment 1 covers aldehyde decomposition catalysts made of a zeolite having a structure selected from MFI and BEA.

[0035] The following commercially available zeolites were used for Embodiment 1.

Reference Example 1

[0036] NH.sub.4-MFI zeolite (HSZ-830NHA from Tosoh Corporation with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 28)

Reference Example 2

[0037] NH.sub.4-BEA zeolite (CP814E from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 25)

Comparative Example 1

[0038] NH.sub.4-CHA zeolite (ZD8028 from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 30)

Comparative Example 2

[0039] NH.sub.4-MOR zeolite (HSZ-643NHA from Tosoh Corporation with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 18)

[0040] Catalyst Performance Test

[0041] Catalyst performance test was performed on the catalysts of Reference Examples 1 and 2 and Comparative Examples 1 and 2. The catalysts of Reference Examples 1 and 2 and Comparative Examples 1 and 2 were press-molded and then ground to mesh sizes 28 to 14.

[0042] FIG. 1 is a flow chart of a test apparatus used for catalyst performance tests.

[0043] The catalyst obtained as described above was filled into a stainless reactor (1) having an inner diameter of 10.6 mm.

[0044] The reactor (1) filled with the catalyst receives a test gas at an upper portion thereof through a line (2) and discharges the gas treated with the aldehyde decomposition catalyst at a lower portion thereof through a line (3).

[0045] The test gas received by the reactor (1) through the line (2) is prepared by mixing the air from a line (4) with N.sub.2 gas from a line (5). The line (4) and the line (5) are provided with a valve (6) and a valve (7), respectively. The degree of opening of the valve (6) and the valve (7) can be adjusted to control the flow rate of respective gases, thereby to control the gas flow rate and the mixture ratio.

[0046] The mixed gas is introduced into an upper portion of a heater (9) through a line (8), and the gas is heated to a predetermined temperature and fed from a lower portion of the heater (9) to the reactor (1) through the line (2).

[0047] An aldehyde solution (a formaldehyde solution for Embodiment 1, hereinafter referred to as the formaldehyde solution) is fed to the upper portion of the reactor (1) through a line (10).

[0048] The formaldehyde solution to be introduced into the reactor (1) is pumped up from a formaldehyde solution tank (11) by a liquid metering pump (12) and then merged from the line (10) into the line (2).

[0049] The treated gas discharged from the reactor (1) is discharged out through the line (3), while partially fed to gas analysis through a line (13).

[0050] Table 1 shows the test conditions applied to the test performed with the test apparatus of FIG. 1.

TABLE-US-00001 TABLE 1 Gas Component: O.sub.2 14% Gas Component: N.sub.2 Balance Formaldehyde 100 ppmvd Moisture 5% Gas Flow Rate 1 L/min Amount of Catalyst 1.0 g Space Velocity (SV) 60,000/h Reaction Temperature 250 C., 300 C.

[0051] In Table 1, the term Balance indicates that N.sub.2 is added such that the total gas composition is 100%, that is, the gas composition other than O.sub.2, formaldehyde, and moisture is occupied by N.sub.2.

[0052] The gas analysis was performed by measuring the outlet formaldehyde concentration with a gas detector tube. Based on the measurements by the gas detector tube, the decomposition rate indicating the formaldehyde decomposition performance of the catalyst was calculated from Formula (1) below.

[00001]$\begin{array}{cc}\mathrm{Decomposition}.Math..Math.\mathrm{Rate}=\frac{\mathrm{Formaldehyde}\left(\mathrm{in}\right)-\mathrm{Formaldehyde}\left(\mathrm{out}\right)}{\mathrm{Formaldehyde}\left(\mathrm{in}\right)}100& \mathrm{Formula}.Math..Math.\left(1\right)\end{array}$

[0053] In Formula (1) above, formaldehyde (in) refers to the concentration of formaldehyde in the gas before introduction into the reactor (1), and formaldehyde (out) refers to the concentration of formaldehyde in the gas discharged from the reactor (1).

[0054] Result

[0055] FIG. 2 shows a graph of the decomposition rate calculated in the above test, and Table 2 shows the calculated decomposition rates.

TABLE-US-00002 TABLE 2 Decomposition Rate (%) Zeolite 250 C. 300 C. Reference Example 1 MFI 75 85 Reference Example 2 BEA 63 70 Comparative Example 1 CHA 50 50 Comparative Example 2 MOR 40

[0056] As shown in FIG. 2 and Table 2, Reference Examples 1 and 2 achieved satisfactory decomposition rates exceeding 60% under the temperature conditions of both 250 C. and 300 C. By contrast, Comparative Examples 1 and 2 achieved some degrees of decomposition rates that were unsatisfactory as compared to those of Reference Examples 1 and 2.

Reference Example 3

[0057] Next, MOR zeolite was used to compare the decomposition rates of zeolites in different cation forms, H, NH.sub.4, and Na.

[0058] The test apparatus used was the same as in FIG. 1, and the temperature conditions of the gas fed into the reactor were 150 C., 175 C., 200 C., 225 C., 250 C., and 300 C. The decomposition rates were calculated by Formula (1) described above.

[0059] Result

[0060] FIG. 3 shows a graph of the decomposition rate calculated in the above test, and Table 3 shows the calculated decomposition rates.

TABLE-US-00003 TABLE 3 Cation Decomposition Rate (%) Form 150 C. 175 C. 200 C. 220 C. 250 C. H 20 35 25 10 35 NH.sub.4 45 50 35 60 40 Na 40 25 25 10 25

[0061] As shown in FIG. 3 and Table 3, NH.sub.4-MOR zeolite achieved stable and satisfactory decomposition rates over the whole temperature range, rated as the best form.

[0062] As described above, the MOR zeolite was used for Reference Example 3 to compare the decomposition rates of NH.sub.4, H, and Na forms. It could be naturally expected or generalized by those skilled in the art that NH.sub.4 form is also the best for a zeolite having a structure selected from MFI and BEA.

Embodiment 2

[0063] Embodiment 2 covers aldehyde decomposition catalysts made of a zeolite having a structure selected from MFI and BEA and carrying at least one metal selected from the group consisting of Cu, Mn, Ce, Zn, Fe, and Zr.

[0064] Each of the catalysts of Embodiment 2 was prepared by addition of 10 g of zeolite into 200 ml of aqueous metal solution, stirring of the solution at 80 C. for three hours, filtration, washing, and drying at 110 C. over a night.

[0065] For simplicity, the following list only includes zeolite species as raw material, aqueous metal solution, and resultant catalyst.

Example 1

[0066] Zeolite species: commercially available NH.sub.4-MFI zeolite (HSZ-830NHA from Tosoh Corporation with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 28)

[0067] Aqueous metal solution: aqueous Cu(NO.sub.3).sub.2 solution

[0068] Catalyst: Cu/MFI catalyst

Example 2

[0069] Zeolite species: commercially available NH.sub.4-MFI zeolite (CBV2314 from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 23)

[0070] Aqueous metal solution: aqueous Mn(NO.sub.3).sub.2 solution

[0071] Catalyst: Mn/MFI catalyst

Example 3

[0072] Zeolite species: commercially available NH.sub.4-BEA zeolite (CP814E from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 25)

[0073] Aqueous metal solution: aqueous Ce(NO.sub.3).sub.3 solution

[0074] Catalyst: Ce/BEA catalyst

Example 4

[0075] Zeolite species: commercially available NH.sub.4-BEA zeolite (CP814E from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 25)

[0076] Aqueous metal solution: aqueous Zn(NO.sub.3).sub.2 solution

[0077] Catalyst: Zn/BEA catalyst

Example 5

[0078] Zeolite species: commercially available NH.sub.4-MFI zeolite (HSZ-830NHA from Tosoh Corporation with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 28)

[0079] Aqueous metal solution: aqueous Fe(NO.sub.3).sub.3 solution

[0080] Catalyst: Fe/MFI catalyst

Example 6

[0081] Zeolite species: commercially available NH.sub.4-BEA zeolite (CP814E from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 25)

[0082] Aqueous metal solution: aqueous Fe(NO.sub.3).sub.3 solution

[0083] Catalyst: Fe/BEA catalyst

Example 7

[0084] Zeolite species: commercially available NH.sub.4-MFI zeolite (HSZ-830NHA from Tosoh Corporation with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 28)

[0085] Aqueous metal solution: aqueous ZrO(NO.sub.3).sub.2 solution

[0086] Catalyst: Zr/MFI catalyst

Example 8

[0087] Zeolite species: commercially available NH.sub.4-BEA zeolite (CP814E from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 25)

[0088] Aqueous metal solution: aqueous ZrO(NO.sub.3).sub.2 solution

[0089] Catalyst: Zr/BEA catalyst

Comparative Example 3

[0090] Zeolite species: commercially available NH.sub.4-MFI zeolite (CBV2314 from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 23)

[0091] Aqueous metal solution: aqueous AgNO.sub.3 solution

[0092] Catalyst: Ag/MFI catalyst

Comparative Example 4

[0093] Zeolite species: commercially available NH.sub.4-BEA zeolite (CP814E from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 25)

[0094] Aqueous metal solution: aqueous Mg(NO.sub.3).sub.2 solution

[0095] Catalyst: Mg/BEA Catalyst

Comparative Example 5

[0096] Zeolite species: commercially available NH.sub.4-MFI zeolite (CBV2314 from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 23)

[0097] Aqueous metal solution: aqueous Ba(NO.sub.3).sub.2 solution

[0098] Catalyst: Ba/MFI catalyst

Comparative Example 6

[0099] Zeolite species: commercially available NH.sub.4-MFI zeolite (CBV2314 from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 23)

[0100] Aqueous metal solution: aqueous La(NO.sub.3).sub.3 solution

[0101] Catalyst: La/MFI catalyst

Comparative Example 7

[0102] Zeolite species: commercially available NH.sub.4-BEA zeolite (CP814E from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 25)

[0103] Aqueous metal solution: aqueous Co(NO.sub.3).sub.2 solution

[0104] Catalyst: Co/BEA catalyst

Comparative Example 8

[0105] Zeolite species: commercially available NH.sub.4-MFI zeolite (CBV2314 from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 23)

[0106] Aqueous metal solution: aqueous SrO(NO.sub.3).sub.2 solution

[0107] Catalyst: Sr/MFI catalyst

Comparative Example 9

[0108] Zeolite species: commercially available NH.sub.4-BEA zeolite (CP814E from Zeolyst International with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 25)

[0109] Aqueous metal solution: aqueous Sr(NO.sub.3).sub.2 solution

[0110] Catalyst: Sr/BEA catalyst

Comparative Example 10

[0111] Zeolite species: commercially available H-ZSM-5(MFI) zeolite (CBV8020 from PQ Co. with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 70)

[0112] Aqueous metal solution: aqueous nitric acid solution of dinitro diamine platinum [Pt(NH.sub.3).sub.2(NO.sub.3).sub.2]

[0113] Catalyst: Pt/ZSM-5(MFI8030)

Comparative Example 11

[0114] Zeolite species: commercially available H-ZSM-5(MFI) zeolite (CBV3020 from PQ Co. with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 35)

[0115] Aqueous metal solution: aqueous nitric acid solution of dinitro diamine platinum [Pt(NH.sub.3).sub.2(NO.sub.3).sub.2]

[0116] Catalyst: Pt/ZSM-5(MFI3030)

Comparative Example 12

[0117] Zeolite species: commercially available mordenite zeolite (PQ511 from PQ Co. with a SiO.sub.2/Al.sub.2O.sub.3 molar ratio of 12.8)

[0118] Aqueous metal solution: aqueous nitric acid solution of dinitro diamine platinum [Pt(NH.sub.3).sub.2(NO.sub.3).sub.2]

[0119] Catalyst: Pt/mordenite

[0120] For Comparative Examples 10 to 12, the catalysts were prepared using the same zeolite species and aqueous metal solutions by the same method as disclosed in Japanese Unexamined Patent Application Publication No. Hei 10-309443. The method of preparing the catalysts of Comparative Examples 10 to 12 was slightly different from the method of preparing the catalysts of Examples 1 to 8 and Comparative Examples 3 to 9, but these two methods were essentially the same. Therefore, the results of the performance test of Examples 1 to 8 and Comparative Examples 3 to 9 will now be compared with the results of the test disclosed in Japanese Unexamined Patent Application Publication No. Hei 10-309443.

[0121] Catalyst Performance Test

[0122] Catalyst performance test was performed on the catalysts of Examples 1 to 8 and Comparative Examples 3 to 9. The catalysts of Examples 1 to 8 and Comparative Examples 3 to 9 were press-molded and then ground to mesh sizes 28 to 14.

[0123] The catalysts obtained as described above were filled into a stainless reactor (1) having an inner diameter of 10.6 mm shown in FIG. 1.

[0124] The catalyst performance test was performed using the test apparatus shown in FIG. 1, as for Embodiment 1. Therefore, the subsequent operations correspond to those for Embodiment 1 and thus detailed description thereof will be omitted.

[0125] Table 4 shows the test conditions applied to the test performed with the test apparatus of FIG. 1.

TABLE-US-00004 TABLE 4 Gas Component: O.sub.2 14% Gas Component: N.sub.2 Balance Formaldelyde 100 ppmvd Moisture 5% Gas Flow Rate 2 L/min Amount of Catalyst 1.0 g Space Velocity (SV) 120,000/h Reaction Temperature 250 C., 200 C.

[0126] In Table 4, the space velocity (SV) is a value equal to the amount of gas (m.sup.3/h) to be treated flowing into the reactor divided by the volume (m.sup.3) occupied by the reactor containing the catalyst. As this value is larger, the catalyst is contacted more efficiently.

[0127] The gas analysis was performed in the same manner as for Embodiment 1, and the decomposition rate was calculated by Formula (1), as for Embodiment 1. Detailed description of these operations will be omitted.

[0128] The numerical values to be compared will now be described before test results are presented.

[0129] For Embodiment 2, the space value SV is set at 120,000/h. In general, as the amount of catalyst is larger (SV is smaller), the catalyst achieves better performance. It is reasonable to expect that, when the values of the decomposition rate of Comparative Examples 10 to 12 for SV=100,000/h disclosed in the literature are converted into the values of the decomposition rate that would be obtained for SV=120,000/h, the decomposition performance will be degraded linearly. Therefore, the values disclosed in Japanese Unexamined Patent Application Publication No. Hei 10-309443 that was obtained under the conditions of SV=100,000/h and 200 C. can be converted as in Table 5 below.

TABLE-US-00005 TABLE 5 Decomposition Rate (%) 100,000/h 120,000/h Comparative Example 10 Pt/MFI 57.5 47.9 Comparative Example 11 Pt /MFI 52.7 43.9 Comparative Example 12 Pt/MOR 67.6 58.3

[0130] Therefore, it can be deemed from the comparison with the values converted from those disclosed in the literature that Examples 1 to 8 achieved about the same results as Comparative Examples 10 to 12 when decomposition rates of about 50% are obtained under the conditions of SV=120,000/h and a temperature of 200 C. or 250 C.

[0131] Table 6 shows the results of Examples 1 to 8 and Comparative Examples 3 to 9.

TABLE-US-00006 TABLE 6 Decomposition Rate (%) 250 C. 200 C. Example 1 Cu/MFI 96.1 51.2 Example 2 Mn/MFI 66.4 51.9 Example 3 Ce/BEA 61.3 66.0 Example 4 Zn/BEA 56.0 69.1 Example 5 Fe/MFI 84.0 63.3 Example 6 Fe/BEA 70.2 56.4 Example 7 Zr/MFI 53.8 74.4 Example 8 Zr/BEA 80.4 70.3 Comparative Example 3 Ag/MFI 51.1 51.1 Comparative Example 4 Mg/BEA 51.5 41.7 Comparative Example 5 Ba/MFI 50.8 51.0 Comparative Example 6 La/MIF 56.2 50.9 Comparative Example 7 Co/BEA 50.6 70.2 Comparative Example 8 Sr/MFI 46.2 60.6 Comparative Example 9 Sr/BEA 50.3 63.7

[0132] All the catalysts of Examples 1 to 8 achieved decomposition rates exceeding 50% under the temperature conditions of both 200 C. and 250 C.

[0133] In particular, Example 1 achieved an excellent decomposition rate of 96.1% at 250 C., Example 2 achieved a satisfactory decomposition rate of 66.4% at 250 C., Example 3 achieved satisfactory decomposition rates of 66.0% and 61.3% at 200 C. and 250 C., respectively, Example 4 achieved a satisfactory decomposition rate of 69.1% at 200 C., Example 5 achieved an excellent decomposition rate of 84.0% at 250 C. and a satisfactory decomposition rate of 63.3% at 200 C., Example 6 achieved an excellent decomposition rate of 70.2% at 250 C. and a satisfactory decomposition rate of 56.4% at 200 C., Example 7 achieved a satisfactory decomposition rate of 74.4% at 200 C., Example 8 achieved an excellent decomposition rate of 80.4% at 250 C. and a satisfactory decomposition rate of 70.3% at 200 C.

[0134] In particular, the catalysts made of MFI zeolite carrying Cu or Fe achieved at 250 C. excellent decomposition rates of 96.1% and 84.0%, respectively.

[0135] Further, the catalysts made of BEA zeolite carrying Fe or Zr achieved at 250 C. excellent decomposition rates of 70.2% and 80.4%, respectively.

[0136] By contrast, the catalysts of Comparative Examples 3 to 9, which achieved the decomposition rates of less than 50% or more than 50% depending on the conditions, did not exhibit distinct advantages over the values disclosed in the above literature.

[0137] As described above, the catalysts of Examples 1 to 8 were made of a particular zeolite carrying inexpensive metals instead of expensive metals such as Pt. Thus, it was possible to decompose formaldehyde in the combustion exhaust gas using a smaller amount of catalyst (a higher space velocity) containing inexpensive metals.

Embodiment 3: An Exhaust Gas Treatment Apparatus of a Marine Diesel Engine and an Exhaust Gas Treatment Method Using an Aldehyde Decomposition Catalyst

[0138] An exhaust gas treatment apparatus of a marine diesel engine using the aldehyde decomposition catalyst according to the present invention will now be described with reference to FIG. 4.

[0139] Since a marine diesel engine runs on C fuel oil containing a sulfur component, the combustion exhaust gas thereof contains a sulfur oxide in addition to a nitrogen oxide. The combustion exhaust gas discharged from the diesel engine has a temperature of about 350 C. It is then discharged via a turbocharger and its temperature is reduced to about 200 to 300 C. When the combustion exhaust gas is denitrated by ammonia selective reduction, the sulfur oxide reacts with ammonia to generate ammonium sulfate that may deposit in an exhaust path to block the heat exchanger.

[0140] Since the cause of blocking of the heat exchanger resides in use of ammonia as a reducing agent, some methods substitute alcohol for ammonia as a reducing agent to overcome the problem of blocking. However, when alcohol is contacted with common denitration catalysts made of a zeolite carrying a metal, oxidation of alcohol occurs in addition to denitration reaction. As a result, aldehydes are generated.

[0141] Embodiment 3 covers an exhaust gas treatment apparatus and an exhaust gas treatment method that use a small amount of catalyst to sufficiently decompose aldehydes, which is generated in denitration by denitration catalyst using alcohol as a reducing agent and contained in an exhaust gas having a low temperature of 200 to 300 C.

[0142] As shown in FIG. 4, an exhaust gas having a temperature of about 350 C. is discharged from combustion chambers of a diesel engine (21) and is delivered from an exhaust pipe (22) to the turbine side of a turbocharger (24) via an exhaust reservoir tank (23). As a result of release of heat, the exhaust gas discharged from the turbocharger (24) via the exhaust gas path (28) has a reduced temperature of about 200 to 300 C., and is delivered to a denitration unit (25) including a zeolite carrying a metal. The pressurized air delivered from a compressor of the turbocharger (24) is fed to combustion chambers of the diesel engine (21) via an air-supply reservoir tank (26).

[0143] In the denitration unit (25), an alcohol serving as a reducing agent is injected into the exhaust gas from an injection nozzle (25b) provided at an exhaust gas inlet, and the exhaust gas is contacted with a denitration catalyst (25a), such that oxidation of the alcohol occurs along with the denitration reaction so as to generate aldehydes. The exhaust gas discharged from the denitration unit (25) is delivered to an aldehyde decomposition unit (27) serving as an aldehyde decomposition means. In the aldehyde decomposition unit (27), the exhaust gas is contacted with the aldehyde decomposition catalyst (27a) of any one of Examples 1 to 8 of Embodiment 2, that is, the aldehyde decomposition catalyst (27a) made of a zeolite having a structure selected from MFI and BEA and carrying at least one metal selected from the group consisting of Cu, Mn, Ce, Zn, Fe, and Zr. The aldehydes contained in the exhaust gas is effectively decomposed, and then the exhaust gas is discharged through an exhaust air chimney (29).

[0144] In summary, Embodiment 3 covers an exhaust gas treatment apparatus in which a combustion exhaust gas is discharged from the diesel engine (21) and introduced into a denitration unit (25) via a turbocharger (24), the combustion exhaust gas has a low temperature of 200 to 300 C., an alcohol serving as a reducing agent is fed to the combustion exhaust gas, and the combustion exhaust gas is contacted with a denitration catalyst for denitration, and the exhaust gas treatment apparatus further includes an aldehyde decomposition unit (27) serving as an aldehyde decomposition means having an aldehyde decomposition catalyst for decomposing aldehydes contained in the combustion exhaust gas discharged from the denitration unit (25), and the aldehyde decomposition catalyst is made of a zeolite in a cation form NH.sub.4 having a structure selected from MFI and BEA and carrying at least one metal selected from the group consisting of Cu, Mn, Ce, Zn, Fe, and Zr.

[0145] Further, Embodiment 3 covers an exhaust gas treatment method in which a combustion exhaust gas is discharged from the diesel engine (21) and introduced into a denitration unit (25) via a turbocharger (24), the combustion exhaust gas has a low temperature of 200 to 300 C., an alcohol serving as a reducing agent is fed to the combustion exhaust gas, and the combustion exhaust gas is contacted with a denitration catalyst for denitration. The combustion exhaust gas discharged from the denitration unit (25) is contacted with an aldehyde decomposition catalyst to decompose aldehydes contained in the combustion exhaust gas. The aldehyde decomposition catalyst is made of a zeolite in a cation form NH.sub.4 having a structure selected from MFI and BEA and carrying at least one metal selected from the group consisting of Cu, Mn, Ce, Zn, Fe, and Zr.

[0146] Embodiment 3 covers a method including feeding an alcohol serving as a reducing agent to an exhaust gas discharged from a turbocharger (24) and having a low temperature of about 200 to 300 C. The exhaust gas is contacted with a denitration catalyst to cause oxidation reaction of the alcohol that generates aldehydes, in addition to the denitration reaction. However, the aldehydes can be effectively decomposed by an aldehyde decomposition catalyst made of a zeolite in a cation form NH.sub.4 having a structure selected from MFI and BEA and carrying at least one metal selected from the group consisting of Cu, Mn, Ce, Zn, Fe, and Zr. Thus, it is possible to prevent, for example, formaldehyde, which is harmful to organisms and highly toxic, from being released along with the combustion exhaust gas. Therefore, sufficient aldehyde decomposition performance can be achieved with a small amount of catalyst.

[0147] In Embodiment 3, the denitration unit (25) arranged upstream in the exhaust gas path and the aldehyde decomposition unit (27) arranged downstream in the same are independent from each other. It is also possible to arrange the denitration catalyst (32) upstream in an integrated container (31) and arrange the aldehyde decomposition catalyst (33), serving as an aldehyde decomposition means, downstream in the same, as shown in FIG. 5. The reference numeral (30) denotes an injection nozzle for alcohol.

[0148] The aldehyde decomposition catalyst (27a), (33) used in Embodiment 3 may be in any appropriate form such as powder, particle, granule (including spherical ones), pellet (cylindrical or annular ones), tablet, or honeycomb (monolithic body).

[0149] The foregoing was description of treatment apparatuses for a combustion exhaust gas discharged from marine diesel engines, the treatment apparatus using aldehyde decomposition catalyst Combustion exhaust gas treatment apparatuses having essentially the same structure as the above-described treatment apparatuses can be used on land as treatment apparatuses for a combustion exhaust gas discharged from, for example, diesel engines installed in a power plant. Also, such combustion exhaust gas treatment apparatuses can be suitably applied to internal combustion engines such as dual fuel engines (DF engines) and gas engines. For the DF engines and the gas engines, it is possible to denitrate exhaust gas discharged from, for example, a compressor of a turbocharger and having a temperature of about 200 to 300 C. Further, the combustion exhaust gas treatment apparatuses can be used for denitrating the combustion exhaust gas discharged from combustion facilities such as waste-incineration plants, boilers, and gas turbines.