DEVICE AND METHOD FOR SUPPRESSING FORMATION OF HIGH-MELTING-POINT PIPE-CLOGGING SUBSTANCE

Abstract

The method for suppressing formation of a high-melting-point pipe-clogging substance includes disposing a urea-solution supply pipe (6) configured to supply pressurized air and a urea solution into a pipe through which exhaust gas flows, connecting a urea-solution spray nozzle (7) near a tip of the urea-solution supply pipe (6), providing a mixing section (8) configured to mix the exhaust gas flowing through the pipe and a sprayed urea solution sprayed from the urea-solution spray nozzle (7), circumferentially providing a metal sheet (9) on all or part of an inner wall surface of the pipe in a belt-like manner around the mixing section (8), and forming a hydrolysis catalyst layer (10) configured to promote hydrolysis of urea on an inner surface of the metal sheet (9).

Claims

1. A device for suppressing formation of a high-melting-point pipe-clogging substance, comprising: a urea-solution supply pipe disposed into a pipe through which exhaust gas flows, wherein the urea-solution supply pipe is configured to supply pressurized air and a urea solution, a urea-solution spray nozzle connected near a tip of the urea-solution supply pipe, a mixing section configured to mix the exhaust gas flowing through the pipe and a sprayed urea solution sprayed from the urea-solution spray nozzle, a metal sheet circumferentially provided on all or part of an inner wall surface of the pipe in a belt-like manner around the mixing section, and a hydrolysis catalyst layer formed on an inner surface of the metal sheet, wherein the hydrolysis catalyst layer is configured to promote hydrolysis of urea.

2. The device according to claim 1, wherein the metal sheet is an aluminum metal sheet or a stainless steel metal sheet.

3. The device according to claim 1, wherein the hydrolysis catalyst layer is formed with an oxide containing one or more types of elements selected from among Ti, Al, and Si.

4. The device according to claim 1, wherein the hydrolysis catalyst layer is a TiO.sub.2 catalyst layer.

5. A method for preventing pipe clogging due to a high-melting-point substance, comprising: disposing a urea-solution supply pipe configured to supply pressurized air and a urea solution into a pipe through which exhaust gas flows, connecting a urea-solution spray nozzle near a tip of the urea-solution supply pipe, providing a mixing section configured to mix the exhaust gas flowing through the pipe and a sprayed urea solution sprayed from the urea-solution spray nozzle, and forming a hydrolysis catalyst layer on an inner surface of a metal sheet circumferentially provided on all or part of an inner wall surface of the pipe in a belt-like manner around the mixing section, wherein the hydrolysis catalyst layer is configured to promote hydrolysis of urea, wherein the urea solution sprayed from the urea-solution spray nozzle is brought into contact with the hydrolysis catalyst layer to promote hydrolysis of isocyanic acid (HN═C═O) and cyanic acid (HOCN), thereby producing ammonia and also decreasing an amount of cyanuric acid formed.

6. The method according to claim 5, wherein the hydrolysis catalyst layer is formed with an oxide containing one or more types of elements selected from among Ti, Al, and Si.

7. The method according to claim 5, wherein the hydrolysis catalyst layer is a TiO.sub.2 catalyst layer.

8. The method according to claim 7, wherein the sprayed urea solution sprayed from the urea-solution spray nozzle is brought into contact with the hydrolysis catalyst layer to promote a hydrolysis reaction through which isocyanic acid and cyanic acid, which are byproducts other than ammonia produced by thermal decomposition of urea, and moisture in the atmosphere are hydrolyzed to be converted into ammonia and carbon dioxide, whereby isocyanic acid and cyanic acid for the isocyanic acid and the cyanic acid to be polymerized into cyanuric acid decrease, and consequently the amount of cyanuric acid formed from urea is decreased.

9. The method according to claim 6, wherein the sprayed urea solution sprayed from the urea-solution spray nozzle is brought into contact with the hydrolysis catalyst layer to promote a hydrolysis reaction through which isocyanic acid and cyanic acid, which are byproducts other than ammonia produced by thermal decomposition of urea, and moisture in the atmosphere are hydrolyzed to be converted into ammonia and carbon dioxide, whereby isocyanic acid and cyanic acid for the isocyanic acid and the cyanic acid to be polymerized into cyanuric acid decrease, and consequently the amount of cyanuric acid formed from urea is decreased.

10. The method according to claim 5, wherein the sprayed urea solution sprayed from the urea-solution spray nozzle is brought into contact with the hydrolysis catalyst layer to promote a hydrolysis reaction through which isocyanic acid and cyanic acid, which are byproducts other than ammonia produced by thermal decomposition of urea, and moisture in the atmosphere are hydrolyzed to be converted into ammonia and carbon dioxide, whereby isocyanic acid and cyanic acid for the isocyanic acid and the cyanic acid to be polymerized into cyanuric acid decrease, and consequently the amount of cyanuric acid formed from urea is decreased.

11. The device according to claim 2, wherein the hydrolysis catalyst layer is formed with an oxide containing one or more types of elements selected from among Ti, Al, and Si.

12. The device according to claim 2, wherein the hydrolysis catalyst layer is a TiO.sub.2 catalyst layer.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0036] FIG. 1 is an explanatory diagram illustrating an example of a device for suppressing formation of a high-melting-point piping-clogging substance according to the present invention.

[0037] FIG. 2 is a schematic sectional view illustrating an example of a catalyst sheet according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] An embodiment of a device for suppressing formation of a high-melting-point piping-clogging substance according to the present invention will now be described with reference to FIG. 1.

[0039] FIG. 1 is an explanatory diagram illustrating an example of the device for suppressing formation of a high-melting-point pipe-clogging substance according to the present invention.

[0040] In FIG. 1, the numeral “1” denotes a diesel engine, and the numeral “2” denotes an exhaust gas pipe through which exhaust gas discharged from the diesel engine 1 is sent.

[0041] The numeral “3” denotes a hydrolysis device for a urea solution, and is also called a vaporizer. The hydrolysis device 3 is provided in a vaporization pipe 4. An exhaust-gas inlet 5 is provided at an inlet of the vaporization pipe 4, and exhaust gas is introduced into the vaporization pipe 4 through the inlet 5.

[0042] Into the vaporization pipe 4, a urea-solution supply pipe 6 configured to supply pressurized air (compressed air) and a urea solution is disposed, and a urea-solution spray nozzle 7 is provided near a tip of the urea-solution supply pipe 6. The urea-solution spray nozzle 7 is configured to be able to spray the urea solution into the vaporization pipe 4.

[0043] The numeral “8” denotes a mixing section configured to mix the exhaust gas flowing through the vaporization pipe 4 and the sprayed urea solution sprayed from the urea-solution spray nozzle.

[0044] A metal sheet 9 is circumferentially provided on all or part of the inner wall surface of the vaporization pipe 4 in a belt-like manner around the mixing section 8. On the inner surface of the metal sheet 9, a hydrolysis catalyst layer 10 configured to promote hydrolysis of urea is formed. A laminated structure with the hydrolysis catalyst layer 10 formed on the metal sheet 9 is preferably formed in a sheet shape.

[0045] The metal sheet 9 is preferably an aluminum metal sheet or a stainless steel metal sheet, for example.

[0046] A catalyst material used for the hydrolysis catalyst layer 10 may be any hydrolysis catalyst for urea, and specifically, a metal oxide that functions as a catalyst to promote the hydrolysis of urea is preferred.

[0047] Examples of the metal oxide includes oxides containing one or more types of elements selected from among Ti, Al, and Si (Al.sub.2O.sub.3, SiO.sub.2, Al.sub.2O.sub.3—SiO.sub.2, TiO.sub.2, etc.). TiO.sub.2 is preferred from the viewpoint of availability and a good balance between safety and catalytic performance.

[0048] When the exhaust gas and the sprayed urea solution are mixed in the mixing section 8, the following hydrolysis reaction occurs.

(NH.sub.2).sub.2CO+H.sub.2O.fwdarw.2NH.sub.3+CO.sub.2

[0049] The method of forming the hydrolysis catalyst layer 10 by providing the hydrolysis catalyst on the metal sheet 9 is not limited to a particular one as long as the hydrolysis catalyst can be immobilized on the metal sheet 9 to form the hydrolysis catalyst layer 10. The hydrolysis catalyst layer 10 can be formed by, for example, mixing a dispersion solution with titanium oxide as a catalyst to prepare a hydrolysis-catalyst coating solution, and applying this coating solution to the metal sheet 9 such as an aluminum metal sheet or a stainless steel metal sheet.

[0050] In the present embodiment, the hydrolysis catalyst may be provided directly on the inner wall of a metal exhaust gas pipe instead of on the metal sheet 9. As the method of providing the hydrolysis catalyst on the metal sheet 9 or on the inner wall of the metal exhaust gas pipe, various methods such as brushing, dipping, spraying, thermal spraying, and CVD as described above in addition to the application of the hydrolysis-catalyst coating solution may be used.

[0051] From the viewpoint of bringing the urea solution into contact with the catalyst, a position where the metal sheet 9 with the hydrolysis catalyst provided and the hydrolysis catalyst layer 10 formed thereon is provided, or a position of the inner wall of the pipe where the hydrolysis catalyst is provided is preferably near a position where the urea-solution spray nozzle 7 is disposed inside the pipe. For a turbocharged engine, the urea-solution spray nozzle 7 is disposed near a position of the exhaust gas pipe between the combustion chamber and the inlet of the turbocharger or a position of the exhaust gas pipe between the combustion chamber and the outlet of the turbocharger. Even in these cases, the hydrolysis catalyst is preferably disposed near the urea-solution spray nozzle 7.

[0052] As illustrated in FIG. 1, in the hydrolysis device 3, urea is hydrolyzed to produce NH.sub.3 through the above hydrolysis reaction, and in a denitration device 11, the exhaust gas containing NO.sub.x and NH.sub.3 is reduced and purified into N.sub.2 through the following reduction reaction with a denitration catalyst.

4NO+4NH.sub.3+O.sub.2.fwdarw.4N.sub.2+6H.sub.2O

6NO.sub.2+8NH.sub.3.fwdarw.7N.sub.2+12H.sub.2O

[0053] The denitration catalyst is not limited to a particular one, and a catalyst is used 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 support such as: TiO.sub.2; binary composite oxide such as SiO.sub.2—TiO.sub.2, WO.sub.3—TiO.sub.2, SiO.sub.2—TiO.sub.2, or Al.sub.2O.sub.3—SiO.sub.2; or ternary composite oxide such as WO.sub.3—SiO.sub.2—TiO.sub.2, or Mo.sub.3—SiO.sub.2—TiO.sub.2, and reduces NO.sub.x into nitrogen gas in the presence of NH.sub.3 (reducing agent) for purification.

[0054] In the present embodiment, a temperature regulator 12 is preferably provided on the outer periphery of the vaporization pipe 4 so as to cover the vaporization pipe 4. The temperature regulator 12 is preferably a pipe heating mantle, for example.

[0055] The following describes a method for preventing pipe clogging due to a high-melting-point substance according to the present invention.

[0056] The method for preventing pipe clogging due to a high-melting-point substance will be described, which uses the device for suppressing formation of a high-melting-point pipe-clogging substance illustrated in FIG. 1.

[0057] Specifically, a heating system in which cyanuric acid is not formed by thermal decomposition of urea (at a temperature of 30° C. to below 130° C., preferably below 100° C. from the viewpoint of preventing crystal deposition), the urea solution sprayed from a urea-solution spray nozzle is brought into contact with the hydrolysis catalyst layer at a temperature of 135° C. to 350° C., more preferably 150° C. to 250° C. to promote a hydrolysis reaction through which isocyanic acid (HN═C═O) and cyanic acid (HOCN), which are byproducts other than ammonia produced by thermal decomposition of urea, and moisture in the atmosphere are hydrolyzed to be converted into ammonia and carbon dioxide. Through the hydrolysis reaction thus promoted, isocyanic acid (HN═C═O) and cyanic acid (HOCN) to be polymerized into cyanuric acid decrease. Consequently, the amount of cyanuric acid formed from urea is decreased, whereby pipe clogging due to a high-melting-point substance can be prevented.

[0058] As a method of adjusting the heating temperature of urea to a temperature at which cyanuric acid is not formed, the temperature of the vaporization pipe 4 can be adjusted by the temperature regulator 12 provided on the outer periphery of the exhaust gas pipe as illustrated in FIG. 1. This allows the temperature of exhaust gas introduced into the vaporization pipe 4 to be adjusted. By providing the temperature regulator 12, the temperature of the exhaust gas can be adjusted when the need for adjustment arises. Consequently, the amount of cyanuric acid formed decreases, and pipe clogging due to a high-melting-point substance can be prevented.

[0059] In the present embodiment, it is also preferable to provide an air pipe (not illustrated) around the outer periphery of the vaporization pipe 4 (double-pipe structure) so as to allow compressed air to flow through this air pipe. The air volume may be adjusted in conjunction with an exhaust-gas temperature sensor. In this case, it is also preferable to provide the temperature regulator 12 on the outer periphery of the air pipe provided on the outer periphery of the vaporization pipe.

[0060] According to the present invention, in the thermal decomposition of urea by the heat of exhaust gas, isocyanic acid (HN═C═O) and cyanic acid (HOCN) to be polymerized into cyanuric acid decrease, and consequently the amount of cyanuric acid formed from the urea is decreased, whereby the amount of ammonia supply can be increased. This eliminates the possibility of decrease in denitration efficiency due to reductant hydrogen source supply loss caused by reductant hydrogen source retention in the pipe, or decrease in NO.sub.x reduction performance at a downstream location due to coating of the surface of the denitration catalyst or clogging of apertures of the honeycomb catalyst. Furthermore, by decreasing the amount of cyanuric acid formed, pipe clogging or blocking due to a high-melting-point substance in a location upstream of a catalytic reaction tube can be prevented, and troubles of decrease in engine power output due to increased back pressure in the exhaust gas pipe and of engine stalling in the worst case can be prevented.

EXAMPLES

[0061] The following describes Examples of the present invention. However, the present invention is not limited to the Examples.

Example 1

[0062] An experiment of suppressing formation of a high-melting-point pipe-clogging substance by using a NO.sub.x removal device illustrated in FIG. 1 with a hydrolysis device and a denitration device for exhaust gas emitted from a marine diesel engine was conducted.

[0063] 1. Experimental Conditions

[0064] (1) Hydrolysis Device [0065] Exhaust gas volume: SV 90,000/h [0066] Pipe for hydrolysis device: heating at 200° C. (heating by pipe heating mantle at 250° C.) [0067] Hydrolysis catalyst: A TiO.sub.2 catalyst was applied in a sheet shape onto an aluminum metal sheet having a thickness of 0.1 mm to prepare a TiO.sub.2 urea hydrolysis catalyst sheet having a total dry thickness of 0.105 mm to 0.200 mm (hereinafter referred to as TiO.sub.2 catalyst AL sheet if necessary).

[0068] (2) Denitration Device [0069] Denitration catalyst: A TiO.sub.2 catalyst was applied to a metal honeycomb body by immersion coating to prepare a honeycomb catalyst as a denitration catalyst. This catalyst has a denitration function and a hydrolysis function.

[0070] 2. Experiment [0071] (1) An experiment was conducted as follows. From 300° C. that is a usual gas temperature in a system achieved when the heating temperature of the pipe heating mantle is set to 450° C., the usual gas temperature in the system was intentionally lowered by 100° C., and a urea solution was sprayed to accelerate formation of cyanuric acid. Specifically, the heating temperature of the pipe heating mantle was set to 250° C., and the behavior of cyanuric acid formation at a gas temperature of 200° C. in the system was examined. [0072] (2) Urea supply conditions [0073] a) The supply amount (g) of 32.5 wt %-urea solution of 4H is given in Table 1. [0074] b) The supply flow rate (g/min) of pure urea is given in Table 1. [0075] (3) Formation rate of high-melting-point substance (cyanuric acid) [0076] a) The amount (g) of a urea-derived high-melting-point substance (cyanuric acid) formed (after 240 min) was 6.12 g (see Table 1). [0077] b) The amount of the urea-derived high-melting-point substance (cyanuric acid) formed per unit time (g/min) was 0.0255 g/min (see Table 1). [0078] (4) The conversion ratio (%) of the supplied urea into the high-melting-point substance (cyanuric acid) was 3.9% (w/w) (see Table 1).

Comparative Example 1

[0079] An experiment was conducted in the same manner as in Example 1, except that only the aluminum sheet was used without providing a catalyst on the aluminum sheet in Example 1.

[0080] The results are given in Table 1.

TABLE-US-00001 TABLE 1 Comparative Example 1 Example 1 TiO.sub.2 Blank catalyst (Al sheet) Al sheet Urea Supply amount (g) of 32.5 480 480 supply wt %-urea solution of 4H conditions Supply flow rate (g/min) of 0.65 0.65 pure urea Formation Amount (g) of urea-derived 10.50 6.12 rate of high-melting-point substance high- formed after 240 min melting- Amount of urea-derived 0.0438 0.0255 point high-melting-point substance substance formed per unit time (g/min) Conversion ratio of supplied 6.7% 3.9% urea into high-melting- point substance (w/w)

[0081] (Evaluation)

[0082] From the experimental results in Table 1, As the effect of the catalyst in suppressing formation of the high-melting-point substance (cyanuric acid), it was found that the conversion ratio of the supplied urea into the high-melting-point substance (cyanuric acid) decreased from 6.7% to 3.9%, and thus the formation of the high-melting-point substance (cyanuric acid) decreased by 42%.

Example 2

[0083] An experiment of suppressing formation of a high-melting-point pipe-clogging substance by using a NO.sub.x removal device illustrated in FIG. 1 with a hydrolysis device and a denitration device for exhaust gas emitted from a marine diesel engine was conducted.

[0084] 1. Experimental Conditions

[0085] (1) Hydrolysis Device [0086] Exhaust gas volume: SV 90,000/h [0087] Pipe for hydrolysis device: heating at 200° C. (heating by pipe heating mantle at 250° C.) [0088] Hydrolysis catalyst: A TiO.sub.2 catalyst was applied in a sheet shape onto a stainless steel metal sheet having a thickness of 0.1 mm to prepare a TiO.sub.2 urea hydrolysis catalyst sheet with a total dry thickness of 0.105 mm to 0.200 mm (hereinafter referred to as TiO.sub.2 catalyst SUS sheet if necessary).

[0089] (2) Denitration Device [0090] Denitration catalyst: A TiO.sub.2 catalyst was applied to a metal honeycomb body by immersion coating to prepare a honeycomb catalyst as a denitration catalyst. This catalyst has a denitration function and a hydrolysis function.

[0091] 2. Experiment [0092] (1) An experiment was conducted as follows. At a usual gas temperature of 200° C. in the system lowered by 100° C. from 300° C. that is a usual gas temperature in a system achieved when the heating temperature of the pipe heating mantle is set to 450° C., a urea solution was sprayed to accelerate formation of cyanuric acid. Specifically, the heating temperature of the pipe heating mantle was set to 250° C., and the behavior of cyanuric acid formation at a gas temperature of 200° C. in the system was examined. [0093] (2) Urea supply conditions [0094] a) The supply amount (g) of 32.5 wt %-urea solution of 4H is given in Table 2. [0095] b) The supply flow rate (g/min) of pure urea is given in Table 2. [0096] (3) Formation rate of high-melting-point substance (cyanuric acid) [0097] a) The amount (g) of a urea-derived high-melting-point substance (cyanuric acid) formed (after 240 min) was 1.92 g (see Table 2). [0098] b) The amount of the urea-derived high-melting-point substance (cyanuric acid) formed per unit time (g/min) was g/min (see Table 2). [0099] (4) The conversion ratio (%) of the supplied urea into the high-melting-point substance (cyanuric acid) was 1.23% (w/w) (see Table 2).

Comparative Example 2

[0100] An experiment was conducted in the same manner as in Example 2, except that only the stainless steel sheet was used without providing a catalyst on the stainless steel sheet in Example 2.

[0101] The results are given in Table 2.

TABLE-US-00002 TABLE 2 Comparative Example 2 Example 2 TiO.sub.2 Blank catalyst (SUS sheet) SUS sheet Urea Supply amount (g) of 32.5 480 480 supply wt %-urea solution of 4H conditions Supply flow rate (g/min) of 0.65 0.65 pure urea Formation Amount (g) of Urea- 3.39 1.92 rate of derived high-melting-point high- substance after 240 min melting- Amount of urea-derived 0.0141 0.00800 point high-melting-point substance substance formed per unit time (g/min) Conversion ratio of supplied 2.17% 1.23% urea into high-melting- point substance (w/w)

[0102] (Evaluation)

[0103] From the experimental results in Table 2, the conversion ratio of the supplied urea into the high-melting-point substance (cyanuric acid) was 2.17% in Comparative Example 2.

[0104] As the effect of the TiO.sub.2 catalyst coated on the SUS sheet in suppressing formation of the high-melting-point substance (cyanuric acid), it was found that the conversion ratio of the supplied urea into the high-melting-point substance (cyanuric acid) decreased from 2.17% to 1.23%, and thus the formation of the high-melting-point substance (cyanuric acid) decreased by 43%.

Examples 3 to 5

[0105] Examples 3 to 5 were also tested in the same manner as in Example 2, except that an Al.sub.2O.sub.3 catalyst, an aluminum silicate oxide (Al.sub.2O.sub.3—SiO.sub.2) catalyst, and a silica (SiO.sub.2) catalyst were used instead of the TiO.sub.2 catalyst of Example 2. Table 3 gives the results of the effects of the SUS metal sheets coated with the respective four catalysts of Examples 2 to 5 in suppressing formation of the high-melting-point substance (cyanuric acid) with respect to the SUS metal sheet with no catalyst in Comparative Example 2.

TABLE-US-00003 TABLE 3 Catalyst sheet type Suppression effect Example 2 TiO.sub.2 catalyst-coated SUS sheet +43% Example 3 Al.sub.2O.sub.3 catalyst-coated SUS sheet +40% Example 4 Aluminum silicate oxide catalyst- +40% coated SUS Sheet Example 5 Silica catalyst-coated SUS sheet +10%

[0106] (Evaluation)

[0107] Among these porous metal oxide catalysts coated on the surface of the SUS metal sheet, it was found that the effects of the TiO.sub.2-based catalyst, the Al.sub.2O.sub.3 catalyst, and the aluminum silicate oxide (Al.sub.2O.sub.3—SiO.sub.2) catalyst in suppressing the formation of the urea-derived high-melting-point substance (cyanuric acid) were relatively high, and the effect of the TiO.sub.2-based catalyst was highest.

EXPLANATIONS OF LETTERS OR NUMERALS

[0108] 1 diesel engine [0109] 2 exhaust gas pipe [0110] 3 hydrolysis device for urea solution (vaporizer) [0111] 4 vaporization pipe [0112] 5 exhaust gas inlet [0113] 6 urea-solution supply pipe [0114] 7 urea-solution spray nozzle [0115] 8 mixing section [0116] 9 metal sheet [0117] 10 hydrolysis catalyst layer [0118] 11 denitration device [0119] 12 temperature regulator