Patent Application
20230383682
The present invention relates to a device for suppressing formation (generation) of a high-melting-point pipe-clogging substance and a method for preventing pipe clogging due to a high-melting-point substance, and in particular, relates to a device for suppressing formation of a high-melting-point pipe-clogging substance and a method for preventing pipe clogging due to a high-melting-point substance, which can prevent pipe clogging due to scale in a pipe caused by a urea-derived high-melting-point substance formed by thermal decomposition of urea.
Exhaust gas emitted from diesel engines contains pollutants such as hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx), and particulate matter (PM).
Among these pollutants, NOx is difficult to be purified with oxidation catalysts or three-way catalysts used practically in gasoline-fueled cars, and a selective reduction type NOx catalyst, which is a denitration catalyst, has been studied as a promising catalyst that can purify NOx (Patent Document 1).
The denitration catalyst produces nitrogen gas (N2) through the following reaction of NOx, such as NO and NO2, with ammonia (reductant), thereby contributing to removal of NOx.
4NO+4NH3+O2→4N2+6H2O (1)
NO+NO2+2NH3→2N2+3H2O (2)
6NO2+8NH3→7N2+12H2O (3)
As a method of supplying ammonia as a reductant, a method of adding urea from a urea-solution tank into an exhaust system upstream of the denitration catalyst thereby producing ammonia to be used is known.
According to Patent Document 1, as a method of supplying ammonia as a reductant, the method of adding urea from a urea-solution tank into the exhaust system upstream of the denitration catalyst thereby producing ammonia to be used is known, but urea is hydrolyzed by heat from exhaust gas or by a hydrolysis catalyst to produce ammonia. However, it is pointed out that, through thermal decomposition of urea by heat of exhaust gas, the urea changes into high-melting-point substances such as cyanuric acid, isocyanic acid, and melamine, causing a problem that the decomposition efficiency decreases and the NOx reduction performance at a downstream location decreases, and also a problem of clogging of pipes and the like with high-melting-point substances.
However, Patent Document 1 only points out that all high-melting-point substances such as cyanuric acid, isocyanic acid, and melamine have a problem of pipe clogging, but does not clarify what substances contribute to the pipe clogging due to scale in a pipe.
It is also known that thermal decomposition of urea produces isocyanic acid and cyanic acid, produces cyanuric acid (six-membered ring), and also produces melamine (six-membered ring).
According to studies made by the inventor of the present invention, it was found, through qualitative analysis conducted by the X-ray diffraction method, that a component that clogs a urea-solution injection nozzle or a component that clogs an exhaust gas pipe when urea is sprayed into the exhaust gas pipe is cyanuric acid (six-membered ring).
In the thermal decomposition of urea, in a process in which cyanuric acid is formed, isocyanic acid and cyanic acid are produced by heating at 150° C. to 300° C., a part of the isocyanic acid and the cyanic acid is slow in reaction speed, and is polymerized (trimerized) into cyanuric acid at about 150° C. to about 300° C. In another reaction pathway, when urea is heated at 135° C. (its melting point) or higher, ammonia is intermolecularly liberated to form isocyanic acid and cyanic acid, a part of which forms biuret (intermediate). Furthermore, when the temperature has been increased to 196° C., this intermediate is decomposed into cyanuric acid.
Among the substances formed during these thermal decomposition processes of urea, it is difficult to cause hydrolysis in six-membered rings such as melamine and cyanuric acid.
The inventor focused on isocyanic acid and cyanic acid that have not yet become multimeric, and found that when hydrolysis of isocyanic acid and cyanic acid is promoted before polymerization of isocyanic acid and cyanic acid to form cyanuric acid, the amount of cyanuric acid formed decreases. Thus, the present invention has been made.
In view of this, it is an object of the present invention to provide a device for suppressing formation of a high-melting-point pipe-clogging substance and a method for preventing pipe clogging due to a high-melting-point substance by promoting hydrolysis of isocyanic acid and cyanic acid and decreasing the amount of cyanuric acid formed.
Furthermore, other objects of the present invention will become apparent upon reading the following description.
The above objects are achieved by each of the following inventions.
According to the present invention, it is possible to provide the device for suppressing formation of a high-melting-point pipe-clogging substance and the method for preventing pipe clogging due to a high melting-point substance by promoting hydrolysis of isocyanic acid and cyanic acid and decreasing the amount of cyanuric acid formed.
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
In
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.
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.
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.
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.
The metal sheet 9 is preferably an aluminum metal sheet or a stainless steel metal sheet, for example.
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.
Examples of the metal oxide includes oxides containing one or more types of elements selected from among Ti, Al, and Si (Al2O3, SiO2, Al2O3—SiO2, TiO2, etc.). TiO2 is preferred from the viewpoint of availability and a good balance between safety and catalytic performance.
When the exhaust gas and the sprayed urea solution are mixed in the mixing section 8, the following hydrolysis reaction occurs.
(NH2)2CO+H2O→2NH3+CO2
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.
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.
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.
As illustrated in
4NO+4NH3+O2→4N2+6H2O
6NO2+8NH3→7N2+12H2O
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: TiO2; binary composite oxide such as SiO2—TiO2, WO3—TiO2, SiO2—TiO2, or Al2O3—SiO2; or ternary composite oxide such as WO3—SiO2—TiO2, or Mo3—SiO2—TiO2, and reduces NOx into nitrogen gas in the presence of NH3 (reducing agent) for purification.
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.
The following describes a method for preventing pipe clogging due to a high-melting-point substance according to the present invention.
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
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.
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
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.
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 NOx 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.
The following describes Examples of the present invention. However, the present invention is not limited to the Examples.
An experiment of suppressing formation of a high-melting-point pipe-clogging substance by using a NOx removal device illustrated in
1. Experimental Conditions
(1) Hydrolysis Device
(2) Denitration Device
2. Experiment
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.
The results are given in Table 1.
(Evaluation)
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%.
An experiment of suppressing formation of a high-melting-point pipe-clogging substance by using a NOx removal device illustrated in
1. Experimental Conditions
(1) Hydrolysis Device
(2) Denitration Device
2. Experiment
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.
The results are given in Table 2.
(Evaluation)
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.
As the effect of the TiO2 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 were also tested in the same manner as in Example 2, except that an Al2O3 catalyst, an aluminum silicate oxide (Al2O3—SiO2) catalyst, and a silica (SiO2) catalyst were used instead of the TiO2 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.
(Evaluation)
Among these porous metal oxide catalysts coated on the surface of the SUS metal sheet, it was found that the effects of the TiO2-based catalyst, the Al2O3 catalyst, and the aluminum silicate oxide (Al2O3—SiO2) catalyst in suppressing the formation of the urea-derived high-melting-point substance (cyanuric acid) were relatively high, and the effect of the TiO2-based catalyst was highest.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-176139 | Oct 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/036976 | 10/6/2021 | WO |
