JP2001157822A - Method for removing nitrogen oxide in combustion exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for removing nitrogen oxide which is suitable for purifying exhaust gas of an automobile at a relatively low temperature wherein in a method for removing nitrogen oxides in combustion exhaust gas by a catalytic reduction process by means of ammonia gas, a high pressure ammonia storage tank is not used, and as the generation of decomposed ammo nia gas due to decomposition of urea, high temperature heating or hydrolytic catalysts are not used to vaporize urea and a relatively low temperature can be used. SOLUTION: As an ammonia generating reducing agent, at least any one of the group consisting of ammonium carbamate, ammonium carbonate, and ammonium hydrogencarbonate is heated and decomposed to generate a mixed gas of ammonia and carbon dioxide, which are mixed with a combustion exhaust gas to remove nitrogen oxides by a catalytic reduction reaction, following which a part of the exhaust gas is used as a heating source for generation of decomposed ammonia of a reducing agent.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for removing nitrogen oxides (hereinafter referred to as NOx) in combustion exhaust gas. More particularly, the present invention relates to a method for reducing and decomposing and removing nitrogen oxides in combustion exhaust gas with ammonia gas.

[0002]

2. Description of the Related Art Conventionally, techniques for treating NOx contained in combustion exhaust gas have been required in various fields. For example, NOx present in exhaust gas from boilers and automobiles causes photochemical smog which is harmful to the human body, and also causes acid rain.Therefore, it has been demanded to effectively remove NOx in exhaust gas, All vehicles are subject to emission regulations. For example, in commercial vehicles such as heavy trucks and buses, the NOx regulation value in exhaust gas is currently 5 g in Japan.
/ Kwh, but it will be reduced to 3.38 / kwh in 2003. To cope with this, it has become necessary to develop a more efficient ONx-free system.

As a method for removing NOx, a selective catalytic reduction method using ammonia has been put to practical use. In this method, ammonia gas is mixed with combustion exhaust gas, and NOx is removed by passing the mixture through a denitration catalyst layer. At this time, the necessary ammonia is 1 mol per 1 mol of NO,
It is efficient. However, this method usually supplies ammonia gas directly from the ammonia storage container,
Since high-pressure liquefied ammonia is handled, the device must have sufficient pressure resistance, and the safety of the device, such as measures against leakage of ammonia, which is a toxic gas, tends to be problematic, resulting in high costs.

The most common reducing agent for NOx removal instead of ammonia is an aqueous urea solution. The aqueous urea solution is odorless and harmless even if it leaks. For example, in the method of JP-A-11-171535, the aqueous urea solution is converted to ammonia-carbon dioxide gas in the hydrolysis catalyst layer, and the ammonia-carbon dioxide gas contains NOx. It is mixed with flue gas and catalytically reduced on a denitration catalyst. In the method of JP-A-6-280546, an aqueous urea solution is sprayed immediately after the combustion cylinder, and is catalytically reduced by a denitration catalyst layer provided for each exhaust passage. In this case, the exhaust gas is 5
Since the temperature is as high as 00 ° C. or higher, no hydrolysis catalyst is used. In each case, since the aqueous solution is used, the urea concentration is 3
The maximum is around 0%, and the volume efficiency is poor. In addition, since urea solids precipitate from the aqueous urea solution remaining in the pipe at the time of cooling, troubles such as clogging of the pipe are expected.

[0005] As a method for improving the volumetric efficiency, there is a method in which urea solid is used as it is as a reducing agent. JP 2
In -261519, a urea solid is fed by a screw feeder, and the urea gas is vaporized by being brought into direct contact with exhaust gas to produce urea gas. Then, an ammonia-carbon dioxide gas is generated and used in a hydrolysis catalyst layer. The generated ammonia-carbon dioxide gas reduces NOx in the denitration catalyst layer. In this method, 100% can be used as the urea concentration, so that the volume efficiency is the maximum. However, in this method, since the solid is fed, it is difficult to control the flow rate and heat energy consumption for vaporizing the solid is large. In addition, since urea is deliquescent, it is likely to deliquesce due to humidity and cause clogging due to solidification.

[0006]

As is apparent from the above description,
Conventionally, in the reduction and removal of NOx by the catalytic reduction method using ammonia gas, there are problems in the method of directly using ammonia gas from a high-pressure ammonia storage tank and the method of using ammonia gas decomposed by a urea hydrolysis catalyst. Therefore, the object of the present invention is no mechanical trouble such as clogging, no need for a hydrolysis catalyst,
It is an object of the present invention to provide a NOx removal system that can generate ammonia gas at a relatively low temperature, requires less energy, is easy to control, and can be applied to exhaust gas from a small-scale internal combustion engine such as a vehicle.

[0007]

That is, according to the method of the present invention for removing nitrogen oxides from flue gas, ammonia gas is mixed with flue gas containing nitrogen oxides and the nitrogen oxides are contacted in the presence of a catalyst. A method for reducing and removing, characterized in that at least one of ammonium carbamate, ammonium carbonate, and ammonium bicarbonate is heated and decomposed as an ammonia gas source, and the generated ammonia gas is mixed with combustion exhaust gas. It is.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, the decomposition temperature of a low-temperature decomposable compound used as an ammonia generating source of a reducing agent is 59 ° C. for ammonium carbamate, 58 ° C. for ammonium carbonate, and 35 to 60 ° C. for ammonium bicarbonate. The heating energy can be greatly reduced as compared with the case where the boiling point of urea is 192 ° C. Since these compounds are solid at room temperature, the volumetric efficiency is 100%. In addition, since it is decomposed by heating to generate ammonia and carbon dioxide, a hydrolysis catalyst is not required. Further, since the gas is completely gasified, there is no risk of clogging, and the flow rate can be easily controlled. These will fully satisfy the above objectives. Further, the heat decomposition heat source of the ammonia generation source includes 1 after the catalytic reduction reaction.
No additional heating energy is required by utilizing the heat of the exhaust gas at 00 to 200 ° C.

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, a flue gas 1 discharged from a combustion chamber is mixed with ammonia gas from an ammonia generator 8, enters a denitration reactor 2, and performs a selective reduction reaction in a catalyst layer to remove N.
Oxed. The reduction reaction is performed at 300 to 400 ° C.
The unreacted NOx of the combustion exhaust gas after the denitration reaction is measured by the NOx concentration meter 7, whereby the required amount of ammonia gas is calculated and controlled by the calculator 6 and the ammonia flow control valve 5. Further, a part of the combustion exhaust gas after the denitration reaction is divided and controlled in the exhaust gas branching device 3 and used for heating the ammonia generating substance in the ammonia generator to generate a necessary amount of ammonia.

[0010]

The present invention will be described more specifically with reference to the following examples.

Example 1 A 30 liter jacketed pressurized container was filled with 20 kg of ammonium carbamate. Nitrogen was passed through the jacket at 150 ° C. as a simulated exhaust gas for heating, and the internal temperature of the pressurized container was controlled at 70 ° C. to thermally decompose ammonium carbamate to generate ammonia and carbon dioxide. On the other hand, NO: 1800 pp
A simulated exhaust gas having a composition of m, O ₂ : 5%, H ₂ O: 1.3%, N ₂ : balance amount was used. Exhaust gas temperature is 300
° C. The exhaust gas flow rate was 500 Nm ³ / h, and 20 liters of a vanadium-tungsten-based catalyst was filled as a denitration catalyst. The ammonia generated earlier in this exhaust gas
The mixed gas of carbon dioxide was continuously supplied to the denitration catalyst layer. The amount of the ammonia-carbon dioxide mixed gas is determined by the molar ratio NH ₃ / NO between the NO amount and the ammonia amount in the exhaust gas.
It controlled so that NO might be set to 1.0. The temperature of the denitration catalyst layer was 350 ° C. The NO concentration in the exhaust gas after denitration was 450 ppm, the NOx removal rate was 75%, and stable operation was possible for 10 hours.

Example 2 A test was conducted in the same manner as in Example 1 except that the ammonia generating substance was ammonium carbonate. NOx removal rate is 73
%, And stable operation could be performed for 10 hours.

Example 3 A test was conducted in the same manner as in Example 1 except that the ammonia generating substance was ammonium bicarbonate. NOx removal rate is 7
It was 2%, and stable operation could be performed for 10 hours.

[0014]

According to the method of the present invention, the volumetric efficiency of the ammonia generating source as the reducing agent is good, there is no risk of clogging of the reducing agent, and there is no need for a hydrolysis catalyst and ammonia is generated with low energy. Thus, the flow rate can be easily controlled, and a system suitable for an internal combustion engine such as a vehicle can be provided.

[Brief description of the drawings]

FIG. 1 is a flow chart showing one embodiment of a method for removing nitrogen oxides from combustion exhaust gas according to the present invention.

[Explanation of symbols]

 1 ···· Combustion exhaust gas 2 ···· Denitration reactor 3 ···· Exhaust gas branch device 4 ···· DeNOx exhaust gas 5 ····· Ammonia flow rate control valve 6・ Calculator 7 ・ ・ ・ ・ ・ ・ ・ NOx total 8 ・ ・ ・ ・ ・ ・ ・ Ammonia generator

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 3G091 AA02 AB04 AB05 BA14 BA36 CA08 CA17 DA01 DA02 DB10 DB11 GB01W 4D048 AA06 AB02 AC03 AC09 BA23X BA27X CC53 4G069 AA02 AA04 BC54B BC60B CA03 CA08 CA13 CD04

Claims (2)

[Claims] 1. A method for removing nitrogen oxides by catalytically reducing and removing nitrogen oxides in the presence of a catalyst by mixing ammonia gas with a combustion exhaust gas containing nitrogen oxides. A method for removing nitrogen oxides from combustion exhaust gas, comprising subjecting at least one type to thermal decomposition and mixing the generated ammonia gas with the combustion exhaust gas. 2. The method according to claim 1, wherein the flue gas after the catalytic reduction reaction is used as a heat source for thermal decomposition of ammonium carbamate, ammonium carbonate and ammonium bicarbonate. Removal method.