JP2004041965A - System for removing nitrogen oxide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a system for decomposing or removing nitrogen oxide contained in the exhaust gas from a combustion equipment with an electrochemical cell using a solid electrolytic membrane from the start-up of a combustion equipment. <P>SOLUTION: In the method and the system for decomposing or removing nitrogen oxide, the exhaust gas from the combustion equipment is pretreated by using a nitrogen oxide adsorbing material for adsorbing nitrogen oxide in a low temperature region until the temperature of the exhaust gas rises and releasing the adsorbed nitrogen oxide in a high temperature region after the temperature of the exhaust gas is risen. The pretreated exhaust gas is treated by the electrochemical cell. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for removing nitrogen oxides for purifying nitrogen oxides and a system for removing the same. More specifically, for example, the present invention relates to, for example, a combustion engine such as a lean engine or a diesel engine that frequently starts and stops. The present invention relates to a nitrogen oxide removal method and a nitrogen oxide removal method capable of reliably removing nitrogen oxides in exhaust gas immediately after starting a combustor at a low temperature of an exhaust gas.
[0002]
[Prior art]
In general, the method of purifying nitrogen oxides generated from a gasoline engine is mainly a method using a three-way catalyst. However, in lean-burn engines and diesel engines that can improve fuel efficiency, the excess amount of oxygen in the combustion exhaust gas causes a problem of a sharp decrease in catalytic activity due to the adsorption of oxygen to the three-way catalyst. It cannot be purified. On the other hand, using a solid electrolyte membrane having oxygen ion conductivity and passing a current therethrough, oxygen in exhaust gas is removed without being adsorbed on the catalyst surface. Further, as a catalyst reactor proposed, for example, a system that removes surface oxygen by simultaneously applying a voltage to a solid electrolyte sandwiched between electrodes on both sides to simultaneously decompose nitrogen oxides into oxygen and nitrogen. Are known.
[0003]
Here, some prior art documents are presented. Electrochemical Soc. , 122, 869 (1975) show that platinum electrodes are formed on both surfaces of zirconia stabilized with scandium oxide and decomposed into nitrogen oxides and oxygen by applying a voltage. Also, J.I. Chem. Soc. Faraday Trans. , 91, 1995 (1995), a palladium electrode is formed on both surfaces of zirconia stabilized with yttrium oxide, and by applying a voltage, nitrogen and nitrogen are mixed in a mixed gas of nitrogen oxide, hydrocarbon and oxygen. It has been shown to decompose into oxygen.
[0004]
However, in an electrochemical cell using a solid electrolyte membrane, nitrogen oxides can be decomposed or removed only by applying a voltage, but in order to increase the ionic conductivity of the solid electrolyte, the temperature must be raised to 400 ° C. or higher. There is a problem. In addition, particularly when the temperature of the exhaust gas is low immediately after the start of the combustor, the electrochemical cell does not exhibit sufficient performance and has a problem that nitrogen oxides cannot be temporarily removed. This is an important issue for lean engines, diesel engines, etc.
[0005]
[Problems to be solved by the invention]
In such a situation, the present inventors, in view of the above prior art, have drastically solved the problems in the above conventional art, In the course of intensive research with the aim of developing a new nitrogen oxide removal method and a removal system capable of reliably removing nitrogen oxides, nitrogen oxides are kept at a low temperature range from room temperature to 400 ° C. That the intended purpose can be achieved by providing a nitrogen oxide adsorbing section comprising a nitrogen oxide adsorbing material that adsorbs nitrogen and releases nitrogen oxide in a high temperature region exceeding 400 ° C. in an upstream portion of the electrochemical cell. After further heading and further research, the present invention was completed.
That is, the present invention has been made as a technical problem to improve the problem in the technology for removing nitrogen oxides in exhaust gas by the above-described electrochemical cell, and it is intended that when the exhaust gas immediately after the start of the combustor has a low temperature. It is an object of the present invention to provide a method for reliably removing nitrogen oxides from the exhaust gas from the exhaust gas and a system for removing the same.
[0006]
[Means for Solving the Problems]
The present invention for solving the above problems is constituted by the following technical means.
(1) A method for removing nitrogen oxides in exhaust gas by an electrochemical cell for decomposing or removing nitrogen oxides,
(A) Nitrogen oxidation that adsorbs nitrogen oxide in the exhaust gas from the combustor in a low temperature region before the temperature of the exhaust gas rises and releases nitrogen oxide in a high temperature region after the temperature of the exhaust gas rises Pretreatment using a material adsorbent,
(B) treating the pretreated exhaust gas in an electrochemical cell;
A method for removing nitrogen oxides.
(2) pretreating with a nitrogen oxide adsorbent that adsorbs nitrogen oxides in a low temperature range from room temperature to the operating temperature of the electrochemical cell and releases nitrogen oxides in a high temperature range equal to or higher than the operating temperature; The method for removing nitrogen oxides according to (1).
(3) The pretreatment according to the above (2), wherein the nitrogen oxide is adsorbed in a low temperature range from room temperature to 400 ° C., and the pretreatment is performed using a nitrogen oxide adsorbent which releases nitrogen oxide in a high temperature range exceeding 400 ° C. A method for removing nitrogen oxides.
(4) In an electrochemical cell section composed of an electrochemical cell for decomposing or removing nitrogen oxides, a nitrogen oxide adsorbing section composed of a nitrogen oxide adsorbing material is provided upstream of the electrochemical cell. Nitrogen oxide removal system.
(5) A device for decomposing or removing nitrogen oxides by using an electrochemical cell composed of at least three layers of a solid electrolyte of an oxygen ion conductor, a cathode, and an anode, wherein a gas inlet portion of the device is used. The nitrogen oxide removing system according to the above (4), wherein a nitrogen oxide adsorbing section is provided in the system.
(6) a nitrogen oxide adsorbing section that adsorbs nitrogen oxides in a low temperature range from room temperature to the operating temperature of the electrochemical cell and releases nitrogen oxides in a high temperature range above the operating temperature; The nitrogen oxide removing system according to the above (4) or (5), wherein
(7) The nitrogen oxide adsorbing portion is made of a nitrogen oxide adsorbing material that adsorbs nitrogen oxide in a low temperature range from room temperature to 400 ° C. and releases nitrogen oxide in a high temperature range exceeding 400 ° C. The nitrogen oxide removing system according to the above (6).
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, the present invention will be described in more detail.
The method of the present invention is a method of removing nitrogen oxides in exhaust gas by an electrochemical cell that decomposes or removes nitrogen oxides.The exhaust gas from the combustor is used in advance until the temperature of the exhaust gas increases. Nitrogen oxides are adsorbed in a low temperature range, and the pretreatment is performed using a nitrogen oxide adsorbent that releases nitrogen oxides in a high temperature range after the temperature of the exhaust gas is increased. A method for removing nitrogen oxides. Further, in the system of the present invention, in the electrochemical cell section composed of an electrochemical cell for decomposing or removing nitrogen oxides, a nitrogen oxide adsorbing section composed of a nitrogen oxide adsorbing material is provided upstream of the electrochemical cell. A nitrogen oxide removing system.
[0008]
As the nitrogen oxide adsorbing material, nitrogen oxide adsorbing has a function of adsorbing nitrogen oxide in a low temperature range from room temperature to an operating temperature of an electrochemical cell and releasing nitrogen oxide in a high temperature range not lower than the operating temperature. For example, it is preferable to use a nitrogen oxide adsorbent that adsorbs nitrogen oxides in a low temperature range from room temperature to 400 ° C. and releases nitrogen oxides in a high temperature range exceeding 400 ° C. That is, when the temperature of the exhaust gas immediately after the start of the combustor is from room temperature to 400 ° C., the ion conductivity of the electrochemical cell is low because the temperature of the solid electrolyte of the electrochemical cell is low. Therefore, in the present invention, the nitrogen oxides in the exhaust gas are adsorbed by the nitrogen oxide adsorbent in a low temperature range from room temperature to 400 ° C., and the temperature of the exhaust gas rises and the nitrogen oxides in the high temperature range exceeding 400 ° C. By releasing the nitrogen oxides adsorbed on the oxide adsorbent, during that time, the heat also raises the temperature of the solid electrolyte of the electrochemical cell, increasing its ionic conductivity and reducing the nitrogen oxides. At this stage, the nitrogen oxides released from the nitrogen oxide adsorbent are decomposed in the electrochemical cell.
[0009]
In the present invention, the nitrogen oxide adsorbent used in the nitrogen oxide adsorbing section is preferably, for example, activated carbon, zeolite, silica gel, silica or alumina containing an alkali metal, silica or alumina containing an alkaline earth metal, Examples include basic diatomaceous earth, alkaline earth metal-containing copper oxide and iron oxide, transition metal-containing zirconia, and manganese oxide compounds. However, in the present invention, the nitrogen oxide adsorbing material is not limited to these, as long as it adsorbs nitrogen oxide at a predetermined temperature and releases nitrogen oxide at a predetermined temperature. Can be used as well. Further, in the present invention, these materials can be appropriately combined to construct and use a nitrogen oxide adsorbent having arbitrary adsorption and release characteristics.
[0010]
In the present invention, the form of the nitrogen oxide adsorbent used in the nitrogen oxide adsorbing section is preferably, but not limited to, a powder, a porous body, a foam, or a honeycomb. In the case of powder, the adsorbent can be used, for example, carried on a ceramic honeycomb or a metal honeycomb. Similarly, in the case of a porous body or a foamed body, these can be pulverized and supported on a honeycomb for use, but their use form is not particularly limited. In the present invention, the electrochemical cell includes at least three layers of a solid electrolyte of an oxygen ion conductor, a cathode, and an anode electrode, and applies a voltage between these electrodes to convert nitrogen oxides into electricity. As long as it has a function of chemically reducing to nitrogen and oxygen, an appropriate one is used. The decomposition of nitrogen oxides by the electrochemical cell depends on the oxygen ion conductivity of the solid electrolyte used. In the electrochemical cell, for example, when the temperature exceeds 400 ° C., the oxygen ion conductivity increases, The object can be sufficiently decomposed. However, in the low temperature range of 400 ° C. or lower when the exhaust gas immediately after the start of the combustor is low, the oxygen ion conductivity of the solid electrolyte is low and nitrogen oxides cannot be sufficiently decomposed. In the present invention, the nitrogen oxide adsorbent has a function of adsorbing / releasing nitrogen oxides in exhaust gas in consideration of the operating temperature of the electrochemical cell to be used and matching the operating temperature. It is desirable to select and use as appropriate.
[0011]
In the present invention, the solid electrolyte material of the oxygen ion conductor used in the electrochemical cell unit is not particularly limited as long as it has oxygen ion conductivity, but is preferably, for example, oxidized. Examples include zirconia stabilized with yttrium or scandium oxide, ceria and lanthanum gallate stabilized with gadolinium oxide or samarium oxide. Further, in the present invention, the cathode material used in the electrochemical cell section is not particularly limited as long as it has electron conductivity, but preferably, for example, gold, silver, platinum, palladium, Examples thereof include metals such as nickel, and metal oxides such as cobalt oxide, nickel oxide, copper oxide, lanthanum chromite, lanthanum manganite, and lanthanum cobaltite. Further, these may be used as a mixture of an electron conductive material and an ion conductive material, or a laminated structure.
[0012]
Further, in the present invention, the anode material used in the electrochemical cell portion is not particularly limited as long as it has electron conductivity, but preferably, for example, gold, silver, platinum, palladium, Examples thereof include metals such as nickel, and metal oxides such as cobalt oxide, nickel oxide, copper oxide, lanthanum chromite, lanthanum manganite, and lanthanum cobaltite. Further, these may be used as a mixture of an electron conductive material and an ion conductive material, or a laminated structure.
[0013]
The nitrogen oxide adsorption part and the electrochemical cell part which constitute the nitrogen oxide removal system of the present invention are preferably connected by, for example, an exhaust pipe. In this case, the interval at which the nitrogen oxide adsorption section and the electrochemical cell section are connected by the exhaust pipe can be arbitrarily adjusted according to the temperature distribution of the exhaust gas. Further, depending on the temperature of the exhaust gas, the nitrogen oxide adsorbing section and the electrochemical cell section may be housed in a unit in the same chamber and may be integrally formed, and these structures are not particularly limited. Further, these specific configurations are not particularly limited, and can be arbitrarily designed according to the purpose of use.
[0014]
[Action]
The present invention provides a method for adsorbing nitrogen oxides in advance in a low temperature region until the temperature of the exhaust gas rises, and releasing nitrogen oxides in a high temperature region after the temperature of the exhaust gas rises. After pretreatment using an oxide adsorbent, the pretreated exhaust gas is treated in an electrochemical cell. In the present invention, by adopting such a configuration, when the exhaust gas immediately after the start of the combustor is at a low temperature, the nitrogen oxides in the exhaust gas are adsorbed by the nitrogen oxide adsorbent, and the temperature of the exhaust gas rises and the electrochemical When the operating temperature of the cell has been reached, the nitrogen oxides are released from the adsorbent, whereby the nitrogen oxides in the exhaust gas can be reliably removed immediately after the start of the combustor. According to the present invention, by appropriately selecting and using a nitrogen oxide adsorbent having a predetermined nitrogen oxide adsorption / desorption characteristic, the exhaust gas immediately after the start of the combustor is cooled to a low temperature in the exhaust gas from the combustor. It is possible to remove nitrogen oxides with high accuracy and high efficiency, so that the emission of nitrogen oxides can be suppressed from the start of the combustor.
[0015]
【Example】
Next, the present invention will be specifically described based on examples, but the present invention is not limited to only these examples.
Example 1
(1) Configuration of Nitrogen Oxide Removal System FIG. 1 shows a system configuration diagram of a nitrogen oxide removal system including a nitrogen oxide adsorption section and an electrochemical cell section according to an embodiment of the present invention. Exhaust gas discharged from the combustor passes through the nitrogen oxide adsorption section 1 and is supplied to the electrochemical cell section 2. In the electrochemical cell section 2, in a high temperature region where the ionic conductivity of the solid electrolyte is high, nitrogen oxides in the introduced exhaust gas are decomposed and discharged as purified gas. While the temperature of the solid electrolyte in the electrochemical cell is low and the ionic conductivity is low, such as when the combustor is started, nitrogen oxides in the exhaust gas discharged from the combustor are absorbed by the nitrogen oxide adsorbing section 1, Reduces nitrogen oxide emissions. The nitrogen oxides absorbed by the nitrogen oxide adsorption unit 1 are released from the nitrogen oxide adsorption unit 1 when the temperature of the exhaust gas rises and reaches the operating temperature of the electrochemical cell unit 2. The released nitrogen oxides are supplied to the electrochemical cell unit 2 together with the nitrogen oxides in the exhaust gas, decomposed in the electrochemical cell unit 2, and discharged as purified gas.
[0016]
(2) Method for Removing Nitrogen Oxide The nitrogen oxide adsorbent of the nitrogen oxide adsorbing section 1 is a lithium silicate foam, the solid electrolyte of the electrochemical cell section 2 is zirconia stabilized with yttrium oxide, and the cathode is Nitrogen oxide purification experiments were performed using a composite of nickel oxide, nickel, platinum, and zirconia stabilized with yttrium oxide, and the anode as zirconia stabilized with platinum and yttrium oxide. Helium-balanced model exhaust gas containing 1000 ppm of nitric oxide and 3% of oxygen was flowed at a flow rate of 50 ml / min. The electrochemical cell section 2 has a nitrogen oxide purifying ability of 90% or more at 600 ° C. under the above conditions. While applying a voltage to the electrochemical cell, the temperature of the system was raised to 600 ° C. in 10 minutes, and the nitrogen oxide concentration of the outlet gas was measured with a chemiluminescence NOx meter.
[0017]
(3) Results By adopting the above configuration, a nitrogen oxide purification rate of 90% or more was obtained even in a low temperature range from normal temperature to 400 ° C. or less. On the other hand, as a comparative example, a gas was directly supplied to the electrochemical cell unit 2 without passing through the nitrogen oxide adsorption unit 1 and a similar experiment was performed. The purification rate gradually increased at a high temperature range of 0% or more, and reached 600 ° C., and the purification rate of nitrogen oxides exceeded 90%. These results indicate that the method and system of the present invention is useful as a technology for treating nitrogen oxides in exhaust gas, especially when the exhaust gas immediately after the start of the combustor is at a low temperature.
[0018]
【The invention's effect】
As described above in detail, the present invention relates to a method for removing nitrogen oxides and a system for removing the same, and the present invention has the following effects.
(1) Nitrogen oxides in exhaust gas from a combustor can be reliably removed.
(2) Nitrogen oxides in the exhaust gas can be treated immediately after the start of the combustor when the temperature of the exhaust gas is low.
(3) In the method and system for removing nitrogen oxides of the present invention, nitrogen oxides in exhaust gas from combustors such as phosphorus engines and diesel engines that frequently start and stop are removed from the time of startup. Can be.
(4) Therefore, the emission of nitrogen oxides from the combustor can be reliably suppressed not only in the steady operation but also in the non-steady operation, whereby the environmental load can be significantly reduced.
[Brief description of the drawings]
FIG. 1 is an example of a system configuration diagram of a nitrogen oxide removing system including a nitrogen oxide adsorbing section and an electrochemical cell section of the present invention.
[Explanation of symbols]
1 Nitrogen oxide adsorption section 2 Electrochemical cell section

Claims (7)

A method for removing nitrogen oxides in exhaust gas by an electrochemical cell that decomposes or removes nitrogen oxides,
(1) Nitrogen oxidation in which exhaust gas from a combustor is adsorbed in advance in a low temperature region until the temperature of the exhaust gas rises, and is released in a high temperature region after the exhaust gas temperature rises. Pretreatment using a material adsorbent,
(2) treating the pretreated exhaust gas in an electrochemical cell,
A method for removing nitrogen oxides. The pretreatment is performed using a nitrogen oxide adsorbent that adsorbs nitrogen oxides in a low temperature range from room temperature to an operating temperature of an electrochemical cell and releases nitrogen oxides in a high temperature range not less than the operating temperature. Method for removing nitrogen oxides. The nitrogen oxide according to claim 2, wherein the nitrogen oxide is adsorbed in a low temperature range from room temperature to 400 ° C and is pretreated using a nitrogen oxide adsorbent that releases nitrogen oxide in a high temperature range exceeding 400 ° C. Removal method. In an electrochemical cell section comprising an electrochemical cell for decomposing or removing nitrogen oxides, a nitrogen oxide adsorbing section comprising a nitrogen oxide adsorbing material is provided upstream of the electrochemical cell. Removal system. An apparatus for decomposing or removing nitrogen oxides using an electrochemical cell composed of at least three layers of a solid electrolyte of an oxygen ion conductor, a cathode, and an anode. The nitrogen oxide removal system according to claim 4, further comprising a substance adsorbing section. The nitrogen oxide adsorbing section is composed of a nitrogen oxide adsorbent that adsorbs nitrogen oxides in a low temperature range from room temperature to the operating temperature of the electrochemical cell and releases nitrogen oxides in a high temperature range above the operating temperature. The nitrogen oxide removing system according to claim 4 or 5, wherein: The nitrogen oxide adsorbing part is made of a nitrogen oxide adsorbing material that adsorbs nitrogen oxide in a low temperature range from room temperature to 400 ° C. and releases nitrogen oxide in a high temperature range exceeding 400 ° C. 7. The nitrogen oxide removal system according to 6.

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