JPH04305227A - Electrochemical exhaust gas treating system - Google Patents

Abstract

PURPOSE:To reduce the NOx incorporated in exhaust gases so as to make the gases pollution-free and to simultaneously generate power by installing an anode and a cathode and a carbonic acid ion conductive electrolyte and separators interposed therebetween, supplying exhaust gases to the cathode side and supplying a reducing gas to the anode side. CONSTITUTION:The anode 2 and cathode 3 are disposed to hold the carbonic acid ion conductive electrolyte 1 therebetween and the anode separator 4 and cathode separator 5 which supply reactive gases to the anode 2 and the cathode 3, respectively, are provided to constitute a unit cell. NOx and CO2-contg. gases 7 are supplied to the cathode 3 side of the cell and the reducing gas 6, such as H2, is supplied to the anode 2. The harmful gases, such as NOx, contained in industrial waste gases, are treated by an electrochemical reaction and are made pollution-free. The electrons generated by the electrochemical oxidation reduction reaction are taken to the outside and are converted to electric energy. In addition, the CO2 in the exhaust gases is concentrated and is efficiently removed.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention treats harmful gases such as nitrogen oxides (NOx) contained in industrial exhaust gas by electrochemical reactions to render them harmless, and also removes electrons generated by electrochemical redox reactions. The present invention relates to an electrochemical exhaust gas treatment system that is suitable for extracting CO2 into an external circuit and converting it into electrical energy, as well as concentrating and efficiently removing CO2 in combustion exhaust gas.

0002

[Prior art] Nitrogen oxides (N) contained in combustion exhaust gas, etc.
When emitted into the atmosphere, substances such as Ox) become the cause of acid rain and photochemical smock, and strict emission regulations have been established due to pollution and global environmental issues. Also, carbon dioxide gas (CO2
), etc., are attracting attention as substances that cause global warming and ozone layer destruction, and reducing their emissions has become an urgent issue.

[0003] Thermal power plant exhaust gas, high-temperature gas turbine exhaust gas, and the like contain several hundred ppm to several thousand ppm of NOx, and on the other hand, contain several percent to several tens of percent of CO2. Conventionally,
The methods proposed for removing NOx from exhaust gas can be roughly divided into wet methods and dry methods. The wet method is mainly an absorption adsorption method, in which NO is oxidized to NO2 and absorbed and dissolved in an alkaline aqueous solution. In this case, oxidation is O in the gas phase.
3 and ClO2 are used, and in the liquid phase KMnO4,
This is carried out using NaClO2 or HNO3. In addition, absorption methods using iron(II) complex solutions have also been proposed, but all absorption methods have problems in the post-treatment of the absorption solution.

On the other hand, a molten salt absorption method using the reaction between alkali molten salt and NO has also been proposed (Pollution Prevention Equipment and Equipment Editorial Committee: Dictionary of Pollution Prevention Equipment and Equipment, p. 459), but NO is oxidized to NO2. In addition, a high denitrification rate cannot be obtained due to problems such as a low oxidation rate for gases containing low concentration NOX such as combustion exhaust gas.
It is not effective as a method for treating large volumes of exhaust gas generated by absorbing and regenerating the gas in a molten state.

[0005] The mainstream dry methods are catalytic cracking and selective reduction, and the former involves direct catalytic decomposition of NOx using a Co3O4, YBa2 Cu3Oy catalyst. The latter is a selective reduction denitrification method using ammonia using a VO-TiO catalyst. Further, a method of reducing NO2 to NO through an electrochemical reaction is proposed in Japanese Patent Application Laid-Open No. 51-86735. According to the proposed method, NO
This method reduces NOx to NOx and then oxidizes it to NO2 again for recycling, and cannot reduce NOx in the combustion exhaust gas to make it harmless.

By the way, NOx in general boiler exhaust gas
Of these, NO2 is several ppm to several tens of ppm, NO is several tens of ppm to several hundred ppm, and NO is present in overwhelmingly large amounts, so the proposed method is suitable as a method for detoxifying NOx in combustion exhaust gas. Not. Furthermore, in order to reduce NOX and SOX in exhaust gas from thermal power plants, etc.
tanleyH. have attempted to introduce it into the cathode side of a proton-conducting electrolyte fuel cell using an acidic electrolyte (Environmental Pro
gress (Vol. 5, No. 4). In this method,
It is necessary to use an expensive noble metal group catalyst in the electrode, which is difficult to implement in large-capacity exhaust gas treatment.

As described above, various flue gas denitrification methods have been proposed, but only the selective reduction denitrification method using ammonia is currently in practical use. On the other hand, CO2 treatment methods include the alkali absorption method, amine absorption method, calcium oxide adsorption method, and zeolite adsorption method, but gas treatment is not easy when the amount of exhaust gas is large, such as thermal power plant exhaust gas, and a new concentration method is required. Development of technology is desired.

[0008]

[Problem to be Solved by the Invention] NOx, CO2, SOx, etc. contained in industrial exhaust gas etc. must be removed as much as possible in view of pollution problems and global environmental problems. The purpose of the present invention is to electrochemically reduce NOx contained in exhaust gas, etc., to make it harmless using a system similar to a molten carbonate fuel cell, and at the same time extract electrons generated by the electrochemical reaction to an external circuit. In addition to generating electricity using electrochemical reactions, the objective is to efficiently remove CO2 from large volumes of exhaust gas by utilizing the CO2 concentration effect associated with electrochemical reactions.

[0009]

[Means for Solving the Problems] The above object is, as shown in FIG.
and a cathode 3, and an anode separator 4 and a cathode separator 5 for supplying reaction gas to the anode 2 and the cathode 3, respectively, constitute a unit cell, and the cathode side of the cell to NOx and CO2
A containing gas 7 is supplied, and a reducing gas 6 such as H2 is supplied to the anode, as shown in the following formulas (Chemical Formula 1) to (Chemical Formula 3) or (Chemical Formula 4) to (Chemical Formula 6) This can be achieved by performing an electrochemical redox reaction.

1) When NOx is NO, cathode: NO
+CO2 +2e──→CO32− +1/2N2−−
------- (Chemical formula 1) Anode: H2 + CO3
2- ────→H2 O+CO2 +2e------
--- (Chemical formula 2) Overall: NO + H2 ────
─→H2O+1/2N2−−−−−−−−−− (Chemical formula 3) 2) When NOx is NO2, cathode: NO2 +2CO2 +4e→2CO32−
+1/2N2---(Chemical formula 4) Anode: 2
H2 +2CO32- ──→2H2 O+2CO2
+4e--- (Chemical formula 5) Overall: NO2 +
2H2 ---→2H2 O+1/2N2----
---(Chemical 6) In other words, the cathode obtains electrons from the external circuit and reduces NOx to N2, and also CO2
oxidizes to CO32- ions. CO32- ions move to the anode via the electrolyte. At the anode,
CO32- ions are reduced to CO2 by H2 and generate electrons. These electrons are taken out to an external circuit and returned to the cathode. Here, the electron flow taken out to the external circuit is used as electrical energy by providing a load on the way.

[0011] When CO2 or NOx supplied to the cathode is mixed with a large amount of N2, etc., the N2, etc. that do not participate in the electrochemical reaction flow out of the system from the cathode outlet, and CO2 is selectively Since it moves to the anode side, a high concentration of CO2 is obtained at the anode outlet. Therefore, if this system is applied, CO2 can be substantially concentrated and CO2 treatment becomes easy.

By the way, this system is made possible by CO32- ion conduction from the cathode side to the anode side. In the present invention, carbonate is used as the ionically conductive electrolyte. As the carbonate, one or a mixed carbonate of two or more selected from lithium carbonate, potassium carbonate, sodium carbonate, barium carbonate, strontium carbonate, calcium carbonate, etc. is preferable, and a eutectic mixture with a melting point depression is particularly preferable. . By using an ion-conducting electrolyte fuel cell as the base, which uses carbonate as the electrolyte, it becomes possible to use base metals in the electrodes. In addition, because the battery operating temperature is high at around 650°C, high-temperature combustion exhaust gas can be directly introduced.
This is a desirable form as a system for effectively utilizing waste heat.

[0013] Furthermore, as a result of extensive studies on the cathode in order to efficiently perform the electrochemical reduction reaction of NOx at the cathode, the present inventors found that the porosity of the cathode was 50 to 80V.
ol% porous electronically conductive oxide was found to be effective. If the porosity is less than 50 vol%, the reaction gas will not be sufficiently diffused into the electrode, resulting in poor performance. Also, 80vo
When the amount exceeded 1%, the electrode strength significantly decreased, which was undesirable. This electronically conductive oxide has a first component of nickel oxide or cobalt oxide, and a second component of one or two selected from silver, lithium, chromium, copper, iron, and aluminum. A composite oxide electrode formed containing more than one metal or oxide showed good performance. The amount of the second component added is 0.5 to 20 wt% (wt%
) is good, and if it is less than 0.5 wt%, the effect of addition is small;
When the content exceeds 0 wt%, an increase in electrical resistance and a decrease in electrode activity were observed for components other than silver. When silver exceeds 20 wt%, a slight decrease in electrical resistance is observed, but there is no particular change in electrode activity.

On the other hand, a porous metal electrode or a porous composite electrode with a porosity of 40 to 75 vol% was effective as an electrode for efficiently carrying out the electrochemical reduction reaction of CO32- ions at the anode. . Porosity is 40vo
If it was less than 1%, the reaction gas would not be sufficiently diffused into the electrode, resulting in poor performance. Moreover, if it exceeds 75 vol%, the voids in the electrode will increase, the amount of electrode catalyst per unit area will decrease, and the reaction will not proceed sufficiently. As for the material of the porous metal electrode or porous composite electrode, the first component is nickel or copper, and the second component is one or more metals selected from aluminum, zirconium, magnesium, and yttrium. or formed by oxides. The purpose of adding the second component here is to stably exhibit electrode activity over a long period of time, and an alloy or an intermetallic compound consisting of the first component and the second component is preferable, but the second component is in the form of an oxide. It may also be a composite electrode dispersed within the first component. The amount of these second components added is 2 to 50 wt%, preferably 2 to 20 wt%. If the amount is less than 2 wt%, the purpose of addition will not be achieved. If it exceeds 50 wt%, the electrode activity may decrease, which is not preferable.

[0015] By the way, general combustion exhaust gas, etc. contains NO.
In addition to x and CO2, 3 to 10 vol% of O2 coexists. The coexistence of O2 is rather advantageous for obtaining high electrical energy. When such NOx, CO2 and O2 are mixed, it is assumed that the electrochemical redox reaction at the cathode proceeds as shown in the following equation. NO+1/2O2+2CO2+4e─→2CO32
- +1/2N2--(Chemical formula 7) The oxidation reaction of CO2 by NOx and O2 is as shown in the following equation, and the free energy of formation (ΔG) in this reaction is expressed as JanafTh
The results obtained using the emochemical Tables values are as follows.

2NO+2CO2 +4e ---→2CO32-
+N −−−−−−−−−−−−−− (Chemical formula 8)
NO2 +2CO2 +4e ──→2CO32-
+1/2N −−−−−−−−(Chemical formula 9) O2 +
2CO2 +4e ───→2CO32− −−−−
−−−−−−−−−−−−−−−−(Chemical formula 10) The free energy of formation (ΔG) is expressed by the formula (Chemical formula 8)−−−−−−−−−−ΔG: 21.6k
cal/mol (Chemical formula 9) ----------Δ
G: 29.8 kcal/mol (Chemical formula 10)---
-------ΔG: 40.5kcal/mol

(ΔG at 650°C) If we assume that the smaller the value of ΔG, the easier it is to proceed in equilibrium, it is predicted that the electrochemical oxidation reaction of CO2 will take priority in the order of NO>NO2>O2, resulting in selective reduction of NOx. The reaction progresses.

In the electrochemical harmful gas treatment system of the present invention, in order to extract a large amount of electrical energy, simply introducing the combustion exhaust gas directly into the cathode may not be stoichiometrically sufficient. In such a case, the problem can be solved by supplementing the insufficient gas from outside. In addition, if NOx and O2, which are necessary to oxidize CO2 to CO32- ions, do not coexist in the combustion exhaust gas, air 9 or the like is introduced from the outside into the combustion exhaust gas 8 on the cathode side as shown in Fig. 2. It is effective to supply the The amount of O2 supplied varies somewhat depending on the amount of NOx present, but an amount equivalent to 1/2 mole of CO2 in the combustion exhaust gas is sufficient. Further, as shown in FIG. 3, the objective can also be achieved by adding combustion exhaust gas 8 to air 9 and supplying it to the cathode.

By the way, at the anode, a reducing gas such as H2 or CO is supplied to electrochemically reduce CO32- ions to CO2. As the anode gas, H2-rich gas obtained by reforming natural gas or naphtha, or coal gasification gas containing a large amount of CO can be used. As mentioned above, the anode exhaust gas contains a high concentration of CO2, so by installing a CO2 device in the anode gas exhaust line as shown in Figure 4, it becomes possible to efficiently recover CO2 in the combustion exhaust gas. . The recovered CO2 is a casso-
It is preferable to return the waste to the power side for effective use in order to improve power generation efficiency.

The CO2 recovery method is not particularly limited, and conventional alkali absorption methods, calcium carbonate adsorption methods, PSA methods, etc. can be applied. Combustion exhaust gas generally contains CO2, O2, NOx, N2, etc., as well as SOx.
Contains dust. It is preferable to remove SOx and dust that have an adverse effect on the cathode before supplying them to the cathode side.

[0020]

[Operation] A cell is constructed by arranging an ion-conducting electrolyte between an anode and a cathode, and harmful gases including nitrogen oxides (NOx) and CO2 are supplied to the cathode side. - A reducing gas such as H2 is supplied to the side to promote an electrochemical redox reaction to remove NOx, etc.
At the same time, the electrons generated by the redox reaction are taken out to an external circuit and converted into electrical energy.

[0021] In addition, as another method of utilizing the present invention, by applying an oxidation active catalyst to the cathode, it is also possible to electrochemically oxidize NO and absorb it into molten carbonate to remove NO from exhaust gas. It is.

[0022]

【Example】

[0023]

[Example 1] 10wt with a porosity of 70vol% on the anode
%Al-remaining Ni electrode, the cathode has a porosity of 75vo.
A porous lithium aluminate plate with a thickness of 1.7 mm was impregnated with Li2 CO3 + K2 CO3 (62:38 molar ratio) mixed carbonate as the electrolyte, and the electrode area was A single cell of 64 cm2 was constructed. After heating this cell to 650°C, the cathode
A power generation test was conducted by supplying a mixed gas of 0 vol% NO-20 vol% CO2-residual N2 to the anode, and a mixed gas of 50 vol% H2-residual N2 to the anode at a rate of 500 ml/min. Figure 5 shows the results of determining the relationship between load current density and voltage.
Shown below. A mixed gas of NO and CO2 was supplied to the cathode, and an electrochemical reaction proceeded to obtain electrical energy.

[0024]

[Example 2] During a power generation test using the cell used in Example 1, the gas at the cathode outlet was measured using a chemiluminescence analyzer to determine NOx.
A quantitative analysis was conducted. The analysis results are shown in Table 1, and a high denitrification rate was obtained.

[0025]

[Table 1]

[0026]

[Example 3] In this example, a cathode electrode was studied. A porous Ni sintered plate was impregnated with silver nitrate, lithium nitrate, chromium nitrate, copper nitrate, iron nitrate, and aluminum nitrate in an amount equivalent to 5 wt% each as a metal oxide as a second component, dried, and then heated at 800°C in air. Bake for 1 hour 2
A component-based NiO+α electrode was created. These electrodes had a thickness of about 0.7 mm and a porosity of 68 to 71 vol%. These electrodes are used as cathodes, and the anode is made of 10 wt% Al-
Using residual Ni electrode, Li2 CO3 + K2 as electrolyte
CO3 (62:38 molar ratio) mixed carbonate to a thickness of 1.9
A single cell with an electrode area of 64 cm2 was constructed by impregnating a porous lithium aluminate plate with a thickness of 64 cm2. Set this cell to 650
After raising the temperature to ℃, 20vol%NO-20v was applied to the cathode.
ol% CO2 - remaining N2 mixed gas, 5% to the anode
0 vol% H2 - remaining N2 mixed gas at 500% each
A power generation test was conducted by supplying ml/min. Load current density 9
Cell voltage at 0 mA/cm and N at cathode exit
Table 2 shows the results of analyzing the O2 concentration.

[0027]

[Table 2]

[0028]

[Example 4] In this example, an anode electrode was studied. A porous nickel sintered plate was impregnated with aluminum nitrate, zirconyl nitrate, magnesium nitrate, and yttrium nitrate in an amount equivalent to 8 wt% as metal oxides as the second component, dried, and then heated at 800°C for 0.5 hours in an H2 atmosphere. Fired. The thickness of these electrodes was about 0.6 mm, and the porosity was 65 to 69 vol%. Using these electrodes as anodes, the cathode has a thickness of about 0.7 mm and a porosity of 70.
A porous lithium aluminate plate with a thickness of 1.9 mm was impregnated with a mixed carbonate of Li2 CO3 + K2 CO3 (62:38 molar ratio) as an electrolyte, and an electrode area of 64 cm2 was prepared using vol% 5wt% Ag-residue NiO. A single cell was constructed. After raising the temperature of this cell to 650°C in the same manner as in Example 3, 20 vol% NO-20 vol% CO was added to the cathode.
2 - Residual N2 mixed gas, 50 vol% to the anode
H2 - remaining N2 mixed gas at 500ml/mi each
A power generation test was conducted by supplying n. Load current density 90mA/c
Table 3 shows the results of analyzing the cell voltage and NOx concentration at the cathode outlet.

[0029]

[Table 3]

[0030]

[Example 5] 10wt with a porosity of 70vol% on the anode
%Al-remaining Ni electrode, the cathode has a porosity of 75vo.
A porous lithium aluminate plate with a thickness of 1.7 mm was impregnated with a mixed carbonate of Li2 CO3 + K2 CO3 (62:38 molar ratio) as an electrolyte using a 1% 5 wt% Ag-remaining NiO electrode. A single cell with an electrode area of 100 cm2 was constructed.

After heating this cell to 650°C, the cathode
2vol%NO-14vol%O-30vol
%CO2 - residual N2 mixed gas at 460ml/min,
A power generation test was conducted by supplying a mixed gas of 80 vol% H2 and residual N2 to the anode at a rate of 164 ml/min. Cell voltage at load current density 150 mA/cm, NO concentration of cathode outlet gas, and CO of anode outlet gas
2 The concentration (dry base) measurement results are shown in Table 4. 12
Watt electrical output and 99% denitrification rate as well as 62% CO
2 concentration gas was obtained.

[0032]

[Table 4]

[0033]

[Example 6] 2 wt% of porosity of 65 vol% on the anode
Using a MgO-remaining Ni electrode, the cathode has a porosity of 75vo.
Li2 CO3 + K2 CO3 (62:38
A porous lithium aluminate plate with a thickness of 1.5 mm was impregnated with the mixed carbonate (molar ratio) to form a single cell with an electrode area of 100 cm2.

After heating this cell to 650°C, the cathode
0.012vol%NO simulating boiler exhaust gas
-7vol%O2 -12vol%CO2 -Remaining N2
Mixed gas 1150ml/min, LNG for anode
70vol%H2-11vol% as reformed simulated gas
A power generation test was conducted by supplying a H2O-residual N2 mixed gas at a rate of 187 ml/min.

Table 5 shows the results of measuring the cell voltage and the NO concentration of the cathode outlet gas at a load current density of 150 mA/cm2. A cell voltage of 0.75V and a denitrification rate of 99% or more were obtained.

[0036]

[Table 5]

[0037]

[Embodiment 7] FIG. 6 shows an example in which the electrochemical exhaust gas treatment system of the type shown in Examples 1 to 6 is used as an electrochemical treatment system for combustion exhaust gas of a thermal power plant. As shown in the figure, this system introduces fuel 14 and air 19 into a boiler 15 and burns them to generate steam to operate a steam turbine 21. After the combustion exhaust gas is purified by a dust collector 16 and a desulfurizer 17, it is introduced into the cathode 3 of the cell. On the other hand, H2-rich gas produced by the fuel reformer 18 is introduced into the anode 2 of the cell to allow an electrochemical redox reaction to proceed and to extract electric current. A CO2 removal device 11 is installed in the anode discharge line 10.
Collect O2. Some of the CO2 recovered is CO2
The gas is refluxed to the cathode 3 via a gas supply line 13. Air 19 is also added to the combustion exhaust gas via an air make-up line 20. This system eliminates nitrogen oxides (NO) contained in exhaust gas.
It processes harmful gases such as It can be removed efficiently.

[0038]

Embodiment 8 FIG. 7 similarly shows a system for electrochemically treating exhaust gas from a high temperature gas turbine. Air 19 is pressurized by a compressor 24 and introduced together with fuel 14 into a high-temperature gas turbine 22 to drive a generator 23, and the combustion exhaust gas is introduced into the cathode 3 of the cell. On the other hand, fuel reformer 1
The H2 rich gas produced in step 8 is applied to the anode 2 of the cell.
The electrochemical oxidation-reduction reaction proceeds and current is extracted. A CO2 removal device 11 is provided in the anode discharge line 10 to recover CO2. A portion of the recovered CO2 is refluxed to the cathode 3 via the CO2 gas supply line 13. Air 19 is also added to the combustion exhaust gas via an air make-up line 20. With this system, nitrogen oxides (N
Ox) NO in high-temperature gas turbine exhaust gas with high content
It detoxifies harmful gases such as x by electrochemical reactions, takes electrons generated by electrochemical redox reactions to an external circuit, converts them into electrical energy, and condenses CO2 in combustion exhaust gas for efficient removal. I can do that.

[0039]

Effects of the Invention According to the present invention, harmful gases such as nitrogen oxides (NOx) contained in industrial exhaust gas are treated by electrochemical reactions to render them harmless, and electrons generated by electrochemical redox reactions are The CO2 in the combustion exhaust gas can be extracted and converted into electrical energy by an external circuit, and can be efficiently removed by concentrating it.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of a unit cell used in the exhaust gas treatment system of the present invention.

FIG. 2 is a schematic cross-sectional view of a unit cell when air is added to combustion exhaust gas.

FIG. 3 is a schematic cross-sectional view of a unit cell when combustion exhaust gas is added to air.

FIG. 4 is a diagram showing the overall configuration of the exhaust gas treatment system of the present invention.

FIG. 5 is a diagram showing the relationship between load current density and voltage of a test cell.

FIG. 6 is a diagram showing an electrochemical treatment system for thermal power plant combustion exhaust gas according to the present invention.

FIG. 7 shows an electrochemical treatment system for high temperature gas turbine exhaust gas according to the present invention.

Claims (11)

[Claims] Claim 1: A cell comprising an anode and a cathode, a carbonate ion conductive electrolyte interposed therebetween, and a separator for supplying a reaction gas to the anode and cathode. , on the cathode side, nitrogen oxides (NOx
) and carbon dioxide (CO2), and a reducing gas is supplied to the anode side to proceed with an electrochemical redox reaction to reduce NOx to N2 and reduce NOx generated by the redox reaction. An electrochemical exhaust gas treatment system characterized by extracting electrons from the exhaust gas into an external circuit and converting them into electrical energy. [Claim 2] The carbonate ion conductive electrolyte is lithium carbonate, potassium carbonate, sodium carbonate, barium carbonate,
2. The electrochemical exhaust gas treatment system according to claim 1, comprising a molten carbonate electrolyte composed of one or more mixed carbonates selected from strontium carbonate and calcium carbonate. 3. Claim 1 or 2, characterized in that a porous electrically conductive oxide electrode is used for the cathode, and a porous metal electrode or a porous composite electrode is used for the anode.
Electrochemical exhaust gas treatment system as described. 4. The porous electrically conductive oxide electrode of the cathode comprises a first component consisting of nickel oxide and one or two selected from silver, lithium, chromium, copper, iron, aluminum, and cobalt. 4. The electrochemical exhaust gas treatment system according to claim 3, wherein the second component is made of one or more metals or oxides. 5. The porous metal electrode or porous composite electrode of the anode comprises a first component consisting of nickel or copper and one or more metals selected from aluminum, zirconium, magnesium, and yttrium, or 4. The electrochemical exhaust gas treatment system according to claim 3, characterized in that it is formed by a second component consisting of an oxide. [Claim 6] On the cathode side, NOx, CO2,
O2-containing gas is supplied, and H2 and CO-containing gas are supplied to the anode side, and in a temperature range of 500 to 800°C, an electrochemical reduction reaction of NOx and CO2 occurs at the cathode.
The electrochemical oxidation reaction of CO proceeds at the anode.
32- The electrochemical exhaust gas treatment system according to any one of claims 1 to 5, wherein NOx is rendered harmless by proceeding with an electrochemical reduction reaction of ions. 7. Combustion exhaust gas containing CO2 and NOx is mixed with O2 gas in an amount equivalent to at least 1/2 mole relative to CO2, and the mixture is supplied to the cathode to cause an electrochemical reaction between NOx, O2, and CO2. NOx
At the same time, CO2 is reduced and rendered harmless, and CO2 is oxidized to CO32- ions, which conduct the ions to the anode side.
The electrochemical reduction reaction of CO22- ions by H2 and CO progresses in the deionizer, concentrating them as CO2, and removing CO2 from the anode exhaust line. At the same time, the electrons generated by the redox reaction are taken out to an external circuit and converted into electrical energy. An electrochemical exhaust gas treatment system characterized by converting. [Claim 8] The combustion exhaust gas containing CO2 and NOx is a thermal power plant combustion exhaust gas,
The electrochemical exhaust gas treatment system according to claim 7. 9. The electrochemical exhaust gas treatment system according to claim 7, wherein the combustion exhaust gas containing CO2 and NOx is high temperature exhaust gas from a gas turbine. 10. The electrochemical exhaust gas treatment system according to claim 7, wherein the combustion exhaust gas containing CO2 and NOx is exhaust gas from a chemical plant or a nuclear fuel reprocessing plant. 11. The electrochemical exhaust gas treatment system according to claim 7, wherein sulfur oxides (SOx) contained in the combustion exhaust gas are removed and then supplied to the cathode.