KR20130035896A - Apparatus for absorbing acidic gas with high efficiency - Google Patents

Abstract

PURPOSE: Acid gas absorption equipment and method are provided to improve entire system efficiency, for example, increasing the absorption rate, reducing the size of an absorption tower, etc. CONSTITUTION: An acid gas absorption method includes the following steps: a step of absorbing acid gas at an absorption tower by using an absorbent; a step of regenerating the absorbent at a separation tower; a step of transferring the absorbent regenerated at the separation tower to the absorption tower through a regenerated absorbent transfer line; a step of dividing a part of the regenerated absorbent through a branch line connected to the regenerated absorbent transfer line; and a step of cooling the divided absorbent through a cooler installed at the branch line and injecting into the absorption tower. The flow quantity of the divided absorbent is controlled through a control valve installed at the branch line. The absorbent cooled at the branch line is injected into the middle part of the absorption tower or between packed layers installed at the absorption tower. A cooler is also installed at the regenerated absorbent transfer line, and cools the regenerated absorbent. [Reference numerals] (AA) Absorption tower upper part; (BB) Adding cooling equipment; (CC) Absorption tower lower part; (DD) Absorption solution; (EE) Temperature change; (FF) Without cooling equipment;

Description

[0001] The present invention relates to a high efficiency acid gas absorption apparatus and method,

The present invention relates to a high efficiency acid gas absorption apparatus and method, and more particularly relates to an absorption separation technique for separating only acid gas from an acid gas such as carbon dioxide mixed with other gases such as nitrogen, The present invention relates to a high efficiency acid gas absorption apparatus and method which improves the absorption tower efficiency by keeping the temperature distribution at a low temperature uniformly.

Since the beginning of industrialization in the early 19th century, the use of fossil fuels such as coal, petroleum, and LNG in the energy industry has increased the use of acid gases such as CO ₂ , CH ₄ , H ₂ S and COS (carbonyl sulfide) The concentration was increased rapidly. These acid gases, especially carbon dioxide, have been shown to warm the globe, and regulations for emissions and treatment are becoming more stringent worldwide. The United Nations Conference on Environment and Development in Rio in Brazil in June 1992 raised international interest in global warming and advanced countries including the United States and Japan agreed to cut global GHG emissions by 5.2% International agreements are underway to reduce acid gas emissions.

Techniques for reducing carbon dioxide emissions include energy saving technologies for reducing carbon dioxide emissions, carbon dioxide capture and storage (CCS) technologies, techniques for using or fixing carbon dioxide, Alternative energy technologies.

Among them, CCS technology is recognized as the most effective way to treat large volumes of greenhouse gases in power plants and industrial facilities. As a result, G-8, Intergovernmental Panel on Climate Change (IPCC) and International Energy Agency (IEA) The organization is actively encouraging the development and utilization of technology.

Among the CCS technologies, carbon dioxide capture technology is an important technology that accounts for a large part of the overall cost. As technology that has been studied so far, it is divided into preburning, burning, and postburning depending on the position of the carbon dioxide treatment. , An adsorption method, a membrane separation method, and a deep-cooling method.

Among them, the absorption method is suitable for gas separation of a large amount of gas and low concentration, so that it can be easily applied to most large industrial plants and power plants, and the process of ABB Lummus Crest Co., USA) and Shady Point (Oklahoma, USA), and related technologies are disclosed in Korean Patent Publication No. 2010-35335.

According to Henry's law, generally expressed by the following formula in the absorption method, the absorption of gas and the acid-alkali reaction are advantageous at a low temperature, and therefore it is desirable to maintain the temperature uniformly at a low temperature in a possible reactor, Do.

pA = HAxe

Here, pA is the partial pressure of the component A, HA is the Henry's constant, and xe is the liquid phase equilibrium composition.

On the other hand, in the absorber, reaction heat is generated during operation. Since the flow of the gas and the flow of the absorbent are opposite to each other, the temperature of the upper portion of the tower is relatively high, Which is an unfavorable factor.

It is an object of the present invention to solve the above-mentioned problems of the prior art, and it is an object of the present invention to improve the efficiency of the entire system by lowering the temperature of the absorption tower and increasing the temperature distribution, , An acid gas absorbing device and a method.

Means for Achieving the Object The object of the present invention is to provide an absorption tower in which an acid gas is absorbed using an absorbent; A stripping tower connected to the absorption tower and regenerating the absorbent; A regenerable absorbent feed line in which the regenerated absorbent in the stripping tower is conveyed to the absorber; A branch line branching from the regenerant absorbent transfer line to the absorption tower; And an absorbent cooler (Lean amine cooler) installed in the branch line.

It is preferable that the configuration of the present invention further includes a control valve provided on the branch line.

In the present invention, the branch line may be connected to an intermediate portion of the absorption tower, and the branch line may be connected between the filling layers provided in the absorption tower.

In addition to the cooler installed in the branch line in the present invention, a lean amine cooler may also be provided in the regenerated absorbent feed line.

As the cooling medium of the absorbent cooler in the present invention, cooling water may be used, and cooling water of the absorbent cooler may be cooled using seawater. That is, the absorbent can be cooled by an indirect cooling method using seawater. Particularly, in the case of a facility located on the beach, seawater is readily available as a good cooling source.

The apparatus for absorbing acid gas according to the present invention comprises a seawater supply source for indirect cooling of an absorbent using seawater; A seawater transfer line connected to a seawater source; A coolant transfer line connected to the sorbent cooler; And a cooling water cooling heat exchanger installed at the intersection of the sea water conveyance line and the cooling water conveyance line for cooling the cooling water of the absorbent cooler using seawater.

In the present invention, the absorbent is at least one selected from the group consisting of an amine type, an amino acid salt, an inorganic salt type solution, an aqueous ammonia solution and a metal ion salt solution, and the acid gas is at least one selected from CO ₂ , H ₂ S, SO ₂ , NO ₂ , More than species.

The present invention also relates to a method for producing an adsorbent, comprising the steps of: absorbing an acidic gas using an absorbent in an absorption tower; Regenerating the sorbent in the stripping tower; Transferring the regenerated absorbent in the demoulding tower to the absorption tower through a regenerant absorbent transfer line; Branching a portion of the absorbent regenerated through the branch line connected to the regenerant absorbent transfer line; And cooling the absorbent branched through the cooler installed in the branch line and injecting the absorbent into the absorption tower.

In the method according to the present invention, the flow rate of the absorbent branched through the control valve provided in the branch line can be adjusted.

In the method according to the present invention, the absorbent cooled in the branch line can be injected into the middle portion of the absorption tower, and the absorbent cooled in the branch line can be injected between the packed bed provided in the absorption tower.

In the process according to the invention, the regenerant absorbent can also be cooled by providing a cooler in the regenerant absorbent transfer line.

The method of absorbing acidic gas according to the present invention comprises the steps of: supplying seawater from a seawater supply source to a heat exchanger for cooling water through a seawater transfer line for indirect cooling of the absorbent using seawater; Supplying the cooling water used in the absorbent cooler to the heat exchanger for cooling water through the cooling water transfer line; Cooling the cooling water using seawater in a heat exchanger for cooling water; And feeding the cooled cooling water to the absorbent cooler

The present invention has the following effects.

First, by lowering the absorption temperature and inducing an even distribution of temperature in the absorption tower, it is possible to reduce the volume of the exhaust gas and increase the absorption rate in the absorption tower, thereby reducing the size of the absorption tower and cooling the exhaust gas at a high temperature. The installation cost can be reduced. The reaction rate in the upper and lower parts of the absorber is lowered to the upper part due to the CO ₂ concentration distribution (see FIG. 3, Gary T.rochelle and etc., "Rate-Based Process Modeling Study of CO2 Capture with Aqueous Monoethanolamine Solution" , Ind. Eng. Chem. Res., Vol. 48, 9233-9246, 2009), and the temperature distribution can be made uniform by injecting the fresh absorbent of the stripping tower (regenerating tower)

Second, since the temperature of the absorption liquid is low, more acid gas can be absorbed, so that the circulating flow of the absorption liquid can be reduced during the operation of the system, and the operation energy can be reduced.

Third, processing can be performed without cooling the exhaust gas flowing into the absorption tower, so that the related cooling facility and operation cost can be greatly saved and the site can be saved. In the conventional absorption process, the operation temperature of the absorption tower is 40 ° C., and it is impossible to directly process the exhaust gas temperature (around 53 ° C.) flowing into the absorption tower. Therefore, a cooling facility has to be additionally installed. However, when the technique of the present invention is used, the efficiency of the absorption tower is increased, and the treatment can be performed without cooling the exhaust gas, thereby reducing the cost.

Fourthly, it is possible to reduce the energy of removal. It is possible to achieve the desired acid gas removal efficiency even if the removal efficiency of the acid gas is relatively lowered in the demixing column because the absorption tower efficiency is increased.

Fifth, by indirectly cooling the absorbent with seawater, the absorbent can be improved in cooling efficiency.

1 is a configuration diagram of an acidic gas absorption apparatus according to an embodiment of the present invention.
FIG. 2 is a graph comparing the temperature distribution in the absorber with and without a cooling system.
3 is a graph showing the CO ₂ reaction rate according to the absorption tower height.
FIG. 4 illustrates an indirect cooling system for cooling the cooling water of an absorbent cooler using seawater.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, so that those skilled in the art can easily carry out the present invention. Other objects, features, and operational advantages, including the purpose, operation, and effect of the present invention will become more apparent from the description of the preferred embodiments.

It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory only and are not to be construed as limiting the scope of the invention as disclosed in the accompanying claims. It is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and similarities, many of which are within the scope of the present invention.

Also, terms and words used in the description and claims of the present invention are defined based on the principle that the inventor can properly define the concept of a term in order to explain its invention in the best way, And should not be construed as limited to only the prior art, and should be construed in a meaning and concept consistent with the technical idea of the present invention. For example, the terms relating to directions are set on the basis of the position represented on the drawing for convenience of explanation.

1 is a configuration diagram of an acidic gas absorption apparatus according to an embodiment of the present invention.

1, the construction of an acid gas absorber according to an embodiment of the present invention includes an absorption tower 30, a saturated absorbent conveyance line 40, a heat exchanger 50, a stripping tower 60, The reboiler 70, the condenser 80, the reflux drum 90, the regenerant absorbent transfer line 100, the main absorbent cooler 110, the branch line 120, the control valve 130, the secondary absorbent cooler 140 ) And the like.

The operation of the acid gas absorber according to one embodiment of the present invention having the above-described configuration is as follows.

Sulfur dioxide from the desulfurizer (10) (SO ₂₎ such that the removed exhaust gas (G1) is supplied to the bottom of the absorption tower 30 through the blower 20, is absorbent to the top of the absorption column 30 . The exhaust gas G1 may be a combustion exhaust gas containing an acid gas, and the acid gas may be at least one selected from CO ₂ , H ₂ S, SO ₂ , NO ₂ , and COS. The absorption agent is not particularly limited, and for example, amine, amino acid salt, inorganic salt solution, aqueous ammonia solution and ionic salt solution may be used alone or in combination. At this time, the absorbent is preferably used as an aqueous solution having a volume fraction ranging from 1 to 50 vol%.

The absorption process basically consists of an absorber 30 for absorbing the acid gas and a stripper 60 for removing the acidic gas absorbed by the absorbent. As the absorbent, an amine-based organic absorbent is mainly used. In view of economical efficiency, the absorbent absorbs the acidic gas from the absorption tower 30 and is removed from the demolition tower 60 to be recycled.

As the temperature of the absorption tower 30 is lowered by Henry's law, the acid gas is absorbed more easily. Therefore, the absorbent having passed through the high temperature demolder 60 is passed through a series of heat exchangers (Lean / Rich Amine Exchanger) Allow the temperature to drop sufficiently. This absorbent comes into contact with the acidic gas coming from the lower part of the absorption tower 30 and generates heat while the acidic gas reacts with the absorbent by the acid-alkali reaction. In the absence of the cooling facility, 2, that is, the temperature at the upper part of the absorber 30 is maximized, which adversely affects the reaction rate and the acid gas removal rate. Therefore, it is very important to improve the efficiency of the absorption tower 30 and the acid gas removal rate by making the temperature distribution within the absorption tower 30 uniform.

In the absorption tower (30), the absorbent and the exhaust gas (G1) are in countercurrent contact. At least one, preferably two or more, filling layers 31 and 32 may be provided in the absorber 30 to increase the contact between the absorbent and the exhaust gas G1. The absorbent absorbs acidic gas such as carbon dioxide contained in the exhaust gas (G1) to generate an acid gas saturated absorbent, and is discharged to the lower end of the absorption tower (30). Unreacted gases such as nitrogen and oxygen in the exhaust gas G1 that have not reacted with the absorbent are discharged through the upper portion of the absorption tower 30. [ The operation temperature of the absorption tower 30 may vary depending on the type of the absorbent. For example, the operation temperature of the absorption tower 30 may be maintained in a range of 40 to 60 ° C. On the other hand, it is possible to prevent the absorber or steam from being spoken by using the circulating washing water or using the replenishing water supplied so as to match the water balance of the process.

The absorbent saturated with the acid gas is conveyed to the demolition tower 60 along the saturated absorbent conveying line 40 by the saturated absorbent conveying pump P1 so that the heat exchanger 50 installed in the middle of the conveying line 40 regenerates And is injected into the deodorizing tower 60 in a preheated state through heat exchange with the absorbent. The regeneration of the saturated absorbent is carried out in the demixing column 60 at a high temperature (100 to 140 ° C) and a pressure of atmospheric pressure or so. The saturated absorbent injected into the upper part of the stripping tower (60) is moved to the lower part of the stripping tower (60) while the acid gas is stripped by the heat energy supplied from the reboiler (70) and the absorbent is regenerated. In order to maintain the regeneration condition, a heat source such as turbine water vapor is supplied to the reboiler 70, and heat energy is consumed in this process.

The energy supplied to the stripping tower 60 removes the acidic gas chemically bonded to the saturated absorbent and the mixed gas of the removed acidic gas and water vapor passes through the condenser 80 and the reflux drum 90, Gas (G3) and condensed water. The acid gas G3 is transferred to a separate recovery process or treatment process and used depending on the storage or use, and the condensed water is recovered and supplied to the demolition tower 60 by the transfer pump P3.

The recovered absorbent is transferred to the absorption tower 30 along the regenerated absorbent conveyance line 100 by the regenerant absorbent conveyance pump P2. The regenerated absorbent is conveyed to the absorption tower 30 by the heat exchanger 50 installed in the middle of the conveyance line 100, And is recycled to the absorption tower 30 in a cooled state through heat exchange with the saturated absorbent.

The regenerant sorbent feed line 100 is provided with a main sorbent cooler 110 which is injected into the top of the absorption tower 30 after the regenerated sorbent is further cooled.

The regenerant absorbent transfer line 100 is provided with a branch line 120 to divide a part of the regenerant absorbent and the regenerant absorbent cooler 140 is installed in the branch line 120, The cooled regenerated absorbent is further cooled.

A control valve 130 is provided in the branch line 120 in front of the auxiliary absorbent cooler 140 to control the flow rate of the regenerated absorbent that branches to the branch line 120.

The regenerated absorbent cooled in the auxiliary absorbent cooler 140 is injected into the space between the middle portions of the absorption tower 30, preferably between the filling layers 31 and 32, so that the temperature distribution in the absorption tower 30 can be made uniform .

The cooling medium used for the main absorbent cooler 110 and the auxiliary absorbent cooler 140 is not particularly limited, but conventional cooling water is preferably used, and seawater is preferably used for cooling the cooling water.

Table 1 below shows the distribution of seawater in Korea (Source: General Environmental Survey and Evaluation Report, 2009 ~ 2011, KEPCO Power Research Institute). The domestic seawater temperature is low enough so that absorbent coolers 110 and 140 ) Is cool enough to cool the hot coolant that is discharged after being used.

location
West Coast
(Yeonggwang nuclear power) South coast
(Around Kori nuclear power plant) East coast
(Around Uljin nuclear power)
Seawater temperature (℃)

3.8 to 34.4
9.9 to 26.7
10.0 to 30.2

4 illustrates an indirect cooling system for cooling the cooling water of an absorber cooler using seawater, the system including a seawater source 150, a seawater transfer line 170, a heat exchanger 180 for cooling water, Line 190 and the like.

The seawater source 150 may be an ocean or a seawater storage tank. C.W. means circulation water, and sea water is used as circulation water.

The seawater transfer line 170 is connected to the seawater supply source 150 to supply the seawater to the heat exchanger 180 for cooling water cooling.

The cooling water transfer line 190 is connected to the absorbent coolers 110 and 140 to supply the cooling water discharged after use to the cooling water cooling heat exchanger 180.

The cooling water cooling heat exchanger 180 is installed at the intersection of the sea water conveyance line 170 and the cooling water conveyance line 190 and cools the cooling water of the absorbent coolers 110 and 140 using relatively cool seawater.

In the present invention, it is preferable to use seawater to cool the cooling water used in the main absorbent cooler 110 for cooling the absorbent and the auxiliary absorbent cooler 140. That is, the seawater cools the cooling water, and the cooling water cools the absorbent. In short, seawater can be used to indirectly cool the absorbent.

For this purpose, it is preferable to additionally provide a heat exchanger 180 for cooling water in the existing heat exchanger 160, as shown in FIG. In the cooling water cooling heat exchanger 180, the sea water conveyance line 170 and the cooling water conveyance line 190 intersect with each other, whereby the relatively cool seawater is heated and the cooling water flowing into the temperature higher than the seawater is cooled.

Specifically, the cooling water used in the main absorbent cooler 110 and the auxiliary absorbent cooler 140 is discharged from the coolers 110 and 140 while being heated while being heated while cooling the absorbent. The hot water thus cooled flows into the cooling water cooling heat exchanger 180 through the cooling water transfer line 190 and is cooled by the relatively cool seawater and then supplied to the main absorbent cooler 110 and the auxiliary absorbent cooler 140 . This process is then repeated.

In order to uniformize the temperature distribution in the absorption tower 30, which is a problem in the absorption separation process utilized when separating only the acid gas from a gas in which a large amount of acid gas such as carbon dioxide exists in a mixture state, ) Is fed to the middle part of the absorption tower 30 where the temperature is greatly increased. In order to efficiently operate the branched absorbent, a control valve (not shown) is installed in front of the heat exchanger (130) to optimize the operating conditions of the system such as the flow rate control.

Therefore, in the present invention, by partially branching the absorbent from the existing cooler (Lean Amine Cooler), the temperature of the absorbent is further lowered in the absorbent cooler, and then supplied to the center of the absorbent column 30 having a relatively high temperature, The temperature of the tower 30 is lowered and the temperature distribution in the absorption tower 30 is evened.

In addition, by increasing the absorption rate in the absorption tower 30 and reducing the size of the absorption tower 30, it is possible to lower the area and the installation cost, and to absorb more acid gas at a relatively small absorbent flow rate, The flow rate can be reduced. Therefore, the efficiency of the acidic gas absorption system is greatly improved, such as a decrease in operating energy. Since the reaction efficiency and the temperature control of the absorption tower 30 are increased, the exhaust gas flowing into the absorption tower 30 need not be cooled separately, and the cost of the related equipment can be greatly reduced.

The present invention is directed to lowering and evenly distributing the temperature distribution in the absorber 30 to increase the efficiency of the absorber 30 and to partially separate the absorber from the existing cooler, And the temperature is lowered by the supply to the center, so that the temperature distribution due to the addition of the cooling equipment in Fig. 2 is obtained. At this time, the amount of the absorbing agent that branches into the additional cooler is determined according to the operation state of the absorber 30, so that it is adjusted by the additional automatic control valve 130 installed. Further, by cooling the cooling water supplied to the absorbent cooler and the additional branch absorbent cooler using seawater, the cooling effect of the absorbent can be maximized.

In the absorber 30, reaction heat is generated. Since the flow of the gas and the flow of the absorbent are opposite to each other, the temperature in the upper portion of the absorber 30 has a temperature gradient, Is an unfavorable factor for the efficient use of. In order to solve such a problem, in the present invention, after a part of the absorbent from the existing cooler (Lean Amine Cooler) is partially branched, an absorbent whose temperature is sufficiently lowered through the automatic control valve 130 and the additional cooler is supplied to the center of the absorption tower The temperature of the absorber 30 can be increased by increasing the temperature of the absorber 30 so that the size and the installation cost of the absorber 30 can be reduced, It is possible to absorb more acid gas due to the even temperature distribution in the absorber 30, thereby reducing the circulating flow rate of the absorbent, thereby reducing the operating energy. In particular, in the conventional absorption process, The temperature of the flue gas (around 53 DEG C) of the flue gas in the absorber 30 was not able to be processed and it was necessary to drive down to 40 DEG C to achieve a predetermined removal rate. Becomes possible to process without cooling the exhaust gas flowing into it is possible essentially to reduce the tremendous cost of equipment and the like associated cooling equipment and operating cost and the site, it is possible to obtain an effect of reducing the stripping energy.

10: desulfurizer 20: blower
30: absorber 31, 32: filling layer
40: Saturated absorbent transfer line 50: Heat exchanger
60: demounting tower 70: reboiler
80: condenser 90: reflux drum
100: regenerable absorbent transfer line 110: main absorbent cooler
120: branch line 130: control valve
140: secondary absorbent cooler 150: seawater source
160: Existing Heat Exchanger 170: Sea Water Transfer Line
180: Heat exchanger for cooling water 190: Cooling water transfer line

Claims (18)

An absorption tower for absorbing acidic gas using an absorbent;
A stripping tower connected to the absorption tower and regenerating the absorbent;
A regenerable absorbent feed line in which the regenerated absorbent in the stripping tower is conveyed to the absorber;
A branch line branching from the regenerant absorbent transfer line to the absorption tower; And
And an absorbent cooler installed in the branch line. The method according to claim 1,
Further comprising a control valve installed in said branch line. 3. The method according to claim 1 or 2,
Wherein the branch line is connected to an intermediate portion of the absorber. 3. The method according to claim 1 or 2,
Wherein the branch line is connected between the filling layers provided in the absorption tower. 3. The method according to claim 1 or 2,
Characterized in that a lean amine cooler is additionally provided in the regenerated absorbent conveyance line. 3. The method according to claim 1 or 2,
Wherein the cooling medium of the absorbent cooler uses cooling water. 3. The method according to claim 1 or 2,
Seawater supply;
A seawater transfer line connected to a seawater source;
A coolant transfer line connected to the sorbent cooler; And
Further comprising a cooling water cooling heat exchanger installed at an intersection of the sea water conveyance line and the cooling water conveyance line for cooling the cooling water of the absorbent cooler using seawater. 3. The method according to claim 1 or 2,
Wherein the absorbent comprises at least one selected from an amine type, an amino acid salt, an inorganic salt type solution, an aqueous ammonia solution and a metal ion salt solution. 3. The method according to claim 1 or 2,
Wherein the acid gas is at least one selected from the group consisting of CO ₂ , H ₂ S, SO ₂ , NO ₂ , and COS. Absorbing the acidic gas in the absorber using an absorbent;
Regenerating the sorbent in the stripping tower;
Transferring the regenerated absorbent in the demoulding tower to the absorption tower through a regenerant absorbent transfer line;
Branching a portion of the absorbent regenerated through the branch line connected to the regenerant absorbent transfer line; And
And cooling the branched absorbent through a cooler installed in the branch line, and then injecting the cooled absorbent into the absorption tower. 11. The method of claim 10,
Wherein the flow rate of the absorbent branched through the control valve provided in the branch line is adjusted. 11. The method of claim 10,
And the absorbent cooled in the branch line is injected into an intermediate portion of the absorption tower. 11. The method of claim 10,
And the absorbent cooled in the branch line is injected between the packed bed provided in the absorber. 11. The method of claim 10,
And a cooler is also provided on the regenerant absorbent conveying line to cool the regenerated absorbent. 11. The method of claim 10,
Wherein the cooling medium of the absorbent cooler is cooling water. 11. The method of claim 10,
Supplying seawater from a seawater source to a heat exchanger for cooling water through a seawater transfer line;
Supplying the cooling water used in the absorbent cooler to the heat exchanger for cooling water through the cooling water transfer line;
Cooling the cooling water using seawater in a heat exchanger for cooling water; And
And supplying the cooled cooling water to the absorber cooler. ≪ RTI ID = 0.0 > 11. < / RTI > 11. The method of claim 10,
Wherein the absorbent comprises at least one selected from the group consisting of an amine series, an amino acid salt, an inorganic salt series solution, ammonia water, and a metal ion salt solution. 11. The method of claim 10,
Wherein the acid gas is at least one selected from the group consisting of CO ₂ , H ₂ S, SO ₂ , NO ₂ , and COS.

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