Method for treating combustion exhaust gas

Abstract: A method for recovering carbon dioxide by absorbing carbon dioxide present in a combustion exhaust gas using an aqueous alkanolamine solution, comprising the step of bringing a combustion exhaust gas from which carbon dioxide has been absorbed and removed into contact with water containing carbon dioxide. A method for treating a combustion exhaust gas for denitration using ammonia as a reducing agent and for removal of carbon dioxide by absorption with an aqueous alkanolamine solution, which method comprising the steps of recovering ammonia present in the combustion exhaust gas after the carbon dioxide removal, and using the recovered ammonia as a reducing agent for the denitration. A method for removing CO₂ from a combustion exhaust gas comprising the step of bringing the combustion exhaust gas into contact under atmospheric pressure with an aqueous monoethanolamine solution having a concentration of 35% by weight or more.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating a combustion exhaust gas which has a denitration step for removing NO x (nitrogen oxides) present in the combustion exhaust gas and a CO 2 removal step of removing CO 2 using an aqueous alkanolamine solution in which ammonia (NH 3 ) present in the combustion exhaust gas after the CO 2 removal treatment is recovered and then used as a reducing agent in the denitration step. 2. Description of the Related Art In recent years, a greenhouse effect by CO 2 is indicated as one cause of the warming of the earth, and its prompt resolution is globally required in order to protect earth environment. Sources of CO 2 extend over almost all fields of human activities in which fossil fuels are burned, and there is a tendency that regulations on the discharge of CO 2 will be further tightened in the future. Thus, for power generation facilities such as thermoelectric power plants in which a large amount of the fossil fuel is used, intensive research efforts are being made on methods for recovering CO 2 in a combustion exhaust gas by bringing the combustion exhaust gas coming from a boiler into contact with an aqueous alkanolamine solution or the like, and on methods for storing the recovered CO 2 without discharging it into the atmosphere. Fig. 7 shows one example of a prior art process for recovering CO 2 present in a combustion exhaust gas by the use of an aqueous alkanolamine solution. In Fig. 7, only major devices and parts are shown, and auxiliary devices are omitted. Examples of alkanolamines which can be used to recover CO 2 include aqueous solutions of monoethanolamine, diethanolamine, triethanolamine, dimethyldiethanolamine, diisopropanolamine and diglycolamine, and aqueous mixed solutions thereof. In general, an aqueous solution of monoethanolamine is preferred. In Fig. 7, a combustion exhaust gas which comes from a boiler or the like and which is to be discharged through a chimney has normally a temperature of 100 to 150°C, and this gas is introduced into a combustion exhaust gas cooling column 1 through line 5 and then brought into contact with a cooling water circulated through line 12 by a pump 11. The cooling water is cooled by a heat exchanger 13 and used repeatedly. Usually, the combustion exhaust gas cooled to a temperature in the range of about 40 to 80°C is introduced into a CO 2 absorbing column 2 through line 6. An aqueous solution of an alkanolamine, e.g., monoethanolamine, at a concentration of 20 to 30% by weight is conducted to the CO 2 absorbing column 2 through line 9. The aqueous monoethanolamine solution is brought into counter current contact with the combustion exhaust gas, and the aqueous monoethanolamine solution having absorbed CO 2 is then fed from a lower portion of the column to a regeneration column 3 through line 7 for its regeneration. In order to prevent monoethanolamine from being discharged together with the remaining combustion exhaust gas (hereinafter referred to also as "the treated exhaust gas" at times) from which CO 2 has been absorbed, a water washing and cooling section for the treated exhaust gas is provided in the upper portion of the CO 2 absorbing column 2, and this section is equipped with a pump 15, a heat exchanger 16 and a tray 18 to which water is fed through line 19. The treated exhaust gas which has been washed and cooled with water in these devices is discharged to the atmosphere through line 8. In the aqueous monoethanolamine solution regeneration column 3, the aqueous monoethanolamine solution is regenerated by heating the same with steam fed through a reboiler 4, and it is then returned to the absorbing column 2 through line 9. CO 2 and water vapor are led together to a recovery step through line 10. In addition, a cooling system for the combustion exhaust gas illustrated in Fig. 7 is mainly used for a fuel, such as LNG, containing a large amount of hydrogen. According to this cooling system, steam derived from the fuel and present in the combustion exhaust gas is cooled and becomes condensed water. This condensed water is accumulated as the cooling water circulating through line 12. Therefore, an excess of the cooling water is constantly discharged from the system through line 14 using a water level meter and the like (not shown) located in the lower portion of the combustion exhaust gas cooling column 1. As a cooling system for the combustion exhaust gas, there is another system utilizing a humidification cooling technique suitable for boilers which use fuel containing a large amount of carbon, such as coal or fuel oil. In this system, no heat exchanger 13 is usually disposed, and circulating water is simply brought into contact with a combustion exhaust gas to cause the water evaporate for the purpose of cooling. Since the circulating water is gradually lost, it is necessary to replenish water from the outside. In the above-mentioned process, ammonia is detected in the treated exhaust gas discharged through line 8, though its amount is very small. It is presumed that this ammonia in the treated exhaust gas is attributable to the decomposition of part of alkanolamine during the treatment processes. Another origin of the detected ammonia is considered to be residue of ammonia which has been added to the fuel for the purpose of decreasing the amount of NO x in the combustion exhaust gas. In any case, if ammonia, though its amount is very small, is discharged into the atmosphere together with the treated exhaust gas without any additional treatment, another environmental problem of bad odor may occur. Therefore, this ammonia has to be removed. However, ammonia is present in the treated exhaust gas in very small amounts, and thus how ammonia can effectively be removed has been a problem yet to be solved. Further, when burned, fossil fuel generates pollutants, such as NO x (nitrogen oxides) and SO x (sulfur oxides), though the degree of this pollutant generation depends upon the kind of the fossil fuel. These pollutants cause air pollution and acid rain, and regulations on their discharge will be further tightened. With this current trend, measures have been taken so that a combustion exhaust gas coming from a boiler is treated by denitration and desulfurization processes. Among these measures, a method is known in which denitration is carried out using NH 3 (ammonia) as a reducing agent under the presence of a catalyst to decompose NO x into nitrogen and water. The concentration of NH 3 used is usually in the range of 50 to 150 ppm, depending upon the amount of NO x in the combustion exhaust gas. As described above, NH 3 is used as a reducing agent for NO x in the denitration of a combustion exhaust gas. Therefore, in a denitrating apparatus for the combustion exhaust gas, a storage tank for liquid NH 3 is usually disposed. In order to store liquid NH 3 , however, a low temperature and a high pressure must be maintained, and NH 3 itself is poisonous. In addition, NH 3 is combustible, and when mixed with air produces an explosive mixed gas. Thus, NH 3 is controlled under various laws and regulations in Japan, such as laws related to fire regulations, handling of high pressure gases, and control of noxious odor. Therefore, in the denitration step in which NO x is removed from the combustion gas, special storage and feed facilities for NH 3 , which is difficult to handle, are required. On the other hand, in the above-mentioned CO 2 removal step, ammonia (NH 3 ) is detected, even though its amount is small, in the combustion exhaust gas which has been subjected to a CO 2 removal treatment. It may be presumed that the NH 3 is generated by the decomposition of part of alkanolamine in the process system. If NH 3 , though present in small amounts, is discharged into the atmosphere together with the CO 2 -free exhaust gas without any additional treatment, another environmental problem may occur. Therefore, NH 3 has to be recovered and disposed without any environmental harm. As mentioned above, the concentration of an aqueous solution of monoethanolamine (MEA), which is a kind of alkanolamine, for absorbing CO 2 is normally up to about 30% by weight. In a process for removing CO 2 from a combustion exhaust gas by an aqueous MEA solution, it is desirable to decrease energy used in the process as much as possible. In particular, the aqueous MEA solution is circulated and repeatedly used in the system, and therefore it is beneficial to decrease the volume of the aqueous MEA solution for the purpose of reducing an electric power consumption of a pump or the like. Moreover, in order to regenerate a reusable aqueous MEA solution from the aqueous MEA solution having absorbed CO 2 , a large amount of heat energy is necessary, and therefore it is important from an economical viewpoint to reduce the consumption of heat energy. EP-A-0 359 991 discloses that CO 2 and, if necessary, H 2 S are removed from a gas which contains CO 2 and may contain H 2 S with an absorption liquid by a process in which the gas is treated in an absorption zone with the absorption liquid, the treated gas is removed from the absorption zone, the absorption liquid which is obtained from the absorption zone is then regenerated, at the same time the absorption liquid is let down in one or more let-down stages and a let-down gas is taken off at the top at least one of the first to penultimate let-down stages, and the partially regenerated absorption liquid obtained from the last let-down stage is, if required, fed to a stripping zone for further regeneration, one or more acid gas streams which contain the CO 2 and may contain H 2 S are removed from the regeneration stage, and the regenerated absorption liquid is recycled to the absorption zone, wherein a bleed stream of completely or partially regenerated absorption liquid is fed to one or more of the let-down stages from which the let-down gas is removed, at a point above the feed of the absorption liquid to be let down. The present inventors have made an intensive investigation on possible ways to remove ammonia produced in a recovery process shown in Fig. 7 in which CO 2 in a combustion exhaust gas is absorbed and recovered using an alkanolamine. They have found that it is effective to use water occurring in the recovery process and containing a small amount of CO 2 as an ammonia absorbing liquid. The present inventors have made an intensive investigation on the above-mentioned problem of handling NH 3 associated with the conventional denitration processes for combustion exhaust gas and the above-mentioned problem of recovering and disposing NH 3 formed during CO 2 removal, which has recently attracted much attention. As a result, they have found that these problems can be simultaneously resolved by recovering NH 3 present in the combustion exhaust gas subjected to a CO 2 removal treatment and then using the same in the denitration step. Therefore, the invention is directed to a method for treating a combustion exhaust gas which has a denitration step for denitrating the combustion exhaust gas using NH 3 as a reducing agent and a CO 2 removal step for removing CO 2 present in the combustion exhaust gas by bringing the combustion exhaust gas into contact with an aqueous alkanolamine solution, in which ammonia present in the combustion exhaust gas having been subjected to the CO 2 removal treatment by the contact with the aqueous alkanolamine solution is recovered and then used as the reducing agent in the denitration step. According to the invention, NH 3 present in a combustion exhaust gas which has been subjected to a CO 2 removal treatment can be recovered and then used as a reducing agent in the denitration step, so that a storage installation for liquid NH 3 becomes unnecessary in the denitration process. Furthermore, most of NH 3 present in the combustion exhaust gas after being subjected to the CO 2 removal treatment and being discharged from the CO 2 removal step can be recovered, whereby an environmental problem associated with NH 3 which secondarily occurs in small amounts can be solved. The present inventors have intensively investigated into the possibility of reducing energy consumed during the process of removing CO 2 from a combustion exhaust gas by the use of an aqueous MEA solution, and as a result, they have found that the employment of the aqueous MEA solution having a certain concentration is effective. A further aspect of the present invention resides in bringing the combustion exhaust gas into contact with an aqueous MEA solution having a concentration of 35% by weight or more under atmospheric pressure. Accordingly, in the process for absorbing CO 2 contained in the combustion exhaust gas, an aqueous MEA solution having a concentration of 35% by weight or more is used, whereby the amount of the circulating aqueous MEA solution can be decreased. In addition, in the regeneration of the aqueous MEA solution, the amount of a required steam (heat energy) can also be decreased. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an illustrative diagram of the CO 2 recovery method according to the present invention; Fig. 2 shows one example of a treatment process of a combustion exhaust gas according to the present invention; Fig. 3 shows one example of the CO 2 removal process; Fig. 4 is an illustrative view of the embodiment according to dependent claim 7 of the present invention; Fig. 5 is a graph showing relations among a liquid/gas ratio, a CO 2 absorption ratio and a CO 2 loading value at various concentrations of an aqueous MEA solution; Fig. 6 is a graph showing a relation between the concentration of the aqueous MEA solution and the amount of a steam necessary for regeneration; and Fig. 7 is an illustrative diagram of one embodiment of a process which can be employed in a conventional CO 2 recovery method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In Fig. 1, only major devices and parts are shown, and relatively minor fittings are omitted. The like symbols in Fig. 7 denote the like members as in Fig. 1. In the method for recovering CO 2 according to Fig. 1 of the present invention, the treated exhaust gas after CO 2 absorption is brought into contact with water containing CO 2 . In this case, this water containing CO 2 preferably contains 20 ppm or more, preferably 25 ppm or more of CO 2 . When ammonia in the treated exhaust gas is brought into contact with the water containing CO 2 , either of reactions represented by the following formulae takes place, and ammonia is absorbed by the water to form ammonium carbonates: The ammonium carbonates formed in these reactions are stable in an aqueous solution. Since ammonia can be removed in the form of these ammonium carbonates, discharge of ammonia into the atmosphere may be prevented. The short-term contact of the treated exhaust gas with water containing CO 2 , particularly one containing 20 ppm or more of CO 2 , permits the effective removal of ammonia from the treated exhaust gas (exhaust gas after the absorption of CO 2 ) as stable ammonium salts. The water containing CO 2 , one containing 20 ppm or more of CO 2 in particular, may be cooling water which has been brought into contact with combustion exhaust gas so as to cool the high-temperature combustion exhaust gas or, more specifically, cooling water circulated by means of the pump 11. In particular, it may also be part or all of condensed cooling water which is to be discharged from the system as excess water through line 14. Fig. 1 shows an embodiment in which part of excess water coming through line 14 is introduced into the CO 2 absorbing column 2 through line 17. In the above-mentioned humidification cooling method which is not shown, part of water (which has been brought into contact with the combustion exhaust gas) circulating for cooling by humidification can be drawn and used. As described above, the above-mentioned water usually contains 20 ppm of CO 2 , depending upon a kind of fuel to be used in the boiler, cooling conditions of the combustion exhaust gas, circulation and reuse conditions of the cooling water and the like. Therefore, such water can be directly used in the method of the present invention without any additional treatment. When the concentration of CO 2 is low, for example, at 20 ppm or less, owing to the above-mentioned conditions, part of recovered CO 2 can be used to easily increase the concentration of dissolved CO 2 . For example, there can be provided in line 17 a contact device 22 in which water is brought into contact with CO 2 fed through line 21, so that the CO 2 concentration in water fed to the absorbing column 2 through line 23 can be adjusted to the preferable concentration range or more. Also when CO 2 -containing water, particularly water containing CO 2 at a preferable concentration, is prepared from water containing almost no CO 2 , the contact device 22 for contact with CO 2 can be used. CO 2 fed through line 21 may be just CO 2 , or CO 2 gas mixed with steam or present in water. The amount of CO 2 for this purpose can be obtained by using a very small portion of CO 2 recovered in the aqueous monoethanolamine solution regeneration column 3. The number of moles of CO 2 which is fed to the top of the CO 2 absorbing column 2 through line 23 per unit time should be determined considering the stoichiometry of the above-mentioned chemical formulae (1) and (2), and the conversion of ammonia into ammonium salts which depends upon contact conditions at the column top. However, the number of moles of CO 2 should preferably be equal to or more than that of ammonia which is discharged through line 8 per unit time. The CO 2 -containing water fed through line 23 is brought into contact with the treated exhaust gas at the top of the absorbing column 2 using an ordinary gas-liquid contact method which uses a tray 24 as in the embodiment of Fig. 1, so as to absorb ammonia present therein, and the water containing ammonia is then led to effluent treating facilities or the like installed outside the CO 2 absorbing and recovering system through line 25. Since ammonia is discharged from the system this way, ammonium salts are no longer accumulated in the system, and ammonia is prevented from recurring by the decomposition of the accumulated ammonium salts. Now, the present invention will be described more specifically with reference to Experimental and Comparative Examples. (Experimental Example 1) A process shown in Fig. 1 was employed to absorb CO 2 in combustion exhaust gas of natural gas. In this case, however, CO 2 was not fed through line 21. Under conditions that CO 2 -containing water was not fed to the top of the CO 2 absorbing column 2 through line 17, the discharge volume of the treated exhaust gas was 555 Nm 3 /hr, and the concentration of ammonia was 39.3 ppm, which corresponded to an ammonia discharge volume of 9.74 × 10 -4 kg mole/hr. Next, while CO 2 was absorbed under the above-mentioned conditions, excess water (50°C; atmospheric pressure; CO 2 concentration of 28 ppm) coming from a combustion exhaust gas cooling column 1 was fed through line 17 at a ratio of 30 kg/hr. At this time, an ammonia concentration in the treated exhaust gas discharged through line 8 was 25 ppm. (Comparative Example 1) The same procedure as in the above-mentioned Experimental Example 1 was repeated except that water containing no CO 2 was fed through line 17. In this case, the ammonia concentration in the treated exhaust gas discharged through line 8 was 37 ppm. This represents only a small decrease in the amount of ammonia present in the treated exhaust gas. Next, the method according to claim 1 of the present invention will be described in detail with reference to Figs. 2 and 3. Fig. 2 shows one example of a treatment process for a combustion exhaust gas to which the present invention is applied. In Fig. 2, A indicates a boiler in which coal, naphtha or fuel oil is used as a fuel, B a denitration step, C a dust collection step containing an electrostatic precipitator, D a desulfurization step, E a CO 2 removal step, F a chimney, and G a transport line for recovered NH 3 . The temperature of the combustion exhaust gas led to the denitration step is usually from 200 to 400°C, depending upon kinds of selected fuel and boiler. Examples of the NO x present in the combustion exhaust gas include NO, NO 2 and N 2 O, but the major component of the NO x is NO. The concentration of NO in the combustion exhaust gas depends upon the kind of fuel and conditions for combustion. The NO concentration is usually from 50 to 100 ppm when LNG is burned, from 100 to 150 ppm if fuel oil is burned, and from 200 to 500 ppm if coal is burned. The ratio of NO which can be removed in the denitration step is usually from about 50 to 80%. An example of a catalyst which can be used in the denitration step is a catalyst in which titanium oxide is used as a carrier and an oxide of a transition metal, such as vanadium pentaoxide, is used as an active component. The supply of ammonia (NH 3 ) which serves as a reducing agent depends upon a desired degree of denitration, but NH 3 may be injected to the denitration step so that the concentration of NH 3 becomes about 50 to 80% of the NO x concentration. An optimum temperature for starting the injection of NH 3 depends upon the kind of fuel, but if LNG is used, the optimum temperature would be between 200 and 400°C. In the present invention, NH 3 normally required in the denitration step can be supplemented with NH 3 recovered in the CO 2 removal step, depending upon operating conditions at the CO 2 removal step. Usually, during the treatment of the combustion exhaust gas to which the present invention is applied, the combustion exhaust gas is first subjected to desulfurization in a desulfurization step, and the thus treated combustion exhaust gas is then brought into contact with an aqueous alkanolamine solution for CO 2 removal at the CO 2 removal step, as shown in Fig. 2. For this desulfurization, various dry methods and wet methods have been heretofore suggested, but a typical technique is the so-called wet lime-gypsum method in which SO x is absorbed by an aqueous slurry of limestone (calcium carbonate) powder, and then recovered in the form of gypsum. Examples of the aqueous alkanolamine solution which can be used to absorb CO 2 in the CO 2 removal step include aqueous solutions of monoethanolamine, diethanolamine, triethanolamine, dimethyldiethanolamine, diisopropanolamine and diglycolamine, and aqueous mixtures thereof. Usually, the aqueous monoethanolamine (MEA) solution is preferably used. No particular restriction is placed on a process with which CO 2 present in the combustion exhaust gas is removed therefrom using an aqueous alkanolamine solution, above all, an aqueous MEA solution, but one preferable example of the process will be described in reference to Fig. 3. In Fig. 3, only major devices and parts are shown, and less important parts are omitted for ease of understanding. In Fig. 3, reference numeral 201 indicates a CO 2 removing column, 202 a lower filled portion, 203 an upper filled portion or a tray, 204 a combustion exhaust gas feed inlet of the CO 2 removing column, 205 a discharge outlet for the CO 2 -free combustion exhaust gas, 206 an aqueous MEA solution feed inlet, 207 a first nozzle, 208 a cooler for the combustion exhaust gas disposed as needed, 209 a nozzle, 210 a filled portion, 211 a humidifying and cooling water circulating pump, 212 a feed line for supplemental water, 213 a discharge pump for an aqueous MEA solution for absorbing CO 2 , 214 a heat exchanger, 215 an aqueous MEA solution regeneration column (hereinafter referred to as "a regeneration column" at times), 216 a first nozzle, 217 a lower filled portion, 218 a regeneration heater (reboiler), 219 an upper filled portion, 220 a reflux water pump, 221 a CO 2 separator, 222 a discharge line for recovered CO 2 , 223 a reflux cooler for the regeneration column, 224 a second nozzle, 225 a feed line of reflux water from the regeneration column, 226 a combustion exhaust gas feed blower, 227 a cooler, and 228 a reclaimer (accumulated-heat stable-salt removing device). Moreover, reference numeral 229 indicates a second nozzle of the CO 2 removing column 201, 230 a filled portion for NH 3 removal, 231 a tray, 238 a third nozzle, 232 an NH 3 separating drum, 233 a steam feed line, 234 a cooling water feed line, 235 a cooler, 236 a CO 2 feed line, 237 a CO 2 mixer, and G the NH 3 transport line shown also in Fig. 2. In Fig. 3, the combustion exhaust gas which has passed through the desulfurization step is introduced into the cooler 208 by means of the combustion exhaust gas feed blower 226 and then brought into contact with humidifying and cooling water from the nozzle 209 in the filled portion 201 to become humidified and cooled. The cooled combustion exhaust gas is introduced into the CO 2 removing column 201 through the combustion exhaust gas feed inlet 204 of the CO 2 removing column. The humidifying and cooling water which has been brought into the combustion exhaust gas is stored in the lower portion of the combustion exhaust gas cooler 208 and circulated through the nozzle 209 by means of the pump 211. The humidifying and cooling water is gradually lost as it humidifies and cools the combustion exhaust gas, and thus replenished through the feed line 212 for water supply. The combustion exhaust gas introduced into the CO 2 removing column 201 is brought into contact with the aqueous MEA solution of a given concentration fed through the first nozzle 207 in a counter current manner in the lower filled portion 202, whereby CO 2 in the combustion exhaust gas is absorbed and removed therefrom by the aqueous MEA solution. The combustion exhaust gas after the CO 2 removal treatment streams toward the upper filled portion 203. The aqueous MEA solution fed to the CO 2 removing column 201 absorbs CO 2 , and the temperature of the aqueous MEA solution becomes higher than its temperature at the feed inlet 206 owing to reaction heat generated by the absorption. Then, the aqueous MEA solution is conducted to the heat exchanger 214 by means of the discharge pump 213 for the aqueous MEA solution, heated therein, and then led to the regeneration column 215 through the first nozzle 216. In the regeneration column 215, the aqueous MEA solution is regenerated in its lower filled portion 217 by the heat from the regeneration heater 218, and the thus regenerated aqueous MEA solution is cooled by the heat exchanger 214 and then returned to the CO 2 removing column 201. If necessary, the solution is further conducted through the cooler 227 disposed between the heat exchanger 214 and the aqueous MEA solution feed inlet 206 before returned to the CO 2 removing column 201. In the upper portion of the regeneration column 215, CO 2 separated from the aqueous MEA solution is brought into contact with reflux water fed through the second nozzle 224 and then cooled by the regeneration column reflux cooler 223. Subsequently, in the CO 2 separator 221, CO 2 is separated from the reflux water formed by the condensation of water vapor delivered with CO 2 and then led to a CO 2 recovery step through the discharge line 222 for recovered CO 2 . As the operation of the CO 2 recovery step continues, part of MEA forms stable salts and accumulates in the system. In order to prevent this accumulation, the reclaimer 228 in Fig. 2 is periodically operated to treat, for example, a liquid present at the bottom of the regeneration column 215. In the CO 2 removal process shown in Fig. 3, the combustion exhaust gas having been subjected to the CO 2 removal treatment by the contact with the aqueous alkanolamine (MEA) solution contains NH 3 , as described above. The content of NH 3 is not constant but depends upon the operating conditions of the CO 2 removal step. However, it is usually from 30 to 80 ppm. In the present invention, NH 3 present in the combustion exhaust gas subjected to the CO 2 removal treatment is recovered, and then conducted to a denitration step through the NH 3 transport line in Fig. 2. As a process for recovering NH 3 from the combustion exhaust gas which has been subjected to the CO 2 removal treatment, it is preferred to bring the combustion exhaust gas into contact with water containing CO 2 in the CO 2 removing column 201. The CO 2 containing water may be obtained, for example, by circulating the water for NH 3 absorption through the third nozzle 238, the filled portion 230 for NH 3 removal, the tray 231, the NH 3 separating drum 232 and the third nozzle 238 to mix the water with CO 2 fed through the CO 2 feed line 236 in the CO 2 mixer 237, as shown in the top of the CO 2 removing column 201 in Fig. 2. Other examples of the water containing CO 2 include a regeneration column reflux water fed through the second nozzle 229 of the CO 2 removing column 201 and a humidifying and cooling water of the cooler 208 for the combustion exhaust gas. It is possible to make any of these waters contain CO 2 in a saturated state or at near-saturation concentrations at a given temperature. For example, when the temperature is about 40°C, water can contain CO 2 at a concentration of about 400 ppm. The combustion exhaust gas from which CO 2 has been removed in the lower filled portion 202 of the CO 2 removing column 201 usually has a temperature of 50 to 80°C and contains a relatively large amount of an MEA vapor corresponding to a vapor partial pressure at this temperature, a trace amount of NH 3 and water vapor. This combustion exhaust gas streams upward and comes into contact with the above-mentioned reflux water fed from the regeneration column 215 through the regeneration column reflux water feed line 225. Since MEA and NH 3 are weak bases, they are easily absorbed by the regeneration column reflux water when the regeneration column reflux water containing weakly acidic CO 2 in a substantially saturated state comes into contact with the combustion exhaust gas after the CO 2 removal treatment in the upper contact portion 203. However, NH 3 is considered to be a decomposition and deterioration product of MEA, and after NH 3 has been accumulated in the system and saturated, NH 3 is discharged in the form of a vapor to the upper portion of the upper filled portion 203. In the preferable embodiment of the present invention, this NH 3 vapor is absorbed by the water containing CO 2 in the top of the CO 2 removing column, taken out from the system, delivered to the denitration step B shown in Fig. 2, and then utilized, as described above. NH 3 in the combustion exhaust gas subjected to the CO 2 removal treatment is reacted with CO 2 in the above-mentioned water containing CO 2 . As a result, either of the reactions represented by the following formulas takes place, and ammonia is absorbed by the water to form ammonium carbonates: Any of the ammonium carbonates formed according to the above-mentioned reaction formulas decomposes into NH 3 , water and CO 2 at 70°C or higher, and therefore, in the process shown as an example in Fig. 3, steam at a temperature of 100°C or higher can be blown through the steam feed line 233 into the water containing NH 3 taken out from the CO 2 removing column 201 to easily regenerate NH 3 . Together with similarly regenerated gaseous CO 2 , the thus regenerated NH 3 is separated from the water in the NH 3 separating drum 232, and then delivered to the denitration step B shown in Fig. 2 through the NH 3 transport line G. The water from which the regenerated NH 3 has been removed is cooled in the cooler 235. Subsequently, in the CO 2 mixer 237, this water again dissolves CO 2 fed through the CO 2 feed line 236, and it is conducted to the third nozzle 238 located at the top of the CO 2 removing column 201. According to the invention, a storage installation for NH 3 , which is troublesome to handle, becomes unnecessary in the denitration process, and also NH 3 is effectively prevented from being discharged from the system through the discharge outlet 205 for the combustion exhaust gas after the CO 2 removal treatment. The NH 3 recovered in the CO 2 removal process can be effectively utilized as the reducing agent in the denitration process. The effectiveness of the method according to claim 1 of the invention will be shown in detail with reference to Experimental Example 2. (Experimental Example 2) A combustion exhaust gas was treated using the steps and equipment shown in Figs. 2 and 3. In the denitration step B, TiO 2 was used as a denitrating catalyst and a denitration temperature was set to 300°C. In this case, NH 3 used as a reducing agent was all prepared by the absorption and recovery of NH 3 using regeneration column reflux water coming from the CO 2 removal step, and then by the regeneration with steam of 1 kg/cm 2 G. Various conditions are shown in Table 1. Item Units Amount of combustion exhaust gas discharged from boiler A Nm 3 /H 2213 Pressure of combustion exhaust gas Atmos. pressure Temp. of combustion exhaust gas °C 100 CO 2 conc. in combustion exhaust gas mole% 8.6 NO x conc. in combustion exhaust gas ppm 50 Water conc. in combustion exhaust gas mole% 17.3 NO x conc. in combustion exhaust gas at outlet of denitration step B ppm 10 Denitration ratio % 80 Temp. of denitration step 300 Amount of combustion exhaust gas at inlet of CO 2 removing column Nm 3 /H 1752 CO 2 conc. in combustion exhaust gas at inlet of CO 2 removing column mole% 8.3 Water conc. in combustion exhaust gas at inlet of CO 2 removing column mole% 19.5 Temp. of combustion exhaust gas at inlet of CO 2 removing column °C 61 Amount of aqueous MEA solution at aqueous MEA solution feed inlet kg/H 4367 Temp. of aqueous MEA solution at aqueous MEA solution feed inlet °C 38 Temp. of aqueous MEA solution for absorbing CO 2 °C 64 CO 2 absorption ratio % 90 Amount of combustion exhaust gas after CO 2 treatment in CO 2 removing column °C 61 NH 3 conc. in combustion exhaust gas after CO 2 treatment in CO 2 removing column ppm 40 MEA vapor conc. in combustion exhaust gas after CO 2 removal treatment at outlet of CO 2 removing column ppm 0.1 Feed amount of NH 3 to denitration step B through transport line G kg·mole/H 4.3×10 -3 Temp. of water fed through third nozzle °C 50 Amount of water fed through third nozzle kg/H 450 CO 2 conc. in water fed through third nozzle ppm 400 One example of a process which can be employed in a method for removing CO 2 from a combustion exhaust gas according to dependent claim 7 of the present invention will be described with reference to Fig. 4. In Fig. 4, only major parts are shown, and less important fittings are omitted. In Fig. 4, reference numeral 301 indicates a CO 2 removing column, 302 a lower filled portion, 303 an upper filled portion or a tray, 304 a combustion exhaust gas feed inlet of the CO 2 removing column, 305 a combustion exhaust gas discharge outlet of the CO 2 removing column, 306 an aqueous MEA solution feed inlet, 307 a nozzle, 308 a reflux liquid feed inlet, 309 a nozzle, 310 a cooler for the combustion exhaust gas, 311 a nozzle, 312 a filled portion, 313 a humidifying and cooling water circulating pump, 314 a supplemental water feed line, 315 a discharge pump for the aqueous MEA solution for absorbing CO 2 , 316 a heat exchanger, 317 an aqueous MEA solution regenerating column (hereinafter also called "a regenerating column"), 318 a nozzle, 319 a lower filled portion, 320 a regenerating heater (a reboiler), 321 an upper filled portion, 322 a reflux water pump, 323 a CO 2 separator, 324 a discharge line for recovered CO 2 , 325 a reflux cooler of the regenerating column, 326 a nozzle, 327 a regenerating column reflux water feed line, and 328 a combustion exhaust gas feed blower. The combustion exhaust gas is introduced into the cooler 310 for the combustion exhaust gas usually at 100 to 150°C by means of the combustion exhaust gas feed blower 328 and then brought into contact with humidifying and cooling water from the nozzle 311 in the filled portion 312 to be humidified and cooled. The cooled combustion exhaust gas is then introduced into the CO 2 removing column 301 through the combustion exhaust gas feed inlet 304 of the CO 2 removing column. At this time, the temperature of the combustion exhaust gas at the inlet of the CO 2 removing column 301 is usually from 50 to 80°C, depending upon the kind of fuel and cooling conditions. The humidifying and cooling water which has been brought into contact with the combustion exhaust gas is stored in the lower portion of the cooler 310 for the combustion exhaust gas and then circulated through the nozzle 311 by means of the pump 313. The humidifying and cooling water is gradually lost as it humidifies and cools the combustion exhaust gas, and thus water is replenished through the supplemental water feed line 314. The combustion exhaust gas introduced into the CO 2 removing column 301 is brought into contract with the aqueous MEA solution having a certain concentration and fed through the nozzle 307 in a counter current manner in the lower filled portion 302, whereby CO 2 in the combustion exhaust gas is absorbed and removed therefrom by the aqueous MEA solution. The combustion exhaust gas from which the CO 2 has been removed streams toward the upper filled portion 303. The aqueous MEA solution fed to the CO 2 removing column 301 absorbs CO 2 , and consequently the temperature of the aqueous MEA solution becomes higher than its temperature at the feed inlet 306 owing to reaction heat generated by the absorption. Then, the aqueous MEA solution is delivered to the heat exchanger 316 by means of the discharge pump 315 for the aqueous MEA solution for absorbing CO 2 , heated therein, and then led to the regenerating column 317. The temperature of the aqueous MEA solution which is fed through the aqueous MEA solution feed inlet 306 is adjusted so that the temperature of the combustion exhaust gas may be at the same level at the inlet and at the outlet of the CO 2 removing column 301, whereby a water balance can be maintained in the CO 2 removing column 301 and further in the whole system shown in Fig. 4. Furthermore, even if the combustion exhaust gas discharged from the CO 2 removing column 301 has a high temperature, the escape of MEA from the system can be effectively prevented by feeding reflux water from the regenerating column 317 to the upper portion of the CO 2 removing column 301 through the reflux liquid feed inlet 308. In the regenerating column 317, the aqueous MEA solution is regenerated by heat from the regenerating heater 320, and the thus regenerated aqueous MEA solution is cooled by the heat exchanger 316 and then returned to the aqueous MEA solution feed inlet 306 of the CO 2 removing column 301. In the upper portion of the regenerating column 317, CO 2 separated from the aqueous MEA solution is brought into contact with the reflux water fed through the nozzle 326 and then cooled by the regenerating column reflux cooler 325. Then, in the CO 2 separator 323, CO 2 is separated from the reflux water formed by the condensation of water vapor delivered with CO 2 and then led to a CO 2 recovery step through the discharge line 324 for recovered CO 2 . The reflux water is returned to the regenerating column 317 by means of the reflux water pump 322. In the process shown in Fig. 4, a part of this reflux water is fed to the CO 2 removing column 301 through the regenerating column reflux water feed line 327. The reflux water is water separated from CO 2 in the CO 2 separator 323, and therefore this water contains CO 2 at a concentration of saturation or near saturation at a given temperature. For example, when the temperature of the reflux water separated in the CO 2 separator 323 is about 40°C, the reflux water may contain CO 2 at a concentration of about 400 ppm. The combustion exhaust gas from which CO 2 has been removed in the lower filled portion 302 of the CO 2 removing column 301 usually has a high temperature of 50 to 80°C and contains relatively large amounts of MEA vapor and water vapor corresponding to partial vapor pressures at this temperature. This combustion exhaust gas streams upward and is brought into contact with the above-mentioned reflux water fed from the regenerating column 317 through the regenerating column reflux water feed line 327. MEA is a weak base, and therefore, by bringing the reflux water containing weakly acidic CO 2 in a substantially saturated state with the CO 2 -free combustion exhaust gas in the upper contact portion 303, MEA can be effectively prevented from being discharged into the atmosphere. After the reflux water which is fed to the CO 2 removing column 301 is brought into contact with the CO 2 -free combustion exhaust gas to absorb MEA, this water falls through the CO 2 removing column 301, and it is mixed with the aqueous MEA solution and then returned to the regenerating column 317. The reflux water which is fed to the CO 2 removing column 301 is a part of the whole reflux water, and the reflux water fed thereto is subsequently returned to the regenerating column 317. Thus, there is no problem from the viewpoint of water balance in the system. Incidentally, since the system is in a highly corrosive state owing to the presence of CO 2 , oxygen, hydrogen and the like, it is preferred to add an inhibitor for corrosion prevention to the system, as necessary. According to dependent claim 7, the aqueous MEA solution having a concentration of 35% by weight or more is used in the CO 2 removal process shown in Fig. 4 in which CO 2 is removed from the combustion exhaust gas under atmospheric pressure. The preferable concentration of the aqueous MEA solution depends upon a recovery ratio of CO 2 present in the combustion exhaust gas, but it is preferably 40% or more, more preferably in the range of 40 to 45%. Here, the above-mentioned "atmospheric pressure" is understood to also mean such a pressure as that applied by the pump in order to feed the combustion exhaust gas into the CO 2 removing column 301. In order to maintain a predetermined value of the CO 2 absorption ratio which is defined as an absorption and removal ratio of CO 2 present in the combustion exhaust gas by the aqueous MEA solution, for example, an absorption ratio of 90% in the CO 2 removal column 301 under constant conditions of the combustion exhaust gas such as the temperature of the combustion exhaust gas, the CO 2 concentration in the combustion exhaust gas and the flow rate of the combustion exhaust gas, it is presumed that the amount of the required aqueous MEA solution decreases as the concentration of the aqueous MEA solution increases. However, the present inventors have found that decreases in the amount of the aqueous MEA solution do not follow a linear (1st order) function with respect to increases in the concentration, and that a liquid/gas ratio can be even further decreased as the concentration of the aqueous solution increases. The above-mentioned "liquid/gas ratio" means the amount (liter) of the aqueous MEA solution which is used for a unit volume (Nm 3 ) of the combustion exhaust gas under the standard conditions. Also with regard to the number of moles (CO 2 loading value) of CO 2 absorbed by a unit mole of MEA, a similar tendency appears. That is, when the conditions of the combustion exhaust gas and the concentration of the aqueous MEA solution to be used are kept constant and when the liquid/gas ratio is decreased (or the amount of the aqueous MEA solution is decreased), the CO 2 loading value increases. In short, when the liquid/gas ratio is decreased, a reaction ratio between MEA and CO 2 is improved. The present inventors have inspected a relation between the liquid/gas ratios and the CO 2 loading values at various concentrations of the aqueous MEA solution, and as a result, they have found that in order to achieve a given loading value, the liquid/gas ratio must be decreased below a certain value, but this value for the liquid/gas ratio decreases as the concentration of the aqueous MEA solution increases. These facts mean that the amount of the aqueous MEA solution necessary to achieve a given CO 2 absorption ratio or a given loading value is smaller than a value that would be indicated if a liner relationship is assumed between the required amount and concentration of the MEA solution, as the concentration of the aqueous MEA solution becomes higher. Therefore, increasing the concentration of the aqueous MEA solution to be used results in significantly larger decreases in the amount of the aqueous MEA solution circulated in the process. Furthermore, the aqueous MEA solution into which CO 2 is absorbed is regenerated in the regenerating column 317. It has been found that the quantity of heat necessary to regenerate a unit amount of MEA can be decreased as the concentration of the aqueous MEA solution becomes higher, under conditions that only the concentration of the aqueous MEA solution is changed, keeping constant other variables such as the loading value of CO 2 constant and a CO 2 removal ratio. For the regeneration of the aqueous MEA solution, a large amount of heat energy is required, and therefore, as is apparent from the above, sufficient energy saving can be achieved, as the concentration of the aqueous MEA solution is raised. When CO 2 present in the combustion exhaust gas is removed therefrom according to the method of the present invention, a process used is by no means limited to the example shown in Fig. 4. In the process shown in Fig. 4, however, the reflux water of the regenerating column is fed to the CO 2 removal column 301 and then brought into contact with the CO 2 -free combustion exhaust gas, whereby MEA is effectively prevented from escaping from the system together with the CO 2 -free combustion exhaust gas. Therefore, the process in Fig. 4 can be preferably employed as a process for the method of the present invention. In the process shown in Fig. 4, the combustion exhaust gas is cooled by humidification, but an alternative way can also be used in which the cooling water cooled by the cooler is brought into contact with the combustion exhaust gas. However, according to the process shown in Fig. 4, the temperature of the CO 2 -free combustion exhaust gas rises, and the gas comes to contain a large amount of an MEA vapor corresponding to this temperature, but the loss of the MEA vapor can be effectively prevented by using the regenerating column reflux water containing CO 2 . Therefore, a cooler becomes unnecessary for the cooling water, and thus the process in Fig. 4 is advantageous from an economical standpoint. The method according to dependent claim 7 of the present invention will be described in further detail with reference to Experimental Examples 3 and 4. (Experimental Example 3) Fig. 5 shows the results of an experiment in which CO 2 in a combustion exhaust gas is absorbed by an aqueous MEA solution in a small-scale apparatus of Fig. 4. That is, Fig. 5 is a graph showing the absorption ratio of CO 2 (left ordinate axis, units = %) and CO 2 loading value (right ordinate axis, units = CO 2 moles/MEA mole) measured with varying liquid/gas ratio (abscissa, units = liters/m 3 N) under the following experimental conditions using aqueous MEA solutions having various concentrations. Experimental Conditions Temperature of the combustion exhaust gas to be fed to the CO 2 removal column 301: 60°C Concentration of CO 2 in the same gas : 9.0-9.2 vol.% Concentration of oxygen in the same gas: 3.2-3.5 vol.% Flow rate of the same gas: 2.0 Nm 3 /H Temperature of the aqueous MEA solution to be fed to the CO 2 removal column 1: 60°C It is apparent from Fig. 5 that, for example, a value of the liquid/gas ratio necessary to secure an absorption ratio of 95% is 1.9 in the case of the aqueous MEA solution at a concentration of 30%, but it is 0.8 in the case of the aqueous MEA solution at a concentration of 60%, and so in the latter case, the liquid/gas ratio is less than 1/2 of the former case. Furthermore, in order to secure a loading value of, for example, 0.5, the required liquid/gas ratio is 1.75 in the case of the aqueous MEA solution at a concentration of 30%, but it is 0.8 in the case of the aqueous MEA solution at a concentration of 60%, and hence the latter liquid/gas ratio is less than 1/2 of the former. (Experimental Example 4) CO 2 was absorbed and recovered from a combustion exhaust gas of an LNG-fired boiler by the use of an apparatus in Fig. 4. In this case, a relation between the concentration of an aqueous MEA solution and the amount of steam necessary for regeneration was inspected keeping a recovery ratio of CO 2 constant. Experimental Conditions Temperature of the combustion exhaust gas to be fed to the CO 2 removal column 301: 60°C Flow rate of the same gas: 600 Nm 3 /H Concentration of CO 2 in the same gas: 8.56 vol.% Steam used in a regenerating column: Saturated steam of 2.3 kg/cm 2 G The results are set forth in Fig. 6. In Fig. 6, an abscissa axis denotes the concentration of an aqueous MEA solution (% by weight), and an ordinate axis denotes the consumption [kg/Nm 3 (CO 2 )] of a steam necessary for the regeneration. As indicated by Fig. 6, in the case of a recovery ratio of 85%, the consumption of the steam decreases with increases in the concentration of the aqueous MEA solution, and it becomes substantially constant at a concentration of about 50% by weight. With the concentration of the MEA solution being 50% by weight, 8.5% of steam can be saved compared with the case of 30% by weight. In the case of a recovery ratio of 90%, the consumption of the steam decreases with increases in the concentration of the aqueous MEA solution, and when the concentration of the aqueous MEA solution is in excess of 40% by weight, there is a tendency that the consumption of the steam increases again. Thus, when the aqueous MEA solution at a concentration of about 40% by weight is used, the amount of the steam can be decreased as much as 7.8%, as compared with the case where the aqueous MEA solution at a concentration of about 30% by weight is used. Anyway, it can be understood that the concentration of the aqueous MEA solution is preferably 40% or more, more preferably in the range of 40 to 45%.

Claims: A method for treating a combustion exhaust gas, comprising the steps of: denitrating a combustion exhaust gas using ammonia as a reducing agent; removing carbon dioxide present in said combustion exhaust gas by bringing said combustion exhaust gas into contact with an aqueous alkanolamine solution; recovering ammonia present in said combustion exhaust gas having been subjected to said carbon dioxide removal by bringing said exhaust gas into contact with water containing carbon dioxide; and using said recovered ammonia as a reducing agent in said denitrating step. The method claimed in claim 1, characterized in that said combustion exhaust gas is desulfurized before said carbon dioxide removal step. The method claimed in claim 1 or 2, characterized in that the ammonia is recovered from said water which has been brought into contact with said exhaust gas by blowing steam into said water. The method claimed in any of claims 1 to 3, characterized in that said water containing carbon dioxide contains carbon dioxide near its saturation concentration. The method claimed in claim 1, characterized in that the aqueous alkanolamine solution which is reusable is regenerated by separating carbon dioxide from said solution having absorbed carbon dioxide and having been discharged from said carbon dioxide removal step; the carbon dioxide gas separated during said regenerating step and water vapor accompanying said carbon dioxide gas are cooled wherein at least part of the reflux water occurring in said cooling step is supplied to said regenerating step and the remaining of said reflux water is supplied to said carbon dioxide removal step. The method claimed in claim 1 or 4, characterized in that said water containing carbon dioxide is either one of condensed cooling water for said combustion exhaust gas or cooling water which has been brought into contact with said combustion exhaust gas, and supplied to a top portion of a reaction column in which said carbon dioxide removal step is carried out. The method claimed in claim 1, characterized in that the combustion exhaust gas is brought into contact with an aqueous monoethanolamine solution under atmospheric pressure wherein said monoethanolamine solution has a concentration of 35 % by weight or more.

Inventors: FUJII MASUMI, HOTTA YOSHITSUGU, IIJIMA MASAKI, KARASAKI NUTSUNORI, KAWASAKI MASAMI, KITAMURA KOICHI, KOBAYASHI KENJI, MITSUOKA SHIGEAKI, SEZO TOURU, SHIMOJO SHIGERU, SUDA TAIICHIRO, YOSHIDA KUNIHIKO, FUJII, MASUMI, HOTTA, YOSHITSUGU, IIJIMA, MASAKI, KARASAKI, MUTSUNORI, KAWASAKI, MASAMI, KITAMURA, KOICHI, KOBAYASHI, KENJI, MITSUOKA, SHIGEAKI, SETO, TOURU, SHIMOJO, SHIGERU, SUDA, TAIICHIRO, YOSHIDA, KUNIHIKO, FUJII, MASUMI, C/O THE KANSAI ELECTRIC POWER, HOTTA, YOSHITSUGU, C/O THE KANSAI ELECTRIC POWER, IIJIMA, MASAKI, C/O MITSUBISHI JUKOGYO K. K., Karasaki, Nutsunori, c/o Mitsubishi Jukogyo K. K., Kawasaki, Masami, c/o Mitsubishi Jukogyo K. K., KITAMURA, KOICHI, C/O THE KANSAI ELECTRIC POWER, Kobayashi, Kenji, c/o The Kansai Electric Power, MITSUOKA, SHIGEAKI, MITSUBISHI JUKOGYO K. K., SEZO, TOURU, MITSUBISHI JUKOGYO K. K., SHIMOJO, SHIGERU, C/O THE KANSAI ELECTRIC POWER, SUDA, TAIICHIRO, C/O THE KANSAI ELECTRIC POWER, YOSHIDA, KUNIHIKO, C/O THE KANSAI ELECTRIC POWER
Applicants: KANSAI ELECTRIC POWER CO, MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA, THE KANSAI ELECTRIC POWER CO., INC.
Application year: 1993
Main classification: B01D53
IPC codes: B01D53