Abstract:
A method of heating a used absorption liquid resulting from a carbon dioxide absorption stage in which a CO 2  comprising gas stream is contacted with an absorption liquid comprising ammonia, wherein the used absorption liquid is subjected to heat exchange, in a heat exchanger, with a fluid of a desulfurization stage in which a SO 2  comprising flue gas is contacted with a reagent composition comprising an alkaline compound.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/287,221 filed Dec. 17, 2009, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application relates to processes and systems for desulfurization of, and removal of carbon dioxide from, gas mixtures and to a method of heating a used absorption liquid resulting from a carbon dioxide absorption stage in which a CO 2  comprising gas stream is contacted with an absorption liquid comprising ammonia. 
       BACKGROUND 
       [0003]    Environmental concern or other air quality control needs render desirable the desulfurization of, and removal of carbon dioxide from, gas mixtures, such as flue gases released from oil- or coal-fired steam generators or other power production plants. 
         [0004]    Desulfurization of a gas mixture refers to removal of sulfur dioxide (SO 2 ) from the mixture. Wet as well as dry processes for desulfurization are available. In a typical wet flue gas desulfurization process (WFGD), the flue gas is contacted with an alkali slurry in an absorption tower. In the presence of water and oxygen, SO 2  reacts with the alkali to form a sulfur based salt. In some cases, the sulfur salt is precipitated as a solid. Flue gas depleted of SO 2  leaves the absorption tower at the adiabatic saturation temperature, whereas the sulfur salt byproduct formed is recovered from the slurry and is used for commercial purposes or is sent to disposal. A typical dry flue gas desulfurization process (DFGD) is based on the reaction between SO 2  and Ca(OH) 2  in humid conditions. A humidified mixture of hydrated lime and reaction product is injected into an absorber and cools the inlet flue gas by evaporation. The cooled flue gas then flows to a dust collector where the particles in the flue gas are removed and recycled back for mixing with hydrated lime. 
         [0005]    Removal of carbon dioxide (CO 2 ) from a gas mixture is often referred to as CO 2  capture. In a typical ammonia solvent based process for removal of CO 2  from a flue gas, the flue gas enters an absorption tower where it is contacted with an absorption liquid comprising ammonia. Flue gas depleted of CO 2  leaves the absorption tower whereas used absorption liquid is sent to regeneration. During regeneration, CO 2  is released from the used absorption liquid. Since the absorption of CO 2  from the flue gas in the absorption liquid is often operated at a temperature of the absorption liquid of below room temperature, this process may be referred to as the chilled ammonia process. During regeneration of the absorption liquid, the temperature of the used absorption liquid is raised, typically by heat exchange with steam or with the regenerated adsorption liquid returned from the regenerator, from a lower temperature employed during absorption to a higher temperature facilitating release of CO 2  from the used absorption liquid. 
         [0006]    WO 2006/022885 (U.S. patent application Ser. No. 11/632,537, filed Jan. 16, 2007, and which is incorporated by reference herein in its entirety) discloses one such method of removing carbon dioxide from a flue gas, which method includes capturing carbon dioxide from the flue gas in a CO 2  absorber by means of an ammoniated solution or slurry. The CO 2  is absorbed by the ammoniated solution in the absorber at a reduced temperature of between about 0° C. and 20° C., after which the ammoniated solution is regenerated in a regenerator under elevated pressure and temperature to allow the CO 2  to escape the ammoniated solution as gaseous carbon dioxide of high purity. 
         [0007]    It is of continuous interest to improve energy efficiency or reduce energy consumption related to desulfurization of, and removal of carbon dioxide from, gas mixtures. There is a need to improve the energy efficiency, such as to lower the steam consumption, involved in heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. 
       SUMMARY 
       [0008]    According to aspects illustrated herein, there is provided a process for desulfurization of, and removal of carbon dioxide from, gas mixtures, comprising the steps of:
   (a) contacting in a desulfurization stage a flue gas comprising SO 2  with a scrubbing liquor comprising an alkaline compound and, optionally, with oxidation air, to remove SO 2  from the flue gas;   (b) contacting in a carbon dioxide absorption stage a second gas stream comprising CO 2  with an absorption liquid comprising ammonia, to remove CO 2  from the second gas stream;   (c) subjecting the scrubbing liquid, the flue gas or the oxidation air to heat exchange, in a heat exchanger, with used absorption liquid resulting from step (b), to heat the used absorption liquid; and   (d) regenerating the absorption liquid by releasing CO 2  from the heated used absorption liquid resulting from step (c).   
 
         [0013]    In this process related to wet desulfurization of, and removal of carbon dioxide from, gas mixtures, the energy efficiency involved in heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia has thus been improved by utilization of heat from the desulfurization stage. The flue gas may be a flue gas resulting from combustion of a fossil fuel. It is a common desire to reduce the SO 2  and/or the CO 2  content of a flue gas before emitting it to the atmosphere. 
         [0014]    In the desulfurization stage, the alkaline compound of the scrubbing liquor may be an alkaline calcium compound, such as CaCO 3  or Ca(OH) 2 , or another alkaline reagent, such as ammonia, sodium, lime, seawater or flyash, providing the alkalinity needed in the absorption reaction. In the case of the alkaline compound being CaCO 3 , fresh scrubbing liquor may conveniently be prepared from limestone and water. Presence of oxygen during the desulfurization stage may improve the SO 2  removal process and byproduct characteristics. A sufficient amount of oxygen may be present in the flue gas. Alternatively, oxygen may be supplied to the desulfurization stage as oxidation air, such as in the form of compressed air, to produce a commercial byproduct, to improve the byproduct dewatering characteristics or to reduce the likelihood of scale formation. Typically, for a limestone based scrubbing liquor comprising CaCO 3 , although the process is not bound to this theory, removal of SO 2  from the flue gas is effectuated according to the following reaction formula. 
         [0000]      SO 2 +CaCO 3 +½O 2 +H 2 O→CaSO 4 .2H 2 O+CO 2  
 
         [0000]    The flue gas may enter the desulfurization stage at room temperature or above, often at about 120-200° C., preferably about 150-200° C. Fresh scrubbing liquor may enter the desulfurization stage at ambient temperature or above. The optional oxidation air may conveniently enter the desulfurization stage at its compression temperature of about 140-250° C., preferably about 150-200° C., or after cooling by an aftercooler or by water spray to about 60-70° C. In the desulfurization stage, the flue gas may be contacted with the scrubbing liquor in a counter-current flow, the scrubbing liquor typically being sprayed into the flue gas in a spray tower. The bottom of the spray tower may serve as a reservoir for scrubbing liquor, to which reservoir fresh scrubbing liquor as well as scrubbing liquor that has been in contact with the flue gas may be passed and from which reservoir scrubbing liquid for spraying as well as for further treatment, such as separation of solids formed during the desulfurization stage, may be withdrawn. 
         [0015]    In the carbon dioxide absorption stage, an absorption liquid comprising ammonia absorbs CO 2  from a gas stream comprising CO 2 . The CO 2  from the gas stream may form salts, such as ammonium carbonate, ammonium bicarbonate and ammonium carbamate, in the form of a solution or slurry with ammonia from the absorption liquid, thus removing CO 2  from the gas stream. The gas stream may be cooled before entering the carbon dioxide absorption stage. Preferably, the gas stream is cooled below ambient (room) temperature before entering the CO 2  absorption tower. For example, the flue gas may be cooled below 25° C., preferably below 20° C., and optionally below 10° C. in step (a). The absorption liquid may enter the carbon dioxide absorption stage at a temperature below about 25° C. preferably below 20° C., and optionally below 10° C. In the carbon dioxide absorption stage, the gas stream may be contacted with the absorption liquid in a counter-current flow, the absorption liquid typically being sprayed into the second gas stream in a packed tower. Used absorption liquid may leave the carbon dioxide absorption stage at a temperature below about 25° C. Reactions during the carbon dioxide absorption stage are reversible when heat is applied. Thus, regeneration of absorption liquid is performed by releasing CO 2  from used absorption liquid, i.e. from absorption liquid that has been contacted with the second gas stream and has absorbed CO 2 . The temperature of the absorption liquid during regeneration may be up to about 175° C., and be higher than during CO 2  absorption. 
         [0016]    In order to heat the used absorption liquid, the scrubbing liquor, the flue gas or the oxidation air is subjected to heat exchange, in a heat exchanger, with used absorption liquid resulting from the carbon dioxide absorption stage. The heat exchange involves transfer of heat from a fluid of relatively higher temperature, i.e. from the scrubbing liquor, the flue gas or the oxidation air of the flue gas desulfurization process, to the used absorption liquid of a relatively lower temperature in a heat exchanger. The heat exchanger is, as conventionally understood, a device for transferring heat from one fluid to another without allowing them to mix. With regard to design parameters such as temperature difference and flow rates of the fluids in question, a skilled person is able to select a suitable type of heat exchanger. The heat exchanger may, e.g., be of plate, tubular or coil type. 
         [0017]    The process may be designed so that in step (c) the scrubbing liquor is subjected to heat exchange, in a heat exchanger, with used absorption liquid resulting from step (b). Thus, heat from the scrubbing liquor of the desulfurization stage is utilized in the heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. Such reagent liquid may have a temperature of about 45-65° C. As an example, the heat exchange may take place between scrubbing liquor present in a reservoir formed, e.g., at the bottom of an absorption tower and the used absorption liquid. Such reagent liquid may have a temperature of about 45-65° C. As another example, the heat exchange may take place between scrubbing liquor withdrawn from said reservoir and for spraying into the first gas stream in said absorption tower and the used absorption liquid. Such scrubbing liquor may have a temperature of about 45-65° C. 
         [0018]    The process may be designed so that scrubbing liquor is withdrawn from the desulfurization stage of step (a) and solids formed during step (a) are removed from the withdrawn scrubbing liquor, before the withdrawn scrubbing liquor is subjected to the heat exchange of step (c). Since a scrubbing liquor of reduced solids content is utilized, operation of the heat exchanger can be expected to occur with reduced risk of clogging. Such reagent liquid may have a temperature of about 45-65° C. As an example, the heat exchange may take place between the scrubbing liquor after solids have been removed in a hydroclone and/or a belt filter. Such scrubbing liquor may have a temperature of about 45-65° C. The heat exchange may take place along a conduit withdrawing such solids-reduced scrubbing liquor from said hydroclone or belt filter or it may take place in a tank collecting such solids-reduced scrubbing liquor. 
         [0019]    The process may be designed so that in step (c) the flue gas is subjected to heat exchange, in a heat exchanger, with used absorption liquid resulting from step (b). Thus, heat from the flue gas is utilized in the heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. The flue gas may be at room temperature or above, often at about 120-200° C., preferably about 150-200° C. 
         [0020]    The process may be designed so that in step (a) the flue gas is contacted with the scrubbing liquor and with the oxidation air. Presence of oxidation air during the desulfurization stage may improve the SO 2  removal process and byproduct characteristics. Oxidation air may be supplied in the form of compressed air at a pressure of about 1-3 bar. Such a process may be further designed so that in step (c) the oxidation air is subjected to heat exchange, in a heat exchanger, with used absorption liquid resulting from step (b). Thus, heat from the oxidation air is utilized in the heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. The oxidation air may be at its compression temperature of about 140-250° C., preferably about 150-200° C., or at a temperature reached after cooling by an aftercooler or by water spray of about 60-70° C. 
         [0021]    The process may be designed so that in step (b) the gas stream is essentially the SO 2  depleted flue gas resulting from step (a). Accordingly, a process for desulfurization of, and removal of carbon dioxide from, the same gas mixture is achieved. In such a process, CO 2  resulting from the desulfurization stage may be removed from the gas mixture. The gas stream being “essentially” the SO 2  depleted flue gas resulting from step (a) means, e.g., that other contaminants may be removed from the gas stream between steps (a) and (b) or that the temperature of the gas stream may be changed between said steps. Typically, the temperature of the SO 2  depleted flue gas is lowered before it enters step (b). 
         [0022]    According to other aspects illustrated herein, there is provided a system for desulfurization of, and removal of carbon dioxide from, gas mixtures, comprising a desulfurization system comprising 
         [0023]    a desulfurization vessel for contacting a flue gas with a scrubbing liquor; 
         [0024]    a withdrawal circuit arranged to withdraw scrubbing liquor from the desulfurization vessel, 
         [0025]    a gas conduit arranged to supply the flue gas to the desulfurization vessel; and 
         [0026]    optionally, an air conduit arranged to supply oxidation air to the desulfurization vessel; 
         [0000]    and a carbon dioxide removal system comprising 
         [0027]    a CO 2  absorber for contacting a second gas stream with an absorption liquid, the CO 2  absorber having an inlet for absorption liquid and an outlet for used absorption liquid; and 
         [0028]    a regenerator for regenerating the absorption liquid by release of CO 2  from used absorption liquid, the regenerator having an inlet for used absorption liquid and an outlet for regenerated absorption liquid, 
         [0000]    wherein the outlet for used absorption liquid of the CO 2  absorber and the inlet for used absorption liquid of the regenerator are operatively connected via a heat exchanger, the heat exchanger further being operatively arranged in the desulfurization system. 
         [0029]    In this system related to wet desulfurization of, and removal of carbon dioxide from, gas mixtures, the energy efficiency involved in heating of a used absorption liquid resulting from the CO 2  in an absorption liquid comprising ammonia has thus been improved by utilization of heat from the desulfurization system. The desulfurization system and the carbon dioxide removal system operate according to the principles set forth for the wet desulfurization process discussed above. 
         [0030]    The heat exchanger is, as conventionally understood, a device for transferring heat from one fluid to another without allowing them to mix. With regard to design parameters such as temperature difference and flow rates of the fluids in question, a skilled person is able to select a suitable type of heat exchanger. The heat exchanger may, e.g., be of plate, tubular or coil type. The system may be designed so that the heat exchanger further being operatively arranged in the desulfurization system is operatively arranged in the desulfurization vessel. As an example, the heat exchange may take place between scrubbing liquor present at the bottom of the desulfurization vessel and the used absorption liquid. 
         [0031]    The system may be designed so that the heat exchanger further being operatively arranged in the desulfurization system is operatively arranged in the withdrawal circuit. As an example, the heat exchange may take place between scrubbing liquor withdrawn from the desulfurization vessel and for spraying into the flue gas in said desulfurization vessel and the used absorption liquid. As another example, the heat exchange may take place between the scrubbing liquor after solids have been removed in a hydroclone and/or a belt filter and the used absorption liquid. The heat exchanger may be operatively arranged in a conduit withdrawing such solids-reduced scrubbing liquor from said hydroclone or belt filter or the heat exchanger may be operatively arranged in a tank collecting such solids-reduced scrubbing liquor. 
         [0032]    The system may be designed so that the heat exchanger further being operatively arranged in the desulfurization system is operatively arranged in the gas conduit 
         [0033]    The system may be designed so that the desulfurization system comprises the air conduit. In such system, the heat exchanger further being operatively arranged in the desulfurization system may be operatively arranged in the air conduit. 
         [0034]    According to other aspects illustrated herein, there is provided a process for desulfurization of, and removal of carbon dioxide from, gas mixtures, comprising the steps of:
   (a) preparing a reagent mixture by combining lime and water, to obtain a solution of slaked lime, and mixing the solution of slaked lime with a particulate, to obtain a reagent mixture;   (b) contacting in a desulfurization stage a flue gas comprising SO 2  with the reagent mixture, to remove SO 2  from the flue gas;   (c) contacting in a carbon dioxide absorption stage a gas stream comprising CO 2  with an absorption liquid comprising ammonia, to remove CO 2  from the gas stream;   (d) subjecting the solution of slaked lime or the flue gas to heat exchange, in a heat exchanger, with used absorption liquid resulting from step (c), to heat the used absorption liquid; and   (e) regenerating the absorption liquid by releasing CO 2  from the heated used absorption liquid resulting from step (d).   
 
         [0040]    In this process related to dry desulfurization of, and removal of carbon dioxide from, gas mixtures, the energy efficiency involved in heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia has thus been improved by utilization of heat from the desulfurization stage or from the preparation of a reagent mixture for the desulfurization stage. The flue gas may be a flue gas resulting from combustion of a fossil fuel. It is a common desire to reduce the SO 2  and/or the CO 2  content of a flue gas before emitting it to the atmosphere. 
         [0041]    In the desulfurization stage, the reagent mixture comprises slaked lime. The reagent mixture used is thus conveniently prepared by combining lime and water, to obtain a solution of slaked lime, and mixing the solution of slaked lime with a particulate, to obtain a reagent mixture. The combination of lime and water to obtain a solution of slaked lime is an exothermic reaction, which may result in a solution of slaked lime having a temperature of about 50-65° C. The particulate used is typically the product of a reaction between SO 2  and Ca(OH) 2  in humid conditions, i.e. a product of the desulfurization stage. Removal of SO 2  from the first gas stream may thus be effectuated by reaction of the SO 2  with Ca(OH) 2  in humid conditions and subsequent filtering off of the particulate reaction product. The flue gas may enter the desulfurization stage at room temperature or above, often at about 120-200° C., preferably about 150-200° C. In the desulfurization stage, the flue gas may be contacted with the reagent mixture in a spray dryer type tower. Particulate reaction product may be collected from the bottom of the tower. The SO 2  depleted gas stream leaving the tower may pass a filter, such as a fabric filter, where further reaction product is collected. 
         [0042]    In the carbon dioxide absorption stage, an absorption liquid comprising ammonia absorbs CO 2  from a gas stream comprising CO 2 . The CO 2  from the gas stream may form salts, such as ammonium carbonate, ammonium bicarbonate and ammonium carbamate, in the form of a solution or slurry with ammonia from the absorption liquid, thus removing CO 2  from the gas stream. The gas stream may be cooled before entering the carbon dioxide absorption stage. Preferably, the gas stream is cooled below ambient (room) temperature before entering the CO 2  absorption tower. For example, the flue gas may be cooled below 25° C., preferably below 20° C., and optionally below 10° C. in step (a). The absorption liquid may enter the carbon dioxide absorption stage at a temperature below about 25° C. preferably below 20° C., and optionally below 10° C. In the carbon dioxide absorption stage, the gas stream may be contacted with the absorption liquid in a counter-current flow, the absorption liquid typically being sprayed into the second gas stream in a packed tower. Used absorption liquid may leave the carbon dioxide absorption stage at a temperature below about 25° C. Reactions during the carbon dioxide absorption stage are reversible when heat is applied. Thus, regeneration of absorption liquid is performed by releasing CO 2  from used absorption liquid, i.e. from absorption liquid that has been contacted with the second gas stream and has absorbed CO 2 . The temperature of the absorption liquid during regeneration may be up to about 175° C., and be higher than during CO 2  absorption. 
         [0043]    In order to heat the used absorption liquid, the solution of slaked lime or the flue gas is subjected to heat exchange, in a heat exchanger, with used absorption liquid resulting from the carbon dioxide absorption stage. The heat exchange involves transfer of heat from a fluid of relatively higher temperature, i.e. from the solution of slaked lime or the flue gas, to the used absorption liquid of a relatively lower temperature in a heat exchanger. The heat exchanger is, as conventionally understood, a device for transferring heat from one fluid to another without allowing them to mix. With regard to design parameters such as temperature difference and flow rates of the fluids in question, a skilled person is able to select a suitable type of heat exchanger. The heat exchanger may, e.g., be of plate, tubular or coil type. 
         [0044]    The process may be designed so that in step (d) the solution of slaked lime is subjected to heat exchange, in a heat exchanger, with used absorption liquid resulting from step (c). Thus, heat from the preparation of a reagent mixture for the desulfurization stage is utilized in the heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. Such solution of slaked lime may have a temperature of about 50-65° C. As an example, the heat exchange may take place between solution of slaked lime present in a tank, such as a storage or feed tank, and the used absorption liquid. As another example, the heat exchange may take place between solution of slaked lime flowing through a conduit and the used absorption liquid. 
         [0045]    The process may be designed so that in step (d) the flue gas is subjected to heat exchange, in a heat exchanger, with used absorption liquid resulting from step (c). Thus, heat from the flue gas is utilized in the heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. The flue gas may be at room temperature or above, often at about 120-200° C., preferably about 150-200° C. 
         [0046]    The process may be designed so that in step (c) the gas stream is essentially the SO 2  flue gas resulting from step (b). Accordingly, a process for desulfurization of, and removal of carbon dioxide from, the same gas mixture is achieved. In such a process, CO 2  resulting from the desulfurization stage may be removed from the gas mixture. The second gas stream being “essentially” the SO 2  depleted flue gas resulting from step (b) means, e.g., that other contaminants may be removed from the gas stream between steps (b) and (c) or that the temperature of the gas stream may be changed between said steps. Typically, the temperature of the SO 2  depleted flue gas is lowered before it enters step (b). 
         [0047]    According to other aspects illustrated herein, there is provided a system for desulfurization of, and removal of carbon dioxide from, gas mixtures, comprising a desulfurization system comprising 
         [0048]    a slaker adapted for preparation of slaked lime and arranged to receive lime and water; 
         [0049]    a feed circuit adapted for storage and supply of slaked lime and arranged to receive slaked lime from the slaker; 
         [0050]    a mixer adapted for preparation of a reagent mixture and arranged to receive slaked lime from the feed circuit; 
         [0051]    a desulfurization vessel adapted for contacting a flue gas with a reagent mixture and arranged to receive a reagent mixture from the mixer; and 
         [0052]    a gas conduit arranged to supply the flue gas to the desulfurization vessel, and a carbon dioxide removal system comprising 
         [0053]    a CO 2  absorber for contacting a gas stream with an absorption liquid, the CO 2  absorber having an inlet for absorption liquid and an outlet for used absorption liquid; and 
         [0054]    a regenerator for regenerating the absorption liquid by release of CO 2  from used absorption liquid, the regenerator having an inlet for used absorption liquid and an outlet for regenerated absorption liquid, 
         [0000]    wherein the outlet for used absorption liquid of the CO 2  absorber and the inlet for used absorption liquid of the regenerator are operatively connected via a heat exchanger, the heat exchanger further being operatively arranged in the desulfurization system. 
         [0055]    In this system related to dry desulfurization of, and removal of carbon dioxide from, gas mixtures, the energy efficiency involved in heating of a used absorption liquid resulting from the CO 2  in an absorption liquid comprising ammonia has thus been improved by utilization of heat from the desulfurization system. The desulfurization system and the carbon dioxide removal system operate according to the principles set forth for the dry desulfurization process discussed above. 
         [0056]    The heat exchanger is, as conventionally understood, a device for transferring heat from one fluid to another without allowing them to mix. With regard to design parameters such as temperature difference and flow rates of the fluids in question, a skilled person is able to select a suitable type of heat exchanger. The heat exchanger may, e.g., be of plate, tubular or coil type. 
         [0057]    The system may be designed so that the heat exchanger further being operatively arranged in the desulfurization system is operatively arranged in the feed circuit. As an example, the heat exchange may take place between solution of slaked lime flowing through a conduit and the used absorption liquid. In such a system, the feed circuit may comprise a tank and the heat exchanger may be operatively arranged in the tank. As an example, the heat exchange may take place between solution of slaked lime present in a tank, such as a storage or feed tank, and the used absorption liquid. 
         [0058]    The system may be designed so that the heat exchanger further being operatively arranged in the desulfurization system is operatively arranged in the gas conduit. 
         [0059]    According to other aspects illustrated herein, there is provided a method of heating a used absorption liquid resulting from a carbon dioxide absorption stage in which a CO 2  comprising gas stream is contacted with an absorption liquid comprising ammonia, wherein the used absorption liquid is subjected to heat exchange, in a heat exchanger, with a fluid of a desulfurization stage in which a SO 2  comprising flue gas is contacted with a reagent composition comprising an alkaline compound. 
         [0060]    In this method related to desulfurization of, and removal of carbon dioxide from, gas mixtures, the energy efficiency involved in heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia has thus been improved by utilization of heat from a fluid of a desulfurization stage. The flue gas may be a flue gas resulting from combustion of a fossil fuel. It is a common desire to reduce the SO 2  and/or the CO 2  content of a flue gas before emitting it to the atmosphere. 
         [0061]    In the carbon dioxide absorption stage, an absorption liquid comprising ammonia absorbs CO 2  from a gas stream comprising CO 2 . The CO 2  from the gas stream may form salts, such as ammonium carbonate, ammonium bicarbonate and ammonium carbamate, in the form of a solution or slurry with ammonia from the absorption liquid, thus removing CO 2  from the gas stream. The gas stream may be cooled before entering the carbon dioxide absorption stage. Preferably, the gas stream is cooled below ambient (room) temperature before entering the CO2 absorption tower. For example, the flue gas may be cooled below 25° C., preferably below 20° C., and optionally below 10° C. in step (a). The absorption liquid may enter the carbon dioxide absorption stage at a temperature below about 25° C. preferably below 20° C., and optionally below 10° C. In the carbon dioxide absorption stage, the gas stream may be contacted with the absorption liquid in a counter-current flow, the absorption liquid typically being sprayed into the second gas stream in a packed tower. Used absorption liquid may leave the carbon dioxide absorption stage at a temperature below about 25° C. 
         [0062]    In the desulfurization stage, the alkaline compound of the reagent composition may be an alkaline calcium compound, such as CaCO 3  or Ca(OH) 2 , or another alkaline reagent, such as ammonia, sodium, lime, seawater or flyash, providing the alkalinity needed in the absorption reaction. The SO 2  comprising flue gas may enter the desulfurization stage at room temperature or above, often at about 120-200° C., preferably about 150-200° C. In the desulfurization stage, the SO 2  comprising flue gas may be contacted with the reagent composition in a spray tower absorber or a spray dryer type tower. 
         [0063]    In order to heat the used absorption liquid, a fluid of the desulfurization stage is subjected to heat exchange, in a heat exchanger, with used absorption liquid resulting from the carbon dioxide absorption stage. The heat exchange involves transfer of heat from a fluid of relatively higher temperature, i.e. from the fluid of the desulfurization stage, to the used absorption liquid of a relatively lower temperature in a heat exchanger. The heat exchanger is, as conventionally understood, a device for transferring heat from one fluid to another without allowing them to mix. With regard to design parameters such as temperature difference and flow rates of the fluids in question, a skilled person is able to select a suitable type of heat exchanger. The heat exchanger may, e.g., be of plate, tubular or coil type. 
         [0064]    In one embodiment of the method, the fluid is a scrubbing liquor comprising an alkaline compound. The alkaline calcium compound may be an alkaline calcium compound, such as CaCO 3  or Ca(OH) 2 , or another alkaline reagent, such as ammonia, sodium, lime, seawater or flyash, providing the alkalinity needed in the absorption reaction. Thus, heat from a fluid of the desulfurization stage is utilized in the heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. Such scrubbing liquor may have a temperature of about 45-65° C. As an example, the heat exchange may take place between scrubbing liquor present in a reservoir formed at the bottom of an absorption tower and the used absorption liquid. As another example, the heat exchange may take place between scrubbing liquor withdrawn from said reservoir and for spraying into the flue gas in said absorption tower and the used absorption liquid. Such reagent liquid may have a temperature of about 45-65° C. 
         [0065]    In such embodiment, solids formed during the desulfurization stage may be removed from the scrubbing liquor before the scrubbing liquor is subjected to the heat exchange. Since a scrubbing liquor of reduced solids content is utilized, operation of the heat exchanger can be expected to occur with reduced risk of clogging. Such reagent liquid may have a temperature of about 45-65° C. As an example, the heat exchange may take place between the scrubbing liquor after solids have been removed in a hydroclone and/or a belt filter. Such scrubbing liquor may have a temperature of about 45-65° C. The heat exchange may take place along a conduit withdrawing such solids-reduced scrubbing liquor from said hydroclone or belt filter or the heat exchange may take place in a tank collecting such solids-reduced scrubbing liquor. 
         [0066]    In another embodiment of the method, the fluid is the SO 2  comprising flue gas. Thus, heat from a fluid of the desulfurization stage is utilized in the heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. The SO 2  comprising flue gas may be at room temperature or above, often at about 120-200° C., preferably about 150-200° C. 
         [0067]    In another embodiment of the method, the fluid is oxidation air. Thus, heat from a fluid of the desulfurization stage is utilized in the heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. Presence of oxidation air during the desulfurization stage may improve the SO 2  removal process and byproduct characteristics. Oxidation air may be supplied in the form of compressed air at a pressure of about 1-3 bar. The oxidation air may be at its compression temperature of about 140-250° C., preferably about 150-200° C., or at a temperature reached after cooling by an aftercooler or by water spray of about 60-70° C. 
         [0068]    In another embodiment of the method, the fluid is a solution of slaked lime. Thus, heat from a fluid of the desulfurization stage is utilized in the heating of a used absorption liquid resulting from absorption of CO 2  in an absorption liquid comprising ammonia. Such solution of slaked lime may have a temperature of about 50-65° C. As an example, the heat exchange may take place between solution of slaked lime present in a tank, such as a storage or feed tank, and the used absorption liquid. As another example, the heat exchange may take place between solution of slaked lime flowing through a conduit and the used absorption liquid. 
         [0069]    The method may be designed so that the CO 2  comprising gas stream is essentially the gas stream resulting from desulfurization of the SO 2  comprising flue gas. Accordingly, a method for desulfurization of, and removal of carbon dioxide from, the same gas mixture is achieved. In such a process, CO 2  resulting from the desulfurization stage may be removed from the gas mixture. The CO 2  comprising gas stream being “essentially” the gas stream resulting from desulfurization of the SO 2  comprising flue gas means, e.g., that other contaminants may be removed from the gas stream between the desulfurization stage and the carbon dioxide absorption stage or that the temperature of the gas stream may be changed between said stages. Typically, the temperature of the SO 2  depleted gas stream is lowered before it enters the carbon dioxide absorption stage. 
         [0070]    Reactions during the carbon dioxide absorption stage are reversible when heat is applied. Thus, regeneration of absorption liquid is performed by releasing CO 2  from used absorption liquid, i.e. from absorption liquid that has been contacted with the second gas stream and has absorbed CO 2 . The temperature of the absorption liquid during regeneration may be up to about 175° C., and be higher than during CO 2  absorption. 
         [0071]    The above described and other features are exemplified by the following figures and detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0072]    Referring now to the figures, wherein the like elements are numbered alike: 
           [0073]      FIG. 1  is a schematical representation of a prior art system for wet desulfurization of, and removal of carbon dioxide from, gas mixtures; 
           [0074]      FIG. 2  is a schematical representation of an exemplary system for wet desulfurization of, and removal of carbon dioxide from, gas mixtures: 
           [0075]      FIG. 3  is a schematical representation of exemplary systems for wet desulfurization of, and removal of carbon dioxide from, gas mixtures; and 
           [0076]      FIG. 4  is a schematical representation of exemplary systems for dry desulfurization of, and removal of carbon dioxide from, gas mixtures. 
       
    
    
     DETAILED DESCRIPTION 
       [0077]      FIG. 1  illustrates a prior art system for wet desulfurization of, and removal of carbon dioxide from, a flue gas. The prior art system comprises a system  100  for desulfurization and a system  200  for removal of carbon dioxide. 
         [0078]    Flue gas enters a spray tower absorber  102  via an electrostatic precipitator  104  and a gas conduit  106 . Fresh scrubbing liquor is prepared from limestone and water in a ball mill  108  and enters the spray tower absorber  102 . Water is also supplied directly to the spray tower absorber  102 . Oxidation air is supplied to the spray tower absorber  102  via an air conduit  110 . Scrubbing liquor is passed through a circuit  112  and sprayed into the flue gas in the spray tower absorber  102 . By reaction in the spray tower absorber  102  of SO 2  with CaCO 3  in the presence of water and air, SO 2  is removed from the flue gas and gypsum is formed. Scrubbing liquor comprising gypsum is withdrawn from the spray tower absorber  102  to a hydroclone  114  and a subsequent belt filter  116 , from which gypsum is discharged. Scrubbing liquor recovered from the hydroclone  114  and the belt filter  116  is returned to the spray tower absorber  102  in a reclaim water circuit  118  via a water tank  120 . Flue gas depleted of SO 2  leaves the system  100  for desulfurization via a conduit  122 . 
         [0079]    Flue gas depleted of SO 2  enters an absorption tower  202  via the conduit  122  and a cooler  204 . An absorption liquid comprising ammonia enters the absorption tower  202  via a conduit  206  and is sprayed into the flue gas in the absorption tower  202 . By absorption in the absorption tower  202  of CO 2  in the absorption liquid, CO 2  is removed from the flue gas. Flue gas depleted of CO 2  passes an ammonia condenser  208  and leaves the system  200  for removal of carbon dioxide via a conduit  210 . Ammonia recovered in the ammonia condenser  208  is returned to the absorption tower  202  via a conduit  212 . Used absorption liquid is withdrawn from the absorption tower  202  to a regenerator  214  via a heat exchanger  216 . The used absorption liquid in the regenerator  214  is circulated in a circuit  218  via a heat exchanger  220 . In the heat exchanger  220  steam is used for heating of the used absorption liquid. By release of CO 2  from the used absorption liquid, the absorption liquid is regenerated. Regenerated absorption liquid is recycled to the absorption tower  202  via the heat exchanger  216  and the conduit  206 . Released CO 2  leaves the regenerator via a conduit  222 . 
         [0080]      FIG. 2  illustrates an exemplary system for wet desulfurization of, and removal of carbon dioxide from, a flue gas. The system comprises a system  300  for desulfurization and a system  400  for removal of carbon dioxide. 
         [0081]    Flue gas enters a spray tower absorber  102  via an electrostatic precipitator  104  and a gas conduit  106 . Fresh scrubbing liquor is prepared from limestone and water in a ball mill  108  and enters the spray tower absorber  102 . Water is also supplied directly to the spray tower absorber  102 . Oxidation air is supplied to the spray tower absorber  102  via an air conduit  110 . Scrubbing liquor is passed through a circuit  112  and sprayed into the flue gas in the spray tower absorber  102 . By reaction in the spray tower absorber  102  of SO 2  with CaCO 3  in the presence of water and air, SO 2  is removed from the flue gas and gypsum is formed. Scrubbing liquor comprising gypsum is withdrawn from the spray tower absorber  102  to a hydroclone  114  and a subsequent belt filter  116 , from which gypsum is discharged. Scrubbing liquor recovered from the hydroclone  114  and the belt filter  116  is returned to the spray tower absorber  102  in a reclaim water circuit  118  via a water tank  120  and a heat exchanger  420 . The heat exchanger  420  is further described below. Flue gas depleted of SO 2  leaves the system  300  for desulfurization via a conduit  122 . 
         [0082]    Flue gas depleted of SO 2  enters an absorption tower  202  via the conduit  122  and a cooler  204 . An absorption liquid comprising ammonia enters the absorption tower  202  via a conduit  206  and is sprayed into the flue gas in the absorption tower  202 . By absorption in the absorption tower  202  of CO 2  in the absorption liquid, CO 2  is removed from the flue gas. Flue gas depleted of CO 2  passes an ammonia condenser  208  and leaves the system  400  for removal of carbon dioxide via a conduit  210 . Ammonia recovered in the ammonia condenser  208  is returned to the absorption tower  202  via a conduit  212 . Used absorption liquid is withdrawn from the absorption tower  202  to a regenerator  214  via a heat exchanger  216 . The used absorption liquid in the regenerator  214  is circulated in a circuit  418  via the heat exchanger  420 . The heat exchanger  420  is operatively connected to the reclaim water circuit  118 . In the heat exchanger  420  reagent liquid recovered from the hydroclone  114  and the belt filter  116  is used for heating of the used absorption liquid. By release of CO 2  from the used absorption liquid, the absorption liquid is regenerated. Regenerated absorption liquid is recycled to the absorption tower  202  via the heat exchanger  216  and the conduit  206 . Released CO 2  leaves the regenerator via a conduit  222 . 
         [0083]      FIG. 3  illustrates exemplary systems for wet desulfurization of, and removal of carbon dioxide from, a flue gas. The systems comprise a system  300  for desulfurization and a system  400  for removal of carbon dioxide. Alternative positions of the heat exchanger in the circuit  418  have been indicated by reference numbers  420   a ,  420   b ,  420   c ,  420   d  and  420   e.    
         [0084]    In a first alternative, a heat exchanger  420   a  in the circuit  418  is positioned in the water tank  120 . In the heat exchanger  420   a  scrubbing liquor recovered from the hydroclone  114  and the belt filter  116  and collected in the water tank  120  is used for heating of the used absorption liquid. 
         [0085]    In a second alternative, a heat exchanger  420   b  in the circuit  418  is positioned in the spray tower absorber  102 . In the heat exchanger  420   b  scrubbing liquor present in the spray tower absorber  102  is used for heating of the used absorption liquid. 
         [0086]    In a third alternative, a heat exchanger  420   c  in the circuit  418  is operatively connected to the circuit  112 . In the heat exchanger  420   c  scrubbing liquor to be sprayed into the flue gas in the spray tower absorber  102  is used for heating of the used absorption liquid. 
         [0087]    In a fourth alternative, a heat exchanger  420   d  in the circuit  418  is positioned in the gas conduit  106 . In the heat exchanger  420   d  flue gas entering the spray tower absorber  102  is used for heating of the used absorption liquid. 
         [0088]    In a fifth alternative, a heat exchanger  420   e  in the circuit  418  is positioned in the air conduit  110 . In the heat exchanger  420   e  air supplied to the spray tower absorber  102  is used for heating of the used absorption liquid. 
         [0089]      FIG. 4  illustrates exemplary systems for dry desulfurization of, and removal of carbon dioxide from, a flue gas. The systems comprise a system  500  for desulfurization and a system  600  for removal of carbon dioxide. 
         [0090]    Flue gas enters a spray dryer type tower  502  via a gas conduit  504 . A reagent mixture is also supplied to the tower  502 . By reaction in the tower  502  of SO 2  with Ca(OH) 2  in humid conditions, SO 2  is removed from the flue gas and a reaction product in the form of a particulate is formed. Flue gas depleted of SO 2  is fed to a fabric filter  506  from which particulate is discharged. A solution of slaked lime is prepared from lime and water in a slaker  508 . The solution of slaked lime is passed to a mixer  510  in a circuit  512  via a tank  514  and a heat exchanger  620 . The heat exchanger  620  is further described below. The reagent mixture is prepared in the mixer  510  by mixing the solution of slaked lime with particulate resulting from the fabric filter  506 . The reagent mixture is supplied to the tower  502 . Flue gas depleted of SO 2  leaves the system  500  for desulfurization via a conduit  516 . 
         [0091]    Flue gas depleted of SO 2  enters an absorption tower  202  via the conduit  516  and a cooler  204 . An absorption liquid comprising ammonia enters the absorption tower  202  via a conduit  206  and is sprayed into the flue gas in the absorption tower  202 . By absorption in the absorption tower  202  of CO 2  in the absorption liquid, CO 2  is removed from the flue gas. Flue gas depleted of CO 2  passes an ammonia condenser  208  and leaves the system  600  for removal of carbon dioxide via a conduit  210 . Ammonia recovered in the ammonia condenser  208  is returned to the absorption tower  202  via a conduit  212 . Used absorption liquid is withdrawn from the absorption tower  202  to a regenerator  214  via a heat exchanger  216 . The used absorption liquid in the regenerator  214  is circulated in a circuit  418  via the heat exchanger  620 . The heat exchanger  620  is operatively connected to the circuit  512 . In the heat exchanger  620  the solution of slaked lime is used for heating of the used absorption liquid. By release of CO 2  from the used absorption liquid, the absorption liquid is regenerated. Regenerated absorption liquid is recycled to the absorption tower  202  via the heat exchanger  216  and the conduit  206 . Released CO 2  leaves the regenerator via a conduit  222 . 
         [0092]    Alternative positions of the heat exchanger in the circuit  418  have been indicated by reference numbers  620   a  and  620   b.    
         [0093]    In a first alternative, a heat exchanger  620   a  in the circuit  418  is positioned in the tank  514 . In the heat exchanger  620   a  a solution of slaked lime collected in the tank  514  is used for heating of the used absorption liquid. 
         [0094]    In a second alternative, a heat exchanger  620   b  in the circuit  418  is positioned in the gas conduit  504 . In the heat exchanger  620   b  flue gas entering the tower  502  is used for heating of the used absorption liquid. 
         [0095]    While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.