Abstract:
A system for treating a flue gas from a combustion process comprises an absorber vessel configured to receive an aqueous ammonia solvent stream lean in CO 2  and a flue gas stream having CO 2 , the aqueous ammonia solvent stream and the flue gas stream in contact in the absorber vessel in a counter-current arrangement to provide an outlet stream rich in CO 2 ; a desorber configured to strip the CO 2  from the outlet stream rich in CO 2  from the absorber vessel at a temperature less than 100 degrees C. and to return the resultant aqueous ammonia solvent stream lean in CO 2  to the absorber vessel; a source of heat configured to supply heat to the desorber; and a CO 2  sequestration system for sequestering CO 2  stripped by the desorber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This patent application claims benefit under 35 U.S.C. §119(e) of co-pending U.S. Provisional Patent Application No. 61/617,879 entitled “FLUE GAS TREATMENT SYSTEM WITH AMMONIA SOLVENT FOR CAPTURE OF CARBON DIOXIDE,” filed Mar. 30, 2012, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure generally relates to a treatment system that utilizes an ammonia solvent for the capture of carbon dioxide (CO 2 ) from a flue gas and more particularly relates to a flue gas treatment system that absorbs CO 2  into an ammonia solvent and regenerates the CO 2  at low pressure and temperature. 
       BACKGROUND 
       [0003]    The combustion of carbon- and hydrogen-containing fuel such as oil, coal, and natural gas generally results in the production of a flue gas stream containing contaminant emissions in the form of particulates, hydrocarbons, SO x , NO x , and the like. Awareness regarding the effects of these contaminants on the environment has generally called for the enforcement of stringent limits on emissions thereof into the atmosphere. As such, those that combust such fuels must find more efficient ways to remove contaminants before venting the flue gas stream to the atmosphere. 
         [0004]    One particular environmental contaminant is CO 2 , which is typically referred to as a “greenhouse gas.” Although CO 2  is considered an atmospheric contaminant, it has various beneficial uses, and so it is often absorbed from flue gas into a solvent, regenerated from the solvent, captured, and compressed for use. The efficient capture of CO 2  by such a process requires a balancing of the energy requirements for the actual regeneration of the CO 2  from the solvent against the energy requirements for the compression of the CO 2 . Regeneration of the CO 2  at relatively high pressures and temperatures using steam to desorb or strip the solvent from the CO 2  reduces the electric power requirements for compression but detrimentally affects the stability of the solvent in which the CO 2  is entrained. Conversely, regeneration of the CO 2  at relatively low pressure and temperature increases the energy needed for compression of the CO 2 . Thus, the selection of a suitable pressure for the regeneration of the CO 2  using steam stripping in a power plant is generally dictated by the steam cycle in the plant and the quality of the steam at the point at which steam is extracted from the steam cycle. For most absorption/desorption schemes utilizing steam stripping, the steam quality is constrained by the production of water vapor to achieve stripping of the CO 2  from the solvent. This means that when the regeneration is carried out at atmospheric pressure, the steam extraction of the CO 2  takes place at temperatures above 100 degrees C. 
       SUMMARY 
       [0005]    According to one aspect disclosed herein, a system for treating a flue gas from a combustion process comprises an absorber vessel configured to receive an aqueous ammonia solvent stream lean in CO 2  and a flue gas stream having CO 2 , the aqueous ammonia solvent stream and the flue gas stream contacting in the absorber vessel in a counter-current arrangement to provide an outlet solvent stream rich in CO 2 . The system also comprises a desorber configured to strip the CO 2  from the outlet solvent stream rich in CO 2  produced in the absorber vessel at a temperature less than 100 degrees C. and return the resultant aqueous ammonia solvent stream lean in CO 2  to the absorber vessel. The system further comprises a source of heat configured to supply heat to the desorber and a CO 2  sequestration system for sequestering CO 2  stripped from the outlet solvent stream rich in CO 2  in the desorber. 
         [0006]    According to other aspects disclosed herein, a CO 2  capture system comprises a packed column comprising a vessel and a packing material therein, the packed column configured to receive an aqueous ammonia solvent stream lean in CO 2  at an upper portion thereof and a flue gas stream having CO 2  at a lower portion thereof, the aqueous ammonia solvent stream and the flue gas stream being in contact in the packed column in a counter-current arrangement to provide an outlet solvent stream rich in CO 2 . The system also comprises a desorber configured to strip the CO 2  from the outlet solvent stream rich in CO 2  produced in the packed column at a temperature less than 100 degrees C. and return the resultant aqueous ammonia solvent stream lean in CO 2  to the upper portion of the packed column. A source of heat is configured to supply heat to the desorber. 
         [0007]    According to still other aspects disclosed herein, a method for removing CO 2  from a flue gas stream comprises the steps of contacting an aqueous ammonia solvent stream lean in CO 2  with a flue gas stream having CO 2 , the aqueous ammonia solvent stream and the flue gas stream being in contact in the absorber vessel in a counter-current arrangement. An outlet stream is directed from the absorber vessel to a desorber, the outlet stream being rich in CO 2  absorbed from the flue gas. The desorber is heated using a source of heat, and the CO 2  is stripped from the outlet stream rich in CO 2  at a temperature less than 100 degrees C. to remove at least a portion of the CO 2  therefrom to produce the aqueous ammonia solvent stream lean in CO 2 . The method also includes the steps of sequestering the CO 2  stripped from the outlet stream rich in CO 2  and returning the resultant aqueous ammonia solvent stream lean in CO 2  to the absorber vessel. 
         [0008]    The above described and other features are exemplified by the following Figures and detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Referring now to the Figures, which are exemplary embodiments, and wherein the like elements are numbered alike: 
           [0010]      FIG. 1  is a schematic representation of a flue gas processing system for the capture of CO 2 ; and 
           [0011]      FIG. 2  is a schematic representation of a CO 2  capture system and heat transfer system of the flue gas processing system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    As illustrated in  FIG. 1 , a system for treating a flue gas containing CO 2  by the capture of CO 2  from the flue gas is designated generally by the reference number  10  and is hereinafter referred to as “system  10 .” In system  10 , CO 2  is captured from a flue gas containing CO 2  by absorption utilizing a solvent, and then removed from the solvent at relatively low temperatures utilizing waste heat, e.g., heat from a power generation plant, heat from solar energy, or heat from geothermal energy. The solvent is selected such that the capture and sequestration of the CO 2  takes place at relatively low pressure. By using heat from various sources such as waste heat, a solvent having relatively low volatility, and a relatively low system pressure, the use of a steam cycle (such as for example, that of a power generation plant utilizing system  10 ) to remove CO 2  from the solvent may be avoided. In system  10 , the volatility and system pressure are low as compared to similar systems for treating flue gas containing CO 2 . 
         [0013]    The system  10  includes a flue gas pre-processing stage  12  that receives a flue gas stream  14  from a boiler, a furnace, or the like. The flue gas stream  14  contains CO 2 . The flue gas pre-processing stage  12  may include one or more devices such as, but not limited to, a scrubber, a dust removal system, a pre-heater, or the like. From the flue gas pre-processing stage  12 , the flue gas stream  14  is directed to a CO 2  capture system  20  that utilizes an aqueous ammonia solvent that allows for CO 2  capture from the flue gas stream  14  and CO 2  stripping from the aqueous ammonia solvent for CO 2  regeneration. Once a desired portion of the CO 2  is regenerated (as CO 2  stream  22 ), the regenerated CO 2  is sequestered in a CO 2  sequestration apparatus  24 . Upon capture of CO 2  from flue gas stream  14 , a treated flue gas stream  26  is produced and conveyed to an exhaust stack  28 . The CO 2  capture system  20  is in fluid communication with a heat transfer system  30  that allows for heat transfer between the aqueous ammonia solvent streams flowing to and from the CO 2  capture system  20 . The various components of system  10 , such as the flue gas pre-processing stage  12 , the CO 2  capture system  20 , the exhaust stack  28 , the heat transfer system  30 , and the CO 2  sequestration apparatus  24 , are fluidly connected. 
         [0014]    The aqueous ammonia solvent is an ionic ammonia solution that is about 10 weight percent (wt. %) ammonia based on ammonium carbonates, ammonium bicarbonates, and/or ammonium carbamates. 
         [0015]    As illustrated in  FIG. 2 , the CO 2  capture system  20  includes an absorber vessel  32  in which the aqueous ammonia solvent contacts the flue gas stream  14 . The absorber vessel  32  is a packed column with an interior area  32 A containing packing material  32 B either arranged in a structured configuration or randomly dumped within interior area  32 A of the absorber vessel  32 . In contacting the aqueous ammonia solution with the flue gas stream  14 , the same are mixed in a counter-current arrangement within absorber vessel  32 . In particular, the aqueous ammonia stream flowing into the absorber vessel  32 , which is hereinafter referred to as the absorber inlet stream  34 , is received by the absorber vessel  32  and is distributed within the upper portion or top  32 C of the absorber vessel  32  via a liquid distribution system (not shown). The flue gas stream  14  is introduced to the absorber vessel  32  at or near the bottom  32 D thereof. Because the aqueous ammonia solvent is introduced at or near the top  32 C of the absorber vessel  32  using the liquid distribution system, the aqueous ammonia solvent is substantially evenly distributed over the complete horizontal cross-section of the interior area  32 A of absorber vessel  32 , thereby allowing the aqueous ammonia solvent to permeate the packing material  32 B and flow downwardly in a substantially even manner contacting the flue gas stream  14  flowing upwardly through the packing material  32 B and interior area  32 A. 
         [0016]    The absorber inlet stream  34  is rich in ammonia and lean in CO 2 , which allows it to absorb CO 2  from the flue gas stream  14 . Absorbing CO 2  from the flue gas stream  14  increases the concentration of CO 2  in the aqueous ammonia solvent and thus renders it “rich in CO 2 .” Once discharged from the absorber vessel  32 , the aqueous ammonia solvent rich in CO 2 , hereinafter referred to as the absorber outlet stream  38 , is directed to the heat transfer system  30 . 
         [0017]    The heat transfer system  30  is a heat exchanger. The heat exchanger may be, but is not limited to, a plate-and-frame design. In the heat exchanger, the absorber outlet stream  38  is heated and directed to a desorber  40 , which strips the CO 2  from the absorber outlet stream  38  to regenerate the CO 2  and the aqueous ammonia solvent lean in CO 2 . 
         [0018]    Still referring to  FIG. 2 , the desorber  40  includes a vessel  42  and a reboiler  50  that provides heat to the vessel  42 . The vessel  42  is any suitable container with a hollow interior area  42 A, for example, a generally hollow cylindrically-shaped column having gas-liquid contacting devices  42 B suitable for facilitating mass transfer. Such gas-liquid contacting devices  42 B include, but are not limited to, random packing material, structured packing material, and trays. The reboiler  50  receives a takeoff stream  46  comprising aqueous ammonia solvent substantially free of CO 2  from the bottom  40 A of the desorber  40 , heats the takeoff stream  46 , and returns a heated return stream  52  to the desorber  40 . 
         [0019]    Because the system  10  utilizes aqueous ammonia solvent that vaporizes at a temperature lower than that for water at any given pressure, the reboiler  50  operates upon receiving heat from a heat source  45 , which can comprise any suitable source of heat, including waste heat from a plant process. The heat source  45  is not limited to waste heat from a plant process, but rather the heat may result from any source including, but not limited to, a plant steam cycle, a geothermal source, or solar heat. In so heating the reboiler  50 , the desorber  40  operates at atmospheric pressure to strip ammonia at a temperature below that of the boiling point of water (less than 100 degrees C.), such that ammonia is effectively vaporized from the heated absorber outlet stream  38  (the CO 2 -rich aqueous ammonia solvent) and subsequently condensed in either the packing material  42 B or on the trays  42 B of the desorber  40 , thereby regenerating the CO 2 . 
         [0020]    After condensing the ammonia from the CO 2 -rich aqueous ammonia solvent in the desorber  40 , an overhead CO 2  stream  54  is taken from the top  40 A of the desorber  40  and directed to a reflux drum  56 . Because the overhead CO 2  stream  54  contains some amount of ammonia vapor, the reflux drum  56  allows the ammonia vapors to condense and be returned to the upper portion or top  40 A of the desorber  40  via an overhead return stream  58 . 
         [0021]    From the reflux drum  56 , CO 2  is removed and sequestered in the CO 2  sequestration apparatus  24 . Any suitable method of sequestering the CO 2  may be used. For example, the CO 2  may be reacted with a metal oxide to produce a carbonate, which may be stored as a solid. 
         [0022]    From the reboiler  50 , an ammonia solvent takeoff stream  60  is directed back to the heat transfer system  30 . The ammonia solvent takeoff stream  60  is substantially free of CO 2  and is close to the boiling point of the aqueous ammonia solvent. The heat transfer system  30  is configured such that upon receiving the ammonia solvent takeoff stream  60 , heat is transferred from the ammonia solvent takeoff stream  60  to the absorber outlet stream  38 , thus cooling the ammonia solvent takeoff stream  60  and heating the absorber outlet stream  38  flowing to the desorber  40 . 
         [0023]    The cooled ammonia solvent takeoff stream (hereinafter designated by the reference number  64 , flows from the heat transfer system  30  to a chiller  66 , which further cools the ammonia solvent  64  to produce chilled solvent  64 A. The chilled solvent  64 A is analyzed using a formulator  70  or any other suitable apparatus to determine the amount (e.g., mole ratio) of CO 2 . The formulator  70  may also adjust the composition of the chilled solvent  64 A by (optionally) adding makeup aqueous ammonia solvent  74  calculated to have a particular molar concentration to render the chilled solvent  64 A from the formulator  70  (which corresponds to the absorber inlet stream  34 ) of a desired concentration of ammonia for use in the absorber vessel  32 . 
         [0024]    By controlling the operating temperature of the absorber vessel  32 , the operating pressure of the desorber  40 , the molar concentration of the aqueous ammonia solvent (e.g., by adjusting the operating temperature and flow rates of the solvent through the reboiler  50 ), solvent and flue gas flow rates, and the amount of makeup aqueous ammonia solvent  74  added in the formulator  70 , the system  10  can be operated using waste heat, heat from solar sources, heat from geothermal sources, or other thermal sources. Furthermore, the system  10  can be advantageously operated with the reboiler  50  and/or the desorber  40  at ambient pressure and a temperature of less than about 100 degrees C. under a lean loading of less than about 0.332 mole/mole. Also, the capture of CO 2  at relatively low temperatures can be adjusted to obtain a desired amount of CO 2  at the sequestration apparatus  24 . 
       EXAMPLE 
       [0025]    Using the CO 2  capture system  20 , several different processes of capturing CO 2  were simulated to demonstrate the impact of CO 2  regeneration pressure on the overall process performance. In such simulations, the reboiler  50  was operated at pressures ranging from 10 bar down to 1 bar, and analyses were made at various pressures to determine effective CO 2  capture rates. The desorber  40  was heated solely through the reboiler  50 . The aqueous ammonia solvent contained about 10 wt. % ammonia, and the solvent temperature at the inlet of the absorber (absorber inlet stream  34 ) was about 5 degrees C. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                 Process conditions for different reboiler pressures. 
               
             
          
           
               
                   
                   
                   
                 des 
                 abs 
                 total 
                 CO2 
               
               
                 Preboiler 
                 lean loading 
                 Treb 
                 NH3out 
                 NH3out 
                 NH3out 
                 capture 
               
               
                 [bar] 
                 [mole/mole] 
                 [° C.] 
                 [kg/s] 
                 [kg/s] 
                 [kg/s] 
                 [—] 
               
               
                   
               
             
          
           
               
                 10 
                 0.251 
                 136.8 
                 1.13 
                 10.96 
                 12.09 
                 0.916 
               
               
                 7 
                 0.258 
                 127.2 
                 1.44 
                 10.67 
                 12.11 
                 0.908 
               
               
                 5 
                 0.266 
                 118.3 
                 1.74 
                 10.41 
                 12.15 
                 0.899 
               
               
                 4 
                 0.269 
                 112.6 
                 1.96 
                 10.32 
                 12.29 
                 0.896 
               
               
                 3 
                 0.275 
                 105.2 
                 2.23 
                 10.08 
                 12.31 
                 0.889 
               
               
                 2 
                 0.281 
                 95.0 
                 2.63 
                 9.89 
                 12.52 
                 0.883 
               
               
                 1.5 
                 0.292 
                 87.8 
                 3.01 
                 9.41 
                 12.42 
                 0.876 
               
               
                 1.3 
                 0.307 
                 84.7 
                 3.38 
                 8.81 
                 12.20 
                 0.847 
               
               
                 1.1 
                 0.323 
                 80.0 
                 3.87 
                 8.07 
                 11.94 
                 0.823 
               
               
                 1 
                 0.332 
                 77.7 
                 4.14 
                 7.68 
                 11.82 
                 0.810 
               
               
                   
               
             
          
         
       
     
         [0026]    Because the aqueous ammonia solvent is of -high volatility as compared to water, the amount of heat needed to raise the solvent to a suitable temperature for stripping of the CO 2  therefrom is less than the amount needed to raise water to a suitable temperature for stripping of the CO 2 . 
         [0027]    As seen in the above Table, acceptable capture rates of CO 2  above 80% were achieved from the desorber  40  with reboiler temperatures as low as about 78 degrees C. In particular, at atmospheric pressure, a CO 2  capture rate of 81.0% was desirably achieved at 77.7 degrees C. Also, the amount of ammonia exiting the CO 2  capture system  20  remains substantially unchanged for a marked decrease in reboiler temperature and pressure, while the bulk of the emissions has shifted from the absorber vessel  32  to the overhead CO 2  stream  54  due to the lean loading of the solvent. This is advantageous as the cycling of the aqueous ammonia solvent throughout the system  10  is favored by the conditions in the desorber  40 , such as the lower volumetric flow rates of gas. Furthermore, it is contemplated that the use of a heat source other than waste heat to heat the reboiler  50  will result in a substantial increase in energy input without returning a corresponding increase in output in the form of CO 2  captured. 
         [0028]    While the invention has been disclosed and described with respect to the detailed embodiments hereof, it will be understood by those of skill 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, 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 embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of the foregoing description.