Abstract:
A method for processing flue-gas, in an exemplary embodiment, includes providing an absorber unit having a membrane contactor, channeling a combustion flue gas along a first surface of the membrane contactor, and channeling an ammonia-based liquid reagent along a second opposing surface of the membrane contactor. The method also includes partially separating the ammonia-based liquid from the flue gas such that the ammonia-based liquid and the flue gas contact at gas-liquid interface areas, defined by a plurality of pores of the membrane contactor, to separate CO 2  from the flue gas by a chemical absorption of CO 2  within the ammonia-based liquid to produce a CO 2 -rich ammonia-based liquid.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to industrial combustion systems, and more particularly to methods and systems for removing carbon dioxide (CO 2 ) from combustion flue gases. 
         [0002]    At least some known carbon separation technologies intervene at different points in coal and/or natural gas systems. For example, carbon separation technologies that separate CO 2  from combustion flue gases are generally known as post-combustion carbon separation technologies. Known post-combustion carbon separation technologies include processes such as, but not limited to, physical absorption, cryogenic separation, solid sorbent separation, chemical looping combustion, chemical absorption, and/or membrane separation. 
         [0003]    Some known chemical absorption processes attempt to remove CO 2  from the flue gases by an exothermic reaction of CO 2  with separation solvents, for example, potassium carbonate, sodium hydroxide, and amine-based solvents. Known amine-based liquids may include alkanol amines, for example, diethanolamine, triethanolamine, activated methyl diethanolamine, and monoethanolamines (MEA). During a known chemical absorption process, for example, a flue gas and an amine-based liquid such as MEA counter-currently flow within an absorber (scrubber). The flue gas may enter the scrubber near a bottom end, flow upward, and exit near an opposing top end. The liquid may enter the scrubber near the top end, flow downward, and exit near the bottom end. 
         [0004]    A CO 2 -rich liquid amine-based solution is formed by a chemical reaction of the flue gas and the MEA liquid in the scrubber. The CO 2 -rich liquid is then channeled to a desorber (stripper). The stripper heats the CO 2 -rich liquid to reverse the chemical reaction such that the absorbed CO 2  is released from the liquid. The released CO 2  may be subsequently compressed and transported to storage, and the CO 2 -lean liquid may be recycled and reused in the scrubber. 
         [0005]    The combustion flue gas stream generally includes a smaller volume of CO 2  as compared to a larger volume of the flue gas. Known scrubbers generally require equipment sizes capable of processing the larger volumes of flue gas. During processing within known scrubbers, the flue gas is dispersed into the liquid causing gas bubbles to be formed within the liquid. The CO 2  absorption amount of the liquid partially depends on a total gas-liquid contact area, which is the sum of the surface areas of the gas bubbles. The liquid may absorb CO 2  and other impurities, for example, carbon oxysulfide and carbon bisulfide. Such known impurities may cause foaming of the liquid and/or liquid degradation due to irreversible reactions with the impurities. Also, a driving force that is required to separate the CO 2  from the flue gas is determined based on a concentration (density) of flue gas components. The scrubber footprint and stripper regeneration energy increases capital cost, operating costs, and energy consumption. A plant capacity is also reduced because of electrical power consumption in known chemical absorption processes 
         [0006]    Some known membrane separation processes include porous membranes that allow selective permeation of gases. The CO 2  absorption amount in the liquid partially depends on the physical interaction between flue gases and membrane materials, for example, polyimide and polyolefin. Membrane materials and pore sizes partially affect the degree in which one flue gas component permeates the pores as compared to other flue gas components. Compression of the flue gas is generally used to provide the driving force for permeation because a driving force that is used to separate the CO 2  from the flue gas is a pressure differential across the membrane. Therefore, a separation solvent and a stripper are not required for membrane CO 2  separation as compared to known chemical absorption processes. Additional compression of the separated CO 2  may be used for CO 2  transport and/or storage. Although known membrane separation processes generally use smaller scrubber sizes, such known scrubbers may produce a lesser amount of separated CO 2  as compared to CO 2  released using a chemical absorption process. As such, multiple recycling and processing of the flue gas may be needed in a smaller scrubber to achieve the same degree of CO 2  separation as a larger scrubber that processes a similar amount of flue gas. The additional flue gas processing and compression further increase energy consumption and costs. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0007]    In one aspect, a method for processing flue-gas is provided. The method includes providing an absorber unit having a membrane contactor; channeling a combustion flue gas along a first surface of the membrane contactor; and channeling an ammonia-based liquid reagent along a second opposing surface of the membrane contactor. The method also includes partially separating the ammonia-based liquid from the flue gas such that the ammonia-based liquid and the flue gas contact at gas-liquid interface areas, defined by a plurality of pores of the membrane contactor, to separate CO 2  from the flue gas by a chemical absorption of CO 2  within the ammonia-based liquid to produce a CO 2 -rich ammonia-based liquid. 
         [0008]    In another aspect, a combustion system is provided that includes an absorber unit. The absorber unit includes a housing for channeling a combustion flue gas; an ammonia-based liquid reagent channeled within the housing; and a membrane contactor coupled within the housing to separate the ammonia-based liquid from the flue gas. The membrane contactor includes a plurality of pores defining gas-liquid interface areas, with the ammonia-based liquid and the flue gas contacting at the gas-liquid interface areas to separate carbon dioxide from the flue gas by a chemical absorption of CO 2  within the ammonia-based liquid to provide a CO 2 -rich ammonia-based liquid. The combustion system also includes a desorber unit coupled to the absorber unit, so that the desorber unit receives and releases CO 2  from the CO 2 -rich ammonia-based liquid. 
         [0009]    In another aspect, a flue-gas processing system is provided. The flue-gas processing system includes a housing for channeling a combustion flue gas; an ammonia-based liquid reagent channeled within the housing; and a membrane contactor coupled within the housing to separate the ammonia based liquid from the flue gas. The membrane contactor includes a plurality of pores defining gas-liquid interface areas. The ammonia-based liquid and the flue gas contact at the gas-liquid interface areas to separate carbon dioxide from the flue gas by a chemical absorption of CO 2  within the ammonia-based liquid to provide a CO 2 -rich ammonia-based liquid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram of an exemplary combustion system including a carbon dioxide scrubber; and 
           [0011]      FIG. 2  is a schematic diagram of the exemplary CO 2  scrubber shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    The exemplary methods and systems described herein overcome the disadvantages of known post-combustion carbon separation technologies by combining ammonia-based chemical absorption processes and hydrophobic membrane contactors. 
         [0013]      FIG. 1  is a schematic diagram of an exemplary combustion system  100  that includes a furnace/boiler  110 , optional pollution control devices  120 , a CO 2  absorber (scrubber)  130 , a preheater  140 , a desorber (stripper)  150 , and a steam generator  160 . Furnace/boiler  110  serves as a combustion chamber that includes fuel injection ports  112 , air injection ports  114 , and a combustion zone  116 . In the exemplary embodiment, at least one fuel injection port  112  and at least one air injection port  114  are coupled to furnace/boiler  110  to inject fuel and air, respectively, into combustion zone  116 . 
         [0014]    After combustion of the fuel, a generated combustion exhaust gas, also known as a combustion flue gas, is optionally channeled in a transport stream into pollution control devices  120 , for example, electrostatic precipitators, and/or filter baghouses, to undergo various environmental clean-up processes prior to CO 2  separation within scrubber  130 . The pollution control devices  120  may include environmental clean-up processes that facilitate removing impurities, for example, nitrogen oxides (NO x ), sulfur oxides (SO x ), fly-ash, oxygen, and/or other particulate matter present in the flue gas which may cause liquid absorption degradation. 
         [0015]    Any remaining flue gas is then channeled to scrubber  130 . Scrubber  130  processes the flue gas by using an ammonia-based liquid to facilitate separating CO 2  from the flue gas (later described in more detail). After flue gas processing, the CO 2 -free flue gas is optionally discharged to the ambient air, and the CO 2 -rich ammonia-based liquid solution is channeled to preheater  140 . 
         [0016]    The CO 2 -rich ammonia-based liquid is then channeled to stripper  150 . Stripper  150  reduces pressure and/or increases temperature of the CO 2 -rich liquid to facilitate releasing CO 2  from the liquid. In the exemplary embodiment, steam generator  160  produces steam to reverse the chemical reaction by heating the CO 2 -rich liquid to facilitate regenerating the CO 2  from the liquid. The released CO 2  is then compressed and/or dried for storage and/or transportation. The CO 2 -free ammonia-based liquid is then recycled and channeled to scrubber  130  for subsequent flue gas processing. 
         [0017]      FIG. 2  is a schematic diagram of scrubber  130  (shown in  FIG. 1 ). In the exemplary embodiment, scrubber  130  operates to integrate membrane separation and ammonia-based chemical absorption technologies to capture CO 2  from the flue gas. Scrubber  130  includes gas-liquid membrane contactors  170  fabricated from hydrophobic material, for example, polytetrafluoroethylene (PTFE), including expanded polytetrafluoroethylene (ePTFE). Membrane contactors  170  each include a surface  172 , an opposing surface  174 , and micro-pores  178  that extend through contactor  170  and receive flue gas during processing. It should be appreciated that membrane contactors  170  may be fabricated as inert hollow fibers, substantially planar sheets, and/or other known structures packaged in a tube-and-shell arrangement, spiral-wound module, and/or other known configurations. Because membrane contactors  170  are densely packaged as hollow fiber and/or modular units in the exemplary embodiment, membrane contactors  170  provide a greater gas-liquid contact area per unit volume as compared to known scrubbers implementing only chemical absorption processes. As such, a smaller footprint of scrubber  130  can be customized and/or retrofitted to known industrial combustion systems. Also, PTFE membrane contactors  170  increase the surface area contact between the gas and liquid streams, making absorption more efficient than traditional absorption column configurations. 
         [0018]    In the exemplary embodiment, membrane contactors  170  facilitate non-selective flow of flue gas components  176  into micro-pores  178  defined therein as compared to known porous separation membranes that selectively facilitate passage of flue gas components. Because membrane contactors  170  are hydrophobic, membrane contactors  170  facilitate preventing convective liquid flow across membrane contactor  170 . Based on such hydrophobic and micro-porous material characteristics, membrane contactors  170  facilitate contacting flue gas components  176  and an ammonia-based liquid without dispersion of one phase in another. 
         [0019]    Because membrane contactor  170  acts as a gas permeable barrier between the gas and liquid phases, membrane contactors  170  do not actually separate CO 2  from flue gas as compared to known porous separation membranes. Instead, ammonia-based liquid  182  provides the exemplary hybrid process CO 2  separation selectivity. In the exemplary embodiment, membrane contactors  170  act as contacting mediums between flue gas  176  and ammonia-based liquid  182  to facilitate CO 2  separation from flue gas  176  based on a chemical absorption process. In other words, CO 2  separation within scrubber  130  is determined by a reaction of flue gas  176  with the ammonia-based liquid separating agent within scrubber  130  as discussed in greater detail herein. 
         [0020]    In the exemplary embodiment, the contact area between flue gas  176  and ammonia-based liquid  182  is an area sum of all gas-liquid interfaces  180  of all membrane contactors  170  provided within the scrubber. A packing density of membrane contactors  170  and/or micro-pores  178  provide a larger overall gas-liquid surface area contact as compared to total gas bubble surface areas caused by gas dispersion using known chemical absorption processes. As a result, membrane contactors  170  facilitate increasing CO 2  absorption efficiency as compared to known chemical absorption processes. 
         [0021]    In the exemplary embodiment, membrane contactor  170  is fabricated from hydrophobic materials such as PTFE. Particularly, membrane contactor is fabricated from expanded polytetrafluoroethylene (ePTFE). Expanded polytetrafluoroethylene has a surface area of about 10 to about 100 m 2 /gr and a void volume of about 90% to about 95%. The surface area and void content of expanded polytetrafluoroethylene permits flue  176  gas to contact ammonia-based liquid  182 . 
         [0022]    The PTFE material is suitable for flue gas processing operations as discussed with respect to the exemplary embodiment. For example, flue gas  176  generally has negligible amounts of hydrocarbon and large amounts of CO 2 . Also, in flue gas processing operations within scrubber  130 , flue gas  176  is generally processed at about 50° C. to about 100° C., in another embodiment, about 50° C. to about 80° C., and all subranges therebetween. Because CO 2  from flue gas  176  is absorbed by ammonia-based liquid  182  at ambient conditions, for example, about 50° C., pre-heating CO 2  and/or ammonia-based liquid  182  is not needed as compared to known systems known systems that attempt to separate CO 2  from flue gas using other liquid separating agents. As a result, ammonia-based liquid  182  facilitates reducing operating cost associated with CO 2  absorption using other liquid separating agents. 
         [0023]    Because PTFE is generally inert to flue gas components, membrane contactor  170  is fabricated from PTFE and/or other similar materials that are inert to flue gas  176 . As a result of PTFE material, membrane contactor  170  does not swell under flue gas processing operations as compared to known off-shore oil drilling and/or sweetening operations. Also, PTFE facilitates controlling sizes of micro-pores  178  to control gas-liquid contact areas for increasing CO 2  absorption efficiency. 
         [0024]    In the exemplary embodiment, ammonia-based liquid  182  also facilitates increasing CO 2  absorption efficiency as compared to known chemical absorption processes. Ammonia-based liquid  182  has higher reaction energies and absorption capabilities as compared to other types of known liquid separating agents such as amine-based liquids. As a result of using ammonia as a liquid separating agent, ammonia-based liquid  182  facilitates reducing an amount of liquid that is capable of absorbing approximately equal amounts of CO 2  as compared to other types of known liquid separating agents such as amine-based liquids. Because less ammonia-based liquid  182  is required for CO 2  absorption, less heat/steam is required to regenerate CO 2  from ammonia-based liquid  182  as compared to known systems that attempt to regenerate CO 2  from other liquid separating agents. As such, a reduction in cost associated with regeneration is facilitated. 
         [0025]    During flue gas processing in the exemplary embodiment, flue gas  176  and ammonia-based liquid  182  are channeled on opposite sides of membrane contactor  170  provided within scrubber  130 . Although flue gas  176  and ammonia-based liquid  182  are illustrated as counter-current flows, it should be appreciated that flue gas  176  and ammonia-based liquid  182  may flow concurrently in a same direction. As describe above, mass CO 2  transfer from flue gas  176  occurs by diffusion of CO 2  through gas-liquid interfaces  180  using ammonia-based liquid  182  in a chemical absorption process as discussed in greater detail herein. 
         [0026]    In the exemplary embodiment, a partial pressure gradient is applied within scrubber  130  to transfer CO 2  mass through diffusion from a gas phase to a liquid phase by lowering a pressure of the flue gas portion that is in contact with ammonia-based liquid  182 . It should be appreciated that a vacuum, inert gas, and/or other driving force can create the necessary driving force for CO 2  mass transfer. By controlling pressure differences between flue gas  176  and ammonia-based liquid  182 , a portion of flue gas  176  is immobilized in micro-pores  178  of membrane contactor  170  so that each gas-liquid interface  180  is located at a mouth of each micro-pore  178 . 
         [0027]    Because membrane contactors  170  serve as partitions between flue gas  176  and ammonia-based liquid  182 , a gas-liquid contact area is not disturbed by variations in flow rates. As a result of membrane contactor  170  and pressure control, scrubber  130  facilitates transferring flue gas  176  and ammonia-based liquid  182  over a wider range of independent flow rates as compared to flow rates of gas and liquids used in known scrubber systems. Membrane contactor  170  and pressure control also facilitate reducing flooding, channeling, and/or back-mixing of ammonia-based liquid  182  into micro-pores  178 . As such, scrubber  130  can tolerate a wider range of variations in flue gas processing conditions as compared to known scrubber systems. Membrane contactors  170  also facilitate reducing foaming and/or solvent degradation by reducing dispersion of potential ammonia-degrading impurities in flue gas  176  in ammonia-based liquid  182 . Because the driving force for CO 2  separation from flue gas  176  is a pressure gradient, a density difference between gas and liquid components are not required. As a result, membrane contactors  170  require no selectivity to CO 2  separation from flue gas  176  as compared to known porous separation membranes. 
         [0028]    In the exemplary embodiment, scrubber  130  includes a hybrid of membrane contactor  170  and ammonia-based liquid to facilitate CO 2  separation from flue gas  176 . Because scrubber  130  includes membrane contactor  170 , a gas-liquid surface contact area is increased as compared to known scrubber systems that attempt to separate CO 2  only by known chemical absorption processes therein. As a result, membrane contactors  170  may be densely packaged as bundles and/or modules to facilitate reducing a scrubber size as compared to known scrubber systems. Because membrane contactor  170  is fabricated from a micro-porous hydrophobic material such as PTFE, membrane contactor  170  acts as an inert material barrier in flue gas processing. Because ammonia-based liquid  182  is used as a liquid separating agent, the amount of liquid for chemical absorption of a given amount of CO 2  is less than an amount required for other liquid separating agents. As a result, use of ammonia facilitates reducing a scrubber size as compared to a size of known scrubber systems. Also, use of ammonia-based liquid  182  facilitates reducing an amount of regeneration energy used to release CO 2  in stripper  150  as compared to regeneration of CO 2  from other liquid separating agents. Overall, scrubber  130  facilitates reducing equipment size and/or capital costs associated with flue gas processing to separate, absorb, and release CO 2  to reduce such emissions. 
         [0029]    Exemplary embodiments of scrubbers are described in detail above. The scrubbers are not limited to use with the specified combustion systems described herein, but rather, the combustors can be utilized independently and separately from other combustion system components described herein. Although the exemplary methods and systems are described above with respect to coal or natural gas plants, it should be appreciated that the exemplary scrubber systems and methods are also applicable to other combustion systems such as, but not limited to, combustion engines. Moreover, the invention is not limited to the embodiments of the combustors described in detail above. Rather, other variations of scrubber embodiments may be utilized within the spirit and scope of the claims. 
         [0030]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.