Abstract:
A method of capturing and sequestering a gas species from a fossil fuel-fired power plant flue gas is disclosed. The method includes the step of providing an apparatus having a vessel adapted to be pressurized and a hollow fiber membrane contained in the vessel and having a sorbent embedded therein. The method further includes the steps of subjecting the hollow fiber membrane to a flow of flue gas, removing one or more gas species from the flue gas with the hollow fiber membrane, and regenerating the sorbent contained in the hollow fiber membrane.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a method and apparatus for rapid adsorption-desorption CO 2  capture. 
     The capture and sequestration of CO 2  from fossil fuel-fired power plant flue gas is an important step in controlling global warming due to fossil plant energy production. 
     CO 2  separation and capture from flue gases of various stationary sources can be described by either post-combustion, pre-combustion, or oxy-combustion configurations. In the post-combustion configuration, CO 2  is captured from the flue gas after the fuel is combusted. When air is used as an oxidant, the combustion flue gas is diluted with the nitrogen in the air; thus, the CO 2  concentration in the post combustion flue gas is usually low and ranges from 10-15% by volume for a pulverized coal (PC) fired power plant. For each MW of electric generation capacity, a PC boiler produces a flue gas volume of about 3,500 acfm and emits roughly 1 ton of CO 2  each hour. Due to the low concentration of CO 2  in the flue gas, low operating pressure, and large volume of flue gas to treat, the post-combustion configuration requires larger equipment and, hence, a higher capital cost. 
     One option available for lowering the capital cost is post-combustion capture using sorbent beds to adsorb the CO 2  from the flue gas. However, current sorbent bed technologies require very large beds of granular sorbents, are difficult to regenerate, and require high energy consumption. Based on the operation modes, adsorption processes include temperature swing adsorption (TSA), pressure swing adsorption (PSA), and vacuum swing adsorption (VSA). VSA is a PSA process in nature. Adsorption processes have several process configurations, such as fixed bed, moving bed, fluidized bed, and simulated moving bed (SMB). Most of the TSA and PSA processes employ the fixed bed configuration.  FIGS. 1 and 2  show typical TSA and PSA adsorption processes for CO 2  separation. 
     In adsorption processes, gases or vapors can be captured through chemical or physical interaction with a porous solid adsorbent such as zeolite or activated carbon. Gas separation is achieved when certain species are preferentially adsorbed and subsequently regenerated at high purity. 
     For TSA applications, CO 2  is generally adsorbed at temperatures between 10° C. and 60° C. while regeneration is conducted at greater than 100° C. With large beds, it takes a long time to heat up (to regenerate, such as using steam) and cool down (for adsorption, such as using air) due to heat transfer limitations. The steam used for heating also attacks some of the sorbents, especially if they condense and collect on the sorbent surface. During the adsorption cycle, mass transfer and diffusion is a rate limiting step requiring very large beds to adsorb CO 2  due to the large quantity of CO 2  to be adsorbed and the large granules needed for the beds. In addition, pressure drop across the beds is also a concern. 
     For example, in a typical TSA implementation for a 500 MW PC power plant, CO 2  emission from the power plant is about 500 ton/hr. The following conditions are assumed for the TSA process:
     CO 2  removal rate from the flue gas: 90%   CO 2  working capacity for the adsorbent: 8% (could be significantly lower)   Adsorption/desorption cycle time in TSA: 2 hours   Bed utilization: 90%   

     According to these assumptions, the total amount of sorbent required is:
 
Total sorbent=500*90%*2*/8%/90%=12,500 (ton).
 
     This results in a large amount of sorbent. Using the design parameters given in Table 1 below, the minimum number of columns needed is:
 
Number of Columns=12500/(π*3.1*3.1*32/4)/0.8=65
 
The total bed volume (65 columns of sorbent material) required is 15, 600 m 3 .
 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Working 
                   
                 Sorbent 
                   
               
               
                   
                 Column 
                   
                 capacity (g 
                 Cycle 
                 packing 
               
               
                   
                 diameter 
                 Height 
                 CO 2 /g 
                 time 
                 density 
                 Columns 
               
               
                   
                 (m) 
                 (m) 
                 sorbent) 
                 (hour) 
                 (ton/m 3 ) 
                 required 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 TSA 
                 3.1 
                 32 
                 0.08 
                 2 
                 0.8 
                 65 
               
               
                   
               
             
          
         
       
     
     It should be pointed out that the adsorption-desorption cycle time may vary. However, considering a column size of 3.1 m in diameter and 32 m in height the total cycle time (mostly because of heating and cooling the adsorbent during regeneration) of 2 hours for a TSA process is very moderate. Real cycle time may be well above 2 hours and the column number will accordingly increase. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other shortcomings of the prior art are addressed by the present invention, which provides an apparatus and method for the capture and sequestration of CO 2 . 
     According to one aspect of the invention, an apparatus for capture and sequestration of CO 2  from coal-fired power plant flue gas includes a polymer matrix embedded with a sorbent suitable for removing CO 2  from the flue gas and a spacer mated with the polymer matrix. The spacer is adapted to create channels between adjacent portions of the polymer matrix such that the flue gas flows through the channels and comes in contact with the sorbent. 
     According to another aspect of the invention, an apparatus for capture and sequestration of gas species from coal-fired power plant flue gas includes a vessel adapted to be pressurized and a hollow fiber membrane contained in the vessel. The vessel includes a lumen-side having first and second lumen-side ports disposed at opposing ends of the vessel and a shell-side having first and second shell-side ports disposed on opposing sides of a shell of the vessel. The hollow fiber membrane includes a sorbent embedded in a wall of the membrane for removing one or more species of a gas from the flue gas. 
     According to another aspect of the invention, a method of capturing and sequestering a gas species from coal-fired power plant flue gas includes the steps of providing a polymer matrix embedded with a sorbent, placing the polymer matrix parallel to a flow of flue gas, and subjecting the flue gas to the polymer matrix to allow the sorbent to remove one or more gas species from the flue gas. The polymer matrix is adapted to be heated and cooled rapidly. 
     According to another aspect of the invention, a method of capturing and sequestering a gas species from a coal-fired power plant flue gas includes the steps of providing an apparatus having a vessel adapted to be pressurized and a hollow fiber membrane contained in the vessel. The hollow fiber membrane includes a sorbent embedded therein. The method further including the steps of subjecting the hollow fiber membrane to a flow of flue gas, removing one or more gas species from the flue gas with the hollow fiber membrane, and regenerating the sorbent contained in the hollow fiber membrane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a schematic of a temperature swing adsorption (TSA) process; 
         FIG. 2  is a schematic of a pressure swing adsorption (PSA) process; 
         FIG. 3  shows a prior art polytetrafluoroethylene (PTFE) tape embedded with small sorbent granuals; 
         FIG. 4  shows a polymer matrix apparatus according to an embodiment of the invention; 
         FIG. 5  shows a top view of the polymer matrix apparatus of  FIG. 4 ; 
         FIG. 6  shows a hollow fiber membrane according to an embodiment of the invention; and 
         FIG. 7  shows membrane module for containing the hollow fiber membrane. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A prior art process for embedding small sorbent granules in a polytetrafluoroethylene (PTFE) polymer matrix was developed by Gore,  FIG. 3 . The PTFE matrix stabilizes the sorbent while allowing for ready access to the sorbent particles. The presence of the PTFE, a hydrophobic polymer, keeps water away from the sorbent surface. This helps to prevent any sorbent pore pluggage or chemical attack. Since sorbents can be readily embedded in relatively thin sheets of a PTFE tape, any gas phase components can easily access the sorbents with minimal mass transfer resistance. 
     In adsorption-based separation processes, the two primary means to drive the separation are temperature and/or pressure. In the process, an adsorbent bed is subjected to changes in either or both of these to affect a separation of a mixture into its components. 
     In prior work, it has been shown that the key to minimizing the size of the adsorbent bed is to use devices that (1) setup flow channels that allow for convective flow of the gas mixture that is to be separated as well as the separated components, and (2) hold the adsorbent particles between the flow channels and yet allow for convective flow (more preferably) or diffusive flow (less preferably) around the particles. By constructing adsorbent modules that offer both attributes, the adsorbent bed size can be reduced by a factor of as much as 100. 
     Referring now to the present invention,  FIGS. 4 and 5 , a sorbent embedded module for CO 2  capture is shown generally at reference numeral  20 . The module  20  may be placed in a flue gas duct, as shown, or in any other suitable position. Generally, a suitable CO 2  sorbent is embedded in a polymer matrix similar to that developed by Gore and used in a TSA process. One major advantage is that the thin sheets of the polymer matrix can be heated and cooled rapidly. During heating, direct steam injection can be used and during cooling, water, instead of air, can be used due to the hydrophobic nature of the PTFE. The ability to cool with water and the thin profile of the polymer matrix allows for very rapid cooling. Further, since the sheets can be oriented parallel to the flow, the pressure drop is minimal compared to a fixed bed design. Some of the key benefits of the current invention for CO 2  capture are described below:
         1) Reduction in mass transfer resistance for CO 2  adsorption;   2) Significant reduction in regenerating and cooling cycle time;   3) Low pressure drop;   4) Small footprint;   5) Can be used with any good CO 2  sorbent; and   6) Less affected by flue gas components such as water, SOx, mercury etc.       

     A comparison of the typical TSA described in table (1) with the PTFE rapid TSA, implemented for a similar 500 MW PC power plant emitting 500 ton/hr of CO 2  was performed. The following conditions are assumed for the PTFE rapid TSA:
         CO 2  removal rate from the flue gas: 90%   CO 2  working capacity for the adsorbent: 2% (conservative estimate)   Adsorption/desorption cycle time in TSA: 6 minutes (0.1 hours)   Sorbent Utilization 60% (conservative estimate)       

     According to these assumptions, the total amount of PTFE with embedded sorbent required can be calculated:
 
Total sorbent=500*90%*0.1/2%/60%=3,750 tons.
 
3,750 tons is 30% of the original 12,500 tons of sorbent required by the typical TSA process, thus, only 30% of the material would be required.
 
     The apparatus  20  includes sheets of PTFE tape  21  embedded with sorbent and spacers  22  to provide channels  23  between adjacent sheets of PTFE tape  21 . The spacers  22  may be of a flexible material and may include a matrix of channels or other suitable configuration to allow the flow of gas therethrough. This configuration allows the flue gas to flow through the channels  23  of the adjacent sheets  21  such that the flue gas comes into contact with the sorbents contained in the tape  21 . Assuming the bulk density of apparatus  20  to be approximately 0.5 tons/m 3  (tape with spacer material), the apparatus  20  would require 3,750/0.5=7,500 m 3  of material for a 500 MWe plant compared to 15,600 m 3  for a typical TSA sorbent bed, Table 1. The material and sizing savings are directly related to the capital cost of the system. 
     The major reason why the size of the system is more than halved despite less favorable assumptions for the working capacity and sorbent utilization in the material is the very short cycle time that is achievable through heating and cooling by steam injection and direct water cooling. This process advance is also applicable to other sorbents with higher working capacities that can be embedded into the PTFE. With higher performance sorbents, there will be further decreases in both the size of the system and the energy required to regenerate the CO 2 . 
     Referring now to  FIGS. 6 and 7 , a hollow fiber membrane embedded with an adsorbent  31  is shown at reference numeral  30 . The adsorbent particles are embedded into wall  32  of the membrane  30 . In a limiting case, the membrane may be made entirely of the adsorbent itself assuming the adsorbent&#39;s physical properties allow the manufacturing of a suitable membrane structure. A pre-determined number of hollow fiber membranes are formed into fiber membrane bundle  33  and are contained in a membrane module or vessel  40 ,  FIG. 7 . The vessel  40  is adapted to be pressurized and includes a shell  48  and a pair of ends  49  and  50 . In a process, the gas mixture is fed either into a lumen-side  41  having first and second lumen-side ports  43  and  44  at opposing ends of the vessel  40  or a shell-side  42  having first and second shell-side ports  46  and  47  disposed on opposing sides of the shell  48  of the module  40 , resulting in several process configuration options. 
     In a first option, the gas mixture would flow through the lumen-side by entering the first lumen port  43  and blocking the second lumen port  44  or exit port. The gas would then be pressurized within the lumen-side  41  and start to flow through the membrane walls  32  which contain the adsorbent. The adsorbent would selectively adsorb one or more species from the gas mixture and the non-adsorbing species would be collected from the shell-side port  47  and gradually removed. This flow would continue until the adsorbent is essentially saturated with the adsorbed species upon which time the flow would be stopped and switched to a second fiber membrane bundle  33 . Once the flow has stopped, the lumen-side  41  would be depressurized and a back-flow of the non-adsorbing gas from the shell-side  42  would be used to regenerate the adsorbent. This 4-step cycle is common in pressure swing cycles. 
     A second option is to feed the gas through the shell-side  42  and collect the non-adsorbing gas in the lumen-side  41 . The overall process steps are the same for the first and second options, i.e., pressurize, flow, depressurize, and purge. 
     Both of the above options show an example with pressure as the driving force for adsorption. A similar set of options exists for thermal swing. 
     In thermal swing, the 4 steps are cooling, flow, heating, and purge. For example, a cool gas mixture (say flue gas at 50 C) is introduced into the lumen-side  41  of the fiber membrane bundles  33  with the port  44  blocked. The gas would then flow through the membrane walls  32  as described where the adsorbent would selectively adsorb one or more species in the mixture and let the remaining gas mixture species permeate into the shell-side  42  where it would be collected through port  47 . Once the adsorbent is saturated, the adsorbent would be heated (to say 120 C) to regenerate the adsorbent. This heating could be done via steam directly injected into the membrane module  40  or external heating for instance. 
     Likewise, the gas mixture can be initially introduced into the shell-side  42  and collected in the lumen-side  41 . 
     In all four options (two for pressure swing and two for thermal swing), the bed length is the membrane wall  32  itself. For many membrane bundles  33 , this can be made thin, e.g., a few hundred microns. Such small adsorbent bed lengths, i.e., thin membrane walls, are key to enabling rapid frequency pressure and/or thermal swing cycles which in turn dramatically reduce the size of the adsorbent bed. 
     The specific geometry of the bed is not critical to this application, but what is critical is the size of the bed: it must be as small/thin as possible. Eventually, the thickness of the bed will be dictated by other factors in the process, e.g., valves that are able to switch on and off at sufficiently rapid frequencies to allow for high-frequency shifts in gas flows. 
     For CO 2  separation from flue gas, short-bed, high-frequency cycles are going to be favorable because of the extraordinary large volumes of flue gas emitted from power plants. This cycle can be achieved in modules and devices that offer short adsorbent bed path lengths. Such short-bed path lengths can be achieved in a multitude of geometries such as the hollow fiber bundle shown above. 
     The foregoing has described an apparatus and method for rapid adsorption-desorption CO 2  capture. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.