Post-Combustion CO2 Capture with Solid Amine Sorbents

中国环境学会  2011年 06月21日

  Alexander Chuang (presenter, and Felipe Guzman 
  Akron Sorbent LLC, 411 Wolf Ledges, Suite 105, Akron, OH, 44325-2103
  Matt Isenberg, Jak Tanthana, and Steven S. C. Chuang (  
  Department of Chemical Engineering, The University of Akron
  Akron, OH 44325-3906

  Coal-fired power plant contributed nearly 40% of anthropogenic CO2 emitted to atmosphere.  The solid amine processes for capturing CO2 from coal-fired power plants hold a great promise for (i) reduction of the energy need for the sorbent regeneration due to its low heat capacity, (ii) avoidance of equipment corrosion, and (iii) increase in the rate of CO2 adsorption/absorption and desorption due to elimination of slow absorption from gas to liquid and diffusion steps in liquid phase.  To meet the requirements of CO2 capture capacity and stability of the sorbent under multiple cycles of CO2 capture (i.e., adsorption/desorption), we have examined a wide range of oxide- and activated carbon-supported organic amines with various additives.  SiO2 was found to be an effective oxide support, providing high surface area and hydrophilicity.  By properly fine-tuning the concentration of additives, we have developed a sorbent with a CO2 capture capacity of more than 1.5 mmol/g and a thermal stability of more than 500 CO2 adsorption/desorption cycles with less than 14% degradation.   In this paper, we report the method of a cyclic CO2 capture study with adsorption at 40 °C and desorption at 100 °C over an additive-amine/SiO2 sorbent and discuss the approach for design and construction of CO2 dual adsorbers.  The economic feasibility and the overall cost of CO2 capture will be further evaluated and discussed in a subsequent paper.  


  The increase in atmospheric CO2 over past centuries is a result of the growing use of fossil fuels.  The CO2 concentration in the atmosphere has increased from 280 ppm during the pre-industrial time to about 390 ppm in 2010.1  The CO2 concentration is expected to continue increasing until a non-carbon containing fuel takes over as the dominant energy source.2  The continuous rise in CO2 concentration and its linkage to global warming demands cost-effective approaches to stabilize the CO2 concentration in the atmosphere. 
  Examination of various sources of CO2 emission revealed that more than 33% of global CO2 emissions originates from coal-fired power plants, representing the largest stationary source of CO2.3  Extensive research has been focused on CO2 capture and carbon sequestration technologies (i.e., CCS) to capture and store carbon dioxide (CO2) that would otherwise reside in the atmosphere for long periods of time.  The direct capture of CO2 from the highly concentrated and large volume CO2 stationary source is technically feasible and could be cost-effective for sequestrating CO2.   
  Depending on operating conditions and the type of coal burned, CO2 concentration in the power plant flue gas varies from 10-15 %, NO from 1,500-2,500 ppm, and SO2 from 500-2,000 ppm.4  NO is removed by the selective catalytic reduction while SO2 is captured by the wet lime scrubber and CO2 is vented to the atmosphere.5, 6        The available approaches for CO2 capture and separation in Fig. 1 include absorption of CO2 in aqueous amines, adsorption on solid sorbents. cryogenics, membrane separation, and microbial/algal systems,.7-18  
  The absorption of CO2 is carried out in a packed column where the CO2 stream is injected at the bottom of the column while the aqueous amine (R1R2NH) is sprayed from the top of the column at 40 °C.  CO2 is absorbed in the aqueous amines to form carbamates and bicarbonates by the following reactions: 
  2 R1R2NH  +  CO2             R1R2NH2+  +  R1R2NCOO- (carbamate)                   (1)
  R1R2NH  +  H2O  +  CO2           R1R2NH2+  +  HCO3- (bicarbonate)                    (2)
  In absence of water, one mole of CO2 reacts with two moles of amine to form carbamate by reaction 1; in presence of water, one mole of CO2 reacts with one mole of amine to form bicarbonate by reaction 2.7, 13, 19, 20  Aqueous alkanolamines have also been widely used to capture CO2 by the following reaction:
  C2H4OHNH2 + CO2     + H2O           C2H4OHNH3+  + HCO3-                                                       (3)
  CO2-containing aqueous amine is regenerated by striping at about 110 °C.
  The membrane process for CO2 separation  involves use of either polymer membranes or amine-modified inorganic oxide membranes.8,21  The membrane process is based on the high permeability of CO2 through the membrane compared to other gases.  The membrane process is efficient in separating CO2 from the gas stream with a small volumetric flow rate and a low CO2 concentration.8  As the amount (i.e., flux) of CO2 required for permeating through the membrane increases, the membrane efficiency for the CO2 separation decreases and the rate of membrane degradation accelerates due to the presence of contaminants in the feed stream.8,21,22  The cryogenic process is very costly; the microbial/algal process is very slow. 
   Solid sorbents based on amine-treated polymers have been used for years to capture CO2 from closed environments, such as space shuttles and submarines, containing less than 1% CO2.13, 23  A feed stream containing CO2 is pumped through a packed bed of solid sorbent; CO2 adsorbs on the sorbent by interaction with the amine functional groups.24  The sorbent is regenerated by temperature swing adsorption or pressure swing adsorption or a combination of both processes.13
   The current aqueous amine and membrane technologies are cost-effective for separation of CO2 from natural gas in liquefaction process and ammonia synthesis process due to the high value of end products.25,26  However, when applied for CO2 capture from coal-fired power plants, the cost of electricity increases by more than 70%.3  The cost of CO2 sequestration can be reduced if an effective CO2 capture sorbent is developed which has (i) high CO2 adsorption capacity (> 2,000 mole/g), (ii) long term regeneration capacity in power plant flue gas environment, and (iii) low energy requirement for regeneration compared to large amount of energy required for aqueous amine process.27
    The amine-grafted mesoporous silica exhibited the highest CO2 capture capacities under flowing (i.e, dynamic) conditions between 20 – 60 °C compared to other solid sorbents reported in literature.7,9,11,13,27,28  Other low temperature solid sorbents including activated carbons, carbon nanotubes, and zeolites have shown CO2 adsorption capacity at room or low temperatures.29-34  Due to the physisorption nature of CO2 adsorbed on these solid materials, a slight increase in temperature can cause a significant decrease in CO2 sorption capacity.  Thus, these materials will not be effective and reliable for CO2 capture under practical condition where temperature fluctuation occurs.  In this paper, we report the results of a cyclic CO2 capture study with adsorption at 40 °C and desorption at 100 °C over an additive-amine/SiO2 sorbent and discuss the approach for design and construction of CO2 dual adsorbers.  The economic feasibility and the overall cost of CO2 capture will be further evaluated and discussed in a subsequent paper.  


  Immobilized amine on SiO2 was prepared by mixing 10 g of fume silica with 30 cm3 of 40 vol% amine in ethanol.  The resultant mixture was placed in an oven at 95 °C for 30 min to remove ethanol and obtain a granule white powder.  Additives were added to amine solution to further improve the amine stability.  Additive compositions will be further revealed in a patent application.  
  Testing apparatus:  The experimental apparatus, shown in Figure. 2, consists of (i) a gas manifold with a 4-port and 6-port mass flow controllers, (ii) a tubular adsorber with movable heating jacket, (iii) a Pfeiffer QMS 200 quadruple mass spectrometer (MS), and (iv) a signal input-output module for controlling the 4-port, 6-port valve position, and heating rate.  The valves and temperature of the adsorber were controlled by a computer with an interface (National Instrument).  The unique feature of this testing apparatus is the movable heating jacket which allows each adsorption/desorption cycle to be completed within 30 min.   
  Cyclic CO2 adsorption/desorption studies: One gram of the amine sorbent was placed in the tubular reactor.  Adsorption of CO2 on the sorbent was carried out at 40 °C by switching (i) the reactor inlet flow from Ar (150 cm3/min) to a CO2/air mixture (100 cm3/min, 15 vol%) for 3 min and (ii) back to Ar for 2 min to purge out residual CO2.  The sorbent was heated to
  100 °C or 115 °C at 5 °C/min and held constant at desorption temperature for 2 min.  The temperature was then decreased to 40 °C at 10 °C/min to complete one CO2 adsorption/desorption cycle.  The reactor effluent composition was monitored by MS and CO2 profile (m/e=44) was calibrated by CO2 pulse injection of 3, 5, and 10 cm3 via 6-port valve.  The responding (i.e., calibration) factors was calculated by dividing the area under the CO2 profile and the amount of CO2 injected.  The CO2 capture capacity of the fresh sorbent and the sorbent after 500 adsorption/desorption cycles was further verified by weighing the sorbent.  The weight of the sorbent prior to exposure of CO2 at 25 °C was subtracted from that of after the exposure of CO2, resulting in amount of CO2 adsorbed on the sorbent.  The sorbent was heated to 100 °C to perform the CO2 desorption  
  Results and Discussion:

  Figure 3(a) shows the MS profiles of CO2, N2, O2, and Ar during the first CO2 adsorption/desorption cycle over additives-amine/SiO2 which were prepared by organic amine,  polyethylene glycol, and a number of additives.  The immediate drop in Ar intensity together with sudden rise of N2 and O2 intensities occurred when the inlet reactor flow switched from Ar to CO2/air.  The delayed rise in CO2 intensity with respect to rapid rise of the O2 and N2 profiles indicates the saturation (breakthrough) of the sorbent sites by CO2.  Switching the inlet flow from CO2/air back to Ar caused immediate drop in N2, and O2, intensities to baseline and gradual drop in CO2 intensity.  Heating the sorbent during the TPD caused gradual increase in the CO2 MS intensity.  CO2 capture capacity at the first cycle (i.e., fresh sorbent) was determined to be 1.61 mmol/g-sorb.  Figure 3(b) shows that a 21 °C average temperature rise was observed after switching Ar to CO2/air, indicating that CO2 adsorption is a highly exothermic process.   
  Figure 4(a) shows the CO2 MS profile during cycles 90 to 110 of CO2 over additives-amine/SiO2.   The figure was enlarged to show the broad peak of CO2 produced from TPD and omitting the higher portion of the intensity during adsorption.  The TPD CO2 profile shows many humps with nearly equal size, indicating that the degradation did not occurred to a discernable extent.  Figure 4(b) shows the corresponding temperature profile (which is the average of the temperature at the top and bottom of the sorbent bed) during these 20 cycles.  The extent of the temperature rise in each adsorption/desorption cycle is nearly constant.  Figure 4(c) and 4(d) shows the overall CO2 MS profile and the average temperature profile (average of the temperature at the top and bottom of the bed during the 500 cycles).  The line EF was drawn over the CO2 TPD peaks in Figure 4(c) to give a visual indication to show the absence of observable degradation.  The lack of significant degradation is also evidenced by the consistent temperature rise during CO2 adsorption, shown in Fig 4(b). 
    Due to an unexpected disturbance to the sorbent bed, the CO2 TPD peak did not reach the EF line from cycle 480 onward to the end of cycle 500.  Table 1 shows the CO2 capture capacity results.  CO2 capture capacity at cycle 500 was determined to be 1.39 mmol-CO2/ g-sorb or roughly 87% of original CO2 capture capacity.  Additional cycles with CO2 at 25 °C were performed over the fresh sorbent to verify the effect of adsorption temperature on the CO2 capture capacity of the sorbent.  As expected, lowering the adsorption temperature increased CO2
     Adsorber Development
  Fig. 5(a) illustrates the typical CO2 capture system which consists of a dual adsorber scheme for CO2 adsorption and adsorbent regeneration (CO2 desorption).  In this thermal swing process, CO2 capture take place by flowing flue gas over the additive-amine/SiO2 layer or pellet at temperatures below 55 °C where adsorption of CO2 occurs and the N2 and O2 flow through.  Regeneration is achieved by heating the sorbent at temperatures above 100 °C to desorb CO2.  Due to the external heat, the amines at the external surface tend to get overheated and degraded. 
  Like many catalytic reaction processes, the adsorber can be operated in the mode of fixed bed, fluidized bed, and monolith.  Although the fixed bed is easy to set up, it suffers from high pressure drop and difficulty in temperature control for highly exothermic processes such as CO2 capture processes.  The monolith in Figure 5(b) and fluidized bed in Figure 5(c) provide the excellent heat and mass transfer as well as low pressure drop.  The monolith, consisting of straight channels, requires extensive fabrication; the fluidized bed requires the proper control of fluidizing conditions.  The selection of the mode of the operation will call for detailed analysis of the fixed equipment and operation cost as well as the sorbent cost.   

  The capture of CO2 from large point-sources is an essential step toward developing large-scale CCS (carbon capture and storage) operations.  The uniqueness of Akron’s sorbent is its thermal stability which allows more than 500 CO2 adsorption/desorption cycles with less than 14% degradation.  The solid amine sorbent could (i) reduce the energy need for the sorbent regeneration due to its low heat capacity (ii) avoid equipment corrosion due to exposure to liquid amine, and (iii) increase the rate of CO2 adsorption due to elimination of slow absorption and diffusion steps.  The low cost of raw materials for the synthesis of these dual-amine sorbents could lead to a breakthrough in technology for the effective capture of CO2 from coal fired power plants. 
  This work was supported by U.S. Department of Energy under DE-FC26-07NT43086 and FirstEnergy Corp. 
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