Development of Novel Carbon Sorbents for CO<sub>2</sub> Capture (2014)

An innovative, low-cost, and low-energy-consuming carbon dioxide (CO{sub 2}) capture technology was developed, based on CO{sub 2}adsorption on a high-capacity and durable carbon sorbent. This report describes the (1) performance of the concept on a bench-scale system; (2) results of parametric tests to determine the optimum operating conditions; (3) results of the testing with a flue gas from coal-fired boilers; and (4) evaluation of the technical and economic viability of the technology. The process uses a falling bed of carbon sorbent microbeads to separate the flue gas into two streams: a CO{sub 2} -lean flue gas stream from which > 90% of the CP{sub 2} is removed and a pure stream of CO{sub 2} that is ready for compression and sequestration. The carbo sorbent microbeads have several unique properties such as high CO{sub 2} capacity, low heat of adsorption and desorption (25 to 28 kJ/mole), mechanically robust, and rapid adsorption and desorption rates. The capture of CO{sub 2} from the flue gas is performed at near ambient temperatures in whic the sorbent microbeads flow down by gravity counter-current with the up-flow of the flue gas. The adsorbed CO{sub 2} is stripped by heating the CO{sub 2}-loaded sorbent to - 100°C, in contact with low-pressure (- 5 psig) steam in a section at the bottom of the adsorber. The regenerated sorben is dehydrated of adsorbed moisture, cooled, and lifted back to the adsorber. The CO{sub 2} from the desorber is essentially pure and can be dehydrated, compressed, and transported to a sequestration site. Bench-scale tests using a simulated flue gas showed that the integrated system can be operated to provide > 90% CO{sub 2} capture from a 15% CO{sub 2} stream in the adsorber and produce > 98% CO{sub 2} at the outlet of the stripper. Long-term tests ( 1,000 cycles) showed that the system can be operated reliably without sorbent agglomeration or attrition. The bench-scale reactor was also operated using a flue gas stream from a coal-fired boil at the University of Toledo campus for about 135 h, comprising 7,000 cycles of adsorption and desorption using the desulfurized flue gas that contained only 4.5% v/v CO{sub 2}. A capture efficiency of 85 to 95% CO{sub 2} was achieved under steady-state conditi ons. The CO{sub 2} adsorption capacity did not change significantly during the field test, as determined from the CO{sub 2} adsorptio isotherms of fresh and used sorbents. The process is also being tested using the flue gas from a PC-fired power plant at the National Carbon Capture Center (NCCC), Wilsonville, AL. The cost of electricity was calculated for CO{sub 2} capture using the carbon sorbent and compared with the no-CO{sub 2} capture and CO{sub 2} capture with an amine-based system. The increase i the levelized cost of electricity (L-COE) is about 37% for CO{sub 2} capture using the carbon sorbent in comparison to 80% for an amine-based system, demonstrating the economic advantage of C capture using the carbon sorbent. The 37% increase in the L-COE corresponds to a cost of capture of $30/ton of CO{sub 2}, including compression costs, capital cost for the capture system, and increased plant operating and capital costs to make up for reduced plant efficiency. Preliminary sensitivity analyses showed capital costs, pressure drops in the adsorber, and steam requirement for the regenerator are the major variables in determining the cost of CO{sub 2} capture. The results indicate that further long-term testing with a flue gas from a pulverized coal­ fired boiler should be performed to obtain additional data relating to the effects of flue gas contaminants, the ability to reduce pressure drop by using alternate structural packing , and the use of low-cost construction materials.