The use of fossil fuels for electricity, power, or heat, as well as the extraction of natural gas can create emissions of carbon dioxide (CO2). CO2 emissions are increasingly targeted by regulatory authorities.
There are many different solid sorbents under development for CO2 capture from flue gas streams, and while the entire field is extensive, a few sorbent related patents are mentioned only for reference. For example, Siriwardane discussed amine-treated sorbents (U.S. Pat. No. 6,908,497) that could be used for CO2 capture at low temperature, which could be regenerated by heating to temperatures in excess of 35° C. In addition, Gray et al. discussed a new method for making low-cost dry amine sorbents (U.S. Pat. No. 6,547,854). Sayari also used amine functionalization of mesoporous silica to create a potential CO2 sorbent (US 2006/0165574). Finally, Tirio proposed the use of an ion exchange resin for CO2 capture (US Application 2011/0088550).
While the sorbent is important to the effectiveness and costs related to CO2 capture, the process and related equipment are also of high importance. Several different groups have proposed different process configurations for CO2 capture. The most relevant works that utilize a temperature swing (possibly in addition to a pressure or partial pressure swing) are discussed. Several proposals have been made to utilize a process where the sorbent remains stationary, such as using fixed beds that can be operated for either adsorption or regeneration (U.S. Pat. No. 6,755,892 and WO 02/06637). One concern with a fixed bed system is that the actual bed and support structure itself would need to be heated up and cooled down for each regeneration and adsorption step, respectively. This can be avoided by moving the sorbent between separate separator and regenerator vessels. A moving bed system with cross flow was proposed by Pennline et al. (U.S. Pat. No. 6,387,337). While the proposed moving bed system offers low pressure drop, the contact time between the gas and sorbent is low; in addition, heat removal during adsorption is difficult. To increase the contact time between the sorbent and gas and maximize the CO2 delta loading of the sorbent, Knaebel proposed using a counter-current reactor with internal cooling during adsorption (US 2006/0230930). Knaebel also proposed to lower the sensible heat requirements by transferring heat from the hot sorbent after it is regenerated to the cool sorbent entering the separator. While the counter current design would effectively maximize the CO2 loading on the sorbent, heat and mass transfer in such a system are unlikely to be sufficient to manage the heat generated during the exothermic adsorption and the heat lost due to the endothermic regeneration. Finally, gas/solids contacting in a moving bed system can be inefficient. It is highly desirable for CO2 adsorption/regeneration reactors to demonstrate effective mass and heat transfer.