Introduction and evaluation of a double loop staged fluidized bed system for post combustion CO2 capture using solid sorbents in a continuous temperature swing adsorption process
Abstract A double loop staged fluidized bed system operated with solid amine sorbents is proposed and evaluated for temperature swing adsorption (TSA) CO2 separation from flue gas streams. The proposed system features effective counter current movement of gas and sorbent streams in both gas solid contactors, i.e. the adsorber and the desorber. The counter current movement enables the realization of axial CO2 concentration profiles within both contactors that is exploited within the TSA process to reach high CO2 capture rates as well as concentrated CO2 from the desorber. The system comprises two multi stage fluidized bed contactors where the sorbents move downwards through a series of bubbling fluidized bed stages. The stages are made up by nozzle or sieve plates and feature downcomers or dip tubes to allow for downward directed particle flow. The gas is fed at the bottom and serves as fluidizing agent in the stages. The particles are removed from the bottom stage through chutes and are directed through a sealing device (loop seal or purged moving bed) towards a transporting riser. The particles are lifted up, separated from the transport gas via gravitational or centrifugal separators and sent to the respective other contacting device through simple chutes. Lean/rich heat transfer may be possible using fluidized bed heat exchangers within the dense bed phase of both transport risers. The process is thermodynamically evaluated for a well studied sorbent material consisting of a porous silica support that is functionalized with a mixture of two different amines (PEI+APTES) It is shown that application of the double loop staged system leads to a significant reduction of sorbent circulation rate and stripping steam demand compared to single stage fluidized bed systems and thus to a significant reduction of the energy demand of the CO2 capture process. It is found that when implementing five stages in each contacting device, it is possible to capture 90% of the CO2 from a typical flue gas stream (10 vol% CO2 content) even when the process is operated with moderate sorbent circulation and stripping steam rates. The corresponding overall heat requirement of the capture process without accounting for CO2 compression and effective heat integration is 3.5 MJ per kg CO2 captured and, thus, equal or even below the values reported for liquid amine capture systems. Improved heat transfer through immersed fluidized bed heat exchangers as well as a significantly lower number of theoretical stages indicate that the proposed TSA contacting devices will allow for a more compact design as compared to gas liquid systems, which would lead to reduced capital costs. In conclusion, CO2 capture within the proposed TSA system using solid amine sorbents may be regarded as a promising post combustion CO2 capture technology.