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Capture of CO2 from pre-combustion gas by adsorption processes at high temperatures
thesisposted on 09.02.2017, 05:32 by Xiao, Gongkui
The capture and storage of CO2 from coal have received considerable attention due to a growing demand for green use of carbon-based resources concerning its global warming effect. There are usually two ways of capturing CO2 from coal-related gases according to locations of capture blocks, post-combustion and pre-combustion. The conventional approach for pre-combustion CO2 capture is liquid absorption. However, one potential limitation of this process is that the liquid scrubbing must occur at relatively low temperatures (near ambient). By contrast, adsorbent based processes offer the potential of higher temperature capture eliminating the intermediate cooling and reheating steps. Therefore there is a motivation to develop high temperature adsorption processes. This project investigates the capture of CO2 by solid adsorbents from pre-combustion coal gasification syngas in an Integrated Gasification Combined Cycle (IGCC) process at high temperatures. The project covers materials development and process testing on both a real gasifier and a synthetic pre-combustion gas stream provided in our laboratory. Adsorbents screening was performed with real coal gasification syngas from a pilot coal gasifier. Breakthrough and regeneration experiments with hydrotalcite MG 70, calcium chabazite, and zeolite 13X were performed on the custom-made gas separation apparatus at 120 °C and 200 °C. Zeolite 13X was found to be a potentially good adsorbent for CO2 separation provided T<200 °C. Thereafter, different VPSA processes using zeolite 13X were designed to pursue a CO2 purity of >95% with synthetic syngas. The designed processes were then validated with real coal gasification gas. However, simulation results for the separation from full scale syngas showed that using zeolite 13X to reach a purity of > 95% would be very costly. High temperature adsorbents composed of MgO and K2CO3 were prepared and characterized by various means. These prepared adsorbents possessed high CO2 sorption capacity and cycling stability both under dry and wet conditions. An adsorption model was proposed for the prepared adsorbent and provided an adequate description of the adsorption and desorption experiments performed on a thermogravimetric analyser. The derived isotherm from the proposed model was also able to represent the experimental isothermal adsorption data well. Although the CO2 adsorption kinetics of these adsorbents were higher than other reported high temperature adsorbents, they were still much slower than widely used physical adsorbents, which makes it currently not very efficient for pressure swing adsorption processes. In future work, efforts will be focused on increasing the adsorption and desorption kinetics by, for example making these adsorbents to nanoscale, possibly supported on porous substrates.