Investigation of adsorbent-based warm carbon dioxide capture technology for IGCC system

Abstract

Integrated gasification combined cycle with CO₂ capture and sequestration (IGCC-CCS) emerges as one of the most promising technologies for reducing CO₂ emission from coal power plant without reducing thermal efficiency significantly. However the high capital cost of these plants has limited their deployment. The current solvent-based low-temperature CO₂ capture process (Selexol process) is energy and capital intensive contributing to the problem. Sorbent-based warm CO₂ capture has been predicted to be a key enabling technology for lowering down the costs of IGCC-CCS. However, no commercial adsorbents or processes exist for these warm CO₂ separations. My thesis work has been devoted to developing a solid sorbent and CO₂ capture process which can capture CO₂ at an elevated temperature in IGCC system. By combining experimental methods and quantum calculation, I have successfully identified and invented one new sorbent material. The sorbent for warm CO₂ capture containing magnesium oxide was developed using incipient wetness impregnation. The reversible adsorption isotherm, cyclic stability, and sorption rate were measured using a custom-built high pressure microbalance system and a thermogravimetric analyzer. Experimental data indicate the sorbent has a fairly large regenerable capacity in 180-240 °C temperature range, fast kinetics, low heat of adsorption, and stable working capacity for at least 84 cycles. The new sorbent performs better than synthetic hydrotalcite and K²CO³-promoted hydrotalcite in the temperature range of interest. To assess the applicability of CO₂ removal technology to IGCC via a warm pressure swing adsorption (PSA) process based on our newly invented sorbent which has good cyclic sorption-desorption performance at an elevated temperature, a 16-step warm PSA process was simulated using Aspen Adsorption based on the real sorbent properties. I used the model to fully explore the intercorrelation between hydrogen recovery, CO₂ capture percentage, regeneration pressure of sorbent, and steam requirement. Their tradeoff effects on IGCC efficiency were investigated by integrating the PSA process into the plant-wide IGCC simulation using Aspen Plus. On the basis of our analysis, IGCC/warm PSA using our new sorbent can produce slightly higher thermal efficiencies than IGCC/cold Selexol. In order to achieve this, warm PSA needs a narrow range of process parameters to have a good balance between the hydrogen loss, steam consumption and work requirement for CO₂ compression. Sensitivity analysis is finally conducted to point out the future direction for making warm syngas cleanup more applicable. Further research is needed toward synthesizing new sorbent materials with higher working capacity and improved mass transfer, a better PSA configuration with higher H2 recovery and less steam consumption, new desulfurization process with reduced H2 consumption, and better heat integration. The development in this research would help further improving the efficiency and economics of IGCC/CCS. Overall, my thesis work provides a rigorous analysis framework for identifying and assessing warm CO₂ capture by sorbents in an IGCC system. This adsorbent-based warm CO₂ capture technology developed in my work can potentially help make IGCC/CCS more affordable and acceptable.