Kinetic Study of the CO2 Capture using Calcium-Based Sorbents and its Application for Hydrogen Production in Gasification Process

Abstract

Carbon dioxide (CO2) capture and sequestration (CCS) from flue gas obtained from fossil fuel power plants appears to be essential to decrease CO2 emissions into the atmosphere. Calcium-looping cycle represents a promising technology for CO2 capture. The intrinsic kinetics of the carbonation reaction for the calcines from two types of limestone (Havelock and Cadomin) was studied. The order of the carbonation reaction changed from first order to zero order when the CO2 partial pressure exceeded ~ 0.7 atm for both types of limestone. The rate of carbonation reaction increased with increasing temperature up to 675°C and decreased with further increases. The obtained intrinsic kinetic parameters (e.g., order of reaction, activation energy, and per-exponential factor), were applied in global kinetic study of the carbonation reaction. The global kinetic study performed using the grain model and the changing grain size (CGS) model. In the CGS model, the physical structure of the sorbent particle changes during the reaction. The overall sorbent conversion obtained with the CGS model showed better agreement with the experimental data than the results obtained using the grain model. The presence of sawdust as a biomass during calcination step decreased the negative effects of calcium-based sorbent sintering by enhancement of calcium dioxide (CaO) surface area, which resulted in an improvement in the carbonation initial rate. Four CO2 carriers included one calcined natural limestone (Cadomin) and three modified calcium-based sorbents; Cadomin-calcium aluminate cement pellets (CD-CA-14), Cadomin-silica-sol pellets (CD-Si), and Cadomin-mesostructred silica core/shell pellets (CD-CS), were examined through 31 carbonation–calcination cycles. The presence of the Mayenite phase (Ca12Al14O33) in the CD-CA-14 pellets led to slower decay in sorbent activity with increased numbers of cycles. The CD-CS pellets showed the best performance in the retention of CO2 uptake activity with only 46% activity loss. Finally, CO2 capture sorbents were integrated with steam gasification of coal to capture CO2, which consequently produced high purity hydrogen. The experimental results showed that the presence of sorbent in the steam gasification of coal enhanced the molar fraction of H2 to more than 80%, with almost all CO2 fixed into the sorbent structure. The steam gasification of coal integrated with CO2 capture exhibited an optimal condition at 675°C.