Carbon dioxide removal in power systems using calcium-based sorbents

Bench scale studies were carried out, focusing on the application of calcium-based sorbents in fossil-fuel-fired combustion and gasification systems, with conditions ranging from atmospheric to elevated pressures and at practical combustion and gasification temperatures.; In the kinetic study of CaO carbonation, the reaction order changed abruptly from first- to zero-order when the CO2 partial pressure exceeded the equilibrium value by more than -10 kPa. A Langmuir mechanism successfully explained the experimental information, with the intermediate complex CaO•CO 2 postulated to saturate CaO sites immediately at high CO2 partial pressure. The activation energies for rate constants were found to be 29 +/- 4 kJ/mol and 24 +/- 6 kJ/mol for Strassburg limestone and Arctic dolomite, respectively. A discrete-pore-size-distribution-based model was formulated, with the aid of which the kinetic study was extended to obtain diffusivities through the solid product layer formed during carbonation, with activation energies of 215 and 187 kJ/mol for the limestone and dolomite, respectively.; Sorbent cyclic CO2 removal ability was investigated based on pore size distribution measurements. Several important features observed from measurements could be predicted by a mechanistic model which included simultaneous sintering and calcination in the fixed bed. It was found that the decay in the reversibility of limestone capture/regeneration was insensitive to operating conditions; the achievable carbonation extent of each cycle depends on the <∼220 nm pore volume that decreases monotonically during cycling.; Co-capture of SO2 and CO2 was attempted at fluidized bed combustion temperatures. Parametric studies with Strassburg limestone and Arctic dolomite found that the presence of SO2 impeded carbonation even at low concentrations of SO2 relative to CO2 concentrations. This finding is significant for the application of calcium-based sorbents in fluidized-bed combustors (FBCs), given the initial assumption that sulphur should not be problematic given the low sulfur/carbon ratio in fuels. The mechanism of the impeding effect of SO2 was investigated for seven sorbents at both atmospheric and elevated pressures. It was found that direct sulphation becomes dominant after completion of an initial fast stage of carbonation, enveloping the sorbents and inhibiting further carbonation. Among the techniques tested, increasing the CO2 partial pressure was found to be the most helpful way to improve sorbent reversibility.; It was also shown that often-cycled sorbents can be reactivated and achieve improved reversibility by the use of low-temperature steam or liquid water. CO is not appropriate as an agent to cyclically regenerate CaO from CaSO 4 because of slow regeneration of CaSO4. Among the inert dopants tested, only Al2O3 mixed with CaO at a 1:1 molar ratio, was able to achieve satisfactory CO2 capture reversibility.; As extensions of the applications of calcium-based sorbents, sequential SO2 and CO2 capture were investigated for fluidized bed combustion. Among the four options examined, the best was found to be to apply spent sorbent after cyclic CO2 capture to remove SO 2 from atmospheric pressure combustors.; A novel concept of co-capture of CO2 and H2S in a gasifier-based process was also investigated. Unlike the findings for co-capture of SO2 and CO2, no obvious impeding effect of H 2S was observed on cyclic CO2 capture. Parametric studies indicated that it should be feasible to co-capture H2S and CO2. With CO2 sorbents in a gasifier, one-step hydrogen production via gasification should be achievable.