UBC Theses and Dissertations
CO₂ removal in power systems using calcium-based sorbents Sun, Ping
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•CO₂ 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 CaSO₄ 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 SO₂ 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 H₂S 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.
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