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Limestone as a sorbent for CO2 capture and its application in enhanced biomass gasification Butler, James


Global greenhouse gas emissions continue to increase despite the knowledge that the rise in atmospheric concentrations will have devastating effects on climate and human lives. Carbon dioxide capture and storage can be a stop-gap measure to mitigate CO₂ emissions from existing fossil fuel facilities during their gradual replacement by low-carbon alternatives such as biomass. Calcium oxide-based CO₂ capture is a relatively mature technology, ready for implementation. Limestone CaO precursor is relatively low-cost and readily available. A thorough understanding of the CaO-CO₂ reaction and its reversibility over multiple cycles is required to aid in design, improve efficiency and reduce costs of industrial capture processes. A novel method of CaO cycling involving pressure swing is demonstrated which was found to give improved calcium utilization up to 16.1%, after 250 carbonation/calcination cycles. The kinetics of pressure swing cycling are examined, and a mechanism to describe the loss in calcium utilization resulting from cycling, is presented linking the morphological changes of sorbent particles to the decay in calcium utilization. Coupling CaO-based CO₂ capture and storage with energy production from biomass has the potential for energy production with negative CO₂ emissions. Biomass is a carbon neutral source of energy and through gasification can be converted in a variety of energy carriers. Biomass was steam-gasified in a semi-batch fashion in a fluidized bed of CaO, which absorbed CO₂ as it was produced, resulting in a 55% increase in hydrogen production and decreases in CO, CH₄, CO₂ and higher hydrocarbons of 63%, 16%, 47% and 4% respectively. Limestone enhanced gasification (L.E.G.) of biomass also increased carbon and hydrogen utilization efficiencies. Cycling of CaO between gasification/carbonation and calcination was conducted in a single reactor by switching the mass flows from biomass and steam to air, up to eight cycles. Syngas composition and gasification efficiency were only marginally affected by cycling, reducing H₂ concentration by less than 5%. The degree to which the sorbent was re-calcined had a greater impact on system operation. A simple equilibrium model is provided to predict syngas composition.

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