Increasing concerns over growing CO 2 levels in the atmosphere have led to a worldwide demand for efficient, cost-effective, and clean carbon capture technologies. One of these technologies is the Carbonation-Calcination Reaction (CCR) process, which utilizes a calcium-based sorbent in a high-temperature reaction (carbonation) to capture the CO 2 from the flue gas stream and releases a pure stream of CO 2 in the subsequent calcination reaction that can be sequestered. A 120 KWth subpilot-scale combustion plant utilizing coal at 20 pph along with natural gas has been established at The Ohio State University to test the CCR process. Experimental studies on CO 2 capture using calcium-based sorbents have been performed at this facility. Greater than 99% CO 2 and SO 2 capture has been achieved at the subpilot-scale facility on a once-through basis at a Ca:C mole ratio of 1.6. In addition, the sorbent reactivity is maintained over multiple cycles by the incorporation of a sorbent reactivation hydration step in the carbonation-calcination cycle.
Clean-coal technologies that include carbon dioxide and sulfur capture during the production of electric power, liquid fuels, and hydrogen represent a major thrust area. The calcium looping process (CLP) is one such technology that is being developed to convert syngas obtained from coal gasification to hydrogen using a regenerable calcium oxide sorbent. It integrates the water−gas shift reaction with in situ carbon dioxide, sulfur, and halide removal at high temperatures while eliminating the need for a water−gas shift catalyst and reducing the overall footprint of the hydrogen production process. The CLP comprises three reactors: the carbonation reactor, where the thermodynamic constraint of the water−gas shift reaction is overcome by the constant removal of the carbon dioxide product and high-purity hydrogen is produced with contaminant removal; the calciner, where the calcium sorbent is regenerated and a sequestration-ready carbon dioxide stream is produced; and the hydrator, where the calcined sorbent is reactivated to improve its recyclability. In this article, the reaction chemistry occurring in the three calcium looping reactors and the performance of the reactors at various process conditions are presented through thermodynamic and experimental analyses. High-purity H2 with less than 1 ppm of H2S is obtained in the carbonation stage at a stoichiometric steam-to-carbon ratio at high pressures. Although calcination of the sorbent under realistic conditions causes severe sintering and a loss in reactivity, sorbent reactivation by hydration is effective in restoring sorbent reactivity.
The calcium looping process (CLP) is one of the clean coal technologies being developed for the production of hydrogen (H2) and electricity from coal-derived syngas. It integrates the water−gas shift reaction with in situ carbon dioxide (CO2), sulfur, and halide removal in a single-stage reactor. In the CLP, a regenerable calcium-based sorbent is used to react with and remove CO2, sulfur, and halide impurities from the synthesis gas during the production of H2. The removal of CO2 creates a favorable equilibrium and drives the water−gas shift reaction forward per Le Chatelier’s principle enabling the production of high-purity H2. In this investigation, the feasibility and optimum process conditions for the production of H2 in the absence of a water−gas shift catalyst have been described. Calcium oxide (CaO) sorbent has been found to enhance H2 yield to a large extent even in the absence of a water−gas shift catalyst. Specifically, at high pressures, high carbon monoxide (CO) conversion and H2 purity (>99%) have been obtained in the absence of a water−gas shift catalyst at near-stoichiometric steam to carbon (S:C) ratios.
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