A comparative process study of chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) for solid fuels

A solid-fuel combustion system based on chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU) has the potential to assist in the capture of CO₂ from coal-fired power plants. In both processes an air separation unit is not required, and the flue gas streams from CLC and CLOU contain primarily carbon dioxide and water, which facilitates CO₂ capture. CLOU offers a potential advantage for solid fuels as it uses combustion reactions. The O₂ for the combustion reactions in CLOU is supplied from the reduction of a metal oxide (e.g. CuO). Iron-based materials are being considered for oxygen carriers in CLC, wherein the coal is gasified, and subsequently the product gas is oxidized to CO₂ and H₂O by reaction with the circulating oxygen carrier. CLOU affords faster coal char oxidation reaction rates, as compared to CLC coal gasification reactions, but CuO-based materials for CLOU will necessarily be more expensive. Furthermore, the stability of CuO-based oxygen carrier materials is also an important concern. In this paper, ASPEN PLUS process engineering models were developed for combustion of a Wyoming Powder River Basin coal using an iron-based oxygen carrier for CLC and a copper-based oxygen carrier for CLOU. The objective of these process models was to evaluate the material and energy requirements for a process development unit by incorporating insights from previously reported kinetic studies on laboratory scale units. A relative economic analysis has also been performed to address key technical challenges which will subsequently help in addressing the development of CLC and CLOU for solid fuels. Due to slower char gasification reaction times, CLC requires a larger reactor, which results in a relatively higher capital cost. It also manifests in a higher pressure drop and consequently higher energy costs for fluidizing the oxygen carrier.