A Novel Hybrid Chemical-Looping Oxy Combustor Process for the Combustion of Solid and Gaseous Fuels: Thermodynamic Analysis

Abstract

The larger reactor volume, additional oxygen polishing unit, and carbon stripper for the separation of oxygen carriers and ash in the chemical looping combustion (CLC) and/or chemical looping oxygen uncoupling (CLOU) processes for solid fuels are anticipated not only to incur operational complexity but also to increase the capital and operating costs. As an alternative, this paper proposes a novel hybrid process, called “Chemical Looping Oxy Combustor (CLOC)”. This novel process provides an integration of chemical looping air separation (CLAS) with fluidized bed oxy-fuel combustion and is expected to eliminate the need for an additional oxygen polishing unit and carbon stripper. It can be retrofitted to any existing coal circulating fluidized bed (CFB) at low cost. The other advantages of CLOC includes less solid handling issues, flexibility in handling low-grade coal with high moisture, no/less contamination of oxygen carriers, no/less slip of CO2/SOx in an air reactor, low energy penalty, etc. Also, in the CLOC process, coal combustion will occur in a separate fluidized bed combustor with relatively faster kinetics, because of the availability of high oxygen concentration (i.e., ∼25–28 vol %), which eliminates the need for a larger fuel reactor volume. In the current paper, thermodynamic simulations of CLOC process using Cu-, Mn-, and Co-based metal oxide oxygen carriers were performed. Their performances were also compared against the conventional air-firing and oxy-firing technologies, e.g., oxy-fuel combustion integrated with cryogenic air separation unit (CASU) and CLOU. It was identified that the CLOC process needs external heat for reduction reactor provided by either direct or indirect methane combustion. Moreover, a maximum plant thermal efficiency was achieved for CLOC using Cu-based oxygen carrier. The energy penalty of the CLOC process, compared with the air-firing base case, was found to be ∼2%–3%, which is ∼4–5 times smaller than those of the CASU cases and only half of that of the CLOU process, indicating that CLOC offers a promising option for the combustion of solid fuels.