Chemical looping based co-combustion of high ash Indian coal and rice straw operating under CO2 in-situ gasification mode

Abstract

Chemical looping combustion (CLC) is a promising technology paving the way for the inherent CO2 capture. As Indian coals are enriched in ash content, their utilization in the CLC technology under direct fueled condition is challenging due to the ash layer resistance in the reactivity of metal oxides. Alternatively, the blending of biomass with high ash coal (HAC) can enhance fuel reactivity with metal oxides. In this context, the present study is focused on the co-utilization of high ash Indian coal (ash 33 wt %) and rice straw (RS) in the CLC process using Fe2O3 particles as metal oxides in a fixed bed reactor under CO2 based in-situ gasification mode of operation. The intrinsic reactivity of ash, char, and volatile matter of the solid fuels with Fe2O3 is assessed individually. The obtained CLC based experimental results showed that the CO2 capture efficiency and gas conversion are found to be increased by 9.7% and 6%, respectively, during the co-utilization of HAC and RS. The reactivity of H2, CO, and CH4 with Fe2O3 is also assessed under the co-combustion mode of the CLC process. The XRD results showed that the formation of calcium ferrite (CaFe2O4) had enhanced the conversion of char during the CLC process. A higher reactivity between char and Fe2O3 (solid-solid interaction) is observed at high operating temperatures with 13% increase in the weight loss under the co-combustion mode of the CLC process. Further, the kinetic parameters are evaluated for the CLC and non-CLC based reactions under various operating temperature regimes. Among the kinetic models considered, the shrinking core model is found to be the best fit, compared to the homogenous progression model. The chemical reaction is the slowest step at the operating temperatures in the range of 200–500 °C; ash layer diffusion is found as the rate controlling step at high operating temperatures (800–1000 °C); whereas film diffusion controlled mechanism is dominant at 700–800 °C.