Identifying the contribution of rich-CO2/H2O gasification on the char conversion in typical atmospheres of chemical looping with oxygen uncoupling via single particle simulation

Abstract

Char conversion is an important but also complicated step during the combustion of solid fuels (i.e. coal and biomass) in chemical looping with oxygen uncoupling (CLOU). Potentially, both the lean-O2 oxidation and rich-CO2/H2O gasification reactions of char contribute to the char conversion. However, the contribution degree of char gasification reaction has rarely been quantified in typical CLOU atmosphere. In this work, the single particle simulation is conducted to theoretically understand the effects of different operation conditions on the char conversion, and to identify the contribution degree of rich-CO2/H2O gasification in CLOU. A 10-step heterogeneous reaction mechanism is used for the gas-char reactions, and the random pore model is adopted to account for the evolution of porous char structure as well as its impact on reaction rates. The detailed conversion processes of chars with different sizes are simulated in various concentrations of gasification/oxidation agents and at different temperatures. The amounts of char consumed by lean-O2 combustion, rich-CO2 gasification, and rich-H2O gasification are assumed to be proportional to the amounts of surface adsorbed species generated by char reacting with O2, CO2, and H2O, and a formula to quantitatively calculate the relative contributions of the gasification reactions (both with CO2 and steam) is finally proposed. The results indicate that the gasification reactions (occurring in the inner core of the char particle) hardly influence the oxidation reaction (dominating the heterogeneous reaction in the external layer of the char particle) under CLOU conditions. The CO2-char gasification contributes less than 3% on the char conversion, while the contribution of H2O-char gasification can even reach 2–18%. The char-H2O gasification contributes more significantly (18% for 0.5 vol.% O2, and 2% for 4 vol.% O2) under the O2-deficient conditions. Generally, the overall contribution of gasification reactions on the char conversion is between 1 and 20% in CLOU, which depends on the reaction temperature, gas composition, and char particle size.