The competition/inhibition effect of H2O/CO2-char gasification in typical in situ gasification-chemical looping combustion (iG-CLC) conditions via particle-resolved simulation

Abstract

In the fuel reactor of in-situ gasification chemical looping combustion (iG-CLC), the solid fuel (e.g., coal, biomass) goes through two typical processes: devolatilization and char gasification. Then the pyrolysis and gasification products react with oxygen carrier to produce CO2 and H2O ideally. The slow char gasification rate during iG-CLC is responsible for combustion efficiency. Therefore, a deeper understanding of char conversion characteristics is required to optimize iG-CLC conditions. In this paper, a particle-resolved simulation with detailed heterogeneous reactions is conducted, in which the effects of char particle size and reaction temperature on char conversion characteristics are carefully studied. To understand the competition between CO2 gasification and H2O gasification, single-particle simulations are carried out under mixed and separate atmospheres. The char gasification rate in CO2/H2O mixtures (rmix) is lower than the sum of the char-H2O gasification rate (rH2O) and the char-CO2 gasification rate (rCO2). We find that the competition for active sites between char-H2O gasification and char-CO2 gasification is not significant, while the inhibition of gasification products play an important role in char-CO2 gasification, particularly the inhibition of CO (from char-H2O or char-CO2 gasification), due to the accumulation of H2 and CO inside the char particle. The inhibition effect is quantified via particle-resolved simulations and then a quantitative formula is attained to describe the complex relationship between H2O gasification and CO2 gasification. Then the point-source model for H2O/CO2-char gasification is developed for computational fluid dynamics simulations. Straightforward cloud diagrams are presented to explore the relationship between conversion rate characteristics and physicochemical properties of char, providing useful information for the optimization of iG-CLC conditions.