Abstract
In a chemical looping combustion (CLC) system, gasification kinetics of char holds immense importance being the rate-limiting reaction in the fuel reactor. This paper studied the gasification kinetics of char derived from Victorian Brown Coal (VBC) in a fluidised bed reactor which mimics the fuel reactor conditions of a CLC process. Mass of char, char particle size and gas flow conditions were optimised to ensure the gasification reaction free from mass transfer limitations. Effect of oxygen carrier, hematite, being the bed material was also studied. The experiments were conducted in the temperature range of 800C-950C, which is a typical range for fuel reactor. The experimental results were modelled with the help of grain model (GM) and random pore model (RPM) to analyse the kinetic parameters. Activation energy was found to be around 177 kJ/mol in sand bed and 175.5 kJ/mol in the hematite bed. Reaction in hematite bed was found to be 42% faster on average compared to the reaction in a sand bed. Fastest total conversion of char took as low as 4.1 minutes in hematite bed at 950C. While catalytic effect of hematite was ruled out due to insignificant change in activation energy, it was concluded that increase in CO2 partial pressure at the vicinity of char particle enhanced the reaction rate in the case of hematite bed. This study has generated relevant information for the CLC of Victorian Brown Coal with hematite as the oxygen carrier.
A theoretical analysis of the burn-out times of an isothermal particle of coal char under air-firing, gasification and oxyfuel combustion in fluidised beds