Analysis of the Rate of Combustion of Biomass Char in a Fluidised Bed of CLOU Particles

Abstract This research concerns the combustion of biomass char in a fluidised bed using Chemical Looping Combustion with Oxygen Uncoupling (CLOU). To evaluate the influence of the CLOU material on the rate of combustion, an analytical model has been developed, based on a simplified scenario, i.e. gaseous mass transfer external to the fuel particle was taken as a stagnant system, ignoring advective flow. The combustion of a char particle was modelled as a shrinking particle. Results from the model were compared with experiments performed by combusting char from birch-wood in a fluidised bed (i.d. 30 mm) of an active oxygen carrier (CuO supported on mayenite) or inert SiO2 sand. The experiments were undertaken with a partial pressure of oxygen, pO2, close to the equilibrium pressure of O2 of the Cu-based oxygen carrier. Despite the same pO2 for both experiments, the presence of the reaction of oxygen uncoupling from the oxygen carrier resulted in a significant increase in the combustion rate of char. As a result, at 1173 K, the burn-out time of 0.1 g of char particles with the oxygen carrier was around five times faster than with SiO2. The results from the analytical model of CLOU agreed with the experimental observations despite the simplified assumption of a stagnant system, viz. a system in which the mass transfer boundary layer, δ , is infinite. This is because the char combustion in CLOU depends, in fact, on gaseous mass transfer across an effective boundary thickness, δ e , rather than a conventional boundary thickness for the system, δ . At 1023 K when the oxygen uncoupling did not occur, the enhancement in the rate of reaction due to the presence of the CLOU particle was less significant. The model was used to investigate the apparent enhancement of the combustion rate in CLOU, and possible explanations, including improved mass transfer, are discussed.