The Combustion of Biomass Char in a Fluidised Bed: The Influence of CLOU Particles on the Rate of Combustion
Session chaired by Pr. Epaminondas Mastorakos
This research concerns the combustion of biomass char in a fluidised bed using Chemical Looping Combustion with Oxygen Uncoupling (CLOU). Here, gaseous oxygen for reaction comes from a solid oxide, called an oxygen carrier. To evaluate the influence of the CLOU material on the rate of combustion, experiments were performed in a fluidised bed (i.d. 30 mm) using either an active oxygen carrier (CuO supported on mayenite) or inert SiO2 sand, as the bed material. To avoid other mass transfer effects in the fluidised bed, a small mass of birch wood char (≤ 0.1 g) was combusted in the bed, 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 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 in the CLOU setup was around five times faster than in the inert bed of SiO2. To investigate the origin of this apparent enhancement of rate of combustion of the birch char in CLOU, 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 model accounted for: (1) the combustion of char with oxygen to CO and CO2, (2) the homogeneous reaction of CO with oxygen, and (3) gaseous oxygen release from the oxygen carrier. The combustion of a char particle was modelled as a shrinking particle. 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, is infinite. This agreement results from a creation of an effective boundary thickness, rather than a conventional boundary thickness for the system, . Moreover, in CLOU, is independent of , provided . Finally, the modelling work evidences that the rate of combustion in CLOU was enhanced due to the improved mass transfer of O2 from the bulk phase to the surface of the char particle, i.e. the interphase mass transfer.
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