External Bed Materials for the Oxy-Fuel Combustion of Biomass in a Bubbling Fluidized Bed

This paper presents an experimental study of various bed materials for oxy-fuel combustion of biomass in a bubbling fluidized bed (BFB). For the experimental evaluation, a silica sand with three particle size distributions (PSDs) and a lightweight ceramic aggregate (LWA) of expanded clay with two particle size distributions were chosen as potentially suitable materials. The experiments of biomass combustion in oxy-fuel mode with these materials were carried out to study their suitability as external bed materials. The experiments were performed using a 30 kWth lab-scale BFB facility with A1 wooden pellets as a fuel. The selected materials were able to form a sufficiently stable fluidized bed within all experiments. There were no significant temperature fluctuations within the experimental facility or dynamic changes in the flue gas composition during the combustion process, hence the combustion process can be considered stable. The fluidized bed temperature was controlled at approx. 800 °C and the volumetric fraction of O2 in dry off-gas at 10 %. The resulting CO2 volumetric fraction in dry off-gas was about 85 %. At these conditions, there were no problems with sintering and with the formation of agglomerates. In case of ceramic materials, it was possible to control the combustion process in a significantly wider range of operating parameters. Compared to the silica sand, the experimental facility could be operated at a lower excess of oxygen (below 6 % in dry off-gas) without significant influence on the temperatures in the facility. In terms of own consumption of the combustion facility, ceramic materials seem to be more suitable, since they are lighter. The pressure drop of the fluidized bed made of LWA is lower than of the bed of the same volume made of silica sand, which means that less power is required to drive the fluidization fan.