Experimental investigation on the combustion and heat transfer characteristics of wide size biomass co-firing in 0.2 MW circulating fluidized bed

In order to reduce emissions and enhance combustion efficiency and bed-to-wall heat transfer in circulating fluidized bed (CFB), wide size crushed biomass pellets were adopted for co-firing. The influences of biomass share on bed pressure, temperature profiles, fly ash loss, heat transfer, and gaseous pollutant emissions were studied by experiments performed in a 0.2 MW pilot scale CFB. Results show that with higher biomass share, more uniform axis temperature profiles, lower fly ash loss, less gaseous emission and higher bed-to-wall heat transfer can be achieved, but the effect of biomass share on bed pressure and porosity is minor. As biomass share increases, the reducing atmosphere formed at the upper part of the dense zone is enhanced, and the temperature at the lower part is decreased. This inhibits the formation of NO, N2O and SO2 in the dense zone, while promotes their reduction, so lower gaseous emissions are obtained. Biomass char with small diameter that generated in the dense zone can be consumed soon after the introduction of secondary air, which achieves lower fly ash loss. The fine particles yielded from biomass combustion reduce the thickness of the heat-conduction gas layer between clusters and water wall, and the heat transfer coefficient can be improved by 10%.