Interactive processes in the fluidized bed combustion of high-volatile solid fuels

The combustion of two high-volatile solid fuels (a biomass and a tyre-derived fuel) in bubbling fluidized bed is analyzed in the framework of the combustion-comminution-elutriation network recently proposed to follow the fate of fixed carbon during solid fuels combustion in fluidized bed (Arena et al., 1996). A simple model is developed based on the assumption that fixed carbon present in the bed can be lumped into a coarse particles phase and a fine particles phase. The model takes into account the volatile matter released from the fuel and allows incomplete combustion of the volatiles in the bed. Results of computations show that the two fuels behave in a rather different way during combustion in fluidized bed: coarse particles of the biomass generate huge amounts of fines which are almost completely burned in the bed due to their high reactivity, leading to a very high combustion efficiency. On the other side TDF undergoes complete comminution to elutriable fines upon devolatilization and the combustion efficiency depends on the competition between parallel fines combustion and elutriation processes. The influence of air staging into a primary and a secondary stream on the combustion efficiency is assessed. This strategy for optimization of the combustion efficiency can be very effective especially for the tyre-derived fuel.