A model of the dynamics of a fluidized bed combustor burning biomass

A dynamical model of an atm., bubbling, fluidized bed combustor of biomass is presented. The model, based on one previously developed for the steady combustion of high-volatile solids, accounts for the fragmentation and attrition of fuel particles, the segregation and postcombustion of volatile matter above the bed, as well as thermal feedback from the splashing region to the bed. The model was used to assess how the dynamic behavior of the combustor varies with some of the operating parameters. To this end, a bifurcation anal. was first used to study the effect of selected parameters on the no. and quality of steady state solns. Moreover, direct integration of the governing equations provided a simulation of the dynamic behavior of the combustor after perturbing the parameters. Results of the bifurcation anal. indicated that extinction may take place through limit point bifurcations when varying the moisture content of the biomass and the flow rates of feed or air. Dynamic simulations showed that the bed temp. changes slowly when a stepwise change is imposed on one of the parameters. Either a new steady state or extinction eventually results, depending on the stepwise change. While relaxation of the bed temp. occurs rather slowly, the dynamics of the splashing region and of the freeboard are much faster, due to the shorter time-scales assocd. with homogeneous oxidn. reactions. The relaxation time of the bed is detd. by the heat capacity of the fluidized solids and by the fraction of the heat released recycling to the bed as thermal feedback.