Estimation of the temperature in the bottom region of a fluidized-bed furnace burning biomass

Paper in proceeding, 2006

The heat and mass balance of the bottom region of a fluidized bed boiler burning biomass is established with the aim of determining the furnace temperature. The influence on the temperature by the flow of solids in the upper part of the bottom region is investigated. Heat and mass balances of gas and fuel were applied over a control volume containing the bottom region, i.e. both the bed and the adjacent splash zone. The mass balance is based on given input flow rates of gas and fuel and concentrations in the gas leaving the control volume. The measured gas concentrations were used to determine the rate of fuel burnout inside the control volume. Pressure drop measurements along the height of the furnace estimate the solids concentration. Experiments were performed in a furnace with a cross-section area of 2.2 m2. The walls of the furnace are refractory lined up to a height of 2 m and, for simplicity, the control volume is chosen from the air distributor up to this height. Two fuel mixtures were used: one biomass and one of biomass and coal. The results show that the gas temperature at the top of the control volume strongly depends on the flow-rate of fine particles present in the fluid, even when the volume fraction of particles is small. The particles carry heat to the freeboard, where they transfer heat to the walls before returning to the bottom region. This process has previously been neglected in models of fluidized-bed combustion under bubbling conditions, overestimating the fluid temperature.