Heat Balance Modeling of a Stationary Fluidized-Bed Furnace Burning Biomass

Doctoral thesis, 2004

The influence of biomass fuel properties on the heat balance of the bottom region (bed and splash zone) of a commercial-scale bubbling fluidized-bed furnace is investigated theoretically and experimentally. The experiments have been carried out at the Chalmers 12 MWth research boiler, operated under non-circulating conditions during most of the experiments. The different parts of the work are summarized as follows. A two-phase flow model of the bed and the splash zone in a boiler is presented. The combustion rate in the bed is estimated by global kinetic expressions, limited by the gas exchange between oxygen-rich bubbles and a fuel-rich emulsion phase, whereas the combustion rate above the bed is determined from turbulence properties. A heat balance of the bottom region shows that the average temperature of the gas leaving the bottom region strongly depends on the flow rate of fine particles present in the fluid. The particles carry heat to the freeboard, where they transfer heat to the boiler walls before returning to the bottom region. A method is proposed for estimation of the effective lateral dispersion of fuel in a fluidized-bed combustor. By correlating the drying of the fuel particles and the measured moisture concentrations above the bed, the effective lateral dispersion coefficient of the fuel particles is determined. This coefficient was estimated to be on the order of 0.1m2/s, which is considerably higher than predicted by most of the expressions given in the literature. The local air ratios in the furnace are estimated by fluctuating signals from zirconia cell probes, which are compared to simultaneous gas concentrations of extracted gas samples. The time fraction during which the fluctuating zirconia cell signal shows oxidizing gas conditions is strongly correlated with the local air ratio of the gas. When this correlation is determined, the fluctuating signals from zirconia-cell sensors can be used to obtain the air ratio at different heights in the furnace. This provides a cheap and robust technique for on-line monitoring of the gas conditions in the furnace when, for example, optimizing the operation of a fluidized-bed boiler to reduce nitrous oxide emissions.