Pressurized downdraft combustion of woodchips

Abstract

A novel biomass fired combustor for a gas turbine powered cogeneration system has been investigated. Experimental results are summarized and a numerical model of the combustor is presented. A 23 cm i.d., downdraft, packed bed combustor has been tested with 2 cm woodchips at pressures to 5 atm with air preheated to 500 K. The woodchips are supported by a bed of refractory gravel. Burning rates over 100 kg/(hr*m2), dry basis at 13% moisture were obtained. Pressure drop in the bed was about 10%. Particulate emissions averaged 42 ppm. A one dimensional, unsteady, two-phase, reacting, numerical model of the combustor is presented which predicts the fuel burning rate as a function of fuel properties and flow variables. The burning rate is sensitive to combustor presure, air to fuel ratio, inlet air preheat and fuel properties. Temperature and species profiles were obtained. The reaction zone thickness is typically about 25 mm at 4 atm pressure. Good agreement between the model and experiments was obtained at 1–5 atm and air/fuel of 15–30. Solid and gas temperature, fuel density and species profiles in the combustor were obtained. In contrast to a packed bed a gasifier which has an extended reaction zone, the air flows are high and the reaction zone is thin. At 4 atm pressure the reaction zone thickness is typically about 25 mm.