A fixed bed, downdraft combustor for solid fuels applicable to gas turbine cogeneration or combined cycles is described. The combustor has a refractory gravel bed, with fuel placed on top of the gravel and burning at the fuel-gravel interface. The gravel retains fuel particles in the combustion zone, preventing carryover of unburned char. Combustion temperature is held below the ash fusion temperature by using high excess air through the bed, thereby minimizing particulate ash agglomeration.
Previous work on fixed bed, downdraft combustors and coal fired gas turbines is reviewed. Recent results from test firing a 480 cm2 (0.5 ft2) gravel bed combustor at atmospheric pressure with 2 cm (0.75 in.) wood chips are presented. Heat release rates of 3500 MJ/hr m2 (300,000 Btu/hr ft2) and higher were obtained. Carbon carryover in the exhaust was negligible. Approximately 85 percent of the particulate ash emissions was less than 10 microns.
