Influences of coal size and coal-feeding location in co-firing with rice husks on performance of a short-combustion-chamber fluidized-bed combustor (SFBC)

This study examined the combustion characteristics and performance of rice husks co-fired with coal in a short-combustion-chamber fluidized-bed combustor (SFBC) with a 225 kWth capacity. Rice husks were the main fuel, and coal was a supplementary fuel in the experiments. The effects of coal size (< 5 mm and 5–10 mm) and coal-feed location (above or below a recirculating ring) on combustion performance were investigated. Various co-combustion tests of rice husks with coal were performed, with different thermal percentages (10, 15, 20, and 25%) of coal. The results were compared to firing 100% rice husks alone. With the assistance of a stirring blade and a recirculating ring, good combustion was feasible without using any inert materials mixed into the bed. tion efficiency in an excess of 98% was readily achievable. CO and SO2 emissions (at 6% O2) were in the range 64–104 and 10–22 ppm, respectively, while NOx emissions were in the range of 208–281 ppm. Although the CO and SO2 emissions were acceptable, combustion of 100% rice husks and co-combustion with < 20% coal failed to comply with Thai NOx emission limits. Therefore, to minimize NOx emissions (208–244 ppm, at 6% O2), coal of both sizes was introduced below the recirculating ring. The results demonstrated that the thermal percentage of coal in the fuel mixture should be 20–25%.