OPTIMIZATION OF SULFUR RETENTION IN ASH WHEN COCOMBUSTING HIGH-SULFURFUELS AND BIOMASS FUELS IN A SMALL PILOT-SCALE FLUIDIZED-BED

Previous chemical equilibrium calculations as well as experimental res ults concerning cocombustion of high sulfur containing fuels and bioma ss fuels have shown that a significant retention of sulfur can be obta ined in the ash. The processes are, however, complex and to minimize t he SO2 emissions, experiments were performed, according to statistical experimental designs, in a small pilot scale fluidized bed (5 kW). He reby, a sulfur retention of 70-75% for a peat-wood fuel mixture and 90 -85% for a mixture of coal and an energy crop (Lucerne) were obtained. The products CaSO4 and 3K2SO4.Na2SO4 were identified in the ashes and dust. Fuel feeding rate (load), primary air ratio and total air flow were identified as the most influential operating factors, and bed tem perature and oxygen concentration seem to be the most crucial physical -chemical factors for the sulfur retention. The NO emissions were also decreased by the SO2 reducing measures, although the fraction of prim ary air was increased. This was probably due to a lower bed temperatur e at optimum sulfur capture conditions. The SO2 emissions could be red uced without any increase in CO or NO emissions. Some preliminary meas urements of the slagging tendency showed no increased slagging with in creasing sulfate formation.