THE CO-FIRING OF PULVERIZED BITUMINOUS COALS WITH STRAW, WASTE PAPER AND MUNICIPAL SEWAGE-SLUDGE

The combustion of pulverised coal with biomass waste materials for ele ctricity generation has potential for addressing several areas of conc ern relating to waste disposal and reduction of harmful emissions. It is in this context that the European Union Clean Coal Technology progr amme; APAS, is executed. As one of the participants, the IFRF has perf ormed semi-industrial scale studies on the co-firing of different coal s with straw, waste paper and municipal sewage sludge. The single burn er experiments were performed at the scale of 2.5 MW(t) with a swirl s tabilised Aerodynamically Air Staged Burner (AASB) in a boiler chamber simulator with internal dimensions of 2 x 2 x 6.3 m. The pulverised c oals studied are bituminous coals of high and medium volatile content, and low and high sulphur content. The biomass fuels studied are straw and waste paper. The co-firing ratios varied from 0% to 100% straw. D ifferent coals showed similar trends in NOx and SO2 emissions. The mai n parameters affecting the NOx, SO2 emissions and burnout were the co- firing ratio, coal type and flame type. Preferential burning (lower bu rnout) was observed in the 20-40% straw/coal co-firing range and trend s were different for high and medium volatile coal. The investigation into the co-firing of pulverised coal with dried sewage sludge has dem onstrated that the sewage may be milled and fired successfully as a pu lverised fuel. Different coals were fired with the dried sewage in the co-firing range 0-30%, 50%, 70% and 100%. In addition, the effect of air and fuel staging on burner performance was also established. NOx c ould be reduced by similar to 60% with fuel placement (i.e. by varying the mode of fuel injection), and by 70-80% with the introduction of a ir staging. In general it can be concluded that the co-firing of pulve rised coal and straw is possible although handling, chopping and feedi ng of the biomass waste can be difficult. Further studies are required on burnout optimisation. Go-firing of municipal sewage sludge with pu lverised coal indicated that it is feasible to co-fire sewage sludge w ith pulverised coal. Operational difficulties such as slagging and fou ling are of major concern because of the high ash content of the sewag e feed. Increased concentrations of NOx, relative to a 100% coal flame , were observed due to the higher concentrations of fuel-bound nitroge n.