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Abstract
The combustion of pulverised coal with biomass waste materials for electricity generation has potential for addressing several areas of concern relating to waste disposal and reduction of harmful emissions. It is in this context that the European Union Clean Coal Technology programme; APAS, is executed. As one of the participants, the IFRF has performed semi-industrial scale studies on the co-firing of different coals with straw, waste paper and municipal sewage sludge.
The single burner experiments were performed at the scale of 2.5 MW, with a swirl stabilised Aerodynamically Air Staged Burner (AASB) in a boiler chamber simulator with internal dimensions of 2 × 2 × 6.3 m. The pulverised coals 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. Different coals showed similar trends in NOx and SO2 emissions. The main parameters affecting the NOx SO2 emissions and burnout were the co-firing ratio, coal type and flame type. Preferential burning (lower burnout) was observed in the 20-40 % straw/coal co-firing range and trends were different for high and medium volatile coal.
The investigation into the co-firing of pulverised coal with dried sewage sludge has demonstrated that the sewage may be milled and fired successfully as a pulverised 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 could be reduced by ∼60% with fuel placement (i.e. by varying the mode of fuel injection), and by 70-80% with the introduction of air staging.
In general it can be concluded that the co-firing of pulverised coal and straw is possible although handling, chopping and feeding of the biomass waste can be difficult. Further studies are required on burnout optimisation. Co-firing of municipal sewage sludge with pulverised coal indicated that it is feasible to co-fire sewage sludge with pulverised coal. Operational difficulties such as slagging and fouling are of major concern because of the high ash content of the sewage feed. Increased concentrations of NOx, relative to a 100% coal flame, were observed due to the higher concentrations of fuel-bound nitrogen.