Effect of co-combustion of biomass on emissions in pulverized fuel furnaces

Biomass not only has a considerable potential as an additional fuel source but it also shows a reasonable cost level in comparison to other renewable energies. The practicable fuel types are both residual material from forestry and agriculture, such as wood or straw, and especially cultivated reproducible feedstock such as Miscanthus Sinensis, whole cereal plants, poplars, or willows. Besides, as single fuel, it is also considered to be sensible to utilize biomass in co-combustion in existing firing systems, such as pc-fired power stations. In order to promote the employment of biogenetic fuels, the European Union, within the framework of the APAS Clean Coal Technology Programme, initiated a research project concerning the co-combustion of biomass in existing firing systems. The focus of the investigations in experimental and industrial-scale plants was to establish the impact of co-combusting biomass and sewage sludge in coal-fired systems with regard to combustion behaviour and pollutant formation. The investigations were carried out on laboratory scale and pilot scale, as well as in large-scale plants. The task of the Institut fur Verfahrenstechnik und Dampfkesselwesen (IVD), University of Stuttgart, within the EU clean coal technology programme, was to investigate the effects of co-combustion of solid biomass (Miscanthus, straw) and municipal sewage sludge together with the primary fuel hard coal. The experimental plant used was a semi-industrial pulverized fuel test rig (0.5 MW). Within the scope of this publication, a synopsis shall be given of the outcome of the project, with special regard to the effect of co-combustion on emissions. The investigations reveal that biomass addition has a positive effect on emissions. Since biomass in most cases contains considerably less sulphur than coal, an increasing biomass share in the thermal output makes the SO₂ emissions decrease proportionally. In addition, SO₂ can partly be captured in the ash by the alkaline-earth fractions of the biomass ash. As for sewage sludge, the emissions of SO₂ correlate with the sulphur content of the fuel and, hence, rise with an increasing share of this biomass. Due to the high volatile content of the biomass, low NO_x emissions can be achieved both by air staging and by reburning.