Energy recovery from waste: Comparison of different technology combinations

Energy recovery from waste can follow several routes. The most common one is waste direct combustion associated with conventional energy recovery in a steam turbine cycle. The combustion can be applied directly to Municipal Solid Waste or can be applied to a stream of selected waste obtained by means of mechanical sorting of Municipal Solid Waste, using several technologies for the combustion, the most common of which is mobile grate combustor. Besides the direct combustion of waste, alternative possibilities for thermal treatment are gasification and pyrolysis. These processes require being fed by a homogeneous combustible fraction obtained by mechanical sorting and supply as output one or more combustible streams, available for energy recovery. When Municipal Solid Waste mechanical sorting is applied, besides the combustible fraction stream, a humid fraction is also obtained, characterised by a high presence of organic biodegradable fraction. At present the fate for this stream is biological aerobic stabilisation, but another option, to push energy recovery also from this stream, is biological anaerobic digestion, which can be applied through different technologies (wet and dry digestion). Through this process a biogas with elevated content of methane can be produced and supplied to engines for energy recovery. The above-mentioned technologies can be combined in several schemes to optimise the overall energy recovery. The combination of schemes will be analysed in this chapter in reference to a study case characterised by an average waste material composition. The comparison will be carried out using some indicators of the overall energy recovery for each scheme.