Modelling Trace Element Emissions in Co-Gasification of Sewage Sludge With Coal

Gasification has attracted considerable interest from water utilities as a sewage sludge disposal option, with the advantages of waste volume reduction, pathogen destruction and energy recovery. Co-gasification with coal in a larger plant (>10 MW_t) employing a gas turbine for energy recovery may reduce the risk and cost of this option. However, controlling the release of trace elements such as Pb and Zn in the gas produced may be necessary to avoid corrosion, and to meet environmental requirements. A thermodynamic equilibrium model has been used to make predictions of the speciation of trace elements in the fuel gas from co-gasification of sewage sludge with coal. Experimental data from a pilot scale 2 MW_t sewage sludge/coal co-gasification plant with a hot gas filter was used to test the validity of these predictions. No significant amount of Be, Co, Cu, V and Zn was predicted to be in the form of gaseous phase species, and this was confirmed by the experimental data. On the other hand, Hg and Se were predicted to be only present in gas phase species, and this was also confirmed experimentally. The elements As, B, Cd, Pb, Sb and Sn were all predicted to form a larger amount of gaseous species than was observed in the experimental measurements. Refinement of the predictions for As and B by inclusion of specific minor/trace element interactions with Ni and Ca respectively gave a better agreement with the experimental data. Whilst the experimentally-observed lowering of Pb emissions by reduction of the gas cleaning temperature from 580 °C to 450 °C was qualitatively predicted, the concentration of Pb in the fine dust removed by the hot gas filter indicates condensation at higher temperatures than predicted. The absence of thermodynamic data for the more complex minerals and adsorbed species that may be formed is thought to account for some of these differences.