Trace Element Distribution in Sewage Sludge Gasification:  Source and Temperature Effects

Constraints within European Union (EU) countries on sewage sludge disposal routes are growing as former options meet with increasing environmental, legislative, and economic pressure. Gasification of sewage sludge for heat and power generation in combined heat and power (CHP) applications is an attractive concept that provides an environmentally acceptable, efficient, and economically viable means of generating energy from a waste disposal problem. The final solid residues are pathogen-free but may contain toxic elements such as barium, copper, mercury, lead, and zinc at levels that could make their disposal to landfills costly as well as environmentally unsound. Elements such as barium, copper, mercury, lead, and zinc are present in sewage sludges at levels significant to the disposal of the residual streams from a gasifier. The distribution of barium, copper, mercury, lead, and zinc to the ash residue streams has been studied in an air-blown laboratory-scale spouted-bed gasifier that is fueled by crushed, dried sewage sludge pellets. The gasifier was operated at temperatures of 770-960degreesC, and samples of the solid residues were collected. In this study, measurements of trace element concentrations have been used to determine their overall retention in the solid streams, as well as their relative depletion from the coarser bed residue and enrichment in the fines carried to the gas-cleaning system. The effect of the gasifier bed temperature and the type of sewage sludge has been investigated. Under all of the conditions studied, no mercury retention in the solid residues was observed. Cobalt, copper, manganese, and vanadium were neither depleted from the bed residue nor enriched in the fines. The extent of barium, lead, and zinc depletion from the bed residue varies with sludge type, and the enrichment of lead in the fines seems to be enhanced by gasifier bed temperatures in excess of 900degreesC. The observed behavior of these elements is discussed in relation to their speciation, as predicted by thermodynamic equilibrium modeling. The potential implications of these findings for process design, operating conditions, and residue disposal are discussed.