Entrained-flow gasification of sewage sludge to reduce heavy metals in the remaining ash

Containing essential plant nutrients like P, sewage sludge is a potential raw material for recovering nutrients. However, sewage sludge contains organic pollutants and heavy metals. Thermal treatment can crack organic pollutants and reduce heavy metals. This study investigated whether entrained-flow gasification, despite its short residence time, enables the evaporation of heavy metals to be a suitable thermal treatment method for sewage sludge. Thermodynamic equilibrium calculations predicted how the reaction atmosphere, temperature, and additives affect the evaporation of heavy metals during thermal treatment. Sewage sludge was gasified under different atmospheres, temperatures, and with seventeen additives. Remaining ash was analyzed for its Cd, Cu, Cr, Hg, Ni, and Pb content. Using different types of sewage sludge, one high in Fe and one high in Al, findings were reproduced, while including As, Tl, and Zn in the investigation. In line with thermodynamic equilibrium calculations, halogen-containing additives promoted the evaporation of some heavy metals. The most significant factor influencing heavy metal evaporation was an increase in gasification temperature. At studied conditions, entrained-flow gasification evaporated over 90% of Cd, Hg, Pb, 75% of Zn, and 50% of Cr, As, Tl. Evaporation of Cu and Ni could not be determined conclusively. Entrained flow gasification of three kinds of sewage sludge for P recovery Influence of atmosphere, temperature and up to seventeen additives on the ash Thermodynamic equilibrium calculations to estimate influences on the evaporation Halogen containing additives support the evaporation of heavy metals in gasification Gasification temperature increases the evaporation of heavy metals most significantly