Environmental Impact of Sewage Sludge Co-Gasification With Coal

The aim of this work is to study the minimization of the environmental impact associated with the gasification of sewage sludge. Co-gasification of a poor quality coal with dried sewage sludge was undertaken on a lab-scale gasifier. The presence of sewage sludge in co-gasification with coal led to higher energy conversions and the gas produced had greater hydrocarbons content. Both temperature and air flow rate were increased with the objective of giving rise to higher energy conversion, thus producing more gas, maximizing hydrogen formation and lowering hydrocarbons, char and tar contents. Due to the adverse effects of the presence of N, S and Cl pollutant precursors leading to the presence of highly corrosive compounds in the syngas, their conversion was evaluated. The increase of sewage sludge content in the feedstock was found to enhance the production of NH3, but working at higher temperatures and with greater Equivalence Ratio (ER) promoted the destruction of NH3. Production of H2S was similar for sewage sludge and coal and seemed to have been decreased by the presence of large quantities of ashes containing Ca and Fe. Temperature had no significant influence on H2S, but higher ER lowered H2S in the syngas. HCl was low and did not varied significantly for the conditions studied. The bed char residues were characterized and their leachability behaviour was evaluated. Heavy metals supplied with the fuel were mostly retained in solid residues, Pb and Hg being the most volatiles at 850°C. However, leachability of these metals was found to be below the analytical detection levels and only small quantities of SO42− and Cl− were released. The bed char residues were found to have variable concentrations of residual C, hence they could be used either as fuels to provide energy for gasification reactions or as sorbents to capture volatile heavy metals.