Partitioning of mercury and other trace elements from coal and waste-derived fuels during fluidised bed pyrolysis

The potential releases of toxic trace elements such as mercury, lead and arsenic call for emission control during fluidised bed (FB) combustion, pyrolysis or gasification of waste-derived fuels and fossil fuels. Control measures for sulphur oxides, nitrogen oxides and particulates effectively remove many other pollutants from the exhaust gases as well, but mercury and several other trace elements are already problematic and this situation will only worsen with time. Besides the effect of temperature, gas atmosphere and halogens, the presence of other species, for example metal oxides, have an effect on under which conditions and in what form trace elements are released from fuels. Understanding the events of trace elements release from solid fuels during the pyrolysis or char combustion stage will provide a key to manipulating their partitioning and controlling their emissions. Pyrolysis experiments were made with coal, sewage sludge and automotive shredder residue (ASR) in a two-stage fluidised bed combustion (FBC) facility. An Ontario Hydro measurement train plus an additional sampling system were used to measure mercury and around fifteen other trace elements in the gases, and also char samples were taken and analysed. Results from these experiments are presented. An issue that is addressed explicitely is the bed material, which may be contaminated with significant amounts of toxic trace elements.