Trace element emissions from co-combustion of secondary fuels with coal: A comparison of bench-scale experimental data with predictions of a thermodynamic equilibrium model

Trace element emissions from the co-combustion of coal with biomass and waste secondary fuels have been measured, under conditions relevant to commercial fluidized bed combustors, using a novel, bench-scale, suspension-firing reactor. Experiments have been conducted using two coals (one Polish, one Colombian), four biomass fuels (wood-bark, straw, pulp sludge, and paper sludge), and three waste fuels (agricultural waste, sewage sludge and plastic waste). Concentrations of eighteen trace elements have been measured in these raw fuels and a variety of combustion and co-combustion ashes, using inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES), plus an atomic absorption based mercury determination devise. The influence of chlorine and sulfur on trace element release from combustion has been tested also, in the case of wood-bark, by injecting first HCl and then SO2 into the reactor during combustion. Experimental data have been compared with the predictions of a thermodynamic equilibrium model throughout this study. The Metallurgical and Thermochemical Databank (MTDATA) Gibbs free energy minimization software has been used to predict the speciation of individual trace elements. The trace elements have been ranked according to their average retention in combustion ashes, the most volatile being Hg and Se, followed by Cd, Tl, Pb, and As. Potentially problematic trace element emissions have been noted in certain cases, e.g., Cu and Zn from wood-bark, As and Pb from Polish coal, and Cd and Hg from sewage sludge. The injection of HCl served to decrease the retention by ash of the elements Cd, Cu, Zn, Mn, and Ba, while injection of SO2 increased the retention by ash of As and Hg, but decreased that of Cd.