Trace element behavior during co-combustion of sewage sludge with polish coal

The co-combustion of sewage sludge (up to 30% by energy content) with coal has been investigated experimentally in a bench-scale suspension-firing reactor, at temperatures representative of commercial fluidized bed combustors. The emphasis has been on the study of the behavior of the potentially most harmful metals present in the raw fuels in trace quantities; data for As, Cd, Cr, Cu, Hg, Ni, Pb, Se, V, and Zn are reported. Metal concentrations have been quantified using inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES), plus an atomic absorption-based method for Hg. Interpretation of these data has been facilitated by the use of a thermodynamic equilibrium model, based on Gibbs free energy minimization; Metallurgical and Thermochemical Databank (MTDATA). The addition of sewage sludge to coal combustion is seen to lead to increased losses of Cd and Hg from the system, due to the significantly higher content of these elements in the raw sludge and the failure of combustion ashes to retain them in appreciable quantities. However, some benefit has been observed in the cases of As, Pb, and Se, where the addition of sewage sludge is seen to improve retention of these elements in the combustion ash. In the case of the other five elements studies, addition of sludge appears neutral in terms of potential metal loss to the atmosphere.