Co-firing of Single, Binary, and Ternary Fuel Blends: Comparing Synergies within Trace Element Partitioning Arrived at by Thermodynamic Equilibrium Modeling and Experimental Measurements

The suitability of thermodynamic equilibrium modeling as a predictive tool in assessing the partitioning of trace elements in co-fired fuel blends and the synergistic effects involved in these processes were evaluated. The relationships between modeled results and experimental data obtained by the combustion of three separate fuels (Polish coal, sewage sludge, and straw) as well as their binary and ternary blends in a laboratory-scale suspension-firing reactor have been examined. The percentage of trace elements retained in the combustion ash as a proportion contained in the initial fuel was calculated, as well as the partitioning of each trace element between the bottom and sinter ash. Synergistic influences during the co-firing of fuel blends in both the modeled and empirical data were appraised for 13 trace elements, considered to be of primary toxicological importance, viz. As, Be, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sc, V, and Zn. Elements that are thermodynamically predicted to form solid oxides (Be, V, Cr, and Mn) were generally found to be retained in the ash. Elements that were predicted to form gaseous oxides (Sc) appeared to volatilize in reasonable agreement with thermodynamic calculations. For elements predicted to form a gaseous chloride (Zn and Pb), there was competition between chloride formation and oxidation. Cd and Mo were predicted to form volatile hydroxides. These appeared to be kinetically constrained. As and Ni oxidize to form solid species with minor components in the fuel, such as Mg or Fe, with which they are already associated. Hg was found to be volatile as an atomic species, and a positive retention synergy was found for Hg when combusting the ternary blend.