Leaching Hierarchies in Co-combustion Residues

Abstract

The leaching propensities from co-combustion residues of 10 trace elements have been evaluated. The elements assessed were Be, V, Cr, Zn, As, Se, Cd, Ba, Hg, and Pb. Eight fuels varying from coal blends to coal and secondary fuel mixtures to ternary mixtures were co-combusted in two reactor configurations (a fixed-bed “hot-rod” reactor and a muffle furnace) and at two temperatures (850 and 950 °C). It was thus possible to assess the effect of fuel type, reactor configuration, and temperature on the leaching propensity of trace elements and establish potential extent-of-leaching hierarchies. The trace-element content in each fuel was determined, as was the trace element in the resulting postcombustion ash. The ash was subjected to a miniaturized toxicity characteristic leaching procedure (TCLP) developed for this study, and the trace element content in the leachate was analyzed. This leaching test gives a good measure of the potential toxicity of leachates from landfill. Percentage retentions of elements in the ashes and leachates have subsequently been calculated. Mercury and selenium were almost completely volatilized during combustion and, therfore, were largely absent from the ashes, in all cases. For the other trace elements, it was not possible to establish a hierarchy of relative trace-element retention. Retention was primarily a function of the combustion method, with no clear effect of temperature retention being observed. The measured trace-element retentions have been compared to those predicted by thermodynamic equilibrium modeling, using the MTDATA software. The model successfully predicted the measured values in many cases; however, many anomalies were also noted. This suggests that thermodynamic modeling is best used for interpreting experimental data rather than as a stand-alone design tool. From trace-element analysis in the leachates, an extent-of-leaching hierarchy could be established. The elements that underwent low degrees of leaching were Zn, Hg, Pb. Those that underwent low to moderate leaching were Be, Cr, and Cd, and those that were leached to a great extent were V, As, Se, and Ba. This hierarchy was observed for all fuels and conditions studied. Leaching was found to be a strong function of the combustion temperature and combustion method. The type of fuel used in this study did not appear to influence the degree of leaching. When assessing the potential toxicity of leachate from co-combustion residues, Zn, Hg, and Pb may be deemed of least concern, while a greater emphasis should be placed in mitigating the release of the remaining elements.