Laboratory study of trace element vaporization from combustion of pulverized coal

Small-scale combustion experiments were conducted to study the vaporization of trace elements during the combustion of pulverized coal. The combustion process was sampled at high temperatures (1423 K) corresponding to in-flame conditions. Rapid quenching and dilution of the combustion products were followed by a cascade impactor to collect size-segregated ash samples. Based on the comparison of concentrations of refractory elements in size-segregated ash with their bulk concentrations, we conclude that the ash having diameters less than approximately 0.4 micrometers represents the material which vaporizes during combustion in our combustion furnace. Good mass balance closure (90% to 140%) was obtained overall for ash in the sampling system. The volatile elements As, Sb, Se, and Zn showed mass balance closures significantly less than 100%. Thermochemical equilibrium predictions reported in the literature indicate that these elements should be completely volatile in the flame, although As is predicted to form condensed calcium arsenate at the sampling temperature in our system. Little chromium was found in the vaporization mode in our experiments. The uniformly low volatility of chromium suggests that chromium may react with oxides in the bulk ash. An upper bound on the amount of the more volatile elements which vaporized was computed. This upper bound accounted for the amount of the element in the finest ash and the material "losses'' indicated by the mass balance closure. The results of these experiments suggest almost complete vaporization of certain trace elements (Se, Zn) from coal combustion in the flame zone, in accordance with theoretical equilibrium predictions. Other elements (As, Sb) appeared considerably less volatile and may react with constituents in the bulk ash at combustion temperatures. (C) 2000 Elsevier Science B.V. All rights reserved.