TRACE-METAL TRANSFORMATION MECHANISMS DURING COAL COMBUSTION

Mechanisms governing the fate of trace metals during coal combustion a re reviewed, and new theoretical results interpreting existing data ar e presented. Emphasis is on predicting the size-segregated speciation of trace metals in pulverized coal-fired power plant effluents. This f acet, which determines how trace metals originally in coal impact the environment, is controlled by fuel composition and combustion conditio ns. Multicomponent equilibrium calculations are used to predict vapori zation/condensation temperatures for antimony, arsenic, beryllium, cad mium, chromium, lead, mercury, nickel, and selenium compounds in coal combustion flue gases, for a representative Illinois No. 6 coal. Exper imental data show that equilibrium provides a good guide on the effect of chlorine on the partitioning of pure nickel, cadmium, and lead sal ts, introduced separately into a gaseous turbulent diffusion flame wit hin an 82 kW combustor. Metal nuclei coagulation mechanisms are examin ed using existing computer codes, and these predict that coagulation d oes not allow condensed metal nuclei to be scavenged by existing coal ash particles. Rather, literature data on trace metal enrichment on sm all particles are consistent with processes of reactive scavenging of metals by larger particles, and it is suggested that these processes m ight be exploited further to convert these metals into environmentally benign forms.