Environ. Scl. Technol. 1993, 27, 1595-1601
The Relationship between de Novo Synthesis of Polychlorinated Dlbenzo-pdioxins and Dibenzofurans and Low-Temperature Carbon Gasification in Fly Ash Michael S. Mllllgan and Elmer Altwlcker'
Isermann Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Four fly ashes, three from municipal solid waste incinerators (MSWI) and one from a coal-fired power plant, were studied for their potential to promote carbon gasification and de novo synthesis of PCDD/F, chlorobenzenes, and chlorophenols from native carbon precursors in the temperature range 275-350 "C. A fixed-bed tubular reactor employing mixtures of oxygen and nitrogen was used in this laboratory study. The incinerator fly ashes showed steadily increasing yields of chlorobenzenes with temperature and an optimum temperature window for PCDD/F formation in the range 300-325 "C. The rate of PCDD/F formation for one of the fly ashes, found to be 2X pg (PCDD/F) g1min-I, was constant for reaction times from 5 to 30 min. In addition, gasification rates of native carbon in the incinerator fly ashes were at least an order of magnitude greater than those of pure carbon, suggesting catalytic activity. Coal fly ash was inactive to both PCDD/F formation and carbon gasification. A possible correlation was noted between de nouo synthesis of PCDD/F and carbon gasification rates. Introduction
predicted from extrapolation of higher temperature data at 450-550 "C. In addition, Stieglitz et al. (12)and Schwarz and Stieglitz (13)have recently shown that CO2 is evolved from carbon in fly ash at temperatures near 300 "C. Evidence of catalytic carbon gasification in fly ash could lead to inferences of catalytic mechanisms responsible for PCDD/F formation. These two mechanisms may be coupled, whereby PCDD/F molecules are formed from surface C-0 complexes-intermediates to carbon gasification (14)-0r even gas-phase CO and C02 precursors (15). The carbon gasification and PCDD/F formation mechanisms may not share intermediates, but instead share the same catalytic elements responsible for these reactions on fly ash. In the lower temperature (
