Kinetics of MSWI Fly Ash Thermal Degradation. 2. Mechanism of Native Carbon Gasification ELENA COLLINA, MARINA LASAGNI, MASSIMO TETTAMANTI, AND DEMETRIO PITEA* Dipartim ento di Scienze dell’Am biente e del Territorio (DISAT), Universita ´ degli Studi di Milano-Bicocca, Piazza della Scienza 1, 20126 Milano, Italy A generalized kinetic model for fly ash native carbon oxidation is developed. It is shown that the conversion of fly ash native carbon to CO 2 is the result of two simultaneous processes taking place on fly ash surface: the first process (rate constant k 2 ) is the direct oxygen transfer from a metal oxide site to a vacant carbon active site leading to immediate carbon gasification; the second one is dissociative oxygen chemisorption (k 1 ) followed by C(O) complex intermediate uncatalyzed gasification (k 3 ). The model is validated using the kinetic data from model systems and those reported in Part 1 for four different fly ashes. The rate constants, k 1 , k 2 , and k 3 , together with activation and thermodynamic parameters are calculated. For each fly ash, the rate determining step is the complex intermediate oxidation.The nature of the interaction between native carbon and the fly ash surface is the key factor for C and C(O) complex formation and gasification. Introduction In Part 1 of this series (1), the native carbon thermal degradation of four Municipal Solid Waste Incinerators, MSWIs, fly ash samples was studied. The samples were selected to be representative of two countries (Denmark: FA1 and FA2a,b; Italy: FA3), three MSWIs (Reno-Nord: FA1; Re-Sud: Fa2a,b; Zama: FA3), and different initial carbon contents (the TOC 0 ranged between 2000 and 8000 ppm). We have demonstrated that the main chemical process is carbon oxidation to CO2 and that, under our experimental conditions,the carbon for the reactions does not come from organic compounds but originate from MSWI fly ash native carbon. At each temperature in the range 200-600 °C, the experimental total organic carbon, TOC-time data were processed with a sum of two exponentials (deconvolution procedure) i.e., in terms oftwo pseudo-first-order reactions Ri and Rj (eq 1 in ref 1). The key experimental information for the formulation of a reaction scheme is the temperature dependence of the preexponential Ci and Cj parameters (Ci + Cj ) TOC 0 ); Ci, related to the slower Ri process, decreased, while Cj,related to Rj,increased with increasingtemperature. In this work we propose a reaction mechanism for native carbon thermal degradation based both on the elaboration of Part 1 (1) experimental results and the previous one obtained on C-SiO2 model system (2). Background For carbon gasification, it is reported (3) that in the 450-850 °C range there are three reaction regimes: at temperatures between 450 and 550 °C (zone I; activation energy of about 200 kJ m ol -1 ), the overall reaction rate is controlled by the inherent chemicalreactivityofthe carbon;in the temperature range between 550 and 650 °C (zone II; activation energy of about 90 kJ mol -1 ), the reaction rate is controlled by both surface chemistry and internal mass transport (pore diffu- sion); at temperatures between 650 and 850 °C (zone III; activation energy above 10 kJ mol -1 ), the controlling factor is usually external mass transport. For fly ash reactions , it was suggested (4) that external mass transfer limitations are not important under simulated postcombustion conditions . In fact, fly ash surface areas are generallyquite low(1-10 m 2 g -1 ),suggestingthat a significant pore structure is not present; accordingly, internal mass transfer limitations may also be not important. Carbon gasification can result from both of the two following processes or their combination. One possible process is the direct impingement of oxygen onto vacant carbon active sites, leading to some immediate carbon gasification (3). The active sites on carbon are believed to be the unsaturated carbon atoms with one or two free sp 2 orbitals located at the edge of graphitic sheets or other structural defects,whereas the carbon atoms inside the basalgraphitic plane (saturated carbon atoms)are almost inactive (3).Thus, the reactivityisdetermined bythe availabilityofcarbon active sites in the carbon structure which in turns depends on the structural and topochemical characteristics of carbon such ascarbon type,crystallite size,porosity,lattice imperfections, and surface properties (5). In the second process, elementary reactions describing the mechanism details of the chemical reaction scheme are as follows. (i) Gaseous Oxygen Adsorption and Surface Diffusion. Molecular oxygen can be dissociatively chemisorbed on carbon active sites (uncatalyzed adsorption)or metallic sites (catalyzed adsorption) (6). The hypothesized equations for the dissociative chemisorption of O2 are where C is a free site in carbon structure; C(O) a surface complex; Me a metallic site, and MeO the metal oxide. After step 2a, the adsorbed oxygen can migrate from a metallic site to a carbon site: During the course of low-temperature carbon gasification, a considerable amount ofoxygen complexescan build up on the carbon surface. It was reported (3) that at 200 °Croughly half of the O2 consumed in the gasification processes goes to gaseous CO and CO2, and about halfchemisorbs strongly on the carbon surface as stable oxygen complexes. Surface coverage by these oxygen complexes increases with carbon burnoff and can reach nearly 100% at 75% burnoff (7). (ii) Carbon Gasification . The reactions for intermediate oxygenated complexes gasification can be schematized: *Corresponding author phone: +39-02-26603253; fax: +39-02- 70638129; e-mail: d.pitea@csrsrc.mi.cnr.it. C + 1 / 2 O 2 f C(O) (1) Me + 1 / 2 O 2 f MeO (2a) MeO + C f Me + C(O) (2b) Environ. Sci. Technol. 2000, 34, 137-142 10.1021/es9812655 CCC: $19.00 © 2000 American Chemical Society VOL. 34, NO. 1, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 137 Published on Web 12/02/1999