Kinetics of MSWI Fly Ash Thermal Degradation. 2. Mechanism of Native Carbon Gasification

A generalized kinetic model for fly ash native carbon oxidation is developed. It is shown that the conversion of fly ash native carbon to CO2 is the result of two simultaneous processes taking place on fly ash surface: the first process (rate constant k2) 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 (k1) followed by C(O) complex intermediate uncatalyzed gasification (k3). 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, k1, k2, and k3, 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.