Semi-global intrinsic kinetics for char combustion modeling

Abstract

This paper addresses the form of simple rate laws used to describe intrinsic char oxidation within practical combustion and gasification codes. The modern literature is surveyed for near-atmospheric kinetic studies that report global reaction order and/or activation energy. The resulting data set together with mechanistic insights drawn from fundamental surface studies are used to assess various semi-global kinetic models. There is strong evidence in the accumulated data for high reaction order (0.6–1) below 900 K, much weaker but significant evidence for low order above 1200 K, and some suggestion of transition toward another high-order regime at temperatures above about 1600 K. Neither global power-law kinetics nor semi-global Langmuir–Hinshelwood kinetics can describe this temperature-dependent behavior. A three-step semi-global mechanism is proposed whose simple rate law describes the major trends in reaction order, activation energy, and CO/CO 2 ratio from 600 to 2000 K. The three-step model includes reaction between gaseous oxygen and surface complex, C(O), which is key to describing the high reaction orders widely reported in the low temperature Zone I regime.