Mechanisms of extinction and near-extinction in pulverized solid fuel combustion

Previous experimental studies of the late stages of pulverized solid fuel combustion have reported extinction-like events that greatly increase the time required to achieve the high conversions required in industrial practice. The present paper examines the complete mass and energy transport processes that govern char particle temperature and considers a number of separate mechanisms that can lead to extinction and extinction-like phenomena. The analysis confirms the conclusions reached by Essenhigh, Fortsch, and Klimesh that a transition from zone II to zone I can occur under pulverized coal combustion conditions and that this transition must be described explicitly to correctly model extinction behavior. The analysis further concludes that only some of the observed extinction-like events are associated with multiple pseudosteady-state solutions to the single-particle energy balance as described by the classical Semenov theory, others being continuous changes in particle temperature or “near-extinction” events. A model is developed, CBK8, that describes a large set of data on extinction and near-extinction for chars from pulverized coals of various rank at gas temperatures from 1100–1600 K and oxygen concentrations from 3–19 vol %. The model with the same parameter set also describes the thermogravimetric analysis (TGA) data of Jenkins et al. at 500°C for a wide range of U.S. coals, thus unifying reactivity measurements spanning over 1000 °C in particle temperature and 4–5 orders of magnitude in reaction rate.