Kinetic studies of ignition of coal char particle suspension in hot air

Abstract

Coal ignition has been a focal point in the pursuit of improved energy efficiency and reduced emissions. This study focuses on investigating the kinetic characteristics of char ignition within a closed reactor. The combustion process is modeled using a one-step global reaction in a quiescent air environment. The results show that particle ignition time increases with equivalence ratio and particle size, while decreases with gas initial pressure and initial temperature. Particle ignition temperature is minimally affected by the particle size, slightly influenced by the gas initial pressure and initial temperature, but substantially influenced by the equivalence ratio. In the fuel-rich condition, the coal char combustion exhibits a thermal runaway stage, primarily driven by the escalating particle temperature, followed by a decaying stage, mainly influenced by the depleting oxygen mass fraction. The dominant kinetic factors affecting the heat release rate (HRR) peak are detailly investigated: the HRR peak shows an initial increase and subsequent decrease with the equivalence ratio, which is mainly controlled by the total particle number and particle temperature; Particle size affects the HRR peak through altering the total particle specific surface area, i.e., the product of the particle number and the particle diameter squared; the gas initial pressure and temperature all affect the HRR peak through directly altering the gas density. Additionally, larger retention coefficient leads to faster particle ignition and higher HRR peak. These findings provide valuable insights into the regulations of coal combustion processes, particularly in scenarios involving elevated pressure and temperature, such as coal/mine explosions.