An Approximate Model of Heat Treatment and Ignition of Coal in Small Cyclones

Abstract

An approximate physical model of coal heat pretreatment with external heat input in a cyclone-type unit is presented and its governing principles are examined. The model makes it possible to predict the time of moisture release from the fuel (or evaporation), the temperature of gas suspension and of fuel combustion, and the composition and volume of the combustion products. This enables one to determine the design and layout of the furnace extension for various process conditions. The model is based on the following assumptions: the processes are quasi-stationary, the heat capacities and heat transfer coefficients are constant and determined at an average process temperature, coal particles are isothermal, the gas suspension is uniform, ash components are inert, and the flow is one-dimensional. In addition, the model includes only the reactions governing the combustion processes. By an example of Kansk-Achinsk coal’s heat treatment, it has been established that the yield of combustible volatiles can be 30–40% (by mass) at a gasification degree of 0.35–0.60. The time of solid phase ignition is approximately 0.8 s from the onset of the process. In all cases, the products of heat treatment of coals at different phases of metamorphism contain at least 25% of combustible volatiles, thereby securing ignition of the solid phase. Basic design features of a small-size furnace extension acting as a boiler burner for steam-turbine power units are determined.