Volatility model for coal dust flame propagation and extinguishment

Abstract

A theoretical analysis is presented for the propagation and extinguishment of coal dust flames and of dust and gas flames containing inhibitor powders. The analysis is based on the established mechanisms for homogeneous flame propagation and the well known concept of a constant limit flame temperature for a given class of homogeneous fuels. The analysis is expanded to phase-heterogeneous systems such as coal dust by means of a volatility model. The analysis includes the singly heterogeneous system of a solid fuel dust in air; the singly heterogeneous solid inhibitor dust in a homogeneous fuel-air flame: and the doubly heterogeneous system consisting of a solid fuel and inhibitor dust mixture in air. The data for measured explosion pressures, flammability limits, and extinguishant requirements for heterogeneous systems are shown to be consistent with the established mechanisms and processes for homogeneous flame propagation provided that one adds an additional process: the heating and devolatilization of the solid fuel or inhibitor. The limitations on the rates of devolatilization of the solid particles become rate controlling at high burning velocities, at high dust loadings, and for large particle sizes. Devolatization rates are controlled by the intrinsic devolatilization rate constant for the solid fuel or inhibitor and the effective heating flux of the approaching flame front. The effective vield of volatiles is a function of those factors, the decomposition chemistry, and the time available for devolatilization. The fraction of the total volatiles that can be generated in the time available is the β-factor, and it determines the effective yield of fuel or inhibitor that participates in the flame propagation process. The data for explosion pressure, Flammability limits, and extinguishant requirements are readily understood in terms of those β-factors.