Coal dust flames: A review and development of a model for flame propagation

Prediction of ignition temperature of a single coal particle

Propagation of laminar pulverized coal-air flames

Coal particles in combustion can pyrolyse, swell, graphitise, and react internally or externally or both. Mechanisms determining or controlling the reaction rate can include: boundary layer diffusion (generally controlling for particles greater than 100 microns), pore diffusion, or surface reaction with alternatives of adsorption control or desorption control, or some intermediate. Present evidence on particles in flames is that swelling in oxidizing conditions is likely to be relatively mild, about 10% in diameter. Particles are sufficiently porous for reaction to be in either Zone II (parital penetration) or Zone I (full penetration) with reaction in the latter case at effectively constant diameter. Reaction orders, which provide information on the relative dominance of the adsorption and desorption processes, are uncertain. Numerical values favor 1/2 to unity; very low temperature chemisorption of oxygen on carbon, however, (at−70°C) implying a very low adsorption activation energy, indicates otherwise. There is limited evidence that the reaction order is zero to 1/2, with activation energies of 20 to 40 kcal identified with desorption. Coal flames propagate by several mechanisms. Rates of heating in all cases (including the fixed bed) are in the range 10 to 106 or 107 deg C/sec. In explosion flames, propagation is by turbulent exchange, probably with heterogeneous ignition; in full flame furnaces, propagation is by radiation if there is no backmix flow, with either heterogeneous ignition or prior pyrolysis; in small flame systems, propagation is by downstream pyrolysis and upstream diffusion. Effective research is still in the data-gathering and mechanism-determining stage. Qualitative mechanisms are being elucidated by quantitative investigations. Accurate predictive quantitative theories, however, do not yet exist; neither is it yet possible to predict with any confidence the combustion behavior of one coal from another. Work is moving steadily in that direction, however.

S. Bandyopadhyay

Papers

Combustion in coal dust flames