The rate of burning of small concentrations of fine graphite powder has been measured in a flat, laminar weak methane-air flame at sub-atmospheric pressure. Measurements were obtained with a laser doppler system, that not only measured particle velocities, but also particle concentrations and size distributions through the flame. Particle temperatures were measured by the two colour method and gas temperatures with thermocouples. An appreciable increase in the oxidation rate was observed in the flame reaction zone, attributable to the reaction zone transient species, O, H, and OH. A computational study of the flame, with comprehensive chemical kinetics and detailed representation of the transport fluxes, yielded the concentrations of all species through the flame. The observed rates of graphite oxidation are kinetically explained in terms of rates of reaction of all species with graphite. The observed elevations of particle above gas temperature in the reaction zone are higher than would be expected if the heating were due only to reactions in which the carbon surface was attacked. Amongst other possible explanations there is that of heating by exothermic radical recombination on the carbon surface.

The oxidation of graphite powder in flame reaction zones

Influence of Coal Dust on Premixed Turbulent Methane-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.

Combustion and flame propagation in coal systems: A review

Laminar dust flames in a reduced-gravity environment

A mathematical model for the combustion of a porous carbon particle

A convenient apparatus for dispersing uniform clouds of solid particles for laboratory dust flame experiments has been tested. The combustion characteristics of premixed laminar graphite-oxygen-nitrogen flames have been studied at atmospheric pressure both with and without added hydrocarbon. The variation of particle burning time with initial oxygen partial pressure and oxygen/ fuel ratio has been described by first-order combustion kinetics with due consideration for oxygen depletion during burn-out. Flame velocity measurements indicate very slow propagation compared with hydrocarbon sprays and aluminum clouds of similar particle diameter. Added chlorine did not measurably affect graphite combustion up to concentrations of 2.5 vol. % in the presence of added hydrocarbon, but less than 1.0 vol. % of chlorine extinguished pure graphite-oxygen flames. The addition of 1.5 to 4.75 wt.% of lead acetate visibly stabilized the graphite-oxygen flame, but the effect on the flame velocity and burning rate ...

James T. Bryant

Papers

The Combustion of Premixed Laminar Graphite Dust Flames at Atmospheric Pressure