Laminar burning velocities of CH 4—air-graphite mixtures and coal dusts

Abstract

Lean CH 4 —air flames have been seeded with different amounts of graphite and laminar burning velocities measured. Burning rates of the graphite particles, together with particle and gas temperatures also were measured through the flame. The profile of net chemical heat release rate against gas temperature was found from the kinetically modelled chemical heat release rate, assumed independent of the added graphite, and the heat transferred to the gas from the particles as a consequence of their oxidation by the gas phase species. Laminar burning velocities were calculated from the net heat release rate profile from Spalding's expression, which involved eigenvalue evaluation from the centroid expression. The burning velocities so obtained were in good agreement with those measured. This suggests a means of predicting the burning velocities of pulverised coal. The approach adopted for this rests upon rapid devolatilisation and mixing, the assumption that the volatiles are entirely composed of CH 4 , and that the presence of char does not change the gas phase composition and kinetics. Char oxidation rates were found from the gas phase species concentrations. Radiative loss from the particles also was computed as they moved through the flame. From such considerations the net heat release rate profiles could be evaluated and, from these, the associated values of burning velocity. The values of burning velocities of pulverised coal mixtures found in this way were compared with those of experiment. Agreement was good, provided devolatilisation and mixing with air were sufficiently rapid and the char did not create a heat sink sufficient to perturb the assumed uncoupled gas phase kinetics. In practice, these conditions seemed to be fulfilled for particles sizes of about 10 μm diameter or less, with volatile matter content greater than about 25%.