COMBUSTION OF COAL DUST-AIR MIXTURES (FLAMMABILITY, RADIATION, VOLATILE)

Coal dust flammability studies are essential in order to evaluate propagation limit concentrations and the fundamental burning velocity as a function of dust concentration. This work presents such data for Pittsburgh seam bituminous coal dust using a relatively large scale horizontal flammability apparatus, the Gravity Tumbler Flammability Tube (GTFT). The reported lean limit (based on propagating a steady-state constant pressure flame) was measured as 0.42 kg/m('3), a factor of two or three greater than that reported from the modified Hartmann bomb apparatus. A maximum steady-state flame speed was measured as 34 cm/sec, at a concentration of 0.9 kg/m('3), a value greater than that observed when studying similar dusts with held flame facilities. Consistent indirect evidence was observed to indicate a rich flammability limit of 1.5 kg/m('3), but this could not be conclusively proven with the GTFT facility.;A combustion model describing the fundamental coal dust flame propagation phenomena was also developed in this work. The model includes heterogeneous combustion, pyrolysis of the coal, and homogeneous combustion of volatile matter. Molecular diffusion, conduction, and the optically thick limit for radiation heat transfer were also included in the model. Coal (fuel) rich mixtures in air were considered for equivalence ratios of 3 to 8. Predicted burning velocities for 50 (mu)m particles of coal with 36% volatile matter indicated a broad maximum of 37 cm/s at an equivalence ratio of 4 (0.367 kg/m('3)). The minimum computed velocity was 21 cm/s at (phi) = 8 (0.733 kg/m('3)). The burning velocity was found to increase somewhat as the particle size decreased. The chemical kinetics model was highly simplified, but based on experimental information. The predicted flame temperatures and structures compared well with experimental data. The structure of the flames were found to be strongly influenced by radiative heat transfer. Flame thicknesses were about 10 cm.