AUTOIGNITION TEMPERATURES FOR COAL PARTICLES DISPERSED IN AIR

The apparent discrepancies in the data for the ignition temperature of coal particles measured within extended isothermal sources can be resolved, in part, by considering the marked difference in the mixture test volumes used by the various investigators. Data obtained with systems whose mixture test volumes are too small compared to their isothermal source volumes are not considered here because they are too strongly 'overdriven' to be of practical use. The consequences of ignition events in such overdriven systems are of trivial significance for the system as a whole. Data are presented and analysed for the autoignition temperatures of dispersed carbonaceous dusts in a 1.21 furnace whose test volume and source volume are comparable, and in which an ignition event leads to a dust explosion involving drastic changes in the composition, temperature and pressure of both the test volume and the source volume. In the limit of fine particle sizes, the minimum autoignition temperatures for carbonaceous dust clouds in air vary from as low as 400-degrees-C for the totally volatilizable polyethylene to as high as 925-degrees-C for a non-volatile graphite, with coal values at intermediate temperatures depending on the coal's rank or yield of combustible volatiles. On the basis of the available data presented for the coals, for the oil shale dusts and for artificial mixtures of polyethylene and graphite, it is concluded that the pyrolysis and devolatilization process is generally rate controlling.