Ignition of a dispersed coal particle stream and measurement of ultrafine particle size distributions

The ignition behavior of a dispersed coal particle stream with ambient gas temperatures from 1300 to 1800 K and an oxygen concentration of 20% are studied. Dispersed PRB subbituminous coal in the size range of 106-150 µm are injected into a Hencken flat-flame burner. High-speed, high-resolution imaging reveals homogeneous and heterogeneous ignition modes, depending on particle size. An ignition mode transitioning from homogenous to heterogeneous is predominant for large particles at all temperatures. After volatile combustion is complete, the large particles undergo re-ignition of char after a characteristic induction time. The induction time decreases with increase in ambient gas temperature from 1300 to 1800 K. The observed ignition delays are 9.6, 4.7 and 4.1 ms for ambient gas temperatures 1300, 1600 and 1800 K respectively. Real-time particle size distributions and concentrations are obtained using a high-resolution electrical low pressure impactor (HR-ELPI+). The results suggested that soot dominates the ultrafine particle range due to the high volatile content of PRB subbituminous coal.