The effects of natural gas cofiring on the ignition delay of pulverized coal and coke particles

This paper presents the results of a study designed to determine the effects of natural gas cofiring on particle ignition delay for variously sized pulverized coal and coke particles exposed to realistic combustor conditions. A fluidized bed feeder injects small numbers of particles (typically three to five) into a drop tube furnace at temperatures from 1300K to 1500K with heating rates up to 10⁵ K/sec. Individual particle ignition times are recorded using an optical sensor at the furnace entrance and a photomultiplier tube at the furnace exit. Ignition delay measurements were performed for various inlet gas velocities, particle volatilities and gas compositions (including variations in oxygen, methane, natural gas, nitrogen and carbon dioxide concentrations). Ignition measurements with particles of different volatile contents, ranging from 7.5% to 36.1%, show that addition of 1% methane by volume reduces the ignition delay of low volatile particles to a level similar to the ignition delays for high volatile coal of the same particle size. Experimental results are compared with ignition delays predicted by using a thermal model of particle behavior coupled with two ignition models—one model based on energy absorption and the other based on devolatilization. The thermal model includes the effects of gas phase combustion, particle size and swelling, gas and particle velocity and temperature. The energy-ignition model requires an experimentally determined ignition energy for each tested coal. The devolatilization-ignition model predicts ignition delay using a single value for the minimum volatile concentration required for ignition for all tested coals. Both ignition models accurately predict the measured ignition delay for various volatile contents and sizes in cofiring experiments.