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 e ffects of natural gas cofiring on particle ignition delay for variousl y sized pulverized coal and coke particles exposed to realistic combus tor conditions. A fluidized bed feeder injects small numbers of partic les (typically three to five) into a drop tube furnace at temperatures from 1300 K to 1500 K with heating rates up to 10(5) K/sec. Individua l particle ignition times are recorded using an optical sensor at the furnace entrance and a photomultiplier tube at the furnace exit. Ignit ion 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 concentratio ns). Ignition measurements with particles of different volatile conten ts, ranging from 7.5% to 36.1%, show that addition of 1% methane by vo lume reduces the ignition delay of low volatile particles to a level s imilar to the ignition delays for high volatile coal of the same parti cle size. Experimental results are compared with ignition delays predi cted by using a thermal model of particle behavior coupled with two ig nition models-one model based on energy absorption and the other based on devolatilization. The thermal model includes the effects of gas ph ase 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 mi nimum volatile concentration required for ignition for all tested coal s. Both ignition models accurately predict the measured ignition delay for various volatile contents and sizes in cofiring experiments.