Ignition characteristics of single coal particles from three different ranks in O_2/N_2 and O_2/CO_2 atmospheres

摘要

This work assessed the ignition behavior of single-coal and single-char particles in O_2/N_2 and O_2/CO_2 atmospheres, with oxygen mole fractions in the range of 20-100%. Fuels included four pulverized coals from three different ranks (one high-volatile bituminous, one sub-bituminous and two lignites) as well as chars prepared from two of the coals. Particles of 75-90 μm were injected in a bench-scale, transparent drop-tube furnace (DTF), electrically-heated to 1400 K. Optical access of the furnace allowed the ignition of individual particles to be observed with high-speed cinematography. A particle's ignition delay was defined as the time lapsed from the instant when the particle exited the injector to the onset of its luminous combustion in the furnace. The experiments were conducted at two different gas conditions inside the furnace: (a) quiescent gas condition (i.e., no flow or inactive flow) and, (b) an active gas flow condition in both the injector and the furnace. In the former case, the axial gas temperatures in the DTF were found to be similar in the N_2 and CO_2 background cases and, consequently, small differences were observed in the ignition delay and in the ignition behavior of coal particles in O_2/CO_2 and O_2/N_2 atmospheres. These small differences were easily accounted for by disparities in the physical properties of the background gases. Increasing the oxygen mole fraction in either N_2 or CO_2 reduced the ignition delay mildly. To the contrary, in the latter case, i.e., under the active gas flow condition the axial gas temperatures in the DTF were found to be disparate in the N_2 and CO_2 background cases. The ignition delay of coal particles was drastically prolonged in the slow-heating O_2/CO_2 atmospheres, relative to the faster-heating O_2/N_2 atmospheres, particularly at high-diluent mole fractions. Photographic evidence showed that under active gas flow, which is relevant to practical applications in utility furnaces, ignition of volatiles in envelope flames was suppressed in the presence of CO_2. As a result, whereas in the quiescent gas condition, bituminous and sub-bituminous coal particles experienced homogeneous ignition in both O_2/N_2 and O_2/CO_2 atmospheres, in the active gas flow condition heterogeneous ignition was evident in O_2/ CO_2. Lignite coal particles often fragmented before ignition; this increased the likelihood of heterogeneous ignition in both O_2/N_2 and O_2/CO_2, in either active or inactive gas flows.