Ignition predictions of isolated coal particles by different ignition criteria and devolatilization models

Abstract

The prediction accuracy of coal ignition characteristics depends on the sub-model selections. A detailed modeling study on the influences of ignition criteria and devolatilization models on the ignition characteristics of isolated coal particles is conducted using a one-dimensional transient ignition model. The assessed ignition criteria include the thermal explosion theory (TET), the transient adiabatic criterion (TAC) and the flammability limit criterion (FLC), while the assessed the devolatilization models include the chemical percolation devolatilization (CPD) one and two competing reaction (TCR) one. Under each combination, the ignition characteristics under different oxygen concentration, particle size, and surrounding temperature are studied. The prediction results are compared with microgravity experimental data obtained from the 3.6 s drop tower in National Microgravity Laboratory Center of China (NMLC). Modeling results reveal that the particle center temperature is more reasonable to be used as the characteristic temperature in the ignition study. When devolatilization process is described by CPD model, for homogenous ignition FLC is more accurate to predict ignition temperature and time than TAC at a higher XO2 and a high Tw, while TAC is more accurate to predict ignition temperature at a large particle size. Using FLC, against TAC, the transient of ignition mode from homogenous to combined happens at a ∼ 80 K higher temperature and ∼ 50 μm larger particle size, while that from combined to heterogenous happens at 250 μm rather than 400 μm. When FLC is used as the ignition criterion, selection of devolatilization model has more notable effect on the prediction of ignition temperature and time. Furthermore, the combination of FLC and CPD is more accurate to predict ignition temperature and time at a higher XO2. Two diagrams are respectively given to guide the selection the ignition criterion and identify the ignition mode at a wide range of conditions. In addition, the mechanisms of the associated discrepancy are discussed.