A multi-step reaction scheme to simulate self-heating ignition of coal: Effects of oxygen adsorption and smouldering combustion

Abstract

Self-heating ignition has been a fire hazard in coal production, transportation, and storage for decades. Self-heating ignition of coal is driven by two exothermic processes which are chemically and thermodynamically different: adsorption of oxygen and heterogeneous combustion (smouldering). In classical self-heating theory and previous computational studies, a lumped one-step reaction was used. However, this scheme does not differentiate the aforementioned two processes. This study develops a computational model that incorporates a 4-step reaction scheme, encompassing both adsorption and smouldering, to simulate self-heating ignition. The kinetic parameters for a bituminous coal are first obtained through inverse-modelling of thermogravimetric experimental data from the literature. Based on the 4-step reaction scheme and kinetic parameters, we simulate two sets of hot plate experiments from the literature and predict the critical ignition temperature of different sample thicknesses. These predictions are compared with the predictions using a 1-step reaction scheme. Predictions based on both schemes show a good agreement with experiments when sample thickness(L) is less than 20 mm. However, the accuracy of the model with1-step scheme decreases as the sample thickness increases. The critical ignition temperatures predicted by the 1-step scheme become significantly higher than the 4-step scheme when L > 20 mm and at L = 127 mm the difference is over 12%. According to the simulation results of the 4-step scheme, at the large-scale scenarios, adsorption is the dominant reaction before ignition and the acceleration of smouldering occurs afterwards. As 1-step reaction scheme does not differentiate adsorption and smouldering, a 4-step scheme is more suitable for realistic and large scale scenarios.