Simulation of underground coal gasification ignition in deep coal seam based on transitional diffusion mechanism: Influence of inlet temperature and O₂

Underground coal gasification (UCG) in deep coal seam was attracting more and more attentions all over the world due to its characteristics of safety, clean and low carbon. In the study, the ignition of UCG in deep coal seam was simulated by employing computational fluid dynamics (CFD). A new method setting gas diffusion in coal seam as transitional diffusion (TD) was put forward, and the TD and Fickian diffusion (FD) mechanism were compared. It was elucidated that TD mechanism was more accurate for UCG model. The default FD mechanism in coal seam overestimated the reaction rate of coal seam. With TD mechanism, coal seam in turn went through external heating controlled (EHC) region and hybrid heating controlled (HHC) region during the ignition of UCG. The demarcation temperature between EHC and HHC regions was 720 K. Then, various inlet temperatures and mass fractions of O-2 were attempted during the simulation of UCG ignition. The time spent by the ignition was shortest with 1100 K inlet temperature and it was similar to 180 s. The less the inlet mass fraction of O-2 was used, the slower combustion rate was. CO2 was the most potential indicator to get access to the underground condition.