Effects of the Steam-To-Oxygen Ratio and the Equivalence Ratio on Underground Coal Gasification

Underground coal gasification is a complex physico-chemical process. UCG thermodynamic equilibrium model was established and validated, and the effects of the steam-to-oxygen ratio (RH2O/O2) and the equivalence ratio (ER) on the gasification temperature and the gaseous products were investigated. At low ER (0.08–0.14), coal is incompletely gasificated when the gasification temperature is less than the corresponding peak temperature. At high ER (0.16–0.30), coal is completely gasificated. With the increase of RH2O/O2, the gasification temperature decreases monotonically and finally tends to approach 850°C at RH2O/O2 = 4.5. H2 reaches high concentration at ER = 0.18–0.24 and RH2O/O2 = 1–3, but the change of CH4 concentration is just the opposite. CO concentration is high at RH2O/O2<1.5. In order to reach high the utilization effectiveness of oxygen, RH2O/O2 has the corresponding optimum value where the utilization effectiveness of oxygen reaches peak value at ER = 0.08–0.2.