"Reverse combustion along a bored hole (76 mm diameter) will be used during Lawrence Livermore Laboratory's next field experiment in underground coal gasification: Hoe Creek III. The enlarged channel produced by the reverse combustion will allow high gas velocities for the forward combustion phase of gasification. In the present experiments , reverse combustion along smaller boreholes (6.35 and 12 .7 mm diameter) was investigated in coal blocks which were 0.5 m in diameter and 0.9 m long. In these experiments reverse combustion did not propagate with air as the feed as . Three air velocities (14, 27, and 32 m/s) were tried in the 6. 35 mm borehole and two velocities (7.5 and 21 m/s) were tried in the 12.7 mm borehole . All experiments were conducted at atmospheric pressure. Reverse combustion did propagate when the feed gas oxygen content was increased to 32%. Rates up to 14 m/ day were found with an inlet gas velocity of 27 m/s. Product gas compositions are reported and discussed."