In situ gasification of coal, a natural example: additional data on the Aldridge Creek coal fire, southeastern British Columbia

Abstract

A burning coal seam at Aldridge Creek, southeastern British Columbia, has been monitored over a 3 year period to document the geometry of the fire front, subsidence history, and gas emissions. The fire front is advancing at about 13.5 m/year at an angle of about 45–70° to the strike of the coal seam. Based on near-surface temperature anomalies, the zone of combustion, carbonization, and gasification appears to extend down the dip of the coal seam about 40 m and to a depth below ground surface of 20 m. The geometry of the fire front is mainly the result of the insulation provided by the overburden, which facilitates higher temperatures in advance of the fire at depth. With advance of the fire front the upper 2–3 m of coal is coked and partly burnt, resulting in subsidence of the overlying strata along concentric faults. Heat is initially conducted and, with opening of fractures, convected with emission gases to the surface. In hotter zones, at temperatures approaching the melting point of the rocks, radiative heat transfer may occur.With advance of the fire front towards the sampling drill holes, a general, although not consistent, decline in O2 and N2 and an increase in CO2, CO, H2, and CH4 were observed in the emission gas. All the gasification products are highly diluted by air and are not in equilibrium. The product gas emissions do not reflect any distinct zonation of combustion, reduction, or pyrolysis–devolatilization accompanying advance of the fire front, such as has been observed in cocurrent in situ gasification experiments. Lack of such a zonation is likely a result of gas leakage and a large throughput of air resulting from the development of open fractures and faults, together with the shallow depth of cover.