In situ gasification of coal, a natural example: history, petrology, and mechanics of combustion

Abstract

A 6 m thick coal seam of the Upper Jurassic and Lower Cretaceous Mist Mountain Formation in the southeastern Canadian Cordillera has been burning since 1936. The upper 3 m of coal is being consumed to an estimated depth of 20 m. Temperatures in excess of 1100 °C are locally reached, resulting in the melting of overlying sandstones and shales. The melted and vitrified rocks contain a new suite of minerals, including diopside, anorthite, cristobalite, and tridymite. Underlying the burnt coal ash is a zone of coke averaging about 10 cm thick, which is in turn underlain by unaltered coal.Within the area of combustion three zones can be distinguished: an advance zone, where open cracks are developed at the ground surface; a zone of active combustion, where volatiles driven off the coal burn en route to the surface and at the mouth of vents; and an abandoned zone marked by vents, some of which act as air intakes. Approximately 1000 t/year of coal is consumed, giving an energy release of about 1 MW. The heat generated is carried both forward and upward by convecting gas, thereby coking the coal and baking the roof rock. Little heat is carried downward, as evident from a sharp decrease in vitrinite reflectance below the zone of combustion.The baked roof rocks are brittle and extensively fractured, providing little roof support. The completely fused and scoriaceous rock and welded associated breccias, on the other hand, have greater coherence. This welding hinders roof collapse and thereby assists the passage of gases.