Infrared thermography applied to spontaneous combustion monitoring of coal tips

Abstract

Spontaneous combustion of coal tips can generate dangerous slips and it is necessary to monitor this phenomena. Aerial IR thermography is one of methods widely used but, in the case of coal tips, it appears costly and not adapted to survey the evolution of reaction. It is for these reasons that the LAMTI in liaison with Charbonnages de France Group develops a method using topographic, thermographic and atmospheric survey that is intended to be more reliable and less expensive. 1. Introduction Coal tips are conical or truncated-cone hills formed by piling up the waste from mining operations. Some of them still possess 5-15% of residual coal, the percentage varying according to the sorting techniques used. The oldest tips thus often contain more residual coal. The porous nature of the tip enables air and water to circulate, causing the coal to oxidise. Any iron sulphides such as marcassite and pyrites present in the tip will tend to catalyse the exothermal oxidation reaction, causing spontaneous combustion of the residual coal [1]. 10% of coal tips are undergoing a process of spontaneous combustion, the effects of which are as follows [2], slipping of the tip, accompanied by clouds of inflammable dust, formation of cavities, emission of noxious gases, creation of water-gas, a compound that explodes in air. Most failures occur during heavy rains, with penetrating water having the following additional direct effects (increase in the density of the materials, drop in shear strength, increase in pore pressures, downward movements, creating destabilising forces). The effects can be particularly catastrophic. Slips are not classic rotational ones but, on account of the high water content in the schists, are more similar to mud slides; these can be deadly, as experienced in Virginia in the 1940s. There is also the added problem of major atmospheric pollution due to the release of sulphurous fumes [3]. As it is very expensive to fight coal tip fires, attempts have been made since the second half of the 20th century to prevent such fires during the formation of coal tips by taking precautions, such as for example not creating them above carbonate materials or spreading them out in order to reduce internal pressures. In the case of tips formed before this period, spreading them can be dangerous and costly if high-risk areas are not known, and monitoring then appears to be a necessary step.