INVESTIGATION OF SELF-SUSTAINING COMBUSTION OF A COAL WASTE HEAP IN SCOTLAND

Coal mining was widespread in the central belt of Scotland from 1830 until the 1970’s and created a legacy of waste heaps or ‘bings’ that still dot the landscape. High content of coal fines and carbonaceous shales, make bings very prone to self-heating and smoldering combustion. A 30 m high waste heap at Bogside, North Lanarkshire, Scotland, started to smoulder in 2009, approximately 80 years after the closure of the pit.

Induced smouldering, a flameless combustion phenomena, of soils contaminated with PAHs, heavy oils and other organic compounds is a promising technology for the remediation of contaminated land, but has hitherto been conducted on an experimental scale up to several hundred tonnes. Moreover, coal fires smouldering underground are a worldwide problem that result in significant release of carbon and pollutants into the atmosphere. The large scale phenomena of burning bings provide a unique opportunity to study a ‘natural’ analogue for smouldering processes.

Chemical, geotechnical and physical parameters of the Bogside Bing have been studied. A combustion front is moving from west to east along the axis of the bing at an approximate rate of 1m/month. Three well-defined zones were identified and mapped using thermal imagery and temperature probes: the undisturbed zone, the preheating plus drying zone and the combustion zone. The subsurface fire results in a detrimental effect to the vegetation and structural integrity of the heap. Spread of the combustion is accompanied by the development of vents ahead of the front, fissures that run parallel to the direction of heating and smaller landslips along the flanks. Changes to the heap's soil mechanics induced by the smouldering front create a network of fissures, some running deep, that supply the front with enough air to sustain the process.

Analysis of gas from the vents, show elevated CO₂, CO, CH₄ and SO₂, and partially depleted in oxygen. All these are indicative of smouldering activity within the bing. The primary environmental concerns are likely to be from SO₂ release and metals leaching from waste material (i.e. Pb, Se, Cr). The stability of the structure may be compromised as smouldering progresses. Bogside Bing continues to release products of combustion and represents an accidental source of fossil fuel burning.