Spontaneous combustion of coal waste tips as a natural analogue for smouldering remediation methods.

Coal mining was widespread in the central belt of Scotland from 1830 until the 1970’s and created a legacy of waste tips 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 and waste dumping. Induced smouldering, a flameless combustion process, of soils contaminated with tar, PAHs, heavy oils and other organic compounds is a promising technology for the remediation of brown field sites, but has hitherto been conducted on an experimental scale of several hundred tonnes. But the large scale phenomena of burning bings provide a unique opportunity to study a ‘natural’ analogue for smouldering remediation.
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 tip at an approximate rate of 1m/month and 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 is resulting 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 and fissures that run parallel to the direction of heating and smaller landslips along the flanks. It is probable that changes to the soil mechanics’ of the tip 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 deep into the tip. The primary environmental concern is likely to be from SO2 release, although the stability of the structure may be compromised. Understanding the development of internal fissures and their role in sustaining the combustion process is crucial to utilizing smouldering as a remediation method for large areas of land contaminated by organic compounds. Bogside Bing continues to release products of combustion and represents an accidental source of fossil fuel burning.