Expanding energy consumption and the discovery of coal 17th century had the coal mining industry in northern France thrive over centuries. Underground coal mining activities, however, also produced large amounts of coal waste, which was deposited in heaps (about 600) near coal mines, towns, and roads, with little or no environmental concern. The composition of these coal waste heaps was largely dependent on the mining method and type of equipment used in their construction, and the sequence of tipping. The coal waste heaps in the Nord-Pas-de-Calais region vary in size and shape. The oldest heaps are elongated; those of the early 20th century are mostly cone-shaped. They consist of shale, sandstone, and coal, deposited in sedimentary basins during the Upper Carboniferous 335 to 295 million years ago. The predominant geological material in most heaps is carbonaceous shale, often constituting over 85% of all waste. Its mineralogical composition may differ, but pyrite occurs in almost colliery waste material.
Coal waste heaps are complex structures, made up of a mixture of materials with different reactivities toward oxygen and different broad particle size distribution. Where particles are large, air flux can easily dissipate heat; in fine materials, air stagnates and allows heat to build up gradually. However, stagnat air also means that fires caused by accumulative heating will stop as soon as the oxygen in the material is consumed. Waste distribution in coal waste heaps depends both on the layering of the mined soil as well as on the dumping method used, each resulting in a different overall layering and mixing of materials in the heap. For example, dragline mining will produce a sequence of layers different from that of shovel mining (Carras et al. 1999). Particles of different sizes may be segregated over time as surface material erodes down the slop.
Broadly speaking, coal waste heaps are structures of reactive material which allow gas and heat transport. Oxygen (O2) and water (H2O) vapor are the decisive factors for spontaneous combustion. Fires caused by self-heating are indeed a moderately common occurrence in coal mining. They received considerable attention in the past from the perspective of mine safety, but are now coming under closer scrutiny as a source of greenhouse gases (GHGs). Coal and carbonaceous rock debris react with atmospheric O2 in a heat-generating process. Under certain conditions, the heat accumulates and raises the temperature inside the material. This rise in temperature increases the rate of chemical reaction and thus, again, the rate at which heat is generated. Should the heating remain unchecked, combustion will result.
Spontaneous combustion poses significant safety, geo-technical and environmental hazards; it is at the origin of gas spots (vents), ground subsidence, surface cavities, noxious gases, secondary minerals, and thermal metamorphosis of rock debris. In places where gas and water vapor is discharged along open cracks and vents, solid minerals condensate on unbaked and baked shale debris.
Gas emission from the two burning coal waste heaps nos. 76 and 83 in the French region of Nord-Pas-de-Calais can be described, in very broad terms, as similar to those from large-scale coal seam fires. The main emission gas is CO2 (7%), with smaller amounth of NOx and SO2. Products of incomplete coal combustion, namely CO and CH4, are also present. Other hydrocarbons were not detected.
Carbonaceous shales are the main component of coal waste heaps. They contain pyrite and organic carbon, which contribute to SO2, CO2, and CO formation during combustion. Rock debris is thus a significant source of pollutant gas; it also needs to be considered in calculation of CO2 emissions for climate change scenarios.
NH3 released from fire sites reacts with O2 to form NOx (NO and NO2) gases, which may precipitate as NO3- and NO2-. Secondary minerals resulting from chemical reaction of these NH3 derivates, must be studied in more detail as they, too, can act as environmental (air, water, soil) pollutants.
O2 concentrations in a range from <1 to 19% were measured at both sites. This is below the stipulation of the International Mining Law, which requires a minimum proportion of 19.5% oxygen for safe working. Various gases emitted from the heaps may impact directly on the well-being of people.
As the total amount of GHGs in the atmosphere accumulates, and the global climate heats up ever more noticeably, it becomes imperative to carry out direct and regular field gas measurement at coal fire sites (i. e. coal seams and coal waste heaps) to identify gas compositions; quantify emission gases, especially GHGs; and indicate the location and extent of active combustion zones.
Precautions must be taken for those working – as scientist or miners – on burning coal waste heaps. The danger of accident and poisoning is omnipresent; suitable working attire is an absolute must (e. g. gas masks).