GASES EMITTED FROM SPONTANEOUS COAL FIRES: COMPOSITION AND POTENTIAL ENVIRONMENTAL IMPACTS

Spontaneous combustion of coal in mines, waste piles, or in situ outcrops produces CO, Hg, H₂S, C1 to C9 aliphatic compounds, aromatic compounds, and typical combustion products (e.g., CO₂, NO_x). In certain situations, such as meteorological inversions, human populations can be directly impacted by coal fire emissions. This study presents organic and inorganic gas compositional data from three subbituminous coal bed fires in the Powder River Basin of Wyoming, three bituminous coal fires in eastern Kentucky, and a burning bituminous coal waste pile in northern Alabama.

Gas composition and temperature varies dramatically between vents at the same fire and among the sites. For example, benzene concentrations ranged five orders of magnitude (0.0008-29 ppmv) and in some cases exceeded human exposure limits (0.5 ppmv for four hours). Likewise, for a single fire in Wyoming, Hg concentrations ranged from 110 to 12,000 ng m^-3 and vent temperatures varied from 4.1 to >300°C. Whereas these Hg concentrations are below exposure standards, they do suggest that coal fires may be a significant source of hazardous compounds when summed across large coal-bearing regions.

Significant variations in the trend of aliphatic concentrations, where compounds were lumped by carbon number, suggest that their abundances may be controlled by differences in source material (e.g., maceral composition and coal rank) and combustion efficacy. Aliphatics larger than C10 (e.g., nonane) were not present in detectable concentrations in any samples; this observation agrees with GC-FID analyses of Kentucky coal samples, which showed a loss of C9-C20 compounds during combustion. Ratios of the total concentrations of aliphatic versus aromatic compounds varied substantially between sites (52-130,000), but generally varied less then an order of magnitude between vents for the same fire, suggesting control by general coal chemistry/environment.

Future research directions in the potential environmental impacts of spontaneous coal combustion include characterizing compounds in other types of coals and regions, receptor exposure sampling for gaseous and particulate compounds, toxicological exposure of biological media to coal-fire generated compounds, and epidemiological examination of possible health impacts near coal fires.