Measuring CO₂ emissions from coal fires in the U.S.

Uncontrolled coal fires pose multiple threats to the environment due to the emission of greenhouse gases, mercury, and other toxic or potentially toxic substances. The contribution of coal fires to global atmospheric greenhouse and toxic gas budgets is poorly known, but potentially significant. In an effort to quantify the magnitude of coal-fire emissions, a ground-based approach was developed to calculate fluxes of CO₂ from several coal fires in the U.S. This approach combines vent measurements of CO₂, CO, CH ₄, H₂S and Hg emissions with a gridded series of CO ₂ soil flux point measurements. This combination was applied in May, 2009 at three active coal fires in the Powder River Basin, Wyoming: the Welch Ranch fire, the Hotchkiss fire, and the Ankney fire. At the Welch Ranch fire, measurements of 12 vents gave a total vent emission of 6.8 ± 2.4 Mg CO₂ d^-1. An additional 39 soil flux sampling sites were measured, and the results were interpolated using an inverse distance weighting algorithm. When summed, the interpolated results give a preliminary estimate of 1.0 Mg CO₂ d^-1 or the CO₂ diffuse flux , bringing the total CO₂ emitted from the Welch Ranch Fire to just under 8 Mg d^-1. Similar calculations are underway for the Hotchkiss fire. For the Ankney Fire, access and safety concerns limited measurement coverage so that only partial fluxes were obtained. Ground-based measurements for the three Powder River Basin Coal Fires are being compared to airborne tropospheric CO₂ emission estimates and thermal infrared images of the three fires that are contemporaneous with ground sampling. This combined sampling approach follows a reconnaissance investigation of a burning coal waste pile in the town of Mulga, in northern Alabama near Birmingham. Temperature measurement over the Mulga coal fire show localized hot spots in excess of 250 °C. Diffuse fluxes of CO₂ vary by three orders of magnitude in four traverses across the burning coal pile. Within these traverses, CO ₂ flux shows a moderate, but statistically significant correlation with temperature (r²=0.56; p<0.01) that could potentially be used to estimate CO₂ diffuse flux from portions of the fire outside of the traverses, but where soil temperature was determined.