The concentration of carbon dioxide (CO₂) in the atmosphere has dramatically increased from the start of the industrial revolution in the mid-1700s to present levels exceeding 400 ppm. Carbon dioxide derived from fossil fuel combustion is a greenhouse gas and a major contributor to on-going climate change. Carbon and oxygen stable isotope geochemistry is a useful tool to help model and predict the contributions of anthropogenic sources of CO₂ in the global carbon cycle. Surprisingly few studies have addressed the carbon and oxygen isotopic composition of CO₂ derived from coal combustion. The goal of this study is to document the relationships between the carbon and oxygen isotope signatures of coal and signatures of the CO₂ produced from laboratory coal combustion in atmospheric conditions.

Six coal samples were selected that represent various geologic ages (Carboniferous to Tertiary) and coal ranks (lignite to bituminous). Duplicate splits of the six coal samples were ignited and partially combusted in the laboratory at atmospheric conditions. The resulting coal-combustion gases were collected and the molecular composition of the collected gases and isotopic analyses of δ¹³C of CO₂, δ¹³C of CH₄, and δ¹⁸O of CO₂ were analysed by a commercial laboratory. Splits (~ 1 g) of the un-combusted dried ground coal samples were analyzed for δ¹³C and δ¹⁸O by the U.S. Geological Survey Reston Stable Isotope Laboratory.

The major findings of this preliminary work indicate that the isotopic signatures of δ¹³C (relative to the Vienna Pee Dee Belemnite scale, VPDB) of CO₂ resulting from coal combustion are similar to the δ¹³C_VPDB signature of the bulk coal (− 28.46 to − 23.86 ‰) and are not similar to atmospheric δ¹³C_VPDB of CO₂ (~ − 8 ‰, see http://www.esrl.noaa.gov/gmd/outreach/isotopes/c13tellsus.html). The δ¹⁸O values of bulk coal are strongly correlated to the coal dry ash yields and appear to have little or no influence on the δ¹⁸O values of CO₂ resulting from coal combustion in open atmospheric conditions. There is a wide range of δ¹³C values of coal reported in the literature and the δ¹³C values from this study generally follow reported ranges for higher plants over geologic time. The values of δ¹⁸O (relative to Vienna Standard Mean Ocean Water) of CO₂ derived from atmospheric combustion of coal and other high-carbon fuels (peat and coal) range from + 19.03 to + 27.03‰ and are similar to atmospheric oxygen δ¹⁸O_VSMOW values which average + 23.8‰. Further work is needed on a broader set of samples to better define the relationships between coal composition and combustion-derived gases.