The Generation of Nitrogen and Methane from Coal as Revealed by Pyrolysis Experiments: Implications for Natural Gas Compositions

Bernhard M. Krooss, Erdem F. Idiz, Brian Horsfield

Westphalian coals from N. Germany in the maturity range from 0.7 - 6 % vitrinite reflectance (high volatile bituminous coal to meta-anthracite) have been examined with respect to their hydrocarbon and nitrogen generation potential by open and closed system pyrolysis. Experimental methane yields were highest for high volatile bituminous coals, with 33 mg/g TOC and 50 mg/g TOC determined from open and closed system pyrolysis, respectively. These quantities are much lower than predicted by mass balance calculations for naturally matured coal samples (150 - 200 Std m³/t TOC). The discrepancy is probably due to significant amounts of molecular hydrogen (H₂) and wet gases (C₂-C₄) being produced during laboratory pyrolysis. Hydrogen wa not quantified in this study but wet gases made up 40% of the total hydrocarbon gases in pyrolysates of high volatile bituminous coals and 2% in meta-anthracites.

Open-system pyrolysis experiments with heating rates between 0.1 and 2 °C/min showed two distinct generation peaks for N₂. At a heating rate of 0.1°C/min these occurred between 550 - 800 °C and above 900 °C. Nitrogen losses from coals on pyrolysis up to 1000°C ranged between 50 and 100%. As was the case with hydrocarbon gas precursors, significant amounts of nitrogen were evolved from bituminous coals in the form of tar. Kinetic parameters for the generation of methane and N₂ were derived from open system nonisothermal pyrolysis experiments. At geologic heating rates the onset of N₂ generation from coals is expected to occur between 250 and 300°C.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995