Liberation of molecular hydrogen (H2) and methane (CH4) during non-isothermal pyrolysis of shales and coals: Systematics and quantification

Abstract

Open system non-isothermal (programmed) pyrolysis experiments have been conducted to investigate the pyrolytic liberation of molecular hydrogen (H2) and methane (CH4) from carbonaceous shales and coals of different rank and organo- and lithofacies types. The aim was to explore the origin and generation pathways of H2 and their relation to methane generation mechanisms. The experiments were performed at two heating rates (0.5 and 1°C/min) up to a final temperature of 1200°C. CH4 and H2 generation usually did not proceed simultaneously but with a distinct temperature offset. Molar yields of H2 under open system pyrolysis conditions were always considerably higher than the corresponding CH4 yields. While coaly material showed one single, broad H2 generation peak, H2 evolution from shales revealed complex release patterns.The pyrograms (gas generation rate as a function of temperature) were classified according to the number and shape of peaks and the maximum temperatures of the main phases' gas liberation. H2 pyrograms were tentatively subdivided into five groups (A–E) according to peak patterns, reflecting different thermal stabilities of precursor moieties.Based on previous research on H2 generation mechanisms, different evolution pathways are proposed to explain the observed H2 release patterns. For shales, the dominant mechanism shifts from cracking of hetero-bonds and demethylation in Group A towards aromatization and condensation in Groups B and C. In coals, aromatization and condensation are considered to be the predominant H2 evolution mechanisms. The molar yields of H2 correlate like the CH4 yields with the TOC and Rock-Eval S2 values of the samples, indicating a purely organic origin. The results have implications for hydrogen balance considerations during thermal maturation of coals and gas/oil shales.