Systematics of pyrolytic N2 and CH4 release from peat and coals of different thermal maturity

Abstract

Open system non-isothermal (programmed) pyrolysis with a constant heating rate of 0.5 °C/min up to a final temperature of 1200 °C was used to study the liberation of molecular nitrogen (N2) and methane (CH4) from organic-rich sedimentary matter of different thermal maturity. The sample set analysed comprised peats, lignites, bituminous coals and anthracites. Temperature ranges, shapes and intensities of pyrolytic peaks in the CH4 and N2 pyrograms are sensitive indicators of the chemical composition of the sedimentary organic matter and may serve as characteristic fingerprints of organofacies and depositional environment. Within the coalification sequence, an increase in total organic carbon (TOC)-normalized methane yields was observed from peat to medium volatile bituminous coal, followed by a distinct decrease for the anthracites. The onset of methane and nitrogen liberation temperatures shifted towards higher temperatures with increasing rank, reflecting the increase in thermal stability of the residual organic matter. A systematic temperature shift was also observed for the methane generation maximum (peak). The N2 pyrograms exhibit bimodal generation curves (two maxima) up to the bituminous coal rank, while essentially only one single high-temperature N2 generation maximum persists at the anthracite level. This is attributed to a predominance of nitrogen-containing moieties (e.g. pyridinic and pyrrolic) of high thermal stability. The N2 pyrograms of the lignites show a higher degree of complexity, indicating a substantial variability in nitrogen-containing compound groups. Characteristic N2 spikes were found in the Miocene lignites of the Lower Rhine Basin, similar to those observed previously in Greek lignites. The origin of these “anomalies” and their relation to depositional environment and diagenesis still remains unknown.