Variations in the stable isotope ratios of specific aromatic and aliphatic hydrocarbons from coal conversion processes

Abstract

To establish the scope for applying gas chromatography–isotope ratio mass spectrometry (δ¹³C GC–IRMS) to molecular recognition problems in coal utilisation, ¹³C/¹²C isotope ratios were determined for n-alkanes and polycyclic aromatic hydrocarbons (PAHs) as a function of coal rank and process conditions. Six coals ranging from a lignite to a low volatile bituminous coal were subjected to chloroform extraction, fixed-bed pyrolysis under hydrogen pressure (hydropyrolysis) and fluidised-bed (flash) pyrolysis. No significant variations in the stable isotope ratios of n-alkanes were evident as a function of either rank or conversion regime. In contrast, the isotope ratios of PAHs show large variations with those for hydropyrolysis (–23 to –25‰) being similar to the bulk values of the initial coals and being isotopically heavier (less negative) than their fluidised-bed pyrolysis counterparts by 2–3‰. However, the PAHs from fluidised-bed pyrolysis, which resemble closely those obtained from high temperature coal carbonisation, are still heavier (by 2–3‰) than those from diesel particulates and coal gasification and combustion residues. This provides a firm basis for the source apportionment of airborne PAHs in the proximity of coking plants, particularly with no major variations in the PAH isotope ratios being found as a function of rank.