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.