The effect of syngas on tar quality and quantity in pyrolysis of a typical South African inertinite-rich coal

The influence of gas atmosphere on the quantity and quality of tar obtained from thermochemical processing of an inertinite-rich South African coal is reported in this work. The effect of syngas and steam on the produced tar is addressed. Experiments were carried out in a fixed bed reactor at 30 bar and four gas atmospheres: N2, N2 + steam, H2/CO and H2/CO + steam. Tar yield increased when H2/CO was used instead of N2. Additionally, the introduction of steam to the reaction atmosphere together with N2 or H2/CO gave rise to higher tar yields than those obtained under dry N2 or H2/CO alone. Tar quality was evaluated by size exclusion chromatography, simulated distillation, gas chromatography-mass spectrometry and infrared spectrometry. Tar evolved under H2/CO atmosphere presented lighter molecular weight distribution than that obtained under N2 atmosphere. Moreover, the presence of steam in the reaction atmosphere gave rise to tars with slightly broader molecular weight distribution, shifted towards heavier species. Although all tars exhibited wide variety of aromatic compounds, tar from H2/CO experiment showed the largest degree of aromatization. The addition of steam to H2/CO atmosphere yielded tar with lower degree of aromatization and oxygenated groups. In contrast, tar with higher degree of aromatization and oxygen functional groups were obtained when steam was added to N2 atmosphere.