The characterisation of tars produced during the gasification of sewage sludge in a spouted bed reactor

Tars released by the gasification of sewage sludge in a spouted bed gasifier have been collected, quantified and characterised. The amounts collected were low which shows that the volatiles contained in the feedstock were cracked efficiently under the conditions in the gasifier. The presence of a high concentration of steam in the gasifying medium enhanced the conversion further. Within the 760-980 degreesC range, the amount present did not show any variation with temperature of formation. GC/MS of a limited number of samples showed that the tars it had an aromatic character, as expected at the temperatures used. Approximately, 60-70% of the tars evaporated below 450 degreesC. Thermogravimetric analysis of the tar-trap samples has allowed an estimation of the boiling point distributions of the samples. For the majority of the tar samples, from both types of sewage, between 60 and 70% of the tar boiled at < 450 degreesC. The Biogran tar exhibited a trend of an increase in the <250 degreesC fraction and an increase in the >450 degreesC fraction, with increasing temperature of formation. The Danish tar-trap samples showed a decreasing trend in both fractions with increasing temperature. These data give an indication of the way the tars progressively break down under the gasifier conditions and infers that steam can have an impact on the reaction pathway. Changes in the composition of the tars with temperature were also identified using size exclusion chromatography (SEC) and by ultra violet fluorescence spectroscopy (UV-F). SEC showed that the molecular sizes of lighter material (retained peak in the chromatogram) decreased with increasing temperature, whereas that of the heavier material (excluded peak) generally increased. The large area of the retained peaks compared with the excluded peaks showed that lower mass material formed the majority of the tar samples. The mass-averaged molecular weights of the retained material showed a trend of decreasing values with increased gasifier temperature. The UV-F spectra of samples indicated the presence of higher concentrations of larger polynuclear aromatic ring systems, heterocyclic structures and alkyl or heteroatom substitutents as the temperature of formation increased. (C) 2004 Elsevier Ltd. All rights reserved.