Characterisation of tar from sewage sludge gasification. Influence of gasifying conditions: Temperature, throughput, steam and use of primary catalysts

This work shows the influence of parameters such as temperature, throughput (TR), the gasifying agent and the use of catalysts (olivine, alumina and dolomite) on the products resulting from the gasification of sewage sludge on a fluidised bed. An increase in temperature of between 750 and 850 °C rises the production of combustible gas and reduces the production of gravimetric tar (by 65%, at TR = 110 kg/h m2, and 49% at TR = 322 kg/h m2), while the increase in TR at a given temperature produces the contrary effects. An analysis of the composition of the tar detected by gas chromatography (GC), shows that higher temperatures increase the concentration of polycyclic aromatic hydrocarbons (PAH). In spite of all the catalysts being active in tar removal, dolomite showed the most marked results in the air tests, with a reduction in the production of gravimetric tar of 40-50% and a conversion of the heavy PAHs (4-7 aromatic rings) of up to 90%. The combined use of a catalyst and air + steam increases the H2 and CO2 content in the synthesis gas and reduces the CO, CH4 and CnHm production, in addition to improving by 20% the catalysts performance in gravimetric tar removal compared to the tests with catalyst and air. Under the evaluated gasification conditions, the dew point of the gas was never below 108 °C, which means that technical problems could be expected due to tar condensation if the synthesis gas were to be used in combustion engines without applying additional tar removal measures.