Scale-up of microwave heating process for the production of bio-oil from sewage sludge

A pilot-scale microwave heating apparatus was constructed for the production of bio-oil from sewage sludge, and the effects of important microwave processing parameters and chemical additives on the quality and yield of bio-oils were investigated. It was found that bio-oil was mainly formed at the pyrolysis temperature range of 200–400 °C. A higher heating rate (faster pyrolysis) not only increased the yield of bio-oil, but also improved the quality of bio-oil according to the elemental composition and calorific values. The maximum bio-oil yield was 30.4% of organic fraction, obtained from the pyrolysis of original sewage sludge at microwave radiation power of 8.8 kW and final pyrolysis temperature of 500 °C. All of five simple additives (KOH, H2SO4, H3BO3, ZnCl2, and FeSO4) reduced the bio-oil yield, but the composition and property of bio-oil varied with the additive types greatly. KOH, H2SO4, H3BO3 and FeSO4 were found to improve the quality of bio-oils remarkably according to the calorific value, density, viscosity and carbon content of bio-oils, but ZnCl2 treatment went against that. GC–MS analysis of the bio-oils showed that, alkali treatment promoted the formation of alkanes and monoaromatics, while acid treatment favored the formation of heterocyclics, ketones, alcohols and nitriles. Compared with sulfate slat FeSO4, chloride salt ZnCl2 was a better catalyst for selective catalytic pyrolysis of sewage sludge. The addition of ZnCl2 only promoted the formation reactions of a few kinds of nitriles and ketones remarkably. It is technologically feasible to produce bio-oil form microwave-induced pyrolysis of sewage sludge by optimizing pyrolysis conditions and selecting appropriate additives.