Co-pyrolysis of sewage sludge and pinewood sawdust: the synergistic effect and bio-oil characteristics

Co-pyrolysis with biomass is a promising solution to obtain high-quality products for sewage sludge (SS) treatment. In the present study, a thermogravimetric analyzer (TGA) and fixed bed reactor were used to investigate the synergistic effect and bio-oil characteristics during co-pyrolysis of SS and pinewood sawdust (WS). The results show a significant interaction between SS and WS, and the most apparent synergistic effect appears at the WS proportion of 50 wt. %. Co-pyrolysis of SS with WS promotes the generation of bio-oil while reducing gas formation, but less effect can be observed on the char yield. In the case of 50 wt. % WS addition, the maximum bio-oil yield (40.54%) was obtained at 550 °C, which was increased by 13.65% relative to the calculated value. Moreover, at 550 °C, the production of acids, sugars, and nitrogen-containing compounds during co-pyrolysis decreased by 47.68%, 54.34%, and 71.89%, respectively. In the meanwhile, the phenolic compounds are 29.92% higher than the theoretical value. Besides, high temperatures can decrease acids, alcohols, ketones, esters, and sugars and increase hydrocarbon production during co-pyrolysis. The above results indicated that the addition of WS can effectively improve the quality of bio-oil from SS pyrolysis.