Co-gasification of Acid Hydrolysis Residues and Sewage Sludge in a Downdraft Fixed Gasifier with CaO as an In-Bed Additive

Co-gasification experiments of acid hydrolysis residues (AHR) and sewage sludge (SS) both derived from a lignocellulosic ethanol plant were preliminarily investigated in a downdraft fixed gasifier at atmosphere pressure. Attention was focused on the effects of the gasifier bed temperature (600–800 °C), SS content (0–100 wt %), and equivalence ratio (ER, 0.15–0.30) on the gas quality and cold gas efficiency (CGE). The results showed that the optimal low heating value (LHV) of fuel gas of 6.83 MJ/Nm3, CGE of 70.68%, and low tar content of 5.84 g/Nm3 were obtained at 800 °C, SS content of 50 wt %, CaO/C (molar ratio) of 1.0, and ER of 0.22. A high temperature was favorable for high-quality gas production, and the optimum temperature was 800 °C, at which CaO mainly played the catalyst role but not the CO2 sorbent. Synergetic effects occurred in the co-gasification, and gas quality could be improved in an appropriate proportion by blending 50 wt % SS with AHR. ER ranging from 0.20 to 0.22 improved gas quality and CGE; however, a higher ER favored the decrease of the tar content, and the optimal tar content of 4.00 g/Nm3 was obtained with ER = 0.30. The results indicated the potential technology for air co-gasification of AHR and SS as an option for energy recovery and waste disposal as well as for light industry with agricultural feedstock.