Co-liquefaction of sewage sludge and rice straw/wood sawdust: The effect of process parameters on the yields/properties of bio-oil and biochar products

Liquefaction is a promising technology for the preparation of bio-oil and biochar products from sewage sludge (SS). Co-liquefaction of SS and agricultural/forestry biomass is likely a potential method for obtaining high yield/quality of bio-oil and biochar. In this work, rice straw (RS) and wood sawdust (WS) were selected as representatives of agricultural/forestry biomass and the effects of main process parameters during the co-liquefaction of SS-RS/WS on the yields/properties of bio-oil and biochar were explored in details. The proper reaction temperature, solid-liquid ratio and reaction time for the co-liquefaction of SS-RS/WS were suggested to be 280–300 °C, 0.1 g/mL and 10–30 min, respectively. Potassium/sodium salt (-OH and -CO3) catalysts could efficiently enhanced the co-liquefaction of SS-RS/WS. The introduction of RS/WS into the liquefaction of SS would reduce the contents of nitrogen and sulfur in bio-oil and enhance the formation of phenolic compounds. In addition, co-liquefaction of SS-RS could promote the percentage of lower-boil-point compounds in bio-oil. Although the thermal stability, surface area and pore volume of biochars slightly decreased, the contents of organic matters in biochars were obviously improved, suggesting higher potential in land application. •Co-liquefaction of sewage sludge and rice straw/wood sawdust was explored.•Proper reaction conditions: 280–300 °C for 10 min with 0.1 g/mL solid-liquid ratio.•Potassium/sodium salts (-OH and -CO3) can be utilized as catalysts.•The contents of heteroatoms (nitrogen and sulfur) in bio-oil were decreased.•The contents of organic matter in biochars were obviously increased.