Catalytic pyrolysis of wheat straw based on dual catalyst CaO/ZSM-5 with acid washing and torrefaction pretreatment to enhance aromatic yield in bio-oils

In this paper, the catalytic pyrolysis integrating combined pretreatment (acid washing and torrefaction) and dual catalysts was adopted to improve the quality of bio-oil and the selectivity of aromatic hydrocarbons. Combined pretreatment is a highly effective method to improve the quality of biomass feedstock. It was capable of the removal of alkali and alkaline earth metals from wheat straw, with a K removal rate of 97.79 %. Under the combined pretreatment, the aromatic hydrocarbon content in bio-oil increased to 68.8 % using the ZSM-5 catalyst alone. Compared with ZSM-5, CaO could remove part of the oxygenated functional groups and had a better acid removal effect, but the aromatic hydrocarbon yield was low to 7.95 %. After combined pretreatment using simulated aqueous phase bio-oil for acid washing, catalytic pyrolysis using CaO/ZSM-5 dual catalysts greatly enhanced the quality of the bio-oil. The oxygenated compounds content was reduced to 17.58 %, and the total hydrocarbon yield was increased to 82.42 %, especially the aromatic hydrocarbons yield was increased to 79.91 %, of which the monocyclic aromatic hydrocarbons were as high as 68.38 %, and the benzene, toluene, and xylene content reaching 49.39 %. Thus, integrating dual catalysts (CaO/ZSM-5) with combined pretreatment can effectively increase the aromatic yield for producing high-quality bio-oil.