Synergistic effect of CaO/HZSM-5 composite catalyst in regulating co-pyrolysis products of rice straw and PVC

Abstract

Co-pyrolysis of biomass and waste plastics is one of the potential technologies to achieve the resourceful utilization of both, not only to prepare high value-added hydrocarbon-rich bio-oil, but also to achieve clean resourceful utilization of wastes and reduce environmental pollution. However, the products complexity limits its further application of co-pyrolysis technology. In this study, rice straw (RS), an agricultural waste, and polyvinyl chloride plastic (PVC) were used to investigate the individual and co-pyrolysis characteristics of them by thermogravimetric-mass spectrometry (TG-MS) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), and the synergistic regulation law of the pyrolysis products by CaO and HZSM-5 composite catalyst. The results show that interactions exist between PVC and RS during the co-pyrolysis process, which promotes the production of aromatic compounds. The relative content of hydrocarbon compounds in tar is as high as 66.78%. The relative content of aromatics reaches 64.17%. Compared with RS pyrolysis, content of oxygenated compounds in the co-pyrolysis tar fraction decreases by 62.05%, which effectively improves oxidative stability of the tar. CaO/HZSM-5 composite catalyst shows better light aromatic selectivity compared with the individual one. The relative content of C₄–C₁₀ hydrocarbons in the pyrolysis tar increases by 2.81% and 5.06%, respectively. Under the synergistic effect of CaO/HZSM-5 composite catalyst, relative content of monocyclic aromatic hydrocarbons in the tar is 34.34%, which is larger than the theoretical calculated value of 33.03%. And relative content of PAHs is 28.21%, which is smaller than the theoretical value of 31.22%. In addition, the CaO/HZSM-5 composite catalyst promotes immobilisation of Cl in the form of chlorides such as CaCl₂ in the char, which significantly reduces the gas-phase release of elemental Cl.