Study on the synergistic effect of CaO/HZSM-5 composite catalyst in regulating the co-pyrolysis products of rice straw and PVC

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 complexity of the products limits the 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 showed that PVC and RS had interactions during the co-pyrolysis process, which promoted the production of aromatic compounds. The relative content of hydrocarbon compounds in bio-oil is as high as 66.78%. The relative content of aromatics reached 64.17%. Compared with RS pyrolysis, the content of oxygenated compounds in the co-pyrolysis bio-oil fraction decreased by 62.05%, which effectively improved the oxidative stability of the bio-oil. The relative content of C₄−C₁₀ hydrocarbons in the pyrolysis product oil increased by 2.81% and 5.06%, respectively, with the CaO/HZSM-5 composite catalyst showing better light aromatic selectivity compared with the individual addition of CaO and HZSM-5 catalysts. Under the synergistic effect of CaO/HZSM-5 composite catalyst, the relative content of monocyclic aromatic hydrocarbons (PAHs) in the bio-oil was 34.34%, which was larger than the theoretical calculated value of 33.03%. And the relative content of PAHs was 28.21%, which was smaller than the theoretically calculated value of 31.22%, showing the better selectivity of monocyclic aromatic hydrocarbons. In addition, the CaO/HZSM-5 composite catalyst promoted the immobilisation of a large amount of elemental chlorine in the form of chlorides such as CaCl₂ in the semicoke, which significantly reduced the gas-phase release of elemental Cl.