Fast Co-pyrolysis of Wood and Plastic : Evaluation of Gases from Primary Reactions

Fast co-pyrolysis (FCP) of wood and plastic offers a promising strategy not only to reduce dependence on fossil fuels for feedstock and chemical production but also to mitigate plastic waste accumulation. This thermochemical process decomposes wood and plastic in the absence of oxygen. It involves complex chemical reactions that yield valuable solid, liquid and gaseous products. These reactions include both primary and secondary reactions. However, limited research has been conducted on gases produced during primary reactions. In this work, micropyrolyser connected with a gas chromatography, a mass spectrometry, a flame ionization detector, and a thermal conductivity detector (Py-GC/MS/FID/TCD) have been employed to analyse gaseous products from primary reactions during both non-catalytic and catalytic FCP of wood and plastic. Primary gaseous products from the fast pyrolysis of cellulose, xylan, and lignin were successfully promoted, showing increased gas yields at higher temperatures and longer residence times. Additionally, fast pyrolysis of low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS) has been performed. Fast pyrolysis of lignocellulosic biomass primarily produces oxygenated compounds such as sugars, phenols, carboxylic acids, aldehydes and ketones. Meanwhile, fast pyrolysis of plastics yields aliphatic and aromatic hydrocarbons, including alkanes, alkenes and alkynes. The underlying reaction mechanisms for primary reactions were proposed. The evaluation of gaseous products from the pyrolysis of individual materials was further used to assess interaction effects during FCP of birch wood with plastics (Wood-LDPE, Wood-PP, and Wood-PS). Significant interactions were observed, with a 90% reduction in oxygenate content in the gases at 75 wt% wood content, regardless of plastic type. Furthermore, catalytic FCP using calcium carbonate (CaCO₃) was conducted to explore the role of in-situ catalysts in enhancing gas yield. Adding 10 wt% CaCO₃ doubled the total gas yield, with a significant increase in volatile compounds containing fewer than 10 carbon atoms. The findings contribute to optimising fuel and chemical production by fine-tuning the wood-to-plastic ratio and selecting appropriate plastic types.