Thermal processing of agricultural waste-based biorefinery residues and plastics for producing sustainable fuels and end of life value

The depletion of fossil fuel reserves and escalating environmental concerns—such as greenhouse gas emissions and plastic pollution—have intensified the global pursuit of renewable and circular energy solutions. Thermochemical conversion technologies, particularly pyrolysis, offer effective pathways for valorising carbon-rich waste into fuels, chemicals, and functional materials. The co-pyrolysis of lignocellulosic biomass with plastic waste exploits synergistic effects between hydrogen-deficient and hydrogen-rich feedstocks, improving product yield and quality. However, feedstock heterogeneity, complex reaction mechanisms, and variability from environmental aging and industrial processing remain key challenges to optimizing and scaling pyrolysis systems. This dissertation presents a systematic research framework addressing these challenges through the investigation of co-pyrolysis behaviour, catalytic upgrading, and kinetic modelling of integrated bio-based and polymeric residues, with an emphasis on process circularity, feedstock interdependency, and environmental resilience. The study began with thermogravimetric analysis (TGA) of various corn stalk tissues (stem, husk, ear, cob, and leaf), high-density polyethylene (HDPE), and their blends. Structural differences in the plant tissues strongly influenced decomposition behaviour and product distribution. The corn cob/HDPE blend achieved the highest yields of valuable chemicals (such as furan derivatives and aromatics) and exhibited the lowest activation energy (149.3 kJ/mol). Machine learning models, including random forest (RF) and gradient boost regression tree (GBRT), were applied to predict mass loss, with RF showing superior performance. Building upon these findings, downstream ethanol processing residue (EPR) from a corn cob-based biorefinery was co-pyrolyzed with HDPE. In situ catalysis with bottom ash (BA) from the same biorefinery reduced activation energy and promoted deoxygenation and aromatic hydrocarbon formation. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) confirmed catalytic improvements in product distribution. To further enhance circularity, wood-plastic composites (WPCs) were examined. TGA and TG-FTIR revealed synergistic effects between EPR and plastic, lowering activation energies and facilitating a one-dimensional diffusion pyrolysis mechanism. Additionally, artificial weathering of WPCs altered plastic crystallinity and pyrolysis behaviour, informing post-consumer management strategies. Overall, this research advances a comprehensive approach to the circular pyrolytic valorisation of biorefinery residues and plastics. Through integrated thermal analysis, catalytic enhancement, kinetic modelling, and machine learning, the findings contribute to the development of efficient, low-emission thermochemical processes for sustainable waste management and renewable energy production.