Zero-waste valorisation of rapeseed through integration of CO₂-assisted pyrolysis and biochar-catalysed transesterification

Global warming is exacerbated by the excessive accumulation of carbon dioxide (CO₂) produced from the combustion of fossil fuels. Oil-bearing biomass is increasingly recognised as a valuable feedstock for biodiesel production, providing a strategy to mitigate the environmental impacts of global warming. Since oil makes up only a fraction of oil-bearing biomass, biodiesel production generates large quantities of carbon-rich residues. In response, this study explored a zero-waste valorisation method for residues of oil-bearing biomass (defatted seeds and cultivated biomass), rapeseed (Brassica napus L.), through pyrolysis. However, the complex composition of liquid pyrolysates hinders their practical applications. As such, further thermolysis into simpler pyrogenic products, particularly syngas, is necessary. Introducing CO₂ into pyrolysis enhanced the thermolysis of volatile matter, discharged from pyrolysis of rapeseed residues, via gas-phase reactions at ≥460 °C, increasing syngas yield (mainly CO) through partial oxidation. To further accelerate the reaction rates, catalytic pyrolysis over a nickel catalyst was adopted, showing a 4.44-fold increase in CO generation. Biochar produced from CO₂-assisted pyrolysis was employed as a catalyst in the thermally induced transesterification of rapeseed oil. The biodiesel yield catalysed by the biochar reached 96.3 wt% at 130 °C within 1 min, whereas the reference test required 360 °C to achieve a comparable yield of 96.9 wt%. These findings suggest that integrating CO₂-assisted pyrolysis of rapeseed residues with biochar-catalysed transesterification of rapeseed oil offers a sustainable approach for producing carbon–neutral fuels.