Supercritical CO2 extraction of waxes as part of a holistic biorefinery

The scope of this project was to utilise supercritical carbon dioxide (scCO2) extraction technology as a pre-treatment step in a holistic C4 biorefinery in order to extract added value metabolites from C4 agro-residues. The lipid profile of three different types of C4 biomass agro-residues namely miscanthus, maize and sugarcane was studied. Wax extractions with organic solvents and scCO2 were conducted on different parts of the plant. A diverse range of hydrophobic molecules were identified including long-chain fatty acids, n-policosanols, fatty aldehydes, n-alkanes, wax esters, sterols, steroid ketones and triterpenoids. The extracts exhibited significant differences in melting temperatures, highlighting the possibility for utilisation in different applications. The advantages of scCO2 extraction over conventional organic extraction are clearly demonstrated in this work. Herein, the first reported fractionation of maize stover wax gave rise to three waxy fractions. Fraction A consisted of high molecular weight compounds while Fraction B was predominantly phytosterols. Fraction C had the lowest melting profile and the wax was tested as a renewable antifoaming agent. Fraction C successfully reduced foam and had no negative effects on the detergent performance. The extraction of wax from C4 biomass only utilises around 1% of the total biomass. In order to have a systemic view of a C4 biomass processing scenario where scCO2 extraction is integrated into a biorefinery, the effects of scCO2 extraction on the downstream processing of maize stover and miscanthus were studied. In fermentation of maize stover for surfactant production, results show that there was a higher glucose consumption (19%) and greater growth (18%) for the scCO2-extracted maize stover when compared to non-treated maize stover. In fermentation experiments for ethanol production a 40% increase in overall ethanol production for the scCO2-extracted maize stover was obtained. Saccharification results on miscanthus leaves showed a 22% increase in sugar release for scCO2 extracts. Finally, this work cannot be developed further unless an economic evaluation of the manufacturing process is done. This is the first time an economic assessment for the scCO2 extraction of waxes from biomass has been carried out. The cost for miscanthus and maize stover wax extraction was found to be €148/kg of wax and €88.19/kg of wax respectively. If the biomass was pelletised, leaves were solely taken into consideration (4 times the wax yield) and the biomass was combusted following the extraction, then the cost of the wax could be reduced to as much as €8.60/kg of wax for miscanthus and €1.97/kg of wax for maize.