Integrated conversion of agroindustrial residue with high pressure CO2 within the biorefinery concept

Abstract

Sustainable production of energy and other added-value products from biomass-derived polysaccharides is a key challenge of an efficient biorefinery facility. Most technologies for biomass processing are energy demanding and use significant amounts of chemicals and catalysts. The need to develop a process which is devoid of all these shortcomings associated with conventional processes is emphasized. A new approach is demonstrated for an integrated wheat straw biorefinery using a green technology, high-pressure CO₂–H₂O, to produce oligosaccharides from hemicellulose fraction and to enhance the cellulose digestibility for the enzymatic hydrolysis. Over the range of reaction conditions (130, 215, 225 °C and 0 to 54 bar of CO₂), CO₂ adds value to the process by in situ formation of carbonic acid that leads to higher dissolution of hemicellulose into xylo-oligosaccharides and xylose and to the use of less energy in comparison with water-only technologies. Without an additional chemical catalyst, high-pressure CO₂–H₂O outperformed hydrothermal reactions and gave much higher total sugars yield for wheat straw (as high as 84% in comparison with 67.4% with auto-hydrolysis at a 10 °C higher temperature). Apart from the results obtained for valorisation of hemicellulose fraction, both chemical and physical effects of CO₂ coupled to enzymatic hydrolysis resulted in a glucan conversion to glucose yield of 82%, which consists of 26% improvement over those obtained during auto-hydrolysis. The influence of the high pressure reaction on the processed solid was examined by spectroscopic methods (namely Scanning Electron Microscopy and Fourier Transform Infrared Spectroscopy). The obtained results suggest that the high pressure CO₂-based method is a very promising alternative technology allowing integrated biomass processing within the biorefinery concept.