Ligncellulosic biomass chemically consists of three main fractions: cellulose, hemicellulose and lignin. Cellulose is a polymer of glucose (a C6 sugar), which can be used to produce glucose monomers for fermentation to e.g. bioethanol. Hemicellulose is a copolymer of different C5 and C6 sugars including e.g. xylose, mannose and glucose, depending on the type of biomass. Lignin is a branched polymer of aromatic compounds. Both the C5 sugars and the lignin fragments can be used as feedstock for the production of chemicals in a biorefinery.

Representation of spruce lignin

The cellulose present in lignocellulosic biomass is resistant to hydrolysis. Therefore, if you want to produce bioethanol from lignocellulosic biomass (so-called 2nd generation biofuels), the biomass has to be pre-treated in order to enable hydrolysis of the cellulose into sugars. Different pre-treatment technologies have been developed (steam explosion, treatment with acids or bases, etcetera), but the common purpose of these technologies is to break open the lignocellulosic structure.

Schematic representation on biomass pre-treatment (Mosier et al, 2004)

One step further is the fractionation of lignocellulosic biomass. In such a process the biomass is separated into its three constituents (thereby also pre-treating the biomass for cellulose hydrolysis), which can be used for further conversion.

A possible biorefinery including biomass fractionation

ECN is working on two biomass pre-treatment/fractionation technologies: hydrothermal pre-treatment and organosolv fractionation.

Hydrothermal pre-treatment
Within the hydrothermal pre-treatment process, biomass is brought into contact with water at elevated temperature and pressure. Thus, hemicellulose is hydrolysed and sugars or sugar derivatives (such as furfural, used in plastics and solvents) can be produced. The process is autocatalysed by the formation of acetic acid from the carboxylic acid groups present in hemicellulose.

Modified organosolv
The goal of the organosolv process is to achieve full fractionation of lignocellulosic biomass into its three fractions in a sufficient quality for further conversion into biofuels and chemicals. In the organosolv process the biomass is ‘cooked’ in a mixture of an organic solvent and water at typically 150-200 °C and under pressure. Thus, the hemicellulose fraction of the biomass can be hydrolysed and the lignin is extracted from the matrix. Organosolv was developed as an alternative environmentally friendly pulping technology in the last century. ECN has adapted and improved this process for biomass fractionation purposes within the context of a biorefinery.

More information

Click here for publications, presentations and public reports on organosolv.

References

• Fengel, D. & Wegener, G., “Wood, Chemistry, Ultrastructure, Reactions”, 2003.
• Mosier, N. et al., “Features of promising technologies for pre-treatment of lignocellulosic biomass”, Bioresource Technology, 96, 673-86.