Hydrothermal (Liquid Hot Water) Pretreatment of Biomass

Lignocellulosic Biomass

Biomass Hydrolysis

Need for Pretreatment

• Hydrolysis of hemicellulose.
• Partial removal of lignin from the solid biomass.
• A reduction in cellulose recalcitrance to enzymatic hydrolysis.

Effects of Different LHW Conditions

Factors to Consider in LHW Pretreatment

• Extractives - The content of composition of the extractives is important in hydrothermal pretreatment as many of these are likely to also come out in the liquid phase. These may end up as pseudo-lignins that can be sorbed back onto the biomass, hindering subsequent enzymatic hydrolysis. For feedstocks with particularly high extractives content it may be necessary to employ an extractives-removal step prior to LHW pretreatment.
• Hemicellulose Type - Feedstocks with hemicelluloses containing acetyl groups are more suited to LHW pretreatments as the hydrolysis of acetyl groups during the pretreatment generates acetic acid which helps to catalyse the hydrolysis of more hemicellulose. (e.g. hardwoods, herbaceous biomass etc.) removal of lignin from the solid biomass.
• Lignin Content - Biomass with higher lignin contents tend to be more resistant to hydrothermal pretreatments.

Advantages of Hydrothermal Pretreatment

• Environmentally Friendly - As a result of the process not using chemicals.
• Minimial Inhibitor Formation - Particularly at lower process severities.
• Effective on Hemicellulose & Cellulose - In many feedstocks the process effectively hydrolyses the hemicellulose and helps to make the cellulose much more available for cellulase enzymes.
• Can Provide Oligomeric Sugars - There has been a particular interest in recent years in the generation of oligosaccharides, rather than monosaccharides, from lignocellulose. This is due to the potential benefits, to human and/or animal health, of oligomeric sugars included in food/feed or supplements. This is particularly the case for XOS (xylo-oligosaccharides, derived from hemicelluloses with a xylan backbone). LHW pretreatment allows for the optimisation of conditions to target oligomer production from hemicellulose.

Disadvantages of LHW Pretreatment

• CAPEX & Energy Requirements - Due to the need for special reactors that can handle high pressures increases and then the energy demands associated with high temperatures and pressures.
• Unsuitable for Some Feedstocks - For example those with high lignin and/or extractives contents.
• Hydrolysed Hemicellulose in Different Forms - There is likely to be a distribution of the hydrolysed hemicellulose between oligomers, monomers, and (under higher process severeties) degradation products. Subsequent downstream valorisation approaches for the hemicellulose should be compatible with this profile of hydrolysis products.
• Condition and Feedstock Specific - There are many variables to consider in hydrothermal pretreatment (e.g. temperature, pressure, time, etc.). Due to the complexity of lignocellulosic biomass, the best approach for one feedstock may not be suitable for another. Hence, optimisation work is necessary for different feedstocks.

Mechanical Pretreatment

Steam Pretreatment

Acid Pretreatment

Alkali Pretreatment

Organosolv Pretreatment

Other Pretreatments

Pretreatment Homepage