• Hydrothermal (LHW)
    Pretreatment of Biomass
    Bioprocess Development


Lignocellulosic Biomass

Lignocellulosic biomass is defined as a plant, or plant-derived, material that is mostly composed of cellulose, hemicellulose, and lignin. Lignocellulosic feedstocks are highly abundant, covering many biomass types including grasses, wood, energy crops (e.g. Miscanthus and coppices), agricultural residues (e.g. straws and corn stover), and municipal wastes.

Lignocellulosic feedstocks are highly abundant and can often be sourced sustainably, at low cost, without leading to land-use conflicts. As a result, there is currently great interest in obtaining chemicals, fuels, and biomaterials from such biomass.

Biomass Hydrolysis

A major pathway by which many lignocellulosic feedstocks are processed is known as hydrolysis, where sugars are released from the lignocellulosic polysaccharides (i.e. cellulose and hemicellulose).

However, the hydrolysis of lignocellulosic polysaccharides is not easy and is influenced by the complex inter-associations between hemicellulose and cellulose and between these polysaccharides and lignin in the lignocellulosic matrix. In particular, the crystalline nature of much cellulose and the existence of a physical barrier of lignin surrounding the cellulose fibres are said to be major contributors to the recalcitrance of cellulose.

The mechanism of hydrolysis is further complicated by the fact that different process intensities are required for the hydrolysis of cellulose versus hemicellulose. The more intense conditions required for cellulose hydrolysis may degrade the sugars hydrolysed from hemicellulose (to products such as furfural and formic acid).

For this reason, most hydrolysis technologies employ pre-treatment processes that aim to break apart the matrix (and in particular the associations between lignin and cellulose), reduce cellulose crystallinty, and hydrolyse hemicelluloses, hence separating the hydrolysate from cellulose which can then undergo more severe/targeted treatment.

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Need for Pretreatment

For this reason, most hydrolysis technologies employ pre-treatment processes that aim to break apart the matrix (and in particular the associations between lignin and cellulose), reduce cellulose crystallinty, and hydrolyse hemicelluloses, hence separating the hydrolysate from cellulose which can then undergo more severe/targeted treatment.

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Hydrothermal Pretreatment (Liquid Hot Water)

Hydrothermal pretreatment, also know as Liquid Hot Water (LHW) pretreatment, is a mild treatment process that does not use any chemicals in the process. The process does, however, operate at high temperatures and pressures for increased penetration of the solvent (water) into the biomass. The superheated water (150–230 oC) is kept in the liquid state as a result of the high pressures employed by the process.

The general targets of LHW pretreatments are:

Effects of Different LHW Conditions

An advantage of LHW at lower temperatures is that hemicelluloses can be efficiently hydrolysed without significant amounts of degradation products (e.g. furfural from pentoses and hydroxymethylfurfural from hexoses). The products of hemicellulose hydrolysis can be oligosaccharides (sugar oligomers), monosaccharides (e.g. xylose, arabinose), or a combination of these, depending on the process conditions.

As temperatures increase, less hemicelluloses will remain in the polymeric form and there is a tendency for monomers to be favoured over oligomers, however the potential for sugar degradation products to form also increases.

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.

Hydrothermal Pretreatment Bioprocess Development at Celignis

Whether you have an exisiting hydrothermal pretreatment process that you think can be improved, or if you want to evaluate LHW as a pretreatment for your feedstock(s), we are here to help.

Our initial step would involve learning from you what your targets are for the pretreatment and what the subsequent stages of the process, and final products, should be.

Then we would undergo a detailed compositional analysis of the feedstock in order to gain an understanding of its suitability for LHW pretreatment and to consider appropriate process conditions.

We would then work on a design of experiments (DoE) for lab-scale LHW pre-treatments to explore the effects of different process conditions. These experiments would be undertaken using our Dionex ASE 350 pressurised-liquid extraction system which involves reactor vessels up to 100m and can operate at pressures up to 3000psi.

There would be detailed analysis of the liquid and solid outputs of these pretreatments, comparing these results with the targeted aims of the pretreatment and downstream valorisation.

If subsequent enzymatic hydrolysis of the solid residue is the planned next step in the process then we can undertaken lab-scale enzymatic hydrolysis experiments on the residues from this DoE. We can also use the cellulose-derived sugars as a substrate for fermentation, combining the approaches if required (i.e. simultaneous saccharification and fermentation).

If there are also plans for downstream valorisation of the liquid fraction we can also assess the suitability of various hydrolysates from the DoE for such processes.

Following a review of the results from the DoE we can then work on scaling-up the process, in the first step to a 1-litre PARR vessel with motorised stirring, and later to higher TRL levels. The outputs of these pretreatments can also be tested for their downstream valorisations. Any deviations from the outputs at lower TRLs can be assessed and remedying approaches undertaken.

The commercial viability of pretreatment is particularly important and should be considered throughout the project. We usually recommend to our clients that we undertake interative techno-economic analyses (TEA) at various stages of the bioprocess development, so that the experimental and scale-up work are framed in a commercially-relevant context.

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Other Types of Biomass Pretreament

Mechanical Pretreatment

Mechanical pretreatments of biomass usually focus on a reduction in the particle size of the feedstock, allowing for more surface-area availability in downstream processes.

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Steam Pretreatment

Steam pretreatment uses high-pressure steam at elevated temperatures. In steam-explosion pretreatment this is followed by rapid decompression which physically disrupts the biomass structure.

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Acid Pretreatment

This pretreatment involves treating the biomass with a dilute solution of a strong acid at elevated temperatures. A primary target is the hydrolysis of hemicellulose into monomeric sugars.

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Alkali Pretreatment

Alkali pretreatment uses chemicals (e.g. sodium hydroxide, potassium hydroxide, calcium hydroxide) to disrupt the complex structure of lignocellulosic biomass with lignin removal a primary target.

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Organosolv Pretreatment

Organosolv pretreatment fractionates biomass into its three major components (cellulose, hemicellulose, and lignin), with obtaining a lignin of a high-purity and quality being a particular target.

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Other Pretreatments

This page covers some other pretreatments: ionic liquids pretreatment, hydrogen-peroxide pretreatment, ultrasonic pretreatment, microwave pretreatment, fungal pretreatment, and bacterial pretreatment.

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Pretreatment Homepage

Click here to read more about the important factors to consider when designing a biomass pretreatment process and to read about how we undertake our pretreatment development projects with our clients.

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