Biomass Pretreatment Processes

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 cellulose and the existence of a physical lignin barrier surrounding the cellulose fibers 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 hydrolyzed 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 crystallinity, and hydrolyze hemicelluloses, hence separating the hydrolysate from cellulose which can then undergo more severe/targeted treatment.

How to Develop a Sustainable Biomass Pretreatment Process

There are several important considerations when developing a biomass pretreatment technology, including:

Feedstock Chemistry

The chemical composition of lignocellulosic feedstocks varies greatly. This variability is seen not only in the relative proportions of the different polymers of lignocellulose (cellulose ,?hemicellulose ,?lignin ) but also in the compositions of each of these polymers.

The particular chemistry of the feedstock is a crucial factor when deciding on an optimal pretreatment since some types of biomass (due to their chemistry) are unsuitable for certain pretreatment technologies. For instance,?steam explosion ?is much less effective as a pretreatment on?softwoods ?compared with?hardwoods ?because?softwood ?hemicelluloses ?do not contain?acetyl ?groups (which aids the mechanisms of?steam explosion ?pretreatment). Similarly, a feedstock with a very high?lignin ?content may be more suited to?organosolv pretreatments ?that can separate the?lignin ?from the?cellulose , allowing the feedstock to be more accessible to cellulase?enzymes .

The content and composition of?extractives ?are also important considerations when designing a pre-treatment process since these can complicate downstream valorization stages.

Lignocellulose Fractionation

Some pretreatment processes (e.g.?steam explosion ) focus primarily on a disruption of the?lignocellulose ?matrix to allow for it to be more readily hydrolyzed in downstream processes. However, other technologies (e.g. when using acids, alkalis, solvents, or hydrothermal pretreatment) involve the production of two (or more) process streams with the primary lignocellulose polymers (cellulose ,?hemicellulose ,?lignin ) displaying a preference for one of these streams.

For example, an?organosolv approach ?employing acetone as the solvent (as in our?CBE ?project?UNRAVEL ) would result in hemicellulose ?and?lignin ?in the liquid phase with the solid phase mostly composed of?cellulose .

Hence, if your target for biomass valorization is different downstream processes for each polymer then certain types of pretreatments that allow for such fractionation of biomass chemistry should be preferred.

Downstream Technologies

There are different ways in which the output streams (solid and liquid) of the pretreatment process can be valorized. For example, the hemicellulose sugars that become separated into the liquid phase in many pretreatment processes (e.g.?acid pretreatments ?and?hydrothermal pretreatments ) can be biologically processed (e.g.?fermented ?to?ethanol ) or chemically/catalytically processed (e.g. catalytic production of?xylitol ?from?xylose ).

Each type of downstream valorization process will have its preferred conditions and requirements for the composition of the stream being processed. For example, many organisms used for fermentations may have their activities inhibited by the presence of degradation products of the sugars (e.g.?furfural ?is the degradation product of?xylose ?and?arabinose ). This would lead to a preference for a pretreatment that minimizes the formation of inhibitors (for example?hydrothermal pretreatment ?may be favored over an?acid pretreatment ?in this case). Conversely, if you are using a downstream technology that is not affected by these inhibitors then other aspects of the pretreatment become more important (for example, the process with the lower CAPEX/OPEX or the process that provides the highest yields of monomeric sugars).

Commercial & Environmental Viability

For a bioprocess to be commercially viable it is necessary for the revenues associated with it to exceed the operating costs and that these operating margins cover the CAPEX in a reasonable timeframe.

Some pretreatments can have significantly higher CAPEX and/or OPEX than others. However, this does not mean that they are not commercially viable since they may provide more valuable products. An example would be organosolv technologies which typically have higher OPEX costs (due to chemicals usage and the energy costs of product/solvent recovery) and CAPEX costs. However, organosolvs can produce a high-quality lignin with significant revenue-generation potential. For this potential to be realized pretreatment optimization should focus on the use of a feedstock, and the development of a process, that will achieve the product requirements for entering these higher-value markets.

Similarly, bioprocesses targeting products to replace fossil-derived products should be demonstrably more sustainable. Hence, the pretreatment and downstream steps should be optimized so that the greenhouse gas emissions, and environmental impacts of the whole process, represent a significant improvement on the status quo.

At Celignis we are experts in the?techno-economic analysis ?(TEA) of bioprocesses and consider this, as well as the environmental impacts, at all stages of process development.

Types of Biomass Pretreatment

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.

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.

Hydrothermal Pretreatment

Hydrothermal pretreatment, also known as Liquid Hot Water (LHW) pretreatment, does not involve the use of any chemicals but operates at elevated temperatures and pressures

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.

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 as a primary target.

Organosolv Pretreatment

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

Other Pretreatments

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

How Celignis Develops Biomass Pretreatments for Our Clients

1. Understanding Your Requirements

Before undertaking bioprocess projects, we learn from our clients what their targets are from the process as well as whether there are any restrictions or requirements that may need to form the boundaries of the work that we undertake. These help to guide us in preparing a potential bioprocess development project.

For example, in the context of pretreatment, the primary target of one client may be to achieve the greatest possible glucose yields from cellulose , with the valorization of the other main components of biomass (e.g. lignin and hemicellulose ) being of much lesser importance. In contrast, another client may be particularly focused on the production of oligomers from hemicellulose , with the efficient hydrolysis of cellulose being lower down in their list of priorities. Such differing targets are likely to lead to different pretreatment technologies being used in each case and/or in the selection of different process parameter ranges for the experiments to be undertaken.

Similarly, a requirement from a client for no chemicals to be used would narrow down the range of options that we could consider for the pretreatment process.

2. Detailed Feedstock Analysis

A thorough understanding of the chemistry of a feedstock is a crucial component in designing an effective pretreatment process. Detailed compositional analysis is also necessary to accurately determine process yields and efficiencies.

We recommend that each feedstock that is to be investigated for pretreatment be analyzed with one of our in-depth lignocellulose analysis packages (P10 or, ideally, P19 ). We advise that these analyses are undertaken before the start of the bioprocess project as the resulting data will allow us to consider the appropriate pretreatment technologies and process conditions for any particular feedstock.

Hence, based on the analytical results and the client's requirements, the Celignis Bioprocess team will then meet to discuss and prepare a project proposal for biomass pretreatment development and optimization. After this proposal is reviewed by the client, and revised if needed, we are then ready to start work on developing the pretreatment process.

3. Lab-Scale Pretreatments

Our projects usually involve undertaking several pretreatment experiments, covering a variety of process conditions. We follow a scientifically-based Design of Experiments (DoE) protocol where the criteria and boundaries for this DoE are formulated in close collaboration with our clients, considering the chemistry of the feedstock(s) and our understanding of the mechanisms of biomass pretreatment.

We usually recommend that these initial optimization experiments are undertaken at the lab scale (around TRL3) to reduce costs and the length of the project. For each experiment we analyze the solid and liquid outputs of the pretreatment process, leading to a detailed data set where the effects of process conditions on the yield and composition of the various streams can be explored and mapped.

We can also undertake a second iteration of lab-scale experiments to fine-tune the conditions based on the knowledge gained in the initial experiments.

4. Downstream Valorisation Experiments

The analytical results from our lab-scale experiments will provide us with valuable data on the composition of the various output streams (e.g. solid and liquid) from the pretreatment process. However, to fully evaluate these streams for their suitability for the planned applications (for example, ethanol from the cellulosic fraction) we advise that their downstream processing is tested.

We can undertake a wide variety of tests here, based on our client's requirements and the targeted end products. The results from these experiments allow for a more thorough evaluation of the pretreatment process and for the conditions to be optimized according to the final product requirements, rather than just according to the chemical composition of the pretreatment's output streams.

We advise that these initial downstream valorization experiments are also undertaken at the lab scale, using the outputs of Stage 3. This will enable us to gather more data, at lower costs, allowing for the pretreatment process to be optimized most effectively.

5. Validation At Higher TRLs

Once we have concluded our optimization of the pretreatment process conditions at the lab scale we can then test those conditions at higher technology readiness levels (TRLs). The scales at which we can operate are dependent on the type of pretreatment technology employed but can reach up to 100 liters.

We have all of the necessary downstream equipment to efficiently handle the solid and liquid streams arising from these scaled-up activities and we can also undertake scaled-up processing of these different fractions from the treatment, for example, high-TRL enzymatic hydrolysis runs for the solids and fermentation runs for the liquid stream.

If we find that there are differences between the yield and compositions of the different post-pretreatment streams, compared with our lab-scale experiments, then we can explore the potential reasons for these and work on final tweaks to optimize the bioprocess for higher TRLs.

6. Techno-economic Analysis (TEA)

The Celignis team, including Oscar our chief TEA expert, can undertake a detailed techno-economic analysis of the developed process. We apply accurate and realistic costing models to determine the CAPEX and OPEX of simulated and pilot scale processes which are then used to determine key economic indicators such as IRR, NPV, and payback periods.

Within these TEAs, we can consider the pretreatment as a discrete independent module or we can evaluate the biomass valorization process as a whole, evaluating the impact of the pretreatment on the whole biorefinery concept. We also undertake sensitivity analyses to assess the effect of variable costs and revenues on the commercial viability of the pre-treatment process.

Our preferred approach is to include TEA studies at each stage of the development of the pretreatment bioprocess, so that the process can be optimized in a commercially-relevant way, followed by a more detailed TEA after the process has been optimized and tested at higher TRL levels.

Our Bioprocess Development Services can work on the evaluation and optimization of a particular node in the biomass processing technology or can involve the development of a bespoke vertically integrated technology for your chosen feedstock and/or product.

Click here to see how our Bioprocess Development Services work and how we devise and undertake a project.