Degree of biomass substrate solubilization

When evaluating the effect of pretreatment and especially enzymatic hydrolysis of biomass, we normally measure the monomeric sugar produced. At most time, quite a lot of soluble oligomers existing in the hydrolyzates that could not be measured by the standard HPLC analysis, which will lead to lower hydrolysis yield. This can be confirmed by post-dilute acid hydrolysis, showing several fold increase in  monomeric sugars (glucose and xylose) yield. The reason for this is that most of the commercial enzyme cocktails are not for global use and need customization for a specific biomass substrate. Some new technology ends up with strains than can work on low DP soluble sugars. Therefore, measurement of the degree of biomass solubilization after pretreatment or/and enzymatic hydrolysis will be useful information for the estimation of the treatments.  A weight loss is rough, but quick method.

Moderate pretreatment: a practical approach for bioconversion of cellulosic biomass

When pretreating biomass under harsh conditions (extreme pHs and high temperatures), biomass chemical components will degrade into by-products that become inhibitors for the subsequent processing. However, moderate pretreatment can be developed to avoid sugar degradation and toxic compounds formation during pretreatment. As a result, a whole slurry process without inter-stage washing/detoxification can be used, which can significantly reduce capital/operational cost. A moderate pretreatment must remove some lignin but do not break down lignin to simple compounds.It should achieve some defibrillization without much sugar degradation. In a word, It is enough as long as cell wall pores/channels are opened. "Over pretreatment" may improve cellulose accessibility but inevitably cause other chemical components degradation. The right degree of pretreatment is the key. 

Biomass particle size

It is agreed that biomass feedstock particle sizing can impact the economics of cellulosic ethanol commercialization through its effects on conversion yield and energy cost. Physical size reduction can not always achieve expected effect of biomass enzyme digestibility. For example, the thickness of woody biomass play more critical role in chemical pretreatment of wood chips than its length. Defibrillation of grass biomass is more efficient than size reduction in terms of enzymatic hydrolysis.

Therefore, for practical application, an appropriate size reduction needs to be selected based on the biomass type,what chemicals to be used for pretreatment. The smaller does not mean the best!

Cellulosic biomass pretreatment reactor

A news from Biorefiningmagazine.com reported that  a bench-scale horizontal reactor designed and manufactured by AdvanceBio Systems LLC will be in use by Penn State University for their cellulosic pretreatment research. This is great! If looking at the reactors that have been used in bench top studies on biomass pretreatment, most of them can not be use to run the large particle size (un-milled biomass) with good high shear mixing at high solids content.

Structural features lead to two-phase enzymatic hydrolysis for mild pretreated biomass

The limiting factors that affect enzymatic hydrolysis of biomass have been traditionally divided into two groups: substrate-based and enzyme based. The substrate-based factors mainly involve chemical structural features such as the compositions of cellulose, hemicellulose, lignin, and side groups bound to hemicellulose and physical structural features that consist of accessible surface area, crystallinity, the physical distribution of lignin in the biomass matrix, degree of polymerization, pore volume, and biomass particle size.

For mild pretreated biomass, the inital rate of enzymatic hydrolysis will be influenced by the transport of enzymes and the dominant hydrolysis will be on amorphous cellulose. At this phase, the hydrolysis of hemicellulose will correlate with the slow increase of glucose yield. When the hemicellulose removal to some point, say 50% removal, a slight hemicellulose hydrolysis will lead to significant increase in cellulose hydrolysis, i.e. crystal cellulose hydrolysis: A transition point occurs, demonstrating the main or 2nd phase hydrolysis.

At 1st phase, lignin content and side groups in hemicellulose play an important role. Any treatments to remove lignin and peel off side groups and break down hemicellulose will improve enzyme transport though opened channels; At the 2nd phase,cellulose crystallility become the limiting factor, any destruction of cellulose crystallility will speed up the hydrolysis and increase the yield.

Whole slurry hydrolysis: an approach to produce cheap sugar

Almost all of currently reported  biomass pretreatments are those reqyiring post- liquor/solid separation or washing,  or nutralization, or even detoxification before further processing (hydrolysis), which will add additional cost due to the more step unit operations. A cost-effective pretreatment should be low demand of post- pretreatment processing. A "whole slurry" processing or hydrolysis will be ideal case. Therefore, the pretreatment should not be to acidic or too basic, or too harsher that generate too much toxic compounds. I am working on a process that really can realize this goal.

Ionic liquids pretreatment of biomass

Recently more research are conducted on Ionic liquids (< 140 C) pretreatment of biomass, demonstrating

• Converts Cellulose I to cellulose II, improving cellulose digestibility
• Deferulylation of biomass, removal of lignin
•  Cellulose swelling, destruction of cellulose crystal
•  Work for mixed feedstocks-flexibility on biomass resources

Recovering sugars from ILs has been alos investigated. More questions remain on IL pretreatment:

• Is it effective on un-mill biomass such as wood chips and 1-2 inche agricultural residues?
• What about the moisture of wet biomass (in reality, it is hard to dry the biomass with moisture content < 5%) on IL stability?
• The recovery yield? Can be great than 99%?
• Any inhibition or toxic on enzymes and fementation microorganisms?

Benefits from Surfactants (Tween and BSA) During Enzymic Hydrolysis

It appears that Tween improves biomass enaymatic hydrolysis through three effects: enzyme stabilizer, lignocellulose disrupter, and enzyme effector. BSA treatment can improve both cellulase and beta-glucosidase activity due to the non-specific competitive, irreversible adsorption of BSA on lignin.

However, all of the research were based on the current leading pretreatment methods. Scince these pretreatments are really cost non-competitive. Any slight improvements in hydrolysis yield and reduction in enzyme loading is negligible compared with the cost of pretreatment and large scale enzyme production.

A cost effective pretreatment must be developed, which does not require harsher conditions (very acidic and very alkaline, and very high temperature). Is it possible? Yes, if we think the chemistry carefully!

When reading the most recently published review paper on pretreatments, nothing new included. I do not understand that there are so many people working in this field, but most of them just copy the ideas and focus on those so-called leading pretreatments.

I think more education on biomass chemistry is really need to train people/students to develop new approches/solve problems before fundemetal understanding of the problems.

Pretreatment: the last rice straw to knock down the camel?

As a crucial step in the biological conversion to ethanol, biomass pretreatment is often regarded as one of the main economic costs in the process,even described as the second most expensive unit cost in the conversion of lignocellulose. But what pretreatment does it refer to? Sream explosion? Dilute acid? AFEX? Lime? Hot water? organosolv? alkaline? or Ionic liquids? Why not others?

Idealy, a cost effective pretreatment should be the one that uses less chemical at low temperature with little or no inhibitory products. Is it possible? Why not?  For all the pretreatments reported, Soaking Aqueous Ammonia (SAA) is relatively a decent pretreatment method that performed at lower temperature with both glucan and xylan retained in the solids and lower amount of inhibitory compounds released form sugar degradation. After fundemental understanding of linocellulosic chemistry (rather just read the review papers), I believe that a better or improved technology can be potentially developed.

Pretreatment: Low temperature and atmospheric

Here is a paper on low temperature pretreatment. What other methods?

• Alkaline peroxide?
• Oxygen-alkaline treatment?
• Ozone?
• Liquid ammonium?

All of these work but which one is more practical in terms of environmental issue and cost?

What about biopulping process?

Cheap sugar: How can it be cheaper?

It is known that feedstock cost account for significant part of overall ethanol production cost. The key is to obtain cheap fermentable monomer sugars from lignocellulosic biomass with the following factors:
· Monomer sugar recovery yield. If we look at biomass, only ~65-70% of cell wall is carbohydrate that is our target to convert into monomer sugars. The question is how to efficiently hydrolyze it with high yield. Chemically or enzymatically? Currently both cannot achieve high sugar yield and high efficiency. The combination of chemical and enzymatical hydrolysis is the direction. However, more factors need to be considered to achieve high sugar yield when talking about a commercialized technology:
o Recoverable/recyclable chemicals that minimize the chemical cost and waste treatment
o The pH of pretreatment that impacts reactor metallurgy
o The temperature of pretreatment that impacts energy usage
o The robust of enzymes that reduce the loading and increase the efficiency

Cheap sugar: the key for bioethanol to survive

Bioethanol industry is facing another winter time in its history due to current cheap oil/gas price. Most people believe the price of oil will back up again sooner or later. The question is when? The good news is the incentive policy and stimulus fund from the new government that will bring the spring this industry. However, the long term survival will depend on its own economic viability. The key is the cheap sugar and apparently the renewable source is lignocellulosic biomass. Generally only 2/3 of biomass weight is carbohydrate that can be converted into monomeric sugars for fermentation. The question is how to obtain them with a high yield in a cheap way. The current hydrolysis technology is still not good enough to realize this.

Enzymatic hydrolysis is a direction for bioethanol production from lignocellulosic biomass. Ideally a or a combination of chemicals are used to remove both hemicelluloses and delignification simultaneously, the resulting solid is mostly cellulose with more exposed and accessible surface and pores, free chain ends, leading to lower enzyme loading, high enzyme selectivity, and fast rate of hydrolysis.

The cheap sugar is calling on advanced enzyme!

Integration may lower the overall hydrolysis cost

Before the cost of enzyme is down to an economical level, it may be not enough to increase the efficiency and rate of enzymatic hydrolysis of biomass just via a pretreatment.

Because of the nature of hemicelluloses (branched, amorphous, and variety), the severity of pretreatment should be low to avoid hemicelluloses degradation. However the mild pretreatment will not damage cell wall enough and remove part of recalcitrant lignin (the physical barrier and competitive sites for enzyme adsorption). As a result, the efficiency and rate of enzymatic hydrolysis cannot reach the level of what we expect. Ideally, the process should be integrated with several units: pretreatment without washing to pull out hemicelluloses; followed by delignification to remove lignin; enzymatic hydrolysis of delignified biomass with very limited dosage of enzyme to achieve target and high sugar recovery yield. The additional unit may increase capital cost. Considering the saving of enzyme dosage and time, the overall operation and material cost may be lower. If increasing the rate of hydrolysis, the size of equipment can be smaller. Therefore, the integration of process may lead to a overall efficiency.

We plant trees is to have a forest!

The best biomass pretreatment technology

The best biomass pretreatment technology is no pretreatment.

Chemical effects enhanced by charging N2 or CO2 during pretreatment

During biomass pretreatment, introduction of N2 or CO2 into the reactor will enhance the chemical changes of cell wall components due to the mechanical effect.

A recommendation for biomass pretreatment reactor

The most widely used lab pressurized reactors for biomass pretreatment are autoclaves, steam guns, etc, which is hard to scale up.

A reactor like M&K lab digester has been widely used for lab study on pulping. The rapid liquor circulation provides excellent chemical/biomass interactions or mixing. The two-vessel design allow us to use steam for rapid heat-up and the cooling pipe line allows us to cool down fast after treatment. Such a kind of design has been scaled up for pulp and paper industry and should be easily done for the biorefinery.

Integration of pretreatment and enzymatic hydrolysis of biomass

This a new concept: The biomass is first pretreated with water only or with other chemicals. After pretreatment, the treated biomass and slurry is enzymatically hydrolyzed without separating the solid and liquid.

Here is one of the research projects conducted at The BioEnergy Science Center.

Microbial pretreatment of piomass

Biological pretreatment of wood chips has attract great attention and research activities to reduce the mechanical pulping refining energy. The same principle and idea can be applied to biomass pretreatment for lignocellulosic ethanol production.

A review of the literature suggests that fungal pretreatment could potentially lower the severity requirements of acid, temperature and time. These reductions in severity are also expected to result in less biomass degradation and consequently lower inhibitor concentrations compared to conventional thermochemical pretreatment. Furthermore, potential advantages of fungal pretreatment of agricultural residues, such as corn stover, are suggested by its effectiveness
in improving the cellulose digestibility of many types of forage fiber and agricultural wastes.

Hydrothermal treatment of biomass using phosphorous acid as an additive

Instead of using as a solvent, phosphorous acid can be also used as a catalyst for biomass pretreatment.

Based on the study in Japan, the yield of monosaccharide obtained from the rice straw using hydrothermal treatment was low. but when 50 mM phosphoric acid solution was used to hydrothermal system, most of xylan in rice straw was successfully hydrolyzed to xylose and some amount of glucan was also converted into glucose at 160 °C for 15 min. The maximum xylose and glucose yields were 54.1 and 15.0 % at this condition, respectively.

The remained cellulose residue after hydrothermal treatment with phosphoric acid at 160 °C for 15 min was easily saccharified to glucose by enzymatic hydrolysis. After the enzymatic saccharification, the glucose yield was 81.6 %.

After neutralization of hydrolysate with NaOH, the salt formed is sodium phosphate. This salt used as nutrient by microorganisms.

Phosphoric acid pretreatment of biomass

It has been noticed that concentrated phosphoric acid is an ideal cellulose solvent. The advantages of phosphoric acid pretreatment of biomass includes:

• cellulose dissolution by phosphoric acid occurs at low temperatures
• phosphoric acid can dissolve cellulose in the presence of water
• the regenerated cellulose remains in an amorphous form suitable for hydrolysis
• the residual phosphorous acid has no inhibitory effects on the sequential
  hydrolysis and fermentation.

But what about the cost?