Acetyl group in xylan: problem or opportunity?

It is known that ~7 acetyl groups per 10 xylose units in heardwood and straws, which leads to the formation of acetic acid or acetate by peeling off during any thermal and chemical reaction.

It is quite a big amount in the pretreated hydrolyzate, which is toxic to microorganism at certain level. It is not economical by just removing it from the hydrolyzate. It should be recovered or utilized. Distillation is not efficient to recover it from the hydrolyzate. Membrane separation can achieve the goal but the capital and operating cost will kill the biorefinery if ethanol is the only product.

One of the approach is to produce ethanol indirectly, i.e first ferment xylose to acetic acid followed by esterification and hydrogenation. As a result, 2 unit of xylose can produce 10 unit ethanol with 10 unit hydrogen. The acetic acid can be used completely.

The questions are:

1. The yield and efficiency of fermentation to acetic acid

2. The cost of hydrogen

3. Cost of hydrogenation

4. more...

What are other alternatives to revover or remove acetic acid?

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!

Trash Today, Ethanol Tomorrow

Major Advance In Biofuel Technology reported by ScienceDaily.

Ethanol removal from reaction–separation integration

The reaction–separation integration is an attractive alternative for the intensification of ethanol fermentation processes. When ethanol is removed in-situ, the product inhibition will be reduced.The methods to remove ethanol from fermented broth are listed as follows:

· Vacuum extraction, which can be conducted by coupling of fermentor vessel with a vacuum chamber extracting the more volatile ethanol from fermentation broth which allows the partial product removal and the increase of overall process productivity.
· Gas stripping to increase the concentration of sugars in the stream feeding the fermentor and improvement of improves liquid circulation and mass transfer.
· Membrane separation. For example, ceramic membranes can be used to filter cell biomass and remove ethanol during the fermentation.The removed ethanol is then distilled and the resulted bottoms are recycled to the culture broth resulting in a drastic reduction of generated wastewater. The coupling of fermentation with the pervaporation is another case to remove produced ethanol and reduce the natural inhibition of the cell growth caused by high concentrations of ethanol product.

In membrane distillation, aqueous solution is heated for vapor formation, which go through a hydrophobic porous membrane favoring the pass of vapors of ethanol (that is more volatile) related to the vapors of water. The process driving force is the gradient of partial pressures mainly caused by the difference of temperatures across the membrane.
Liquid extraction is to use an extractive biocompatible agent (solvent) that favors the migration of ethanol to solvent phase, a process known as extractive fermentation.

Hopefully,we will have lab or trial date rather than just modeling for the evaluation, right?

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Synergistic effect of toxic chemical compounds in hydrolyzate

The typical inhibitory compounds from lignocellulosic biomass dilute acid pretreatment are sugar degraded products (furfural, HMF, formic acid, and levulinic acid etc), lignin degraded products (vanillin etc), extractives, and some metals.

The maximum concentration of each inhibitor that a microorganism can tolerate varies depends on the factors of types of microorganisms, its adaption to the medium, process, the numbers of inhibitors, and very importantly, their synergistic effect. For example, ethanol production was stimulated by acetic acid only (up to 10 g/l) in medium without furfural, or by furfural only (up to 2 g/l) in medium without acetic acid. However, the combination of both will negatively affect the cell growth, cell mass yield, and ethanol yield, i.e. the toxicity of biomass hemicelluloses hydrolyzates results from the aggregation of several toxic compounds (alcohols, aldehydes, and acid) rather than individual compounds.

Effect of detoxification of dilute-acid pretreatment hydrolyzate

Before detoxification

Detoxification-1

Detoxification-2

After detoxification, almost no sugar loss but significantly remove furfural (HMF) and simple phenolic compounds.

The factors on enzyme transport

Due to the different morphology of cellulase, the rate of transport will be regulated by the following factors:
1. The substrate pore size and shape: the specific surface area accessible to the protein (SSAP)
2. product concentration
3. Substrate adsorption preference
4. Physical barriers

Silica, a problematic metal in of herbaceous biomass

It is known that about 3–10% of total feedstock (dry matter) is the residue remaining after ignition (dry oxidation at 575 ± 25°C) of herbaceous biomass. It is composed of minerals such as silicon, aluminum, calcium, magnesium, potassium, and sodium.

During hot water and acid pretreatment, silica will be extracted and soluble in acid solution. When raising pH using alkali, silica (or silica oxide) will be hydrated to form some kind of flocs. As a result, it will precipitate into cells to cause problems for cell growth and scaling problem for the equipment. It can also form complex with some organic compounds which interfere with the process and even fermentation.

The effect of available cellulose chain ends on the rate of enzymatic hydrolysis

It has been found that available cellulose chain ends directly relate to the rate of enzymatic hydrolysis of biomass, which is controlled by the amount of amorphous regions, pretreatment, and the action of endoglucanases. If a pretreatment just increases the available cellulose surface areas without significant change of cellulose crstallinity, it is possible that the rate usually does not show significant increase. If there is enough cellulose chain ends or not generated fast enough, the increase in enzyme loading often does not show the hydrolysis rate increase.

The best biomass pretreatment technology

The best biomass pretreatment technology is no pretreatment.