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!

The Next Wave: The game-change technology for cheap sugars and biofuels

A news from BiofuelsDigest on Sept. 29,2011.

That’s what Agrivida is up to.

“We are expressing all the cell wall degrading systems in the plant,” explains Agrivida CEO Michael Raab, “as the core part of our technology. We can control the activity of those enzymes so that in the plant we can express all the enzymes in dormant form. After harvest, we activate the enzymes in the material, so you don’t have to pretreat in the same way. It makes the process lower temperature, with a moderate PH, and takes out a lot of capital costs and those high costs of dilute acid pretreatment. Also, we really reduce the enzyme loading.”

Biomass feedstocks with hydrolytic enzymes may enable the industry to lower the cost of both pretreatment and enzyme production/loadings, potentially (hopefully) to solve the problem of producing cheap sugar from the root.

Bioconversions of lignocellulosic biomass: the points we cannot overlook

Pretreatment

• If you pretreat biomass at alkaline conditions, be sure to separate lignin before neutralization. Otherwise, the solublized lignin will be precipitated or re-deposited when pH drops.
• If you pretreat biomass at acidic conditions, be sure the temperature does not exceed 165 C. Otherwise, the lignin will condense and re-distribute through cell wall and become gel coat on the surface of pretreated fiber
• If you pretreat biomass with organosolvents, make sure to recycle/re-use the solvents.


Post-pretreatment
• If run detoxification, make sure the methods to be used with fundamental mechanisms. It is cheap and scalable.
• Recover by-products and easily and economically deal with the chemicals if any.

Enzymatic hydrolysis
• Know what biomass you are using and their sugar composition. Woody and non-woody biomass has different chemical (especially lignin and hemicelluloses) composition.
• Know the pretreatment methods you used. The modification of cell wall structure and chemical composition differ under different thermochemical pretreatments.
• Know what substrate the enzyme cocktail development has been based on. Alkaline and acidic pretreated biomass will end up different cocktail characteristics. The enzyme cocktail may need to be customized in terms of the specific pretreated biomass.


Process
• Avoid or reduce unit operations as much as possible
• Be as simple as possible for process configuration
• Use high solids if possible

Fermentation
• Use cheap nutrients if possible
• Ferment C6 and C5 sugars if possible

Van Krevlen diagram: A useful means of comparing biomass and fossil fuels for their heating values

H/C and O/C ratio for several solid fuels (Van Krevelen diagram).

The lower the respective H/C and O/C ratios,  the greater the energy content of the material, because the lower energy contained in carbon–oxygen and carbon–hydrogen bonds, than in carbon–carbon bonds.

Important factors that can not be overlooked by using biomass as renewable energy sources

It is believed that burning biomass will not contribute carbon dioxide to the atmosphere because replanting harvested biomass ensures that CO2 is absorbed and returned for a cycle of new growth, i.e biomass emits roughly the same amount of carbon during conversion as is taken up during plant growth. But people have to realize that there is the time lag between the instantaneous release of CO2 from burning biomass energy and its eventual uptake as biomass, which can take many years. Therefore, there is a need to recognize this time delay and take appropriate action to mitigate against the lag period. Most imortantly, the action of consuming biomass resources for fuel  and replacement planting needs to occur concurrently so that the overall resultant from the use of biomass does not contribute to a build up of CO2 in the atmosphere from a long term.

Unforfunately, not amny people realize the time lag and many countries have not implement a programme of replacement planting when they already started harvesting the biomass.

Silica bodies: possible physical barriers to chemical penetration during pretreatment

It is known silica content is high for non-woody lignocellulosic biomass.The deposition of silica bodies in biologically engineered craters was found to be a unique feature, which leads to some silica-rich spots on the surface of cell wall. On one hand, the silica will cause scaling problems if not treated properly; on the other hand, they also become barriers for chemical pretreatment. It is neccessary to dissolve the silica through pretreatment. Therefore, the chemicals used and the pH for pretreatment need to be chosen carefully.

Paper mill sludge: a cheap sugar for biofuel/biochemicals

Modern paper companies produce large quantities of sludge when using recycled fiber. These sludges are the residue leftover from the paper recycling process and consist of unusable short cellulose fibers inks and dyes, clay, glues and other residue along with any chemicals used in the recovery process. The main disposal routes for paper sludge are land-spreading as agricultural fertilizer or incineration in CHP plants at the paper mill. However, the shortage of landfill space and more restrictive environmental regulations have made disposal more costly and less desirable.

A recent analysis of the sludges from a carton paperboard mill and a tissue paper mill indicating the cellulose are composed of 50-65% of total dry weight of sludge, which can be an important fiber source for biofuel and biochemical feedstock. These fibers with low amount of lignin make them more digestible for enzyme and the process is much simpler compared with the one from biomass. Prior to hydrolysis, there is a need for pre-cleaning or separation of the contaminants through mechanical/chemical process.

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!

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.

Hydrogen from Biomass

Different process routes of hydrogen-production from biomass can be broadly classified as follows:

1. Thermochemical gasification coupled with water gas shift. Maximum conversion can be achieved. But significant gas conditioning is required and removal of tars is important.

2. Fast pyrolysis followed by reforming of carbohydrate fractions of bio-oil. It produces bio-oil which is the basis of several processes for development of fuels, chemicals and materials.But therr are chances of catalyst deactivation.

3. Direct solar gasification. Good hydrogen yield but requires effective collector plates

4. Miscellaneous novel gasification process.

5. Biomass-derived syn-gas conversion.

6. Supercritical conversion of biomass.Can process sewage sludge, which is difficult to gasify. Require a selection of supercritical medium

7. Microbial conversion of biomass.Waste water can also be treated simultaneously. Also
generates some useful secondary metabolites. Selection of suitable microorganisms is needed.

Genomics strategies on biomass to biofuels

Based on the cover storty on C&EN, BESC's 300 scientists located at federal labs, academic institutions, companies, and nonprofit organizations across the country have been given the challenge of designing a path through the lignocellulosics recalcitrance problem.

Deconstructing lignocellulose to accessible sugars followed by chemical or fermentation processes is considered to be the most practical pathway to biofuels. However, pretreatment is a very expensive step in the process for biofuel production. So their efforts are to eliminate or significantly reduce the amount of pretreatment needed. One of the approaches is to ultimately come up with plants that are more easily digested and might need just a hot-water pretreatment without chemicals. A second approach is to engineer a multitalented microbe that can disassemble the plant cell wall and ferment the resulting sugars into biofuel in one go, a strategy known as consolidated bioprocessing. This type of one-organism, one-pot process could be a major breakthrough for low-cost production of ethanol or other fuels and chemicals.

BESC's approach to conquering recalcitrance has started with a major study on a fast-growing poplar tree from the Pacific Northwest and switchgrass native to the prairies of North America. Because no single parameter characterizes recalcitrance, the standard operating procedure is to screen multiple plant samples under a variety of conditions and measure the amount of sugar produced at the end of each test run.

Chemical firms show growing interest in bio-based production.

"Renewable Feedstocks are in the Bag", written by Bill Gerards, Contributing Editor of ChemicalProcessing.com.

The fate of biomass

Biomass has been traded like a commodity as solid biofuels within Europe, including wood chips, wood pellets, briquettes, logs, sawdust and straw bales etc. The standard and markets have been established.

However, biomass has been utilized and will be utilized more as feedstocks for the production of liquid fuels through thermochemical and biological conversion in the world.

Will be a competition for both fuels? Whatever, biomass will suddenly become a hot commodity like corn when biofuels become a hot industry.