Friday, October 31, 2008
These days, "green" become popular and most widely used word. "Green food", "green energy", "green chemistry", "green economy", "green consumers", "green power","green chemicals", "green drink".....
Green may have many new meanings when people use it. It may mean clean, environmental friendly, healthy, technical advanced, more natural, and more.
But we need to know almost every "green" products are not cheap. Sometimes, it requires more sources or sacrifice more clean and healthy sources to produce one "green" product. Green is good but nowadays we need to know the cost to be "green".
Wednesday, October 29, 2008
Typically, fermentation is conducted at ~30 0C for 24-96 hours. The microorganisms include bacteria (such as Zymomonas molis and E. coli) and yeast (i.e. Saccharomyces cerevisiae).
–Embden-Meyerhof pathway and Entner-Doudorpff pathway
–High yield and ethanol tolerance
»Inability to convert complex carbohydrate to ethanol
»By-products such as sorbitol, glycerol and acetic acid
–Very robust and well suited to SW lignocelluloses (mannose and later galactose)
But traditional baker's yeast only ferment C6 sugars and need engineer it by genetic modification to ferment both C5 and C6 sugars.
When talking algae about its potential being biofuel, , people may focus more on these facts: "algae can flourish in non-arable land or in dirty water, and when it does flourish, its potential oil yield per acre is unmatched by any other terrestrial feedstock". My question is: can we do the transgenetic modification by transfering the genes from high oil yield plants into algae to increase its oild yield? What are the technical barriers?
Tuesday, October 28, 2008
Since the main barrier is the interference of lignin and the crystal structure of cellulose,one approach may be to utilize cellulose swelling agents to destroy the cristal structure; the other one may be to use lignin dissolving agents to remove part of lignin. As a result, the rate of saccharification will be speeded up from either of the treatments.
- high energy content: 110,000 btu/Gal. Ethanol: 84,00 btu/Gal;Gasoline: 115,00 btu/Gal.
- 6 times less evaporation than ethanol and 13.5 times less than gasoline
- cost less than ethanol
- can be made from biomassrenewable and could be produced in distributed biorefineries scattered across the landscape
Now the questions are:
1. Why is the butanol not declared as an alternative fuel sources by government like ethanol?
2. Why aren't there more extensive research and commercialization activities on bio-butanol production?
3. What are the major barriers for its production and commercialization?
Saturday, October 25, 2008
Fig. 1. Processing of Biomass to Bioethanol
Traditionally, most of processes are based on acid hydrolysis of the lignocellulosic materials. The acids includw sulfuric acid, phosphoric acid, formic acid, hydrochloric acid, etc. Concentrated acid hydrolysis can process at ow temperature and achieve high monoer sugar yield. However, it cause significant equipment corrosion and requires energy-demanding acid recovery. Therefore, dilute acid hydrolysis has been studied intensively and still a major process to produce sugars from biomass. Its advantages include low acid consumption, short reaction time: 2-10 min. But the aisadvantages are high reaction temperature (170-220 C), low sugar yield due to sugar decomposition, equipment corrosion, by-product innhibition.
During the last decades, process based on enzymatic hydrolysis has attracted increasing attention due to more selective hydrolysis and the formation of less-by-products. It uses cellulase mix (Cellulase with b-glucosidase etc.) to enzymatically hydrolyze cellulose to monomer sugars. The aximum cellulase activity usually occurs at 50 ±5 0C and a pH of 4.0-5.0. The advantages of enzymatic hydrolysis includes low temperature, specific conversion of cellulose, and high yield and less toxic compounds formation than acid hydrolysis. But the problems still exsiting for this process are the slow conversion cellulose to sugars due to the matrix of hemicelluloses and lignin as well as enzymes. As mentioned before, it needs pretreatment step to expose cellulose or modify the pores in the materials to allow enzymes to penetrate into the fibers and hydrolyze the cellulose to monomer sugars.
Thursday, October 23, 2008
The objective of pretreating lignocellulosics is to alter the cell wall structure of biomass to extract hemicellulose and meanwhile make the cellulose more accessible and amenable to hydrolytic enzymes that can generate fermentable sugars.
Effective pretreatment technologies need to address several important criteria, including: minimize inhibitory compounds formation (e.g. furfural, HMF, aldonic acids, etc.), lignin alterations, minimal energy, capital and operating costs.
Good pretreatments will have a major influence on downstream process such enzyme loading, enzymatic hydrolysis rates, fermentation toxicity, product concentration, mixing power, and waste treatment.
Certainly, limit chemical addition or consumption should be considered for pretreatment because itself is alslo a costly step. But overall cost from the whole process should be more critial than the pretreatment itself.
So far, many pretreatment technologies have been proposed and studied using either inorganic catalysts or organic solvents. Under acid pretreatment conditions, most of the process have been conducted at a temperature above 170 C, which usually causes a problem for lignin re-adsorption or precipitation on the surface of cellulose fiber after cooling down (See Fig). The temperature of 170 C is close or higher lignin glass transition points. As a result, some hydrophobic lignin will melt, re-distribute or migrate. If they are not removed before cooling down, they will deposite on the surface of cellulose fiber forming lignin coating, which will act as a barrier to the cellulase enzymes and also adsorb cellulase enzymes to limit the efficacy of hydrolysis and increase enzyme loading.
(Zeng and Ladisch, 2007)
Therefore, ideally, chemicals used for pretratments have the ability to participate in lignin fragmentation and/or prevent lignin from re-condensation/re-adsorption.
Wednesday, October 22, 2008
- Renewable biomass supply
- Genetic modification of biomass to massively produce energy crops or biomass?
- Cost effective pretreatment technology
- Cost effective, large scale available stable microorganisms
- Effective and low cost product recovery system
- Consumers' preference
Tuesday, October 21, 2008
Sunday, October 19, 2008
After 3 hours enzymatic hydrolysis
After 168 hours enzymatic hydrolysis
(Ref. Zeng and Ladisch, 2007)
Obviously, transport of chemicals or enzymes into the fiber walls is regulated by the sizable pores. It has been found that there is a linear relationship between the initial hydrolyzability of a ligonocellulosic substrate and its accessibility to a molecule of nominal diameter 51 Å (Grethlein et al.,1984 ). However, CBH I (cellobiohydrolase I) from Trichoderma reesei is tadpole shaped proteins with total length of 180 Å and a dimension of the catalytic core: 50x60x40 Å (Divine et al., 1993, 1994; Esterbauer et al., 1991). To initiate and promote its penetration, it requires sort of opened channels at the very beginning. That is why we need pretreatments of biomass before any enzymatic hydrolysis.
Saturday, October 18, 2008
Lignin is one of the major chemical components in plant cell wall. It is an amorphous, cross-linked, 3-dimensional branched polymer (Fig.1) with C9- phenylpropane unit (Fig.2.). The biosynthesis of lignin stems from the polymerization of three types of phenylpropane units as monolignols: coniferyl, sinapyl,and p-coumaryl alcohols (Fig.3). Softwood lignin is composed mainly of coniferyl alcohol units, while hardwood lignin is composed mainly of coniferyl and sinapyl alcohol units.
Fig. 3. Three building blocks of lignin
The three building blocks are connected through C-O-C and C-C linkages to form the 3-D structure, which has the strength to support plants. The exact structure of protolignin is unknown. But the improvements in methods for identifying lignin-degradation products and advancements in spectroscopic methods have enabled scientists to elucidate the predominant structural features of lignin. Fig. 4 depicts some of the common link ages found in soft wood lignin.
Fig. 4. Common linkages in softwood
Th e typical abundance of these types of linkages in softwoods are shown in Fig.5
Fig. 5. Proportions of different types of linkages connecting the phenylpropane units in softwood lignin.
In addition, lignin blocks are also linked with some sugar unit to form Lignin-Carbohydrate Complex (LCC) (Fig. 6). So, lignin also functions like glue to stick different cell structures together.
Fig. 6. Typical LCCs
Because of these structural and chemical characteristics, lignin is the most recalcitrant component of the plant cell wall. In general, the higher the proportion of lignin, the lower the bioavailability of the substrate. Th effect of virgin lignin, redeposited lignin after pretreatment, and LCC on the bioavailability of other cell-wall components is thought to play a large role in the physical restriction mechanism. This require some pretreatments to partially delignify to remove the barrier and open cell wall channels so that hydrolysis chemicals or enzymes can penetrate inside the cell wall more easilly and fast, accordingly speed up the transport and overall reaction rate.
Friday, October 17, 2008
After cellulose, the next major polysaccharide resource is plant hemicelluloses. Unlike cellulose, hemicellulose is a collection of short branched polymers with lower DP values (i.e., typically 50–300), frequently have side chain groups and are essentially amorphous. The types and amoumt of hemicellulose are also species dependent. For example, typical softwood hemicelluloses are galactoglucomannan (Mannans), arabinoglucuronoxylan, arabinogalactan, and pectins; The dominant hemicellulose in softwood are glucomannan; Hardwood hemicelluloses are mainely glucuronxylan (xylans) and glucomannan. The dominant hemicellulose in hardwood is xylan. In spite of these, after hydrolysis, the monomer sugars from hemicellulose and cellulose are arabinose (C5),glucose (C6), mannose (C6), galactose (C6), xylose (C5) as well as some hexuronic acids: glucuronic acid, 4-methyl-glucuronic acid, and galacturonic acid.
Because the hemicellulose is Not crystalline in biomass, they are very accessible to chemicals and very reactive. As a result, they are easily extracted, hydrolyzed, and further degraded during the typical pretreatments of biomass. For example, during acid pretreatment, hemicellulose can be extracted by the cleavages of glycosidic linkages. Meanwhile, the ester and ether linkages are also cleaved to for acetic acid (from acetyl group) and methanol (from methyl group).
Fig. 2. Hemicellulose action under acidic conditions
One of the benefits for hemicellulose extraction is to open the cell wall pores to allow chemicals or enzymes to transport inside the cell wall more easily. On the other side, if not control very well, the monomer sugars produced will further degrade to form the by-products such as aldonic acids, furfural (or HMF), levulinic acid, and formic acid, etc. Both the furfural and weak acids are inhibitors of enzyme metabolism and should be minimized from the treatments.
Fig.3. Sugar degradation under acidic conditions
Wednesday, October 15, 2008
Fig. 2. Chemical composistion of some softwood species
Cellulose is one of the major but simplest chemical components of lignocellulosics, which is a linear polymer of D-glucose units linked by 1,4-beta-D glycosidic bonds with a degree of polymerization (DP) greater than 10,000 (Fig. 3).
Fig. 4. Hydrogen-hydrogen bonds (Sjostrom, E. 1993)
Fig. 5. Crystal and amorphous structure (Sjostrom, E. 1993)
There are currently strong incentives and expectation for increased use of renewable fuels in the transport sector worldwide. BIOETHANOL and BIODISEL are the two major products.
It is not difficult to find that nowadays the most successful technological route for bioethanol production is through fermentation. And currently fuel supplement bioethanol (5-10%) is mainly derived from food-based crop such as corn starch and sucrose. From Ethanol Producer Magazine (2007), almost all of 162 bioethanol producer listed are corn-based plants.
Obviously, from a long term, change is needed for the feedstock resources i.e. utilization of non-food renewable feedstocks. It has been agreed that more efficient biofuel systems are those based on lignocellulosics and novel conversion technologies to attain high level of renewable fuels with great availability, potential lower cost, and the avoidance of the “food or fuel” argument.
However, unlike corn or sugar-based feedstocks, lignocellulosic biomass has heterogeneity in many aspects such as cell wall and ultra-microstructure, chemical components and distribution in cell wall, reaction rate, etc, which leads to the complexity of the treatment on biomass in technology, process, equipment as well as the cost.
The typical plant biomass that can be served as this purpose are:
–Agricultural residues: corn stover, bagasse, wheat and rice straws etc
–Waste wood and forest trimmings
–Energy crops: switchgrass, willow, poplar etc
–Waste paper, paper mill sludge
Native Plant Cell Wall Structure: HeterogeneityTypically, plant cell walls have multi-layered structure that consist of the following three types of layers:
Middle lamella (ML): the first layer formed during cell division and makes up the outer wall of the cell and shared by adjacent cells. It is composed of pectic compounds and protein and has the highest lignin concentration
The combined middle lamella and primary wall is also called compound middle lamella (CML)
Secondary wall (S): formed after cell enlargement is completed. The secondary wall is extremely rigid and provides compression strength but it is often layered. It is made of cellulose, hemicellulose and lignin and has the highest cellulose and hemicellulose concentration.