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With rising demand for alternative fuels, ethanol is emerging as a good option for some good reasons more importantly, it can reduce pollution. The major source for production of ethanol is conversion of biomass by fermentation. Conventionally, ethanol is made from fermentation of Sugarcane or corn. However they are not economical compared with costs of fossil fuels. The recent development is production of ethanol from cellulose due to abundance of the later. However, due to numerous problems associated with separation of cellulose, and also due to high cost involved in hydrolysis, the production is yet to be commercially employed.
The Source materials may include wood waste, crop residues and even some grasses. Ethanol is made from the cell wall components like cellulose, hemicelluloses and lignin. Breakdown of sugar from corn is much easier than the breakdown of sugar from cellulose making the later more complex. Currently the production employes two methods; Biochemical and Thermochemical
Biochemical involves involves Size reduction of raw materials, pretreatment by hydrolysis making use of either dilute/ concentrated acid or by making use of enzymes for conversion of Cellulose to glucose and then fermentation of glucose and pentose by yeast and/or bacteria, and then recovery of ethanol by dehydration.
Thermochemical involves heat and chemicals breakdown cellulose to syngas. The gas obtained can be converted into ethanol thourgh pyrolysis. The advantage of thermochemical process is over conversion of lignin which makes up one third of cellulosic feed stock. The process involves drying, conversion of feed stock to syngas, conversion to liquid by pyrolysis, removal of contaminants and distillation to remove water from ethanol.
Sources of Cellulose
The major sugar feed stock is sugarcane. The other biomass feed stocks rich in sugar includes sorghum, beet, and fruits. Even though it is cheapest to make ethanol from sugar, the sources are within the human food chain and may adversely affect the chain if extensive production to come into existence.
Starch Feed stocks
Another major source for ethanol is starch feedstock. The long chain of glucose molecules in starch can be easily broken down and the starch feed stocks includes maize, wheat, potato and cassava. The Starchy materials will be hydrolyzed with water and heat for breakdown of starch into fermentable sugar.
While both the above sources are within the human food chain thus becoming expensive, this alternative feed stock is the most abundant one. These cellulosic feed stocks comprises of lignin, hemicelluloses, and cellulose. Lignin which provides structural support poses major problem in production of ethanol fro cellulose. The lignin encloses both cellulose and hemicelluloses and extensive pretreatment processes were needed to reach the cellulose and hemicelluloses. Grasses have least lignin and trees have the highest.
Compared with starch, the cellulose has long chains of glucose molecules with a different structural configuration. Hyrolysis is made difficult by these different structural configuration along with the encapsulation of lignin.
The other component hemicelluloses have same long chain of glucose molecules but with an additional component pentose.
Lignin problem in Ethanol production
The major problem with the production of ethanol is lignin. It is Î²-glucosidases that breakdowns the cellulose to sugar. And lignin are known to inhibit Î²-glucosidases. Another major problem is that lignin encloses the cellulose and hemicelluloses and act as a barrier preventing the contact between enzyme and cellulose there by inhibiting the conversion. Enzymes will have non polar cellulose domains which involves in hydrolysis of cellulose. In a recent study, it was found the lignin was able to bind with those non polar domains in the enzymes(7).
Ethanol production process:
Pretreatment is the preparation of cellulosic feed stock for hydrolysis. The main purpose of the pretreatment is to disrupt the natural bonding between cellulose, hemi-cellulose & lignin, decrease crystallinity nature & complex structure. In a study by singh, Delignification process increases the yield of reducing sugars. So any prior process done before hydrolysis to reduce lignin is considered to be pretreatment.
Pretreatment methods classified on physical, chemical, physicochemical and biological.
Physical pretreatment: Physical treatments include size reduction (milling, shredding, mulching) and pyrolysis.
Milling: One such physical pretreatment process is milling i.e., reducing particle size. Particle size is one the key factor for the sugar conversion ratio. Particle size is indirectly proportional to the sugar conversion rate. This is due to the fact that, smaller size particles have higher surface area that in turn helps in higher reaction rate. Singh postulated that particle size to less than 417 microns does not improve the cellulose conversion. A. E. Abasaeed & Y. Y. Lee found from their research that increasing the hardwood cellulose particle size, decreases the glucose yields and increases the reaction time at which maximum yield occurs, using dilute acid hydrolysis.
Pyrolysis: At higher temperature, Cellulose breakdowns and we get gaseous and char products. Under lower temperature pyrolysis, in presence of mild acid (1N H2SO4, 97 degree celcius and 2.5 hours), the pretreatment results in 80-85% conversion of cellulose to reducing sugars. This process is enhanced in presence of limited oxygen and zinc chloride. (Yu and Zhang 2003).
Chemical pretreatment: Chemical pretreatment includes addition of chemicals, which reduces the shielding effect of lignin, reducing crystallinity and increases the cellulose swelling. Major classification includes Ozonolysis, Oxidative delignification, organosolv process and there are number of minor classification.
Ozonolysis: The chemical compound, ozone is used to degrade lignin and hemicellulose. Degradation of lignin results in higher hydrolysis rate. Main advantages of ozonolysis: effectively removes lignin, does not require elevated environmental conditions for the process to take place and it does not produce toxic or inhibitory materials. However, a large quantity of ozone is required, which makes the process expensive.
Oxidative delignification: Peroxidase enzyme present in the plant tissue biodegrades lignin in presence of H202. This pretreatment is found to be working good with sugarcane bagasse. Fifty percent of lignin is solubilized by 2%H2O2 at 30 degree Celsius and 8 hr. (Ye Sun et al. 2002)
Organosolv process: An organic solvent mixture with inorganic catalysts (Hcl & H2SO4) is used to breakdown the linkage between lignin and carbohydrate. Organic solvent used are methanol, ethanol, acetone, ethylene, glycol, etc.. At the end of the process, solvents need to be removed to avoid the inhibitory action on further process.
Physico-chemical pretreatment: These are the new pretreatments found in the last two decades. This type uses the combination of both physical parameters and chemical characteristics.
Steam explosion: Biomass is exposed to high-pressure saturated steam for a certain period and then its pressure is swiftly reduced (260 degree Celsius at 0.69 Mpa). Steam acts on complexity structure of lignocelluloses and causes hemicellulose and lignin transformation. Studies shows that lower temperature, long time process is better when compared to the vice versa. Residence time, temperature and particle size are affecting factors in the process. Addition of H2SO4 improves the hydrolysis rate, decreases inhibitory products and obtains complete removal of hemicellulose.
Advantages of steam explosion: Low energy requirement, no recycling and effective on agricultural residues. The limitations of this pretreatment are that certain inhibitory compounds will be produced by incomplete lignin disruption. Also large quantity of water is needed to remove inhibitory compounds.
Ammonia fiber explosion (AFEX): Similar to steam explosion. Lignocelluloses is subjected to liquid ammonia at reduced temperature (90 degree Celsius) and pressure for a period of time (30mins) and then the pressure is swiftly reduced. Dosage of liquid ammonia is 1-2 kg ammonia/kg dry biomass. AFEX works better with low lignin content and smaller particle size biomass. The disadvantage is that Ammonia has to be recovered for economic issues
Carbondioxide explosion: The explosion of CO2 would form carbonic acid and increase the hydrolysis rate. This process obtains 75% of theoretical glucose, which is relatively low when compared with other two methods.
Irradiation:. Irradiation like electron beam, microwave, gamma irradiation, ultraviolet irradiations is used as the source of irradiation. Acid or Alkali, in small quantities (1-5%), is added to the substrate and then exposed to irradiation. Azyma found that irradiations help in disintegrating the complex structure of Lignocelluloses (Azuma et al., 1984).
Biological pretreatment: Microorganisms such as Brown-rot, whit-rot and soft rot fungi are used to degrade lignin and hemicellulose. Brown-rot can degrade cellulose, where as white and red-rot degrades cellulose and lignin. Biggest advantages of this biological treatment are that it is environmental friendly and requires very less energy input. However, the rate of hydrolysis in most biological process is very low. (A.I. Hatakka. 1984)
One of the major limitations of biomass transportation is its low density. The density ranges from 60-80 kg/m3 for agricultural straws. Due to this, it occupies high volume making the biomass difficult for storage, transportation, utilization and handling. Density increases over 10 times after densification. Baling, pelletization, extrusion and briquetting are the four main types of densification process done on agricultural straw. Baling is a field type densification process, where all other densification processes are industrial type. Pelletizing and briquetting are commonly found in biomass solid fuel industries, often called as 'binderless technologies', which uses either piston press or a screw press.
Hydrolysis is often defined as the chemical reaction type in which polymers of holocelluloses breakdown into monomers. Hydrolysis produces reducing sugars from helocelluloses, which is comprised of cellulose and hemicellulose. Hydrolysis involves exposure of chemicals, enzymes for a period of time at a specific temperature.
Hydrolysis is the chemical reaction which involves conversion of complex cell wall polysaccharides in the feedstocks into simpler sugar for further fermentation into ethanol. Acids and enzymes were used in ethanol production to catalyze this production process.
Two common types of hydrolysis are Acid hydrolysis by either dilute or concentrated acid and Enzyme hydrolysis.
Acid Hydrolysis: Sulphuric acid and Hydrochloric acid are the powerful agents for Acid Hydrolysis. In-between these two, sulphuric acid is predominant, as it was found to be better hydrolytic agent than Hcl. (A.Singh et al. 1984). In general, acid hydrolysis requires either dilute Acid at higher temperature & pressure, or concentrated acid at low temperature. Concentrated acid hydrolysis yield high monomers than dilute acid hydrolysis. Xylan to xylose conversion will be done in dilute acid hydrolysis. After this type of hydrolysis, the feedstock needs to be neutralized.
Enzymatic Hydrolysis: This method of hydrolysis a a recently developed method came into existence by 1970's, while the former one is being used since 19th century.
Enzymatic treatments are preferred to the chemical ones. Cellulase enzyme, synthesized from fungi, bacteria and plants, is the key role for the reaction. Also enzymes are naturally occluding in the palnt proteins. However, pretreatment is ver y much essential for enzyme hydrolysis to break the crystalline lignin so the enzyme can interact with cellulose and hemicelluloses. Glucose concentration is one of the hydrolysis rate-limiting factors in enzymatic hydrolysis. (Lynd et al. 2002).
Fermentation the breakdown of sugar obtained from hydrolysis process into ethanol by action upon microorganism.
Types of fermentation
Separate hydrolysis & Fermentation
Simultaneous Saccharification & Fermentation
Direct Microbial Conversion (Using thermophilic Bacteria)
Seperated Hydrolysis and fermentation: Pretreatment and fermentation are conducted separately and glucose concentration is one of the main hurdles for the process.
Simultaneous sacarification and fermentation (SSF): This process requires less enzyme load and the contamination is reduced. This method is comparatively faster and provide higher yield. Gauss et al., (1976) says that SSF process is already patented and is known as the Gulf SSF process.
Direct Microbial conversion: It combines all the three main processes in lignocelluloses bioconversion (Cellulase production, cellulose hydrolysis and fermentation). However, it has less hydrolysis rate than SSF or SHF. The organism that has been most investigated for DMC method is C. thermocellum. (Kiyoharu F. et al. 1996)
Removing unwanted compounds and improving the ethanol percentage in the final product is called purification. Distillation process is employed for purification. Purification is done in two steps: Rectification, which achieves 96% and dehydration that achieves 99.9%.
Alternatives - Swtich Grass
The problem with the current cellulosic ethanol production is due to lignin. To overcome the problem of lignin, switch grass has been tested over recent years due to its low levels of lignin and high levels of cellulose. Panicum Virgatum (Switch grass) can produce more 700 % energy than what it is supplied with. (16) The perennial plant consumes low amount of water and it wont compete for food with other crops. The important advantage of switch grass is that the 10 % of the genome is dedicated to cell wall and so by genetic modification it is further possible to increase the expression of carbohydrates and ultimately high ethanol yield. (18)