Biochemical Conversion Process of Producing Bioethanol from Lignocellulosic Biomass

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1 International Journal of Microbial Resource Technology, Vol.1, No.1, Original research paper Abstract Biochemical Conversion Process of Producing Bioethanol from Lignocellulosic Biomass Seema Devi, Meenakshi Suhag, Anil Dhaka and Joginder Singh* Laboratory of Environmental Biotechnology A.I. Jat H.M. College, Rohtak , Haryana, INDIA. *Corresponding author: Accepted 16 November 2011, Available online 27 January, 2012 The use of bioethanol can reduce our dependence on fossil fuels, while at the same time decreasing net emissions of carbon dioxide, the main greenhouse gas. However, large-scale production of bioethanol is being increasingly criticized for its use of food sources as raw material. Bioethanol from cellulosic biomass materials (such as agricultural residues, trees, and grasses) is made by first using pretreatment and hydrolysis processes to extract sugars, followed by fermentation of the sugars. Brazil's bioethanol production consumes large quantities of sugar cane, while in the USA, corn is used. Other starch-rich grains, such as wheat and barley, are mostly used in Europe. The use of such sugar-rich biomass causes the escalation of food prices, owing to competition on the market. Therefore, future expansion of biofuel production must be increasingly based on bioethanol from lignocellulosic materials, such as agricultural byproducts, forest residues, industrial waste streams or energy crops. These feedstock s, which are being used in second-generation (2G) bioethanol production, are abundant, and their cost is lower than that of food crops. In Europe, wheat straw has the greatest potential of all agricultural residues because of its wide availability and low cost. Proper pretreatment methods can increase concentrations of fermentable sugars after enzymatic saccharification, thereby improving the efficiency of the whole process. Conversion of glucose as well as xylose to ethanol needs some new fermentation technologies, to make the whole process cost effective. Researchers are working to improve the efficiency and economics of the cellulosic bioethanol production process. In this review, available technologies for bioethanol production from agricultural wastes are discusses. Key words: Ethanol, Lignocellulosic Biomass, Cellulase, Pretreatment Technologies, Biofuels, Fermentation 1. Introduction Bioethanol is the most widely used biofuel today. Cellulosic biomass, a second generation biofuel feedstock, are one of the fastest growing feedstock for biofuels (especially ethanol), and present one of the most exciting possibilities as a future solution to our energy problems, especially that of transportation fuel (Balat et al., 2011). Plant biomass contains approximately 75% polysaccharides, a rich source of sugars. Production of ethanol from this biomass (which is no different from that produced from first generation processes) by fermentation is, however, significantly more complicated than its production from first generation feedstock s such as sugarcane/beet and starch crops such as wheat grain. According to the International Energy Agency, second-generation biofuels are not yet produced commercially, but a considerable number of pilot and demonstration plants have been announced or set up in recent years, with research activities taking place mainly in North America, Europe and a few emerging countries (e.g. Brazil, China, India and Thailand) (International Energy Agency, 2010). The terms first, second and third generation can be used in the contexts of both feedstock s and processes. For instance, corn, cane and maize represent first generation ethanol feedstocks, and fermentation represents first generation ethanol production process. Similarly, Switch grass is one of the popular second generation ethanol feedstocks, 28

2 while the production of cellulosic ethanol represents the second generation process for ethanol. Bioethanol from agricultural waste could be a promising technology though the process has several challenges and limitations such as biomass transport and handling, and efficient pretreatment methods for total delignification of lignocellulosics (Prasad et al., 2007). To efficiently utilize lignocellulosic products, pretreatment is required to hydrolyze the hemicelluloses to make the celluloses more accessible to the enzymes. 2. Biochemical Conversion Process 2.1 Pretreatment The purpose of the pretreatment step is to further increase the surface area of the lignocellulosic material, disrupt the structure of the lignocellulose such that the cellulose component is accessible to hydrolyzing agents and reduce the crystallinity of the cellulose to further facilitate hydrolysis. Depending on the nature of the pretreatment technology selected, this step can also include solubilization of the lignin or the hemicellulose component. Various pretreatment options are available now to fractionate, solubilize, hydrolyze and separate cellulose, hemicellulose, and lignin components. These include physical, physicochemical, chemical and biological pretreatment (Avira et al., 2010). Consequently, the pretreatment process represents a significant cost element of the whole lignocellulosic bioethanol process. 2.2 Feedstock Size Reduction Before pretreatment, the first stage in the production of ethanol from biomass is cleaning followed by mechanical comminution combines chipping, grinding, and milling to break the lignocellulosic materials down to 0.2 to 2 mm and reduce the crystallinity of the materials (Mosier et al., 2005). Size reduction is necessary to provide pumpable slurry and to increase the biomass surface area so that mass transfer effects are minimized during the downstream processes. Techniques for size reduction include hammer, disk and knife milling and are well established (Taherzadeh and Karimi, 2008). 2.3 Hydrolysis In the hydrolysis reaction, the complex chains of sugars that make up the hemi-cellulose are broken, releasing simple sugars. The complex hemi-cellulose sugars are converted to a mix of soluble five-carbon sugars, xylose and arabinose, and soluble six-carbon sugars, mannose and galactose. The rest of hemicelluloses are degraded to weak acids, furan derivates, and phenolics. These compounds, however, are potential fermentation inhibitors. By the action of dilute acids, concentrated acids, and/or enzymes (Cellulase), the glucose yields of cellulose hydrolysis often exceed 90%, but hydrolysis without preceding pretreatment yields typically less than 20% only (Sun and Cheng, 2002). The cellulose hydrolysis reactions can be simply represented as: (C 5 H 8 O 4 )n + n H 2 O n (C 5 H 10 O 5 ) (C 6 H 10 O 5 )n + n H 2 O n (C 6 H 12 O 6 ) The pretreated feedstock can be hydrolysed by two methods (Acid hydrolysis and Enzyme hydrolysis). 2.4 Acid hydrolysis In the past various acid hydrolysis technologies have been developed. The different acid hydrolysis technologies can be divided into two broad categories: i) hydrolysis with concentrated acid at low temperatures; ii) hydrolysis with dilute acid at high temperatures (Brodeur et al., 2011). Acid hydrolysis technologies have a long industrial history but lead to high operating costs and various environmental and corrosion problems. Dilute acid hydrolysis- The dilute acid process is conducted under high temperature and pressure and has a reaction time at a scale of up to minutes, facilitating continuous processing. Concentrated acid hydrolysis- The concentrated acid process uses relatively mild conditions, with a much longer reaction time. 2.5 Enzyme hydrolysis Another basic method of hydrolysis is enzymatic hydrolysis. Enzymes are naturally occurring plant proteins that cause certain chemical reactions to occur. Enzymatic hydrolysis is not commercialized yet but is recognized to be the most promising hydrolysis technology. A reduction of the cost of ethanol production can be achieved by reducing the cost of either the raw materials or the cellulase enzymes. Reducing the cost of cellulase enzyme production is a key issue in the enzymatic hydrolysis of lignocellulosic materials (Kuila et al., 2011). Enzymatic hydrolysis of cellulose is usually 29

3 carried out by cellulase enzymes. During hydrolysis, cellulose is degraded into the reducing sugars that can be fermented by yeasts or bacteria to ethanol (Duff and Murray, 1996; Carlos et al., 2009). 3. Fermentation Process The fermentation of, hydrolyzed product, glucose into ethanol can be carried out using a biocatalyst, called Saccharomyces cerevisiae yeast or Zymomonas mobilis bacteria. Saccharomyces cerevisiae and related species have the ability to utilize a wide range of hexoses such as glucose, fructose, sucrose, galactose, maltose and maltotriose to produce a high yield of ethanol. The fermenting of the biomass is conducted under standard fermenting conditions and will utilize all the major biomass (Mussatto and Teixeira, 2010). Yeasts are minute, often unicellular, fungi. The yeasts used are typically brewers' yeasts. Examples of yeast capable of fermenting the decaying biomass include, but are not limited to, Saccharomyces cerevisiae and Saccharomyces uvarum. Non-Sacharomyces yeasts, also known as non-conventional yeasts, are also used to make a number of commercial products. Some examples of non-conventional yeasts include Kuyberomyces lactis, Yarrowia lipolytica, Hansenula polymorpha and Pichia pastoris (Kuhad et al., 2010). Microorganisms other than yeast can also be useful in making fermentation products. For example, cellulosic ethanol production also utilizes fungi and bacteria. Examples of these cellulolytic fungi include Aspergillus niger, Trichoderma reesei, Trichoderma longibrachiatum and Trichoderma viride. One example of a bacteria used in cellulosic ethanol production is Clostridium thermocellum, Clostridium cellulovorans and Clostridium Ijungdahlii. Mid- to long-term technology under development are expected to improve the fermentation efficiency of the organism, producing higher yields in less time, and an organism requiring less detoxification of the hydrolysate. This process has the advantage of being able to maintain a much higher cell density in the fermentor, thereby increasing ethanol productivity (Begum and Alimon, 2011) 4. Distillation Separation of ethanol from the fermentation solution refers to the stage in which once ethanol begins to form during fermentation, it is isolated from the fermentation solution. The fermentation solution is likely to contain water, ethanol, and the remaining biomass. Distillation was one of the earliest separation techniques used by alchemists and pharmacists. And, generally, distillation, along with chromatography and filtration, is still considered to be a key method of separating and purifying substances. The separated ethanol, which will generally not be fuel-grade, can be concentrated to fuel grade (at least 95% ethanol by volume) via a second distillation (Wyman et al., 2005) A process for producing and recovering light alcohols, particularly ethanol, alcohol mixtures containing ethanol, and ABE mixtures (alcohol mixtures containing acetone, ethanol and butanol ), using a combination of steps including fermentation, first membrane separation, dephlegmation and dehydration by second membrane separation. 5. Ethanol Production Cost The cost of ethanol production and its value depends on plant location, feedstock, production scale, and end use. It also depends on the availability of feedstock, plant location, feedstock transportation cost, method of pretreatment, hydrolysis, fermentation techniques and ethanol market price. Currently, the most important feedstock for the production of ethanol is sugarcane juice in Brazil and corn in the USA. Other starch-rich grains, such as wheat and barley, are mostly used in Europe. Most of the ethanol produced from North America is from cereal grains such as corn and wheat. Improvements in pretreatment processes and breakthrough in enzyme technology will have an impact on the competitive industrial production of fuel ethanol in processes such as continuous process without or with cell recycling. Simultaneous saccharification and cofermentation, utilization of immobilized cells, etc. enable higher yield of ethanol and reactor productivity (Kosaric et al., 1981). Economic conversion of cellulosic biomass to ethanol will also reduce the production cost that can be achieved by making ethanol fermentation process faster, particularly xylose fermentation, reducing the formation of by-products and developing genetically engineered yeasts to produce various high-value byproducts (Ho et al.,1999; Mukherjee et al., 2010; Joshi et al., 2011). 6. Conclusion and Future Direction Ethanol costs could be reduced dramatically if efforts to produce ethanol from biomass are successful. Biomass materials, including forest residue, agricultural residue, energy crops, are abundant and relatively inexpensive, and they are expected to lower 30

4 the cost of producing ethanol and provide stability to supply and price. Biomass has to be fractionated into cellulose, hemicellulose and lignin and the integration of the process byproducts will lead to economically feasible production of ethanol (Badger, 2002). Enzymatic hydrolysis of cellulose appears to have the most potential for achieving the goals, but substantial reductions in the cost of producing cellulase enzymes and improvements in the fermentation of non-glucose sugars to ethanol still are needed. In order to maximize the ethanol yield from lignocellulosic feedstock s it is essentially required that the hemicellulose fraction must be utilized along with the cellulose in order to obtain an economically viable conversion technology. The efficient pretreatment/hydrolysis process for the recovery of maximum amount of fermentable sugars (hexose and pentose) with the minimum or no toxic chemicals is the major challenge and requires advance biotechnological approaches to conquer this problem (Singh et al., 2011).Cellulosic ethanol is a nascent field today, but has enormous potential for the future. The pay-offs are big, and so are the challenges. Researchers are working to improve the efficiency and economics of the cellulosic bioethanol production process. Companies that make an early start in this field stand a chance of reaping significant benefits in future. References Avira, P., Tomas-Pejeo, E. and Ballesteross, M.J. (2010). Pretreatment technologies for an efficient bioethanol production process based on enzymatic hydrolysis: a review. Bioresource Technol. 101(13): Badger, P.C. (2002). Ethanol from cellulose: A general review. Trends new crops new uses Begum, M.F and Alimon A.R. (2011). Bioconversion and saccharification of some lignocellulosic wastes by Aspergillus oryzae ITCC for fermentable sugar production. Elect. J. Biotechnol. 14( 5):1-9. Balat, M. (2011). Production of bioethanol from lignocellulosic materials via the biochemical pathways: a review, Energy Cons. Management. 52: Brodeur, G., Yau, E., Badal, K., Collier J, Ramachandran, K.B. and Ramakrishnan, S., (2011). Chemical and physicochemical pretreatment of lignocellulosic biomass: a Review. Enz. Research., Carlos, A., Cardona, Óscar, J., Sánchez. and Luis, F.G. (2009). Process synthesis for fuel ethanol production. Biotechnol Bioprocess Duff, S.J.B. and Murray, W.D (1996). Bioconversion of forest products industry waste cellulosics to fuel ethanol - a review. Bioresouce. Technol., 55: Gong, C.S., Cao, N.U. and Tsao, G.T. (1999). Ethanol production by renewable sources. Adv. Biochem. Eng. Biotechnol. 65: Ho, N.W.Y., Chen, Z., Brainard. and Sedlak, M (1999). Successful design and development of genetically engineered Saccharomyces yeasts for effective co fermentation of glucose and xylose from cellulosic biomass to fuel ethanol. In: Advances in Biochem. Engg. 65: International Energy Agency, Sustainable Production of Second Generation Bio-Fuels: Potential and Perspectives in Major Economies and Developing Countries Joshi, B., Bhatt, M.R, Sharma, D., Joshi, J., Malla, R. and Sreerama, L. (2011). Lignocellulosic ethanol production: Current practices and recent developments. Biotechnol. Mol. Biol. Review. 6(8): Kosaric, N., Duvnjak, Z. and Stewart, Z.Z. (1981). Fuel ethanol from biomass: production, economics and energy. Adv. Biochem. Engg., 20: Kuhad, R.C., Gupta, R., Khasa, Y.P. and Singh, A. (2010). Bioethanol production from Lantana camara (red sage): Pretreatment, saccharification and fermentation. Bioresource. Technol., 101: Kuila, A,, Mukhopadhyay, M., Tulia, T.K.. and Banerjee, R. (2011). Production of ethanol from lignocellulosics: An enzymatic overview.. Exclusive J. 10: Singh, L.K.., Chaudhary, G., Majumder, C.B. and Ghosh, S. (2011). Utilization of hemicellulosic fraction of lignocellulosic biomaterial for bioethanol production. Adv. App. Sci. Res.2 (5): Mosier, N., Wyman, C., Dale, B., Elander, R., Lee, Y.Y., Holtzapple, M. and Ladisch, M. (2005). Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresource. Technol., 96: Mukherjee, S., Das, P., Giri, S.R.B., Satpute, D., Chakrabarti, T. and Pandey, A. (2010). Commercializing lignocellulosic bioethanol: technology bottlenecks and possible remedies. Biofuels, Bioprod. Bioref., 4: Mussatto, S.I and Teixeira, J.A. (2010). Lignocellulose as raw material in fermentation 31

5 process. Curr. Res. Technol. Edu. Topics App. Microbial Biotechnol. A. Mendez-Vilas (Ed.)., Prasad, S., Singh, A. and Joshi, H.C. (2007). Ethanol as an alternative fuel from agricultural, industrial and urban residues.res. Conservation Recycling. 50: Sun, Y. and Cheng, J. (2002). Hydrolysis of lignocellulosic materials for ethanol production: a review. Bioresource. Technol. 83:1-11. Taherzadeh, M.J. and Karimi, K. (2008). Pretreatment of lignocellulosic wastes to improve ethanol and biogas production: A review. Int. J. Mol. Sci., 9: Wyman, C.E., Dale, B.E., Elander, R.T., Holtzapple, M., Ladisch, M.R., Lee, Y. (2005). Coordinated development of leading biomass pretreatment technologies. Bioresource. Technol. 96: