Bio-ethanol as alternatives source of energy
Bioethanol is obtained from plant biomass based waste materials or renewable sources. It can be used as a fuel, chemical feedstock, and solvent in various industries. It has certain advantages as petroleum substitutes, alcohol can be produced from a number of renewable resources, alcohol as fuel burns cleaner than petroleum this aspect is environmentally more acceptable. It is biodegradable and thus, keeps a check on pollution and it is far less toxic than fossil fuels (Domínguez et al., 2014). Ethanol is a clear, colorless liquid with a characteristic, agreeable odor. In dilute aqueous solution, it has somewhat sweet flavor, but in more concentrated solutions it has a burning taste. Ethanol, CH3CH2OH, is an alcohol, a group of chemical compounds whose molecules contain a hydroxyl group, -OH, bonded to a carbon atom. The word alcohol derives from Arabic al-kuhul, which denotes a fine powder of antimony used as an eye makeup. Alcohol originally referred to any fine powder, but medieval alchemists later applied the term to the refined products of distillation, and this led to the current usage (Wondale Mekonnen, 2012).
Bioethanol production from biomass
Feed-stocks are typically grouped under the heading biomass and include agricultural residues,wood, municipal solid waste and dedicated energy crops (Kim and Dale, 2004). Biomass is seen as an interesting energy source for several reasons, the main reason being the contribution provided by bioenergy to sustainable development (Sanchez and Cardona, 2008). Resources are often available locally, and conversion into secondary energy carriers is feasible without high capital investments (Kunz, 2008). Different feedstock types are available for the production of bioethanol as it can be derived from any biological raw materials that contain sugar, such as sugarcane, sugar beets and potatoes, or materials that can be converted into sugar from starch or cellulose, such as corn, wheat and other cereals (Yacob Gebreyohannes, 2013). Ethanol may be obtained from the agricultural products, produced from food and biomass processing industries that are divided into three groups: raw sugar e.g. sugar beet and sugar cane, raw starch e.g. corn, rye, and triticale and lignocellulose materials such as grass, wood (Gumienna et al., 2015)
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Starch constituent plants
Starch containing crops form an important constituent of the human diet and a large proportion of the food consumed by the world’s population originates from them. In spite of the large number of plants able to produce starch, only a few plants are important for industrial starch processing. The major industrial sources are maize, tapioca, potato,and wheat (Belum et al., 2006). Amongst various starchy materials available throughout the world; corn cob, wheat straw, potato waste, sweet sorghum stalk and purified starch have been successfully utilized for the commercial production of bioethanol (Pervez et al., 2014).
The important of ethanol in biofuel
One method to reduce air pollution is to oxygenated fuel for vehicles. Methyl tert-butyl ether (MTBE) is a group of chemicals commonly known as fuel oxygenates (Fischer et al., 2005). It is a fuel additive to raise the octane number. In addition, it is water-soluble and human carcinogenic. Other oxygenated substances to increase the octane number of the fuel should substitute it. Presently, ethanol as an oxygeneous biomass fuel is considered as a predominant alternative to MTBE for its biodegradable, low toxicity, persistence and regenerates characteristic (Cassada et al ., 2000).
The United States gasoline supply is an ethanol blend and the importance of ethanol use to expected to increase as more health issues are relate to air quality. Ethanol may be produced from many high-energy crops such as sweet sorghum, corn, wheat, barley, sugar cane, sugar beet, cassava, sweet potato etc (Drapcho et al., 2008). Like most biofuel crops, sweet sorghum has the potential to reduce carbon emissions. High contents of carbon dioxide in the atmosphere that are contributing towards global warming were caused by the use of fossil fuels around the world (Blas et al., 2000). India, Europe, Brazil and the United States have resorted to the production of bio-fuels from plants is an attempt to reduce the carbon dioxide levels in the atmosphere (Kim and Dale, 2004).
Bio-ethanol produced from agricultural crops or plants is a clean burning renewable fuel that is used as a substitute fuel for road transport applications (Sanchez and Cardona, 2008). In 2009, worldwide ethanol fuel production reached about 19.5 billion gallon. The market for bio-ethanol road fuel is growing so rapidly that demand is starting to exceed supply. There is an urgent need for alternative plant species that can produce large volumes of biomass for conversion into cost effective bio ethanol on a large industrial scale. Plants used for ethanol production include maize, sugar beet, sugarcane, sweet sorghum and cassava. Food security is a very serious issue in terms of using by food crops for energy production. Crops that were choose for future energy production should thus be able to produce both food and energy (Prasad et al., 2007).
Renewable energy attracts attention for the protection of the environment and supplies our energy needs by reducing dependence on petroleum and non-renewable energy sources. Bio- ethanol, which is one of the energy sources, is known to be a potential alternative to petroleum-derived fuels and has the potential to meet the increasing demand for energy for industrial processes, heating and transportation (Balat et al.,2008). Advanced bio-fuels (may include ethanol derived from cellulose, sugar/starch, or waste material, including crop residue, other vegetative waste material, animal waste, and food waste) production. Petroleum import by approximately $23 billion in 2016 according to the report for “U.S (Fessehaie, 2009).
Energy in all its forms is essential to humanity and is central to the improvement in people’s quality of life. The continuous increase in energy demand, the inevitable decline in the availability of fossil fuels, and the growing concerns about climate change have sparked a number of initiatives from governments around the world to in-crease production of energy from renewable sources. Bio-ethanol obtained from crops or lignocellulose biomass, are getting a lot of attention as a possible option for renewable transportation fuel. Countries with tropical weather condition, such as Brazil, have successfully utilized sugarcane (Saccharum officinarum) for decades as the main feedstock to produce ethanol (Quintero et al., 2008).
However, some studies showed a low power density value. Due to the low cost of sugarcane, other countries in Africa, Latin America, and Asia have plans to increase their production of ethanol from sugarcane. In the United States and Europe, ethanol is mainly produced from corn and grain (Ferguson, 2008). Other starchy crops being utilized are sorghum (Sorghum sp) grain, cassava (Manihot esculenta) and potatoes (Solanium tuberosum) (Liimataine, 2007). However, there is currently a substantial amount of research being done concerning the development of cellulosic bioethanol (Milken et al., 2007), but the process for producing it is not yet at a commercial level. Rapeseed (Brassica sp.), sunflower (Helianthus annus), sugar beet (Beta vulgaris varsaccarifera), wheat (Triticum vulgaris), and potatoes have been considered as potential feedstock for the production of biofuel.
Bioethanol Production and Use Conditions in Ethiopia
Ethanol production in Ethiopia is linked with sugar factories and aimed for import substitute of petroleum products, enhance agricultural development and agro processing, job creation, and export earnings. However, only a small fraction of the potentials is utilized yet and an alternate 5% and 10% ethanol blend has accessed in the capital city of the country. Therefore, in this section a brief look into the history, status, and future targets of bioethanol development program along with the potential, opportunities, and risks of the ongoing and planned development projects in the country are discussed (Fessehaie, 2009).
History and status of bio-ethanol in Ethiopia
Ethiopia started modern sugar industry in 1951 at Wonji sugar factory as a share company founded by Ethiopian government and foreign investors followed by Shoa and Metehara sugar factories in 1962 and 1969 respectively. Five years later, in 1974, all sugar factories became in charge of the government ownership following the change of government and started operating under the Ethiopian Sugar Corporation which has embarked upon alcohol production program from molasses for use as additive to gasoline in 1979 (Fessehaie, 2009)
When the sugar corporation was dissolved by law in 1992, all the factories were re-established as public enterprises and thus Ethiopian Sugar Industry Support Center came into existence in 1998 to provide common support to all factories as a share company of Ethiopian Insurance Corporation, the Development Bank of Ethiopia, and the three sugar factories. Fincha sugar factory was established in 1999, as a third sugar development project in the country. The same year, a technical committee was established consisting of the Ethiopian Sugar Industry Support Center, Fincha sugar factory, Ethiopian Petroleum Enterprise, and Oil distributing companies in Ethiopia to address issues intended for successful commercialization of ethanol fuel (Kassa Mekonnen, 2007).
Microorganisms use for fermentation
Microorganisms involved in fermentation process of bioethanol production, have the ability to use glucose in the absence of oxygen for their energy, producing ethanol and carbon dioxide [50, 51]. This microorganisms property makes them vital role in fermentation technology from the beginning of its history. Some single cell microorganism was used in sugar fermentation. Such as yeast, is one of the oldest practices in biotechnology processing in case of energy and alcohol production. Anaerobic respiration this yeast, widely used for the production of drinking alcohol, namely, beer and wine, in the past time, while, nowadays, this practice is industrially used to produce fuel ethanol from renewable energy sources [52]. Major characteristics of ethanologenic microorganisms to be employed in industrial plants are higher ethanol yield (>90.0% theoretical yield), tolerance to ethanol (>40.0 g/L), good ethanol productivity (>1.0 g/L/h). Also, have good growing rate, simple and inexpensive media, capability of growth in undiluted fermentation broth with resistance to inhibitors, and ability to retard contaminants from growth condition, for example, acidic pH or higher temperature [53].
Microorganisms studied for ethanol production from sugar content feestock were including ; bakery yeast or Saccharomyces cerevisiae [54–57], S. diastaticus [58],Kluyveromyces marxianus [59, 60], Pichia kudriavzevii [26], Escherichia coli strain KO11 and Klebsiella oxytocastrain P2 [61], and Zymomonas mobilis [62–65] have been ability to convert glucose into ethanol . Among these ethanol producing microorganisms, S. cerevisiae is the most attractive choice in fermentation due to its greater efficiency in sugar conversion to alcohol and capability of producing flocs during growth, making it easier to settle or suspend on need [52], and high tolerance to ethanol [66]. The optimum temperature range of S. cerevisiae used for ethanol production is 30–35°C that leads the researchers to search for thermotolerant microorganisms. According to Dhaliwal et al. [26] isolated a strain of thermotolerant yeast (P. kudriavzevii) from sugarcane juice and adapted the cells to galactose that produced more ethanol than the non-adapted cells at 40°C.
Some strains of Saccharomyces cerevisiae are able to grow and produce ethanol at 42-440c (Sree et al., 2000; Edgardo et al., 2008). Further increasing in temperature and pH reduces the percentage of ethanol production and it is mainly due to denaturation of yeast cell, while an excellent temperature for maximum productivity occurs at 320C for maximum strain. It is therefore, necessary to select temperature the optimum temperature at the yeast strain can ferment sugar (Yah et al., 2010). … from LEMAYEHU JOURNAL
Factors Affecting fermentation process
Temperature
As reported that ethanol production depends on fermentation temperature and to some extent its concentration increases with the increase in temperature [99].
However, high temperature is considered as a stress factor for microorganisms and unfavorable for their growth. Microorganisms produce heat-shock proteins in response to the high temperature, which is inactivate their ribosomes. Moreover, microorganisms used in the fermentation process have optimum temperature range for their better growth. Therefore, it is necessary to predetermine an optimum temperature during fermentation for proper microbial growth as well as higher yield of ethanol. It is generally believed that the ideal fermentation temperature range is between 20 and 35°C and high temperature in almost all fermentation processes creates problem [101, 102]. The optimum fermentation temperature for free cells of S. cerevisiae is near 30°C [101, 103].
Effect of ph on ethanol production
PH is one of the key factors for ethanol production, having direct influence on organisms activity and reduce yield of ethanol production through fermentation [108, 109]. In general, H+ concentrations in fermentation broth can change the total charge of plasma membrane affecting the permeability of some essential nutrients into the cells. The optimum pH range for S. cerevisiae used in fermentation for ethanol production is 4.0–5.0 [105, 110]. However, very recently, studied was reported that this well-known yeast could produce ethanol from date juices even at pH 3.8 [111], though the critical pH for this organism is 2.3 [108]. Another, study also reported that the highest ethanol yield was obtained using Z. mobilis adjusting the pH range of the broth as 5.0–6.0 [112]. Different optimum pH range was also reported for several feedstocks such as 2.8 to 3.4 for sugarcane juice [113] and 4.0 to 4.5 for sucrose.
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