Sustainable bio-ethanol production from agro-residues: A review
Introduction
Due to increasing population, the demand of energy is increasing throughout the world. Currently, the primary source of energy is the fossil fuel and non-renewable sources such as natural gas, oil and coal. These have been used for the production of fuel, electricity and others goods [1]. It has been proposed that such resources would be depleted rapidly near future. The extreme consumption of fossil fuels, especially in large urban areas, has caused more pollution due to release of green house gases (GHGs) during the last few decades. The concentration of GHGs in the biosphere has hugely increased [2]. For subsistence of one׳s on the earth must require energy, is the most important part for human beings for their growth and development. And it has been deduced that about 13-fold energy consumption increased in 20th century, which is faster than that of increasing population [3], [4], [5], [6]. The other interference concludes that about one-quarter of world׳s population do not access a fraction of energy [7]. We are consuming both renewable as well as non-renewable energies and due to the overconsumption and exploitation of non-renewable energy resources, eventually, all petroleum reserves will be completely depleted; therefore, people are approaching towards the use of renewable source of energy. Consequently, overconsumption of non-renewable energy sources scaling up the price of oil and exacerbating our environment. According to World Energy Council petroleum, natural gas and coal (non-renewable energy sources), which are the good source of energy, collectively contribute nearly 82% of global energy needs and one fifth of the CO2 emission is due to 60% of petroleum based fossils fuel [8]. Hence to reduce the dependency on these resources a considerable promising shift is needed to utilize the alternative, sustainable as well as renewable sources of energy such as solar, wind, water, biomass and geothermal heat for the energy industry. The chemical industry may depend on biomass as an alternative source in the near future [6]. About 80% world׳s wind energy is produced by California in the form of Electricity and it has been accounted that Denmark, the world׳s second largest producer of wind energy, gained 2% of its power through wind turbine in 1990 [9]. Alternative sources of energy are being used in various countries. Biomass like cellulosic agricultural waste is the most abundant biomass on the earth. Using biomass like cellulosic agricultural waste is the potential promising natural renewable, inexpensive, cost effective and sustainable sources used for considerable and commercial production of bio-energy as bio-ethanol. The renewable fuels such as bio-diesel and bio-hydrogen, derived from sugarcane, corn, switchgrass, algae, etc., can be used as petroleum-based fuels in the future as fossil fuels are going to depleted soon due to higher energy consumption.
The limited amount of such alternative energy sources leading us looking for sustainable energy sources i.e., bio-energy. The concept of bio-energy came by dint of pervasive overexploitation of fossils fuel and alternative resources. Bio-energy is the renewable source of energy using natural resources for the production of sustainable bio-fuels. Bemdes et al. [10] estimated that the potential global bio-energy supply range from less than 100 to over 400 EJ/year for 2050 [11]. Biofuel includes solid, liquid and gas and the major biofuels encompass bio-ethanol, biodiesel, biogas, bio-methanol, bio-syngas (CO+H2), bio-oil, bio-char, bio-hydrogen, Fischer–Tropsch liquids petroleum, and vegetable oil, out of which bio-ethanol and biodiesel are liquid transportation fuel, used as an additive source. Bio-ethanol is a gasoline alternate while biodiesel is a diesel alternate to reduce the GHGs emission when blended as an additive. Bio-ethanol produced about 60% from sugarcane and 40% from other crops, while biodiesel from inedible vegetable oil, waste oil and grease and it was estimated that in 2007 about 60 billion liters bio-fuels produced globally [12]. It has been accounted that bio-ethanol could sink about 90% CO2 and 60–80% SO2 when blend with 95% gasoline [13], [14]. It has also been observed that bio-ethanol is being produced by various biomasses, which are naturally available on the earth. Biomass (bestows just about 14% of world׳s energy), is the fourth largest source of energy after petroleum, coal and natural gas [15].
Countries across the globe have well thought-out and directed state policies toward the improved and cost-effective utilization of biomass for summit their future energy demands in order to meet carbon dioxide decline targets as specified in the Kyoto Protocol as well as to reduce reliance and dependence on the supply of fossil fuels [16]. Since biomass can be used as a huge source for bio-ethanol production, it is generally used to produce both power and heat, usually during combustion. Recently, ethanol is broadly used as liquid bio-fuel for motor vehicles [17], [18]. The significance of ethanol is higher due to various reasons such as global warming and climate change. Bio-ethanol production has been increasing widespread interest at the international, national and regional levels. The worldwide market for bio-ethanol production and demand has entered a phase of rapid, transitional growth. The focus toward renewable sources for power production in various countries of the world has been shifted due to depletion of crude oil reserves. Ethanol has prospective as an important substitute of gasoline in the transport fuel market. On the other hand, the cost of bio-ethanol production is higher as compared to fossil fuels.
The world bio-ethanol production in 2008 was 66.77 billion liters [19]. It has grown to 88.69 billion liters in 2013 and is expected to reach 90.38 billion liters in 2014 [20]. Brazil and the USA are the two major ethanol producing countries of 26.72% and 56.72%, respectively of the world production [19]. Huge scale production of ethanol bio-fuel is mainly depended on sucrose from sugarcane in Brazil or starch, mainly from corn, in USA. Presently, ethanol production depending up on corn, starch and sugar substances may not be popular due to their food and feed value. The price is a significant factor for large scale extension of bio-ethanol production. The green gold petroleum from lignocellulosic wastes avoids the existing struggle of food versus fuel caused by grain dependent bio-ethanol production [18]. Kim and Dale [21] reported that 442 billion liters of bio-ethanol can be produced from lignocellulosic biomass and that total crop residues and wasted crops can produce 491 billion liters of bio-ethanol per year, about 16 times higher than the actual world bio-ethanol production. The cellulosic materials are renewable, low cost and are available in large quantities. It includes crop residues, grasses, sawdust, wood chips, agro-waste etc. Many scientists and researchers have been working on ethanol production from lignocellulosics in the past two decades [16], [22], [23], [24], [25], [26], [27], [28]. Hence, bioethanol production could be the route to the effective utilization of agricultural residue and wastes. Rice straw, wheat straw, corn straw, cotton seed hair, seaweed, paper, pineapple leaf, Banana stem, Jatropha waste, Poplar aspen; Oil palm frond and sugarcane bagasse are the major agro-residue in terms of quantity of biomass available [21]. For bioethenol production from the cellulosic material of agro-residues, three processes like pre-treatment, enzyme hydrolysis and fermentation are required. The pre-treatment is the most important and initial process for separation of free cellulose from agro-residues. Second process of enzyme hydrolysis is also important, which has been done by efficient microbes that have ability to secrete cellulose enzyme [29], [30].This enzyme involves in hydrolysis of cellulose to glucose. Several microbial species of Clostridium, Cellulomonas, Thermonospora, Bacillus, Bacteriodes, Ruminococcus, Erwinia, Acetovibrio, Microbispora, Streptomyces are capable to produce cellulase enzyme. Many fungi such as Trichoderma, Penicillium, Fusarium, Phanerochaete, Humicola, Schizophillum sp. also have been reported for cellulase production [16], [31], [32]. After this, fermentation process is required for conversion of glucose to ethanol by microbes e.g. Saccharomyces cerevisiae, Escherichia coli, Zymomonas mobilis, Pachysolen tannophilus, Candida shehatae, Pichia stipitis, Candida brassicae, Mucor indicus etc. [28], [33], [34], [35], [36], [37]. These processes are required for sustainable bioethenol production from cellulosic material of agro-residues. The main aim of this review is to present a brief overview of the available and accessible technologies for bioethanol production using major cellulosic materials of agro-residue. The use of agro-residues for the production of bioethanol is environment friendly and socially acceptable technology and also reduces the green house gas emission [38].
Section snippets
International scenario
The ‘first oil crisis’ came during the post world war II in 1973 due to soaring demand of petroleum and scaling up burning of transportation bio-fuel more [39]. The World Watch Institute estimated that the world׳s oil consumption increased from 2004–2005 and demand increased by 5.3%, typically in China, US, Canada and UK. US, independently was the world׳s biggest polluter in 2005 and consumed about 140 billion gallons of transportation fuel and more than 308 million metric tons of carbon
Cellulosic material as agro-residues for bioethanol production
Globally many agro-residues have been used to produce bio-ethanol such as rice-straw, wheat straw, sugarcane bagasse, sugarcane tops, cotton stalk, soft bamboo, bamboo processing wastes and all are considered as abundantly available feed stocks [91], [103]. These agro-residues are also utilized as animal fodder, as domestic fuel. The consumption fraction of wheat straw, rice straw and corn straw is too low and varies according to geographical region [21]. Every year huge portion of agricultural
Pretreatment process
Pretreatment is necessary process to unwind cellulose from hemicelluloses and lignin in which cellulose is embedded [55] and make cellulose more susceptible for enzymatic hydrolysis [133]. It is most important and challenging process for the production of biofuel from the agroresudues. Lignocellulosic biomass is composed of three main constituents namely hemicellulose, lignin and cellulose. Pre-treatment methods refer to the solubilization and separation of one or more of these components of
Enzymatic hydrolysis
Enzyme hydrolysis is the critical step for bio-ethanol production where complex carbohydrates are converted to simple monomers. It requires less energy and mild environment conditions compared to acid hydrolysis [150]. In this process, cellulose enzyme is most important enzyme, which is naturally occurring in cellulolytic microbes e.g. Clostridium, Cellulomonas, Thermonospora, Bacillus, Bacteriodes, Ruminococcus, Erwinia, Acetovibrio, Microbispora, Streptomyces and other fungi such as
Fermentation and distillation
Both fermentation and distillation is very vital steps for bio-ethanol production. Many microorganisms have been recognized for fermentation of sugars. However, the industrial utilization of lignocelluloses for bio-ethanol production is delayed due to lack of ideal microorganisms, which can efficiently ferment both pentose and hexose sugars [28]. For a commercially viable ethanol production method, an effective microbe should have broad substrate utilization, high ethanol yield and productivity
Application of bio-ethanol as a bio-energy resource
Bio-energy, a carbon neutral technology is used to convert biomass into energy [127]. Ethanol is mostly used as fuel additives to cut down vehicles, which run on mixture of gasoline and up to 85% ethanol, are now available. Bio-energy encompasses three major domain of sustainable development i.e., economic, environment and social (Table 14) [12]. Bio-ethanol trims down Green house gases emission as well as air pollution, the global climate change and carbon dioxide upsurge [189]. Air pollution
Research gaps and challenges
Bio-energy stores assets of technical information, which requires sustainable as well as carbon-neutral source of energy so as to balance the appropriate maintenance of food supply for securing the higher demand of energy and to reduce the environmental threat. Hence, it would be promising step to choose the energy based as well as agro-waste crops having high sugar level and high cellulose yield [191] and higher biomass yield; therefore it will trim down the rivalry with food production and
Conclusion
Agro-residues biomass has been proposed to be one of the main renewable resources for cost-effectively attractive bio-ethanol production. The hypothetical ethanol yields from sugar and starch are superior compared to lignocelluloses agro-residues; however, these conventional sources are not enough for worldwide bio-ethanol production. In that aspect, agro-residues are renewable, less expensive and in large quantities available on earth crust. For the production of agro-residues, there is no
Acknowledgment
Authors thankful to our Director, Prof. A.S. Raghubanshi, Institute of Environment and Sustainable Development, BHU, Varanasi, to give recommendation and suggestion about research work in field of sustainable bioenergy production from agro-residues. Authors also thankful to Dr. Varenyam Achal, Associate Professor, School of Ecological and Environmental Sciences, East China Normal University, Shanghai, for language and grammatical editing of this manuscript.
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