Potentials of postharvest rice crop residues as a source of biofuel

Potentials of postharvest rice crop residues as a source of biofuel

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Pratyush Kumar Das1, Bidyut Prava Das2 and Patitapaban Dash1 1 Centre for Biotechnology, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, India, 2Rama Devi Women’s University, Bhubaneswar, India

13.1

Introduction

The increase in the demand for energy requirement and the necessity for reducing the environmental waste loads make the situation complex and necessitate immediate action to mitigate these problems. Biofuel is a potential source of alternative energy, directly or as an additive, to reduce the load on the consumption of conventional sources of energy and to reduce the load of wastes generated in the environment due to anthropogenic activities. Production of biofuel from the biodegradable wastes and biomass having least commercial importance is a possible approach for curbing environmental degradation. The use of postharvest rice crop residues, a common example of lignocellulosic biomass, as a biofuel feedstock is encouraging on the basis of food security and keeping the land-use pattern undisturbed. The biofuel such as bioethanol is a product of carbohydrate fermentation. The blending of the unleaded gasoline with bioethanol has the possibility of increasing the engine performance in an automobile. Keeping the growing demand from the domestic sector, the biogas is one of the feasible options to fulfill this demand. In addition to fulfill the energy demands, it could be a better option for protecting the environment from the greenhouse gas emission.

13.2

Focus on rice productivity in India

Rice is the staple food for approximately more than half of the Indian population and forms major share of the total food grains produced in the country. In countries such as India, rice not only holds economic value but also possesses social and cultural significance. The unique feature of the rice crop is its ability to grow in diverse tropical environments, including rainfed lowlands of south and Southeast Asia (Das et al., 2009). Among the top five rice-producing countries in the world during 2005 09, India ranks second in the list after China. Table 13.1 provides the information about the top five rice-producing countries and their average production in million tonnes (mt) per annum during the period of 1980 2014 (Varma, 2017). The year 2018 showed significant changes in the production of rice by some of Refining Biomass Residues for Sustainable Energy and Bioproducts. DOI: https://doi.org/10.1016/B978-0-12-818996-2.00013-2 © 2020 Elsevier Inc. All rights reserved.

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Table 13.1 Average rice production by major rice-producing countries in mt/annum (1980 2014). Countries

Production in mt (approx.)

China India Indonesia Bangladesh Vietnam

180 120 50 35 28

these global leaders. The observed changes have been attributed to several climatic factors and reduction in the land area under cultivation. Table 13.2 provides the production figures of rice by the top five global leaders during 2018, along with real-time annual changes in the production quantity. It also focuses on changes in production estimated to occur in the year 2019 (Childs and Skorbiansky, 2018). Stagnant flooding suppresses or inhibits tillering and enhances lodging (Das et al., 2008). On an average, a person in India consumes around 68.2 kg of milled rice per year. During 2008 09, 42% of the country’s total food grains produced comprised rice only. India’s rice production shows an increase from about 115.4 mt in 1995 to near about 144 mt in the year 2010, with 2008 being the most productive year (GRiSP, 2013). The current decade witnessed a similar trend of upsurge in the production of rice in the country which increased to a high of 168.5 mt as reported by FAOSTAT for the year 2017 (FAOSTAT, 2019). The period was also marked with an improvement in the productivity from 3358.7 to 3848 kg/ha (Table 13.3).

13.3

Overview of postharvest rice crop residues and its conventional disposal

During the postharvest stage of rice cropping, a large amount of residues are produced that have an enormous potential to be used as a natural source for various applications. Statistical data during the period of 2003 13 reveal an enormous increase in the production of rice residues on a global level. Asia leads the queue with maximum residue production, followed by the United States in the North America (Fig. 13.1) (Cherubin et al., 2018). These large amounts of residues being generated are utilized in various ways in different countries and regions. Fig. 13.2 shows the cultivation of rice on Indian landmass and the various usages of the postharvest residues. The annual gross crop residue produced in India is approximately 371 mt of which rice crop residues constitutes up to 57% (Hayashi et al., 2014; Gupta, 2011). Moreover, 70% of the crop residues in India are generated by cereal crops which include wheat, rice, maize, and millets. Rice residues accounts for 34% and thus

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Table 13.2 Production statistics of top five global rice producers for the year 2018 and estimates for the year 2019. Countries

Production in mt (approx.)

Annual changes in mt

Projected annual changes for the year 2019 in mt

China India Indonesia Bangladesh Vietnam

145.98 110.00 37.00 32.65 28.94

1.03 3.21 0.14 2 1.92a 1.54

2 5.18a 2 2.91a 0.30 1.75 0.12

a

Indicates a decline in the production as compared to the previous year. Source: USDA, Foreign Agricultural Service. Production, Supply, & Distribution Online Data Base. USDA, Foreign Agricultural Service. ,https://www.fas.usda.gov/psdonline/psdHome.aspx. (last updated 10.09.18).

Table 13.3 Information on rice production in India (2010 17). Year

Harvested area (ha)

Yield (kg/ha)

Production (mt)

2010 2011 2012 2013 2014 2015 2016 2017

42,862,400 44,010,000 42,754,000 44,135,950 44,110,000 43,390,000 43,190,000 43,789,000

3358.7 3587.8 3690.9 3607.0 3563.8 3607.7 3790.2 3848.0

143.9 157.9 157.8 159.2 157.2 156.5 163.7 168.5

Figure 13.1 Production of postharvest rice residues by various regions of the world.

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Figure 13.2 Cultivation of rice and fate of postharvest rice crop residues: (A) rice crops under cultivation, (B) rice field after harvest, (C) open burning of rice residues on field, (D) collection of rice straws, (E) rice straws being used for thatching of houses, and (F) rice residues being used as a cattle feed.

form a major part of these residues (IARI, 2012; Bhattacharyya et al., 2015; Jain et al., 2014). Rice residue, one of the most generous agrarian biomass obtained after paddy cultivation is an essential part to follow up on (Shafie, 2016; Shiun, 2012). Open burning of rice residues is of common practice in Asia (Singh et al., 2008) and especially in India (Sarkar et al., 1999; Gupta et al., 2004). Such a practice of burning the postharvest rice crop residue is quite common in the north western states of India, especially in Punjab, Haryana, and Uttar Pradesh (Singh et al., 2008; Singh et al., 2011; Badarinath et al., 2008; Roy and Kaur, 2016). Satellite data reveal that about 12.68 million hectare of paddy area in Punjab and 2.08 million hectare of paddy area in Haryana are burnt each and every year (Yadav et al., 2014a,b). Out of 83% of crop residues generated in India in a year, approximately

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43% are rice residues, the major contributor, subsequently followed by wheat (21%) and sugarcane (19%) (Jain et al., 2014) (Fig. 13.3). In a similar fashion, the residues generated from cereal crops accounted for 58% of the total residues. The rice crop residues alone contributed 53% of the cereal crop residues followed by wheat residues (33%) and maize and millets each accounting for 7% of the total cereal residues. The disposal of agricultural residues by open burning is a common practice. Open burning of rice residues ranges in between 8% and 80% across various states of India (Gadde et al., 2009). The biomass of postharvest rice residues pose a great economic value and also offer to be an efficient source of energy. Energy harvested from these residues through various technological interventions can prove as a better alternative to the gradually depleting fossil fuels, thus providing an immediate solution to the increasing concentration of carbon dioxide in the environment (Brar et al., 2000; Lohan et al., 2012; Dhaliwal et al., 2011). Burning of the residues is not only a great economic loss but

Figure 13.3 Percentage of crop residues generated in India per annum.

Table 13.4 Various types of emissions resulting from the burning of rice crop residues and their percentage. Emissions due to burning of rice residues

Percentage emissions

Carbon in form of CO2 Carbon in form of CO Carbon in form of CH4 Nitrogen in form of NOx Nitrogen in form of N2O Sulfur in form of SOx

70 7 0.66 20 2.1 17

Source: Jain, N., Pathak, H., Bhatia, A., 2014. Sustainable management of crop residues in India. Curr. Adv. Agric. Sci. 6, 1 9.

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Table 13.5 Percentage of nutrients loss due to burning of rice crop residues. Nutrients

Percentage of loss due to burning

Carbon (C) Nitrogen (N) Phosphorous (P) Potassium (K) Sulfur (S)

Almost 100 80 90 25 20 50

also disturbs the environmental homeostasis. The burning of crop residues in situ is mainly responsible for the pollution of the environment. It has mainly been observed to emit high concentration of carbon, nitrogen, and sulfur in the form of greenhouse gases such as carbon dioxide, carbon monoxide, methane, nitric oxide, and nitrous oxide into the atmosphere (Table 13.4). The emission of greenhouse gases from the open burning of rice and wheat residues in Punjab during the month of May and October 2005 has been estimated by Badarinath et al. (2006). They suggested a lower emission from burning of wheat residues as compared to that of the rice. This further emphasizes the impact of burning of postharvest rice crop residues on the environment. Not only the gases but open burning of the rice residues also emits particulate matter into the atmosphere, thereby causing the culmination of smoke with fog (usually referred to as smog). This, in turn, causes low or almost no visibility, thus increasing the chances of road accidents (Kumar et al., 2015). Dense fog situations arising as a result of the pollution have a direct impact on vehicular journey time and the fuel consumption and costs (Sidhu et al., 2015). Rice residues offer to be a cheap source of nutrients and burning of the same results in loss of the nutrients present. The nutrients lost in the form of various gaseous and particulate matters (Raison, 1979; Ponnamperuma, 1984; Lefroy et al., 1994), thus resulting in pollution of the atmosphere, have been provided in Table 13.5. Open burning of postharvest rice crop residues results in the burnt ashes being left behind causing blackening of the soil. This holds a negative impact on the soil health. Though the residual ash is a suitable source of potassium and lowers soil acidity, but it also further leads to loss of other essential nutrients and organic carbon content of the soil (Singh et al., 2006; Jat et al., 2009; Mehta et al., 2013). The heat generated from the burning of the rice straw raises the soil temperature to a very high extent, thereby disturbing the soil carbon nitrogen equilibrium (Singh et al., 2010). The harmful pollutants arising from the burning of rice crop residues also have a negative impact on the human health. The pollutants are responsible for causing chronic diseases of the heart and lung ailments. They also cause respiratory issues such as asthma, coughing, thereby affecting the children and pregnant women in particular (Mittal et al., 2009; Long et al., 1998; Singh et al., 2011). Burning of rice crop residues relate to a higher concentration of benzene in the atmosphere. Studies have also revealed the chances of leukemia, aplastic anemia, vertigo, pancytopenia, nausea, headache, and drowsiness due to exposure to

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benzene (Chandra and Sinha, 2016; Duarte-Davidson et al., 2001). It also further leads to reduced red blood cell count in humans, thus deteriorating the oxygen carrying capacity of the blood. Animals are also severely affected. Animals have been found to suffer from corneal irritation and temporary blindness and chronic bronchitis. Pollution of the atmosphere due to rice crop residue burning is also responsible for decreased milk yield in animals. A higher exposure level even sometimes leads to the death of animals. The reason behind the deaths may be attributed to the transformation of hemoglobin to carboxyhemoglobin due to higher concentration of carbon dioxide and carbon monoxide in the animal’s blood (Dikshit and Singh, 2010). Crop friendly pests, essential microorganisms, and certain animals such as snakes, frogs, earthworms residing in the soil thus lose their lives due to the mass burning process (Kumar et al., 2015; Kaur and Rani, 2016). Plants are also greatly affected due to the open burning of rice residues in situ. Small and average-sized plants near the rice fields are also destroyed due to the burning process. Depletion in the number of plant species growing in such area contributes toward the loss of biodiversity of that particular area (Mehta et al., 2013). The rice crop residues are of high economic value; however, their improper disposal via conventional methods leads to various environmental and health complications. Alternatives for burning of rice crop residues must be given priority at the present scenario. Instead of wasting the residues by simply burning them off, efforts should be taken toward production of biofuels such as biogas and ethanol from the large masses of rice crop residues produced in a year during the postharvest period. The efficient and green production of biofuels from rice residues may reduce the dependency of humans on fossil fuels. This may further provide an efficient solution to the problem of fuel scarcity that the earth’s population is deemed to face in the decades to come.

13.4

Transformation of postharvest rice crop residues to biofuel

Biofuel production from renewable resources, mostly agricultural wastes, has been an important topic of interest. Wood and agricultural crop residues that contain lignocellulosic biomass are considered as a potential raw material for production of bioethanol and biodiesel (Kaparaju et al., 2009). Biofuels produced from these raw materials are a promising replacement for the fossil fuels that play a major role in environmental pollution. Rice straw is one of the postharvest residue of the rice crop and is an important renewable bioresource for the production of biofuels. It is suggested that around 205 billion liter of ethanol per year can be produced globally from rice straw accounting for approximately 5% of the total world’s consumption. Rice straws contain carbohydrates in the form of glucose, xylose, arabinose, mannose, and galactose (Roberto et al., 2003; Yoswathana et al., 2010). Fig. 13.4 shows the distribution of various carbohydrates present in the rice straws.

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Figure 13.4 Distribution of various carbohydrate forms in rice straw.

Biofuels can be produced from the cellulosic material of rice residues by converting them into fermentable sugars and then into the fuel product by application of certain microorganisms or enzymes. The availability of the carbohydrates in rice residues is mainly hindered by the presence of lignin in the plant cell wall and needs to be removed. Several pretreatment methods (Fan et al., 1982; Schultz et al., 1983; Hormeyer et al., 2009; Hahn-H¨agerdal et al., 2001; Bollok, 1999; Soni et al., 2010) have been developed to degrade lignin materials and for solubilization of hemicellulases resulting in fermentable sugars (Fig. 13.5). The response of rice straw varieties to various chemical pretreatments shows higher enzymatic digestibility as compared to that of wheat straw (Wu et al., 2013). It reveals that rice straws are a more feasible option to be used as a source of biofuel. Instead of burning them by conventional practices, the use of rice straw as a feedstock for the production of bioethanol would make it the world’s single largest feedstock for the production of second-generation ethanol (Harun et al., 2013; Karimi et al., 2016; Kim and Dale, 2004).

13.5

Biofuel as a renewable source of energy

The development of renewable energy technologies is essential to reduce the demand on conventional energy resources. Fossil fuels have played a major role toward advancement in the field of transportation and industrialization, as an energy source. Their high heating capability, combustion characteristics, widespread

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Figure 13.5 Different pretreatment methods for degradation of lignins and solubilization of hemicellulases in rice residues.

acceptance, and large availability had made them to govern the industrial scenario for more than a century since the invention of diesel engine in 1893 by Dr. Rudolph Diesel. However, with large-scale industrial development, growing populations, and increased demand, the fossil fuel reserves on the earth are gradually depleting and are on the brink of getting exhausted (Ghobadian et al., 2009). The current situation seems to be leading toward an energy crisis in the near future. Furthermore, climatic changes and environmental concerns linked to the burning of fossil fuels have prompted researchers and engineers to look for alternative energy sources such as the biofuels. Thus it can be said that climatic and environmental concerns along with energy security are the major driving forces to stimulate the worldwide development of biofuel along with the agro industries (Hassan and Kalam, 2013).

13.5.1 Feedstocks for biofuel production The two most commonly used biofuels are biodiesel and bioethanol mostly derived from vegetable oils, seeds, and lignocelluloses. Biodiesel can be used as a

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replacement for diesel and bioethanol can be used as supplement for petrol. Common biofuel feedstocks originating from lignocellulosic materials are derived from nonedible crops (Murphy and Kendall, 2015) and agricultural crop residues (Hadar, 2013). Other feedstocks such as algae (USEPA, 2011), corn (Elbehri et al., 2013), physic nut (Koh and Ghazi, 2011), palm (De Gorter et al., 2015), soybeans (Thoenes, 2007), sugarcane (De Gorter et al., 2015), sweet sorghum (Elbehri et al., 2013) are used to some extent. Postharvest rice crop residues are better in this context. Postharvest rice residues are renewable in nature and produced in successive cropping seasons. These are the products of transformation of trapped solar energy.

13.6

Overview of biofuel generation and consumption on a global scale: opportunities and challenges

The world has seen an upsurge in the development, production, and utilization of biofuels in the current century. This may be attributed to the interest generated among nations worldwide to increase the level of self-sufficiency in energy, reduce the costs incurred due to fuel imports, and to further strengthen the domestic agricultural development (Arau´jo, 2017; Kovarik, 2013). In order to control pollution from vehicular emissions and to maintain environmental sustainability, many regions have started to strategically focus on bio-based fuels for transportation purposes (REN21). It can be assumed that the transportation sector, which accounts for one-third of the global energy consumption, half of the global oil consumption, and one-fourth of the carbon dioxide emissions due to combustion of fossil fuels, is the major driving force behind the uprise in interest toward biofuels (IRENA, 2016; International Energy Agency (IEA), 2016).

13.6.1 Global biofuel scenario The global biofuel production for the year 2014 accounted for 127.7 billion liters out of which 74% included fuel ethanol. Fatty acid methyl ester and hydrotreated vegetable oil derived biodiesel accounted for only 23%. However, the growth in biodiesel production has been more than double in comparison to the ethanol fuel produced during the period of 2000 14. Table 13.6 provides the global production statistics of bioethanol and biodiesel for the year 2014. The leading nations in biofuel production in 2014 were the United States, Brazil, Germany, China, and Argentina (REN21). United States and Brazil are the two leading ethanol producers accounting for approximately 82% of total ethanol produced in 2014. However, the biodiesel production is somewhat evenly distributed among different countries and regions. According to the 2015 statistics, Brazil and United States lead the global biofuel scenario responsible for the production of approximately 70% of the world’s biofuel supply (REN21, 2015). Asian countries and the European Union are the emerging markets that have cropped up in the last two decades. The European

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Table 13.6 Global production statistics of bioethanol and biodiesel for the year 2014. Leading bioethanol producers

% Share of production in 2014

Leading biodiesel producers

% Share of production in 2014

United States Brazil Europe China Canada Argentina Thailand India Others

57 25 6 2 2 1 1 1 5

Europe United States Brazil Argentina Indonesia China Thailand Others

29 16 11 10 10 4 4 16

Union (Huenteler and Lee, 2015) and the Asian countries (Arau´jo, 2017) have their own respective choices of sources (Fig. 13.6) as far as the production of biodiesel is concerned.

13.6.2 The Indian biofuel potential There has been a steady growth (approximately 7%) in Indian economy since 2000 (EIA, 2013). This high economic growth is marked with a similar growth in energy demand. According to the report of International Energy Agency (IEA) (2014), the country’s major energy demand will double up by 2030. Oil is the second largest energy source after coal for India accounting for approximately 30.5% of the primary energy demand (BP, 2013). India accounts for 1% of global crude oil production and consumes approximately 3% of the world consumption (Energy Statistics Report (ESR), 2003). Being the fourth largest consumer and importer of crude oil and petroleum products, India’s petroleum demand went up to 3.7 million barrels per day in comparison to petroleum production of 1 million barrels per day (EIA, 2014). This is the reason behind India’s dependency on fuel imports and thus the rising fuel prices in the country. Therefore the need arises to generate alternative fuels such as the biofuels derived from renewable energy feedstock to make the country energy self-sufficient at time of situations such as energy crisis. India being an agriculture-based country, majority of the people are dependent on agricultural related activities, leading to huge amount of residues being produced, that can be used as a potential source for the production of biofuel. From 328 million hectare of land in India, about 43% are planted with crops. The net crop area has remained stable at 140 million hectare since 1970, but there has been a considerable increase in the gross crop area (which measures multiple crops grown per year). The crops are grown in two main seasons, namely, kharif and rabi. Rice and wheat dominate majority of the cropped area followed by others (Fig. 13.7) (MoA, 2012).

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Figure 13.6 Feedstock sources for production of biodiesel in European Union and Asian countries.

Figure 13.7 Distribution of cropped area in India. Source: MoA, 2012. Agricultural Statistics at a Glance. Ministry of Agriculture (MoA), New Delhi.

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Figure 13.8 Area under rice cultivation and production of rice for the year 2010/11 and estimates for the year 2020/21 and 2030/31.

Rice being one of the staple foods of India generates huge amount of residues postharvest and can act as a cheap source of raw material for the production of biofuel. The area under production of rice crop and production thereof for the year 2010 11 and estimates for the year 2020 21 and 2030 31 indicates toward possible increase in residue amount. This shows a promising future of biofuel production in India (Fig. 13.8). The government has started taking keen interest in the biofuel production in India. Considerable numbers of government and private organizations (notable among them are D1 Oil Plc, Reliance Industries Ltd., Godrej Agrovet, Emami Group, Aatmiya Biofuels Pvt. Ltd., Gujarat Oelo Chem Limited, Jain Irrigation System Ltd., Nova Bio Fuels Pvt. Ltd., Sagar Jatropha Oil Extractions Private Limited) are involved in the process of production and distribution of biofuels within the country. A mandate was made for blending 5% ethanol with gasoline in nine states of India in 2003 and has been subsequently enhanced to 20 states in the year 2006. The National Policy on Biofuels, 2010, even further approved 20% blending by the year 2017. The same policy also exempts biofuels from central taxes and duties, thus promoting the biofuel sector in the country (Shinoj et al., 2011). Biofuel has cropped up as a better and alternative solution to the depleting petroleum fuel and the related environmental concern on a global scale. However, just like many inventions, biofuels too have their own pros and cons (Fig. 13.9). Biofuels can be widely available as their production does not depend on extraction at specific sites unlike the fossil fuels. The quantity of emissions from biofuels in the form of carbon dioxide, carbon monoxide, sulfur dioxide, hydrocarbons, and particulate matter is very low as compared to the conventional diesel. Production of biofuels consumes less time and is by far renewable in nature. Biofuels especially

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Figure 13.9 Pros and cons possessed by biofuels.

the biodiesel has higher combustion efficiency, better lubricating qualities, thus reducing the need for maintenance and prolonging engine life (Silitonga et al., 2011; Atabani et al., 2012; Mofijur et al., 2012). Despite having a huge number of benefits, there are many key issues or challenges linked to the biofuel prospect and needs to be addressed at the earliest. There has been a rising competition for crop land between the biofuel-producing crops and the food crops. Increased production of biofuels have led to global increase in the agricultural prices (High Level Panel of Experts of Food Security and Nutrition (HLPE), 2013; Thompson, 2012; Oladosu and Msangi, 2013; Tomei and Helliwell, 2016). Claims regarding emissions are also a major topic of debate. There are certain claims that the net greenhouse gases emission from biofuels can be even more harmful than those by gasoline in the terms of climate effects (Searchinger et al., 2008; Yang and Chen, 2013; Kahn Ribeiro et al., 2012). Biofuel production has been found to indirectly affect the water availability. Seventy percent of the global freshwater is used for agricultural purpose (Fischer et al., 2002). Biofuel production that requires growing biofuel crops will lead to overuse of the already scarce water resources and shortage for use in growing food crops. Growing more amounts of fuel crops directly relates to large-scale use of fertilizers to satisfy the needs of growing population. This may lead to leaching of chemical fertilizers to nearby water bodies and into the ground water, thereby polluting them (Solomon and Bailis, 2014; National Research Council (NRC), 2008). Large pieces of land area are required for cultivating the biofuel crops. This can only be made possible by clearing more and more land and forest areas, thus disrupting the natural habitats for a large number of species (Cowie et al., 2016). Moreover, the biofuels especially the biodiesels are 11 17 times more viscous than the normal diesel

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and leads to problems in pumping, combustion, and atomization in diesel engines. The biodiesels are also not suitable for prolonged storage as it leads to degradation of the fuel components. Biodiesel is found to deposit carbon on engine parts and also leads to excessive wear of the engine, thus requiring the need to fabricate new suitable engines (Hassan and Kalam, 2013).

13.7

Operating conditions for biofuel generation from postharvest rice crop residues

The currently increasing energy demands and the depleting fossil fuel reserves have brought about an increased interest in renewable fuels or the biofuels. Wastes generated from agricultural activities and municipal wastes are being used to produce liquid and gaseous biofuels (Behera et al., 2015). Postharvest rice residues can act as a potential source for the production of biofuel (Fig. 13.10). The residues are rich in lignocellulosic biomasses that are converted to ethanol by various steps such as physical and chemical treatment at first followed by enzyme catalyzed hydrolysis of sugar polymers, fermentation of the sugars into ethanol, and finally distillation of the crude ethanol (Cheng and Timilsina, 2011). On-field production of rice grains in million metric tonnes per annum is correlated with the production of huge amount of rice straw on the cultivable land assigned for rice production. The rice straw is used as a cattle feed, but the high silica and lignin contents reduces its use as a cattle feed. The production of million metric tonnes of postharvest rice crop residues such as rice straw per annum is more than sufficient as has been required as cattle feed. The biomass of surplus amount of rice straw could be used as a promising source of feedstock for biofuel production. The postharvest lignocellulosic rice crop residues are rich in cellulose or hemicellulose. The produced biomass of postharvest rice crop residues such as straw is more than the biomass of the rice grains harvested (Kadam et al., 2000). The high cellulose content of rice straw is a possible factor for its consideration as a feedstock for the production of biofuel. It contains about 390 g of cellulose per kilogram of rice straw biomass (Karimi et al., 2006).

13.7.1 The biofuel production process 13.7.1.1 Lignocellulosic biomass Lignocellulosic biomass accounts for a major portion of the terrestrial biomass present on the earth surface. Approximately 64% of this biomass is derived from nontree plant species such as grasses (Umezawa, 2018; Takeda et al., 2019). The lignocellulosic biomass constitutes the postharvest rice crop residues, such as the straw, which is used as feedstock for biofuel production. The major ingredients of lignocellulosic biomass are cellulose (33% 51%), hemicellulose (19% 34%), and lignin (20% 30%) (van Maris et al., 2006). The rice straw biomass consists of

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Figure 13.10 Utilization of postharvest rice crop residues for production of biofuels.

cellulose (36.20%), hemicellulose (19%), and lignin (9.90%) (Nigam et al., 2009). The cellulose and hemicellulose are polysaccharides, and lignin is a polyphenolic polymer.

13.7.1.2 Pretreatment These polymers of cellulose, hemicellulose, and lignins start to degrade at temperature above 180 C. The pretreatment is essential for the separation of lignin and hemicellulose. Pretreatment is a prerequisite for hydrolysis as it increases the efficiency of enzymatic hydrolysis (Chen et al., 2007). The biological pretreatment of lignocellulosic biomass by Phanerochaete chrysosporium is essential to enhance enzymatic hydrolysis (Zeng et al., 2011).

13.7.1.3 Hydrolysis The hydrolysis of the pretreated lignocellulosic biomass is required for the conversion of polymers such as cellulose, hemicellulose and lignin into monomers. With hydrolysis, the lignocelluloses are converted to monomers such as reducing sugars. The produced monomers include pentose and hexose sugars such as xylose, arabinose, mannose, galactose, glucose, and aromatics alcohols. The cellulose

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enzymes mediated catalysis takes place during hydrolysis. The presence of Humicola sp. during the process of cellulose hydrolysis is promising (Kumar et al., 2008). The use of fungal species especially the white rot fungi has been found to be most effective in the generation of biogas from lignocellulosic residue (Take et al., 2006).

13.7.2 Production of bioethanol 13.7.2.1 Fermentation The fermentation is required for the bioconversion of monomers such as reducing sugars to ethanol. Fermentation of hydrolysis products requires optimal operating conditions. The bacterial species such as Bacillus polymyxa (Singh and Mishra, 1993), Klebsiella aerogenes (Ingram et al., 1998), Clostridium thermocellum (Herrero and Gomez, 1980), Escherichia coli (Yomano et al., 1998) are possible species to be involved during the production of bioethanol from the products of hydrolysis by fermentation. The fungal species such as Saccharomyces cerevisiae (Kuhad et al., 2010) and Pichia stipitis (Gupta et al., 2009) are promising species for production of bioethanol.

13.7.2.2 Distillation and production of bioethanol Distillation is essential for separation of bioethanol from crude products on the basis of its physical properties. It is one of the most predominant forms of purification technique of ethanol being followed by the industries worldwide. During production of bioethanol, a distillation tower separates water from ethanol in an effective manner. Water is removed from the bottom of the tower, whereas the ethanol is collected from the top (Onuki et al., 2008). The product of distillation is an azeotropic mixture of ethanol with insignificant amount of water. The vaporization of ethanol at a low temperature of 78 C helps it during separation from crude products. On the basis of the utility and quality requirement of ethanol, the water could be separated from the azeotropic mixture by azeotropic or extractive distillation (Kumar et al., 2010).

13.7.3 Production of biogas The lignocellulosic biomass from rice residues can also be used to produce biogas. After the enzymatic hydrolysis the treated biomass can also be directly subjected to the process of acidification followed by methanogenesis, respectively. The posthydrolysis processes requires the interaction between different types of microorganisms (Kopsahelis et al., 2018; Sun et al., 2019).

13.7.3.1 Acidification Acidification also known as biological acidification is a vital step in the production of biogas. Higher rates of acidification will lead to higher methane production and

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could be achieved by enhancing the intensity of biomass pretreatment (Sun et al., 2019). The process of acidification comprises two different phases—the acidogenic and the acetogenic. In the acidogenic phase, acidogens classes of microbes convert the hydrolyzed simpler molecules, such as amino acids, sugars, peptides, and others into volatile fatty acids. This phase results in the production of alcohols and organic acids (Gerardi, 2003). The acetogenic microbial populations reduce hydrogen and carbon dioxide to acetic acid. The organic acids and alcohols produced in the previous step are also converted to acetate in this phase, which again acts as a substrate for the methaneproducing microorganisms in the methanogenic phase.

13.7.3.2 Methanogenesis The methanogenic group of microorganisms generates methane by a process wherein carbon dioxide is used as a terminal electron acceptor (Cheremisinoff, 1997). This category of organisms includes bacteria such as Methanosarcina barkeri, Methanothrix soehngenii, and Methanococcus mazei (Weiland, 2010). Methane is produced in this phase via anaerobic digestion that occurs through two different ways—acetoclastic methanogenesis and hydrogenic methanogenesis. The acetoclastic methanogenesis converts acetic acid to methane, while carbon dioxide is converted to methane in association with hydrogen by the hydrogenic methanogenesis (Chen et al., 2008).

13.8

Socioeconomic aspects of biofuel generation from postharvest rice crop residues

The rice crop productivity and generation of rice straw as biofuel feedstock are mostly confined to kharif crop (June July to November December) and rabi crop (January February to April May) (MoA, 2012). After productivity and harvest of rice grains, the quick disposal of rice straw is essential for making the land available for the next agricultural production. The storage of rice straw demands a place to be protected from contact with humid and moist conditions due to its biodegradable nature. Rice straw is produced in bulk amount in all the rice-growing countries of the world. Storage of bulk amount of rice straw for a long span is difficult on the part of the marginal farmers. The agricultural residues such as postharvest rice crop residues are renewable hydrocarbon sources, produced in million tonnes per annum (Claassen et al., 1999). Rice straw has the potential to be treated as a biofuel feedstock for its hydrocarbon constituents having many glycosidic bonds. Rice straw gains its strength as a biofuel feedstock, because of its cellulose, hemicellulose and lignin contents. Burning or biodegradation of rice straw in the open agricultural fields leads to the emission of hydrocarbons, suspended particulate matters, greenhouse gases such as oxides of

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carbon and nitrogen to the open environment (Jain et al., 2014). To protect the human health the quantity of the toxic element should be limited in the components of the environment such as air, water, and soil (Das et al., 2017). All these changes may be the factors behind trapping of heat energy nearer to the earth’s surface, unwanted changes in the quality of the environmental components, significant changes in the climate, and loss of biodiversity (Mehta et al., 2013). Ultimately, it degrades the environment and in addition, causes the loss of energy potentials of rice straw. In addition, the burning of rice straw also has the possibility of nutrient loss from the soil, contamination of the soil with pollutants, and subsequently soil degradation. Contamination of the soil is a complex and burning problem and ultimately needs decontamination (Das et al., 2018). The open challenge in front of us is to reduce the environmental degradation and to reduce the loss of energy stored in rice straw so as to increase the energy pool of the society. The digestion of the rice straw inside the bioreactor reduces the release of harmful digested products and by-products to our environment and subsequently, the possible correlated environmental pollution. In addition, the products and the by-products formed during the digestion of the rice straw feedstock inside the bioreactors could be stored and consumed as biofuel at the time of need for domestic, agricultural, and small-scale industrial applications. Let the common questions arise from the members of the society regarding the use of rice straw as a biofuel feedstock. It gives an opportunity to consider the quantity and quality of rice straw before its application as a biofuel feedstock. It is supported by the fact that rice straw has low nutritional value and that too produced in billion tonnes per annum. Besides, it also reduces the competition between the edible crops and the energy crops and increases the land-use efficiency during the agricultural production process. It is strongly advocating in favor of use of rice straw as a biofuel feedstock. The pilot project experiments in relation to the use of rice straw as a biofuel feedstock is essential for standardization of the process and characterization of the product. The marginal farmers in developing countries such as India are not financially strong. They are not in a position to face the failure of the field-scale application. The pilot project paves the way for success of the field-scale application and minimizes the risk associated with the acceptance of the field-scale applications, socially and economically. The field-scale application of rice straw as biofuel feedstock is a way for possible improvisation of the economic status of the farmers. In addition to the value of crops, they could be able to get as bonus, the price of the generated biofuel. The possible constraints from the economic basis of consideration are costs incurred for transport of rice straw from agricultural fields to the site of bioreactors to generate biofuel. The minimum the expenditure incurred, better the possibility of acceptance by the farmers concerned. The limited use of conventional sources of energy due to the application of biofuel, reduces the expenditure incurred by the farmers for agricultural production purposes and makes the process further economical. To make this field-scale application socially acceptable, awareness program is playing a key role. The involvement of nongovernment organizations and extension

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officers minimizes the communication gap between the government and the persons involved at the field-scale application level. The farmers below the poverty line may not be encouraged to accept the operation of the bioreactor to generate biofuel using rice straw as feedstock. The funding for the experiments and application of pilot-scale and field-scale projects may make this program to be acceptable in the society.

13.9

Conclusion

To meet the requirement of rice for the growing world population by the mid of this century, the most important aspect is to increase the efficiency of agricultural fields under rice cropping. To make this, we should not spare more lands for energy cropping process. To make the energy pool sustainable, it is highly essential to reduce the use of conventional sources of energy and to meet that demand, we have to promote the use of nonconventional sources of energy. One of such nonconventional sources of energy is biofuel generated from rice straw, using it as a feedstock in bioreactor. The use of rice straw as a feedstock for biofuel is a possible way to improve the management of rice straw on the agricultural fields under rice cropping. Besides increasing the production efficiency of agricultural fields under rice cropping, it has the potential to reduce environmental pollution and to increase the energy pool. The second generation biofuel production from the postharvest rice crop residues increases the effective conversion of waste to energy and reduces the volume of incremental waste load on the earth surface. It may be able to restrict the emission of greenhouse gases to the atmosphere. It is better in comparison to the first generation biofuel, as it is not disturbing the available land-use practices. The use of postharvest rice crop residues as a feedstock for biofuel production is promising under the changing demand to generation ratio of energy resources in our environment. The availability of postharvest rice crop residues as a feedstock is essential for the successful generation of bioethanol as well as the biogas.

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Further reading Singh, C.P., Panigrahy, S., 2011. Characterisation of residue burning from agricultural system in India using space based observations. J. Indian Soc. Remote Sens. 39 (3), 423 429. Renewable Energy Network 21 (REN21), 2016. Global Status Report. REN21, Paris, France.