Biofuels from agricultural wastes

CHAPTER

Biofuels from agricultural wastes

5

Lopa Pattanaik, Falguni Pattnaik, Devesh Kumar Saxena and Satya Narayan Naik Centre for Rural Development and Technology, Indian Institute of Technology Delhi, New Delhi, India

5.1 INTRODUCTION Fast depletion of fossil fuel is due to its excessive use, causing ecological degradation and environmental pollution. Therefore, researchers are focused on utilization of renewable energy. Among the various renewable energy sources (solar, wind, water, and nuclear energy), bio-based energy or biofuel has gained major importance due to its application as fuel and in other value-added chemicals (Li-Beisson and Peltier, 2013). Biofuel is the fuel derived from living organisms primarily from plants and microorganisms. Biofuel derived from natural sources could help in reducing greenhouse gases from the atmosphere and helping in maintaining the carbon balance in the environment (Naik et al., 2010). The two major categories of biofuels are primary and secondary biofuels. Primary biofuels are the unprocessed, natural source used for generation of electricity and heating. Some examples of primary biofuels are firewood, plants, forest materials, animal waste, and crop residue; whereas, secondary biofuels are derived from processed biomass and are further divided into three categories: first-, second-, and third-generation biofuels, primarily based on the feedstock used (Rodionova et al., 2017). First-generation biofuels are directly produced from food crops such as corn, sugarcane, soyabean, and rapeseed, which limits food production on account of its use of arable lands. Second-generation biofuels focuses on the lignocellulosic biomass and its wastes, and nonedible plants or their parts. The advantages of second-generation biofuel are that it is nonfood-based, comparatively cheap, and it utilizes waste biomass (Saini et al., 2015). Lignocellulosic biomass can directly be used as a primary fuel for burning and generation of heat and electricity. This can also be processed to produce different biofuels like bioethanol, biogas, biohydrogen, and biodiesel. The lignocellulosic biomass can be categorized under several groups, that is, whole plants (dedicated energy crops, perennial grasses, and aquatic plants), agricultural residues (leaves, stovers, straws, husk, pods, seeds, baggases, roots, cobs, and seed pods), agro-waste (solid cattle manure), forest biomass

Second and Third Generation of Feedstocks. DOI: https://doi.org/10.1016/B978-0-12-815162-4.00005-7 Copyright © 2019 Angelo Basile and Francesco Dalena. Published by Elsevier Inc. All rights reserved.

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(soft wood and hard wood), forest wastes (wood chips, slashes, branches from dead trees and forest thinning, sawdust, pruning residues), industrial wastes (chemical pulps and primary wastewater solids), and municipal solid wastes (food waste, newspaper, kraft paper, and sorted refuse) (Zabed et al., 2016). Among other lignocellulosic biomasses, utilization of agricultural residue for biofuel production is the current main research of interest throughout the world. It is reported that the global production of plant biomass is approximately 200 3 109 tons/year, where nearly 8 3 109 2 20 3 109 tons/year can be used for biofuel (Kuhad and Singh, 1993; Zabed et al., 2017). The estimated amount of agricultural wastes contribution toward global lignocellulosic biomass is 1:5 3 1011 tons annually (Gupta and Verma, 2015). Without further reutilization of it, these wastes are simply dumped, left on the field or incinerated, which creates further environmental pollution (Chandra et al., 2012; Saini et al., 2015). Therefore, utilizing these wastes as a resources for generation of biofuel is an environmentally friendly alternative. The other advantage of using agricultural wastes as a source of biofuel is reduction of dependency on woody biomass from forest, which results in reduction of deforestation. Again, the availability of crop residues is more consistent due to their short harvest period (Kim and Dale, 2004; Limayem and Ricke, 2012).

5.2 CLASSIFICATION OF AGRICULTURAL WASTES Agricultural wastes are the residues obtained from production and processing of agricultural products such as crops, fruits, vegetables, meat, poultry, and dairy products (Obi et al., 2016). Broadly, agricultural wastes can be divided into four categories, that is, crop residues, agro-industry wastes, livestock wastes, and fruit and vegetable wastes (FVWs) (Fig. 5.1).

FIGURE 5.1 Classification of agricultural wastes.

5.2 Classification of Agricultural Wastes

5.2.1 CROP RESIDUES The waste residues generated from direct agricultural production at the field level are mostly crop residues like leaves, stovers, straws, and seed pods, etc. Currently, the global annual estimated production of crop residues is 2802 million tons (Zabed et al., 2016). Agricultural residues obtained from crop residues are the most abundant and cheapest organic waste, which can be easily transformed into different value-added products. Globally, there are three major crop residues being used for bioethanol production, that is, rice straw, wheat straw, and corn stover. These crops are available throughout the year and a very small portion is utilized as fodder or biofuel production, with the rest burned and hence causing serious environmental problems. Rice straw is one of the most abundant and promising biomasses in the world, with a global production of 731 million tons/year, and with Asia being the major producer (Sarkar et al., 2012). In India, rice straw waste is expected to be 221.8 million tons/year by 2030 (Kumar et al., 2018). Wheat straw is the residue of harvested wheat with an estimated annual yield of 1 3 tons/acre (Kim and Dale, 2004). The global annual production of wheat straw is 354.34 million tons. Corn stover or corn straw is one of the most promising crop residues for lignocellulosic ethanol, with an estimated production rate of 4.0 tons/acre and global annual production of 128.02 million tons (Kim and Dale, 2004; Sarkar et al., 2012). The other crop residues obtained from barley, sorghum, and oats also contribute toward agricultural wastes.

5.2.2 AGRO-INDUSTRY WASTES The second category of agricultural waste includes agro-industry processing waste. This includes byproducts produced from food processing industries, such as vegetable and fruit peels, fruit pomace after extraction of juice, starch residue from starch-manufacturing industries, sugarcane bagasse, molasses from sugar manufacturing industries, deoiled seed cake from edible oil manufacturing industries, chicken skin, egg, meat, and animal fat from slaughterhouses and meat processing industries. Sugarcane bagasse is one of the major agro-industrial wasted obtained from sugar industries after extraction of juice. The global availability of sugarcane bagasse is 180.73 million tons (Saini et al., 2015). The waste produced from the palm oil industries, which is the world’s largest edible oil, is nearly 35.19 million tons from 85.84 million tons of fresh fruit palm bunches(Shuit et al., 2009; Sukiran et al., 2017). Other agro-industrial wastes include apple pomace, orange peel, and other fruit wastes obtain from fruit juice, cider, and other food processing units. The annual production of apples is 84,630,275 tons worldwide (Hijosa-Valsero et al., 2017). The production of apple pomace is 25% 30% of the total biomass (Dhillon et al., 2013). Other than that some nonfood-based agroindustries like deoiled seed cake obtained from nonedible oil plants like Jatropha curcas and Pongamia pinnata can also be considered as agro-industry wastes.

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5.2.3 LIVESTOCK WASTES The three major types of livestock wastes include liquid manure (urinary waste), solid manure (farmyard manure), and wastewater (collected from farms’ process water, silage juices, liquid manure, bedding, and disinfectants). The European agricultural sector annually produces 1500 million tons of animal manure. Of these, 1284 million tons are cattle manure and 295 million tons are pig manure (HolmNielsen et al., 2009). Untreated manure can cause major air and water pollution. The leaching of nutrient-rich (primarily nitrogen and phosphorous) liquid manure, wastewater, and pathogen contamination can cause surface water pollution. However, solid manure can cause air pollution by contributing towards greenhouse gases (releasing up to 18% CO2 and 37% CH4) (Holm-Nielsen et al., 2009).

5.2.4 FRUIT AND VEGETABLE WASTES Other than crop residues, FVWs include unprocessed fruits and vegetables like mango, pineapple, tomato, jack fruit, banana, and orange, etc. and are also an important part of agricultural wastes. The generation of FVWs from wholesale markets and food processing industries is huge. The landfill disposal of these organic wastes can be difficult and it is a major concern for environmental pollution due its highly perishable nature (Viswanath et al., 1992). Again, the large production of FVWs increases the operating cost of the markets (Scano et al., 2014). The wastes generated from fruit and vegetable processing, packing, distribution, and consumption in India, the Philippines, China, and the United States are approximately 1.81, 6.53, 32.0, and 15.0 million tons, respectively (Wadhwa and Bakshi, 2013). In India, the annual production of FVW is approximately 5.6 million tons and currently these wastes are disposed of by dumping on the outskirts of cities (Bouallagui et al., 2005).

5.3 COMPOSITION OF AGRICULTURAL WASTES In the context of renewable energy (biofuels), the physical and chemical profilings of the biomass are necessary to assess its potential as feedstock for conversion into biofuels. Biomass composition mainly includes proximate, ultimate, lignocellulose composition and biochemical analysis. Other than that, metal and mineral contents and the energy content of biomass also imply several route for biomass conversion. The proximate composition includes moisture, fixed carbon, volatile solid, and ash. For thermochemical conversion, the feedstock should have low moisture and ash content (Garcı´a et al., 2012). In contrast, the high fixed carbon and volatile matter represent the high energy and organic content of feedstock, respectively, and are suitable for biochemical conversion (Angelidaki and Sanders, 2004; Vassilev et al., 2010). The ultimate analysis represents the fuel efficacy of the feedstock (Singh et al., 2017). Again, the C/N ratio derived from

5.4 Routes for Production of Biofuels Using Agricultural Wastes

ultimate analysis also represents the suitability of the feedstock for biogas and biohydrogen production. In Table 5.1, the proximate and ultimate compositions of several agricultural wastes have been provided. Apart from the preliminary analysis, the compositional estimation is needed for designing the process of biofuel production. Compositional analysis mainly consists of the lignocellulosic content (cellulose, hemicellulose, and lignin), and biochemical content of feedstock (protein, total carbohydrates, lipids, etc.). From Table 5.2, it can be observed that most of the agricultural residue contains around 80 85% lignocellulosic content, which is cellulose, hemicellulose, and lignin. The higher cellulose and hemicellulose contents are responsible for the production of bioalcohols. The crop residues contain 30% 50% cellulose and 20% 38% hemicellulose. Whereas, the lignin content in crop varies from 7% 21%. The agro-processing wastes contain a broad range of cellulose (21% 45%), hemicellulose (15% 33%), and lignin (5% 24%), depending on the different sources of origin. Waste rice bran contains a low lignin content (5%) and has been proved to be a potential feedstock for bioethanol production. However, high-lignin-containing sugarcane bagasse needs to be pretreated for the removal of lignin prior to ethanol production (Sun and Cheng, 2002). The carbohydrate-rich substrate has been preferred for production of biohydrogen, biogas, and alcohols due to its easy degradation. The wastes generated from the industrial processing units contain carbohydrate contents ranging between 40% 85% of the total solid (TS). Similarly, the livestock wastes like cattle manure also contain a carbohydrate content of around 50% 60%, for which the livestock manure is the efficient feedstock for biogas production (Møller et al., 2004). Likewise, the protein and lipid contents are also essential to determine the biogas potential of different agricultural waste feedstocks (Angelidaki and Sanders, 2004).

5.4 ROUTES FOR PRODUCTION OF BIOFUELS USING AGRICULTURAL WASTES Agricultural wastes consist of lignocellulosic composition, which can be utilized for biofuel production either in biochemical or thermochemical conversion pathways (Fig. 5.2). Feedstocks containing high moisture (.30%), C/N ratio (,30). and rich in cellulose and hemicellulose content are preferred for biochemical conversion for biofuel production. Whereas, feedstocks containing moisture of ,30%, a C/N ratio of .30, and rich in lignin are preferred for thermochemical conversion and subsequent treatment for biofuel production. The biochemical route involves the use of different microorganisms and enzymes to break down the biomass into intermediates (sugars, amino acids, or short-chain fatty acids) for conversion into liquid or gaseous fuel, such as biogas, bioethanol, biobutanol, and biodiesel. The thermochemical route is direct and utilizes individually or a

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Table 5.1 Proximate and Ultimate Composition of Agricultural Wastes Ultimate Analysis (wt.%)

Feedstock

C

Proximate Analysis (wt.%)

H

O

N

S

Moisture

Fixed Carbon

Ash

Total Volatile Solid (VS)

Heating Value (MJ/kg)

References

Crop Residue Rice straw

34.0 41.5

4.6 6.7

32.8 41.2

0.2 0.8

0.1 0.2

4.2 6

14.5

8.2 16.0

71.6 92.8

14.5 15.5

Wheat straw

41.7 46.7

5.1 6.3

34.1 51.4

0.4 0.5

0.1 0.3

4.4 8.4

17.3

7.3 12.8

74.4 92.7

17 18.9

Corn stover

35.2 45.6

5.4 6.3

43.4 45.7

0.3 0.8

0.1 0.3

5.3 7.4

16.9

4.2 6.3

86.5 96.8

16.2 16.5

Barley straw

49.4

6.2

43.6

0.7

0.13

19.79

5.3 9.8

76.2

16.42

Oat straw

48.8

6.0

44.6

0.5

0.08

19.53

5.9

74.48

3.20 4.34

79 83.66

18.61 18.73

9.20

18.8

64.30

13.36

19.1

2.4

78.50

19.80

5.38

Kumar et al. (2018), Worasuwannarak et al. (2007) Bridgeman et al. (2008), Kumar et al. (2018) Kumar et al. (2018), Vassilev et al. (2010) Serrano et al. (2011), Vassilev et al. (2010) Mani et al. (2011), Vassilev et al. (2010)

Agro-Industrial Waste Sugarcane bagasse

45.48

5.96

45.21

0.15

16.07

Rice bran

38.69

5.40

0.67

0.21

7.70

Coffe husk

47.50

6.40

43.7

Channiwala & Parikh (2002), Tsai et al. (2006) Ji-Lu (2007), Lu et al. (2008) Suarez et al. (2000), Yin (2011)

Pongamia pinnata deoiled seed cake Jatropha curcas seed cake Olive deoiled cake

47.80

6.50

5.50

17.20

81 85.3

Chandra et al. (2012)

48.80

6.20

3.85

9.7

84 86.4

Chandra et al. (2012)

53.70

6.70

36.20

0.60

45.0 48.8

6.40 7.33

47.3 40.2

0.0 0.25

0.68 1.06

16.23

17.1

60.26

0.76

2.99

3.23

2.46

0.82

19.30

34.6

2.8

62.10

21.60

Demirbas and Ilten (2004), Yin (2011)

8.65 18.3

3.02 7.54

71.2 79.7

19.0

Omar et al. (2011)

Agro-Industrial Waste Oil palm empty fruit bunch Orange peel

67

Jekayinfa and Omisakin (2005)

Livestock Waste Cattle manure

29.00

38.6

45.1

50 72

Sweeten et al. (2003)

Table 5.2 Lignocellulosic and Biochemical Composition of Agricultural Wastes Lignocellulosic Composition (wt.%) Feed stocks

Cellulose

Hemicellulose

Lignin

Biochemical Composition (wt.%) Carbohydrates

Protein

Lipids

References

5.9

Banik and Nandi (2004), Vassilev et al. (2012), Watanabe et al. (1993), Worasuwannarak et al. (2007) Adapa et al. (2009), Ding et al. (2012), Vassilev et al. (2012) Kaliyan and Morey (2010), Li et al. (2015), Mani et al. (2006), Vassilev et al. (2012) Adapa et al. (2009), Aqsha et al. (2014), Summerell and Burgess (1989), Vassilev et al. (2012) Adapa et al. (2009), Aqsha et al. (2014), Vassilev et al. (2012)

Crop Residue Rice straw

30.3 52.3

19.8 31.6

7.2 12.8

Wheat straw Corn stover

32.9 44.5

37.8 33.2

8.5 22.3

31.3 49.4

21.1 26.2

3.1 8.8

Barley straw

29.2 48.6

35.8 29.7

Oat straw

37.6 44.8

3.48

5.34

3.6 8.7

0.7 1.3

6.7 21.7

3.62

1.91

23.3 33.4

12.9 21.8

5.34

1.65

43.6 45.8

31.3 33.5

18.1 22.9

Vassilev et al. (2012)

44.6

27.1

20.7

Kim and Day (2011)

39

31

4

7.9

Agro-Industrial Waste Sugarcane bagasse Sweet sorghoum bagasse Rice bran

23.58

14.6 15.4

16.1 23.8

Al-Shorgani et al. (2012), AbdulHamid and Luan (2000), Cara et al. (1992)

Coffee husk Jatropha curcas deoiled seed cake Olive deoiled cake Oil palm empty fruit bunch Apple pomace

24.5 43

7 29.7

9 23.7

58 85

8 11

0.5 3

56.31

17.47

23.91

Elshaarani et al. (2013)

22.0

18.2

50.0

Demirbas and Ilten (2004)

23.7 63

21.6 33

29.2 36.6

Omar et al. (2011)

21.22

14.75

18.50

55.86

Vassilev et al. (2012)

Hijosa-Valsero et al. (2017)

Agro-Industrial Waste Orange peel

Jekayinfa and Omisakin (2005), Joshi et al. (2015)

38 40

6.95

62.46

15.09

6.85

52.08

23.9

14.3

Livestock Waste Cattle manure Pig manure

32.7

24.5

42.8

Møller et al. (2004), Vassilev et al. (2012) Møller et al. (2004)

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CHAPTER 5 Biofuels from agricultural wastes

FIGURE 5.2 Routes for biofuel production.

combination of heat and chemicals for production of syngas (combination of H2 and CO), bio-oil, biochar, and biocoal. Although the thermochemical route utilizes a broad range of wastes, from the point of view of fossil fuel consumption and greenhouse gas emissions, the biochemical route is preferable to the thermochemical route (Mu et al., 2010). In this chapter we discuss the different biochemical routes for biofuel production.

5.5 BIOCHEMICAL ROUTES FOR BIOFUEL PRODUCTION 5.5.1 IMPORTANT STEPS IN BIOCHEMICAL ROUTES FOR BIOFUEL PRODUCTION 5.5.1.1 Pretreatment Pretreatment is one of the essential techniques for processing lignocellulosic biomass in biofuel production. The lignocellulosic biomass consists of three primary structural constituents, that is, cellulose, hemicellulose, and lignin, which remains as a compact matrix form, hindering the accessibility of microbes/enzymes for degradation and hydrolysis. Therefore, the pretreatment process helps to delignify the biomass, decomposes the hemicellulose, and increases the porosity of the biomass, which subsequently increases the surface area and decreases the crystallinity of cellulosic moiety (Sarkar et al., 2012). These aspects are useful in ascertaining the efficacy of any pretreatment process. The pretreatment methods can be broadly divided into four different categories, that is, physical, physiochemical, chemical, and biological (Fig. 5.3).

5.5.1.1.1 Physical pretreatment The preliminary step for biomass processing is reduction of biomass size into fine powder for enhancing the better accessibility of enzymes and microbes during the hydrolysis process. Various physical techniques, such as wet milling, dry milling, ball milling, and compression milling are used for comminution of lignocellulosic

5.5 Biochemical Routes for Biofuel Production

FIGURE. 5.3. Pretreatment methods for lignocellulosic biomass.

biomass. A reduction in size helps to decrease the crystallinity nature of cellulose, which is further manifested in terms of an increase in product yield (Sun and Cheng, 2002; Zabed et al., 2016). Other than size reduction, treatment of biomass by radiations, like microwave and ultrasonic irradiation, is also helpful in pretreating the biomass and is often considered for green techniques (Ethaib et al., 2015). In microwave irradiation, electromagnetic radiation goes into the biomass matrix and weakens the interactions between different components of the lignocellulosic biomass (Hu and Wen, 2008). This is used as an auxiliary technique with other chemical pretreatment methods for an enhanced effect (Binod et al., 2012). The introduction of microwave irradiation as a pretreatment method enhances the hydrolysis of rice straw and sugar cane bagasse by nearly 2 and 3 folds, respectively (Ooshima et al., 1984; Sarkar et al., 2012). Ultrasound pretreatment is carried out with a sonicator by using ultrasonic sound waves. Like microwave techniques, ultrasonic treatment acts as an accompanying technique with other conventional techniques such as acid and alkali pretreatment (Subhedar and Gogate, 2016). For instance, ultrasound-assisted alkali pretreatment is very useful to weaken the cellulosic bonds and assists in delignification (Subhedar and Gogate, 2014). Pyrolysis has also been used as a pretreatment method and is carried out at temperatures greater than 300 C, in which cellulose is rapidly degraded to char and gaseous products (H2 and CO). The residual char is further treated by water or with mild acid, and the water leachate contains carbon source, which can be converted to bioethanol (Sarkar et al., 2012). Although the physical pretreatment methods are not suitable for delignification process, these methods are applicable for depolymerization of lignocellulosic matrix (Zabed et al., 2016).

5.5.1.1.2 Chemical pretreatment The popular and conventional chemicals being used for pretreatment of lignocellulosic biomass are acids and alkalis. Acid pretreatment using different mineral and organic acids like HCl, H2SO4, and peroxyacetic acid is effective in hydrolyzing hemicellulose and cellulose, whereas alkali pretreatment with NaOH and KOH is

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applied in dissolving lignin (Nwosu-Obieogu et al., 2016). Though the acid and alkali pretreatments are certainly efficient methods for pretreatment, these methods are not being used nowadays due to their hazardous effects on the environment. Besides acid and alkali treatments, other chemical pretreatment techniques like organosolv and ozonlysis have been recently developed. These techniques majorly focus on removal of lignin from the biomass matrix. Organosolv is an alternative method for the delignification process, especially being used in the pulping industry. It replaces the kraft pulping process due to its hazardous effect. In this method, organic solvents like alcohols and acetone are used to solubilize lignin (de la Torre et al., 2013). A recent study on the pretreatment of corn stover using alkali-mediated methanol found that it enhanced glucan and xylan yield (Yuan et al., 2018). Glycerol, a byproduct from biodiesel production and a cheap solvent, is also used for pretreatment of sugarcane bagasse along with acid, for removal of lignin (Terán Hilares et al., 2017). Similarly to organosolv, ozonolysis and ionic liquid pretreatments are innovative methods of chemical pretreatment (Alvira et al., 2010). In ozonolysis, ozone gas is implemented to selectively solubilize lignin. This method is very effective for high-lignin-containing agricultural wastes like bagasse (Travaini et al., 2013). Ionic liquids are organic solvents with long-chain organic cations. Although, this method is expensive, the simultaneous solubilization of lignin and hydrolysis of the cellulosic fiber has an advantage in the ethanol and butanol production processes (Elgharbawy et al., 2016).

5.5.1.1.3 Physiochemical pretreatment Other than physical and chemical pretreatment methods, different physiochemical treatments like steam explosion, subcritical water, supercritical CO2, and ammonia fiber explosion (AFEX) are also found to be effective against pretreatment of lignocellulosic biomass. Steam explosion is the process of treating the biomass with steam at an elevated temperature and pressure, which separates the fibers by entering into the biomass matrix (Sanchez and Cardona, 2008). This pretreatment could increase the xylan yield to 43% 55% by hydrolyzing hemicellulose (Intanakul et al., 2003). The steam explosion, along with acid or alkali solution, is used to enhance the pretreatment efficiency (Alvira et al., 2010). Liquid hot water or subcritical water treatment (temperature: 170 C 230 C, pressure: 5 bar) is an efficient technique for treating different kinds of lignocellulosic agricultural residues, including corncobs, sugarcane bagasse, corn stover, and wheat straw (Garrote et al., 2001; Laser et al., 2002; Mosier et al., 2005; Pe´rez et al., 2008). Another method of pretreatment is the supercritical CO2, in which the biomass is treated in the CO2 medium at supercritical region (temperature more than 31.1 C and pressure more than 73 bar), which increases the digestibility of biomass by efficiently removing lignin (Alvira et al., 2010). In place of water and CO2, NH3 can also be used as the pretreatment medium and the method is termed as AFEX. This method is similar to subcritical water treatment, but in addition to decomposing hemicellulose it also removes lignin due to the alkalinity of the medium.

5.5 Biochemical Routes for Biofuel Production

AFEX-pretreated corn stover increased the enzymatic saccharification with a glucose yield of 98% (Teymouri et al., 2004).

5.5.1.1.4 Biological pretreatment Biological pretreatment relies on microbial-assisted delignification and decomposition of hemicellulose, which subsequently improve the yield of hydrolysis (Prasad et al., 2007). In this section, some instances of biological pretreatments on agricultural residues have been taken into consideration. Fungal consortium removes lignin from corn stover, enhancing hydrolysis sevenfold (Song et al., 2013). Microorganisms like Irpex lacteus and Phanerochaete chrysosporium have been proved to be efficient to increase the yield of reducing sugar from the straw, corn stalk, and rice husk by partially or fully removing the lignin (Cianchetta et al., 2014; Du et al., 2011; Potumarthi et al., 2013; Taha et al., 2015).

5.5.1.2 Detoxification Detoxification is highly crucial in biofuel production, especially in bioethanol and biobutanol production. It is used for the removal of lignocellulosic-derived byproducts, which acts as an inhibitor for enzymes and microorganisms during fermentation. Due to harsh pretreatment conditions like dilute-acid hydrolysis or steam explosion of lignocellulosic biomass, different byproducts are derived from dehydration of pentose and hexose sugars. Furfural, 5-hydroxymethyl furfural (HMF), and phenolic compounds are generated from degradation of hexose, xylose, and partial degradation of lignin (Palmqvist and Hahn-Ha¨gerdal, 2000). Aliphatic acids, such as acetic, formic, and levulinic acid are formed by degradation of HMF or deacetylation of hemicellulose. These inhibitors cause degradation of cellular DNA, inhibit the synthesis of RNA and protein, and as a result reduce the enzymatic and biological activities (Parawira and Tekere, 2011). Several treatment approaches have been developed for removal of inhibitors and detoxification of hydrolysates. Those approaches include treatments with chemicals (alkali Ca (OH)2, NaOH, NH4OH, reducing agent—dithionite, dithiothreitol, and sulfite), liquid liquid extraction (ethyl acetate, supercritical CO2 extraction, trialkylamine), liquid solid extraction (activated carbon, ion exchange lignin), microbial (Coniochaeta ligniaria, Trichoderma reesei, Reibacillus thermosphaericus, Ureibacillus thermosphaericus, and Saccharomyces cerevisiae), and enzymatic treatment (laccase and peroxidase) (Jo¨nsson et al., 2013). Apart from these treatments, certain alternative techniques are also adapted to overcome the inhibitor problems. These alternative approaches include selection of feedstock (less recalcitrant biomass, which generates less inhibitors), indigenous microorganisms, inoculum size, and genetically engineered microbes (Jo¨nsson et al., 2013). Each approach has its own advantages and limitations. However, the selection of a cost-effective and easy approach for detoxification is necessary to improve the product yield.

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5.5.1.3 Hydrolysis In the hydrolysis step, the pretreated lignocellulosic biomasses containing cellulose and hemicellulose are degraded to the monomeric form. Hydrolysis of lignocellulosic biomass can be carried out either by using acid or enzyme. The acidic hydrolysis can be performed in two ways: (1) dilute acid at high temperature and high pressure for short duration and (2) concentrated acid at low temperature for a longer duration. Dilute acid treatment is carried out at a low concentration of H2SO4 (0.5% 1.5%) and a temperature range of 120 C 160 C. Dilute acid treatment generally targets hemicellulose degradation and also acts as a pretreatment for cellulose. Whereas, for the degradation of cellulose, two-stage hydrolysis is preferred. In the two-stage hydrolysis process, the first stage is conducted at a lower temperature of less than 180 C for degradation of hemicellulose and the second stage is conducted at a higher temperature (230 C 240 C) for conversion of cellulose into glucose (Balat, 2011; Zabed et al., 2016). The concentrated acid hydrolysis is applied both for cellulose and hemicellulose degradation. Acids, such as 41% HCl, 100% trifluoroacetic acid, and 70% 90% H2SO4 with longer reaction time, are used for this process. Concentrated acid hydrolysis, which gives nearly 100% conversion of cellulose and hemicellulose to sugar is cost-effective in comparison to dilute acid hydrolysis. However, the major limitations of this treatment are corrosion of equipment and that it is environmentally hazardous (Balat, 2011). Moreover, generation of byproducts in acid hydrolysis hinders the recovery of product after fermentation. Enzymatic hydrolysis overcomes the limitations of acid hydrolysis by being nontoxic or corrosion-free with less formation of inhibitory byproducts. In enzymatic hydrolysis, mixtures of commercial enzymes or whole-cell microorganisms, producing a cocktail of different enzymes are being used for the degradation of lignocellulosic compounds. The three major groups of enzymes used for hydrolysis of lignocellulosic biomass are cellulases, hemicellulases, and lignanases. Cellulase includes three different type of enzymes (endoglucanase, exoglucanase, and β-glucosidase), which act on cellulose to form glucose. The endoglucanase (endo-1-4-β-glucanase or carboxymethyl cellulase) basically acts on amorphous cellulose. It randomly cleaves β-1-4-glycosidic linkage on glucose polymer chain, producing small chains of reducing and nonreducing ends. The exoglucanase or cellobiohydrolase acts on these reducing and nonreducing ends to produce cellobiose. Then the β-glucosidase acts on the cellobiose to release glucose. Several species of bacteria (Bacillus, Clostridium, Cellulomonas, Bacteriodes, Ruminococcus, Erwinia, Acetovibrio, Microbispora, Streptomyces, and Thermonospora) and fungus (Trichoderma, Penicillium, Fusarium, Phanerochaete, Humicola, and Schizophillum) have been reported to produce cellulase enzymes. The hemicellulases are a more complex mixture of enzymes, which act on heterogeneous hemicellulose to produce pentose sugars (xylose and arabinose), hexose sugars (glucose, mannose and galactose), and uronic acid. Generally, the hemicellulase enzyme system includes endoxylanase or endo-1,4-β-xylanase (hydrolyse main chains of xylan and β-xylan), xylan 1,4-β-xylan esterases

5.5 Biochemical Routes for Biofuel Production

(attacks xylo-oligosaccharides into xylose), ferulic and p-coumaric esterases, α-arabinofuranosidases and α-glucuronidase, α-1-arabinofuranosidase, acetylxylan esterase (attack the acetyl substitutions on the xylose moieties), and α-4-methyl glucuronosidases (Taha et al., 2016). As with cellulose- and hemicellulose-degrading enzymes, the lignin-degrading enzymes also play an important role in enhancing the access of hydrolytic enzymes for degradation of cellulose and hemicellulose. Some of the lignin-degrading enzymes are lignin peroxidases, manganese peroxidases, and laccases (Taha et al., 2016; Zabed et al., 2016). The enzymatic hydrolysis process overlay depends on two factors: (1) enzyme-associated factors and (2) substrate-associated factors. The enzymeassociated factors include type, source, and efficiency of enzyme. Whereas, the substrate-associated factors include composition and structure of feedstock, degree of cellulose crystallinity, particle size, and porosity.

5.5.1.4 Fermentation Sugars (hexose and pentose) released during the hydrolysis or saccharification process can be converted into several value-added products by fermentation using several microorganisms. The fermentation process is performed in a batch, semicontinuous, or continuous mode or it can be integrated with the saccharification stage to improve the yield by reducing the process cost. Various approaches for integration of the saccharification stage with fermentation are as follows: (1) separate or sequential hydrolysis and fermentation (SHF), (2) simultaneous saccharification and fermentation (SSF), (3) simultaneous saccharification and cofermentation (SSCF), and (4) consolidated bioprocessing (CBP) or direct microbial conversion (DMC). In SHF, saccharification of biomass and fermentation of saccharified biomass are carried out in a two-stage process. There is minimal interaction between these two stages, and for each stage the operating conditions can be optimized. The product (ethanol and butanol) yield in SHF is lower due to two major limitations, that is, end-product inhibitions and contamination. In SSF, hydrolysis of cellulose and fermentation of glucose occurs simultaneously in the same reactor. It avoids the limitations of product inhibition resulting in higher yield as compared to SHF. Moreover, SSF is more cost-effective, with a lower enzymatic loading rate. Simultaneous optimization of process conditions for fermentation and for enzymatic hydrolysis is the major limitations of the SSF process. S. cerevisiae and Zymomonas mobilis are the two most common ethanol fermenting microorganisms. However, S. cerevisiae can only ferment hexose sugar. The other microorganisms that simultaneously use both pentose and hexose sugars are Pichia stipitis, Candida shehatae, Pachysolan tannophilus, and Kluyveromyces marixianus. The third strategy for fermentation is SSCF, which includes cofermentation of hexoses and pentoses. For SSCF, the compatibility of microorganisms in terms of operating temperature and pH is very much essential. However, suitable microorganisms performing cofermentation of both hexose and pentose are not widespread, which is the greatest obstacle. A suitable microorganism reported for SSCF is coculture of C. shehatae and

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S. cerevisiae (das Neves et al., 2007). In the final fermentation method, that is, CBP or DMC, all required enzymes, starting from hydrolysis to fermentation are produced by a single or a group of microorganisms. The CBP is a compatible and cost-effective solution toward hydrolysis and fermentation, which avoids the purchase and production of individual enzymes. However, the resulting product yield by this process is comparatively very low with a longer fermentation time. Bacteria such as Clostridium thermocellum and some fungi including Neurospora crassa, Fusarium oxysporum, and Paecilomyces sp. have shown this type of activity (Balat, 2007). Butanol fermentation has similar process operation steps to ethanol fermentation, except the microorganisms used and the fermentation conditions. The fermentation occurs in strict anaerobic conditions and the efficient strains used for this process are Clostridium saccharoperbutylacetonicum, Clostridium beijerinckii, Clostridium acetobutylicum, and Clostridium saccharobutylicum (Ezeji et al., 2003; Kujawska et al., 2015). Butanol production is also commonly called acetone butanol ethanol (ABE) fermentation, which produces acetone, butanol, and ethanol in the ratio of 3:6:1, respectively (Kujawska et al., 2015).

5.5.1.5 Anaerobic digestion Anaerobic digestion is a series of complex biological treatments of organic substances, carried out by a mixed anaerobic culture. The whole process can be divided into four phases: (1) hydrolysis, (2) acidogenesis, (3) acetogenesis, and (4) methanogenesis (Fig. 5.4). In hydrolysis, complex compounds like carbohydrate, proteins, and lipids are broken down to simple monomeric form, like sugars, amino acids, and fatty acids. This break down is carried out by a hydrolase enzyme, secreted by facultative or obligatory anaerobes. In the acidogenic phase, monomers developed in the hydrolysis phase are converted into organic acids (butyric acid, propionic acid, and acetic acid), alcohols, and hydrogen and carbon dioxide. In this phase, generation of volatile fatty acids (VFA) along with ammonia and hydrogen sulfide occurs. The CO2 and H2 produced in the acidogenic phase are reduced to form acetic acid in the acetogenic phase by homoacetogenic microorganisms. Furthermore, the alcohols and organic acids are converted to acetate. This acetate acts as a substrate for methanogenic bacteria in the methanogenesis phase and formation of methane (CH4) along with CO2 and H2O occurs (Chandra et al., 2012). The major interfering compound during the anaerobic digestion process is H2, which inhibits the formation of acetate. However, under optimum conditions, methanogenic bacteria consume H2 for production of CH4. Typically, in anaerobic digestion, the generation of biogas occurs; whose primary compositions are CH4 (55% 75%) and CO2 (25% 45%). The other gases (N2, H2, O2, and H2S) remain in insignificant form. Through in situ gas purification techniques, the methane yield can rise up to 90%. In a biorefinery approach, the reforming of CH4 into liquid methanol can be another option for transforming gaseous fuel into liquid fuel (Naik et al., 2010). Additionally, the digested slurry generated through this process can be used as a potential source of fertilizer or manure.

FIGURE 5.4 Integrated process for production of biofuels.

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CHAPTER 5 Biofuels from agricultural wastes

5.5.1.6 Dark fermentation The suitable and efficient technology for hydrogen production by utilizing organic wastes is dark fermentation (Ghimire et al., 2015). In dark fermentation (DF) processes, molecular hydrogen (H2) is formed by degradation of major carbohydraterich substances by facultative or obligate anaerobes by following the similar steps of anaerobic digestion (except the methanogenic stage). Under anaerobic conditions, oxidation of organic substrates generates electrons, which are accepted by protons (H1) to form H2 (Das and Veziroglu, 2008). Theoretically, H2 production can be estimated from different organic substrates like sugars, carbohydrates, proteins, and lipids (Ntaikou et al., 2010).

5.5.1.7 Transesterification Among a number of methods for biodiesel production, transesterification is the most acceptable one due to its higher productivity and low production cost (Ma and Hanna, 1999). A comparative study between the different methods for the production of biodiesel is given in Table 5.2. A brief description of the transesterification process along with several instances of biodiesel productions from the different agro-based wastes is given here. Transesterification is the chemical reaction between triglycerides (ester) and alcohols (methanol and ethanol) to produce methyl or ethyl ester of long-chain fatty acids. The chemical equation of the transesterification reaction involved in biodiesel production process is given in Fig. 5.5A. The above reaction usually proceeds in the presence of acid (Ramadhas et al., 2005) or base (Meher et al., 2006) as catalysts. Several modified catalysts, like solid catalysts (Leung et al., 2010), and enzymes like lipase (Du et al., 2004) are being used to enhance the conversion. A basic diagram of the biodiesel production process is given in Fig. 5.4. Generally, the transesterification reaction is a single-step process. However, processing of oils with higher free fatty acid (FFA) is difficult because it causes several interferences in the transesterification process (Tyson, 2002). For example, FFAs form soap (Ghadge and Raheman, 2005; Naik et al., 2008) in the presence of the base catalysts used in the reaction of transesterification, which decreases the fuel quality (Meher et al., 2006; Van Gerpen, 2005). Therefore, the high FFA oils need to be pretreated before the transesterification process. Therefore, for high FFA oils like karanja (P. pinnata), a two-step process is needed (Naik et al., 2008). In the two-step process, usually esterification of the oil is carried out before the transesterification reaction, which is termed as the pretreatment of high FFA oils. The reaction of esterification converts FFAs present in oil into their corresponding esters, which decreases the formation of soap during transesterification (Tyson, 2002). This pretreatment (esterification) reaction is catalyzed by acid, base, or solid catalysts as that of the transesterification reaction. The chemical equation of the acid-catalyzed esterification reaction is given in Fig. 5.5B.

5.5 Biochemical Routes for Biofuel Production

FIGURE 5.5 (A) Transesterification reaction in a biodiesel production process. (B) Esterification reaction involved in a two-step process.

After the pretreatment process, the resulting oil with low FFA is reacted with methanol or ethanol in the presence of suitable acid or base catalysts (mineral acids, NaOH, KOH, solid catalysts, enzyme) to produce fatty acid methyl or ethyl ester (Leung et al., 2010; Wei Du et al., 2004; de Arau´jo et al., 2013). This is designated as biodiesel because of its compatibility with properties of diesel as per the ASTM standards (ASTM-D 6751) (Extension, 2012).

5.5.2 INTEGRATED PROCESS FOR PRODUCTION OF BIOFUELS An integrated process in biofuel production could enhance byproduct utilization and add value. In a particular biofuel production process, the selective microorganisms or enzymes prefer specific substrate constituents over others. In the case of bioethanol/biobutanol production the hemicellulose constituents are easily degraded compared to cellulose, leaving behind lignin-rich residue, which is utilized in the thermochemical conversion process to generate heat and electric energy (Fig. 5.4). Other than lignin residue, partially degraded cellulose containing solid residue along with liquid waste generated after the ethanol/butanol purification process are used as substrate for biogas production. Likewise, the fiber- and nutrient-rich digested slurry that remains after anaerobic digestion is utilized either for bioethanol or biofertilizer production. After biodiesel production the residual deoiled cake is used as a source for biogas production.

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CHAPTER 5 Biofuels from agricultural wastes

5.5.3 AGRICULTURAL WASTES AS FEEDSTOCK FOR PRODUCTION OF BIOFUELS 5.5.3.1 Bioethanol production using agricultural wastes Bioethanol is a renewable alternative to fossil fuel in the transportation sector. The current global ethanol production is 28,375 million gallons, which has almost doubled in the past 10 years. The major bioethanol producers are the United States, Brazil, Europe, and China, followed by Canada, Thailand, Argentina, and India. Among them the United States holds 58% of the total global ethanol production (RFA, 2017). As a fuel, bioethanol has two major advantages over petrol: (1) high octane number (108), which helps in early ignition of engine (Balat, 2007) and (2) high oxygen content (34.7%), due to complete combustion of fuel, there is a lower emission of nitrogen oxide and particulate matter (Kar and Deveci, 2006; Krylova et al., 2008). Again, using bioethanol, 80% less emission of CO2 occurs as compared to petrol. As a result, blending of bioethanol with petrol is a preferable option and could be an environmentally friendly substitute to toxic methyl tertiary butyl ether, which is generally used as an octane enhancer for petrol (Green and Lowenbach, 2001; Yao et al., 2009). The estimated global annual bioethanol potential from the three major crop residues, rice straw, wheat straw, and corn stover, are 205, 104, and 58.6 giga liters (GL) from 731.3, 354.34, and 128.02 million tons of available biomass, respectively (Saini et al., 2015). Similarly, bioethanol production from other crop residues, such as barley (3.7 million tons) and oat straw (11 million tons) is 20.6 and 3.16 GL, respectively (Kim and Dale, 2004). The bioethanol potential of the biomass majorly resides on the cellulose and hemicellulose composition. Whereas, the lignin content causes the major hindrance in hydrolysis and biological conversion into ethanol. Besides the lignocellulosic composition, other mineral and biochemical contents, such as high ash and silica content in rice straw, and a significant alkali and protein content in wheat straw and corn stover, cause a limitation to the thermochemical conversion of biomass into ethanol (Binod et al., 2010). Therefore, an initial pretreatment is mandatory in bioethanol production using any crop residues (straw and stover). In the case of crop residue such as rice straw and wheat straw, alkaline pretreatment has been found to be very effective not only for fractionation of lignin, but also for removing silica. From the dissolved state of silica (silicates), amorphous silica can be recovered and utilized for different value-added products. Besides silica, removal of other compounds like acetyl group and phenolic compounds prior to pretreatment helps in decreasing the generation of toxic compounds (HMF, furfural, and acetic acid) by improving the ethanol yield (Yuan et al., 2018). In the case of rice straw an ethanol yield of 61.3% (29.1 g/L) has been obtained due to alkali and microwave pretreatment followed by hydrolysis and fermentation using cellulase from T. reesei and S. cerevisiae in SSF. In contrast, only alkali (40% 74% theoretical ethanol) and acid (30% theoretical ethanol) pretreatment results in a lower yield (Binod et al., 2010). In the case of wheat straw, a sequential two-stage pretreatment

5.5 Biochemical Routes for Biofuel Production

process, which combines alkaline preextraction [8% (w/w) sodium hydroxide at 80 C for 90 minutes] and acid-catalyzed steam treatment [3% (w/w) sulfur dioxide at 151 C for 16 minutes] gives an ethanol yield of 54.5 g/L (Yuan et al., 2018). Using harsh chemicals (combination of phosphoric acid and hydrogen peroxide), ethanol yield from wheat straw increased up to 71.2 g/L (Qiu et al., 2017). Similarly, for corn stover, using various chemical-based pretreatment technologies (alkaline, solvent-based, and ammonia) a maximum of 19% 22% of ethanol has been produced. Meanwhile, biological-based pretreatment (using fungi) gives a much lower ethanol yield of 11% (Zhao et al., 2018). Several microorganisms (yeasts, bacteria, and fungi) have also been investigated for improvement of ethanol yield during fermentation. For wheat straw, S. cerevisiae, Kluyveromyces marxianus native, Pichia stipites, and recombinant strains are the most studied yeast strains for ethanol fermentation. Among them native S. cerevisiae is found to be the best strain for ethanol fermentation. Again, some of the strict anaerobic thermophilic bacteria such as Clostridium sp. and Thermoanaerobacter sp. are also proposed to be suitable for ethanol fermentation at higher temperatures. The major limitation of thermophilic bacteria is their low tolerance to ethanol (.30 g/L) (Talebnia et al., 2010). The fermentation ability of different microbes for wheat straw gives an ethanol yield of 11.8 31.2 g/L (Talebnia et al., 2010). The bioethanol potential of major agro-industry residues, that is, sugarcane bagasse, is 51.3 GL from the available biomass of 180.73 million tons (Saini et al., 2015). Several pretreatment methods have been tried on sugarcane bagasse and a maximum yield of 59.1 g/L was observed in the case of H2SO4 (1.25%) treatment (Cardona et al., 2010). In other agro-industrial processed wastes such as olive mill solid waste/olive deoiled cake has been explored, whose global annual production has been estimated to be 4 3 108 kg of dry matter. The ethanol extracted was 3% of the dry olive deoiled cake (Tayeh et al., 2014). The potentials of some edible deoiled seed cakes such as canola, sunflower, sesame, soy, and peanut are also explored (Balan et al., 2009). The deoiled seed cakes are rich in protein, fiber, and other nutrients. The fiber is rich in cellulose, hemicellulose, and lignin, which is converted into bioethanol by utilizing suitable pretreatment, hydrolysis, and fermentation processes. Similarly, the nonedible seed cake of P. pinnata has been explored for bioethanol (0.088%) production (Muktham et al., 2016). Other than cereal crops and industrial processed wastes, different fruit wastes such as pineapple peel (8.34%), banana peel (7.45%), apple pomace (8.44%), palm oil empty fruit bunch (14.5%), and a mixture of apple and banana (38%) have also been evaluated for their bioethanol potential (Chatanta et al., 2008; Gupta and Verma, 2015).

5.5.3.2 Biogas production using agricultural wastes Biogas/biomethane is a gaseous biofuel and renewable alternative to commercial fuels. The advantages of using biogas over natural gas are as follows: (1) it is

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produced from a natural renewable source and (2) it helps in reducing the pollution caused by organic wastes and does not add any greenhouse gases. Agricultural residues are rich in carbohydrate, protein, lipid, cellulose, and hemicellulose and are used as a substrate for biogas production. The lipid-rich substrate has a higher methane potential as compared to carbohydrate- and protein-rich substrate due to its higher energy content, but has a longer residence time due to its lower bioavailability. Lignocellulosic biomasses, like straws and stovers, are rich in cellulose, hemicellulose, and lignin, which has a higher theoretical methane potential than that practically achieved, due to the complex and tight bond between cellulose, hemicellulose, and lignin. Therefore, pretreatment of crop residue is necessary for improving digestibility in biogas production. The average methane potentials of different crop residues, such as rice straw, wheat straw, and corn stover are 302, 290, and 338 L/kg volatile solids (VS), respectively. Among the various agricultural wastes, animal manure is a suitable feedstock for biogas generation. It has high organic and moisture content, and a low C/N ratio, which is fermented more easily. Under suitable temperature and substrate to incolumn ratio, the manure can be used as monosubstrate in biogas production. The biogas yield of pig manure is 495 mL/g VS with 70% 80% CH4, whereas, for cattle manure it is 398 mL/g VS with 55% 75% CH4 (Li et al., 2016). The codigestion of manure with crop residues having low N content is always preferable for enhancing biogas production. Other than cereal crops, fruits and vegetable wastes are very common types of waste being used for biogas production (Viswanath et al., 1992). However, anaerobic digestion of individual FVWs are difficult due to their high simple sugars content, which promotes fast acidification of the biomass by inhibiting the activity of methanogenic bacteria. Hence, the FVWs are often codigested with nitrogen-rich manure or sewage sludge to improve the biodegradability (Scano et al., 2014). The major inhibitions in anaerobic digestion are generation of free ammonia, sulfides, metal content, and the presence of other organic substances. The ammonia inhibition can be overcome by maintaining an optimum C/N ratio and pH. Maintaining a higher organic loading rate also helps to avoid ammonia inhibition (Karthikeyan et al., 2012). Excess ammonia can be removed from the anaerobic digestion process by air stripping with different inert materials, such as zeolite, activated carbon, and clay (Paul and Dutta, 2018). Sulfide inhibition can be avoided by maintaining an optimum C/S (carbon:sulfide) ratio of substrate. Pretreated biomass with acid and thermal treatment produce hydroxyl methyl furfural and furfural, which inhibit the aerobic digestion process by increasing the nitrogen content by reducing the C/N ratio. A hydrothermal pretreatment could be a solution to the above problem (Paul and Dutta, 2018). Recent developments in biogas production suggest the separation of the anaerobic digestion process into two stages: hydrolysis/acidogenesis and acetogenesis/methanation or multistage for better organic loading, change in temperature, and effective conversion of organic substrate (Achinas et al., 2017).

5.5 Biochemical Routes for Biofuel Production

5.5.3.3 Biohydrogen production using agricultural wastes The hydrogen produced from renewable sources is known as biohydrogen. In molecular form (H2), it contains the highest energy and is the only carbon-free fuel, which produces water as a clean byproduct after combustion (Kotay and Das, 2008). It has several advantages over other renewable fuels in terms of being a clean energy substitute, which can address the global environmental and pollution problems (acid rain, ozone depletion, greenhouse gases) (Kotay and Das, 2008). It is used directly as a combustion fuel in combustion engines or for generation of electricity via fuel cells (Alves et al., 2013). It has broad industrial applications, ranging from synthesis of alcohols, aldehydes, and ammonia to hydrogenation of petroleum, edible oil, coal, and shale oil (Kothari et al., 2012). Biohydrogen can be produced by both autotrophic and heterotrophic conversion using biological compounds as substrate. Autotrophic conversion directly converts solar energy into hydrogen by using photosynthetic microorganisms like microalgae, photosynthetic bacteria, and protists as a photosynthetic reaction medium. In contrast, heterotrophic conversion converts biomolecules (mostly carbohydrates) obtained from biological feedstock into biohydrogen with the help of anaerobes either by photosynthetic or dark fermentation (Ghimire et al., 2015). Biohydrogen is also produced from crop residues (rice straw, wheat straw, barley straw, etc.) by utilizing the cellulose and hemicellulose portion of the biomass. However, hydrogen production from these complex structures is not easy due to their heterogeneous crystalline structure, which inhibits the hydrolytic activity of the microorganisms. Therefore, pretreatment by physical or chemical methods is necessary for enhancing the efficiency of feedstock for biohydrogen production. It is reported that hydrogen production from corn stalk has been increased in the case of pretreated biomass compared to the nonpretreated biomass. The alkali treatment (0.5% NaOH) enhances biohydrogen production (57 mL H2/g VS) almost 19-fold, whereas the acidic treatment (0.2% HCl coupled to heat pretreatment) enhances the biohydrogen (150 mL H2/g VS) production to nearly 50 times the initial value (3 mL H2/g VS). Like acid and alkali treatments, different thermal, steam explosion, and microwave treatments are found to be quite efficient for biohydrogen production (Ivanova et al., 2009; Yokoyama et al., 2007). Biohydrogen production from different crop residues is described in Table 5.3. The broad range of H2 produced depends upon the type of pretreatment used. The biohydrogen production efficiency of livestock waste is comparatively lower than crop residue. The livestock (manure) waste is generally nitrogen-rich (Yin et al., 2014) and produces high ammonia during biohydrogen production, which causes failure of the reactor. It is reported that, with an ammonia concentration above 2 g N/L, the production of biohydrogen is decreased (Cavinato et al., 2012). However, an acclimatized microbial culture in lower ammonia concentration helps in hydrogen production (Salerno et al., 2006). As with nitrogen, livestock waste (swine manure) is also rich is sulfate and can inhibit biohydrogen production due to sulfate reducer bacteria, which are hydrogen consumers. Pretreatment (physical and

125

Table 5.3 Biofuel Potentials of Different Agricultural Wastes Agricultural Waste

Bioethanol Yield

Biomethane Yield

Biobutanol Yield

Biohydrogen Yield

Rice straw

12 29.1a

302c

3.43d

24.8g

Wheat straw

11.8 71.2a

290c

21.42e

1 68f

Corn stover

40 55.27a

338c

50.14e

49 66f

Barley straw

11.9 46.0a

Sugarcane bagasse

23.2 59.1a

References

Crop Residue

47.2e 278c

14.17d

6980

h

Akhtar et al. (2017), Asadi and Zilouei (2017), Binod et al. (2010), Chandra et al. (2012), Gottumukkala et al. (2013), Phitsuwan et al. (2016), Yadav et al. (2011) Chandra et al. (2012), Guo et al. (2010), Qiu et al. (2017), Qureshi et al. (2008), Talebnia et al. (2010), Yuan et al. (2018) Adelabu et al. (2017), Guo et al. (2010), Lau and Dale (2009), Qureshi et al. (2014) García-Aparicio et al. (2011), Han et al. (2013), Saha and Cotta (2010), Qureshi et al. (2014) Cardona et al. (2010), Hu et al. (2018), Pang et al. (2016), Sangyoka et al. (2016)

Agro-Industrial Waste Pongamia pinnata deoiled seed cake Rice bran

0.088b

Apple pomace

8.44b

Orange peel

6b

Rice bran deoiled cake

448c

217c

Chandra et al. (2012), Doshi and Srivastava (2013)

2.75d

545h

12.92e

134.04g

19.5d 5.16d

295h

Taslim et al. (2018), Chandra et al. (2012), Tandon et al. (2018), Zhang et al. (2013) Gupta and Verma (2015), Hijosa-Valsero et al. (2017), Wang et al. (2010) Boluda-Aguilar and López-Gómez (2013), Joshi et al. (2015), Wikandari et al. (2015) Al-Shorgani et al. (2012), Tandon et al. (2018)

Oil palm fruit bunch Livestock waste Cattle manure Pig manure

37.8b

276 340

1.262 2.61d

398c 495c

VS, volatile solids; ABE, acetone butanol ethanol; TS, total solid a L/kg of dry biomass. b %. c L/kg VS. d g/L ABE. e g/L. f mL H2/g VS. g mL H2/g TS. h mL H2/L.

Ibrahim et al. (2012), Kim and Kim (2013), Noomtim and Cheirsilp (2011), Sompong et al. (2012)

65f 14 18f

Guo et al. (2010), Li et al. (2015) Guo et al. (2010), Li et al. (2016)

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CHAPTER 5 Biofuels from agricultural wastes

chemical) of manure and maintaining the thermophilic condition helps to avoid the methanogenic activity during biohydrogen production. The codigestion of manure with carbohydrate-rich substrates again helps in the improvement of biohydrogen production (Wu et al., 2009). FVWs and other carbohydrate-rich industrially processed waste, such as starch-manufacturing waste, potato, sugar beet processing wastewater, brewery whey, cheese whey, etc. are rich in simple sugars and are ideal substrates for biohydrogen production with ranges between 50 290 mL H2/g VS (Guo et al., 2010; Li et al., 2008; Tenca et al., 2011; Venetsaneas et al., 2009; Yokoi et al., 2002). However, in the case of sugar-rich substrate, the easy degradation of substrate and generation of VFA are high, which disturbs the buffer capacity of media and affects the optimum pH (5.0 6.0) responsible for microbial hydrogen production from food waste. For maintaining a weak acidic pH, livestock manure can be a cosubstrate, whose alkaline nature and nutrient-rich condition help in microbial population growth for biohydrogen production (Tenca et al., 2011).

5.5.3.4 Biobutanol production using agricultural wastes High energy content (butanol: 29.2 MJ/L; ethanol: 21.2 MJ/L) and distinct properties (low vapor pressure, hydroscopic nature, less volatility, and less flammability) signify butanol as a superior fuel compared to ethanol and biodiesel (Song et al., 2016; Szulczyk, 2010). Butanol has a similar chain length to gasoline and it can be used directly as a fuel additive or in blending with other petroleum fuel without any engine modifications. ABE fermentation by Clostridium sp. has been one of the most popular routes for biobutanol production. Apart from the conventional substrates, that is, corn or molasses, various biological substrates such as sugar, starch, and biomass have been used for biobutanol production over the past several years. The use of agricultural wastes as well as agro-industrial wastes as feedstock for butanol production has given new potential toward the biorefinery concept. Various agro-based wastes like sugarcane bagasse, cereal straws, waste vegetables, fruit wastes, and food industry wastes are widely used for biobutanol production. Sugarcane bagasse is one of the potential feedstocks for biobutanol production due to its high cellulosic ($40%) and hemicellulose (30% 35%) content. However, the high lignin content of the biomass (18.9%) hindered enzymatic activity for biobutanol production (Sun et al., 2004). Therefore, sugarcane bagasse is pretreated using an alkali solution for delignification, prior to the enzymatic hydrolysis by cellulolytic enzymes like Thermoascusaurantiacus QS 7-2. The hydrolysate was further fermented using Clostridium sp. (C. acetobutylicum GX01), and a yield of 20% 30% of ABE was reported (Pang et al., 2016). In another study, Kong et al. (2016) reported microwave-assisted alkali pretreatment followed by hydrolysis with 80% gamma valerolactone (GVL). GVL is an aprotic solvent, which is used as a substitute for hydrolytic enzymes and mineral acid. Butanol yield in this process was found to be 9.3 g/L along with 4.1 g/L acetone, and 0.86 g/L ethanol, using C. acetobutylicum XY16 (Kong et al., 2016). The cellulosic fiber content

5.5 Biochemical Routes for Biofuel Production

(47.57% cellulose, 15.75% hemicellulose, and 8.66% lignin) in rice straw is found to be quite high in comparison with sugarcane bagasse (Vassilev et al., 2012). Mild acid-pretreated rice straw with sulfuric acid (4 8 w/w%) is saccharified using the enzyme cellulases, followed by fermentation with the strain Clostridium sporogenes BE01 to produce a butanol yield of 3.43 g/L (Gottumukkala et al., 2013). SSF is found to be an efficient process for fermentation of wheat straw (Krishna et al., 2001). In this process, an ABE yield of 21.42 g/L has been obtained from wheat straw using C. beijerinckii P260 strain (Qureshi et al., 2008). The potential of barley straw and corn stover has also been explored for butanol production using C. beijerinckii P260 (Qureshi et al., 2014). In terms of ABE, butanol production was found to be higher in corn stover (50.14 g/L ABE) as compared with barley straw (47.20 g/L ABE). Being an abundant industrial processing waste, palm oil mill effluent is also explored as a potential feedstock for bioalcohol production. This effluent was utilized for butanol production by C. saccharoperbutylacetonicum N1-4 (ATCC 13564). The butanol and total ABE concentrations were found to be 0.9 and 2.09 g/L, respectively (Al-Shorgani et al., 2015). Rice bran and rice bran deoiled cake are also used for biobutanol production. The deoiled rice bran produces more butanol than the rice bran due to its very low oil content. ABE production is from oil-free rice bran, that is, deoiled cake of rice bran is around 88% more than that of rice bran (Al-Shorgani et al., 2012). The pretreatment process has a significant impact on the biobutanol yield. The pretreated rice bran as well as rice bran deoiled cake shows higher butanol yield in comparison to untreated feedstocks (Al-Shorgani et al., 2012). Agricultural process waste, such as brewery industry liquid waste, has been utilized for biobutanol production. In this process, a yield of 10.62 g/L has been obtained after fermentation of sugar concentrate (0.43 g/g) using the strain C. beijerinckii NRRL B-466 (Maiti et al., 2018). Apart from crop residues, vegetable wastes that cause serious disposal concerns have also been investigated for biobutanol production. Attempts have also been made on biobutanol production using vegetable waste, like lettuce due to it being rich in carbohydrates (19.5 g/L) (Procentese et al., 2017). The lettuce waste was pretreated with alkali, and hydrolyzed with an enzyme CellicCTec 2, releasing a sugar concentration of 19.5 g/L, which was further fermented using C. acetobutylicum DSMZ 792. A butanol yield of 1.1 g/L was obtained along with other byproducts (1.44 g/L) in this ABE fermentation process (Procentese et al., 2017).

5.5.3.5 Biodiesel production using agricultural wastes Agricultural processed wastes or agro-industrial wastes like rice bran, coffee ground, waste vegetable oil (from households, restaurants, and agro-food industries, etc.), and deoiled cakes of edible and nonedible oil seeds are potential feedstocks for biodiesel production, due to the residual oil present in them. Bio-oil is extracted from spent coffee powders using green solvents like dimethyl ether (DME). DME is the best option to extract oil from coffee grounds with a high oil

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content (16.8%), which substantiate its potential for biodiesel production by the transesterification process (Sakuragi et al., 2016). Worldwide estimation indicates a production of 38.5 million metric tons (MMT) of rice bran from 482 MMT of rice. The oil content in rice bran is approximately 17.5% (Wilmar, 2017). This unrefined rice bran oil with an FFA content of 8% has enormous potential for biodiesel production. Rice bran oil processed is through a two-step pretreatment process due to its high FFA content. Initially, an acid-catalyzed esterification process was carried out in the presence of methanol and sulfuric acid, followed by the transesterification process of low FFA rice bran oil produced in the first step. The biodiesel yield observed in this process was in the range of 60% 85% with variation in process parameters (reaction time, catalyst %, and methanol %) (Kattimani et al., 2014). Biodiesel from rice bran oil is also produced using solid acid catalyst, which has an advantage over mineral acids or alkali catalyst due to its heterogeneous and reproducibility characteristics (Jitputti et al., 2006). A biodiesel yield of 98.1% has been observed with 4 wt.% of catalyst (K2CO3 modified zeolite) (Taslim et al., 2018). Similarly, a high percentage of biodiesel yield (92%) is achieved using chlorosulfonic acidmodified zirconia through simultaneous esterification and transesterification processes (one-step reaction) from 40% FFA rice bran oil (Zhang et al., 2013). Biodiesel from edible oil seed cake like olive deoiled cake has been explored. The olive oil cake contains 13.75% oil on average and the deoiled cake has been used as the starting material for the production of biodiesel through a two-step process. Preliminarily, the olive oil cake contains 24.5% FFA, which is further decreased to 0.52% through an esterification reaction. The biodiesel yield varied from 40% to 65% by the transesterification reaction (Al-Hamamre, 2011). Nonedible oil seeds like jatropha, simarouba, neem, cottonseed, karanja, etc. have been proved to be the most suitable feedstocks for biodiesel production. However, the technique adopted for the extraction of oil from these seeds has a significant effect on the biodiesel yield. Though many researchers have developed various efficient oil extraction techniques from nonedible seeds, no oil extraction technique has yet reached 100% yield (Sakuragi et al., 2016). The low oil content in cake is more feasible for bio compressed natural gas production and extraction of value-added products like proteins and amino acids (Chandra et al., 2012; Naik et al., 2010; Sa´nchez-Arreola et al., 2015). Efficient use of deoiled cake can have a positive influence on the economy of biofuels.

5.6 CONCLUSION AND FUTURE TRENDS Agricultural wastes are an important aspect of lignocellulosic biomass. Utilization of these wastes for biofuel production depends on their composition, and different processing and conversion techniques. Based on the composition of agricultural wastes, a suitable route for biofuel production can be predicted. Different types of

References

agricultural wastes can be utilized individually or in a mixer (as cosubstrate) to enhance the production of biofuel. The conversion of lignocellulosic biomass into biofuels can be carried out by both biochemical and thermochemical routes. The biochemical route is more environmentally friendly and the byproduct obtained from the biofuel production process can be utilized as value-added product or further utilized as feedstock in the production of other biofuels.

LIST OF ABBREVIATIONS ABE AFEXs C/N C/S CBP DF DMC DME FFA FVWs GL GVL HMF MMT SHF SSCF SSF TS VFA VS

acetone butanol ethanol ammonia fiber explosion carbon/nitrogen carbon/sulfide consolidated bioprocessing dark fermentation direct microbial conversion dimethyl ether free fatty acid fruit and vegetable wastes giga liter gamma valerolactone 5-hydroxymethyl furfural million metric ton separate or sequential hydrolysis and fermentation simultaneous saccharification and cofermentation simultaneous saccharification and fermentation total solid volatile fatty acids volatile solids

ACKNOWLEDGMENT The authors are thankful to Dr. Susant Kumar Padhi and Dr. Pritam Kumar Dikshit for their valuable suggestions.

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