Bioenergy generation from agricultural wastes and enrichment of end products

Bioenergy generation from agricultural wastes and enrichment of end products

15

Goutam Kishore Gupta and Monoj Kumar Mondal Department of Chemical Engineering and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, India

15.1

Introduction

India is an agricultural country and also among the emerging economies of the world with rapid industrialization, urbanizations, and globalizations. At the same time the population of the country has been increasing at an alarming rate and also there is a progression in the standard of living of the people as compared to previous years. This has led to enormous per capita energy requirement as well as a huge amount of waste generation (Guan et al., 2015). The technological advancement and the rising crude oil prices along with the population growth have created surmounting pressure on the available energy resources, that is, fossil fuels (coal, petroleum, and natural gas) (Fig. 15.1) and it is an eminent truth that most of the developing countries, such as India, are dependent on its available energy resources. The main source of energy in India is coal followed by petrol and natural gas. The source-wise energy consumption in India in the year 2014
15 is presented in Fig. 15.2. In the present scenario, India is unable to fulfill the gap between demand 20000

Mega Joules

16000 12000 8000 4000 0

Year

Figure 15.1 Per capita energy consumption in India. Source: Energy Statics 2016. (accessed 12.01.19). Refining Biomass Residues for Sustainable Energy and Bioproducts. DOI: https://doi.org/10.1016/B978-0-12-818996-2.00015-6 © 2020 Elsevier Inc. All rights reserved.

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Crude petroleum

Coal and Lignite

Natural gas

Electricity

3% 8% 42%

47%

Figure 15.2 Source-wise energy consumption during 2014
15. Source: Energy Statics 2016. (accessed 12.01.19).

and supply of energy. The available natural resources are getting used up at a faster rate, and moreover, it is creating pressure on the minds of a future generation. Apart from energy requirement, other problems associated with fossil fuels are the release of harmful toxic gases to the environment which is responsible for global warming (Soltes and Milne, 1988). It is estimated that the population of the world will be around 10 billion by the year 2050 and the energy requirement will also be doubled. The present energy resources will be unable to meet the future energy needs and thus there is a need to search for a cleaner, renewable, recyclable, and environment-friendly energy resource for the upcoming generation. In today’s world, renewable energy (biofuels) is gaining importance (Kim and Dale, 2015) and biomass is being well thought-out to be a most potential resource because of its availability in plenty, less, or negligible cost, clean and sustainable. Main constituents of biomass are mainly cellulose, hemicelluloses, lignin, and other organic extractives which are the elements of carbon, hydrogen, and oxygen and have high-energy value. Among the available energy sources, biomass ranks fourth after coal, petroleum, and natural gas that alone provide 14%
15% of the world’s energy requirement including 38%
43% to the developing nations (Ertas and Alma, 2010). Other benefits of biomass cover carbon neutral, low sulfur, and nitrogen content that account for lesser SOx and NOx productions as compared to fossil fuels. Different forms of biomass are available in India. They are classified based on their availability in the environment. The different sources of biomass include food crops, vegetables, energy crops, lignocellulosic wastes, manures, algae, municipal solid wastes, etc. Classification of different types of biomass in India is presented in Fig. 15.3. The production of biofuels from these biomasses has been divided into four different generations. First generation: Different food crops, such as sorghum, wheat, rice, and corn, are utilized directly as feedstock for biofuel generation. The major drawback with this generation is the fight between fuel versus food as it increases the food prices. Second generation: To overcome the drawbacks of first-generation biofuels and lignocellulosic wastes, such as organic wastes, wood, crop residues, and sugarcane bagasse, were used for the generation of biofuels using different techniques.

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Agricultural wastes: Biomass from crop residues, wheat straw, corn cob, sugarcane baggasse, husks, shells, etc.

Municipal solid waste: Sewage sludge, kitchen wastes, office wastes, fabrics, cattle wastes, clothes, etc.

Forestry wastes

Biomass

Logs, trunks, leaves, tree branches, sawdust residues, bark, etc.

Energy crops: Starch producing species like: root crops, and cassava Woody biomass such as bamboo, and Leuceana

Figure 15.3 Available sources of biomass.

Third generation: In this generation, engineered energy sources, such as algae, are utilized as feedstock for biofuel production. Algae have no relation with the food or other crops and can be easily cultivated in lagoons or open ponds. Fourth generation: In the fourth generation, increased production of biofuel takes place by using metabolically engineered species (engineered algae, bacteria, and other microbes) along with CO2 capture and storage technique. Some of the species contain high lipid contents that can be degraded to polymeric hydrocarbons or other petroleum products. Biomass materials, which have absorbed CO2 while growing, are converted into fuel using the same processes as second-generation biofuels and the production rate is high. The research is at initial stage and requires high investment.

15.2

Scenario of agricultural wastes in India

India has a large extent of agricultural land, so a massive amount of agricultural wastes is produced every year. Management of agricultural wastes in India needs to be looked into, as with the development, enormous amount of wastes are being generated and will be increasing with the population growth. According to MNRE (2009), India produced around 500
550 Mt of agricultural residues (Agarwal et al., 2016) and also to feed around 1.35 billion people in 2025, India has to increase its agricultural production by 25%. Thus it will lead to a huge amount of agricultural waste generation. According to estimated results, 50% of the total agro-residues

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come from wheat, rice, and oilseeds crop. These bulks of agricultural wastes are not only a trouble for collection and management but it also hampers the environment and health of the people. The major techniques followed by farmers today are open dumping, burning of agro-wastes in open atmosphere in the field or in land filling. Although methods adopted by farmers are cheap and easy, it leads to various negative impacts on the agro-system as well as to the environmental climatic conditions. It produces lots of smoke that cause air pollution, particulate matter, smog, and disturbs soil physical, chemical, and biological structure including microbial population and also releases greenhouse gases that lead to global warming (Srivastava et al., 2015). It is the peak time to think on the situation and necessary action to be taken for a better environment. In the present situation, India has 500 Mt of biomass production capacity of which 17,500 MW power can be obtained (Kumar et al., 2015). The government is also promoting setting up of gasification plants for the generation of electricity.

15.3

Characteristics of agricultural wastes

The entire pathway (transportation and handling, pretreatment, conversion route, and enrichment of products) for the utilization of biomass as renewable energy source depends upon its type and physicochemical properties. These physical and chemical properties play a vital role for the design and development of different processes that convert the biomass to biofuel. The properties of the biomass vary depending upon the location, climatic condition, inherent composition, etc., the different physical and chemical characteristics of agricultural wastes are summarized in the following subsections.

15.3.1 Physical properties Agricultural wastes are mostly the lignocellulosic biomass and their properties include particle size, density (particle and bulk), grindability, and flowability. G

G

Particle size—Biomass particles are of irregular shapes and nonuniform size (needle shape, round, leaves shape, etc.) with different surface areas and it affects the feeding rate, mixing and fluidizing conditions, heat and mass transfer behaviors, and also the storage conditions. Variation in shape and size affects the conversion efficiency and energy requirement. Aspect ratio (length/diameter) is a parameter that describes the particle size. When a finely granulated particle converges to a spherical shape, aspect ratio reaches 1 (Shastri et al., 2014). Sieve shaker and digital imaging particle analysis are two known methods for the characterization of particle size. Thus biomass should be comminuted to convert the particle from as-received condition to desirable sizes for the user. Density—Density is defined as mass of the substance divided by its volume (kg/m3). In the field of bioenergy, two types of densities are considered. (1) Particle density and (2) bulk density. The density of the biomass depends upon various parameters, such as moisture content, shape and size of particle, and surface properties.

Bioenergy generation from agricultural wastes and enrichment of end products

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G

341

Particle density refers to the density of the individual particle but usually it is measured for a group of biomass particles. In a group the particle density is the total mass of all particles divided by the volume of the total particles occupying without the pore space volume. Bulk density refers to the mass of the particles to the total volume it occupies including the pore space volume. It plays an important role in the logistics work, that is, handling, transportation, and storage of biomass particles. The bulk density also depends upon the tapping whether it is loose fill or tight fill. Lam et al. (2007) measured the bulk density of switchgrass from 50 to 264 and 68 to 325 kg/m3 for loose and packed fill, respectively. Grindability—Grindability of biomass refers to the resistance to grind and it is an energy consuming process. Lignocellulosic materials are difficult to grind because of fibrous cellulose and lignin. Till today, there is no standard test for grindability of biomass but Hardgrove Grindability Index method is used for the grindability test. The disadvantage of this process is that it requires particle size of the range 0.6
1.2 mm. An alternative, Bond Work Index (BWI), was proposed by Williams et al. (2015) for grindability of biomass. It is defined as the amount of energy required to reduce the infinite particle size to 80% passing 100 µm (Cai et al., 2017). Greater the BWI, more energy is required to grind the material. Flowability—Flowability describes the flow characteristics of biomass, that is, how it flows from one point to other. It also plays an important role for the design and development handling, transportation, and storage. The parameters that describe the flowability of biomass are angle of repose, cohesion coefficient, compressibility index, and flow index (Lumay et al., 2012).

15.3.2 Chemical properties The chemical properties of biomass include its proximate analysis, ultimate analysis, calorific value, and compositional analysis.

15.3.2.1 Proximate analysis Proximate analysis describes the amount of moisture content, ash content, volatile matter, and fixed carbon in the biomass sample. Moisture content is the amount of water (internal and external) present in the biomass sample and it is expressed as the percentage of weight. The presence of moisture in the biomass affects the handling, storage, transportation, and the biomass conversion efficiency. It is determined according to ASTM E1756-08 (2015). The given sample is heated in a muffle furnace at a temperature of 105 C 6 2 C for at least 3 h until we get the constant weight and then the amount of heated sample is deducted from the raw sample to get the amount of moisture. Ash content is one of the most important properties of biomass. It is the amount left over residue after complete combustion of biomass. It is measured according to the ASTM E17551-01 (2015) where dried biomass sample is completely combusted in the muffle furnace in the temperature range of 575 C 6 10 C for 3 h. The percentage of ash content is calculated by diving the amount of ash to the amount of raw biomass sample. Ash contains an enormous amount of inorganic elements,

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such as sodium, potassium, calcium, alumina, silica, and iron, which describes its behavior of forming deposits in the boiler or gasifiers. Volatile matter is the amount of matter except moisture that is liberated when biomass is heated at high temperature in absence of air. It is the additive of condensable vapors and noncondensable gases. Higher amount of volatile matter signifies higher amount of liquid and gaseous biofuel. It is determined according to the ASTM E872-82 (2013) where biomass sample is put inside the crucible with lid to keep away from the air contact and then it is heated in a muffle furnace for devolatilization at temperature of 950 C for 7 min. After cooling in desiccators the weight loss is the amount of volatile matter. Fixed carbon is the amount the solid combustible residue that is left after eliminating the moisture, ash, and volatile matter. It is calculated based on the following equation Fixed carbon ð%Þ 5 100
ðmoisture content 1 ash content 1 volatile matterÞ

15.3.2.2 Ultimate analysis The ultimate analysis gives more widespread results as compared to the proximate analysis. It determines the amount of carbon, hydrogen, nitrogen, and sulfur content in the biomass. This analysis is performed using CHNS analyzer on a dry basis. The amount of oxygen is calculated by subtracting the total amount of abovementioned elements from 100. The typical amount of carbon and hydrogen in agricultural wastes varies from 40 to 50 and 4 to 6 wt.%, respectively.

15.3.2.3 Heating value Heating value (calorific value) of biomass refers to the energy content, that is, the amount of energy stored in the unit mass of biomass and it is expressed as Megajoule/kilogram (MJ/kg). Basically, there are two types of heating value. One is lower heating value (LHV) and the other is high heating value (HHV). LHV is the amount of heat stored in the biomass excluding the latent heat of vaporization of water, whereas HHV is additive of LHV and latent heat of vaporization of water. The standard method and instrument generally used for determination of HHV are ASTM D5865-13 and oxygen bomb calorimeter (Miller et al., 2010). Table 15.1 presents the proximate analysis, ultimate analysis, and HHV of some agricultural wastes.

15.3.2.4 Compositional analysis Lignocellulosic agricultural biomass comprises mainly three components, that is, hemicelluloses, cellulose, and lignin. The quantity of each component can be determined by using Van Soest’s method, National Renewable Energy Protocol method, or Technical Association of the Pulp and Paper Industry method. The amount of the three components varies depending upon the nature and part of the biomass being

Table 15.1 Proximate, ultimate and high heating value (HHV) of different agricultural wastes from literature. Agricultural waste

Proximate analysis (wt.%)

Reference

O

HHV (MJ/ kg)

36.10

17.10

Demirbas and Demirbas (1997) and Vassilev et al. (2012) Azeez et al. (2010) and Savova et al. (2001) Al Arni et al. (2010)

Ultimate analysis (wt.%)

Moisture content

Volatile matter

Fixed carbon

Ash content

C

H

N

Wheat straw

8.5

63

23.50

5.5
13.5

53.90

7

3

Corn cob

9.7

80.6

18.2

1.2
2.8

43.6

5.8

0.7

1.3

48.6

16.9

Sugarcane bagasse Corn stover

8.5

84.0

1.64

4.5
9.0

45.13

6.05

0.3




42.77

18.17

10.6

78.7

17.6

3.7
















17.8

Banana waste Tea waste

7.8 6.5

78.2 85.0

15.6 13.6

11.4 1.4

43.5


6.2


0.86


0.95


42.3


17.1 17.1

Barley straw Cashew nut shell Almond shell Flax straw Rice husk Hazelnut shell

6.9 10.43

78.5 69.31

4.8 19.26

5
9.8 1.0

41.4


6.2


0.63


0.01


51.7


15.7


8.7 7.9
9.0

79.7 80.3 81.6 69.3

4.9 8.8
28.3

2.3 3.0 18.4 4.3

54.7 43.10 36.9 52.30

7.5 6.20 5.0 6.50

0.3 0.68 0.4 5.20

0.3 0.09
9.2

37.4 49.90 37.9 26.8

20.2 17.0 15.29 19.3

12.5

69.1

11.0

7.3

54.1

6.12

5.23




34.53




Babul seeds

S

Demirbas and Demirbas (1997) Sellin et al. (2016) Demirbas and Demirbas (1997) Naik et al. (2010) Das and Ganesh (2003) Caballero et al. (1997) Naik et al. (2010) Raveendran et al. (1995) Demirbas and Demirbas (1997) and HaykiriAcma and Yaman (2010) Garg et al. (2016)

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considered. Accurate component analysis of biomass is essential as it determines the conversion efficiency based on the conversion method. Different components of biomass are described later. Cellulose is a natural polymer and repeating units of six carbon ring D-glucose. The three hydroxyl groups in each unit are linked with one another forming intramolecular and intermolecular hydrogen bonds that provide cellulose, a crystalline structure, and the stability (mechanical and chemical) (Saini et al., 2015). The acetal bonds link the C-1 of one pyranose ring to C-4 of the succeeding ring forming a long chain. The degradation temperature of cellulose is around 240 C
360 C and it mainly produces liquid products after conversion. Hemicellulose is the short and heterogeneous branched chain of polymers (pentose and hexose), which surrounds the cellulose and also links the cellulose with lignin. Unlike cellulose, it has a lower degree of polymerization and is amorphous in nature. Lignin is the aromatic, most complex, and high molecular weight polymer with cross linking of phenolic groups (Lebo et al., 2000). It is the amorphous cross-link resin that serves as a binder for the fibrous cellulose and hemicellulose components. Lignin is located mainly on the exterior of the microfibrils and also covalently bonded to hemicellulose and thus imparts rigidity to cell wall. Because it is covalently bonded, lignin is not readily depolymerized. Apart from these three components, biomass also contains some extractives and inorganic elements. Inorganic elements (Na, K, Mn, Mg, Cl, Al, Zn, Si, etc.) are basically present in the ash whereas extractives include proteins, fats, resins, gums, pectins, etc. that can be extracted using polar and nonpolar solvents (reference). Table 15.2 presents the compositional analysis of some of the agricultural wastes.

15.4

Pretreatment of agricultural waste biomass

Agricultural waste biomass generally requires certain modification in its structure and properties before its utilization in any conversion process. Thus pretreatment of Table 15.2 Lignocellulosic composition of different agricultural wastes from literature. Agricultural waste

Hemicellulose (wt.%)

Cellulose (wt.%)

Lignin (wt.%)

Rice husks Rice straw Wheat straw Corn cob Corn stalk Sugarcane bagasse Tea waste

12.0
29.3 23.0
25.9 23.0
30.0 31.9
36.0 16.8
35.0 28.0
32.0 19.9

28.7
35.6 29.2
34.7 35.0
39.0 33.7
41.2 35.0
39.6 25.0
45.0 30.2

15.4
20.0 17.0
19.0 12.0
16.0 6.1
15.9 7.0
18.4 15.0
25.0 40

Source: Cai, J., He, Y., Yu, X., Banks, S.W., Yang, Y., Zhang, X., et al., 2017. Review of physicochemical properties and analytical characterization of lignocellulosic biomass. Renew. Sustain. Energy Rev. 76, 309
322; Demirbas, A., Demirbas, A., 1997. Calculation of higher heating values of biomass fuels. Fuel 76, 431
434.

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biomass is an essential step to break complex molecular structures to simple monomers for a better output and is based on the pathway followed for the conversion process (Kan et al., 2016). The main objective of the pretreatment is to enhance the surface area, provide ease of accessibility to the enzymes, modify and solubilize the lignin in case of biological processes, and also to minimize the total cost of operation. The various technologies for biomass pretreatment include physical (washing, grinding, extrusion, etc.), thermal (steam explosion, torrefaction, and ultrasound/ microwave irradiation), biological (fungal, enzymatic, etc.), chemical (acid, alkali, ionic liquids), and thermochemical. Effect of pretreatment on biomass is shown in Fig. 15.4. Physical pretreatment of lignocellulosic biomass includes milling, grinding, chopping, extrusion, sonication, and high-pressure homogenization. The primary objective of this process is to disintegrate the biomass into smaller particle size, provide uniformity in its size, and enhance the surface area. The enhanced surface area provides an easy access to bacteria and enzymes in biological processes, whereas in thermochemical conversion, it promotes heat and mass transfer that facilitate uniform temperature within the particles (Kan et al., 2016). Sonication is another physical technique that uses sound energy to disrupt the particles. In a study by Carrere et al. (2010), sonication of biomass before anaerobic digestion (AD) enhanced the biogas yield in both batch and continuous processes. Use of gamma rays is another method; however, it doesn’t change the particle size of the biomass but it cleaves the glycosidic bonds in the biomass that decrease the cellulose crystallinity and enhance the surface area. Extrusion of biomass involves high-pressure treatment that converts biomass into pellet form which decreases the moisture content and enhances the volumetric energy density (Erlinch et al., 2006). The process produces no odor, less energy consumption, but it clogs the equipment. Thermal pretreatment is performed both at lab and industrial scale for easy dewatering, viscosity reduction, and for pathogens removal (Edelmann et al., 2005). For thermochemical conversion, drying and torrefaction are the two important pretreatment methods. Drying involves the removal of moisture from the biomass and increases the efficiency of the process. In torrefaction, biomass is treated thermally in inert atmosphere in the temperature range of 200 C
300 C, where sufficient Cellulose Physical

Lignin

Chemical

Pretreatment

Thermal

Biological Hemicellulose

Figure 15.4 Effect of pretreatment on biomass components.

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oxygen is removed from the biomass including water. Advantages of torrefied biomass include better grindability, high-energy density, lower hygroscopic nature, and better feeding in the reactor. However, torrefaction improved the quality of syngas produced by reducing the carbon dioxide and increasing hydrogen and methane content (Ren et al., 2013). In case of biological conversion processes, treatment of biomass in the temperature range of 50 C
250 C enhances the digestibility of biomass and also removes the pathogens. Thermal treatment includes steam explosion, hydrothermal treatment, liquid hot water, microwave heating, and ultrasound irradiation for biomass degradability. In steam explosion, the biomass is exposed to a hot pressed fluid at high pressure and in the temperature range of 150 C
240 C for a few minutes and then it is depressurized that explode the biomass leading to breakage of carbohydrate linkages which ultimately enhances biomass property (Biswas et al., 2011). Liquid hot water pretreatment is generally operated in the temperature range of around 180 C
190 C with low drying matter content (1%
8%). Hot water cleaves the hemiacetal linkages that liberate acids during biomass hydrolysis and leads to ether linkages breakage in the biomass (Wyman et al., 2005). Microwave heating and ultrasound irradiation are the other alternative pretreatment methods at present. Ultrasound improves the AD process by enhancing the biogas yield whereas microwave heating creates hot spots in the biomass (Bundhoo et al., 2013). Unlike various advantages, the prolonged treatment of biomass at higher temperature can cause unexpected reactions (Maillard reactions) that can form inhibitory substances and decrease the efficiency. Biological pretreatment involves the utilization of different types of enzyme and fungi. When compared, other pretreatment methods are less energy consuming because they are performed at milder conditions and economical (Yu et al., 2013) but are slower as it requires several days. This pretreatment is performed by inoculating the substrate with fungal spores (e.g., white rot basidiomycetes and actinomycetes) or by enzymes (e.g., ferulic acid esterases and hemicellulases) (Lloyd and Wyman, 2005). Enzymes are used for the hydrolysis of lignin. White-rot fungi were used for the degradation of lignin while minimizing the polysaccharide consumption (Sun and Cheng, 2002). Biological pretreatment of agricultural wastes using rot-fungi or rots is a green technique and economical that does not involve any energy input for lignin degradation. Biological pretreatment basically involves the use of various types of fungi for lignocellulosic biomass. Chemical pretreatment includes the involvement of different chemicals, such as acids, alkalis, or ionic liquids to break down the organic components present in the biomass. Basic principle of this pretreatment is to break the lignin
carbohydrate bond and crystalline cellulose structure. Different acids for the pretreatment are H2SO4, HCl, HNO3, and H3PO4. Dilute acid pretreatment can be performed in either batch or continuous mode (Lloyd and Wyman, 2005). The presence of certain elements provides catalytic role and enhances the degradation process but its excessive use sometimes degrades the process by the loss of fermentable sugar or increase in the pH which requires neutralization. Dilute acid
based treatment of lignocellulosic biomass was carried out for the production of furfural (Zeitsch, 2000). In biological conversion processes, alkali pretreatment is more preferred.

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Various alkalis, such as NaOH, Ca(OH)2, and KOH, are used, which degrade the lignin and carbohydrate link. Liquid ammonia
water mixture has been used by many researchers to pretreat lignocellulosic biomass (Dale et al., 1996). It is a dry-to-dry process performed by adding liquid ammonia and prewet substrate in a pressurized reactor and operated for 5
45 min. Apart from this, oxidative and ionic liquid pretreatments are also emerging nowadays. Ionic liquids are green solvents with strong chemical stability, low vapor pressure, and nonflammability and are used to pretreatment of lignocellulosic waste for the production of sugar. Organosolv pretreatment is another technique that is performed in the temperature range of 100 C
250 C with solvents, such as methanol and glycerol. This extracts lignin and solubilized hemicelluloses by breaking the bonds between hemicelluloses and lignin as well as glycosidic bond in hemicellulose (Zhao et al., 2009). Among other pretreatment techniques, thermochemical pretreatment is a combination of both thermal and chemical treatment to improve the product yield and lessen the time required for the conversion.

15.5

Routes for conversion

The rapid development in the country in every sector has increased the use of conventional energy sources and it has also created pressure on the present generation regarding the depletion of fossil fuel reserve. In addition to it, the use of these fuels also releases toxic gases (SOx, NOx, CO2) to the environment, which lead to the change in the environmental conditions and ultimately to the global warming. Therefore, from the last few decades, there has been an increasing interest in the treating biomass as valuable resource rather than a reject. Biomass has enough potential and availability to be converted into valuable products, such as biooil, biogas, chemicals, or fertilizers. In addition, it will put a curtain on biomass disposal or land filling, lessen the environmental problems, and will provide wealth and employment. At present irrespective of various technologies, biochemical and thermochemical conversion processes have gathered much attention in fruitful utilization of agricultural wastes. Depending upon the type, extent, and properties of biomass, any of the technologies can be used. In general, biochemical conversion processes need microbes and bacteria for the biomass degradation, thus biomass with lesser amount of lignin is preferred. For thermochemical processes, lesser amount of moisture is preferred as it reduces the use of extra energy for drying.

15.5.1 Biochemical conversion Biochemical conversion of agricultural biomass waste to bioenergy is an environment-friendly and sustainable technique. It involves the use of different microbes and bacteria that helps in decomposition of biomass waste to energy. AD and fermentation are the two important biochemical processes.

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Refining Biomass Residues for Sustainable Energy and Bioproducts

AD is a biochemical process that is performed in oxygen-free environment with the help of different microorganisms that degrade the organic components into biogas. Biogas is a mixture of carbon dioxide and methane produced from biodegradable material by the enzymatic action of microorganisms under anaerobic condition and also traces of other gases, such as H2S (Bala et al., 2019). Quality of biogas varies from digester to digester according to the type of agricultural waste. There are four steps in AD process: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Each step is carried out by a group of different microorganisms. In the first step, Hydrolytic bacteria (Streptococcus, Bacillus, Enterobacteria, etc.) convert the complex polysaccharides, protein, and fat into sugar, amino acids, and fatty acid with their enzymatic action. The enzymes involved are cellulase, amylase, protease, lipase, etc. (Divya et al., 2015). Further, the degradation of hydrolysis end products is carried out by facultative and obligatory anaerobes (Micrococcus, Syntrophomonas, Pseudomonas, etc.) into fatty acids and organic acids by secreting enzymes, such as acetate kinase, formate hydrogen lyase, and acetaldehyde dehydrogenase (Divya et al., 2015). The organic acids formed in acedogenesis are converted into acetic acid by acetogenic bacteria (Syntrophomonas, Clostridium, Syntrophobacter) in acetogenesis phase by secreting hydrogenases. Methanogenesis is carried out strictly by anaerobes. The methanogenic bacteria (Methanosarcina, Methanococcus, Methanobacteria, etc.) convert the acetic acid into CH4, CO2, traces of H2S, N2, H2, siloxanes, etc. known as biogas by secreting the enzymes formylmethanofuran dehydrogenase, methyl coenzyme m-methyl transferase, etc. (Divya et al., 2015). The AD is a widely used technology for wastes with high moisture content (80%
90%). Different types of substrates, such as animal manure, fruit and vegetable waste, municipal solid waste, agricultural residues, microalgae, and industrial waste water, can be used as substrate for biogas production. AD requires an adequate amount of carbon and nitrogen content to balance the C/N ratio for process stability. The optimum ranges of C/N ratio, pH, and temperature are 20
30, 5.5
8.5, and 30
40 (mesophilic), 50
55 (thermophilic), respectively (Zhang et al., 2016). Biogas yield and composition vary according to the substrate and inoculum used. The main constituents of biogas are CH4 (50%
70%) and CO2 (30%
50%) with other trace gases (Bala et al., 2019). The methane yield achieved from AD of sugarcane bagasse was 299.3 mL/g volatile solids (VS) using 10% ammonia pretreatment (Hashemi et al., 2019), while the biogas production from wheat straw was 615 N mL/g VS after pretreatment at 180 C (Rajput et al., 2018). Biogas produced can be used in spark engines or in turbines. Further, it can also be upgraded to natural gas with the removal of CO2. The left-over solid, that is, digestate, can be used as soil conditioner as it has important nutrients content. Agricultural waste 1 microorganism ! Biogas 1 digestate

G

Fermentation is another biological process that operates in the absence of oxygen and involves the help of microorganisms to convert sugar to basically alcohol, acid, or mixture of gases (CO and H2S). The biomass is converted to smaller particles and then starch is converted to sugar and then to alcohol with the help of enzymes. A variety of substrates, such as starch biomass, lignocellulosic biomass, and algal biomass, is converted into fermentable sugar. However, fermentation of agricultural wastes is difficult as it has complex long-chain polymeric molecules and requires acid or enzymatic hydrolysis before the sugar is fermented to alcohol. Fermentation is lengthy time-consuming process, with chances of contamination of other foreign microorganisms. It is a promising approach but high cost is preventing its movement from laboratory to commercial world. Generally,

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butanol and ethanol are produced by fermentation process. Biobutanol is produced by microorganism Clostridium spp. using sugar produced from different biomass (Li et al., 2019). The process is known as acetone, butanol, and ethanol (ABE) fermentation that comprises two stages of acedogenesis and solventogenesis (Ibrahim et al., 2018). In a study, ABE production achieved from corn processing waste was 19.22 g/L that contained the butanol yield of 11.65 g/L by using Clostridium beijerinckii SE-2 (Zhang and Jia, 2018). Biobutanol has higher heating value, lower volatility, less ignition problems, and viscosity than bioethanol (Ibrahim et al., 2018). Beside these advantages, bioethanol is a well-established technique than biobutanol due to less cost and high production yield than biobutanol. Yeast, such as Saccharomyces cerevisiae, Candida albicans, Pichia stipitis, and Kluyveromyces, are mostly used to generate bioethanol because of their high productivity .1 g/L/h and requirement of simple and inexpensive growth media (Azhar et al., 2017). Basically, three fermentation techniques are used to generate bioethanol, separate hydrolysis and fermentation, simultaneous saccharification and fermentation and simultaneous saccharification and cofermentation. The optimum temperature, pH, and agitation speed range of fermentation are 20 C
35 C, 4
5, and 150
200 rpm, respectively, for S. cerevisiae (Azhar et al., 2017). A yield of 11.6 gEtOH/galgae was achieved from fermentation of industrial algae waste by using S. cerevisiae as fermentation microorganism (Alfonsı´n et al., 2019). Beside yeast, some facultative anaerobic genetically engineered bacteria, such as Zymomonas mobilis, are also used for bioethanol production (Xia et al., 2019). Agricultural waste 1 ezymes ! Alcohol

15.5.2 Thermochemical conversion These processes mainly comprise pyrolysis, gasification, and combustion that require high temperature for the treatment of agricultural wastes into various useful products, such as biooil, syngas, and biochar. G

Pyrolysis is the thermal depolymerization of biomass in an inert atmosphere in continuous supply of heat. It is an endothermic process and is performed in the temperature range of 400 C
700 C. The sources of substrate for biomass pyrolysis are agricultural wastes (corn stover, corn cob, rice husk, wheat straw, etc.), woody biomass (redwood, pine, beech, teak, etc.), energy crops (bamboo, sorghum, etc.), and also municipal solid wastes. The rapid heating of biomass produces vapors that are the mixtures of various hydrocarbons and some part of it can be condensed to give an organic liquid called biooil (Gupta and Mondal, 2019). Biooil is a brown colored viscous complex mixture of large number of organic compounds with some amount of water content. It has the heating value in the range of 20
30 MJ/kg and is also a source of valuable chemical compounds. However, its properties can be enhanced after upgradation using various techniques. The noncondensable gas that leaves the system is a mixture of valuable gases (CO, CO2, H2, CH4, etc.). These gases have good combustion properties and can be used as gaseous fuel. The remaining left-over residue is the carbon-rich compound called biochar. Biochar is a multifunctional material that can be used as solid fuel, adsorbent, sensor, fertilizer, etc. However, product distribution in the pyrolysis process primarily depends upon the compositional analysis of biomass and on the interaction among them. It is also influenced by

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various process parameters, such as temperature, heating rate, inert flow rate, particle size, and residence time. Based on these parameters, it can be classified as slow, fast, or flash pyrolysis. Slow pyrolysis is performed as lower temperature, lower heating rate, and longer vapor residence time, and fast pyrolysis is operated at high temperature, high heating rate, and short vapor residence time. The primary end product for slow pyrolysis is biochar which is around 35%
40%, and biooil and pyrolytic gas are 30% and 30%
35%, respectively, whereas for fast and flash pyrolysis, the primary product is biooil which is around 75%, and biochar and pyrolytic gas are 12% and 13%, respectively (Hossain and Davies, 2013). Agricultural waste 1 heat 1 inertðN2 =ArÞ ! Biooil 1 biochar 1 pyrolytic gas G

Gasification is another thermochemical process that is performed in the partial oxidative (air, oxygen, or carbon dioxide) atmosphere at certain high-temperature range from 800 C to 1000 C. The substrates for the gasification are similar to that are used in pyrolysis process, such as agricultural wastes, woody biomass, and energy crops. The main end product of the gasification process is syngas (CO 1 H2) around 85% along with some amount of tar with around 5% and biochar around 10% (Bridgwater, 2004; Ram and Mondal, 2018). Some amount of CH4 and other hydrocarbons are also produced. The gases produced have ample amount of calorific value and can be used as fuel for engines or turbines. Syngas produced can be used in the Fisher
Tropsh synthesis for the production of valuable liquid hydrocarbon mixtures (methanol, ethanol, etc.). The process parameters are temperature, heating rate, particle size, biomass feed rate, equivalence ratio, etc. Gasification can be performed in two different modes; fixed and fluidized bed. Fixed bed process gases with lower calorific value (4
6 MJ/N m3). Today, fluidized bed gasification process is mostly preferred since it provides uniform temperature distribution in the gasification zone. Agricultural waste 1 heat 1 partial oxidation ! Syngas 1 tar 1 biochar

G

Combustion is a fully oxidative high-temperature thermal degradation process that involves the conversion of chemical energy of biomass to heat and power along with carbon dioxide and water. The energy produced can be utilized in steam turbines, boilers, furnaces, etc. The substrates for this process are similar as used in pyrolysis and gasification processes. For any biomass to be combusted, the moisture content should be below 50%. The net bioenergy conversion biomass combustion is up to 40%. However, its efficiency can be upgraded by cofiring the biomass with coal. Agricultural waste 1 heat 1 excess air=oxygen ! Heat=power

15.6

Enrichment of end products

The various end products obtained after thermal and biological conversion of biomass contain certain impurities and thus cannot be used directly as energy source.

Bioenergy generation from agricultural wastes and enrichment of end products

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Thus it needs to be treated for an efficient use. The products obtained from different processes require different technologies for its enrichment. G

G

G

For pyrolysis, the major problem is associated with the biooil. Their direct applications as fuels are limited by the problems of high viscosity, high oxygen content and corrosion, and their thermal instability. Therefore biooils should be upgraded using proper methods before they can be used in diesel or gasoline engines. Hydrotreating, hydrocracking, esterification, emulsification, and catalytic pyrolysis are various techniques that can improve the quality of biooil (Xiu and Shahbazi, 2012). Hydrotreating is a nondestructive, simple hydrogenation process that removes the oxygen from the biooil and the common catalysts used for this process are sulfide CoMo/Al2O3 and NiMo/Al2O3 systems. Hydrocracking is another thermal process accompanied by cracking at high temperature at relatively high pressure. Dual function catalysts are used where zeolites provide the cracking function, whereas platinum and tungsten oxide catalyze the reaction. Polar solvents reduce the viscosity of the biooil and also increase the heating value. Also, biooil can be emulsified with other fuel source. Biooil with the help of surfactants can be emulsified with the diesel oil. Thus upgraded biooil can be used in engines or in other fields as a source of energy. For gasification, hydrogen-enriched fuel gas has gathered attention due to the use of syngas after gasification in gas engine or gas turbine
based power generation applications. The enrichment of syngas can be done by catalytic gasification (metal oxides) or with some modification in gasification zone of the gasifier for better interaction of the reactants. End product of gasification contains CO, H2, CH4, and CO2 and traces of NH3, HCl, H2S, HCN, CS2, tar, particulate maters, etc. as impurities. To use it as synthesis gas, these impurities must be removed so that CO and H2 enriched. For this the exit gas from gasification chamber passed through cyclone separator to remove particulate materials followed by water wash, where NH3 and HCl washed away with water. The exit gas, thereafter, should be passed through ZnO guard for H2S followed by active carbon filter for traces of NH3, HCl, and CS2. Finally, it should be passed through silica gel for moisture trap and can be utilized effectively. In AD, raw biogas is a mixture of CH4 (40%
65%), CO2 (35%
55%), traces of H2S, N2, H2, water vapor, and other components (siloxanes, volatile hydrocarbons, chlorinated and fluorinated hydrocarbons, etc.). The fraction of CH4 is the combustible part of biogas and its concentration determines the quality of biogas. Biogas upgradation is the removal of contaminants (H2S and other toxic components, CO2, and water vapor) from biogas stream to provide a methane rich ( . 95%) gas. Biogas upgradation techniques are mainly divided into adsorption, absorption (physical and chemical), and membrane separation (Miltner et al., 2017). Pressure swing adsorption is used to adsorb the CO2, H2S, N2, and O2 from biogas stream on the basis of molecular size and thus providing 96%
98% pure methane. Removal of H2S prior to this process is recommended as it irreversibly binds to adsorbent material causing toxicity to it. Zeolites and activated carbon are the commonly used adsorbents for biogas upgradation. Physical water scrubbing method is the most commonly used technique for biogas purification based on the increased solubility of CO2 and H2S in water in comparison to CH4. The pressurized biogas stream (6
10 bar) is injected into an absorption column that is filled with packing material to increase the gas
liquid mass transfer, from the bottom side of the column, while the water is injected from the top part of the column. Amine scrubbing, caustic solvent scrubbing, and amino acid salt solutions are the separation techniques used for CO2 and H2S removal from biogas stream. Commonly used amines for chemical absorption are monoethanolamine,

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Refining Biomass Residues for Sustainable Energy and Bioproducts

diethanolamine, aminoethoxyethanol (DGA), etc. Amine scrubbing generally consists of an absorber and a stripper unit. The process provides efficient removal of H2S and CO2. In membrane separation, permeable membranes are used that enrich concentration of methane by trapping the other impure gases. In fermentation, distillation is the most common technique used for liquid biofuels upgradation. It works on the principle of volatile nature of substances. The separation of more volatile compounds occurs primarily from less volatile heavy compounds. The various types of distillation, conventional distillation, azeotrpic distillation, extractive distillation, and molecular distillation are used for products upgradation. Other purification technologies include absorption, adsorption, super critical fluid extraction, and liquid
liquid extraction. Currently, adsorption with molecular sieve for bioethanol purification is gaining attention because the process requires less energy input than distillation process. Biobutanol and bioethanol are also purified by membrane technology (pervaporation) and gas stripping method (Pulyalin et al., 2015).

The upgraded end products from the abovementioned processes have utility in various fields and can become an effective substitute for the available energy resources.

15.7

Conclusion

India generates enormous amount of agricultural wastes every year and it affects its economical growth if these wastes are not utilized in a proper way. In appropriate collection, open dumping of these affects the environmental condition and also the human health. Several problems associated with the biomass include high moisture, low calorific value, large particle size, different composition, etc. Effective pretreatment can solve these basic problems but there is a need to think for waste to bioenergy approach for effective utilization of agricultural wastes. Various techniques (pyrolysis, gasification, AD, etc.) are available for the bioenergy generation. Research in the field of upgradation of products to high-energy feedstock and its successful utilization will lessen the load on the fossil fuels. However, awareness among the people about these processes is very less and they pay little attention in utilizing renewable energy sources. Therefore there is a need of promoting R&D activities on utilizing the agricultural wastes that will not only reduce the amount of these wastes but it will also open the doors for bioenergy generation which decrease the gap between demand and supply of energy. MNRE has installed various biomass power generation plants along with many gasification plants in various states of India for effective utilization of local available biomass wastes.

Acknowledgment The authors acknowledge the necessary facilities provided by the Department of Chemical Engineering and Technology, Indian Institute of Technology (BHU), Varanasi, for undertaking this work.

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Further reading Chundawat, S.P.S., Balan, V., Da costa Sousa, L., Dale, B.E., 2010. Thermochemical pretreatment of lignocellulosic biomass. Bioalcohol Prod. 24
72.