The organic matter existing on the Earth’s surface produced by photosynthesis is referred to as biomass. They include all aquatic plants and agricultural waste, municipal solid waste (MSW), sewage, animal wastes, forestry residues, and industrial wastes [29]. Since biomass is an abundant carbon-rich natural resource readily available in the environment utilized effectively to replace fossil fuels [10, 29]. The utilization of biomass would not emit CO2 into the atmosphere, and it is capable of consuming the same amount of carbon in growth that it releases during fuel usage [30]. Biomass has high potential in energy generation as it possesses clean, inexpensive, and renewable energy sources, mainly when derived from agricultural and forest residues [30]. In the world, China, the US, Germany, Brazil, India, Japan, and the UK are the leading countries in power generation from renewable resources [3] (Figure 3.4).
India is one of the leading energy-producing countries in which the state of Punjab was the topmost energy-producing state from biomass, waste, and bagasse (Figure 3.5). Biomass sources are classified generally into four types: first-generation (1G), second-generation (2G), and third-generation (3G), and now it also includes genetically modified biomass as fourth-generation feedstock (4G); and waste [6, 8]. All these biomasses using biochemical, thermochemical, and hydrothermal methods converted into biofuel products, viz., alcohols, biogas, bio-oil, and biodiesel [8]. Principal processes of conversion involve fermentation, transesterification, gasification.
Figure 3.4 Global renewable energy production and consumption in 2020 [3].
3.3.1.1 Sugar and Starch-Based Biomass (First-Generation - 1G)
Ethanol is the predominant biofuel product dependent on sugar (molasses) and starch (grains) as first-generation (1G) feedstock (Table 3.1). The restriction on sugar and starch-rich biomass as petroleum-derived transportation fuels involve increased demand for food supplies with growth in the global population, clearing agricultural land [31, 32]. Because of these concerns, studies have focused on using food crops by-products as a sustainable substrate.
3.3.1.2 Lignocellulosic Biomass (Second-Generation - 2G)
Lignocellulosic biomass, a second-generation (2G) feedstock, is an abundantly available agricultural residue that generates cellulosic ethanol as renewable biomass and ethanol fuel [31] (Table 3.1). Limitation behind this type of biomass conversion includes expensive pretreatment and hydrolysis (specifically via enzymes), lack of pentose and hexose simultaneously saccharifying potential microorganisms. Nowadays, the Indian government wants to establish ethanol production from the non-feedstock substrate (Figure 3.6) rather than first-generation biomass like sugarcane. Second-generation biomass involved in 2G ethanol production in India includes rice straw (47%), cotton stalk (11%), and municipal solid waste (MSW), wheat stalk, soya stalk, bagasse, maize, bamboo, and corn cob (each 6%) utilized as a renewable resource [33].
Figure 3.5 Sustainable renewable energy production from various biomass resources in states of India in 2020 [4].
Figure 3.6 Biomass utilization in India (2019-20) for production of second-generation ethanol (2G) [33].
Table 3.1 List of first-generation (1G) and second-generation (2G) biomass, the process of conversion and biofuel products [27, 32, 34–39].
| Types of biomass | Biomass | Process of conversion | Biofuel products | Reactor/fermentation process | Reference |
| Sugar and starch-based biomass (first-generation - 1G) | cassava bagasse from the cassava starch industry with steep corn liquor | Dilute acid pretreatment and enzyme hydrolysis (glucoamylases and cellulases) | n-butanol | fibrous-bed bioreactor | [32] |
| Sweet potato with dairy cattle manure | Anaerobic co-digestion | Biogas | Semi-continuous digesters | [34] | |
| sugarcane molasses | Anaerobic fermentation by Bacillus species (mesophilic condition) | Biofuel (ethanol, butyric acid, acetic acid, and lactic acid) | - | [35] | |
| Lignocellulosic biomass (Second generation - 2G) | Paddy straw supplemented with fruit waste | Alkali treatment with 2% NaOH/ Hydrolysis by Commercial cellulase enzymes (Palkonol and Palkosoft)/ fermentation by Saccharomyces cerevisiae HAU-1 and Candida sp. | Bioethanol | SSCF | [27] |
| Cassava stem, leaves, and peels | Microwave (MW)assisted alkali pretreatment (300 W, 7 min)/Hydrolysis by triple enzyme cocktail with detoxification/ fermentation by Saccharomyces cerevisiae | Bioethanol | SHF | [36] | |
| Napier grass | Hydrolysis by Aspergillus niger and Trichoderma reesei/fermented by Zymomonas mobilis | Bioethanol | SSCF | [37] | |
| Sugarcane trash | Crude glycerol assisted transition metal and alkali pre-treatment/ hydrolysis by commercial cellulase/fermented by Saccharomyces cerevisiae | Bioethanol | - | [38] | |
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