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mainly into two types, traditional/conventional and modern (Gupta et al. 2014, pp. 1–17). Conventional use is the direct utilization of source i.e. biomass (e.g. wood, agricultural waste, traditional charcoal etc.) combustion to obtain energy. Modern use is the indirect use i.e. conversion of biomass using different technologies into biofuels. Apart from biofuels, other recent bioenergy technologies includes bio‐refineries (a range of bio‐products are produced consuming biomass), biogas (obtained via anaerobic digestion of bio‐residues) and wood pellet heating systems. Bioenergy can be stored for a long period as opposed to other renewable energy sources. By 2015, its share in total final energy consumption was nearly 10%, and 1.4% in worldwide electricity production. Approximately, 80% of bioenergy is consumed traditionally in the developing countries i.e. for cooking, space heating, and lighting (Lund et al. 2016, pp. 36–38). In 2019, bioenergy power generation capacity was 124 GW with a 5% growth in the year. Its contribution is roughly 5% in the worldwide renewable generation capacity, and China contributed more than half (3.3 GW) of new capacity added (6 GW) in the same year (IRENA 2020b).

Large‐scale production of biofuels can be achieved when these are produced using agricultural crops, crop residues, agro‐industries wastes, forests residue etc. At present, production cost of biofuels is high, conversion efficiency is low and its cost‐effectiveness is also low compared with other energy sources. To mitigate these issues, scientific community is doing lot of research worldwide. Advances are taking place in technologies for pre‐treatment of lignocelluloses and cost‐effective production method for biofuels with no or minimum by‐products (Lund et al. 2016, pp. 36–38). Bioenergy can significantly contribute in energy sector in the twenty‐first century. It will be a key component for the development of rural areas, agricultural sector and forestry.

      2.3.5.1 Biopellets and Biogas

      Biopellets are the simplest form of bioenergy yet competitive to crude oil or natural gas for the household uses in Western countries. Research in pelletization process of leaves in order to use them as additives or substitute to wood provides an alternative for biopellets production (Verma et al. 2012). Transforming biomass to biogas is a modern use of bioenergy, anaerobic digestion of biomass using a digestor leads to formation of biogas. Different types of digestors used for the anaerobic digestion are fixed dome, floating drum and plug flow (Gupta et al. 2014, pp. 1–17). Chemical composition of biogas is a function of the type of feedstock, design of digestor used and processing conditions (Lund et al. 2016, pp. 36–38). Replacing wood and coal with biogas in rural areas will decrease the emission of GHGs (Pathak et al. 2009). Moreover, biogas and biomethane produced from medium and large‐scale plants can substitute the natural gas usage as fuel in transportation, leading to decarbonization (IRENA 2019b).

      2.3.5.2 Bioethanol and Biodiesel

      Most frequently used biofuel is bioethanol. It can be formed from three types of feed stocks, namely sugar‐based (most common), starchy and lignocellulosic (require pre‐treatment). Production of bioethanol from sugar‐based crops was the most common method till 2003 and contributed nearly 60% in the worldwide production (Gupta et al. 2014, pp. 1–17). After sugar‐based crops (sugarcane, sorghum etc.), starch feedstock (corn, wheat, barley etc.) is used for bioethanol production followed by lignocellulosic feedstock (wood, straw, corncob etc.). Chemically lignocelluloses are mainly composed of carbohydrates (cellulose and hemi‐cellulose) and lignin along with some other quantities of biomass. Owing to the varying composition of lignocelluloses, several pre‐treatment processes were explored to obtain fermentable sugars. Advance researches are conducted for the various pre‐treatment processes because usage of lignocellulosic feedstock can lead to cost‐effective biofuel (Lund et al. 2016, pp. 36–38). Pre‐treatment process can be biological, physical and chemical depending upon the source and means of conversion procedure. Among the different methods for pre‐treatment, biological method which is a microbial process consumes less energy compared with the other two processes and moreover, a safe and environment‐friendly process (Potumarthi et al. 2013). Physical pre‐treatment can be done via mechanical comminution, steam explosion, ultrasonic radiation and extrusion process (Gupta et al. 2014, pp. 1–17). Under the chemical pre‐treatment methods, acid hydrolysis, alkaline hydrolysis, ammonia hydrolysis and ozonolysis are different available methods. Use of bioethanol‐blended gasoline in automobiles offers a viable option to cut down the use of petrol. This will also reduce the GHG emission along with contribution in achieving goals of Paris agreement.

Biodiesel is another biofuel that can be produced from oil‐based feedstock, animal fat wastes and waste cooking oils. Animal fat wastes are commonly obtained from chicken, cow, pork lard, fish and other animals. Oil‐based feedstock can be edible (soybean, corn etc.) or non‐edible. But, according to food first principle, use of edible feedstock is not promoted worldwide. However, continuous evolving technologies for converting biomass to fuels have enabled the bioenergy industry to utilize more of lignocellulosic biomass. Technological developments open doors for new types of feedstock for bioenergy production e.g. forest biomass, agricultural residue, perennial grasses and trees and even municipal waste (Lund et al. 2016, pp. 36–38).

      2.3.5.3 Advanced or 2G Biofuels

Currently, biofuels production is based on edible feedstock termed as conventional biofuels or first‐generation (1G) biofuels. In the last 20 years, conventional biofuels were promoted by the policymakers in view of changing climate conditions. However, after 2007, various discussions have started worldwide on the sustainability issues of these biofuels (IRENA 2019b). Biofuel usage was raising concern on food security, price of food and feed, and land use change. Production of 1G biofuels has negative impact on food resources, and there is a high probability of land use change and indirect land use change because of high demand of edible feedstock. Recent advances in production technologies lead to advanced biofuels or second‐generation (2G) biofuels, future of sustainable bioenergy (IRENA 2019b). Advanced biofuels are derived from non‐edible feedstock and other biomass such as agricultural waste, animal fat wastes and municipal wastes etc. On the basis of production method adopted by advanced biofuel industry, they can be classified into four categories which are tabulated in Table 2.1.

      Potential research is going on in search of algae (3G) biofuels which are based on algae consumption for biofuel production. Recent study (www.energy.gov) has found that algae could be richer in biofuel production compared with conventional feedstocks. There are some limitations associated with algae usage such as its mass production can increase the total cost by 40% (Oh et al. 2018). Thus, more inputs are needed from scientific community in future to pave the path for 3G biofuels.

Table 2.1 Categorization of advanced biofuel industry based on production process.

      Source: Based on Ref. IRENA (2019b).

Method	Biomass type	Biofuel
Microbial conversion	Lignocellulosic e.g. stalks, corn stover etc.	Bioethanol or biobutanol
Transesterification	Waste and/or non‐edible vegetable oils or animal fats	Fatty acids and methyl esters (FAME) i.e. biodiesel
Hydro‐treatment followed by alkane isomerization and cracking	Waste and/or non‐edible vegetable oils or animal fats	Drop‐in fuels (HVO/HEFA)*
Thermochemical/ gasification	Waste and/or non‐edible vegetable oils or animal fats	Biocrude/ syngas (converted to renewable gasoline)

      Note1:

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