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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: HVO – hydro‐treated vegetable oils and HEFA – hydro‐processed esters and fatty acids.