**8.4 Biofuel production**

Plant residues contain cellulose, hemicelluloses and lignin with small fractions of sugars, pectin, protein, nitrogenous, lipids, tanins and inorganic materials [58]. Lgnin mostly provides the structural support and is almost resistant to chemical reactions and biological degradation compared to cellulose and hemicelluloses and thus resists fermentation [59–61]. In crop plants, the nonfood portion such as stalk, husk, straw, stover and bare corn cob contain lignocellulosic biomass. As in agriculture, cereals occupy the maximum area and production so also the largest quantity of such lignocellulosic materials. The residue management in cereal-cereal system such as in rice-rice and rice-maize/wheat is the biggest challenge before researchers. Very often the farmers opt for onsite open burning of the crop residues to get rid of huge biomass with higher lignocellulosic materials in it [62]. But with the advent of innovative green energy technologies, such so called wastes are now converted into precious biofuels to mitigate the growing demands.

Biofuels are produced through pretreatment of lignocellulosic materials by fungi, bacteria and enzymes that break down the lignin, a complex polymer and degrade cellulose and hemicelluloses to corresponding monomers and sugars for effective fermentation and fuel conversion [63]. The pretreatment is mostly chemical or biological but it could be mechanical and physicochemical too that result in increased surface area and porosity, and decrease in crystalinity. Biomass degradation results into ethanol, biodiesel, biobutanol, syngas, and woodtar/oil. The ethanol produced from crop residues is known as 2G bioetahol. Depending on the feedstock and process design, several by-products such as stillage, evaporator condensate and solubles, spent cake and/or distiller's grains are produced which can be used in agricultural amendment, civil construction or sanitary landfills. Stillage is a nutrient rich biodegradable material rich in both total suspended solids (TSS) and Chemical Oxygen Demand (COD) that requires significant processing for remediation. Lignin, a waste product from bioethaol plant is used for generating heat energy required for other processes and thus the final produce is in a form of ash. Ash is alkaline in reaction with significant quantities of Si, P, K, Ca, AL, Fe, and Mg in it which can very well be used in agriculture. In **Figures 4** and **5**, the harvested paddy straw is gathered by square baler and stacked in the collection centre at Thuapali village of Bargarh distract in Odisha, India as a pilot study programme under the direct supervision of the BPCL, India.

**Figure 5.** *Stacking of square bales of rice straw in stockyard in Bargarh district of Odisha.*
