**2.4 Biofuels**

The second-generation biofuels, commonly prepared from inedible crops, woody crops or lignocellulosic biomass, agro waste, or unwanted plant, are potent reply to the food versus fuel feud as they utilize leftover portion of agro waste. Inedible feedstock is commonly used for the second-generation biofuels, i.e., jatropha, grasses, wastes vegetable oil, wood chips, etc. Alcohol generation from rapid growth plants could be produced by enzymatic activities to isolate out the sugars from lignin fibers of the biomass. Syngas, a mixture of hydrogen and carbon monoxide, can be synthesized on thermochemical treatment of biomass. Hydrogen thus prepared can be used as fuel, and other hydrocarbons can be used as add-on to the gasoline [63] . Recently, most of the gasoline available is blended with certain percentage of ethanol to reduce carbon footprint. The effective conversion of cellulose into ethanol has got major prospective due to the ubiquitous obtainability, plentitude, and comparable inexpensive cellulosic plant materials. The banana residue includes banana fruit (pulp and peels) and lignocellulosic biomass can be a potential source for biofuels [64]. Srivastava et al. [65] had successfully utilized Saccharomyces cerevisiae for generation of bioethanol out of rice husk up to yield of 250 mg/g dry biomass after 6 days of fermentation. Singh et al. [66] had enzymatically hydrolyzed the pretreated rice husk with alkali under microwave condition for the generation of biofuel. They have successfully utilized Scheffersomyces stipites and S. cerevisiae yeast for the fermentation. The ethanol production with S. cerevisiae was to be 0.3–0.39 g/g; with Scheffersomyces stipites, waste 0.24–0.35 g/g, respectively. Chukwuma et al. [67] had adopted fermentation process of rice husk using Aspergillus fumigatus, Aspergillus niger, and Saccharomyces cerevisae for the generation of biofuel. On fermentation with Aspergillus fumigatus, treating rice husks shows the at most cellulose of 45 ± 3.31%, hemicelluloses of 31 ± 3.00%, reducing sugar of 2.60 ± 0.30%, carbohydrate of 19.52 ± 10.05%, and non-reducing sugar of 16.92 ± 9.75% producing ethanol yield of 6.60 ± 0.48% with palm wine yeast, while 5.60 ± 0.42% yield was with bakers. Slow pyrolysis activity by thermogravimetric analysis had been investigated to estimate and compare the effective utilization agro waste such as corncob, rice husk, wood chips, wheat straw, bagasse, etc., for biofuel conversion [68]. The corncob was observed to deteriorate with an enhanced rate over lower temperature. On the whole, the activation energy was observed to be enhanced at the reduced temperature range (250–400°C), and that was reduced in the enhanced temperature range (450–600°C). The corncob had been observed to be a suitable contender out of the rest of the wastes for pyrolysis with activation energy of 29.71 and 4.23 kJ/mol in reduced and enhanced temperature range, respectively. Buenrostro-Figueroa et al. [69] had used *Kluyveromyces marxianus* for fermentation of mango fruit for ethanol generation. *K. marxianus* in Tommy Atkins mango juice exhibits encourage finding over Haden mango juice. The finding showss that of 4 g/l/day, a yield of up to about 49% of ethanol and a process efficiency of about 80%. Mihajlovski et al. [70] had utilized Streptomyces fulvissimus for fermentation of lignocellulosic waste such as wheat bran, barley bran, and rye bran, to obtain alcohol. Rye bran observed to be one of the most perfect waste substrates that can be used for bioconversion. Najafi et al. [71] had

#### *Perspective Chapter: Environmental-Friendly Agro Waste Management DOI: http://dx.doi.org/10.5772/intechopen.107505*

reported enzymatic potential of the bacterial strain S. fulvissimus during the hydrolysis of lignocellulosic agro waste such as rice, wheat, sugar cane, barley, and corn for the generation of ethanol. Pistachio waste such as pruning trees, green (soft) shell, and hard shell could be transformed into beneficial fuel using the fermentation processes, anaerobic digestions, and thermochemical degradation (i.e., pyrolysis) methods for production of biofuels [72]. Yuliansyah et al. [73] had evaluated the feasibility of upgrading oil palm fronds and trunks for their decomposition behavior over hydrothermal treatment to generate solid biofuels. The rice straw biomass is constituted of different variety of biopolymers, mainly cellulose, hemicellulose, and lignin. Through the hydrolysis of cellulose and hemicellulose, monomeric sugars are liberated that can be converted into ethanol by fermentation as an alternative to biogas by anaerobic digestion. Laobussararak et al. had utilized the bacterium *Zymomonas mobilis* and distillery yeast, *S. cerevisiae* and a co-culture of *Z. mobilis* and *S. cerevisiae* for fermentation of rice straw waste for production of ethanol [74]. The rice straw had been treated with 2% sodium hydroxide solution, then followed by enzymatic hydrolysis making use of cellulase prior to the fermentation. It was found to be that 2% NaOH pretreatment is perfectly suitable for the rice straw waste as a type of pretreatment context able to generate the high cellulosic content about of 88.96% and diminishing sugar content of 9.18 g/l. The distillery yeast was found to be a befitting microorganism for the generation of ethanol out of the rice straw, as ethanol yield on enzymatic hydrolysis found to be 15.94–19.73%, 20.48–35.70%, and 21.56–29.89% for the bacterium, yeast, and co-culture, respectively. Kumar et al. [75] had comprehensively investigated over green solvent-pretreated rice straw and cellobiose fermenting yeast strain Clavispora for production of cellulosic ethanol. Green solvent (cholinechloride/glycerol) treated rice straw leads to maximum reduction of sugars about 226.7 g/l with a saccharified capability of about 87.1% at 20% solids loading and 12FPU cellicctec2. The generation of ethanol yield of 36.7 g/l was found out of 8% of glucose within 36 h with a conversion capability up to 90.1%. Sasaki et al. [76] had successfully studied that the perfluoropolymer membrane has been suitable used in vapor permeation to isolate aqueous ethanol from combined product obtained out of rice straw with recombinant S. cerevisiae. Kluyveromyces sp. is explored as thermophilic ethanologen, which effectually makes use of hexose for the fermentation of ethanol at high temperature (45–50°C) [77]. The rice straw waste had been hydrolyzed at temperature of 140°C along with dilute H2SO4 of 0.6%v/v over 90 min for utmost retrieval of pentose monomer yield of 12.52 g/100 g. Using commercial cellulose, saccharification efficiency was observed to be 79 ± 0.05% with acid-hydrolyzed biomass. The fermentation of saccharified broth utilizing thermophilic yeast Kluyveromyces sp. with cell recycle produced ethanol with an overall yield and productivity of 93.5 ± 0.05% and 0.90 ± 0.2 g/l/h, respectively, and with a negligible amount residual sugar found in fermentation broth. Assis Castro et al. [78] had investigated multiple approaches of saccharification as well as fermentation utilizing rice straw waste that is pretreated with dilute acid for ethanol production using thermo-tolerant yeast Kluyveromyces marxianus. On concurrently saccharification and fermentation, in the absence of type of any pre-hydrolysis, it was observed to be as the utmost perfect condition owing to the enhanced ethanol generation (1.4 g/l. h), about two times more in contrast to the alternate approach. Mahajan et al. [79] had accessed glycosyl hydrolases produced by different thermophilic fungal strains for the saccharification of alkali as well as biologically (Trametes hirusita/Myrothecium roridum) treated Parthenium hysterophorus (carrot grass) as well as rice straw waste. The integrative examination of hydrolysates observed clear-cut outline of hexose,

pentose, and oligomeric sugars. Malbranchea cinnamomea was utmost orderly origin of glycosyl hydrolases producing 283.8, 35.9, 129.6, 27,193, 4.66, 7.26 (units/gds) of endoglucanase, cellobiohydrolase, b-glucosidase, xylanase, a-arabinofuranosidase, and b-xylosidase, respectively. The fermentation of outcome hydrolysates having glucose/xylose was competently yield of ethanol by *S. cerevisiae* due to the presence of xylose isomerase (0.8 units/gds) activity in culture extract of *M. cinnamomea* resulting in generation of 16.5 and 15.0 g/l of ethanol from alkali-treated rice straw and carrot grass, respectively. Sasaki et al. [80] had utilized a xylose-fermenting *S. cerevisiae* strain in ethanol fermentation activities for accessing in perfect usage of hemicellulose generate from rice straw waste. The xylose fermenting recombinant *S. cerevisiae* helps in generating bioethanol yield of about 34.5 ± 2.2 g/l. Momayez et al. [81] had investigated utilizing the liquid anaerobic digester, the biogas liquid waste, for the pretreatment process of the rice straw at different ambience. The rice straw had been pretreated at varying temperature 130–190°C for ½/1 h duration and put through to enzymatic hydrolysis, simultaneous saccharification and fermentation, dry anaerobic digestion, and liquid anaerobic digestion. The hydrolysis is enhanced by 100%, while the yield of ethanol enhanced by 125% on treating the rice straw waste at temperature of 190°C over 1 hour. There was also enhancement of yield of methane in 24 and 26% on using pretreatment process of rice straw through liquid anaerobic digestion and dry anaerobic digestion. Molaverdia et al. [82] had utilized Mucor indicus fungus for fermentation of rice, which was pretreated with 0.5 M Na2CO3 solution over 3–10 h to enable improving the efficiency of ethanol production. The maximum ethanol yield of about 99.4 g/l generated from the pretreated rice straw waste on simultaneous saccharification and fermentation for about 10 h. Whereas the ethanol yield of about 66.3 g/l was generated on moderate enzymes loading for fermentation about 12 h. Lü et al. [83] studied improving ethanol generation by the pretreating the rice straw waste with the microwave-assisted FeCl3 solution followed by applying simultaneous saccharification and fermentation using *S. cerevisiae* and Pichia stipites. The concentration of ethanol is about 5.51 g/l on fermentation. Trametes hirsute, a white rot fungus, was competed of directly fermenting starch, wheat bran, and rice straw, for generating ethanol in the absence of acid or enzymatic hydrolysis [84]. *T. hirsuta* appeared comparable xylose consumption and ethanol production with a yield of 0.44 g/g. On growing the fungus in a medium containing 20 g/l starch, wheat bran, or rice straw, it was observed that ethanol yield of 9.1, 4.3, and 3.0 g/l, respectively. Karimi et al. [85] had studied for ethanol production out-of rice straw, pretreated with dilute acid, with the yields of 74, 68, and 61% using Rhizopus oryzae, Mucor indicus, and *S. cerevisiae*, respectively. The yield of ethanol was found to be 74 and 68% while using *R. oryzae* and *M. indicus*, respectively. Kaur et al [86] had reported low-cost process involving solid state fermentation of rice straw producing high titers of cellulases and hemicellulases for hydrolysis of alkali pretreated rice straw leading to ethanol yield of 15.6 g/l. Sharma et al. [87] had utilized the charred wood ash from Acacia nilotica as the diversified base catalyst. The wood ash catalyst that charred at 800°C shows an improved catalytic effect owing to its enhanced surface area, which leads to produce a 98.7% biodiesel transformation. It was also observed that the wood ash catalyst was found to be steady for the reaction of jatropha oil without any leaching of catalyst material. Uprety et al. [88] had investigated synthesis of biodiesel from palm oil utilization of ash from Birch bark as a diversified catalyst. It was observed that the biodiesel yield of 69.70% in the effective context like a catalyst load of 3 wt.% while a methanol to oil molar ratio of 12:1 at temperature of 60°C within 3 h. Betiku et al. [89] reported

*Perspective Chapter: Environmental-Friendly Agro Waste Management DOI: http://dx.doi.org/10.5772/intechopen.107505*

synthesis of biodiesel from *Azadirachta indica* oil transesterification utilizing a catalyst obtained on calcination of cocoa pod husk ash. It was observed that the cocoa pod husk ash catalyst charred over temperature of 700°C producing good interest owing to its high potassium content of about 59.2% and formation of the microstructure. While taking catalyst amount of 0.65 wt.% and a methanol: oil ratio of 0.73 (v/v) at temperature of 65°C within 57 min, the yield of biodiesel transformation was found to be 99.3%. Vadery et al. [90] had investigated generation of biodiesel from jatropha oil by utilizing a catalyst on calcination of coconut husk. While taking catalyst amount of 7 wt.% and a methanol: oil ratio of 0.73 (v/v) at temperature of 45°C within 45 min, the yield of biodiesel transformation was found to be 99.86%. The present demand of the time is to find out the perfect catalyst that can be both eco-friendly and economical aside from showing excellent catalytic activity for synthesis of biodiesel. Nowadays, a lot of attention is given on the designing of perfect green catalysts derived out of agro-based waste for the trans-esterification of vegetable oils, banana stem ash [91, 92], and banana peel ash [93–97], are few examples of the catalysts, obtained from agro-based waste, which had been perfectly made use of as basic catalysts for the generation of biodiesel.
