**4. Biofuels**

Biofuels face competition with other food resources such as wheat, grain, corn, cassava, and palm oil. To address this issue, researchers started evaluating other renewable resources that do not face this competition. The second generation of biofuels is obtained from unutilized biomass and biomass waste resources, which are abundantly available. The biomass waste generated from agricultural and forestry sectors, such as burning fuel or dumping of disposal waste, commonly ends up with minimum utilization. Moreover, the unutilized biomass, such as weed, grass, and bamboos, can be converted into biofuels. The second-generation biofuels have been widely introduced with various biorefinery processes, including steam explosion pretreatment for bioethanol and biogas production.

#### **4.1 Ethanol**

The second-generation ethanol production is highly dependent on the availability of cellulose and hemicellulose from LCC. Steam explosion is an effective pretreatment for breaking LCC into cellulose and hemicellulose, which can be readily converted into ethanol. This pretreatment is widely used because of its ability to break LCC into cellulose and hemicellulose obtained from the conversion of lignocellulosic biomass into ethanol. The general routes for ethanol production via steam explosion pretreatment are described in **Figure 3**. Nakamura [9] converted rice straw into ethanol through steam explosion, followed by water extraction and enzymatic saccharification and fermentation. They reported 86% theoretical ethanol production from the substrate by using steam explosion and *Trichoderma viride* cellulase or Meicelase and *Aspergillus aculeatus cellulase* or Acucelase, using *Pichia stipites*. Sasaki [10] converted wood chips and acorns of *Quercus acutissima* into acetone, butanol, ethanol (ABE) by steam explosion pretreatment, followed by direct conversion using the separate hydrolysis and fermentation (SHF) and SSF methods. The SSF method yielded 100% ABE conversion rate, with 0.112 g/L/h ABE productivity for 196 h of incubation time with 60 g/l concentration of the initial substrate. In another study, Sasaki [11] subjected the pruned branches of pear trees to steam explosion pretreatment, followed by SSF, to produce ethanol. They compared the steam-exploded substrates with and without water and methanol extraction in terms of the amount of ethanol produced. The result showed that the SSF with water and methanol extraction achieved 76% of the theoretical production with 99.7% of glucose recovery, whereas the sample not subjected to methanol and water extraction majorly produced sugar and no ethanol. This result

#### **Figure 3***.*

*Production routes for ethanol produced from steam explosion.*

was attributed to fermentation inhibitors contained in the substrate as compound fractions obtained from steam explosion pretreatment.

**Table 1** shows the steam explosion-based ethanol production methods. The most significant problem incurred in ethanol production using steam explosion pretreatment is the yield of a fermentation inhibitor as a derived product of steam explosion. Ando [24] identified the influences of an aromatic monomer in steam explosion on ethanol production via *Saccharomyces cerevisiae*, which could be reduced by washing the inhibitors with a solvent, converting them into inactive compounds using biological or chemical methods, improving the steam explosion conditions to minimize the inhibitor formation, and screening for yeasts that resist the inhibitors. Asada [19] used the SSF method for spent of shitake mushroom medium for evaluating the effect of reducing the formation of fermentation inhibitors with and without water extraction. They reported 87.6% of theoretical ethanol yield, which produced 15.9 g ethanol from 100 g substrate. In another study, Asada [15] used steam explosion, followed by water extraction, in comparison with mechanical grinding with a ball mill, for converting disposable chopstick obtained from aspen into ethanol. This resulted in 20 FPU/g samples, and the continuous fermentation using the SSF method resulted in 520 and 598 mg-glucose/g-dry samples from grinding and steam explosion pretreatment, respectively. This method yielded 79% of theoretical ethanol production with 241 mg-ethanol/g-dry. In addition, Scholl [20] used steam explosion pretreatment for converting the elephant grass *Pennisetum purpureum* under various pressures and steaming times, followed by vacuum draining and washing with water to remove the inhibitor material and enzymatic saccharification using cellulase and xylanase obtained from *P. echinulatum*. Thereafter, the fermentation process was continued using *Saccharomyces cerevisiae* as the steam-exploded elephant grass, followed by water washing and using cellulose enzymes with 10 FPU/g total solid having 4 wt% substrate concentration, which did not result in a feasible production (110.45 μl/g). To overcome the toxicity of phenolic compounds obtained from the steam-exploded substrate, which act as an inhibitor for ethanol production, various treatments have been reported, such as the use of laccase produced from C*oriolopsis rigida* and *Trametes villosa*. The detoxification by laccases considerably improves the ethanol fermentation, which in turn reduces the toxic effect on *S. cerevisiae* [18]. Asada [17] converted cedar into ethanol by using steam explosion pretreatment in


#### *Biorefinery System of Lignocellulosic Biomass Using Steam Explosion DOI: http://dx.doi.org/10.5772/intechopen.98544*


**Table 1.**

*Steam explosion-based ethanol production methods.*

*Cellulose Science and Derivatives*

comparison with water and methanol extract, followed by the use of the simultaneous saccharification, detoxification, and fermentation (SSDF) method combined with the detoxification process using *U. thermosphaericus* A1 as a biological fermentation inhibitor degrading agent to degrade the inhibitory material produced from the steam explosion pretreatment. They concluded that the fermentation inhibitor degradation in low concentration was necessary to produce ethanol from steamexploded lignocellulosic biomass, where the steam-exploded cedar produced glucose alone due to the saccharification end product.
