**4.2 Biogas**

**Figure 4** shows the cellulose conversion into methane gas. The methanogenic reaction of cellulose or other forms of carbohydrates through the hydrolysis process yields monosaccharides, which are fermented to produce acetic acid, hydrogen, and CO2 [25]. From the fermented products, acetolactic methanogenesis converts the acetic acid and hydrogenotrophic methanogenesis converts the hydrogen and CO2; the conversion process includes reforming the acetic acid, hydrogen, and CO2 into methane [26]. Steam explosion pretreatment has been widely used for biogas production. Take [27] subjected Japanese cedar chips to 4.51 MPa steam explosion for 5 min for optimal methane production. Kobayashi [28] used bamboo to produce methane by 3.53 MPa steam explosion (243°C) using the sludge obtained from sewage treatment for microbial seed under mesophilic condition (37°C), which resulted in 80% theoretical yield with 423 ml obtained from 1 gr of cellulose and hemicellulose for 25 days of total cumulative production. Mulat [29] combined steam explosion pretreatment for lignocellulosic biomass and bioaugmentation using *Caldicellulosiruptor bescii*, which enhanced the methane production under thermophilic conditions by 140% in 50 days with low dosages of *Caldicellulosiruptor bescii* inoculum (2–5%). Sholahuddin [30] subjected rice husk to a combination of steam explosion pretreatment at 2.52 MPa and 224°C followed by water extraction and activated cow dung as the inoculum without co-digestion, at 37°C. This yielded 96.1% of stochiometric prediction of methane production with 199 ml/g of total solid, which contained 41% of cellulose only for 22 days, and all the liquid and solid residues were used as the substrate. Steam explosion pretreatment was also used for grass, such as reed, which can be used as a potential raw material for biogas because of its abundance. Lizasoain [31] subjected reed biomass (*Phragmites australis*) for biogas feedstock to steam explosion under various temperatures, pressures, and steaming times, where the 200°C and 15 min combination increased the methane yield by 85% compared to the untreated samples. Furthermore, Dererie [32] used oat straw for combined biogas and ethanol production with steam explosion pretreatment and other chemical treatment; as per the result, the residue of ethanol fermentation from steam-exploded oat straw produced higher methane than that produced by unfermented steam-exploded oat straw. They concluded that the fermentation ethanol process acts as an additional pretreatment for methane production.

**Figure 4.** *Cellulose conversion into methane gas.*

Methane production from lignocellulosic feedstock through steam explosion pretreatment provides a wide spectrum of total conversion. Methane is converted not only from cellulose and hemicellulose but the aromatic lignin fractions also contribute to the methane production. Moreover, steam explosion facilitates better anaerobic digestion by disrupting the lignin structure [33], which can be converted into methane. However, the anaerobic degradation of the aromatic compound incurs several difficulties in the degradation process [34], and several studies have reportedly observed anaerobic lignin degradation [35, 36]. The aromatic lignin heteropolymers mainly comprise two monolignols, which are methoxylated to various degrees: synapyl and coniferile alcohols; these monolignols are fused into lignin in the unit syringyl (S) and guaiacyl (G) forms, respectively [37]. The depolymerized monomeric unit of lignin (i.e., S) is converted into vanillin and the G unit is converted to syringaldehyde [38]. Syringaldehyde can produce a high methane yield [19, 24, 39]. Barakat [40] demonstrated the combination of xylose and the lignin fraction, such as aromatic compounds syringaldehyde and vanillin; cellulose and hemicellulose fractions, such as HMF; and furfural and xylose. The combination of xylose and syringaldehyde yielded the highest methane production, followed by the combination of xylose and furfural compared with xylose alone.
