**3. Steam explosion pretreatment**

The steam explosion pretreatment for lignocellulosic biomass that was used for methane conversion was widely reported in several studies with various experiment conditions and biomass feedstock. The steam explosion pretreatment was reported as stand-alone pretreatment for lignocellulosic biomass for methane production feedstock. Kobayashi et al., [39] used the abundant and fast-growing such as bamboo as a feedstock for methane production by an anaerobic fermentation process which operated in mesophilic condition (37°C), where the steam explosion was set in 3.53 MPa (243°C) for 0,1, 3, 5, 10, and 20 minutes of steaming time. The 5 minutes steaming time was produced the higher methane with 215 ml/g, that amount was 80% from the methane amount prediction that calculated from cellulose and hemicellulose amount from 1 gram of steam-exploded bamboo. Wu et al. [40] was used palm oil mill waste such as empty fruit bunches and palm oil fronds to convert it by steam explosion treatment at 1.5 MPa for 1 minute of steaming time. They concluded that steam explosion pretreatment enhanced the biogas production, and improved the energy values (gross energy, digestible energy, metabolic energy, net energy for maintenance, and net energy for lactation) from palm oil frond and empty fruit bunches. Lizasoain et al. [41] were used reed biomass for methane conversion using various steam explosion conditions from 160 to 220°C with 5, 10, 15, and 20 minutes of steaming time resulted in the severity factor from 2.47 to 4.83. That study has resulted in the 89% enhancement compared to untreated feedstock from steam explosion condition at 200°C for 15 minutes. Theuretzbacher et al. [42] reported utilizing wheat straw as a methane production feedstock by steam explosion pretreatment at 140, 160, and 178°C with 30, 60, and 120 minutes of steaming time which resulted in various severity factors from 2.7 to 4.4 SF Log (R0). The highest methane production was from 140°C for 60 minutes steaming time at 3.0 SF Log (R0) which produced 286 ln kgVS-1. Steinbach et al. [43] were used steam explosion for rice straw to produce biogas by various severity factor 3.05 to 5.29 (S0) from 162 to 240°C for 12 to 30 minutes steaming time. They concluded the moderate severity increases the methane production whereas severe condition dramatically drops the methane production caused by an inhibitor that formed in high severity condition. Those were similar conditions to the report from Lizasoain et al. [41]. Vivekanand et al. [44] was used the rape straw steam-exploded with emphasizing in chemical composition changes under various severity (3.5 to 5) that could impact the methane production under mesophilic conditions for 81 days. The steam-exploded rape straw was treated under 190 to 230°C with 5, 10, and 15 minutes. They concluded that the formation of the inhibitor compounds does not impact methane production. The other report explained the compounds that could be formed from biomass steam-exploded were known as an inhibitor such as HMF and furfural, also the other lignin-derived products in form of phenol and polyphenol as lignin polymers and/or lignin oligomers such as vanillin and syringaldehyde could also convert into methane. They resulted that the HMF could produce methane 450 ml CH4/gMV, furfural 430 ml CH4/gMV, syringaldehyde 453 ml CH4/gMV, vanillin 105 ml CH4/gMV. This study also reported examining the various lignin polymers such as, organosolv, lignosulfonates, and kraft lignins that could produce 14 to 46 ml CH4/gMV. They concluded that a higher syringyl/guaiacyl ratio that generated the syringaldehyde and vanillin by partial depolymerization of lignin polymer, and lower molecular weight of lignin polymer could conduct high methane production [36]. The steam explosion pretreatment was reported to generate the low molecular lignin in line with the increased degree of the SF values [1, 5, 45–47].

The comparation between steam-explosion pretreatment and other pretreatment was reported in several studies. Take et al. [48] reported examined the Japanese cedar chip *Cryptomeria japonica* as methane production feedstock by comparation psychochemical pretreatment using steam explosion at various pressure conditions at 3.53 MPa (243°C) and 4.51 MPa (258°C) for 5 minutes steaming, steaming pretreatment at 170°C for 30 minutes, and biological pretreatment using *Ischnoderma resinosum, Fomitella fraxinea, Mycoleptodonoides aitchisonii, Trichaptum abietinum, Cyathus stercoreus*, and *Trametes hirsute*. The result from the steam explosion pretreatment could produce the highest methane with 180 ml/g with less energy use, compare with steam treatment and biological pretreatment from *Cyathus stercoreus* which only produces methane in 45 and 43 ml/g, respectively. The combination pretreatment between the steam explosion and other treatments for methane production feedstock was reported in several studies. Sholahuddin et al. [16] reported to utilize agricultural biomass waste i.e., rice husk which was treated using steam explosion at 2.53 MPa (224°C) for 5 and 7 minutes steaming time followed by water extraction for methane production, and without water extraction. The water extraction as a dilution treatment to lower the pH condition that increased due to acid formation of psychochemical effect and reduce the concentration of fermentation inhibitor produced from the physicochemical pretreatment into under the threshold. The anaerobic digestion was operated in mesophilic condition (37°C) by using activated

#### *Steam Explosion Pretreatment: Biomass Waste Utilization for Methane Production DOI: http://dx.doi.org/10.5772/intechopen.102850*

cow dung for the inoculum which is naturally rich in cellulolytic microflora resulted produced 199 ml/g methane from 41% cellulose content, that amount was reached 96.1% of methane conversion from the prediction, compared to without water extraction which only 28 ml/g of methane from the same steam explosion condition i.e., 7 minutes of steaming time. Theuretzbacher et al. [49] reported to use a wheat straw for methane production by combination pretreatment using biological using *Scheffersomyces stipitis* and thermo-mechanical using steam explosion at 180, 200, and 220°C, those combinations was examined to reduce the thermal energy input where the biological condition could facilitate the steam explosion to break the LCC in low-temperature condition. The highest methane production from the combination of biological pretreatment and steam explosion 250 and 252 lnkg VS-1 with no significance from 180 and 200°C, respectively. Bauer et al. [50] used late-harvested hay that pretreated using steam explosion at 160 to 220°C for 5, 10, and 15 minutes steaming time followed by enzymatic hydrolysis using b ß-glucosidases and hemicellulases that resulted in high yield glucose from 220°C for 15 minutes, xylose 175°C for 10 minutes. Those combinations resulted in 15.9% methane enhancement compared to the untreated. The anaerobic digestion. The anaerobic digestion. Matsakas et al. [51] reported to used hybrid pretreatment which combines the steam explosion and organosolv where the 99,8% ethanol with 1:2 ratio between biomass chips: ethanol, and heated in 200°C (1.519 MPa) with 15, 30, and 60 minutes of steaming time. The organosolv-steam-exploded products were filtrated using vacuum filtration, and the solid residue was washed using ethanol and dried and continued to the anaerobic digestion process. Weber et al. [52] used steam explosion with temperature setting 142, 164, and 179°C (0,38, 0,68, and 0,98 MPa), after that the solid reside of steamexploded was separated by centrifuge. The methane production was prepared by a 2:1 ratio between inoculum and substrate. Li et al. [53] used corn stover by sequent pretreatment using potassium hydroxide (KOH) 0.5 and 1.5% and steam explosion under 1.2 MPa for 10 minutes steaming time. Those combinations were applied to improve the digestion ability of biomass which resulted in 80% improvement from untreated corn stover with 258 ml/gvs from 1.5% KOH and 1.2 MPa for 10 minutes steaming time, where the only steam exploded and KOH treatment in the same condition was only produced 143.8 and 208.6 ml/gvs, respectively.
