**1. Introduction**

The carbon neutrality or the net-zero carbon dioxide emission could be fulfilled by the way to used energy and fuel from biomass resources. The plant from the agricultural and forestry sector could help the achievement of balancing the carbon dioxide from the utilization of biomass waste produced from its process. Other than that, the utilization of biomass waste could counter the production of greenhouses gas (GHG) produced from the biomass waste dumping process. The conversion from the biomass waste into methane through anaerobic digestion could maintain the GHG release from biomass waste. The use of biomass waste as carbon neutral resources can

be through biomass conversion by steam explosion pretreatment, anaerobic digestion where the biogas could use for LNG substitution for household use, for power generation fuel which produces the electricity that could fulfill the self-sufficient off-grid and for the on-grid electricity system. The biogas also could convert into hydrogen for transportation fuel and other utilization. The compressed biogas with methanerich (CH4) and hydrogen (H2) was potentially utilized as secondary energy, which is widely introduced in several sectors such as public transportation, household application, and other application (**Figure 1**).

The steam explosion pretreatment was commonly used for biomass treatment to break the recalcitrant of lignin carbohydrate complex (LCC) or lignin-carbohydrate polymer which is the main structure of biomass in addition to other content such as resin that makes biomass known as a substrate that is difficult to convert into biofuel through the digestion process or as a source of lignin and cellulose base of biomaterials. The steam explosion also generated the cellulose and low molecular lignin that could be utilized as a biomaterial, where the low molecular lignin could be separated by an extraction process using various types of solvents such as water, ethanol, and acetone and used as polymer-based substitute products such as epoxy resin and thermosetting resin by converting low molecular lignin into lignin-epoxy resin or using it directly as a curing agent [1–6]. The steam-exploded lignocellulosic biomass also could be utilized as an antioxidant resource which is rich in polyphenol content [7–11], and its cellulose content also could utilize as cellulose-nanofiber (CNF) resource that is widely used for sustainable biomaterials [12–14].

As the psychochemical pretreatment, the steam explosion could break the LCC and also change the chemical content as a derivative product of the content of cellulose, hemicellulose, lignin, and other specific contents that differ from one biomass to another. The steam explosion pretreatment which is based on the hydrothermal pretreatment method with high pressure and short retention time then suddenly depressurized to make the explosion effect from the pressure differences between the pressure of the steaming chamber and the normal pressure of the explosion chamber [15]. The explosion effect disrupts the structure of LCC fibrils which break

#### **Figure 1.**

*Carbon neutral biomass waste and unutilized biomass anaerobic digestion scheme via steam explosion pretreatment.*

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

its polymer chain and become small particle size that could facilitate the digestion process easily [16]. Other than that, the chemical content from the LCC could change become derived product such as cellulose that could continuedly to be degraded into cellobiose-glucose-HMF(5-(hydroxymethyl)furfural)-levulinic acid; hemicellulose that could degrade into the pentoses (xylose, arabinose) and could continuedly be degraded into furfural and formic acid, the hexoses (mannose, glucose, galactose) that could continue to degrade into HMF and continue into formic acid or levulinic acid, and hemicellulose also could produce acetyl and continue to degrade into acetic acid; the lignin content could degrade into the lignin precursors such as sinapyl alcohol, p-coumaryl alcohol, and coniferyl alcohol, those compound could continuedly degrade into phenolic compounds such as catechol, guaiacol, vanillin, syringaldehyde, 4-hydroxybenzaldehyde, 4-hydroxybenzoic acid, and vanillic acid. The compounds degradation from steam explosion pretreatment was influenced by the temperature, pressure, and steaming time. That condition was influenced by the degree of severity factor (*R*0 or *S*0 SF) which caused from the temperature condition and residence time [17]. The other factor i.e., pH condition was also affected the physiochemical products such as the acid addition as a catalysator, which knownly as combined severity factor (CSF) [18]. Since the severity factor could not faithfully describe the steam explosion disregard the effect of the explosion condition, Yu et al., [19] added a comprehensive factor which quantified the explosion severity that could better describing the steam explosion severity condition by explosion power density (EPD). The severity factor, combined severity factor, and explosion power density could be calculated with the equations:

$$R\_0 = t \propto e^{\left[ (T\_r - 100)/14.75 \right]} \tag{1}$$

Where *T*r represent the temperature reaction (°C), and *t* represent the resident time (minutes) [20].

$$S\_o = \log \int\_0 \exp\left(\frac{T\left[\text{°C}\right] - 100\text{°C}}{14.75}\right) dt\tag{2}$$

The time integral of *S*0 was described the process of non-isothermal heating character [20, 21].

$$\text{CSF} = \log\left(R\_0\right) - pH \tag{3}$$

Where the *Log (R*0*)* as a severity factor value and *pH* represent the pH level after the acid was added [18].

$$P\_c = \frac{\Delta H\_s + \Delta H\_1 + \Delta H\_m}{\left(t \propto V\right)}\tag{4}$$

Where the ∆*H* represent as the enthalpy drop from the steam (*s*), liquid water (*1*), and biomass (*m*), *t* represent the duration of the explosion, and *V* represent the volume of reactor [19].

The derived product from cellulose, hemicellulose, and lignin could affect the fermentation process on the anaerobic digestion as the fermentation inhibitors,

nevertheless, it can be controlled by adjusting the inhibitor threshold. On the other hand, the inhibitor from physicochemical pretreatment product could be handled by detoxification process through biological, physical, or chemical. The biological detoxification via hired the microorganism that could produce enzymes that change the chemical structures of the fermentation inhibitor compounds which present in the biomass hydroxylate [22–24]. The physical detoxification could remove the inhibitor compounds without changing the chemical structure such as using activated charcoal or activated carbon for neutralizing the hydrolysate, and also by an extraction process using trialkyl amine as an alkali detoxication, n-octanol, and kerosene [25–27]. The chemical detoxification was treated by adding the modified pH such as water extraction, sodium hydroxide, and reductive substance [16, 23, 28, 29]. The potential compounds that could be converted into methane from steam-exploded biomass fraction, is not only cellulose, hemicellulose, and monosaccharides compound, the steam-exploded aromatic lignin fraction and its derived product such as syringaldehyde and vanillin also could be converted into methane by the anaerobic digestion process [15, 30–36].

The use of methane as secondary energy has been widely used, such as a substitute for liquified natural gas (LNG) for household networks and as a fuel for transportation. In addition, methane can also be converted into other secondary energy such as hydrogen by separating its carbon and is included in a cheap hydrogen source similar to LNG [37], compared to other hydrogen sources. Other than that, the methane produced from biomass waste and unutilized biomass has several advantages such as renewable, sustainable, and carbon-neutral compared with LNG which included depleted natural resources that cannot be renewable. The common hydrogen conversion system from the methane can be done in several ways such as steam reforming methane (SRM), dry-reforming methane (DRM), catalytic decomposition methane (CDM), and partial-oxidation methane (POM), those systems were widely introduced in laboratory-scale or existing technology industrial used.

In this chapter, we will try to delineate state the art of methane conversion and its derived products from biomass waste and fast-growing unutilized biomass by steam explosion pretreatment. The combination of carbon-neutral resources and environmentally friendly pretreatment could give the alternative perception from only combustion utilization to the system that vaporization the biomass waste and unutilized biomass into more potentially produces more product from one system.

### **2. Potential biomass waste and unutilized wood and non-wood biomass**

The agriculture and forestry industries were producing sustainable and renewable biomass waste which included carbon-neutral resources that could be converted into methane by an anaerobic digestion process. The utilization of biomass waste from this sector also could help to reduce land-use change from the biomass that is mainly used only for the biofuel feedstock. The conversion of the biomass waste into methane is free from quality problems of biomass as combustion fuel that need specific calorimetry and density that could not be fulfilled by all the biomass waste. **Table 1** showed the agricultural commodity that produces biomass waste with minimum utilization such as palm oil, barley, corn, rice, sorghum, wheat, and sugarcane. Other than that, the forestry industry such as pulp and paper mills, and unutilized fast-growing biomass such as reed and grassland are potentially utilized for methane conversion.


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

#### **Table 1.**

*World production agricultural potential commodity with minimum biomass waste utilization [38].*
