1.1 Introduction
Biomass can be defined as a solid organic substance or material derived from living organisms, including plants and animals. Due to the massive production of plant-derived materials, particularly agro-wastes, wood shavings, and forestry residues, and its attractive chemical composition, a large attention is being paid to the exploitation of them for deriving energy and chemicals (Shankar Tumuluru, Sokhansanj, Hess, Wright, & Boardman, 2011). The characteristic material is profoundly employed in the renewable energy generation as a potential resource (Tursi, 2019). Historically, since the mid-18th century, plant biomass is considered the largest energy producer through thermal processing techniques. In recent years, its application in the biofuel for transportation and electricity generation has been increased, particularly in the developed countries, including the United States. It offers several environmental benefits that could reduce CO2 emissions, which is comparatively more with fossil fuels. This material contains stored chemical energy derived via photosynthesis in the presence of sunlight and water. Thus, it is represented as a solid material that can be burned directly to recover heat or renewable fuels (both liquid and gaseous) through various thermochemical techniques. The International Energy Report 2019 details that the agro-industrial residues delivered nearly five quadrillion British thermal units (Btu) of thermal energy, which is estimated to be nearly 5% of the total primary energy used in the United States alone (Newell, Raimi, & Aldana, 2019). Overall, it is accepted as a potentially scalable feedstock for the production of sustainable fuels and chemicals and, moreover, is believed to have the ability to displace petroleum-derived products. The classified biomass sources utilized for energy and other products manufacturing are forest wood and its processing wastes, agricultural crops and its residues, and other biological materials, including municipal solid wastes, animal wastes (manure), and human sewage wastes (Muscat, de Olde, de Boer, & Ripoll-Bosch, 2020).
The attractive inherent fractional composition of the plant-based material (generally referred to as lignocellulose) encouraged the researchers to exploit it for the potential production of value-added chemicals through different technological routes (Gusiatin & Pawłowski, 2016). Therefore, it has been thoroughly assessed for the potential ethanol production via biological fermentation after employing the pretreatment with that impression. At the same time, the substrate offers challenges in successfully commercializing the bioconversion technologies due to its heterogeneous characteristics. Fundamentally, the solid biomass is made up of cell walls (primary and secondary), constituted with cellulose, hemicellulose, and lignin biogenic polymers; therefore, it is represented as the abundant micromolecular biocomponent available on the planet. Indeed, its presence in plants makes the major difference between the animal cell and plant cell, and furthermore, the plasma membrane surrounds the latter’s cell wall. Basically, the membrane functions to provide tensile strength, giving protection against plant stresses (i.e., osmotic and mechanical). It allows the cells to develop turgor pressure (pressure of the cell contents against the development of the cell wall). Mechanistically, the increased turgor pressure leads to plant wilting; therefore, a plant requires enough water supply. Thus, the cell walls help maintain the plant’s stems, leaves, and other structures (Lerouxel, Cavalier, Liepman, & Keegstra, 2006).
1.2 Classification of Plant-Derived Biomass
1.2.1 Woody Biomass
Wood biomass is typically originated from the tree materials and can be classified into forestry residues (leaves and branches), sawmill wastes (sawdust), and wood scraps (construction wastes) (Demirbaş, 2005). However, this material may not be suitable for preparing wood lumber, and thus, it is ascertained as a potential renewable resource belonging to the lignocellulose category (Danish & Ahmad, 2018). Like the vegetative plants, afforestation, regeneration, and sapling are maintenance methods that are usually adopted for continuous production. Typically, the timber process yields the largest portion of woody biomass, accounting for nearly 25%–45% of the harvested wood. However, the characteristic biomass possesses low bulk density and fuel value, and therefore, it increases the transport cost per unit. To reduce its impact on transportation, a general practice of comminution (or chipping) with compaction to make them in bundles is followed to increase bulk density. The advantages of the substrate to be used for energy and chemicals production include that it is a: (a) non-food organic feedstock; therefore, its competition with the agricultural food crops development is relatively low, (b) renewable resource, (c) material that requires lower energy input for growth, and (d) energy source material that, moreover, emits maximum CO2, which is comparable to the net CO2 released during its natural degradation and therefore represented to be a carbon-neutral material. Another subcategory of this type of biomass is the dedicated en...
