**1. Introduction**

We are living in a world of many challenges such as climate changes, polluted environment, resource depletion, and increasing demand for fuel. The use of oil reserves to fulfill our need of fuel has caused many drastic challenges from energy security to change in temperature. Rapid industrialization has increased the demand of petroleum products and consequently has raised the monopoly of few countries, which can manipulate petroleum price and create instability. This may also create environmental problems by emission of greenhouse gases and subsequently effect on climate change. The most important source of energy is petroleum that is largely used in transportation and industries; therefore, viability of liquid fuel is enhanced. As the environmental issues are growing, more research is being conducted to address the problems. The search for alternative source of petrol that is less costly with minimal environmental effects has become the center of attention. For instance, biomass is considered as a sustainable resource that can be utilized in large-scale production of biofuel that can be utilized as an alternative source of fuel and may present solution to environmental problems. Furthermore, relying on fossil fuel could be detrimental as it has been predicted of its depletion by 2050. The total annual primary production of biomass is over 100 billion tonnes of carbon per year, and the energy reserve per metric tonne of biomass is between 1.5E3 and 3E3 kW hours that is sufficient to cater the needs of the world energy requirements [1].

Bioenergy products like bioethanol, biohydrogen, and biodiesel can be obtained from lignocellulose biomass which is considerably large renewable bioresource and obtained from plants. The term "lignocellulosic biomass" is defined as lignin, cellulose, and hemicellulose that constitute the plant cell wall. Strong cross-linking associations are present between these components that cause hindrance in the breakdown of plant cell wall. Polysaccharides and lignin are cross-linked via ester and ether linkages [2–4]. Microfibrils that are formed by cellulose, hemicellulose, and lignin help in the stability of plant cell wall structure [5, 6].

Lignocellulose was first produced from food crop such as corn, oilseed, and sugarcane. But the use of edible feedstock for bioenergy products formation is being discouraged to prevent the rise in food competition. Thus, second-generation biofuels are obtained from plants wastes to avoid competition of land and water resources between energy crops and food crops. Currently, lignocellulose is being produced from wood residues, agricultural residues, food industry residue, grasses, domestic wastes, municipal solid wastes, and nonfood seeds [7–9]. The lignocellulose wastes (LCW) are largest renewable bioresource reservoir on earth that is being wasted as pre and postharvest agricultural wastes. Thus, many steps need to be adopted for use of these renewable resources for the production of bioenergy products. Recovery of many products like enzymes, methane, activated carbon, lipids, resins, methane, carbohydrates, surfactants, resins, organic acids, ethanol, amino acids, degradable plastic composites, biosorbents, biopesticides, and biopromoters can be achieved by utilizing LCW. The added benefits of using LCW besides recovery of different products are the removal of LCW waste from the environment. Also, utilization of LCW eliminates the use of food for bioethanol production. The US government has planned the production of 21 billion gallon of biofuels by 2022 [2, 5]. Biofuel production from lignocellulosic biomass reduces the emission of greenhouse gases.

Pretreatment brings physical, biological, and chemical changes to biomass structure; therefore, it is very important to consider the type of pretreatment. In order to break down the hindrance caused by strong association within the cell wall, pretreatment is an important step which can increase the availability of lignocellulosic biomass for cellulase enzymes, their digestibility, and product yield. Before subjection to enzymatic hydrolysis, pretreatment of biomass can increase the rate of hydrolysis by 3–10-fold. Pretreatment of LCW is not an easy step as it seems after the installation of power generator; pretreatment is the second most costly process at industrial level. In crystalline cellulose, the disruption of hydrogen bonds, cross-linked matrix disruption, and increase in porosity as well as surface area of cellulose are the three tasks that are performed via a suitable pretreatment methods. The outcome of pretreatment also differs due to the difference in the ratio of cell wall components [10, 11]. The option to use dilute acid pretreatment method is more effective against poplar tree bark or corn as compared to the same method used for sweet gum bark or cornstalks. Few requirements of an effective, efficient, and economically suitable pretreatment process that including use of cheap chemicals, very less consumption of chemicals, prevention of hemicellulose and cellulose from denaturation, minimal energy requirement and consumption, cost-effective size reduction process, and reactive cellulosic fiber production are the factors that need to be considered for pretreatment. There are several methods of pretreatment that can be divided into four categories, namely, chemical, physical, biological, and physiochemical pretreatment [12–15].
