**2. Energy and environment: current and difficult situations**

At the moment, the adverse effect on the environment from the burning of fossil fuels, coal and compressed natural gas has become one of the main global issues [34]. Climate change occurs when the greenhouse effect rises, as demonstrated by flash floods, wind storms, heat waves and sudden droughts in a number of nations [35]. In addition, worldwide demand for energy is growing while fossil-fuel energy sources are quickly declining. Fossil fuels are one of the non-renewable energy resources that will be depleted in several decades if large-scale sources of energy are continually used [36]. As shown in **Figure 2**, the world production of fossil oil is at the peak of the production, and it is expected to diminish by the year 2050 [37]. Because of these situations, it is essential to replace petroleum consumption, minimize future expenses and eliminate the adverse effect on health and the environment. Thus, the replacement of non-renewable energy source with renewable resources is imperative to fulfill the needs of the energy demand without causing harm to the environment and mankind [38]. Due to this crisis, various kinds of energy are used to meet the large demand for petroleum-based fuel such as wind turbines, river dams, solar panels, geothermal power and biofuels [39].

The conversion of methane into liquid fuels or greater hydrocarbons has been performed extensively. Bradford and Vannice [40] studied the growth of methanol, formaldehyde, propanol, benzene and other aromatics through direct oxidative conversion of methane. Unfortunately, all the aforementioned processes produce low yields or they are not recommended for an industrial scale. Today, various technologies are available for the production of syngas from natural gas. This gas is a component of precious fluid fuels and chemicals like Fischer-Tropsch oil, methanol and dimethyl ether [41].

**Figure 2.**

*Oil and natural gas production in the NZE (adapted from Ref. [37]).*

#### **2.1 Biogas**

The most significant renewable energy sources in the globe are biomass and hydropower. However, the use of other renewable resources is necessary to
