*2.3. Generations of Biofuels: A Brief Review*

Fuels produced by the biological process are known as biofuel. Primary biofuels are mostly fuelwood, wood chips, pellets, and organic materials, generally used for heating generation, cooking, or electricity purposes in a crude state. Secondary biofuels come from energy crops, agricultural and forestry wastes and by-products, manure, or microbial biomass used extensively in transportation and industrial purposes [35,36]. Innovative research on biomass for secondary biofuel production has been continuous for the last five decades of biofuels production. A wide variety of potential feedstock from all around the world is utilized for biofuels production. Based on the feedstock, biofuels are classified into four groups, namely: first, second, third, and fourth-generation biofuels [37–39].


However, this type of biomass has disadvantages such as its high initial investment for its production. The biofuel produced from algae is less stable than that produced from other sources, mainly because the oil generated by the algae is highly unsaturated, which means it is more volatile at high temperatures, so it is more likely to degrade. Furthermore, the high water quantity is also a problem when lipids have to be extracted from the algal biomass, requiring dewatering via either centrifugation or filtration before extracting lipids [46].

• Fourth generation. These biofuels, which are still in the developmental stage, use bioengineered microorganisms as microalgae, yeast, fungi, and cyanobacteria. Genetically altered crops used to consume more CO<sup>2</sup> from the environment than they emit. These microorganisms are used to produce different fuels, including ethanol, butanol, hydrogen, methane, vegetable oil, biodiesel, isoprene, gasoline, and jet fuel [39]. Fourth-generation biofuel research started in 2006, and significant results have not been published yet in peer-review journals [35]. Figure 1 shows the schematic representation of the integrated biorefinery for 1G to 4G.

**Figure 1.** Schematic representation of integrated biorefinery for first, second, third, and fourth generation of biomass.
