Microalgal fuels: Promising energy reserves for the future

Traditional methods of fuel production and consumption result in the emission of huge quantities of CO₂, leading to a rise in the Earth's surface temperature. This has driven the scientific community to look for alternate sources of fuels that could fulfil the current energy demands, while reducing CO₂ emissions. Microalgae have garnered substantial interest as an excellent biological resource for biorefineries, given their abilities to accumulate diverse CO₂-derived bioproducts, such as various forms of fuels, chemical building blocks, and pharmaceuticals; this could be interpreted as an efficient method for storing solar energy by biological means. Microalgal biomass is free from ethical debates because it does not compete with food production for fertile arable lands and clean water for their cultivation, which makes the entire process highly sustainable. Microalgal biomass-derived fuels for each end-use can be obtained via several processes, including drying, chemical, and physical lipid-extraction methods, anaerobic digestion, and thermochemical treatments (e.g., pyrolysis, torrefaction, hydrothermal carbonization, liquefaction, and gasification). This review aims to critically explore the process layouts, their operation conditions (including catalysts), and expected outputs related to fuel production. It also provides a comprehensive overview of the most notable and promising strategies for the producing energy resources from various species of microalgae. Pretreatment of microalgal biomass, followed by bioconversion strategies, such as hydrothermal carbonization and photo-fermentation, along with genomics-based approaches, can lead to higher yields of microalga-derived solid, liquid, and gaseous fuels.