A critical review on second- and third-generation bioethanol production using microwaved-assisted heating (MAH) pretreatment

In the pursuit of the 7th Sustainable Development Goal (SDG) of the United Nations, it is driven to develop affordable, sustainable, and clean energy. Second (lignocellulosic) and third (microalgae and macroalgae) generation bioethanol appear to be the most favorable renewable feedstocks to meet SDG 7 pursuance. However, several feedstock pretreatments have shown process challenges in terms of yield and product inhibitors. Microwave-assisted heating (MAH) has gained positive attention as a novel alternative method for pretreatment heating in the global community due to its numerous advantages over conventional heating. Therefore, current studies and progress in MAH pretreatment for bioethanol production are critically reviewed. The effects of different parameters on MAH pretreatment are also examined. The application of MAH pretreatment demonstrates rapid reaction exhibiting remarkable improvement in sugar recovery and fewer inhibitors over conventional heating. Despite these advanced results, certain obstacles remain to be addressed and improved until it is ready for industrial scalability. It includes understanding the interaction and relationship between the different parameters like biomass loading, catalysts, temperature, power intensity, contact time, and pressure for reactor design. Moreover, factors such as the non-thermal effect, dielectric properties of different biomass materials, and controlled temperature need to be further explored and evaluated. Thus, continuous research and development efforts on MAH pretreatment are encouraged to become commercially feasible for the applications of bioethanol production. [Display omitted] •Microwave pretreatment reduces time, energy consumption, and increases sugar yield.•Microwave pretreatment parameters are reviewed and discussed.•Ionic liquids are the future catalyst for biomass disruption.•Microwaves help break down polysaccharides and lead to higher yeast weight.•Large reactors with microwaves must be developed further to allow higher production.