Recent developments and future outlooks of hydrodynamic cavitation as an intensification technology for renewable biofuels production

Biofuels represent one of the most interesting candidates to replace fossil fuels and to reduce the global warming issues. Process intensification techniques able to maximize the biofuel yields minimizing the energy consumption are recently studied by researchers with encouraging results. Among them, Hydrodynamic cavitation (HC) appears a promising method, especially when feedstock hard-to-decompose are used (waste oil, lignocellulosic biomass, plastic wastes). This review is focused on the strengths and weaknesses of HC-assisted processes to produce three biofuels (biodiesel, bioethanol and biogas) starting from waste/non-edible oils in the case of biodiesel and lignocellulosic biomass for bioethanol and biogas, where HC is applied in the pretreatment units. The influence of operative conditions and the design of the cavitation device on the HC effectiveness together with three cavitation models were summarized and discussed in detail. Finally, the comparison in terms of yields, energy consumption and process scale with conventional technologies is made, highlighting the challenges and limitations which still need to be addressed before HC-assisted processes can be scaled-up. It is found that the HC- assisted biodiesel production enhances the yields of over 99% in a short period of time starting from different waste oils, with a high-purity level that meet the standards set by EN 14214 and ASTM D6751. The use of the HC-assisted lignocellulosic biomass pretreatment results in high lignin removal (60%) by consuming 4 times less energy than ultrasound-assisted processes (3.65 MJ/kg and 14.4 MJ/kg, respectively), fundamental to achieve a more effective production of bioethanol and biogas from lignocellulosic wastes.