Valorization of alcohol industry residues into solid, gaseous and liquid biofuels: A comprehensive review

Alcohol industry residues (AIRs) are typical organic solid wastes from a major light industry while belong to renewable biomass resource, and their annual accumulation amount is significant and rapidly increasing worldwide. The clean and efficient utilization of AIRs is urgently needed, driving the development of valorization technologies with the waste-to-energy strategy considered as the most practical and sustainable method. In recent decades, significant attention has focused on unlocking the energy potential of AIRs. However, current technologies face significant challenges in producing desirable biofuels from AIRs in an eco-friendly, carbon-neutral, and economically efficient manner, credit to the restriction of their unfavorable properties such as high initial moisture, nitrogen, and oxygen contents. In this review, the energy valorization of AIRs using thermo- and bio-chemical conversion technologies is comprehensively reviewed and compared. The preparation methods, properties, and thermal use of various biofuels are systematically summarized. Low or medium-temperature hydrothermal technology demonstrates significant advantages in converting AIRs into desired biofuels, either as a pretreatment or preparation method. This strategy can produce modified solid biofuel by upgrading and denitrogenating capabilities, and improve biogas and bioethanol yields through boosted hydrolysis reactions. Combining hydrothermal with other conversion methods would be a promising research avenue for valorizing AIRs into energy, enabling the poly-generation of targeted biofuels by selectively regulating the evolution of specific element. This review can offer an overview and insights into effectively managing and utilizing AIRs for researchers in the field.