Reactor for biomass conversion and waste treatment in supercritical water: A review

Hydrothermal treatment of renewable biomass and organic waste using supercritical water (SCW) is a sustainable, green, and efficient route for achieving energy recovery, chemicals production, and harmless treatment. The reactor is undoubtedly the central arena where various SCW reactions perform. This review is the first to focus on the special topic, i.e., SCW reactors for biomass conversion and waste treatment. All reported reactors have been classified into three groups to discuss their individual merits and demerits, with a particular focus on their advanced aspects, to minimize or overcome their inherent technical problems (corrosion, plugging, safety issues, poor accessibility, and high energy cost). The first group-metallic reactors-includes tank and tube reactors. These are the most commonly used reactors for diverse SCW processes, and can be operated in three modes: batch, semi-batch, and continuous. The second group includes visual reactors, which are the most attractive experimental devices for SCW processes because they enable visual observation and in situ optical detection. These include X-ray/neutron-radiography-based reactors, the diamond anvil cell, sapphire window reactor, fused silica/quartz capillary reactors, and analogous transparent reactors. The last group includes specially designed reactors for different SCW processes geared toward industrial applications, such as reactors for SCW-based hydrolysis, upgrading, gasification, and oxidation. This review provides comprehensive information for the non-expert reader to select a suitable experimental device for fundamental research and discusses diverse advanced concepts for the development of industrial reactors.