Hydrothermal techniques for biofuels production

It is well known that fossil fuels are associated with serious limitations and environmental problems. Therefore, nowadays, there is a great interest in alternative energy sources that may help in the transition to a sustainable development. Hydrothermal techniques are prominent ways to convert energy-rich organic wastes and biomass into fuels or other useful materials. The main advantage that these processes provide is a higher capacity than conventional technologies to valorize high moisture content biomass, without requiring a previous drying treatment. In hydrothermal processes, the reaction medium is water at high pressure and temperature. This covers a wide range of operating conditions where the special properties of the water can be exploited to provide a reaction media of a very different nature and tremendous potential for the transformation of organic compounds. This chapter will address the various hydrothermal processes such as carbonization, liquefaction, and gasification developed for biomass conversion into solid, liquid, and gaseous biofuels, respectively. The main characteristics of the techniques and the effect of the operating conditions on the product yields will be examined. Economic aspects will be assessed to identify the challenges for the technology transfer of these processes.