Energy conversion of biomass with supercritical and subcritical water using large-scale plants

Exploiting unused or waste biomass as an alternative fuel is currently receiving much attention because of the potential reductions in CO2 emissions and the lower cost in comparison to expensive fossil fuels. If we are to use biomass domestically or industrially, we must be able to convert biomass to high-quality and easy-to-use liquid, gas, or solid fuels that have high-calorific values, low moisture and ash contents, uniform composition, and suitable for stored over long periods. In biomass treatment, hot and high-pressure water including supercritical and subcritical water is an excellent solvent, as it is clean and safe and its action on biomass can be optimized by varying the temperature and pressure. In this article, the conversion of waste biomass to fuel using hot and high-pressure water is reviewed, and the following examples are presented: the production of large amounts of hydrogen from waste biomass, the production of cheap bioethanol from non-food raw materials, and the production of composite powder fuel from refractory waste biomass in the rubble from the Great East Japan Earthquake. Several promising techniques for the conversion of biomass have been demonstrated in large-scale plants and commercial deployment is expected in the near future.