A method and apparatus for extracting useful energy from biomass fuels as part of a hybrid electricity generating thermal power plant, utilising both a primary heat source, such as coal, gas, oil or nuclear power, and a secondary heat source in the form of biomass, whereby the biomass is oxidised in aqueous solution in a supercritical water oxidation (SCWO) process utilising energy from the primary heat source to heat and compress a feed stream of water to a temperature and pressure at or beyond its critical point.