Heating is one of the main energy consuming processes in countries with moderate temperatures. Heat is mostly produced from fossil fuels such as natural gas and fuel oil. A sustainable alternative energy source is biomass, however, combustion of biomass results in harmful emissions to the environment, such as emission of SO2, CO, NOx, fine particles and Polycyclic Aromatic Hydrocarbons (PAHs). Moreover, the remaining ashes cannot be reused. Biological wood oxidation has been proposed as a new technology for heat production at temperatures around 40-55 °C.