Biomass is a significant renewable energy source. The conversion of woody biomass into a combustible gas provides the opportunity to enhance the efficiency of biomass-based power systems, and allows solid fuels to be used in high-efficiency power generation processes. This paper discusses the energy efficient utilization of biomass by turbo machines under unpressurized conditions, working with the inverted Brayton cycle in which turbine expansion, cooling by heat exchanger and draft by compressor are made in an open cycle mode. Author calls this an “atmospheric pressure turbine (APT)”. Thermodynamic analysis has shown that an electric efficiency up to 25 percent (HHV) and a total energy efficiency of more than 75 percent (HHV) are expected for combined heat and power applications even at small plant capacities (less than 40 kW). By adapting EGR (exhaust gas recirculation), the total cogeneration efficiency is considerably improved by raising the recirculation ratio, and total energy efficiency of 85% is expected when EGR ratio is 0.4.
However, the recirculation ratio is limited by the residual oxygen in the combustor. In the present analysis, in order to remove this limitation the air separation unit (ASU) is used additionally. Therefore, the recirculation ratio of 95% can be attained.