High efficiency power generation from biomass sources using externally fired supercritical CO2 Brayton cycles

In the small to medium power range the main technologies for the conversion of biomass sources into electricity are based either on internal combustion engines or Organic Rankine cycles. Relatively low electric efficiencies are obtained in both cases due to thermodynamic losses in the conversion of biomass into syngas and to the heat transfer between combustion gases and working fluid, respectively. Higher efficiencies can be obtained using the supercritical closed CO2 Brayton cycles, the applications of which are restricted in the literature to nuclear power plants and more recently to concentrating solar power plants. The cascaded configuration of two supercritical CO2 cycles enables to overcome the intrinsic limitation of the single cycle in the effective utilization of the whole heat available from the heat source. The aim of this paper is to evaluate whether this power plant configuration could be a good alternative option in the conversion of biomass sources into electricity, which was never explored in the literature up to now. The focus is on the search of the thermodynamic operating parameters which maximize power output. Results of the optimization procedure show that a total heat recovery efficiency in the range 30-34% can be achieved, which is approximately 5%-points higher than that of the existing biomass power plants in the small to medium power range.