Thermodynamic analysis of a small scale combined cycle for energy generation from carbon neutral biomass

The aim of this paper is to investigate the thermodynamic performance of a novel small-scale power plant that employs a combined cycle for the energy generation from carbon-neutral biomass, such as pruning residues. The combined cycle is composed of an externally fired Joule Brayton cycle followed by a bottoming steam cycle. The topping cycle has the unique particularity of being composed of a cost-effective turbocharger taken from the automotive industry, in place of a more expensive commercial micro-turbine. The turbocharger can be either directly connected to the electric generator (after a few modifications) or coupled (without modifications) with a power turbine moving the generator. The use of solid biomass in the proposed plant is allowed by the presence of an external combustor and a gas-to-gas heat exchanger. The warm flue gases exhausted by the topping cycle are used in a bottoming cycle to produce steam, which can power a steam expander. This paper thermodynamically assesses the novel combined cycle in the configuration for the topping cycle that employs a turbocharger coupled with a power turbine capable of generating 30 kW of electrical power. Furthermore, the comparison between the performance obtained using the bottoming water Rankine cycle and a bottoming Organic Rankine Cycle is provided.