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Biomass Based Indirectly Heated Combined Cycle Plant: Energetic and Exergetic Performance Analyses

P Mondal, S Ghosh

Biomass based power plant can be an excellent option for community scale power generation to overcome the fossils fuel shortage and environmental issues. Thermal performance and sizing of an indirectly heated combined cycle plant employing topping gas turbine (GT) block and bottoming steam turbine (ST) block integrated with biomass gasifier is analysed in this paper. The plant is capable of producing topping 20 kWe fixed output along with bottoming steam turbine output. A mix of Olive tree leaves and pruning is considered as fuel feed. Novelty of the scheme exists in the heating process of the topping GT cycle, which is done in an external combustor-heat exchanger (CHX) unit rather than the conventional combustor of GT cycle. The plant configuration is modelled and analysed using Cycle-Tempo software. Both energetic and exergetic performances are analysed over a range of topping cycle pressure ratio (4-20) and for different turbine inlet air temperatures (TIT=900, 1000 and 1100 deg C). In the base case scenario (topping cycle pressure ratio=4 and TIT= 900 deg C), the system efficiency is about 30.5%. The plant attains maximum efficiency at pressure ratio range 6-9 depending upon the TIT. Required specific air flow, which indicates the size of the ducting and CHX unit of the topping cycle decreases with increase in pressure ratio and at higher TIT’s. From the Second Law analysis it is found that the major exergy losses occur at the gasifier and combustor-heat exchanger units, together account for 44% of the fuel exergy value.