Project: Biomass and Natural Gas

Dept. of Energy Technology
Div. of Heat and Power Technology

Prof: Torsten H. Fransson

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Title: Hybrid Combined Cycles with Air Bottoming Cycle
Author: Maurice Heine /

Report nr: …….

Project: Biomass and Natural Gas

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Supervisor at KTH: Tekn.Lic. Miroslav P. Petrov

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Overall responsible at KTH: Prof. Torsten H. Fransson
Approved at KTH by: Prof. Torsten H. Fransson /

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Overall responsible at industry: --
Industrial partners: --
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Abstract:
Subject of this work is the hybrid dual-fuel combined cycle concept in combined heat and power (CHP) mode with a natural gas fired topping gas turbine or internal combustion engine and a biomass-fired air bottoming cycle.
Combined cycle technology is a well-known and proven concept. The conventional combined power cycle consists of a topping gas turbine or internal combustion engine cycle and a bottoming steam cycle. In the concept investigated in the course of this thesis, the bottoming steam cycle is replaced by an externally fired air turbine cycle, consisting of an air compressor, air heater and air expander. This configuration is expected to be beneficial in terms of simplicity of installation, operation and maintenance as well as lower costs, especially at small-scale applications (< 10 MWel). The cycle configurations investigated have been modelled using the heat balance simulation software PROSIM. The amount of heat input in the topping cycles was fixed to 6.31 MWth. This value was used as basis for the variation of the heat input and therefore of the size of the applied bottoming cycles. In order to enhance the bottoming cycle electrical power output, prospects of implementing intercooling, humidification and water injection technologies were investigated. As a preliminary condition only common technology and standard components were to be utilized in order to ensure low costs. High electrical overall cycle efficiencies were sought during the optimisation process. The trend of electrical and total efficiencies with changing ratio of fuel energy input in the hybrid dual-fuel combined cycle (biomass and natural gas) has been studied.
Results show that with an applied internal combustion engine as topping cycle the electrical efficiency of the hybrid combined cycle - independent of the applied bottoming cycle’s configuration and size – does not exceed that of the stand-alone topping cycle of 38.6%. The overall electrical efficiency of the combined cycle rather decreases with increasing bottoming cycle size. Application of intercooling, humidification and injection techniques in the bottoming cycle diminish this effect, but can not inverse it.
On the other hand several of the hybrid combined cycles with an applied gas turbine topping cycle exceed the gas turbine cycle’s stand-alone electrical efficiency of 31,26%. An optimum is reached at a fuel heat input ratio of 0.60 for biomass in respect to natural gas. At this proportion, the bottoming cycle utilizes the entire rejected mass flow of the topping cycle. Introducing intercooling, humidification and injection facilities in order to improve the turbo machinery performance of the bottoming cycle can increase overall electrical efficiency up to 5.8% - points at the fuel heat input ratio.
Intercooling, humidification and injection facilities applied to the bottoming cycle in general increase overall electricity output, whereas for the two latter ones the total efficiency decreases massively with enlarging the size of the bottoming cycle, since no flue gas condensation was applied and the vaporization energy of the injected water is lost.

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