Small gas turbine technology
Introduction
Small gas turbines, in the power range from 0 to 500 kW, have an electrical efficiency of about 18%.
This fact is mainly due to the scale effect on the aerodynamic components, the assembly clearances and the relatively low turbine inlet temperature compared with large gas turbine.
To increase this efficiency, up to a value, which can permit to the gas turbine to compete with the reciprocating engine and decrease the CO2 emission, the simplest way is to add a recuperator or regenerator on the gas exhaust.
A small gas turbine unit fitted with a recuperator, with an efficiency of 90%, can achieve an electrical efficiency of about 30%, slightly lower than reciprocating engines, which offer in this range of power an efficiency of about 35%.
To pass a new step in efficiency, two different ways are opened:
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Dramatically increase the Turbine Inlet Temperature (TIT), solution, which requires on small gas turbine the development on ceramic components in hot parts.
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Develop new thermodynamic cycles, which are well adapted to a small co-generation gas turbine system.
For the first way, the technology of the ceramic turbine wheel is unfortunately not yet available. As the demand for a more efficient unit is still existing, this presentation describes the choice made of a more efficient cycle and the feasibility of the major components of this cycle to satisfy the stringent requirements in terms of pollutant emissions, production and maintenance costs, and, reliability.
This R and D study was supported by the European Commission as an EC funded contract: ENK5 CT2000 00070.
Acronym: CHEP.
Title: Research and Development of high efficiency components for an intercooled, recuperated CHP gas turbine for Combined Heat and Efficient Power».
Project co-ordinator: Microturbo S.A.
Partners:
Section snippets
Thermodynamic cycle choice
A parametric study was conducted to compare the simple cycle with the recuperative cycle, the others possible cycles known as recuperative-intercooled cycle recuperative-under-pressurised cycle and recuperative inverse cycle. The aerodynamic pathflow of each cycle is given in Fig. 1, Fig. 2, Fig. 3, Fig. 4. To compare the efficiency of different thermodynamic cycles, the max continuous Turbine Inlet Temperature (TIT) was selected at the conservative level of 950 °C (1223 K).
The efficiency of
Thermodynamic rotating components
The thermodynamic rotating components of the 350 kW recuperative intercooled gas turbine are:
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Compressor 1
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Compressor 2, after the intercooling heat exchanger
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Turbine wheel, radial or two stages of axial turbines
The main dimensional characteristics of each component are given in Table 3.
The parametric study of the influence of the pressure ratio repartition between the compressor 1 and 2, for a constant cycle pressure ratio of 6/1, shows that this parameter is not critical.
The cycle efficiency is
High speed generation
The High Speed Generator (HSG) with associated starting and power converters is the classical solution for Microturbine units. Mostly of the existing systems are in the power range from 30 to 100 kW. For the 350 kW Recuperated-Intercooled unit it is necessary to design and prove the high power HSG.
The design of the HSG was made for the turbine max nominal speed of 42 000 rpm (model G185).
The main characteristics are given below:
Recuperator
The thermal exchange efficiency of the recuperator on the cycle performance is of the first importance.
As shown in Fig. 12, a loose of 5% in recuperator efficiency conducts to a drop of 2% on the cycle performance and, by the way, on fuel consumption.
Recuperator efficiency is one of the major criteria to reduce the cycle fuel consumption, but, the size and the cost. Of this component is directly linked to its efficiency and increase of about 40% for an efficiency change from 85% to 90%.
A state
Catalytic combustion chamber
The catalytic combustion configuration is based on a two steps combustion system:
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Step one is a catalytic partial oxidation (CPO) in rich condition, it can be considered as a fuel processing system, without NOx emission.
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Step two is a homogenous combustion operating in very weak conditions. Due to high dilution, combustion, with H2 high percentage, is done at low temperature, thus reducing NOx emissions.
This stepwise combustion is expected to produce very low NOx emission because its both steps
Turbine mechanical arrangement
A preliminary mechanical arrangement of the recuperative intercooled turbine unit is given on the following lay out.
Note that the first compressor and the High Speed Alternator are on the same shaft.
The second stage compressor with the radial turbine are on a second shaft.
The air and gas inlets into the spiral recuperator need to be optimised by a Fluent simulation to achieve the lowest pressure drops.
Conclusion
This study demonstrates the feasibility of a competitive recuperative-intercooled co-generation unit of a nominal power of 350 kW, with the advance technical solutions to achieve the following targets:
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lowest CO2 emission by higher cycle efficiency;
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reduction of NOx and CO emissions by Catalytic Partial Oxidation combustion system;
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low cost, high efficiency spiral, laser welded heat exchanger recuperator;
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High Speed Generator integrated on the turbine shaft to minimise the number of components.
Acknowledgements
We thank the European Commission for the financial support given to the R and D project subject of this paper.
We also acknowledge our partners for their kind co-operation to achieve this project.