Contact information:
Danish Energy Agency: Rikke Næraa, rin@ens.dk Energinet.dk: Rune Grandal, rdg@energinet.dk Author: Dansk Gasteknisk Center
Publication date August 2016
Amendments after publication date
Date Ref. Description
January
2018 05 Combined cycle
gas turbine Additional references have been included
Qualitative description
Brief technology description
Main components of combined-cycle gas turbine (CC-GT) plants include: a gas turbine, a steam turbine, a gear (if needed), a generator, and a heat recovery steam generator (HRSG)/flue gas heat exchanger, see the diagram below.
Figure 1 Diagram showing an example of a CC-GT plant designed for combined heat and power production.
The gas turbine and the steam turbine are shown driving a shared generator. In real plants, the two turbines might drive separate generators. Where the single-shaft configuration contributes with higher reliability, the multi-shaft has a
Page 69| 414
The condenser is cooled by the return water from the district heating network. Since this water is afterwards heated by the flue gas from the gas turbine, the condensation temperature can be fairly low.
The overall energy efficiency depends on the flue gas stack temperature, while the electricity efficiency depends, besides the technical characteristics and the ambient conditions, on the district heating flow temperature. However, some plants do not have the option to sell district heating, and the condenser is therefore cooled by a sea/river/lake or a cooling tower.
If applying heat pumps for extra cooling of the exhaust gas, even higher total fuel efficiency can be reached. Depending on priorities, the flue gas heat pumps can be electrical or absorption type.
The heat recovery steam generator (HRSG) is defined through the number of pressure levels, each producing steam for the steam turbine. Small, medium and large scale units usually have one or two steam pressure stages whereas very large units may have three steam pressure stages. Steam is fed to the turbine both at the inlet and at a later stage between the two adjacent steam turbine sections; this is one of the special features of steam turbines in CC-GT.
Plants being able to shift between condensation mode (power only) and back-pressure mode (power and district heat) include a so-called extraction steam turbine. Such turbines are not available in small sizes, and dual-mode plants are therefore only feasible in large scale.
The power generated by the gas turbine is typically two to three times the power generated by the steam turbine. An extraction steam turbine shifting from full condensation mode at sea temperature to full back-pressure mode at district heat return temperature will typically lose about 10% of its electricity generation capacity. For example, a 40 MW gas turbine combined with a 20 MW steam turbine (condensation mode), loses 2 MW, (10% of 20 MW) or 3% of the total generating capacity (60 MW).
Input
Typical fuels are natural gas and light oil. Some gas turbines can be fuelled with other fuels, such as LPG, biogas etc., and some gas turbines are available in dual-fuel versions (gas/oil).
Gas fired gas turbines need a fuel gas pressure of 20-60 bar, typically aero-derivative gas turbines need higher pressure than industrial gas turbines.
Additional steam from other sources may be fed to the steam turbine section.
Output
Electricity and heat. The heat is most often supplied as hot water.
Typical capacities
The enclosed datasheets cover large scale CC-GT (100 – 400 MW with extraction steam turbine) and medium scale (10 – 100 MW with back pressure steam turbine).
Most CC-GT units has an electric power of > 40 MWe Regulation ability and other power system services
CC-GT units are to some extent able to operate at part load. This will reduce the electrical efficiency and often increase the NOx emission.
If the steam turbine is not running, the gas turbine can still be operated by directing the hot flue gasses through a boiler designed for high temperature or into a bypass stack.
The larger gas turbines for CC-GT installations are usually equipped with variable inlet guide vanes, which will improve the part-load efficiencies in the 85-100 % load range, thus making the part-load efficiencies comparable with conventional steam power plants in this load range. Another means to improve part-load efficiencies is to split the total generation capacity into several CC-GTs. However, this will generally lead to a lower full load efficiency compared to one larger unit.
The NOx emission is generally increased during part load operation.
Some suppliers have developed CC-GT system designs enabling short start up both regarding the electrical output and the steam circuit as well.
Most CC-GT plants installations include a short time heat storage. This leads to more flexibility in production planning.
Advantages/disadvantages Advantages
Large gas turbine based combined-cycle units are world leading with regard to electricity production efficiency among fuel based power production.
Smaller CC-GT units have lower electrical efficiencies compared to larger units. Units below 20 MWe are few and will face close competition with single-cycle gas turbines and reciprocating engines.
Gas fired CC-GTs are characterized by low capital costs, high electricity efficiencies, short construction times and short start-up times.
Disadvantages
The economies of scale are substantial, i.e. the specific cost of plants below 200 MWe increases as capacity decreases.
The high air/fuel ratio for gas turbines leads to lower overall efficiency for a given flue gas cooling temperature compared to steam cycles and cogeneration based on internal combustion engines.
Environment
Gas turbines have continuous combustion with non-cooled walls in the combustion chamber. This means a very complete combustion and low levels of emissions (except for NOx). Developments focusing on the combustor(s) have led to low NOx levels.
Flue gas post-treatment can consist of SCR catalyst systems etc.
Research and development perspectives
Continuous research is done concerning higher inlet temperature at first turbine blades to achieve higher electricity efficiency. This research is focused on materials and/or cooling of blades.
Continuous development for less polluting combustion is taking place. Increasing the turbine inlet temperature may increase the NOx production. To keep a low NOx emission different options are at hand or are being developed, i.e. dry low-NOx burners, catalytic burners etc.
Development to achieve shorter time for service is also being done.
Examples of market standard technology
Large CC-GT units have demonstrated an electrical efficiency of 60 % (LHV reference). Systems are now being offered and built with an electrical efficiency close to 62 %. The units are large units with an output in the 500 – 600 MWe [3].
In 2009, Eon opened one of the most efficient power plants in Europe, the CHP plant Öresundsverket in Malmö, Sweden.
The 440 MW CC-GT has an electrical efficiency of 58% and an overall fuel efficiency in full cogeneration mode of 90%.
The total investment figure for the project was €300 million [12].
Prediction of performance and costs
Gas turbine based combined cycle plants are a well-proven, widespread and available technology, making CC-GT a category 4 technology. Improvements are still being made primarily on the gas and steam turbines used. Developments for faster load response and dynamic capabilities are now also in focus. In [13] examples is given for a large (>250 MWe) CC-GT plant with full GT power in less than 15 minutes and approx. 70 % power supply from the steam turbine. Full steam turbine power is achieved in less than one hour.
Page 71| 414
The expected market in Denmark is limited and declining for the time being. This means that no significant reductions in investment and/or operation/maintenance cost is expected in the years to come. In a longer perspective, gas turbines or gas turbine combined cycle plants may become relevant for green gas based balancing power.
Uncertainty
Uncertainty stated in the tables both covers differences related to the power span covered in the actual table and differences between the various products (manufacturer, quality level, extra equipment, service contract guarantees etc.) on the market.
A span for upper and lower product values is given for the year 2020 situation. No sources are available for the 2050 situation. Hence the values have been estimated by the authors.
Additional remarks
The main rotating parts (the gas turbine, steam turbine and the generator) tend to account for around 45-50% of the investment costs (EPC price), the heat recovery steam generator, condenser and cooling system for around 20%, the balance of plant components for around 15%, the civil works for around 15% and the remainder being miscellaneous other items [10].
Data sheets
Technology Gas turbine, combined cycle, extraction plant
2015 2020 2030 2050 Uncertainty (2020) Uncertainty (2050) Note Ref
Energy/technical data Lower Upper Lower Upper
Generating capacity for one unit (MW) 100 - 500 F
Environment
SO2 (degree of desulphuring, %) 0 0 0 0 0 0 0 0
NOX (g per GJ fuel) 20 15 10 8 10 30 5 15 D 3, 7
CH4 (g per GJ fuel) 1.5 1.5 1.5 1.5 1 8 1 8 G 7
N2O (g per GJ fuel) 1 1 1 1 0.7 1.2 0.7 1.2 G 7
Financial data
Nominal investment (M€/MW) 0.9 0.88 0.83 0.8 0.8 1.2 0.7 1.1 5, 8
- of which equipment 0.7 0.68 0.64 0.61 0.65 1.02 0.6 0.95 10
- of which installation 0.2 0.20 0.19 0.19 0.15 0.18 0.1 0.15 10
Fixed O&M (€/MW/year) 30,000 29,300 27,800 26,000 25,000 35,000 20,000 30,000 B 5
Variable O&M (€/MWh) 4.5 4.4 4.2 4 3 7 3 7 5
Page 73| 414
Technology Gas turbine, combined cycle (back-pressure)
2015 2020 2030 2050 Uncertainty (2020) Uncertainty (2050) Note Ref
Energy/technical data Lower Upper Lower Upper
Generating capacity for one unit (MW) 10 -100 F
Electricity efficiency (condensation mode for
extraction plants), net (%), 50 51 53 55 42 55 45 58 5
Electricity efficiency (condensation mode for
extraction plants), net (%), annual average 47 48 50 52 39 52 42 55 5, 9
Cb coefficient (50oC/100oC) 1.2 1.3 1.4 1.55 0.9 1.6 1.1 1.7
Cv coefficient (50oC/100oC) - - - - - - - - L
Forced outage (%) 3 3 3 3 2 4 2 4 5
Planned outage (weeks per year) 2.5 2.3 2 2 2 4 1.5 4 5
Technical lifetime (years) 25 25 25 25 25 >25 25 >25 E 5, 3
Construction time (years) 2.5 2 2 2 2 3 2 3 5
Space requirement (1000m2/MW) 0.025 0.025 0.025 0.025 0.019 0.038 0.019 0.038 G 3
Plant Dynamic Capabilities
Primary regulation (% per 30 seconds) - - - - - - - - I
Secondary regulation (% per minute) 15 15 15 15 5 15 5 15 C, M 5, 3, 11
Minimum load (% of full load) 40 40 40 40 30 50 30 50 A 5, 3, 11
Warm start-up time (hours) 1 1 1 1 0.5 1.5 0.5 1.5 H 5, 6, 1, 11
Cold start-up time (hours) 2.5 2.5 2.5 2 2 5 1.5 5 5, 6, 1, 11
Environment
SO2 (degree of desulphuring, %) 0 0 0 0 0 0 0 0
NOX (g per GJ fuel) 20 15 10 8 10 30 5 15 D 3, 7
CH4 (g per GJ fuel) 1.5 1.5 1.5 1.5 1 8 1 8 G 7
N2O (g per GJ fuel) 1 1 1 1 0.7 1.2 0.7 1.2 G 7
Financial data
Nominal investment (M€/MW) 1.3 1.3 1.2 1.1 1.1 1.8 0.9 1.6 5, 9
- of which equipment 1 1.0 0.9 0.8 0.8 1.4 0.65 1.25 10
- of which installation 0.3 0.3 0.3 0.3 0.3 0.4 0.25 0.35 10
Fixed O&M (€/MW/year) 30,000 29,300 27,800 26,000 25,000 35,000 20,000 30,000 B 5
Variable O&M (€/MWh) 4.5 4.4 4.2 4 3 7 3 7 5
Notes:
A Low efficiency at low loads and often increased NOx emission B Limited availability of data
C Power related
D Based on Dry Low NOx (DLN) techniques E Technical- and design life most often > 25 years F Electrical output
G CHP configuration, Including DGC assumptions H Manufacturers says down to 30 minute I No data available
J Data on Cv from the 2012 version roughly adjusted for higher electricity efficiency K No known use
L No Relevance for Back Pressure Lay Out
M Upward regulation is typically 10 - 15 %/min, while downward regulation is > 30 % /min
References
[1] Danish Gas Technology Centre, Analysis on Gas Engine and Gas Turbine Dynamics, 2013.
[2] Major Gas Turbine Suppliers product information available on the Web, 2015 [Online].
[3] Data specs from manufacturers (Web), 2015.
[4] Opportunities for Micropower and Fuel Cell/Gas Turbine Hybrid Systems in Industrial Applications, A. D. Little, US, 2000.
[5] Wärtsila Technical Journal in detail 02-2014.
[6] Danish Gas Technology Centre, Analysis and discussion with manufacturers, April 2013.
[7] Danish Gas Technology Centre, Environmental survey, 2012.
[8] Smart Power Generation - The future of electricity production, J. Klimstra & M. Hotakainen, 2011.
[9] IEA: Projected cost of generating electricity, 2010.
[10] UK Electricity Generation Costs Update, M.M. Donald, June 2010.
[11] Siemens: Data on GT-CC, 2010.
[12] EuroHeat&Power, vol. 6, III/2009
[13] Siemens Flex-PlantTM CC-GT concepts, presentations 2013
Page 75| 414