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Open Cycle Gas Turbines

Topic last reviewed: 1 February 2014

A gas turbine is an internal combustion engine that operates with rotary rather than reciprocating motion. Gas turbines are composed of three main components: compressor, combustor and power turbine. In the compressor section, air is drawn in and compressed up to 30 times ambient pressure and directed to the combustor section where fuel is introduced, ignited and burned. Combustors can be either annular, can-annular, or silo. An annular combustor is a doughnut-shaped, single, continuous chamber that encircles the turbine in a plane perpendicular to the air flow. Can-annular combustors are similar to annular combustors, however they incorporate several can shaped combustion chambers rather than a single combustion chamber. Annular and can-annular combustors are based on aircraft turbine technology and are typically used for smaller scale applications. A silo combustor has one or more combustion chambers mounted external to the gas turbine body. Silo combustors are typically larger than annular or can-annular combustors and are used for larger scale operations.

The compressor, combustor and turbine are connected by one or more shafts and are collectively called the gas generator or gas turbine. Figures 1 and 2 [JR1]  below illustrate the typical gas turbine generator configuration and schematic.

 

Figure 1. Open Cycle Gas Turbine Configuration

 

 

Figure 2. Open Cycle Gas Turbine Schematic

The compressor, combustor and turbine are connected by one or more shafts and are collectively called the gas generator or gas turbine. Figures 1 and 2 [JR1]  below illustrate the typical gas turbine generator configuration and schematic.

 

Figure 1. Open Cycle Gas Turbine Configuration

 

 

Figure 2. Open Cycle Gas Turbine Schematic

Technology maturity

Commercially available?: Yes 
Offshore viability: Yes 
Brownfield retrofit?: Yes 
Years experience in the industry: 5-10 

Key Metrics

Range of application:
5 – 375 MW typical size turbines are sold by various manufacturers with higher efficiencies for larger models. Smaller size turbines are typically used for offshore applications due to lower weight
 
Efficiency: 35% - 40%, potentially as high as 46% (see alternatives) 
Guideline capital costs: $389/kW (USD, 2005) [3]. Emergency power units generally have lower efficiency and lower capital cost, while turbines intended for prime power have higher efficiencies and higher capital costs
 
Guideline operational costs: Depending on the turbine size, the total non-fuel O&M costs range from 0.0111 $/kWh for a 1 MW turbine to 0.0042 $/KWh for a 40 MW gas turbine
 
Typical scope of work description: GHG emissions are directly related to the efficiency of the gas turbine. Newer machines are usually more efficient than older ones of the same size and general type and therefore produce less carbon dioxide emissions. Typical Carbon dioxide emissions from a 40 MW gas turbine without heat recovery and operating at 37 percent efficiency are 1.079 lb/MWh [Reference 4].
 
Time to perform engineering and installation: A few months for engineering and several weeks to a few months for construction. This is also heavily dependent on location and size. Larger units at more remote locations can take a lot longer

Decision Drivers

Technical: Footprint: size, weight, plot area are required
Load profile of installation needs to be relatively stable
Turbines up to about 50 MW may be either industrial or modified aeroderivative engines while larger units up to about 330 MW are designed for specific applications
For offshore turbines, optimum size and high power-to-weight ratio are key factors as well as availability, reliability and ruggedness. Also decision is required for large turbine with appropriate backup or smaller number of turbines for specific applications
Operational: Operators need to be trained for turbines only (no steam system training needed)
Driven by fuel gas price versus incremental capital costs

Commercial: Larger size turbines operate at higher efficiency but are not as efficient as the combined cycle system. Negative impacts can be mitigated through the use of alternatives
 
Environmental:

Depends on application. For a 211 MW Gas Turbine Power Plant [Reference 5]:
Capital Cost: $400 to $700/kW
Variable O&M – $29.9/MWh
Fixed O&M – $5.26/kWh

Additional Comments

A variety of fuels can be used. Natural gas is preferred for most plants but LPG, refinery gas, gas oil, diesel and naptha may be used. Aeroderivative and low emission turbines have more specific fuel requirements.

Additional Comments

A variety of fuels can be used. Natural gas is preferred for most plants but LPG, refinery gas, gas oil, diesel and naptha may be used. Aeroderivative and low emission turbines have more specific fuel requirements.

High Efficiency Gas Turbines

Manufacturer  Model  Simple Cycle Efficiency  Combined Cycle Efficiency  Power Produced (simple) (MW) 
 Alstom  GT24 40  58.4  230.7 
Mitsubishi  M501J  41  61.5  327 
General Electric  7FA  38.5  58.5  216 
General Electric  LMS100  44  53.8  103 
Siemens  SGT6-8000H  40  60.75  274 
Siemens  SGT6-2000E  33.9  51.3  112 
Hitachi  H-25  34.8  50.3  32 

Table 1. High Efficiency Gas Turbine Models

Aeroderivative Intercooler Gas Turbines

Intercooler systems work to increase efficiency by allowing for higher pressure ratios in the combustion zone. This is achieved by splitting the compression unit into two sections: the low pressure compressor (LPC) and the high pressure compressor (HPC).  The intake air is first compressed by the LPC, then sent to the intercooler where the pressure is held constant but the temperature is decreased. The air then goes through the HPC and is sent to the combustor. Since the air in the engine cannot exceed a given temperature due to the material used in the turbine, there is traditionally a limit on the pressure ratio, since compressing gas increases its temperature. By cooling the air part way through but not losing any of the pressure gain, the intercooler allows for a second compression to occur, allowing air in the combustor to be within the temperature limits but with a much higher pressure ratio. The higher ratio causes the turbine to generate more power with the same fuel input, increasing the overall efficiency of the turbine.

An example of new innovations to the aeroderivative gas turbine is the 35-65 MW high pressure turbine (HPT) developed by GE [Reference 6]. The LM6000 PG offers a 25 percent simple cycle power increase compared to its predecessor. The applications of these turbines include oil and gas platforms, university cogeneration systems and industrial park combined cycle installations. These turbines are designed to operate on partial power, withstand voltage swings, and be capable of faster dispatching. 

Operational issues/risks

Gas turbines are complex high speed components, with tight dimensional tolerances, operating at very high temperatures. Components are subject to a variety of potential issues.   These include creep, fatigue, erosion and oxidation with impact damage an issue if components fail or following maintenance. Creep may eventually lead to failure but is of most concern because of the dimensional changes it produces in components subject to load and temperature. A major part of maintenance is checking of dimensions and tolerances. Fatigue is or particular concern at areas of stress concentration such as the turbine blade roots.   Therefore, regular inspection and maintenance is a requirement, particularly for gas turbines operating in harsh environments such as offshore applications [Reference 7]. This would include electrical and control systems in addition to the gas turbine itself.

 

 

Opportunities/ business case

The general trend in gas turbine advancement has been towards a combination of higher temperatures and pressures. While such advancements increase the manufacturing cost of the machine, the higher value in terms of greater power output and higher efficiency provides net economic benefits. The industrial gas turbine is a balance between performance and cost that results in the most economic machine for both the user and manufacturer. Applications in the oil and gas industry include pipeline natural gas compression stations in the range of 800 – 1200 psi (5,516-8,274 kPa) compression is required as well as oil pipeline pumping of crude and refined oil. Turbines up to about 50 MW may be either industrial or modified aeroderivative engines while larger units up to about 330 MW are designed for specific purposes. For electric power applications, such as large industrial facilities, simple-cycle gas turbines without heat recovery can provide peaking power in capacity constrained areas, and utilities often place gas turbines in the 5 to 40 MW size range at substations to provide incremental capacity and grid support. A significant number of simple-cycle gas turbine based CHP systems are in operation at a variety of applications including oil recovery, chemicals, paper production, food processing, and universities.

Industry Case Studies

High Efficiency Gas Turbines

The new line of high efficiency gas turbines has been designated the H class, and are currently built by few manufacturers. After an extensive validation process, GE installed their model, the 9H, at Baglan Bay in 2003. This new model increased efficiency by allowing the firing temperatures to increase 200 °F (93.3 °C) higher than previous models, potentially reaching 2,600 °F (1426.7 °C). The plant has been reliably providing up to 530 MW to the UK national grid since then, operating at over 60% efficiency (as part of a combined cycle system) [Reference 8].

Another manufacturer, Siemens, tested their H class model, the SGT5-8000H, at full load in Ingolstadt, Germany in 2008. The gas turbine unit’s efficiency was shown to be 40%, and was part of a combined cycle system reaching a world record 60.75% efficiency [Reference 9]. This plant has been providing power to the German grid since the testing period finished, all at this same efficiency.

The systems that truly showcase all of the new adjustments that can be made to increase efficiency are currently only these H class turbines, which have very large footprints and have specified outputs of 375 MW and higher. However, the technologies behind the H class turbines (advanced materials, improved cooling, etc.) are available on smaller systems. These cases were chosen to illustrate that they are all effective and operational.

Aeroderivative Intercooler Gas Turbines

GE has produced the LMS 100, an extremely high efficiency aeroderivative engine. Operating at up to 44% efficiency at full base load, it generates over 100 MW after a 10 minute start-up. The Groton Generating Station in South Dakota was the first plant to begin using the LMS100, and it has been successfully operational since 2006 [Reference 10]. This technology, while currently available from GE, is the newest and least tested technology identified here. However, due to its successful initial testing and extremely high efficiency for a simple cycle, it is an important alternative to consider.

 

References:

  1. Offshore gas turbines (and major driven equipment) integrity and inspection guidance notes, ESR Technology Lts, for the Health and Safety Executive 2006, Research Report 430.
  2. Davis, L.B., and S.H. Black. "Dry Low NOx Combustion Systems for GE Heavy-Duty Gas Turbines." GE Energy. N.p., n.d. Web. 26 Jul 2013.
  3. Power Generation Technologies. Newnes. P.59. ISBN 9780080480107
  4. Technology Characterization: Gas Turbines, Energy and Environmental Analysis (ICF), December 2008
  5. Cost Report, Cost and Performance Data for Power Generation Technologies, Prepared for the National Renewable Energy Laboratory, Black & Veatch, February 2012.
  6. Aeroderivative Technology: A more efficient use of gas turbine Technology, Wacke, A, General Electric, DRAFT – 2010 – Jan-15.
  7. Wall, Martin, Lee Richard and Frost, Simon. Offshore gas turbines (and major driven equipment) integrity and inspection guidance notes. Research Report, 430, ESR Technology Ltd for the Health and Safety Executive 2006.
  8. "Baglan Bay Power Station, Cardiff, Wales, UK." Power Magazine. July/August. Top Plants (2003): 45-47
  9. Siemens. "H-Class High Performance Siemens Gas Turbine SGT-8000H series: Power-Gen International 2011 - Las Vegas, Nevada." www.energy.Siemens.com. 15 Dec 2011. Web. 26 Jul 2013.
  10. Reale, Michael J., and James K. Prochaska. "New High Efficiency Simple Cycle Gas Turbine - GE's LMS100." . Industrial Application of Gas Turbines Committee, 14 Oct 2005. Web. 29 Jul 2013.