Overview | Technology | Applications | Strengths/Limitations | Related Links

Overview

Combustion turbines (CTs), often called gas turbines, are a mature technology with broad acceptance in industrial and utility power generation applications. CTs are available with electric outputs ranging from a few kW to hundreds of megawatts. The key attributes of CTs include the following:

• Highly efficient when at or near full-load
• Produce very low air emissions compared to reciprocating engines
• Ideal for combined heat and power (CHP) or combined cycle applications
• High energy density (power to weight ratio) and smaller footprint than reciprocating engines
• Proven technology with wide range of currently available products and established service channels

This section explores the key attributes listed above by providing a description of CT technology, the applications for which it is best suited, and its inherent strengths and weaknesses.

Figure 1: Modern Combustion Turbine Plant

Courtesy: Stirling Energy International

Technology

CT technology has been evolving since the 1940's, but the underlying physics has been understood for over a century. All CTs generally follow the same five basic operations:

1. Intake of air
2. Compression of this air through a turbine
3. Heating the compressed air through combustion with fuel
4. Expansion of hot air through a turbine to drive a shaft
5. Hot air exhaust

It follows that the major components of the CT are the intake system, compressor, combustor, power turbine and exhaust. The intake principally consists of an air collection and air filtering system. Since air goes through the entire CT cycle, it must be cleansed of particulates to prevent performance degradation or damage to turbine blades. The air filter can be a basic screen or a more complex self-cleaning device.

The thermodynamic operation of CTs is referred to as the Brayton Cycle, named after George Brayton who first proposed the concept in 1870. The Brayton Cycle is more commonly referred to today as the Simple Cycle. Air is pulled through rotating and fixed blades in the compression turbine, raising both the pressure and temperature of the air. The compressed air is then forced into a combustion chamber where fuel is injected and ignited. Hot gases exiting the combustion chamber expand across rotating and fixed blades in the power turbine.

The hot (1200-2200 degrees Fahrenheit/ 648-1200 degrees Celsius) exhaust gases from the combustor go through nozzles which direct and accelerate the gases towards the blades on the power turbine rotor. The power turbine converts the hot gases into rotational energy via the shaft. In the case of a turbine generator set, the rotating shaft is utilized to drive an electric generator. In designs referred to as cold end drive combustion turbines, a single shaft drives the compressor and generator. In designs referred to as hot end drive, the power turbine section has two turbines on two separate shafts, where the first turbine drives the compressor and the second turbine drives the attached generator. The term "hot end drive" refers to the fact that the generator is located on the exhaust side of the CT.

Finally, the hot gases are directed by the exhaust system to either a stack or heat recovery system. The exhaust system can contain post combustion nitrogen oxides (NOx) or carbon monoxide (CO) control devices with associated emissions detection systems. Heat recovery systems can be used to preheat incoming air or produce steam or hot water. Heat recovery options are discussed later.

Simple Cycle efficiencies for gas turbines available today range from 10 to 43%. CT design varies by manufacturer, but the fundamental principles remain the same and performance varies with factors that alter the mass of air flowing into the turbine. For example, net output power is reduced when there is an increase in atmospheric temperature or site elevation. Efficiency can be increased by decreasing inlet and outlet pressure losses, designing more efficient compressor or power turbines, and achieving higher combustion temperatures.

The compressor is a critical part of the CT. The compressor delivers large volumes of pressurized air to the combustor, but to do so it requires a large amount of energy. Therefore, the compressor must be highly efficient to ensure that output is higher than required energy. For this reason, manufacturers invest heavily to achieve even small increases in compressor efficiency. The typical compressor is a multi-stage axial design with blades in the rotating (rotor) shaft and fixed blades on the non-rotating (stator) case.

The combustion section receives the compressed air, mixes it with fuel, controls combustion, then guides exhaust gases into the power turbine section. Net efficiency increases can be achieved in the combustor through higher firing temperatures. The higher the temperature, the hotter the exhaust gases and greater energy in the exhaust gases that can be converted into work by the power turbine. The downside is that higher combustion temperatures increase emissions of nitrogen oxides (NOx) and place additional stresses on the metallurgy, particularly the combustor exit guide vanes, the first stage blades and nozzles, and the combustor liner.

There have been remarkable gains in materials (both metallurgy and ceramics), thermal barrier coatings (TBC), and cooling design to increase performance. Of these potential options, the use of ceramic components to overcome temperature limitations is particularly interesting as this option allows for very high temperatures while offering potential cost savings in component manufacturing. For decades, the U.S. Department of Energy has funded testing on ceramic liners, nozzles and blades. Commercial introduction of ceramics is not expected in the immediate future, but certain products seem promising. The more traditional methods used to achieve higher combustion temperatures include mixing exotic metals, TBC and advanced designs to increase air cooling. Manufacturers are pushing the operating envelope of CTs with advanced metals in hot section components. The push has been for superalloy single crystals to create first stage blades and vanes, combustors, transitions and shrouds that could exceed operating temperatures of 2,400 F or 1,400 degrees C. The use of advanced TBC to protect metals from high temperatures has shown substantial success. Advanced air cooling designs are the most widely used technique for increasing turbine performance. These designs use air that bypasses the combustion process to provide relatively cool air for the combustion liner and first stage blades. While this cooling air allows for greater output, it does so at the cost of reducing net efficiency.

Figure 2: Typical Industrial Combustion Turbine Cutout

Courtesy of Solar Turbines

In the pursuit of improved performance, manufacturers have explored technologies that do not involve increased firing temperatures and compressor efficiencies. Technologies that can increase performance while lowering emissions and cost include the following:

• Inlet air cooling - Because CT performance is directly proportional to the air mass flow, hot ambient temperatures can dramatically reduce electric output as air expands with heat. Inlet cooling is utilized to reduce the air temperature entering the CT in order to increase output. Inlet air cooling systems consist of inlet cooling coils and spray mist type systems.
• Water injection - In the 1980's and 1990's, water injection was often used to reduce NOx emissions in gas turbines by quenching hot spots within the combustion area where NOx are formed. This method increases power output as well because the water injected decreased temperatures, increasing mass flow. Unfortunately, efficiency suffers because water energy is required to convert water to steam.
• Steam Injected Gas Turbine (STIG) - This technique is similar in concept to water injection, but instead of using water at ambient temperatures STIG uses steam generated from exhaust gases. The benefit of STIG compared to water injection is that there is a smaller reduction in net efficiency. This was a popular configuration before new emissions regulations forced the use of dry (no water) emission control technologies to achieve tighter NOx and CO emissions limits. .

A cycle is a general term used to describe the completion of one process. One of the advantages of CTs is that with the high temperature waste heat, there are a number of additional processes that can occur, making CTs capable of being a multiple cycle technology. Combining cycles add tremendous value to combustion turbine technology. The most common cycles utilized in commercial installations today include the following:

• Simple Cycle - The simple cycle is the typical method of all combustion turbine operation. Traditionally, combustion turbines have been used by utilities and some businesses in simple cycle mode to provide peaking power. The brief construction lead time, low capital cost, and rapid startup capability of CTs make it the clear preference for backup and peaking power applications with output requirements ranging from several thousand kilowatts and above.
• Recuperated Cycle - Recuperation is a cycle where the exhaust gas is utilized to preheat the post compression air to reduce the fuel needed to achieve combustion. This increases the efficiency of the basic simple cycle by 1-10%, enabling overall power generation efficiencies as high as 40% in some advanced gas turbines.
• Cogeneration or Compined Heat and Power (CHP) - CHP is a leading configuration for supplying electricity as well as thermal energy in the form of process steam or chilling for industrial and commercial applications. CHP can achieve net system efficiencies of over 80%, the highest thermal efficiencies of all commercially available technologies today.
• Combined Cycle - The hot exhaust air of a simple cycle CT is guided into a heat recovery steam generator (HRSG) to produce steam. This steam is utilized to drive a steam turbine generator. Combined cycle increases the efficiency from 28-42%, enabling overall power generation efficiencies as high as 60%. This is the standard configuration for new central power station designs.

Figure 3 Typical cogeneration cycle

Courtesy Solar Turbines

There are other advanced cycles currently under development, such as hybrid CTs and fuel cells. However, these cycles are not expected to be commercially available in the next few years.

Applications

As with most power generation technologies, variations in CT design create unique advantages for specific applications. For low operating hours, capital cost is probably a more important consideration than overall efficiency. As the number of expected operating hours increases, efficiency becomes more significant. For example, a facility that operates 24 hours per day with a thermal load would select an industrial or Aero-derivative (describe below) gas turbine installed in a cogeneration cycle. For peaking applications, aero-derivatives are often used for applications with output requirements of 100 MW or less. The basic CT designs are described below:

• Microturbines - The smallest of the family of gas turbines, with commercially available units ranging in output from 25 to 500 kW. Microturbines are utilized in applications ranging from cars to homes to small commercial buildings. Due to their low simple cycle efficiencies, most of these units are manufactured in a recuperated cycle. Cogeneration cycles are also available to further boost overall system efficiency.
• Aero-derivative - As the name implies, these units are derived from jet propulsion engines utilized on commercial aircraft. Aero-derivative units are available with power output ranging from 300-52,000 kW. Typical applications include peaking and cogeneration projects. Because these turbines were originally optimized for aviation, these are designed to be light weight, reliable, efficient, and easily packaged. Aero-derivatives tend to have shorter intervals between overhauls than industrial CTs.
• Industrial - These units do not have the design constraints of aero-derived combustion turbines as they are designed from the bottom up for high efficiency and high reliability, with long periods of continuous operation between overhauls. Industrial turbines have a fundamentally different combustor design than aero-derivative machines. Power output for commercially available units range from a few hundred kW to over 50,000 kW. Industrial turbines are typically installed in CHP applications, but can be found in peaking capacities as well.
• Utility - These heavy duty frame type large CTs are similar to industrial machines, but operate at higher firing temperatures with higher efficiencies and ultimately lower capital cost per kW. Utility CTs range in output from 50,000 to 250,000 kW per unit.

All of these designs can be installed in simple cycle, CHP, or combined cycle. Recuperation cycles are cost effective on a few specific machines. Microturbines provide a nice example for recuperated gas turbine designs. Microturbines were developed from the turbochargers found in auxiliary power units for airplanes and auto-derived applications. Turbochargers are basically small turbines that generate mechanical power from the exhaust of reciprocating engines (see the Reciprocating Engine section for more details on turbocharger operation and functionality). Over the years, automobile manufacturers have dedicated substantial funds to turbocharger research and have developed breakthroughs in efficiency and design. The microturbines manufactured today are available with outputs ranging from 26 kW to over 500kW, but all share key design characteristics. These include a high-speed shaft directly driving a high speed generator. Most designs are recuperated, have excellent thermal efficiencies, modest power output efficiencies, and demonstrated high reliabilities. Microturbine prices are generally higher than reciprocating engines, but are expected to fall as production volumes increase.

Figure 4 Typical Microturbine Installation

Courtesy of Capstone

CTs are a major source of peaking power in the U.S. and Europe, with typical run times of up to a few hundred hours per year. Industrial and institutional customers use peaking plants to defray high time of day electricity charges or where utility tariffs pass through real-time pricing. Peaking CT installations vary by size, location and accessories, with capital costs ranging from $275 to $600 per kW. This wide range in capital costs is caused not only by variations in manufacturer's prices, but also by the multitude of low cost and capital intensive options ranging from fuel gas compressors to inlet air chillers in areas where the equipment is designated for summer peak load operation. Emergency backup power is not a common application for CTs. Compared to CTs, reciprocating engines have advantages of lower cost and faster start times, which is essential for backup purposes.

Cogeneration plants consist of a CT with exhaust gases directed into a heat recovery steam generator (HRSG). As the name implies, HRSGs convert thermal energy into steam, which is then used to generate additional power in a steam turbine generator. Most HRSGs have an option for duct burners that allow for supplemental firing of the CT exhaust gas to increase the steam or hot water output. The high air flow-through of CTs provide a plentiful supply of oxygen for supplemental firing to provide substantial amounts of additional steam or hot water. There are several different types of steam turbines, but the two most often used in CHP applications are backpressure and condensing designs. Backpressure systems function by using high pressure steam to drive a turbine, leaving lower pressure steam or hot water that can be used for other processes. Condensing systems also use high-pressure steam to drive a turbine, but then use a condenser or series of condensers to recover nearly all of the heat energy from the steam, leaving only low or zero pressure discharge. Because of the choice of steam turbines alone, the capital cost of CHP facilities varies widely, with typical installed costs ranging from $700 to $1200 per KW.

Combined cycle plants are the most common central station plant being installed worldwide due to their high efficiency, low cost, and rapid lead time for installation. Utility style, heavy-duty frame turbines are ideal for combined cycle plants. Although the cost of adding a steam turbine doubles the "per kW" capital cost of a plant, the operating cost is lower than any other technology on the market if the plant is operated for 8,000 hours or more per year. There are smaller combined cycle systems that are commercially available today, including GE's LM6000 and steam turbine train, and Solar Turbines' combined CT and ST system.

Strengths and Limitations

CTs clearly dominate in combined cycle applications due to high cycle efficiency and economies of scale compared to wind, solar, reciprocating engines and other DE technologies. For cogeneration applications requiring steam, CTs are also strong competitors because net thermal efficiencies in excess of 80% are possible. Other DE technologies will begin to compete with simple or recuperated cycle turbine systems for applications below 10,000 kW. Below 1,000 kW, reciprocating engines become more competitive. From 1,000 to 10,000 kW, there is a transition from reciprocating engines to CTs in terms of economic competitiveness.

CT performance is adversely impacted by altitude and temperature. Applications must be viewed with these site-specific and temperature-specific factors in mind. If maximum generator output is required during hot summer months, then inlet cooling adds to the overall costs. Other issues to consider include the following:

• As higher firing temperatures are sought to increase performance, higher combustion pressures and fuel gas compression is required.
• As load drops on the CT, the compressor energy use becomes a larger percentage of cycle energy use resulting in poor partial-load efficiencies.

Related Links

These links provide you supplemental information on combustion turbines. The vendor links will provide direct access to the Websites for manufacturers and packagers of combustion turbines. Please refer to these vendor links to obtain specifications.