This article is a presentation of the different technologies now available to produce energies in co-generation way using gas turbines. It gives a description of the different gas turbine technologies, their main applications, their typical efficiencies, costs and operating ranges, advantages and drawbacks and applications.
Gas Turbine Technical description
The gas turbine technology was introduced to power generation industry in the late 1940s. By the early 1990s, gas turbines had become a significant part of the power generation worldwide. In fewer than 50 years, what was originally a jet engine technology has been changed into an essential high-technology solution to many power generation needs. At present time, the installed base of gas turbines in the world is higher than 100 000 MW. Within few years, the gas turbine market should reach 20 000 MW per year.
Gas turbines are made of three main parts:
- one air compressor,
- one combustion chamber,
- one turbine in which mechanical power is produced with the reduction in pressure of the fumes from combustion.
Both the air compressor and turbine are made of several stages (see figure 1).
Gas turbine packages are made of (see figure 2):
- one gas turbine,
- one gearbox,
- one generator,
- one system for air entrance,
- one system for air exhaust,
- one enclosure.
The Brayton cycle for gas turbine represents the gas turbine power cycle. The four steps of this cycle are (see figure 3):
- (1-2) Isentropic compression: ambient pressure air is drawn into the compressor where it is compressed to a higher pressure,
- (2-3) Reversible constant pressure heat addition: fuel is added to the compressed air and the mixture is burnt in a combustion chamber,
- (3-4) Isentropic expansion: the resulting fumes enter the turbine and expand to produce energy,
- (4-1) Reversible constant pressure heat rejection: the fumes are rejected at a pressure near ambient pressure.
Brayton cycle for gas turbine
About 2/3 of the rotational energy produced is required to drive the air compressor in order to maintain sufficient air flow through the turbine to support combustion. The remaining energy produced is converted to electrical energy.
Fuel is injected by multiple fuel injectors for the bigger ones or in one fuel injector for smaller gas turbines. Combustion of the fuel creates high temperatures gases (near 1500 °C), which then expand and induce mechanical rotation on the shaft of the turbine part.
Generally, the turbine shaft rotates at a speed about 15000 rpm. The generator is driven through a reduction gearbox that brings the speed of the turbine shaft down to near 1800 rpm.
Some gas turbines have water or steam injection to control NOx emissions. Water or steam (this steam can be generated by a boiler) is injected into the combustion chamber nozzle steam injection or casing steam injection to lower the combustion temperature and consequently the NOx production. Also this injection permits to boost electrical power output and efficiency.
Typical efficiencies, costs and operating ranges
In order to get a global idea of the typical efficiencies, costs and operating ranges, please see the table below:
| Electric power | |
| Thermal power
(with heat recovery boiler) |
|
| Ratio electric/ thermal power
(with heat recovery boiler) |
|
| Thermal quality
(with heat recovery boiler) |
|
| Pollutant emissions | |
| Disponibility | |
| Cogeneration efficiency | |
| Maintenance cost | |
| Global cost |
We have to underline that gas turbine technology has benefited from significant improvements and so significant increase in power, efficiency and decrease in price (refer to figures 4, 5 and 6):
This is one example of a complete energy balance for the Taurus 60 gas turbine (SOLAR):
Fluids
The following figure presents the different fuels available for gas turbines with the different combustion chamber (fuel system) design associated:
Natural gas is the fuel of choice for gas turbines that have high annual hours of operation (over 4000 hours/year) because of its relatively low emissions and low cost compared to alternative fuels.
Also other gaseous down to heating values have been demonstrated as suitable for gas turbines.
Distillate oil is also used frequently. Liquid gases such as propane and butane are equally suitable.
Technologies
Gas turbines are divided into two groups on the basis of differences in design philosophy although there is now some convergence in design. They are:
-
the aeroderivative gas turbines:
- The aeroderivative gas turbine is derived from aircraft engines and has a low specific weight, a streamlined shape, low fuel consumption and high reliability levels.Aeroderivative gas turbines incur a relatively high investment cost ($ per kW). They require high quality fuel and may experience a fall in output and efficiency after a long period of operation.
-
the industrial or heavy-duty gas turbines:
- The industrial gas turbine, also referred to as the heavy duty or heavy frame gas turbine, is a robust unit constructed for stationary duty and continuous operation. It has a lower efficiency than the aeroderivative type, but usually maintains its performance over a longer period of operation. Maintenance can easily be carried out on site. Maintenance costs are low. The industrial gas turbine usually has a lower specific investment cost than its aeroderivative counterpart. Furthermore, it can use lower quality fuel.
Technologies
Gas turbines are divided into two groups on the basis of differences in design philosophy although there is now some convergence in design. They are:
- the aeroderivative gas turbines:
- The aeroderivative gas turbine is derived from aircraft engines and has a low specific weight, a streamlined shape, low fuel consumption and high reliability levels.Aeroderivative gas turbines incur a relatively high investment cost ($ per kW). They require high quality fuel and may experience a fall in output and efficiency after a long period of operation.
- the industrial or heavy-duty gas turbines:
- The industrial gas turbine, also referred to as the heavy duty or heavy frame gas turbine, is a robust unit constructed for stationary duty and continuous operation. It has a lower efficiency than the aeroderivative type, but usually maintains its performance over a longer period of operation. Maintenance can easily be carried out on site. Maintenance costs are low. The industrial gas turbine usually has a lower specific investment cost than its aeroderivative counterpart. Furthermore, it can use lower quality fuel.
Gas Turbine Typical applications
The gas turbine is used in a wide range of cogeneration applications:
- Industrial applications:
- Oil and gas industries,
- Pulp and paper industries,
- Food and brewery industries,
- Chemicals and pharmaceutical industries,
- Textile industries,
- Motor industries,
- Glass and ceramics industries,
- Plastic and tyre industries,….
The industrial applications segment is comprised of small and medium size turbines generally ranging from 5 to 40 MW.
- Municipal applications:
- Waste incineration plant,
- District heating,
- Hospitals,
- Schools,
- Airports, ….
The municipal applications segment is comprised of smaller size turbines generally ranging from 2 to 10 MW.
Advantages and drawbacks
Gas Turbine Advantages
Cogeneration based on gas turbines has the main following advantages:
- High efficiency for the bigger aeroderivative gas turbines and good efficiency for the bigger heavy-duty gas turbines,
- Reliability and disponibility,
- Low impact on environment compared to engines,
- Decentralised solution for energy production: gas turbines can produce an important quantity of electricity at the end user site,
- Large range of power available on the market,
- Low capital costs (normally 50-70% of steam-turbine plants),
- Low operating and maintenance costs (compared with engines),
These advantages mean gas turbine are the most popular selection for cogeneration over about 3 MW.
Gas Turbine Drawbacks
- Gas turbines need gas feeding at high pressure. For a same level of power aeroderivative gas turbines require higher gas pressure than heavy-duty gas turbine (maxi about 40/50 bars),
- Heavy-duty gas turbines do not allow a very flexible way of operation (with frequent starts and stops).
Future developments and time to commercialisation
Gas turbine manufacturers need to reduce life-cycle costs through improved thermal efficiency, increased output power, and enhanced component durability. At the same time, emissions of such environmental pollutants as NOx and CO must be lowered. Consequently we can expect many improvements in the next few years.
The American government has launched a big program (Vision 21 EnergyPlex) to favour the energy manufacturers in general and in particular to increase gas turbine efficiency and to decrease pollutant emissions of gas turbine. The project is called “Advance Turbine System” ATS.
NOx reductions
Gas turbines currently utilise diffusion flame combustion chamber that operate at about 1800°C. Without emissions controls or cleanup processes, combustion at these temperatures results in NOx emissions of between 75 and 200 ppm.
One current approach for reducing NOx is to reduce the combustion chamber temperature:
- by using wet controls, which involve injecting water or steam into the combustion chamber. NOx emission levels can be reduced to about 42 ppm with water and about 25 ppm with steam injection,
- by using a lean pre-mix or dry-low-NOx (DLN) technology. DLN is a combustion process in which natural gas and air are premixed prior to entering the combustion chamber, resulting in a low fuel to air ratio. Turbine manufacturers utilising this approach have achieved emission levels of approximately 25 ppm, and are undertaking to achieve emission levels in the 10 to 15 ppm ranges in the next product generation.
Maintaining an operating temperature in the combustion chamber at 1500°C or below virtually eliminates production of NOx. But wet controls and DLN technologies are not able to operate at this temperature level, and therefore these methods require post-combustion process cleanup to achieve lower emission levels:
- the most common post-combustion cleanup process is selective catalytic reduction (SCR). SCR reduces NOx emissions by approximately 80%. Capital and operating costs of this approach add significantly to the overall cost of producing power. In addition, the gas turbine operator must store and handle large quantities of ammonia. Such catalytic combustion technology has been under development for several years, and the first full-scale test of the technology took place in 1992.
Increase in electrical efficiency
Solar proposes a new gas turbine, the Mercury 50, with a different thermodynamic cycle: a cycle with a heat exchanger to heat up air before its entrance into the combustion chamber by the heat contained into the fumes exhaust. This cycle has a better electrical efficiency (about 40% for 4.2 MWe) but of course produces heat at a lower temperature (about 350°C) (see figure 9).
This new technology uses ceramic for the hot section and so need less cooling air. Insertion of ceramic parts for the metallic components in the gas turbine hot section contributes to achieve operation at increased turbine rotor inlet temperatures while reducing component cooling.
General Electric is going to propose a new combined cycle that will reach 60% of electrical efficiency (compare to actual maximum 53%). The declared objectives are to reach global efficiency for combined cycle about 70% for natural gas!
Gas Turbine Manufacturers
- AlliedSignal Power Systems
- Allison Engine Company
- Ansaldo Energia Ss.p.A
- ABB Power Generation
- Cooper Rolls-Coberra
- Centrax Gas Turbines
- Dresser Rand
- European Gas turbines
- European Gas Turbines LTD
- European Gas Turbines S.A.
- European Gas Turbines GmbH
- Fiat Avio
- General Electric Power Systems
- Hitachi
- Kvaerner Energy a.s.
- MAN Dezentrale Energiesysteme GmbH
- Mitsubishi
- Nuovo Pignone
- Pratt and Whitney
- Rolls Royce7
- Siemens AG, Power Generation Group
- Solar Turbines Incorporated
- Stewart and Stevenson
- Sulzer
- Thomassen Stewart and Stevenson International b.v.
- Tuma Turbomach SA
- Turbomeca Industrial
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