Combustion Turbine

 

Overview

Operations

Heat Recovery

Efficiency

Environmental

Manufacturers

 

1. Air Intake Section2. Compression Section3. Combustion Section4. Turbine Section5. Exhaust Section6. Exhaust Diffuser

Source:  Siemens web site, 8/01

   

Overview

For an Overview of the Combustion Turbine, see Combustion Turbine    

Operations

A gas turbine  has a compressor to draw in and compress air; a combustor (or burner) to add fuel to heat the compressed air; and a turbine to extract power from the hot air flow. The gas turbine is an internal combustion (IC) engine employing a continuous combustion process.

This differs from the intermittent combustion occurring in diesel and automotive IC engines.  About 2/3rds of the shaft power produced by the turbine is used to run the compressor, leaving about 1/3rd available to turn a genset to produce electrical power.  

Gas Turbine Cycles

A cycle describes what happens to air as it passes into, through, and out of the gas turbine. The cycle usually describes the relationship between the space occupied by the air in the system (called volume, V) and the pressure (P) it is under. The Brayton cycle (1876), shown in graphic form as a pressure-volume diagram, is a representation of the properties of a fixed amount of air as it passes through a gas turbine in operation. These same points are also shown in the engine schematic above.

Air is compressed from point 1 to point 2. This increases the pressure as the volume of space occupied by the air is reduced.

The air is then heated at constant pressure from 2 to 3. This heat is added by injecting fuel into the combustor and igniting it on a continuous basis.

The hot compressed air at point 3 is then allowed to expand (from point 3 to 4) reducing the pressure and temperature and increasing its volume. In the engine, this represents flow through the turbine to point 3' and then flow through the power turbine to point 4 to turn a shaft or a ship’s propeller. The Brayton cycle is completed by a process in which the volume of the air is decreased (temperature decrease) as heat is absorbed into the atmosphere.

A gas turbine that is configured and operated to closely follow the Brayton cycle is called a simple cycle gas turbine. Most aircraft gas turbines operate in a simple configuration since attention must be paid to engine weight and frontal area. However, in land or marine applications, additional equipment can be added to the simple cycle gas turbine, leading to increases in efficiency and/or the output of a unit. Three such modifications are regeneration, intercooling and reheating.

 

Regeneration involves the installation of a heat exchanger (recuperator) through which the turbine exhaust gases pass. The compressed air is then heated in the exhaust gas heat exchanger, before the flow enters the combustor.

 

If the regenerator is well designed (i.e., the heat exchanger effectiveness is high and the pressure drops are small) the efficiency will be increased over the simple cycle value. However, the relatively high cost of such a regenerator must also be taken into account. Regenerators are being used in the gas turbine engines of the M1 Abrams main battle tank of Desert Storm fame, and in experimental gas turbine automobiles. Regenerated gas turbines increase efficiency 5-6% and are even more effective in improved part-load applications.

 

Intercooling also involves the use of a heat exchanger. An intercooler is a heat exchanger that cools compressor gas during the compression process. For instance, if the compressor consists of a high and a low pressure unit, the intercooler could be mounted between them to cool the flow and decrease the work necessary for compression in the high pressure compressor. The cooling fluid could be atmospheric air or water (e.g., sea water in the case of a marine gas turbine). It can be shown that the output of a gas turbine is increased with a well-designed intercooler.

 

Reheating occurs in the turbine and is a way to increase turbine work without changing compressor work or melting the materials from which the turbine is constructed. If a gas turbine has a high pressure and a low pressure turbine at the back end of the machine, a reheater (usually another combustor) can be used to "reheat" the flow between the two turbines. This can increase efficiency

by 1-3%. Reheat in a jet engine is accomplished by adding an afterburner at the turbine exhaust, thereby increasing thrust, at the expense of a greatly increased fuel consumption rate.

Source: Text and graphics in this section has been extracted from the International Gas Turbine Institute web site 5/02. For more information about IGTI see

www.asme.org/igti/index.html

   

 

 

Heat Recovery

Combustion turbines generate a large volume of very hot air. The exhaust is also high in oxygen content as compared to other combustion exhaust streams, as only a small amount of oxygen is required by the combustor relative the total volume available.   Depending on how much thermal energy is required for the application, the turbine exhaust may be supplemented by a duct burner.   A duct burner is a direct fired gas burner located in the turbine exhaust stream. It has a very high efficiency due to the high inlet air temperature, and is used to boost the total available thermal energy. The turbine exhaust boosted by the duct

burner is directed into the waste heat boiler, called the Heat Recovery Steam Generator, or HRSG commonly pronounced as 'HerSig'.   Turbine exhaust can also be ducted directly into hot air processes, such as kilns and material drying systems. This is the least costly first cost, as there is no boiler or steam system to purchase. Turbine exhaust can also be ducted directly into absorption chillers for large cooling loads.   The system will also include a diverter for times when waste heat is not needed. The diverter vents the turbine exhaust to atmosphere; this substantially reduces the system efficiency, as only the electric energy is being used. Single or Simple Cycle electric plants (typical of peaker plants) dump all of their turbine exhaust, as they have no thermal requirements. These plants generally use turbines with recuperators to maximize their electrical efficiency.   The higher the electrical efficiency of the turbine, the lower the available thermal energy in the exhaust. Newer turbines with recuperators, and larger sized turbines, tend to have higher efficiencies.

For more information about the application of Heat Recovery, see the Applications Guide , Industrial Market Section.  

Efficiency

Turbine efficiency and total capacity is highly variable with the inlet air temperature (ambient air temperature if no inlet air cooling is utilized) and local altitude/atmospheric conditions. In a northern climate, turbine capacity can fluctuate as must as 20% from summer (the lowest) to winter (the highest output time), due to cold, denser air.     Single or Simple Cycle turbines have an efficiency of 25% (smaller, unrecuperated) to 40% (larger units with recuperators), when comparing fuel energy in, to electric energy out. The standard measurement is called the Heat Rate, or the BTUs input to make 1 kWh of electric output.   To estimate efficiency based on the Heat Rate, use the formula:                 BTUs/kWh (Absolute) / BTUs/kWh (Turbine) = 3,413/ Heat Rate   Typical Heat Rates are in the 11,000 BTU range, so typical efficiency would be 3,413/11,000 = 31% electrical efficiency.      To estimate total efficiency, add in the BTUs recoverable in the exhaust stream at the temperature and flow conditions of the application. Typical combined thermal and electric efficiency of combustion turbine plants is in the 60% range;

higher if lower temperature thermal energy can be used, such as direct ducting of exhaust into a process. A duct burner can increase over-all system efficiency, as they operate at near 100% efficiency due to the high temperature of their inlet air supply.   For specific turbine performance data, see the manufacturer's web sites for their product brochures, and see the selected PDF files attached to this program, located in the Manufacturer's section below.  

Source: Graphics Solar web site 5/02 

 

Environmental

The primary emission concern of natural gas fired turbines is NOx, and in some cases CO and CO2. Because turbine combustors operate at a very high temperature, uncontrolled turbines produce high levels of NOx. A variety of controls have been developed in an attempt to lower NOx to the 9 ppm required by the strictest regulations.   The most common control methods for NOx is water injection to reduce combustion temperature, and Selective Catalytic Reduction (SCR) an after-

treatment to remove NOx. Another system developed by Catalytica and Kawasaki is called Xonon, which is a unique combustor that operates below the NOx formation temperature. Xonon is currently offered as an option on certain Kawasaki and other small industrial power turbines.   For more information about combustion turbine emissions, see  Environmental Overview       A gas turbine cycle consists of (1) compression of a gas (typically air), (2) addition of heat

energy into the compressed gas by either directly firing or combusting the fuel in the

compressed air or transferring the heat through a heat exchanger into the compressed

gas followed by (3) expansion of the hot pressurized gases in a turbine to produce useful

work. The work required by the compression is supplied by the turbine with the remaining

being available for useful work. Typically, the work required by the compressor is as much

as half of the power developed by the turbine. Clean fuels such as natural gas, distillate

derived from the refining of petroleum and gas derived from gasification may be used for

direct firing in the gas turbine. In some instances heavy oils derived from petroleum that

are a lot "dirtier" because of their metal and sulfur content are also fired directly in the

gas turbine. To minimize the adverse effects of the metals and sulfur, the gas turbine is

operated at lower turbine inlet temperatures resulting in reduced output and efficiency.

Dirty fuels may also be used with the gas turbine by combusting the fuel externally and

transferring the heat into the pressurized working fluid of the gas turbine.

The useful work developed by the turbine may be used directly as mechanical energy or

may be converted into electricity by turning a generator. An aircraft jet engine is a gas

turbine except that the useful work is produced as thrust from the exhaust of the turbine.

Land based gas turbines are of two types: (1) heavy frame engines and (2) aeroderivative

engines. Heavy frame engines are characterized by lower compression ratios (typically

below 15) and tend to be physically large whereas the aeroderivative engines as implied

by the name are derived from jet engines operate at very high compression ratios

(typically in excess of 30) and tend to be very compact.

Air from the compressor is used for cooling the turbine in order to maintain the metal

temperatures within their design limits of 1500 to 1700 deg F or 820 to 930 deg C

depending on the alloys utilized, while the gas flowing through the turbine may be as high

as 2500 deg F or 1370 deg C. The requirement for cooling the turbine limits the ultimate

thermal efficiency of the gas turbine and technologies are being developed in the areas of

materials including ceramics and enhanced cooling effectiveness in order to minimize the

cooling air requirement. With more advanced materials and cooling technologies,

increases in turbine inlet temperature are possible in order to increase the thermal

efficiency of the cycle.

The optimum compression ratio from a thermal efficiency standpoint increases as the

turbine inlet temperature is increased. Some advanced heavy frame gas turbines with

high turbine inlet temperatures, to be introduced in the near future, will have

compression ratios as high as 25.

Other approaches to increasing the efficiency are to incorporate reheating during the

expansion step of the cycle, intercooling the compressor while recovering the intercooler

heat for humidifcation of the compressed air and preheating the humid air in the turbine

exhaust before admitting it into the combustor (HAT Cycle).

The principal pollutants associated with gas turbines are oxides of nitrogen (NOx). Control

strategies for NOx include water or steam injection and premixed burners as well as the

post combustion control by installing a catalytic reduction unit which consists of reacting

the NO with injected ammonia in the presence of a catalyst. Control strategies

incorporated within the combustion process often result in reduced combustion efficiency

and thus increased emissions of carbon monoxide and unburned hydrocarbons. In order

to increase the efficiency while reducing the pollutants formed within the combustor,

computer based models and laser diagnostic techniques are being applied.

Within a typically can combustor of a gas turbine, swirling action imparted to the

incoming air imparts a centrifugal force and creates a low pressure in a region of the

primary zone causing recirculation of the hot combustion products to the primary

combustion zone which imparts stability to the combustion process (the hot combustion

products contain free radicals which serve to initiate the combustion much like a spark

plug in an auto-engine). The CO oxidizes to CO2 in the secondary zone where additional

air is added. The hydrocarbons in the fuel form CO in the "dome" which is the region

around which the circulation occurs in the primary zone and mixing. Pollutants are also

produced in the dome. Fuel rich conditions maintained in the dome minimize NOx

pollutant formation, however, this results in loss of stability of the combustion process,

especially considering different modes of operation including startup, idling and full load.

Variable geometry combustors under development may solve some of these problems.

The exhaust heat from a land based gas turbine may be recovered to generate steam to

produce additional electric power in a steam turbine resulting in a combined cycle or to

provide process or district heating resulting in a cogeneration plant. With current state of

the art gas turbine technology (in 1997), combined cycles with efficiencies in the

neighborhood of 55% on a fuel LHV basis can be achieved and are projected to increase

to 60% within next couple of years.

When gas turbines were first applied in the electric power generation industry some 20

years ago, the majority of the power generated by the gas turbines was for peaking load

service. Since then however, with increases in efficiency and reliability, the gas turbine is

being utilized more and more in base load generation.

H R Simmons SwRI 41Gas TurbineTechnology CenterSouthwest Research Institute