Combustion Turbine-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.

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.

Combustion Turbine

1. Air Intake Section
2. Compression Section
3. Combustion Section
4. Turbine Section
5. Exhaust Section
6. Exhaust Diffuser

Combustion turbines have been used for power generation for decades and range in size from units starting at about 1 MW to over a 100 MW. Units from 1-15 MW are generally referred to as industrial turbines, a term which differentiates them from larger utility grade turbines and smaller microturbines. Combustion turbines have relatively low installation costs, low emissions, high heat recovery, infrequent maintenance requirements, but low electric efficiency. With these traits, combustion turbines are typically used for cogeneration, as peakers, and in combined cycle configurations.