Industrial Gas Turbines

"Illustration 3.2.2-1": The introduction of technologies towards the minimization of emission values (Lit.3.2.2-1) in the exhaust, especially of nitrogen oxides (NOx), influences many other areas. These are represented in the left picture. The black area indicates the author’s evaluation. If the features lie in the gray „problem zone“, disadvantages in these regions, in contrast to engines without equipment facilitating pollutant minimization, are to be expected, depending on the procedure employed. The diagram below shows the typical emission behavior (Lit.3.2.2- 9) of a not specially equipped low pollutant optimized combustor. A performance increase to full load, through a raising of flame temperatures in the combustor, leads to higher nitrogen oxide formation. Simultaneously follows an increased oxidation of the CO and, with it, its decrease. During partial load, the opposite behavior is to be expected. The formation of the important pollutants works in an opposite way ( "Ill. 3.2.1-3") and technical compromises must be searched for, in order to reach an optimum. There are many suitable techniques for this purpose (Lit. 3.2-5 and Lit. 3.2-8), which lessen the NOx formation in the combustor. They are called primary measures, as opposed to measures used in the treatment of the exhaust gas.

Essentially its a question of: Increased primary air supplement: more air is added in the primary zone ( "Ill. 3.2.1-1") and so the combustion temperature is diminished to a sufficiently low niveau. The raising of the CO portion must be avoided, through oxidation in the hot intermediate zone with a longer dwell time.

Injecting water, respectively, steam ( "Ill. 3.2.2-3"): a very effective NOx reduction can be reached in the primary zone through injecting water or steam into it. The temperature is thus sunk to a suitable niveau. Problems can originate through combustion vibration (flame instabilities, see "Ill. 3.2.2-5") and increased CO emission.

Pre mix combustors ( "Ill. 3.2.1-5.1" and "Ill. 3.2.2-2"): fuel and combustion air are well mixed in the primary zone (DLN= Dry Low NOx process). The high air excess takes care of a low temperature niveau. A pronounced mixing zone for the hot gas is in the combustion chamber not present. This technique must also control the tendency to combustion vibrations.

"Illustration 3.2.2-2": The thermal NOx formation is especially dependent on the flame temperature in the combustor and the dwell time of the combustion in the area of higher temperatures (above ca. 1500 °C, "Ill. 3.2.1-3"). The above picture shows a DLE (Dry Low Emissions) combustor for a derivate-industry gas turbine in the performance area of 50 MW. (Lit. 3.2-13). Along with this arrangement a „dry“ principle that is based on the pre-mixing of combustion air with gas towards the reduction of nitrogen oxide is currently desired, as opposed to the bringing in of water or rather steam. This ‘dry’ principle is based on a pre mixing ( "Ill. 3.2.1-5.1") of the combustion air with the fuel (gas). Both the emissions on NOx and CO, can therefore be maintained at their lowest level (compare "Ill. 3.2.2-1"). The low NOx values according to the manufacturer’s instructions are attained, since, through the equal mixing of fuel and air, the flame temperature holds within the permitted frame of low temperatures. Through a relatively long dwell time in the especially big volume annular combustor and missing film cooling in the reaction zone (hot combustor inner walls), the CO amount is decreased through better combustion. This is only possible with suitable , newly developed combustor inner wall structures „A“ ( "Ill. 3.2.1-4"), that can be supported by pressure absorbing casings „B“ outside and an inner casing „C“. Here, tile designs with ceramic coatings are appropriate ( "Ill. 3.2.1-4"). The combustor dome „E“ also shows ceramic thermal barrier coatings on the flame side and consists of many (here three) concentrically ring arranged pre-mixers that are separated from each other through the ring walls „F“. These ring zones behave like separate regions during low preformance. The pre mixer in the air flow, in front of the dome, consists of hollow axial blades „D“ through which the combustion gas in the swirled air flow is brought in. This swirling of the flow supports the flame stability, in order to control the typical instabilities at low flame temperatures ( "Ill. 3.2.2-5").

"Illustration 3.2.2-3": The injection of water and steam can markedly minimze the NOx production during the combustion process (diagram) and improve the effectivity of the cooling air flow (Lit 3.2-11 and Lit. 3.2-12). The injection of liquid water into the combustion system can be carried out as additive to the fuel, injection into the compressor and/or into the combustion chamber.

The black circular surfaces indicate the steam ratio of the overall mass flow rate (ca. 7%) entering at this position (see adjacent percentage details). Thereby the performance data of the engine (up to about 50 % increase) and the efficiency (improvement up to 20%) are positive influenced. Despite those benefits, this process is not unproblematic ( "Ill. 3.2.2-4").

Natural gas, in combination with injection steam, reveals itself as particularly environment friendly and a convenient compromise. The addition of water in the combustion system can occur as a liquid, (adding to the liquid fuels and / or injections of water in the compressor and / or combustor) or as a steam injection (see nearby picture acc. to Lit 3.2-11. This happens in the combustion chamber with the help of special fuel nozzles and in the area of the compressor outlet casings. The injected steam must show particular purity, solid particles may not exceed a particular size (e.g. ca. 20×10- ³ mm) and overall impurities of natrium and potassium (from the fuel, air and steam) should lie below 200×10-9 because of the damaging effect to the hot parts (e.g. sulfidation, "Ill. 3.4-3").

"Illustration 3.2.2-4": Where there is light, there is shade: the injection of steam or water leads ( "Ill. 3.2.2-3"), indeed, to a clear performance increase, but also influences the components of the gas turbine. General problems related to injecting water and steam:

• The dynamic behavior of the combustor is problematic. It comes to flame instabilities and gas pulsation ( "Ill. 3.2.2-5"). These pulsations can damage the components in many ways:
  • Increased rubbing wear (fretting) at the connections of the hot parts like flame tubes of the combustor ( "Ill. 3.2.3-1" and "Ill. 3.2.3-2"), turbine guide vanes and casings.
  • Cracks and fractures through vibration fatigue on gas duct parts and blades. In combustors with a tile lining ( "Ill. 3.2.1-4") , a detachment of tiles can occur.
  • The rotor can be excited by vibration.
• The initiation of undesired, unburned hydrocarbon (UHC) and of CO ( "Ill. 3.2.1-3" and "Ill. 3.2.2-1") is made convenient. This behavior limits the water addition.
• If the injected steam , respectively, the water is contaminated and reveals deposit forming matters (see requirements in "Ill. 3.2.2-3"), the hot parts can be damaged.
  Deposits can block the outlets of the cooling air passages, furthering the overheating of the cooled hot parts ( "Ill. 3.3-2").
  The deposit constituents can react, e.g., with the ceramic of the thermal barrier coating and/or cover the segmentation cracks that are required to compensate heat expansion ( "Ill. 3.2.3-5" and "Ill. 3.2.3-7").
  Should impurities contain sulfur, (e.g., gypsum in hard water), there is the danger of sulfidation ( "Ill. 3.4-2" and "Ill. 3.4-3"), especially in the low pressure, respectively, the power turbine.
• If injected water is not completely steamed or if water impurities form abrasive particles, thermal barrier coatings can erode at an accelerated pace ( "Ill. 3.2.3-8"). Erosion can abrade the protective, thin diffusion coatings or oxide layers speeding up oxidation noticeably
• Should the pressure in the gas flow increase, the cooling air outlet is influenced. Simultaneously, the heat transition is improved, so that at least locally an increased component temperature is to be expected. This influences the life of the hot parts noticeably (see note at pages 2.2- 9 and "Ill. 2.3-2").

An alteration in the pressure niveau of the engine has an effect on the axial bearing load ( "Ill. 2.5-1").

Are oxidation sensitive materials used in the hot region an increased attack is possible. An exception are SiC ceramic particles. They are fixed on the blade tips of shroudless HP-Turbinerotor blades to cut into the even hard rub in coatings. SiC converts at the high temperatures by steam into a gas. With this the rub behavior changes significant.

"Illustration 3.2.2-5": (Lit. 3.2-22 and Lit. 3.2-23): Lowfrequency pressure vibrations (rumble) at around 50-120 Hz are typical for combustion processes. In engine combustion chambers with fuel-injection nozzles, rumble is observed especially during idling. Gas vibrations cause various problems:

• Noise development that is very irritating outside.
• Vibrations that can lead to dynamic fatigue of combustion chamber and turbine components
• Increased dynamic wear
• Efficiency decreases
• Extreme pressure fluctuations that can cause compressor surges.

Although experience has shown that self-increasing pressure vibrations occur more frequently with certain combustion chamber and fuel nozzle configurations, they cannot be predicted with any certainty (Ill. 11.2.2.1-4.2).

The following is a description of the excitement mechanism of gas vibrations in combustion chambers with fuel nozzles: In modern engines, fuel is injected into the combustion chamber with the aid of nozzles. The compressor air is used to spray the fuel (e.g. the principle of an air spray nozzle, Lit. 11.2.2.1-1). During idle, the pressure in the air- and fuel supply systems is relatively low. This causes the mass flows to be more sensitive to pressure fluctuations in the combustion chamber. Because the diffusion of the fuel is largely dependent on the fuel flow rate and the air speed, the pressure fluctuations also change the size distribution of the fuel droplets. A decrease in combustion chamber pressure accelerates the inflowing air and creates smaller droplets. The droplet size determines the time necessary for the vaporization of the fuel. This effect is especially pronounced at the relatively low air temperatures during idling (slower vaporization).

The rapid vaporization of smaller droplets causes them to increase combustion and intensify head development. As a result of the lower gas flow speed during idling, the dwell time becomes longer and more energy is given off in the dilution zone. Irregularities create hot spots that travel along with the gas flow. This type of combustion creates a pressure pulse that is caused by the increased resistance when the larger gas volume (greater flow speed) passes through the tight cross-section of the combustion chamber exit (Phase “1“). This pressure pulse decelerates the airflow in the combustion chamber, i.e., at the air spray nozzle, and creates a smaller spray cone with larger fuel droplets (Phase“2“). Combustion slows and a cold spot is created (“Phase “3“). If a cold spot leaves the combustion chamber and passes through the turbine stator, the smaller gas volumes cause the static pressure in the combustion chamber to decrease. The results are increased spray air speed, a larger spray cone, and small droplets that accelerate combustion (Phase “4“). The conditions for the formation of a hot spot are present again, and Phase “1“ can recur. If pressure fluctuation conditions are right, the vibration can increase itself.

The bottom diagram depicts the described process as a regeneration diagram (Lit. 11.2.2.1-12). The processes during combustion, especially in low NOx combustion chambers with air pre-mixing, is described in detail in Lit. 11.2.2.1-8.