62 ARTICLE / MAKALE lnjection of Steam or Water Steam or water injection is a method by which the output power of a gas turbine cycle can be increased. This has several effects: it increases the flexibility of the gas turbine during par! load operations and significantly decreases emissions of carbon monoxide and unburned hydrocarbons. Steam injection can be carried out using saturated or superheated steam. The introduced steam is usually injected into combustion chamber. However, water injection is a more common method of increasing efficiency than steam injection. Water is typically t c i u n r j r e e e a c s ft e a e d ll t s h i n e a t s o m a a t h s c e s o s n f y l s o s e w t q e u r m e a tn a e c t . e T t h o h e f e w c c o a o m t m e p r p r r i e n e s j s e s s c o e t r d i o o a n u, i t rl b e t u t e t m t o t p h i e i n s r atemperature reduction can be minimized through use ofa regenerator. No more fuel is consumed in this temperature compensation because the process uses heat from the gas turbine exhaust that would otherwise be wasted. Water can also be injected at the compressor inlet. This has the following advantage compared with injection at the compressor outlet: O Because air at the inlet air duct is at about atmospheric pressure, !here is no need to use a high pressure pump. O The inlet air temperature is atmospheric, which means there is no need to warm up the spray water to prevent thermal shock. O There is usually a long distance between the inlet air duct and the compressor inlet, so by the time the atomized water reaches the compressor, it will be thoroughly mixed with the air, so it is homogenized mixture of water and air that will be introduced into the compressor. This reduces impact damage and corrosion effects on compressor components. exhaust 4 power turlılne � 3' inlet Figııı-e 6. Sclıematic ofa gas ıurbine !hal eınploys relıeating (Eııeıgy Solutioııs Centeı) Additional Methods of lncreasing Efficiency Several other methods of increasing efficiency are also employed. Heat recovery shall power Gas turbines generale a large volume of very hol air. This exhaust is also high in oxygen content compared to other combustion exhaust streams because only a small amount of oxygen is required by the combustor relative to the total volume available. Depending on how much thermal energy is required, 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 by the ENERJİ & KOJENERASYON DÜNYASI duct burner is directed into the HRSG. 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 drying 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 those times when the waste heat is not required. The diverter vents the turbine exhaust to the atmosphere. This substantially reduces the system efficiency because only the electrical energy output of the turbine is being used. The higher the electrical efficiency of the turbine the lower the available thermal energy in the exhaust. Newer turbines with recuperators and larger turbines tend to have higher efficiencies. inlet air coo/ing Another method of increasing turbine efficiency is through inlet air cooling. This is most effective in hot, dry climates because in effect it reduces the ambient temperature entering the turbine. Gas turbines operate with a constant volume of air, but the power generated depends on the mass flow of air. Warm air is less dense than cold air, resulting in lower power output. Warm air is also harder to compress than cold air, taking more work from the compressor and thus increasing internal losses. inlet aa ni rdc tohoul isn sg ei lse cstei on sn i toi vf et hteo caomr rbei ce tn ti ncl eotn adiirt i co on so l ai ntgt hsey ssti et em, i s scioteolstpheeciinfilce.t Faoirrfarotmyp1ic5a°l gas turbine, it is possible to c to 5.6 ° ciency by about 2%. There are many Cte, cinhcnroeloagsieins g tfhuaet la eref ficommercially available for turbine air inlet cooling. These technologies can be divided into the following major categories: O Evaporative: wetted media, fogging and wet compression/ overspray O Chillers: mechanical and absorption chillers with or without thermal energy storage O LNG vaporization O Hybrid systems: Reduction of /eakage f/ow One area of potential loss of efficiency lies in the leakage of flow through the gas turbine. This typically comes from inevitable leakage around the tips of the blades and vanes, as well as from other areas that require sealing. Reduction of leakage is a compromise between reducing the leakage flow and allowing the leakage flow 'necessary' to avoid high temperature increases caused by disc friction or heat conduction or both. The most common method of optimizing blade design to achieve the best compromise is through the use of a blade tip shroud. These restrict gas leakage flow across the blade tip by using knife-edge seals designed to rub into a honeycomb seal material that is brazed onto the shroud blocks. However, centrifugal forces on the rotating stages ofa gas turbine are very large. For example, an F-class blade weighing about 3.6 kg will exert a pull of over 440.000 N at operating speed. Typically, shrouds account for about 1 0% of the weight of a blade. Another method of reducing flow leakage at blade tips is through the use of abradable material. The obvious method of reducing this flow leakage is to reduce the clearance between the blade
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