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NFCRC Tutorial: Gas Turbine

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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.

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