A gas turbine is a set of three pieces of equipment with different functions. First an air compressor, where atmospheric air is pressurized and delivered to the second device, a constant pressure combustion chamber, where fuel is injected and burned. Combustion takes place with high excess air. The heated flue gases are then expanded in the third piece of equipment, a turbine, producing power to drive the compressor and excess power is used for driving an external equipment. Due to the large air excess, almost 100 times the fuel mass, the exiting exhaust gas contains a high concentration of nitrogen and oxygen. And since mainly pressure energy is used to release power, this flue gas temperature is relatively near flame temperature. So, exhaust gas can be considered as heated air and ideal for heating purposes.
Gas turbines may use liquid or gaseous fuels, with the highest energy performance being achieved with liquid fuels, but lowest emissions with clean fuel gases like natural gas. Two gas turbines classes are available for power generation -heavy duty industrial and aeroderivative gas turbines. Aeroderivatives were originally developed for aviation and adapted for power generation. Heavy duty gas turbines are specially designed to match industrial use of some liquid fuels, like diesel, operate at stationery conditions such as extended operation period and load variability, reaching high efficiency.
Gas turbines normally need specialized and more frequent maintenance, compared with steam turbines. Aeroderivative types are designed to have the expander part replaced by a spare or a conditioned unit to minimize downtime, while industrial gas turbines are projected to be maintained on site, and this is generally a less expensive procedure.
Energy efficiency opportunities for gas turbine operation are linked to obtaining maximum energy conversion. Guaranteeing inlet gas design conditions like pressure, temperature and composition, prompt turbines to operate at maximum efficiency. Contaminants like ashes and sulfur compounds may result in deposits, which degrade performance and cause corrosion in the turbine expander section. Tracking and maintaining quality combustion can allow the highest possible temperature of hot gas leaving the combustors and increased temperature results in higher power output. A good specification of average local ambient air conditions, particularly temperature, helps the manufacturer provide adjustments that enhance gas turbine output significantly. Higher ambient air temperature impairs compressor performance due to lower air density, and also excessive pressure drop across exhaust gas ducts and stack. Reducing the pressure drop across air filters increases released power even more. An additional option lies here, reducing inlet air temperature by means of a refrigeration system, generated by using part of the wasted heat from the turbine.
The next great energy efficiency option for gas turbine is recovering excess heat from exhaust gas. Since this gas is in essence hot air, as mentioned before, and it can be used for any heating process, even as hot combustion air. It can be sent to a process fired heater or to a heat recovery steam generator. This option being to set a combined cycle, either in an purely unfired heat recovery steam generator, using only the sensible heat to produce steam or a fired one, using the excess air to burn supplementary fuel and raise the steam production. The supplementary fuel choice increases system flexibility to control heat-to-power ratio of the cogen-eration ensemble. The next step in building a fully combined cycle, is just adding a steam turbine.
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