7.1 MODELO DE SIMULACIÓN DE CAUDALES
7.1.2 Calibración del modelo de simulación
Every 100 mm H2O at suction : Every 100 mm H2O at exhaust : 1.42 % power loss 0.42 % power loss
0.45 % increase in Heat Rate 0.42 % increase in Heat Rate
1 °C increase in exhaust temperature 1 °C increase in exhaust temperature Fuel gas influence
Best performance is achieved if natural gas rather than diesel oil is used. In fact, output power under base load power and under equal conditions (environmental, pressure drops , etc.) will be about 2% greater and specific consumption (Heat Rate) between 0.7 and 1% lower, depending on gas turbine model.
These differences will become all the more remarkable if we compare performances obtained with natural gas and with progressively "heavier"fuel types, such as residuals, Bunker C, etc.
This behavior is due to the higher heating power of products originated by the combustion of natural gas, as the latter has a higher content of water vapour, resulting from a higher ratio between hydrogen and carbon, which is typical of methane, the main component part of natural gas.
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Gaseous fuels with a lower calorific value than natural gas (commonly called "low btu gases") can greatly influence the performance of a gas turbine.
In fact, if the calorific value diminishes (Kj/Nm3), the weight of fuel delivered to the combustion chamber must increase to provide the necessary amount of energy (Kj/h).
This addition in the weight of the fluid, which is not even compressed by the compressor, provokes an increase in power (see the definition of useful work at para. 2.3) and a reduction in specific consumption.
In this case, the power absorbed by the compressor is left substantially unvaried. However, in the case of combustion of “low btu gases”, the following side effects must be taken into consideration:
• An increase in the weight of fluid delivered to the turbine increases the compression ratio in the compressor, which must not come too near the surging limit.
• An increase in the fuel delivered often requires larger diameters of tubings and control valves (and, consequently, higher costs). This effect is all the more conspicuous in the case in which also the temperature of a gas and, therefore, its specific volume, are higher (for example, gases produced by coal gassing).
• Gases with a low calorific value are frequently enough saturated with water vapour upstream of the gas turbine combustion system. This provokes an increase in the coefficients of heat transmission by combustion products, and an increase in metal temperature on hot parts of the turbine.
Air extraction from the axial compressor
On some gas turbine applications (chemical processes, pipe blowing during the commissioning stage, etc.) it may become necessary to extract compressed air from the compressor delivery side.
As a general rule, and unless prescribed otherwise in the case of machines originally built for the aviation industry, it is possible to extract as much as 5% of the air delivered by the compressor without the need to alter the turbine layout at all.
It is possible to achieve extraction values ranging between 6 and 20 % , depending on the machine and combustion chamber configuration, if alterations are made to casings, pipings and the control system.
Fig. 2.12 shows how percentages of air extraction influence output power and specific consumption (heat rate), taking into consideration also ambient temperature.
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Fig. 2.12
Steam injection and water injection
Steam or water injection may have the following two purposes: • a reduction in nitrogen oxide (NOx) level.
• an increase in output power. Reducing the nitrogen oxide (NOx) level
The method of steam or water injection was introduced in the early 70s to limit and control the presence of nitrogen oxides or NOX.
Injection is usually performed in the area where the combustion chamber cap is present. The injection system is built in a way to set a limit to the amount of injectable steam or water, in order to safeguard stability and continuity in the combustion process. Anyway, the amount of steam and water injected is sufficient to guarantee a massive reduction in the level of NOx.
According to the amount of steam or water injected into the combustion chamber, which the turbine control system automatically monitors in relation to the NOx level desired, output power will increase consequently to an increase in the mass of fluid delivered through the gas turbine.
In the case of steam injection, the Heat Rate or specific consumption will also decrease for the same reasons that apply to fuel gases with a low calorific value. On the contrary, the latter advantage does not exist in the case of water injection, as here a higher quantity of fuel is needed to vaporize water to the condition that allows it to be injected into the combustion chamber.
HEAT RATE (%)
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In condition of peak duty, with a maximum of 1250 hours/year, it is possible to increase the water delivered through the combustion chamber cap area in order to increase the gas turbine power output. Obviously, this calls for shorter maintenance intervals.
As concerns the maximum water flow rates and maintenance procedures, these must be evaluated case by case, depending on the machine model and its combustion system.
Output power increase
Steam injection for increasing the gas turbine output power has been available and warranted by over 30 years' experience.
Unlike water, steam is injected into the compressor exhaust casing, thus eliminating all limitations imposed in order to safeguard stability in the combustion process. For this reason, the maximum amount of injectable steam is limited to percentage values of the weight of air aspirated by the compressor.
Steam must be overheated, and there must be at least 25 °C difference with respect to the compressor delivery temperature; steam supply limit pressure must be at least 4 bar(g) greater than maximum pressure in the combustion chamber. In the case of steam or water injection, the amount of steam injected in conditin of partial load must be equal to the amount required to abate NOx. Once the load base value is reached, the control system gives the OK to inject the additional steam needed to increase the turbine output power.
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Fig. 2.13 shows the typical effects of steam injection on the output power of a gas turbine (in this case, an MS 5002 gas turbine) as a function of ambient temperature.
Fig. 2.13 - Effects of steam injection on output power (MS5002 Gas Turbine)
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COALESCER/DEMISTER SUMP TANK CELLE AD EVAPORAZIONE COLLETTORE H2OPOMPA DI CIRCOLAZIONE H2O
ARIA REFRIGERATA VERSO IL FILTRO DELL’ARIA
ARIA CALDAAMBIENTE
Evaporative cooling
Curves in fig. 2.9 show clearly how power and efficiency increase as the compressor inlet temperature decreases.
The latter can be reduced artificially by using an evaporative cooler located upstream of the suction filter.
Water, fractioned into drops or in the form of a liquid film, cools the air by evaporating in the cooler as it flows in contrary direction, thus originating an adiabatic-isoenthalpic exchange (see fig. 2.14).
Fig. 2.14 Evaporative cooler
In order to prevent water from being drawn towards the compressor and fouling it, downstreams of the cooler there are one or more stages of drop separators (demisters), which, by inertia, separate any water drops that might be carried away downstream of the cooler by the flow of air aspirated by the turbine.
Fig. 2.15 shows the effects of evaporative cooling on the gas turbine output power and specific consumption.
As can be noted, benefits increase as relative humidity decreases and ambient temperature increases.
Unfortunately, the above requirements are met in environments (for example, deserts), in which water is not always available in the amounts needed by the cooler.
COOLED AIR