322 Gas Turbine Engine Applications

The use of CFCC components in gas turbine engines increases their efficiency, resulting in fuel savings, reduced emissions, reduced downtime, and other benefits. A turbine is a rotary engine that uses a continuous stream of fluid to turn a shaft that drives machinery. A gas turbine engine uses gas as fuel. This engine consists of a rotary bladed shaft passing through a compressor, combustor, and exhaust sections. Air is compressed, mixed with fuel, ignited in the combustor, and then exhausted through rotary blades that spin, driving the upstream compressor as well as downstream machinery. Fuel can be natural-gas, kerosene, or gas rendered from coal. The hot exhaust gases can be used to power pumps, other equipment, electrical generators or to generate steam for industrial processes or both, a so-called cogeneration system.

Natural-gas-fired turbines are slated to provide 80% of new electrical power capacity in the United States. Of the 200 planned power plants, 96% will use natural-gas fuel, most fueling gas turbines.

Turbine manufacturers are interested in reducing downtime and emissions and improving engine efficiency. Engine shutdowns are the bane of utility operators. The resulting severe thermal shocks damage these large expensive engines. This is the primary reason these engines are limited to lower temperature operation resulting in lower efficiency.

Turbine engine efficiency, as with all heat engines, is determined by operating temperature. The higher the temperature, the higher the efficiency. A turbine engine efficiency increase of 0.4% results in fuel savings of $460,000/ year for a 160-megawatt (MW) engine. A 0.5% additional airflow through the combustor (as a result of reduced cooling to the shroud), at base load conditions, could reduce NOX emission levels 10-25%. A 1.25% reduction in pressure drop, as a result of less cooling, could lead to a $370,000 fuel savings per year per engine. One turbine engine is designed with CFCC components with an efficiency increase of 15% because its use allows for near-stoichiometric fuel combustion for increased power without the cooling air requirement penalties associated with metallic structures. Replacing steel with CFCC combustor liners, shrouds, and interstage seals enables this increase in efficiency (Fig. 3.7).

Combustors have inner and outer liners. The inner liner faces the flaming gases. It is a cylinder, perforated to pass fuel that combines with compressed air and ignited in the combustor. Cooling air flows between the inner and outer liners to preserve the inner liner. Diverting air for cooling also reduces efficiency. The more thermally stable CFCC combustor liners lower and potentially eliminate the need for cooling. At a given NOX level, metal liners showed higher CO levels than the CFCC liner. This is attributable to the quenching effect of cooling air. With no cooling air, the CFCC combustor produced NOX levels below 10 ppm (15% oxygen) with low CO emissions. Metallic liners were limited to NOX emissions near 20 ppm.

The very high-temperature gases from the combustor pass through the firststage turbine stator into the first-stage rotor. Concentric to the outer diameter of the rotor blades is a ring of stationary components called shrouds. Shrouds are

CFCC Gas Turbine Engine Components

CFCC Gas Turbine Engine Components

FIGURE 3.7 CFCC thermal stability enables higher operating temperatures resulting in increased efficiency.

a series of open-top, curved walled boxes attached to the engine inner case and concentric to the outer diameter of the rotating blades. They seal between the inner engine case and the end of the rotating blade. The clearance between the ring of shrouds and the rotor blade is minimized to reduce exhaust gas leakage around the end of the blade. The shroud of a 160-MW engine is 2.44 m in diameter, consisting of 96 rectangular segments 7.6 cm wide, 15.2 cm long and 1.3 cm high. Shroud temperatures can reach 1290°C (2354°F) if uncooled. They are presently made of metallic super alloys and require about 1.2% of the compressor output for cooling. CFCC shrouds enable higher temperature operation, reduce the amount of cooling air required by 80%, resulting in a higher efficiency turbine operation, reducing emissions by 10-25%.

CFCCs successfully performed during 100 shutdowns after steady-state operation at 1115°C (2120°F). To create an excessive thermal shock condition, high airflow rates were maintained after the fuel was turned off resulting in dramatic temperature reductions, high thermal stresses, and complex mechanical stresses in the CFCC shroud. CFCCs achieved the primary goal of demonstrating risk reduction to the engine operator. CFCC combustors, compared to metal combus-tors reduce NOx by 47-60% and CO emissions by 33-60%.

Downstream, another type of interstage seal is used. Since the metal rotating blade and the metal seal thermally expand as the engine warms-up, it cannot be designed to end-seal exhaust gases at all operating temperatures. An abradable seal is placed around the inner case surface. The rotating blade expands into and cuts a path in this material, forming a perfect seal, preventing exhaust gas leakage and increasing efficiency. CFCCs possess the appropriate physical properties and heat resistance to perform satisfactorily as this interstage seal. This seal will improve efficiency, resulting in fuel savings of 0.5%.

Malden Mills, a Polartec™ textile mill in Lawrence, Massachuesetts, has a Solar Centaur 50S gas turbine outfitted with CFCC components. The turbine generates steam, electricity and heat. It uses 25-40% less fuel than todays coal-fired plants and emits 40% less carbon dioxide, a greenhouse gas. The CFCC turbine has successfully operated for 16,000 h and continues to perform.

CFCC thermal stability, thermal shock resistance, strength and oxidation resistance is enabling gas turbine engines with higher operating temperatures, increased efficiency, reduced downtime, maintenance, emissions and operating costs. CFCC light weight is also of interest to airborne turbine users where 30% of turbine weight would be eliminated.

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