30

* Current properties Source: Solar Turbines

* Current properties Source: Solar Turbines corrosion and erosion resulting from poor air or fuel quality.

Significant R&D has also been focused on high-temperature ceramics as structural materials for hot section components such as rotor blades, vanes, and combustor liners. Ceramics have the potential to advance gas turbine inlet temperatures beyond metal alloys. Various types of composites are being developed for different components. These include composites with metallic, glassy, and organic materials. Ceramic coatings are being developed to serve as thermal barriers for metallic hot section components.

Advanced ceramics have fairly simple chemistries. Generally, they consist of elements such as silicon, carbon, aluminum, oxygen, and nitrogen, in network arrangements held together by strong chemical bonds. Other elements used in certain compositions are titanium, barium, lead, and zirconium. Advanced ceramics may be classified as either monolithic ceramics or ceramic matrix composites.

Metal matrix composites have properties imparted to the metal by the non-metallic reinforcements used. The cross-linking of dissimilar materials creates new materials with unique properties that are both very strong and very light.

Metal matrix composites and intermetallic composites are not expected to have the resistance to extremely high temperatures of ceramics. However, they are potentially useful materials for intermediate temperature applications in compressor blades and downstream turbine components such as disks, blades, casings and shafts. Titanium-based metal matrix composites and reinforced titanium aluminides are seen as excellent potential structural materials.

Engineered plastics also have potential uses as materials for lower-temperature component applications such as compressor air-inlet ducting and filter housings where corrosion resistance is of primary importance. The strong damping characteristics of these materials may also allow them to be used as materials for vibration control, and their low density and durability make them potential candidates to replace metallic components for various types of seals and bushings. While material costs are expected to

Material

Table 10-3 Potential Advantages of Advanced Materials To Replace Component Advantage

Organic composites

Aluminum, stainless steels

Compressor blades, housings

Weight reduction, increased damping, corrosion resistance

Titanium aluminide intermetallics

Steels, nickel alloys

Compressor cases and blades, LP turbine blades

Lightweight, high strength

Nickel aluminides

Nickel superalloys

Turbine disks, blades

Lightweight, oxidation resistance

Ceramics

Nickel and cobalt-based

Blades, vanes, combustor liners, transition pieces, fuel injector tips, tubular materials

Durability, oxidation resistance, high-temperature strength, lightweight, reduced cost

Ceramic matrix

Nickel-based superralloys

Combustor liners, ceramic recuperator headers

Oxidation resistance, high-temperature strength, lightweight

Source: Solar Turbines

Liquid Fuel

Liquid Fuel

Shaft Power

LP Turbine Power Turbine

Shaft Power

LP Turbine Power Turbine

Fig. 10-90 Arrangement of Potential Future High-Efficiency Gas Turbine System. Source: Solar Turbines remain high, the potential to manufacture composites into complex shapes could result in lower production costs compared with machined metallic assemblies.

Humid Air Turbine Cycle

The Humid Air Turbine (HAT) cycle is currently under development and is expected to be in production shortly. In the HAT cycle, exhaust heat from the gas turbine is used to heat and humidify the combustion air. The HAT cycle will operate with intercooling and high-pressure ratios and is expected to offer a thermal efficiency of about 47% (LHV). The HAT cycle is being designed to operate with natural gas or fuel from a coal gasifier.

Ericsson Cycle

One cycle that may, in the future, offer the potential of efficiency approaching 50% is the Ericsson cycle. This cycle combines the features of regeneration, reheating, and inter-cooling along with steam/fuel reformation. In the ideal Ericsson cycle, both isothermal compression and expansion are combined with regeneration. Cycle efficiency is improved versus the Brayton cycle because isothermal compression requires only about 75% of the energy (per unit of air) required by adiabatic compression. Isothermal expansion is also more efficient than adiabatic expansion.

Figure 10-90 shows a typical arrangement of this potential future gas turbine system. In the ideal cycle, it is assumed that intercooling is used between every blade row of the compressor and that reheat is used between each turbine section. In a simplified version of the cycle, multi-staged compression with intercooling is used to cool the air prior to entering the regenerator. The energy in the steam produced from the cooling water in the intercooler (heat exchanger) is then used in reforming the fuel. This form of heat recovery increases thermal efficiency by returning the sensible heat of the air to the cycle as part of the fuel energy.

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