Fig. 10-86 Open-Case Model of ABB GT24 Showing Sequential Combustion System. Source: ABB

without water or steam for NOx, the single unit is rated at 25,420 kW ISO base and 8,950 Btu/kWh heat rate (LHV) or 38.1% simple-cycle efficiency. This includes an electric generator efficiency of 97.3%. Figure 10-83 shows heat rate versus power output. The two-shaft design allows for relatively good part-load thermal fuel efficiency. At 60% of full load, for example, there is an increase in heat rate of less than 15%.

Figure 10-84 shows expected mechanical drive performance in shaft output and heat rate versus power turbine speed. Capacity and heat rate, which are represented in kW, can be compared to the performance figures shown for the generator-set, with the primary differences being increased power turbine speed at full load and absence of generator losses. Figure 10-85 shows expected mechanical drive service heat rate versus ambient temperature, at ISO conditions, with no inlet or exhaust losses, at a power turbine speed of 5,000 rpm.

Figure 10-86 is an open-case model of an ABB GT24 industrial gas turbine showing location of the two annular chambers used in the low-NOx sequential combustion system described above. Both combustion chambers operate with extremely lean fuel mixtures, allowing the unit to achieve a NOX emissions rate conservatively estimated at well under 25 ppm when operating on natural gas and 42 ppm when operating on No. 2 oil (@ 15% O2). The turbine inlet temperature, based on ISO conditions, is 2,291°F (1,255°C), enabling the electric generation set to achieve a simple-cycle fuel efficiency of 37.5% on an LHV basis, with a heat rate of 9,003 Btu/kWh (9,497 kJ/kWh). Mean temperature of blade materials, a key indicator for reliability and lifetime, as well as any of the hot-gas sections, are maintained at a maximum of 1,652°F (900°C).

At 60 Hz, design generator output is 166 MW The unit

Fig. 10-87 Assembly of Very Large Capacity, Heavy-Duty Industrial Gas Turbine. Source: Siemens Power Corp.

operates at a shaft speed of only 3,600 rpm and achieves a compressor pressure ratio of 30:1. The full-load exhaust mass flow is 833 lbm/s (378 kg/s) at a temperature of 1,130°F (610°C), which is well-suited for combined-cycle operation. Off-design operation is controlled by adjustment of the first three variable stator rows. The exhaust temperature of the first combustor remains unchanged down to about 25% of full load. In this operating mode, the second turbine stage experiences only small thermal transients when the gas turbine is started. At the moment of ignition, it has already been preheated to approximately 70% of the full-load inlet temperature. The dimensions of the gas turbine are 34 ft (10.4 m) long by 13 ft (4.0 m) wide by 15 ft (4.6 m) high and the dry weight is about 197 tons (178.7 metric tons).

The contrast of Figures 10-87 and 10-88 provide insight into the wide range of types and capacities of gas turbines commercially available for field applications. Figure 10-87 shows the assembly of a very large capacity, heavy-duty industrial gas turbine. Figure 10-88 provides a cutaway illustration of a very small-capacity unit, termed a micro turbine, which has been recently introduced to the market. This packaged, recuperative turbine-generator unit, which produces an electrical output of just under 30 kW, has a weight of only 1,082 lbm (490 kg) and a length of about 6 ft (1.8 m). Manufacturer's specifications indicate that it can operate on a variety of fuels, with operating pressures ranging as low as 5 psig (0.35 bar), NOx emissions of 9 ppmv, and sound emissions of 65 dBA at 33 ft (10 m). All rotating components are mounted on a single shaft that rotates at up to 96,000 rpm at full load. The air-cooled generator is designed for operation at 50 or 60 Hz. When operating at a gas pressure of 55 psig (3.9 bar), under ISO conditions, stated thermal efficiency is 26% (LHV) with a total exhaust energy of 277,000 Btu/h (295,000 kJ/h) at a temperature of 520°F (271 °C). The objective of market

Fig. 10-88 Cutaway Illustration of 28 kW, Packaged "Micro" Gas Turbine-Generator Set. Source: Capstone Turbine Corporation

introduction of micro-type turbines such as this one is to compete with small-capacity packaged reciprocating engines. Hence, there is a large focus on compactness of size, ease of installation, and reliability. Such units are also targeted for use in vehicular applications.

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