Gas Turbines

Combustion gas turbine capacities generally start at about 500 hp (375 kW) and range up to several hundred thousand hp (or kW). With the recent commercial market introduction of "micro" turbine technology, applications of 50 hp (37.5 kW) or even lower may be economically viable. Although their use has historically been confined to large facilities with significant steam loads or to applications in remote locations, recent technology improvements, reduced manufacturing costs, and the ability to control air emissions to extremely low levels have made gas turbines increasingly attractive for other mechanical drive applications.

Gas turbine types and operation are discussed in Chapter 10. Distinguishing characteristics include good reliability and high power density (low specific weight per unit of output capacity). Because of high exhaust mass flow rate at temperatures of about 900 to 1000°F (480 to 540°C), high temperature heat recovery is possible, allowing, for example, recovery of half or more of the total energy input in the form of high-pressure steam. Maintenance frequency is relatively low and typical life-cycle O&M costs range from $0.003 to $0.010/hp-h ($0.004 to $0.013/kWh).

Typically, single-shaft turbines may be effectively applied to full-capacity baseloaded applications. However, they do not operate efficiently at variable speeds and are inefficient at part loads. With variable speed operating capability, multi-shaft turbines are more frequently applied to mechanical drive services with significant part-load operational requirements. Multi-shaft units are also advantageous because they can develop the high torque required to quickly accelerate compressors and other driven equipment to full operating speeds.

Augmentation by supplementary firing in the oxygen-rich exhaust can significantly enhance the quantity and temperature of the exhaust gas stream. Steam injection can also be used for power augmentation. Because heat recovery-generated steam can be split between process and power augmentation service, steam injection cycles accommodate wide variations in host demands for thermal and mechanical energy. When thermal loads are small relative to mechanical load, combined cycle, steam injection (STIG) cycle, or recuperation can be used to increase power generation efficiency. Capacity enhancements to gas turbines are discussed in Chapters 10 and 12.

Generally, sound emissions can be attenuated to 80 to 90 dba at a 3-ft (0.9-m) distance with normally available silencers and packaged sound attenuation. NOX emissions can be controlled to a range of 30 to 9 ppmv on models with low-NOX combustors, and even lower with the use of exhaust gas treatment.

Typical simple-cycle thermal fuel efficiency for combustion gas turbine mechanical drive systems range from 17 to 35 (LHV basis), resulting in simple-cycle heat rates of 15,000 to 7,000 Btu/hp-h (20,000 to 10,000 kJ/kWh). With use of steam injection or recuperation, thermal fuel efficiencies of greater than 40% can be achieved.

Typical recovered heat potential ranges from 45 to 60% of the total energy input, depending on simple-cycle efficiency and temperature of the recovered heat. When fuel use displaced by recovered heat is considered, net heat rates, or FCPs, range from 4,800 to 3,300 Btu/hp-h (6,800 to 4,700 kJ/kWh).

Figures 29-13 through 29-16 show different types of gas turbine engine mechanical drive applications. Figure 29-13 is a cross-sectional illustration of a two-shaft unit and Figure 29-14 shows a two-shaft turbine applied to

Fig. 29-13 Cross-Sectional Illustration of Two-Shaft Gas Turbine. Source: Solar Turbines
Fig. 29-14 Two-Shaft Gas Turbine Driving Propane Compressor. Source: Solar Turbines
Fig. 29-15 Heavy-Duty Industrial Gas Turbine Applied in Water Injection Plant. Source: MAN GHH
Fig. 29-16 Recuperated Gas Turbine Compressor Set. Source: Solar Turbines

compressor drive service. Figure 29-15 shows a gas turbine applied to mechanical drive service in a water injection plant and Figure 29-16 shows a recuperator applied to a gas turbine mechanical drive for power augmentation.

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