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Gas Turbine Heat Recovery Steam Generators (HRSGs) are widely used in cogeneration or combined-cycle plants to recover energy from turbine exhaust gases. Gas turbines are widely used in chemical plants, refineries and industrial plants as a source of clean electrical power. Often HRSGs are added to utilize the energy from the exhaust gases to generate low-pressure process steam (in cogeneration plants) or high-pressure, high-temperature superheated steam for use in steam turbines (in combined-cycle plants).
HRSGs have certain features and characteristics that are different from conventional gas or oil fired boilers which will be explained in this series of articles. Understanding these differences will help plant engineers and consultants plan and build better and more efficient cogeneration or combined cycle plants.
- The amount of exhaust gases flowing through a HRSG relative to the steam generation is very large, compared to conventional boilers, as seen below in Table 1.
- The exhaust gas is relatively clean as natural gas, naphtha or distillate oil is usually fired in gas turbines.
- Due to the low temperature of the gases entering the HRSG (850 to 1,050°F in unfired mode) and the associated low log-mean temperature differences in various sections such as the superheater, evaporator and economizer, large surface areas are required. Hence finned tubes are inevitably used to make the HRSG compact.
- The exhaust gases contain about 14 to 16 vol% free oxygen, which enables them to be fired without using additional air. This improves the fuel utilization significantly and additional steam can be generated at nearly 100% efficiency.
- Multiple pressure steam generation may be required to lower the exhaust gas temperature and recover more energy. It is not unusual to see 2 or 3 pressure levels in large plants. The gas/steam temperature profile is set by thermodynamic considerations by what are known as pinch and approach points, which will be discussed later.
- Variations in exhaust gas flow and temperature (due to load or ambient conditions) affect the HRSG performance significantly.
- Different types of designs are available, such as natural circulation, forced and once through.
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Table 1: Gas/Steam Ratios
in Heat Recovery Steam Generators
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System
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Gas inlet temperature,
°F
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Gas/steam ratio
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Unfired
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800-1,000
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5.5 to 7
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Supplementary fired
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1,000-1,700
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2.5 to 5.5
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Furnace fired
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1,700-3,200
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1.2 to 2.5
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Conventional boilers
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3,300-3,600
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1 to 1.2
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[Superheater duty or steam-pressure gas analysis may affect these numbers but they show the trend.]
As an example, if the gas turbine exhaust flow is about 150,000 lb/h at 900°F, it can generate about 20 to 25,000 lb/h of saturated steam at 300 to 600 psig in the unfired mode. If supplementary firing is introduced, the steam generation can be easily doubled. If furnace firing is done, the steam produced will be quadrupled. Procedures to evaluate these flows at various conditions will be discussed later.
By V. Ganapathy
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