100

Inlet Air Temperature. F Eteain Injection Rates, Mlb'hr

GDhp O Ip

25ppm NOx

77hp;43 Ip

Inlet Air Temperature. F Eteain Injection Rates, Mlb'hr

GDhp O Ip

25ppm NOx

77hp;43 Ip

Fig. 12-10 Representative Steam Injection Performance Curves. Source: Cogen Designs, Inc.

Figure 12-10 shows capacity versus steam injection for NOX, CDP, and LP steam for a steam injection gas turbine. In this graphic, NOX control steam is the first to be introduced. It is added up to about 30,000 lbm/h (13,600 kg/h). CDP steam is then added up to about 32,000 lbm/h (14,500 kg/h). At that point, capacity has increased to about 45 MW and LP steam can be added, along with some additional CDP steam. This brings the maximum capacity of the turbine to about 50.5 MW at a total injection rate of 120 Mlbm/h (54,400 kg/h).

The top line in the figure (filled squares) is the sum of the NOX (32 Mlbm/h), CDP (45 Mlbm/h), and LP (43 Mlbm/h) steam. The incremental power production is fairly linear with steam injection, but the slope of the curve depends on the type of steam introduced. Notice NOX and CDP steam show far greater capacity enhancement and efficiency than LP steam. In this case, the LP steam is introduced part way down the power turbine. Since it is not condensed, it is used inefficiently, relative to the performance achievable in a condensing steam turbine.

Figure 12-11 shows the capacity of this same turbine as a function of steam injection rate and ambient temperature. More steam always produces more power, but the increase is greatest around 59°F (15°C). The curves labeled 42 ppm and 25 ppm NOX represent the steam injection rates that achieve these levels of NOX in the exhaust gases. The other curves represent the steam injection rates in Mlbm/h, of high-pressure (625 psig) and

Fig. 12-11 Capacity as a Function of Steam Injection Rate and Ambient Temperature. Source: Cogen Designs, Inc.

lower-pressure (225 psig) steam.

The performance of an Allison 501 KH Cheng cycle system is illustrated in Figure 12-12. In this example, the gas turbine produces 3,587 kW at sea level and 59°F (15°C), with a fuel input of 50.7 MMBtu/h (53,478 MJ/h) on an HHV basis. Steam production, at a condition of 450 psig/550°F (32 bar/288°C), is 21,340 lbm/h (9,679 kg/h) and the simple-cycle heat rate is 14,142 Btu/kWh (14,917 kJ/kWh). The FCP is 7,736 Btu/kWh (8,160 kJ/kWh).

Fig. 12-12 Capacity and Heat Rate of Allison 501 KH vs. Ambient Temperature at Various Steam Injection Rates. Source: Cogen Designs, Inc.

As shown, injecting 18,000 lbm/h (8,165 kg/h) of steam increases power output to 5,374 kW and the fuel input rate to 56.5 MMBtu/h (59,600 MJ/h), for a heat rate of 10,513 Btu/kWh (11,089 kJ/kWh). If the maximum desired power output was constant at 3,587 kW, any excess steam could be injected back into the turbine, resulting in reduced turbine fuel requirements.

Alternatively, 21,340 lbm/h (9,680 kg/h) steam production and 3,587 kW power output can also be accomplished with 36.8 MMBtu/h (38,817 MJ/h) gas turbine fuel input, 10.6 MMBtu/h (11,181 MJ/h) HRSG supplemental firing, and 18,000 lbm/h (8,165 kg/h) steam injection. The ability to produce the same power output at two different firing rates provides great operational flexibility, but makes steam injection cycle optimization analytically challenging.

Fig. 12-13 Typical Arrangement of Cheng Cycle Package. Source: United States Turbine Corp.

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