Comparison of Combined Cycle and Steam Injection Systems

Conventional combined cycles and steam injection systems offer similar performance and operating flexibility in responding to power and thermal load variations. Supplementary firing can be used with either type of system. Critical differences are: • Combined-cycle systems generally have a higher capital cost than steam injection-cycle systems due to the addition of the steam turbine, condenser, and cooling tower. They also usually offer superior heat rates due to the low exhaust pressure achieved in a condensing steam turbine (a steam injection-cycle gas turbine exhausts at atmospheric pressure). Combined-cycle systems also do not require abundant use of treated water because steam is condensed in a closed cycle. Generally, combined cycles are suited for high-load factor applications where water and fuel costs are high. An additional consideration is that the steam

Fig. 12-15 Gas Turbine Applied to Combined-Cycle System with Fired Boiler.

Fig. 12-16 Heat and Material Balance for 50.4 MW Steam Injection System. Source: Cogen Designs, Inc.

turbine can run independent of the prime mover, possibly reducing back-up requirements for facilities that have boilers for steam generation.

• Typically, steam injection-cycle systems produce greater capacity for a given turbine model without supplementary firing and does so at a lower capital cost. The steam injection-cycle system is also a less-complex system with lower space requirements. They do, however, require far more water and an extensive water treatment program due to steam loss in turbine exhaust. Steam injection provides some NOX emissions control, while the gas turbine in the combined-cycle system may require additional investment for NOx control. Steam injection-cycle systems are better suited for low load factor applications, particularly where peak electric rates are high.

Figures 12-16, 12-17, and 12-18 provide comparisons of similar steam injection and combined-cycle systems. Figure 12-16 is a heat and material balance schematic of a steam injection-cycle system. Fuel input is 448.5 MMBtu/h (473,078 MJ/h), producing 50.4 MW of power at a heat rate of 8,895 Btu/kWh (9,382 kJ/kWh) on an HHV basis.

Figure 12-17 is a combined-cycle system featuring the

Fig. 12-17 Heat and Material Balance for 42.3 MW Combined-Cycle System. Source: Cogen Designs, Inc.
Fig. 12-18 Heat and Material Balance for 50.4 MW Combined-Cycle System with Supplementary Firing. Source: Cogen Designs, Inc.

same gas turbine in which both high- and low-pressure steam is sent to a steam turbine. Without steam injection, the turbine has a capacity of about 32.3 MW and a fuel input requirement of 346.9 MMBtu/h (365,910 MJ/h) on an HHV basis. The steam turbine adds an additional 10 MW of capacity, reducing the overall heat rate to 8,196 Btu/kWh (8,645 kJ/kWh). Comparison of these two designs shows greater capacity with the steam injection cycle, but a superior heat rate with the combined cycle.

Figure 12-18 shows a combined cycle system configuration featuring the same gas turbine, but using supplementary firing to match the 50.4 MW capacity of the steam injection system. In this case, the supplementary firing results in a heat rate of 8,876 Btu/kWh (9,362 kJ/kWh), which is higher than the unfired combined-cycle, but still marginally superior to the steam injection-cycle system. However, if a larger, more costly gas turbine is used, the total capacity of the steam injection-cycle system could be matched without supplementary firing, at a superior heat rate of under 8,200 Btu/kWh (8,650 kJ/kWh).

Both combined-cycle and steam injection-cycle systems can be viewed as adding peak shaving capability into an otherwise baseloaded electric cogeneration system. Varying electric and thermal loads, time-of-use and real-time pricing, and seasonal fuel cost fluctuations all figure heavily into cogeneration project economics. Both systems provide flexibility to operate effectively under a wide range of load and energy cost conditions.

An ideal operating strategy uses either a combined-cycle or steam injection system with supplemental firing or a fuel-fired boiler. Gas turbine capacity is selected for baseload electric and steam loads. Supplementary firing is used as steam load increases, and combined-cycle or steam injection-cycle systems are brought on-line as electric load increases. By varying the levels of secondary combustion and secondary electric generation, a wide range of loads can be met. Microprocessor control is of great value in these types of applications.

This simplified comparison shows the typical trade-off of lower capital cost for the steam injection system versus lower operating cost for the combined-cycle system. Conditions of high peak electric rates, inexpensive water, and lower load factor (low utilization of required capacity) would favor the steam injection cycle. Conditions of higher load factor and water costs and, to some extent, higher fuel costs would favor the combined-cycle.

Prime mover control systems regulate input of energy to the prime mover and the conversion of this energy into power in a safe and efficient manner. Regardless of whether the prime mover drives a generator to produce electric power or drives a mechanical load, such as a compressor or pump, energy flows into the prime mover and the prime mover converts it into developed power (pd) and developed torque (td). The driven load exerts a load power (pl) and load torque (tl) in the opposite direction of the developed power. Torque is a measure of force of rotation and power, which is the rate of doing work, is equal to torque times the speed of rotation. Therefore, if speed is held constant, torque and power are proportional.

There are two general modes of prime mover operation:

• Steady state refers to the condition in which pd and td match pl and tl. This could be a no load, full load, or an intermediate-load condition. Under steady-state conditions, no acceleration or change in speed occurs.

• Transient refers to the condition in which pd and td do not match pl and tl. When, for example, the load (tl) is suddenly decreased, td will momentarily remain unchanged. The excess td immediately produces an acceleration of the prime mover and the speed increases.

The essential role of prime mover control is to continue to operate the prime mover safely in a steady-state mode, while responding to the transient situation as fast as possible with a minimum of instability. An isochronous control system holds rotational speed constant under varying load conditions. When a control system allows a change in operating speed that is inversely proportional to a given change in load, it is said to be a droop system.

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