Table 75 Plant Energy Supply System Considerations Example

Process steam demands

Net heat to process at 150 psig sat—158.5 million Btu/hr avg. (peak steam requirements are 10% greater than average values)

Process condensate returns: 45% of the steam delivered at 300°F

Makeup water at 80°F

Plant fuel is fuel oil

Fuel cost is $5/million Btu

Process areas require 30 MW

Purchased power cost is 5 cents/kWh

Fig. 7.17 Effect of different fuel and power costs on cogeneration profitability: Example 1. Basis: Conditions given in Tables 7.3 and 7.4.

some of the plant boiler feed pumps. The net generation of this cogeneration system is 6.32 MW when operating at the average 150-psig process heat demand. A utility tie provides the balance of the power required.

Case 2. (Refer to Figure 7.19). This alternative is a combined cycle using the 25,000-kW gas turbine generator whose performance is given in Table 7.7. An unfired HRSG system provides steam at both 850 psig, 825°F and 150 psig sat. Plant steam requirements in excess of that available from the two-pressure level unfired HRSG system are generated in an oil fired packaged boiler. The steam supplied to the noncondensing turbine is expanded to the 150-psig steam header. The net generation from the overall system is 26.54 MW. A utility tie provides power requirements in excess of that supplied by the cogeneration system. The plant-installed cost estimates for Case 2 include two half-size package boilers. Thus full steam output can be realized with any steam generator out of service for maintenance.

The energy summary, annual operating costs, and economic results are presented in Table 7.6. The results show that the combined cycle provides a GPO of 2.5 years based on the study fuel and purchased power costs. The incremental cost for Case 2 relative to the Base Case is $395/kW compared to $655/kW for Case 1 relative to the Base Case. This favorable incremental investment cost combined with a FCP of 5510 Btu/kWh contribute to the low CPO.

The influence of fuel and power costs other than those given in Table 7.5 on the GPO for cases 1 and 2 is

Fig. 7.18 Simplified schematic and energy-balance diagram: Example 7, Case 1. All numbers are flows in 1000 lb/hr; gross generation, 6.82 MW; powerhouse auxiliaries, 0.50 MW; net generation; 6.32 MW.

Fig. 7.19 Simplified schematic and energy-balance diagram: Example 7, Case 2. All numbers are flows in 1000 lb/hr; gross generation, 26.77 MW, powerhouse auxiliaries, 0.23 MW: net generation, 26.54 MW.

Fig. 7.19 Simplified schematic and energy-balance diagram: Example 7, Case 2. All numbers are flows in 1000 lb/hr; gross generation, 26.77 MW, powerhouse auxiliaries, 0.23 MW: net generation, 26.54 MW.

Table 7.6 Energy and Economic Summary: Example 7

Alternative

Base Case

Case 1

Case 2

Energy summary

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