extraction at 250 psig and 80 psig exhaust pressure. The boiler plant has three half-size units providing the same reliability of steam supply as the Base Case. The feedwater heating system has closed feedwater heaters at 250 psig and 80 psig with a 20 psig deaerating heater. The 20-psig steam is supplied by noncondensing mechanical drive turbines used as powerhouse auxiliary drives. These units are supplied throttle steam from the 250-psig steam header. For this alternative, the utility tie normally provides 4.95 MW. The simplified schematic and energy balance is given in Figure 7.16. The results of this cogeneration example are tabulated in Table 7.4. Included are the annual energy requirements, the 1980 investment costs for each case, and the annual operating cost summary. The investment cost
data presented are for fully operational plants, including offices, stockrooms, machine shop facilities, locker rooms, as well as fire protection and plant security. The cost of land is not included. The incremental investment cost for Case 1 given in Table 7.4 is $17.2 million. Thus the incremental cost is $609/kW for the 28.25-MW cogeneration system. This illustrates the favorable per unit cost for cogeneration systems compared to coal-fired facilities designed to provide kilowatts only, which cost in excess of $1000/kW. The impact of fuel and purchased power costs other than Table 7.3 values on the GPO for this example is shown in Figure 7.17. Equivalent DRR values based on first-year annual operating cost savings can be estimated using Figure 7.15.
Table 7.3 Plant Energy Supply System Considerations: Example 6 ——————————————————————————————————————————————————— Process steam demands Net heat to process at 250 psig. 410°F—317 million Btu/hr avg. Net heat to process at 80 psig, 330°F—208 million Btu/hr avg. (peak requirements are 10% greater than average values) Process condensate returns: 50% of steam delivered at 280°F Makeup water at 80°F Plant fuel is 3.5% sulfur coal Coal and limestone for SO2 scrubbing are available at a total cost of $2/million Btu fired Process area power requirement is 30 MW avg. Purchased power cost is 3.5 cents/kWh ———————————————————————————————————————————————————
Table 7.4 Energy and Economic Summary: Example 6 ——————————————————————————————————————————————————— Alternative Base Case Case 1 ——————————————————————————————————————————————————— Energy summary Boiler fuel (106 Btu/hr HHV) 599 714 Purchased power (MW) 33.20 4.95 Estimated total installed cost (106 $) 57.6 74.8 Annual operating costs (106 $) Fuel and limestone at $2/106 Btu 10.1 12.0 Purchased power at 3.5 cents/kWh 9.8 1.5 Operating labor 0.8 1.1 Maintenance 1.4 1.9 Makeup water 0.3 0.5 Total 22.4 17.0 Annual savings (106 $) Base 5.4 Gross payout period (yrs) Base 3.2 ——————————————————————————————————————————————————— Basis: (1) boiler efficiency is 87%; (2) operation equivalent to 8400 hr/yr at Table 7-3 conditions; (3) maintenance is 2.5% of the estimated total installed cost; (4) makeup water cost for case 1 is 80 cents/1000 gal greater than Base Case water costs; (5) stack gas scrubbing based on limestone system.