Page 146 - Handbook of Energy Engineering Calculations

unit output by 21.8 MS, Case 4 Table 1, but decreases the steam
turbine/generator’s output by about 12.8 MW. Net gain to the CC is 8.4 MW.
But CC plant heat rate also suffers by 4 percent, or 270 Btu/kWh (256.5
kJ/kWh).
Because the steam-injection system requires makeup water as pure as

boiler feedwater, some means to treat up to 350 gal/min (22.1 L/s) of
additional water is necessary. A dual-train demineralizer this size could cost
up to $1.5-million. However, treated water could also be bought from a third

party and stored. Or portable treatment equipment could be rented during
peak periods to reduce capital costs. For the latter case, the average expected
cost for raw and treated water is about $130/h of operation.
This analysis assumes that steam- or water-injection equipment is already
in place for NO , control during distillate-fuel firing. Thus, no additional
x
capital cost is incurred.
When water injection is used for power augmentation or NO control, the
x
recommended water quality may be no more than filtered raw water in some
cases, provided the source meets pH, turbidity, and hardness requirements.
Thus, water-treatment costs may be negligible. Water injection. Case 5 Table

1, can increase the GT output by 15.5 MW.
In Case 5, the bottoming cycle benefits from increased GT-exhaust mass
flow, increasing steam turbine/generator output by about 3.7 MW. Overall,

the CC output increases by 9.4 percent or 19 MW, but the net plant heat rate
suffers by 6.4 percent, or 435 Btu/kWh (413.3 kJ/kWh). Given the higher
increase in the net plant heat rate and lower operating expenses, water
injection is preferred over steam injection in this case.

6. Evaluate supplementary-fired HRSG for this plant

The amount of excess O in a GT exhaust gas generally permits the efficient
2
firing of gaseous and liquid fuels upstream of the HRSG, thereby increasing

the output from the steam bottoming cycle. For this study, two types of
supplementary firing are considered—(1) partial supplementary firing, Case 6
Table 1, and (2) full supplementary firing. Case 7 Table 1.
There are three main drawbacks to supplementary firing for peak power
enhancement, including 910 lower cycle efficiency, (2) higher NO and CO
x
emissions, (3) higher costs for the larger plant equipment required.