Page 147 - Handbook of Energy Engineering Calculations

For this plant, each 100-million Btu/h (29.3 MW) of added supplementary
firing capacity increases the net plant output by 5.5 percent, but increases the
heat rate by 2 percent. The installed cost for supplementary firing can be
significant because all the following equipment is affected: (1) boiler feed
pumps, (2) condensate pumps, (3) steam turbine/generator, (4) steam and

water piping and valves, and (5) selective-catalytic reduction (SCR) system.
Thus, a plant designed for supplementary firing to meet peak-load
requirements will operate in an inefficient, off-design condition for most of

the year.

7. Compare the options studied and evaluate results
Comparing the results in Table 1 shows that mechanical chilling, Case 2,
gives the largest increase in plant output for the least penalty on plant heat
rate—i.e., 18.1 MW output for a net heat rate increase of 55 Btu/kWh (52.3

kJ/kWh). However, this option has the highest estimated installed cost ($3-
million), and has a relatively high incremental installed cost.
Water injection, Case 5 Table 1, has the dual advantage of high added net
output and low installed cost for plants already equipped with water-injection

skids for NO control during distillate-fuel firing. Steam injection. Case 4
x
Table 1, has a significantly higher installed cost because of water-treatment
requirements.

Supplementary firing. Cases 6 and 7 Table 1, proves to be more acceptable
for plants requiring extended periods of increased output, not just seasonal
peaking.

This calculation procedure is the work of M. Boswell, R. Tawney, and R.
Narula, all of Bechtel Corporation, as reported in Power magazine, where it
was edited by Steven Collins. SI values were added by the editor of this
handbook.

Related Calculations. Use of gas turbines for expanding plant capacity or for

repowering older stations is a popular option today. GT capacity can be
installed quickly and economically, compared to conventional steam turbines
and boilers. Further, the GT is environmentally acceptable in most areas. So
long as there is a supply of combustible gas, the GT is aviable alternative that

should be considered in all plant expansion and repowering today, and
especially where environmental conditions are critical.