Page 159 - Boiler_Operators_Handbook,_Second_Edition

144 Boiler Operator’s Handbook

or steam. The temperature profile of a typical HRSG is will normally be lower than design with the duct burner
shown in Figure 4-14 and pinch points are indicated. out of service. Since the duct burner is in a stream of hot
Since the HRSG is designed for these operating con- gas with 400% excess air the only air added to it may be
ditions there are situations during startup and shutdown some for the ignitor. Being a radiant source in an other-
that require special control, especially to prevent steam- wise simply convection flow the duct burner principally
ing in the economizers which would result in water ham- alters the superheat. The mass flow through the HRSG
mer in them along with potential damage to the econo- isn’t altered as much by simple addition of fuel so the ef-
mizers, feedwater control valves, and the drum internals. fect on steam generation isn’t as significant as increasing
At times it’s necessary to maintain flow through the firing of a conventional boiler.
economizers by returning some of the economizer outlet The literature (Power Magazine among others) is
water to the deaerator. High pressure in the deaerator is constantly revealing new techniques and features of com-
normally prevented by a relief valve and vacuum by a bined cycle plant operation and their use is increasing
vacuum breaker. Return of some feedwater to the deaera- rapidly. It’s not just the availability of natural gas, it’s also
tor from the low pressure economizer can also be used to the fact that these plants are more efficient, converting
help stabilize the deaerator pressure. more of the energy in the fuel to electricity than conven-
The duct burner is used primarily to control super- tional boiler plants. That’s more than 40% and almost
heat temperature and the HRSG is designed so superheat 50% conversion efficiency.

Figure 4-14. HRSG PTV profile N ### needs values