HRSG duct firing

[sv13]

Hi,
I'm designing a new HRSG with duct firing. As the duct firing is very heavy, I was wondering:

1. what is the best location for the duct burner in the HRSG (before or after HPSH)

2. if there are any limitation as to the extent of firing.

3. I would prefer not having to use P91, so what is the maximum gas temp I can have after duct firing with T11 or T22 tubes. What drives the material selection - gas or water/steam temperature?
Any ideas?

 

[davefitz]

Now I think I have a clue as to the problems we've been having with our HRGS's lately.

 

[sv13]

Now that's not the answer I was expecting. What is the problem with your HRSG?

 

[davefitz]

sv13;
The best place for the duct burner is determined by optimizing the configuration , by means of running dozens of simulations on the combined cycle simulation programs. Some will size the HRSG and provide acost estimate for each configuration.

If you don't know where to start, usually finding some other jobs already sold will point you in the rigth direction .

Some things to keep in mind:
The modern duct burners now come supplied with a radiation screen to protect the upstream bank, so it is OK to locate the DB immediately downstream of a superheater or reheater bank. The next downstream bank, though ,will see a lot of heat not only form a high fluegas temperature but also from flame radiation- you may need to deliberately add this to the program if you want to accurately simulate what happens.

A smart idea is to locate a boiler banks screen immediately downstream to act as the downstream rdiation shield . But if that is not practical, then ensure the downstream row of tubes includes the effect of the flame radiation. Keep the tube metal temperature within ASME allowables and the fin temperature below oxidation limits.

 

[MisterDonut]

sv13,

I agree with davefitz, and add the following: duct firing temperature is also limited by the construction firing duct casing. The temperature limits of the inside casing materials is one factor. The outer skin temperature is also a factor, usually for safety reasons. High firing temperatures (rule of thumb: 1500ºF or higher) can warrant a water-wall design.

 

[Guest102023]

Not only the location of duct burners but their operation is of importance.