Assist Gas for Flares

[process440]

How can I estimate the flowrate of assist gas (a gas with a higher heating value such as propane or natural gas required for complete combustion of a waste gas stream going to a flare? I have seen guidelines on assist steam for smokeless operation, but I have not run into guidelines for assist gas.
Any information would be greatly appreciated.

 

[TD2K]

I worked on one flare that used assist gas (horizontal ground flare) that was essentially heavy NG, maybe 19 MW. The assist gas max design flow was 10 MMscfd, I believe the smokeless capacity of the tip was about 40 MMscfd from the vendor which would be about 0.25 lbs assist gas/lb flare gas.

This was off an oil processing plant. Flareman likely has the best information for you. What type of gas are you flaring?

 

[process440]

Diesel vapors

 

[juw]

It really depends on the reason for adding gas. In flares its normally supporting combustion of a low HV gas or providing assistance to achieve smokeless flaring. A good guide line is to use 300 Btu/Scf for combustion support for low HV gases to ensure stable combustion. For complete combustion try and aim for 600 Btu/scf but at the same time try and flare at a higher velocity, the closer to 0.5 Mach the better as the air ingress in to the flame at this exit velocity helps combustion also.
I seem to remember that API 521 does give some guidance on the issue although not alot.

 

[MJCronin]

440,

The John Zink company has several "white papers" on designing and specifying flare sytems. Have you rviewed these ?

Try this link...

http://www.johnzink.com/products/flares/html/flar_sel_guide.htm

Let us know what you decide..


MJC

 

[SooCS]

Flaring diesel, you must be boiling off diesel at high temperature. Won't the diesel heating value be in the thousand of btu per scf. I am not a flare specialist but I believe you will need air to assist burning rather then natural gas and definitely not propane.