Smokeless Flare 2

[memo94]

Hello,

Can anyone please provide me some information regarding the standard references that can be used for smokeless flare design?

I am to verfiy a smokeless flare that is working at 30% of the designed flowrate and both of the Air Blower are running (while it was designed to be 1 running / 1 standby) but still we have a black smoky flame.

I have ckecked the Flared Gas composition analysis, current gas is lighter than the design basis gas composition.
I have calculated the theoritical air required for smokeless flame (complete combustion) and I found that the theoritical air needed is 124000 cubic meter/hour while both Air Blower are supplying 88000 cubic meter/hour.

I know that the flame is in open atmosphere and some air is supplied from surronding atmosphere, but I do NOT know in the design of Smokeless flare how much percentage of theoritical air is to be supplied by air blowers!

Your replies is appreciated in advance.



Regards,
Mehran

 

[ash9144]

Looks like you've already identified your problem. Not enough air flow. Fix the blowers.

 

[memo94]

Thanks for your answer. But do you know any standard for design of smokless flare?

Do have any idea about how much of the required air is to be supplied by air Blowers?



Tnahk you!

 

[Montemayor]

I suggest you go to:

http://www.geocities.com/flareman_xs/

or pray and wait for David Shore (a.k.a. "Flareman") to jump on this thread. If anyone does, he certainly has your answers.

 

[25362]

Smoke is produced by cooled down carbon particles in fuel-rich systems. One procedure to suppress smoke, beside premixing fuel and air, is by the addition of steam. Another I've been told about is to distribute the raw gases through a number of small burners.

One explanation (not necessarily the correct one) for the improved efficiency with steam addition is that steam reacts with carbon particles forming CO, CO[sub]2[/sub], and hydrogen, thereby removing the carbon which forms smoke after cooling.

 

[Flareman]

Memo94
I must have been getting vibes from Montemayor because I just signed on today. Thanks to him for the nice remarks but it's always possible that I'll shoot myself in the foot if your flare comes from my employer.

Anyway, my take on this is that "it all depends".

First of all, smoke is produced as a function of gas composition. Hydrocarbons form carbon when they "crack" in the flame. For stable hydrocarbons like single chain alkanes this needs a temperature between 300 to 600 degC. Unstable hydrocarbons and isomers crack almost immediately and fall quickly into carbon.

Air assisted flares try to put enough blown air close enough to the gas flow that the velocities mix the blown air with some atmospheric air as well as the gas so that you don't need a stoichiometric amount.
If you need a rule-of-thumb (something which I'm not too fond of) you can say that natural gas might start at about 25% to 30% stoichiometric, running through to 90% to 100% for ethylene. Interpolate between the two extremes as a function of net stability for the mixture composition.

Then there's a matter of what the inert content is, because the inert suppresses the flame temperature and reduces the rate of carbon formation. If you have a lot of nitrogen, you can take a little extra credit. CO2 counts as 1.82 x nitrogen. If you have water in there, it's even better (2x N2) because you get -OH which is great for smoke suppression becuase it promotes -CHO radicals which are gaseous as opposed to -C which are solid.

Then there's the specific design of flare and the total flame size which all factor into the considerations.

You seem to have 75% of theoretical air right now and making smoke, which suggets to me that you have a high olefinic or aromatic composition.
You could also have a damaged tip. Has it always been the same or is this new?

How easy is it for you to "tweak" the air flow? Many air assisted flares use vaneaxial blowers which can be reset with a different blade setting. Centrifugal blowers may need a speed increase.
That brings me to another issue. Check the actual back pressure against the fan curves. If the fans were designed for one only in service, you may not be getting 2 x the flow with 2 running. Fans tend to be self unloading and selected for a specific flow and back pressure. Twice the flow would need 4 x the pressure and 8 times the horsepower. You should cross check the total current draw against the designed HP to see where you fall on the real operating curve.

Just things to be thinking about. Feel free to ask again but if you get into commercially sensitive issues which may not be appropriate here, contact me though the web page noted above.

David

 

[25362]

Although my experience with flares is negligible, I pressume we can analyze the situation from a combustion process viewpoint.

If I interpreted rightly flareman's last message (for which he merits a star) it is a high hydrogen concentration, either originally present (memo94) or produced by cracking (flareman), that may be the cause, since hydrogen reacts much faster than carbon. If carbon particles aren't burned away they cool down and form smoke.

Thus, either the hydrogen atom concentration is reduced or sufficient oxygen is provided for uniform burning and complete combustion.

It appears that added steam acts as a diluent of hydrogen molecules, minimizes polymerization reactions and forms oxygen compounds which burn at reduced temperatures. The net effect is to prevent cracking.

Flareman's teaching comments are appreciated.

 

[memo94]

Flareman!

Many thanks indeed.
I will go tru these helpful comments and will get back to you if I found something.


25362!

Thank you too.

Mehran