Hydrogen Flashback Prevention

[pamcinto]

If I have a bunsen-burner fed by a hydrogen gas source, how do I make sure that I don't get a flashback to my source?

Also, can anybody tell me or point me to a good reference describing how to size a pressure relief valve?

 

[EdDanzer]

Try a welding supply company.

 

[Flareman]

For the bunsen burner problem, you need extremely careful control over the air inflow and a jet sized for the specific fuel and pressure. If you are just substituting the hydrogen for a previous fuel, the jet may be giving the wrong flow of hydrogen.

Bunsen burners tend to be gas and pressure specific so you should probably make a theoretical analysis of what is happening, to point you in the direction you need to take for your specific case.

To get a flash back, the flame speed at the top of the pipe has to exceed the actual gas velocity. For hydrogen you can suppose a flame speed of about 10 fps when the mixture is stoichiometric reducing to zero (in a bell curve) at the LEL and UEL.

If the hydrogen plus the air exceed the UEL mixture, you are pre-disposed to a flashback any time the velocity is less than the relevant flame speed.

Because the UEL is so high (80% or thereabouts) = only a little air (1 air:4 gas)is needed to give a problem so you probably need to size the jet to give you something like 8 fps in the tube before you even think about opening the air control.
You might also be able to fool the burner by reducing the flame port opening with similar reasoning, if changing the jet is impractical. A flat wire mesh cover may push up the local velocity, but if it begins to get hot, it could itself act as a source of ignition for the gas coming up the tube.

Good luck
David

 

[butlen]

Smply buy a detonation flame arrester suitable for hydrogen and fit it in the line.

 

[j2bprometheus]

One system used on hydrogen-fueled internal combustion engines is shown on

http://www.hydrogencomponents.com/cvi.html.

This system minimizes how much hydrogen is mixed with air and thus minimizes how much is available to burn. Hydrogen-air mixtures burn quite rapidly over a wide range of air-fuel ratios, so be careful.

 

[nbucska]

Addition to J2B:
and the flame is FAST and invisible. A .1 mm torch
burned out my shirt from about 10 inches !

If you can't get flame arrester, you can make one
by filling a pipe with a bundle of .3mm copper wires about 2 inches long.




<nbucska@pcperipherals.com>

 

[25362]

If one intends to burn mixtures of methane (nat'l gas) and hydrogen, one should remember that because of the high reactivity of hydrogen the flame may become longer (!) This happens because hydrogen consumes oxygen depleting it in the less turbulent zones thus delaying the burning of methane, and the flame becomes longer.

A 50:50 mixture has a longer flame than methane alone.
An 85:15 (hydrogen:methane) mix has the same flame length as for methane alone.
Only at more than 85% hydrogen the flame shortens because of the &quot;speed of flame&quot; effect.

 

[Flareman]

25362
I'm interested in your last post.
I understand the argument philosophically but your information suggests that you have hard data. If so, is it published and can you give the reference? Either way, is this related to a diffusion flame or aerated flame?
I'm working on a diffusion flame model at the moment and would appreciate extra data to throw into the analysis.

David

 

[25362]

To Flareman. David, the data is from notes I took some 25 years ago, I must admit I didn't take down the source. If you visit Google you'll find, however, a series of articles dealing with fuel methane-hydrogen mixtures and flame stabilities.
The same old notes of mine mention that CO behaves similarly to hydrogen and also calls attention to a peculiarity of ammonia flames. It is that the length-to-diameter ratio increases with the mol % of ammonia. This seems to be related to the endothermal dissociation it undergoes before burning, a fact that reflects on ammonia's high self-ignition of 1,400 deg F.
I'm not completely sure after such a long time, but think those notes dealt with premixed (primary air) as well diffusion (secondary air) flames.
Sorry for not being able to be of help.

 

[Flareman]

25362
Thanks anyway.
I'll check out what Google will give me.

David

 

[25362]

to David (Flareman) eureka, I found it !
The source is the classical book Furnace Operations by Robert D. Reed - Gulf Publishing Company, copyright 1973, pp. 177+ under the heading Unusual Fuel Characteristics.

 

[Flareman]

25362
Thanks. Now I have it too. Unfortunately it still leaves my questions un-answered but isn't that the way of things?
I appreciate your studious pursuit of this on my behalf.


David

 

[25362]

To flareman, I've found a website that may be of interest to you:

http://imartinez.etsin.upm.es/bk3/c15/Combustion Kinetics.htm

Kindly comment.

 

[Flareman]

Wow!!
A lot of stuff there.
I'll have to grab a print-out and spend an evening or two slogging through it. Looks good at first glance.


David