What is a secondary burn?

[bvpbill]

Hi all:
Newbie here, but not new to burning wood. Been using wood for years.
I would like to know what the secondary burn is and secondary burn tubes.
Not familiar with the terms.
Thanks,
Bill

 

[Jags]

Secondary burn is the "re-burning" of the off gasses that occur during the pre-coaling stages of burning wood.

Burn tubes is one of 2 methods to generate this secondary burn. It is basically a set of tubes at the top of the burn box that gets "super" heated during the burn process. Additional air is feed to these tubes (which have holes in them). When the smoke meets super heated burn tubes with the extra oxygen being pumped in at the same point.....whooof. You got fire. It burns the nasties up (fuel), reduces emissions, and creates a really cool light show at the top of the burn box.

The other method is a catalytic reactor. Does the same thing, just a different way.

 

[bvpbill]

Thanks Jags. Makes sense to me now.
Bill

 

[Jags]

Thanks Jags. Makes sense to me now.
Bill

Good, cuz I wasn't too sure it made sense to ME!

 

[EddyKilowatt]

During the first phase of burning (first two phases, to some degree), wood will boil off burnable gases, and smoke particles, faster than there is oxygen available in the primary air to burn them. Increasing the primary air just increases the primary fire and the boil-off rate, without doing much to promote additional burning of the burnable gasses.

There are two things (at least) that can help burn this boiled-off stuff. The first is to keep it as hot as possible for as long as possible, allowing more of the burn reaction to finish. Thus you see stoves lined with brick and baffles to hold in heat, and designed with long twisting smoke paths and slow gas flow rates to allow a complete burn. The second thing that helps burn the boiled-off stuff is to mix some more air in with it, away from the primary fire, to provide more oxygen that helps the unburned gases to burn. Thus you see secondary air tubes that preheat outside air and mix it in with the hot gases leaving the primary burn zone.

You'll know when the first of these things is happening when you see long pale flames dancing in slow-motion above the wood that is burning in your stove. You'll know that the second -- secondary burn -- is happening when your secondary air tubes look like gas jets with flames shooting out of them... they'll actually look a lot like the burner tube in a gas oven, except the flames will probably be yellow.

Eddy

 

[Thomask9590]

;-) Good explanation thanks.

 

[johnnywarm]

During the first phase of burning (first two phases, to some degree), wood will boil off burnable gases, and smoke particles, faster than there is oxygen available in the primary air to burn them. Increasing the primary air just increases the primary fire and the boil-off rate, without doing much to promote additional burning of the burnable gasses.

There are two things (at least) that can help burn this boiled-off stuff. The first is to keep it as hot as possible for as long as possible, allowing more of the burn reaction to finish. Thus you see stoves lined with brick and baffles to hold in heat, and designed with long twisting smoke paths and slow gas flow rates to allow a complete burn. The second thing that helps burn the boiled-off stuff is to mix some more air in with it, away from the primary fire, to provide more oxygen that helps the unburned gases to burn. Thus you see secondary air tubes that preheat outside air and mix it in with the hot gases leaving the primary burn zone.

You'll know when the first of these things is happening when you see long pale flames dancing in slow-motion above the wood that is burning in your stove. You'll know that the second -- secondary burn -- is happening when your secondary air tubes look like gas jets with flames shooting out of them... they'll actually look a lot like the burner tube in a gas oven, except the flames will probably be yellow.

Eddy

Eddy
Are you saying when you put new wood in you should have the air inlet open all the way??? If so when do you turn the stove down??

Thanks John

 

[EddyKilowatt]

Eddy
Are you saying when you put new wood in you should have the air inlet open all the way??? If so when do you turn the stove down??

Thanks John

Um, I wasn't actually saying that, but... that in fact is what most people do. Start wide open, to get the primary burn going and the wood heated up, then turn it down in stages over a 20-30 minute period. Secondary burn will phase in as the firebox heats up, and as you reduce the primary air (which on most stoves increases draft thru the secondary tubes).

Eddy

 

[johnnywarm]

Eddy
Are you saying when you put new wood in you should have the air inlet open all the way??? If so when do you turn the stove down??

Thanks John

Um, I wasn't actually saying that, but... that in fact is what most people do. Start wide open, to get the primary burn going and the wood heated up, then turn it down in stages over a 20-30 minute period. Secondary burn will phase in as the firebox heats up, and as you reduce the primary air (which on most stoves increases draft thru the secondary tubes).

Eddy

Thanks again Ed

 

[RedRanger]

A secondary burn is slightly less painful than a third degree burn :lol:

 

[cmonSTART]

I thought it was the burn you got the second time, after you didn't learn from the first.

 

[struggle]

Here is a picture of my stove in secondary burn range, the tubes are the ones with holes in them that introduce the air for secondary burn/combustion

If you think about it there is actually quite a bit of air that comes into the stove during secondary burn as my box has four rows of tubes and drilled with holes all the way across. I bet you get more air in secondary burn than you might with primary air or at least darn close.

 

[fossil]

https://www.hearth.com/econtent/index.php/forums/viewthread/26984/

 

[Adios Pantalones]

Last night I rushed it a bit building a pine fire when I got home. The wife and I walked out the door to go to dodgeball (go Team Taco Dip! http://nhssc.com/teams/view/563 , http://www.youtube.com/watch?v=TNdW3AFg4BA ) and we both commented on the smoke.

I grabbed a reasonable white oak round and the wife said "That's going to fix it?"

"Yup" I said confidently, though I didn't really know- I mean a guy is supposed to know about fire.

It worked like a charm- zero smoke out the chimney after that. My firebox likes to be full, and when it is I get no smoke and good secondary action in very short order. No real heavy coal base required.

In my system- fire and unburned gases have to find their way to the top of the wood in the firebox, and the secondaries light off easily once the gases reach the tubes. I only really discovered how easy it was to make this happen this year, since I've been burning dry pine.

 

[SaratogaJJ]

Secondary burn will phase in...as you reduce the primary air (which on most stoves increases draft thru the secondary tubes).

I did not know that....good piece of information to have!

 

[EddyKilowatt]

"Most stoves" might've been overstating it a little... I'm no expert. But it is my understanding that "many" EPA stoves have fixed secondary air supply, i.e. no secondary air adjustment by the user. With primary air open, you pretty much short-circuit the secondary air. With primary air closed, full draft vacuum (such as it is) gets applied to the secondary system, drawing air thru those secondary tubes and holes.

Simple, reliable, and somewhat less likely to be mis-managed by the end user... at the cost of some efficiency during the coaling phase, and some loss of play value (i.e. levers and adjustments to fool around with) for compulsive Hearth.com members. :coolsmile:

Eddy

 

[xman23]

Cleaning burn tubes?

Since were on the subject of secondary burn tubes. Can anyone comment on how they may have cleaned the burn tubes and manifold feeding the air. I have never done this, so now after 8 years I decided to take a look at them. They must need cleaning by now.

On a friends stove, can't recall the brand I did see a chimney cleaning guy pull the tubes by grabbing them with a vice grip, rotating until he found a "T" slot then hammered to the side. If that doesn't make sense, I can explain more.

But that doesn't seam to work on my Jotel Oslo, I'm afraid to hit it to hard.

Any suggestion's?

Tom

 

[bfunk13]

I have been getting some good secondary burns lately.
Not to hijack the thread, but what kind of times can a secondary burn last?
I've been getting about 15 minutes or so. Like most people here, i am still burning small fires
at night and in the morning to heat the place a little. I suppose when it really gets cold, and am
having bigger hotter fires the secondary times will be longer?

Thanks
Brad

 

[chad3]

I'm also curious about cleaning the secondaries. Mine only have a white film on them that seems to easily come off with a quick wipe. Anything else I should try and do? Have good dry wood and will for the rest of the year. Second real year on full burns (first one didn't count as I was still learning), but the scares from out of control??? is now hopefully wiped out and am gaining very good control of the air, etc. Bring on the weather.
Chad

 

[DoubleClutch]

I grabbed a reasonable white oak round

There is nothing reasonable about burning white oak. Please stop.

 

[Shari]

Ummm... new person here, trying to understand 'secondary burn'....

So.... if secondary burn is going on and you have a top feed stove... then it is not a good time to open the lid and add more wood? The stoves I've seen have the burn tubes attached to the upper lid.

(Dummy question, I know..... but I am just trying to learn the ropes!)

Shari

 

[chad3]

Clutch,
I would hate to give you pics of my white oak. I've cut a fair amount of them in the 20" range in the past 6 months. I had a great score and an planning on thanking the lady for doing so. The last tree that I actually took down buried my 28" bar (red oak). Good wood to say the least.
Chad

 

[DoubleClutch]

What is your address Chad?

I am a boat builder, carry a 660 fueled with race gas and don't appreciate people screwing up my trees. ;-/

Just kidding...but you boys up north need to take better care of your white oaks, sir. You're all we have left.

 

[Jags]

Shari - I would suggest that you do not interrupt the secondary burn stage. It is typically the hottest stage of a burn, and you probably don't want to throw "additional" fuel on the fire at that point. Its not that you "can't", but if the secondaries are burning, why would you??

 

[bobforsaken]

I've been playing with my new (to me) stove. ('93 Quadrafire 3100f). Last night I gave it its first mostly full load after I got the fire really going (pine only). I had just gotten the flue thermometer between 350 and 400 degrees when I threw 3 or 4 more smallish logs on. The I closed the door and it started to roar... I almost though I had really goofed and started a chimney fire (which would be a feet since I've only burned 4-5 test fires into the new chimney) Nonetheless, I got worried and choked down the air as far down as I could. Suddenly the fire looked like the Northern Lights and the wood had now flames.. just a bright red glow as a light grayish purplish wave danced in the top of the stove.

The fire started to cool down over the next hour and then when the temp got to about 250 the waves went away completely with plenty of fuel left. I opened the air back up and the fire started raging again.

My question is.. should I be able to get a "secondary only" burn going until the fuel runs out, or did I accomplish the best I could. If I can get a secondary only burn going overnight, I'm sure I'd be able to keep the fire going for the 6 hours I sleep. As it was, with the air about midway, I still had coals in the morn, but the fire was effectively out midway through the night. Or should I not strive for "secondary only" as much as just dialing back the air enough to get a combo of flames off the logs and secondary flames in the top?