Questions about secondary flames.

[warno]

I've been trying different things all season to get my home built OWB to burn more cleanly. With today being my last fire I think I'm on to something, finally.

I'm running a second intake tube off my induction fan that pushes air through my fire grate, which is made of stainless steel pipe, and up to the top of my fire box Trying to get secondary flames to fire off. I think today I finally got it to happen. I took a video of it and would like if someone could tell me if I'm on to something.

Notice the horizontal pipe near the top of my fire box. It's kind of hard to see but There's a flame that comes from the holes every once in awhile. Is that secondary flame?

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I haven't had a fire in over a week so my water was 58 degrees when I started. I lite my fire like normal and my stack smoked pretty good for about 10-15 minutes. I got busy doing something else in that time and when I looked back at my chimney there was nothing coming out. I thought my fire went out but to my surprise it was still burning like crazy. Here's 2 videos of my chimney

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So am I doing better for burning an OWB? Next year with my storage install should be having a clean chimney discharge more often then not I'm thinking.

 

[brenndatomu]

So the video is with the door open, no? On most any EPA firebox the "secondary" burn is interrupted when the door is open, it is gonna be hard to tell how things normally work unless you have a window...or a viewing port. Many industrial boilers have a small (1.5"-2") round port/window that you can peek through to view the fire. Some have a teardrop shaped metal (SS?) cover on the inside that normally covers the port to help keep the glass clean, it can be flipped/turned up when you want to see...

 

[warno]

Yeah that's with door open. So secondary flame doesn't even work alittle with an open door?

 

[brenndatomu]

The draw of the chimney needs to pull air through the secondary air supply tube...with the door open the path of least resistance is through the door...

 

[warno]

So the secondary tubes are just natural draft? Even in a forced air firebox? I was pushing air up through there with my fan.

 

[brenndatomu]

Ahh, OK, yeah if it is being forced than that is different. Is all incoming air blocked off when there is no call for heat? If not then maybe that secondary tube could remain open allowing the smoke from the smoldering wood to burn off...that's more or less what they are doing on the EPA stoves...once the firebox is hot and the wood is charred, the primary air (air at the bottom) is all but cut off, but the secondary air remains open allowing the wood gas to fire off. It makes a TON of heat...might be better to have some storage capacity to run this way so as not t0 boil over

 

[warno]

I'm adding storage for next year, I'm just trying to get this thing to burn cleaner. I didn't think it would be as bad as it was, I was honestly embarrassed by the amount of smoke it produced.

I've got one other idea to try but its going to take a bunch of fabrication to get it done. I'm thinking about putting a vertical plate blocking the exit of the box to the flue. Then build a tunnel above the fire that will go through vertical plate and run preheated air up into the tunnel to help the burn. My thinking is that with the flue gases directed through this tunnel they should mix with the air and burn off because the gases won't be in continuous contact with the cold water jacket walls.

Does this sound like it might work?

 

[brenndatomu]

Does this sound like it might work?

I can't exactly envision the details of it all, but yeah, sounds like there's possibilitys here.
Might be as simple as running a couple pipes from the front to the back, (through the coals) then up, and back toward the front across the ceiling...I'm assuming you have some sort of baffle on the exhaust already?
You make want to search the "pre EPA" stove forum here...there are lots of ideas there on adding secondary air to old school fireboxes...I've seen some that were really well done.
Just FYI, you can use black iron pipe for temporary testing but you'll need to go stainless when you settle on a permanent design...the BIP droops pretty quick once it gets red hot...and that happens fast when the wood gasses fire up!

 

[warno]

I'll look into that. And stainless was for sure my material of choice. That's what my fire grate is made of now.

 

[maple1]

You'll likely be hard pressed to get much secondary burning happening in a water jacketed boiler. The firebox is in essence liquid cooled - big difference from getting secondary burning happening in a stove. That's why gasifier boilers do their secondary combusting in a second refractory lined burn chamber.

 

[Fred61]

The atmosphere is just too cold at that point to have secondary ignition. It's called secondary combustion but you need ignition before you can have combustion. Sometimes we use the words interchangeably.

 

[leon]

If you fill the boiler half full of firebrick like I did with mine
you will have little to no smoke to worry about and you
will not need an "Afterburner Chamber for secondary
combustion as the thermal mass of the firebrick constantly
adds back heat to keep the fire hot and the gasses burned off.

 

[warno]

My firebox is lined half way up with brick. And it still smokes a good amount.

So even if I duct my gasses through a tunnel that's not surrounded by water it won't get hot enough in that tunnel to burn anything?

 

[maple1]

If you can line the tunnel with some refractory, & get the right amount of fresh air to it - you might get some decent secondary action then. But then you'll have a catch-22 of getting the heat to transfer to the water before it exits the smoke pipe. Most gasifiers use heat exchange tubes running through the water jacket to accomplish that, between the secondary burn chamber/tunnel, and the smoke pipe exit. Which I guess is then why most have the primary burn chamber in a downdraft configuration - smoke goes down thru the coal bed to refractory lined secondary burn chamber (passing through the coal bed greatly helps the secondary burn - kind of like atomizing the fuel mix in a carb throat in a gas engine but even better with added heat), then the hot burned gasses can turn & go up through heat exchange tubes to the smoke pipe exit.

 

[TCaldwell]

It might get hot enough however a refractory lined chamber will protect the steel underneath.

 

[Fred61]

Just dreaming here because I've reached my point of incompetence on this.

Your combustion air is now routed over the fire. How about some means of forcing some air under or in the fire where the heat is high enough to burn the smoke. It's my understanding that the red glow on the burning charcoal is about 1800 degrees. Enough to ignite the gasses.

 

[warno]

My boiler has a heat exchanger built into it that is above the fire box before the flue exit. so if I could get the gases to burn off it would be just heat going through that exchanger into my water.

Here's a side shot of my boiler in build stages. Notice the HX above the fire box.



My air is ducted both above and under the fire.

 

[leon]

My firebox is lined half way up with brick. And it still smokes a good amount.

So even if I duct my gasses through a tunnel that's not surrounded by water it won't get hot enough in that tunnel to burn anything?

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Unless you build it like an incinerator with a long path fire brick after burner your wasting your time.

This goes along with why Russian and Finnish stoves are built the way they are and burn so cleanly- they have one long passageway that rises and drops then runs flat and rises then drops and then runs flat and then rises and then drops and then exits to the chimney. At the bottom of each vertical passageway there is a clean out for ashes and soot.

Fill the firebox volume half full of fire brick up to the breach and you will have a hot fire and the fire brick will absorb heat and shed it back into the firebox as well as the water jacket. The hot fire brick will help the draft too.

 

[warno]

Only the top half of my firebox is surrounded by water the jacket doesn't cover the bottom at all. I put firebrick in the button half way up To protect the non-submerged portion of the box and also to help with heat.