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I'm still unclear about the secondary burn process

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by wahoowad, Jan 18, 2006.

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  1. wahoowad

    wahoowad Minister of Fire

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    I have been mesmerized watching the flames through the door of my non-cat Jotul F3CB and have read everything I can find regarding exactly what is happening when I see the the little jets of secondary burn going on. Even here I have read many differing versions. This other site, http://www.chimneys.com, has the most logical explanation although not as thorough as I would like. The author says:

    I gather outside air enters my baffles via a secondary air port I see on the rear of the stove. This means air is exiting my baffles all the time due to the ongoing combustion process. I only see jets of combustion around these baffle holes when I have a full firebox of flames that reach up there. It is as if the flames themselves ignite the combustible gases even though it is suggested that superheated air ignites combustibles gases. I do not ever see a secondary burn take place unless flames are already up there. I would expect to see jets of secondary combustion happen whether flames were tickling up there or not since the gases should be present and the hot air is continually exiting these holes. I would expect to see jets of secondary combustion happen even more so when the flames are not actively fanning as this is when more smoke/gases are escaping.

    I feel like all I am seeing are hot jets of air exiting the baffles and creating a vortex through the flames that are present. I feel I should see spontaneous jets of combustion once the firebox is hot and during the first half of burning my logs when most gases are released.

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  2. Sandor

    Sandor Minister of Fire

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    Oh, the benefits of catalytic combustors....

    You will not see the combustion in the first half of burning your logs because the temp is not high enough to "light off" the second phase of combustion.

    This is the whole argument around catalytic versus non.

    Your observations are identical to what happended with my Regency stove. My new Woodstock with the cat behaves much differently.
  3. Eric Johnson

    Eric Johnson Mod Emeritus

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    I don't doubt what you say, Sandor, but how can you get a clean burn without secondary combustion? Shoot, even my old boiler burns clean during the second half of the burn cycle.
  4. wahoowad

    wahoowad Minister of Fire

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    Sandor, i am trying to learn and certainly am not one to dispute much of anything anybody else has to say, but I have seen my 'vortex' activity from the baffles rather early on in starting a new fire. Especially when I put a hot load of kiln dried kindling in their.
  5. wg_bent

    wg_bent Minister of Fire

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    I think I might be willing to dispute what he said.....but Dispute is probably too strong a word.

    If I have a very hot fire and put a log on, I almost instantly get secondary combustion. You need to look around the web for the stages of wood combustion. The logs immediately give up their gasses in the first phase (I think it's first phase) once the moisture is driven off. If too much moisture is present, that will extinguish the secondary combustion due to the cooling effect of the moisture.

    Catalytic combustors will actually work the same way. If they're not hot enough, there is no burning and a less efficient fire. With them you have the added complexity of needing to get the fire roaring hot in order to light them off. With a non-cat, the whole process takes off when things get hot enough and no cat is damaged if you don't do it right.

    The whole thing of a efficient burn is having things hot enough to burn the smoke. Consider a Tulikivi (or other properly designed masonry heater). They have neither secondary burn air nor a cat. They simply work by having very high combustion chamber temps of up to 1800 degrees

    The way to getting a vary fast secondary burn is the top down method of building a fire if you can get it to work. I've not had good success, but have with a modified approach. Still the largest logs are at the bottom of the fire. The real goal is a very hot firebox in order to ignite the smoke.

    I'm sort of rambling here...I hope somethign I said helped.
  6. Sandor

    Sandor Minister of Fire

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    Well Warren, we do agree.

    In a very small nutshell, the cat causes a chemical process that enables unburnt compounds, from the wood, to ignite at about 500 degrees. Without this process, it happens around 1100 degrees. Thats why a VC Resolute uses a "space age" material, to hold alot of heat, to kick off the secondary burn process. The Regency I used for a decade, used air tubs, on top of the firebox, to introduce extra air to complete the burn process. The top of the firebox, is the hottest part.

    So, the secondary burn is dependent of the temp of the firebox, in either case. Just so happens, that the process happens at a much lower temp with the cat.

    Eric,

    I get a "clean" burn all the time without EVER engaging my combustor. I do this when I'm burning pine. I do not engage the cat while burning soft-woods because the ash would clog the cat, pretty quick.

    On other threads, I have stated I burn alot of pine. (just cleared a lot and its everywhere) From my experience, with a firebox temp of about 900-1000 degrees, I see NO VISIBLE smoke. Does that mean its squeeky clean. I'm sure not. But most of the nasties are consumed before it hits the flue.

    Obviously, manufacturers use different methods to get a clean burn.
  7. Rhone

    Rhone Minister of Fire

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    The secondary burn in my unit is like yours, it needs flames to be rising up to it. Without a flame, it won't ignite. I'm sure you've witnessed the shooting flames, the ones that go in bursts to the top. If I get one, it will light off my secondary burn, and it will sustain itself for about 10 seconds and then go out until a new flame shoots up to light it again.

    It matters most in the beginning of a fire, usually within the first few hours. Eventually the secondary burn starts happening at the logs, I see blue flames off them and no secondary burn action going on, even though it is definetely up to temperature and flames hitting it. Seems only useful when you're first getting a fire going or after a reload before the new wood is up to speed.

    The Cat vs. non-cat thing, if starting a fire from scratch, if you can do the top down method, with secondary burn, from a cold start it simply rules. Within 15 minutes of using the top down method my secondary burn is in full swing. The flames are at the top of the logs working their way down also maximizing the secondary burn. By the time the logs up top are burned and flames aren't reaching the secondary burn it doesn't matter, you're beyond the time the secondary burn is taking place up top. If you can learn and do the top down method with a secondary burn, it really kicks you in turbo.

    A cats particulates gets worse each year. That's why a cat stove has more strict standards to test to, as they're compensating for it slowly getting worse. A cat is also normally deemed more efficient because it engages sooner. From my research both when burning the secondary gases are equal. I think, using the top-down method makes a non-cat more efficient than a cat. From a cold start, using the top down method in 15 minutes my secondary burn is in full swing, which is keeping the top logs burning and they're feeding the secondary burn. With a cat and a cold start, they're not warmed up within 15 minutes and from then on losing ground to a non-cat. Something I've pondered anyway. Using the bottom up method with a cold start I think a cat starts sooner as it can sometimes be an hour before my secondary burn starts up using that method. When I reload, my secondary burn usually starts up as soon as I shut the door. Anyway, try the top down method with a secondary burn, if you can do it, and it's not easy, it's amazing how fast things go. But, you must have dry wood. The one thing I envy of a cat is because it ignites at lower temperatures they normally can have lower air settings than a non-cat which means longer burn times. The one thing I don't like about the cat, you have to wait until you can engage it (babysit). In the end to each there own, no winners or losers.
  8. Sandor

    Sandor Minister of Fire

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    I agree, Rhonemas. Nicely said.
  9. babalu87

    babalu87 New Member

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    I put seasoned wood in my stove, it burns clean
    I put damp wood in my stove it burns with a bit of smoke
    I put wood in my stove it heats our house
  10. DavidV

    DavidV New Member

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    Currently burning Cat stove. Heats the house. Next stove probably a non cat. Because I won't have to spend so much time with it. My total stove investment at this point is less than 400 dollars. Yes I have other wood related expenses. Saw, gas, splitter, axe, maul. yada yada.
    I can't groan about he cost of the stove though and since I saved enough the first year to cover the cost of EVERYTHING related to my wood heat, I would have to say it's been worth it. Plus there are added benifits you can't account for in money savings.
  11. wahoowad

    wahoowad Minister of Fire

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    I guess I was hoping someone could confirm or deny the process I proposed where I state tehre is actually an airflow path through the secondary burn baffles. This would mean there is airflow out each hole whether flames are kissing up there or not. If that secondary air inlet on the back of my stove actually feeds air into my burn baffles, then this certainly is the case.

    That aside, I experimented last night with a choked down fire. Starting with a cold stove, I created a bed of kindling coals. Then I packed the stove and let it burn hot for 30 minutes. Then I choked it down (the best an EPA stove will let you go) and promised myself I would not add any air. The fire pretty much smoldered and I got some smoke on my glass. Then, 45 minutes into this load I got a LOT of secondary burn out of nowhere. It was awesome to watch - really looked liked these burn tubes were blowing fire. The stove temp shot up about 150 degrees. After a half hour of this things slowed back down and I didn't see hardly any actual flames for the rest of the night. I went to bed after 4 hours of this choked down burn - not sure how long the logs lasted although it was a lot longer than my previous hotter burns with more air.

    I'm not happy with the choked down smoldering fire I got - I'm paranoid about creosote buildup. I will have to find a 10 - 25% open setting and just accept a shorter burn time. Thankfully I am not heating my whole house although I am still striving for the most efficient use of my wood as possible.
  12. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Ok Wahoo here is how it works on the F3 CB. The top baffle on the stove is a two piece arrangement. The top part of it is cast iron and the bottom part (the top of the firebox) is stainless steel. There is about an inch of space between them. Secondary air is in fact drawn in through the opening on the back under the heat shield and travels between the cast iron top and stainless bottom. The secondary air is heated by the fire burning in the firebox under the stainless steel. The stainless steel has two rows of air-holes. One row at the top back of the firebox and the other row at the top front. Secondary burn does not occur in the baffle and fire does not shoot out of the holes. What happens is the heated air exiting from the holes in the baffle causes the unburned gases to ignite when it makes contact with them and creates ideal conditions for combustion. This happens in front of the holes and that makes it look like fire is coming out of them. The gases would have burned earlier but there are just too many for the air and the flame in the firebox to ignite so they get taken care of on their way up to the flue by the secondary burn air mixing with them. The stove has to be at around four our five hundred degrees for the heat in the firebox to ignite the mixture at the top when it comes together.

    To ideally get a choked down burn takes really well seasoned wood. If it isn't it will just smolder after you damp it down and then there is nothing heating the box enough to continue teriary burn.
  13. Nokoni

    Nokoni New Member

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    Thank you Brother Bart! That is a very clear explanation. I wondered if flames were coming out of the holes or just air pushing the flame like you said. I'm learning more about this stove every day.
  14. wahoowad

    wahoowad Minister of Fire

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    Brotherbart,

    In general, I concur with your description. Except I personally do not believe the hot air exiting the baffles is alone enough to ignite the unburnt gases. I believe it also requires a flame to be close enough. All the baffle air is providing is oxygen, even if it is heated oxygen. This mixes with the unburnt gases, expands it nicely and the flames touch it off. Otherwise I think there would be more spontaneous combustion happening up there - it would look like a gas grill burner. Instead it is rather random and kicked off by an errant (or sustained) flame reaching up there.

    Either way I appreciate you confirming the air flow paths and such.
  15. wg_bent

    wg_bent Minister of Fire

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    Depending on the stage of the burn..when the wood is giving off it's gasses, (vs the coal stage) my stove will have a continuous rolling ball of flame that appears to be coming from the secondary burn tubes. It doesn't appear to need flame to touch it off. Once it's going, the flame just continues until the wood exhausts it's gas components.

    I do believe that hot air is enough to light the gas. I'm not sure what the light off temp is but if the hot air is in the 1000 degree range, I would not be surprised if thats hot enough to ignite the smoke.
  16. BrotherBart

    BrotherBart Hearth.com LLC Mid-Atlantic Division Staff Member

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    Re-read the part about the heat in the firebox causing the ignition. I said that the air mixes with the gases and causes a combusbible mixture.

    That mixture is then lit off by either flame from the firebox or the temp in the firebox. Secondary combustion can continue for several minutes without a bit of flame in the stove if the stove is hot enough.
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