Flames During Secondary Combustion

[jadm]

I have been searching through old threads and have found a lot about what secondary combustion looks like. I recall reading somewhere awhile ago that secondary combustion is taking place even if you can't see active flames coming out of the air tubes. I can't find that thread and am now wondering if I made it up.

Question One: If no flames are coming out of the air tubes does that mean there is no secondary combustion happening?

Question Two: If it doesn't matter if there are visible flames or not to indicate secondary burn taking place, is their an advantage to having the flames? (ie. Do they lengthen the burn time?

 

[bokehman]

Question One: If no flames are coming out of the air tubes does that mean there is no secondary combustion happening?

Secondary combustion is the burning of smoke. The air tubes supply super-heated air to the upper part of the combustion chamber and this air mixes with the smoke and burns. If the smoke is reaching the air tubes that is where the major part of the secondary combustion will take place, but, if the interface between the secondary air and the smoke is elsewhere in the firebox that is where secondary combustion will take place. This second form is normally sporadic secondary combustion later in the burn cycle.

Question Two: If it doesn't matter if there are visible flames or not to indicate secondary burn taking place, is their an advantage to having the flames? (ie. Do they lengthen the burn time?

If there is secondary combustion you will see flames somewhere in the firebox that are not connected to the firewood. Spontaneous (secondary) combustion occurs when smoke (unburnt fuel) mixes with air (oxidizer) at a temperature above 1100F. So for secondary combustion you need two things: smoke and heat. Anything that removes heat will destroy secondary combustion, for example, firewood that is not seasoned properly contains lots of water and this quenches the firebox. Also, too much primary air quenches the firebox. As for longer burn times the answer is no for any particular stove setting, but, because the secondary combustion produces heat you can burn the wood slower and still produce the same heat as a stove burning faster but with no secondary combustion.

 

[colebrookman]

bokehman Excellent explanation. Nice job. I've been fighting wet wood all season. Last night I used some old dry wood that I had squireled away for emergencies. What a differrence. The secondary flames were poping all over with much more heat and less air needed. I already have next years wood drying so it shoud be a good time burning.

 

[jadm]

Bokehman - I just printed out your definition. Thank-you very much for explaining it so simply that I can understand it.

Would you know why my glass got creosote build up on one side during a strong secondary burn?

Yesterday I had steady rolling secondary flames off of the air tubes without much flame on the wood (enough to act as a wick). The flames were hitting one side of the glass and left the creosote behind.

My understanding is that it should have burned off due to the high temps. that secondary combustion requires....but that didn't happen. ( Insert temp. was approx.550*)

Perplexed

 

[SlyFerret]

Something that I ended up realizing after I started getting used to my stove is that the secondary combustion doesn't last through the entire burn cycle.

From my experience with my stove (an Englander 30), I get secondary starting about the time the stove top temp gets to 350 or so. The secondary combustion lasts for varying lengths of time that depends on the size of the load. Once the majority of the wood has gasified and the load is reaching the coal stage, the secondary goes out, but there is still quite a bit of heat left to be released as the wood finished burning down to coals.

-SF

 

[Jags]

Bokehman - I just printed out your definition. Thank-you very much for explaining it so simply that I can understand it.

Would you know why my glass got creosote build up on one side during a strong secondary burn?

Yesterday I had steady rolling secondary flames off of the air tubes without much flame on the wood (enough to act as a wick). The flames were hitting one side of the glass and left the creosote behind.

My understanding is that it should have burned off due to the high temps. that secondary combustion requires....but that didn't happen. ( Insert temp. was approx.550*)

Perplexed

Flames and with that smoke (gases, particulate matter, etc.) will cool and condense on the glass (because the glass IS cooler than most of the stove). I think you will find that the next time you have a good, up to temp fire, that the discoloration will be burned off. Even if you have proper operating temps, if you have a direct blast of flame to glass, this is common. Air wash systems are supposed to minimize this, but sometimes wood and flames are simply too close. Move your wood back another 2" and I would bet this doesn't happen again. Even if it does, its really no big deal.

 

[brogsie]

Should I be able to see secondary flame in a cat stove?
VC Encore Defiant

 

[bokehman]

I think you will find that the next time you have a good, up to temp fire, that the discoloration will be burned off. Even if you have proper operating temps, if you have a direct blast of flame to glass, this is common. Air wash systems are supposed to minimize this, but sometimes wood and flames are simply too close.

I don´t think the air wash works properly when a stove has just been loaded over warm/semi-hot coals (due to lack of hot air). Yesterday I put in a piece of wood that spat some tar type mess on the glass. Since then the glass has been up to 850F and not removed it.

 

[Jags]

I don´t think the air wash works properly when a stove has just been loaded over warm/semi-hot coals (due to lack of hot air). Yesterday I put in a piece of wood that spat some tar type mess on the glass. Since then the glass has been up to 850F and not removed it.

:gulp: Tar spitting wood?? The air wash is probably still working, but at a much reduced rate. Usually the airwash systems of most stoves (and their volume of air passed over the glass) is directly relational to how much air the stove/fire is consuming. So yes, you are correct in stating that a warm/low fire will not have the same air wash capabilities as a fully established fire.

Did you spit back at it??

 

[Jags]

Should I be able to see secondary flame in a cat stove?
VC Encore Defiant

After you engage the cat (assuming that you are up to proper engagement temp) you would more than likely see some secondary flame. This flame will/can start to fade with the "stages" of the fire. Meaning that if you are at the coaling stage of the burn, you may not see much if any flames, because most of the gases and other nasties have already burned off. So "yes" and sometimes "no". If you never see any secondary flame, I would investigate.

 

[brogsie]

Thanks Jags. My cat is in it's own chamber(out of sight) . Is the flame at the cat? Does the cat just warm up the gas to fire in fire box?

 

[Jags]

Thanks Jags. My cat is in it's own chamber(out of sight) . Is the flame at the cat? Does the cat just warm up the gas to fire in fire box?

I cannot speak specifically of your stove, but normally you will find the flame AT the cat. Where ever gases contact (or get real close) to the "super" heated surface of the cat.

 

[brogsie]

Thanks again Jags

 

[jadm]

Does the cat just warm up the gas to fire in fire box?

My cat just drinks the water I leave by my insert to add humidity to our house. So far I can't tell if she is adding any humidity to the rooms she walks through after she has drained my bowl. ;-P

 

[jadm]

Flames and with that smoke (gases, particulate matter, etc.) will cool and condense on the glass (because the glass IS cooler than most of the stove). I think you will find that the next time you have a good, up to temp fire, that the discoloration will be burned off. Even if you have proper operating temps, if you have a direct blast of flame to glass, this is common. Air wash systems are supposed to minimize this, but sometimes wood and flames are simply too close. Move your wood back another 2" and I would bet this doesn't happen again. Even if it does, its really no big deal.

Jags- My concern is not that the darkened glass looks bad. I assume that if I see creosote in the box or on the glass that it is also being deposited on my liner. Is that a correct assumption? My insert was plenty hot when the glass was discoloring. My goal is to be burning as cleanly and efficiently as possible. If I see brown I figure I'm doing something wrong. :red:

P.S. On the Jotul 550 there are andirons about 4" in that I stack my wood behind so that it is way far off of the glass. Manuf. recommendations....

 

[begreen]

If the glass is getting dark on only one side you should check for air leaks, starting with the door. Do the dollar bill test all the way around the door gasket.

 

[brogsie]

Good one perplexed

 

[Jags]

I double what Begreen said. Also, if your discoloration is ONLY where the flame was hitting, it would lead me to believe that my above statement still holds true, assuming that you don't have air leaks. Has this same area of discoloration happened on other occasions?

Clean your glass, and start over. If you notice the SAME place is always getting discolored, you more than likely DO have an air leak.

If this was a one time deal, clean the glass and load 'er up! She'll be just fine.

 

[bokehman]

Perplexed, are you getting the stove hot? If not that would explain your lack of secondary combustion and dirty glass. How aare you measuring the temp?

 

[jadm]

Thanks for replies.

Discoloration has happened only once,yesterday, which is when I first succeeded in having sustained flames coming out of my air tubes.

I measure temp. with an IR therm. that I point at the 2 side panels right next to the door. (This model doesn't have a good place to stick a magnetic therm.) I generally burn at 500*-550*.

Today I cleaned the glass and it is fine.

I will do the dollar bill test when it cools down....no money to burn after paying for this baby. :lol:

I have noticed that fires don't always burn the same on both sides. One time the right side will have all the flame and then in next load the left side will have the flame. I just figured it was how I had spread the coals and have been trying to pay better attention to that.