I’m wondering if somebody smarter than me can answer something thats been puzzling me. Why is it in a typical secondary combustiom wood stove its often said that the wood gases must reach a temperature of 1100-1200 degrees F in order to combust, but in our gasifiers we can maintain a flare with ambient temperature gases?
You would think with all the insulation and heat inside a stove that they should have no problem staying lit once they light the first time, so what gives? Do our gases only stay lit at such low temps because we’ve cleaned and/or filtered out the dirty gases through our hot char bed first? If this is in fact the case, what exactly are in the dirty/smoky gases that are preventing steady ignition, like chemically speaking?
Could be becuase in a stove much of the oxygen is consumed in primary stage and it is very hard to balance a second stage air intake with a primary. So the upper stage maybe deprived of oxygen and require the higher temperatures. I have no idea if that is true or the case just my theory. My other theory is that there is misconception that they may note sustaining second stage only becuase they are running the primary stage hotter. But if they understood the process like we do, it is more likely that the upper stage is not getting enough air reguardless of the temps at this stage. But also the primary stage must also sustain a core temp of 1400 f in order for pyrolysis stage to occure.
Matt i think you may be right. Probably where they ger confused is that to maintain secondary combustion the internal stove temp must be 1100-1200F. I’m sure like you said its probably a balance of lighting the secondaries and keeping the wood pyrolysing. Any of us who have built a TLUD know full well ypu can get the secondaries to light very quickly, i don’t think the gases need to reach near 1200F for that to happen.
Most likely the magic internal stove temp is around 1200F where the primary air can be turned down to minimal and the reflected heat from the secondary combustion at that stage will be mostly enough on its own to keep the wood pyrolysing without the need for massive amounts of primary air directly feeding the char bed. It’sactually quite ingenious how the baffle arrangements are designed to reflect that heat bacj to the wood before exiting up top, whoever first came up with that idea mustve been one smart cookie.
Probably if you ran your stove to only 800F and then tried to turn down the primary air, yes the secondaries will light, but you won’t have enough reflected heat to keep the wood pyrolising, which means less secondary combustion, which means less heat, and the negative feedback loop will continue until the stove will go out.
I guess one could liken the idea that the stove has to be so hot to work properly to our gasifier system if you look at them as a whole. Sure our gases are nice and cool by the end of the line and we can flare them off nicely, but if you go to our reactors the temperatures are extreme right in the core, and they have to be to continue to pyrolise and produce more gases. A modern secondary burning stove is basically just a gasifier that burns its woodgas right away in the same space they are made, and all the filters and cooling sytem in between is taken out.
JustinH I think you are confusing the real purpose of the secondary re-burning section in a modern wood stove. It is to oxidize burn up excessive smoke.
Smoke is not just made woodgas. It is a lot of water vapor, tars and carbon chains been released and surface formed from the wood.
The primary combustion burning zone has the glowing hot charcoal as its heat driver.
Way up above; the super heated allowed in air has to be the heat driver and the oxidizer.
In the short time I can evolve down my wood stoves lower char bed to in-place gasify out new wood adds the true woodgas made does not visibly re-burn in the secondary area. Those driven out char surfaces converted gasses burn right off the top of the wood. The blue and pink, mauve colored combustion.
The wood stove as a big internally open area I can only do this for a short time. And yes only super hot inside. At a stove damaging temperature level.
Think about it. A gasifier is intentionally a very constraint place with formed guided flows and focused heating.
The better one’s wringing out the woods, oxidization-made and air-in moisture’s separately above. All smoke oxidized burned brown down mid-system.
So even unfiltered. Even uncooled. The output gasses mix is primarily combustable gasses. And passes thru nitrogen.
Apples; versus strained applesauce.
Regards
Steve Unruh
Grrr. Stupid new computer AI changed my first written stove words to “love”. Hmm. Must be reading my feelings. So Siri? you are she, eh. S.U.
So between the water vapours, tars, longer carbon chains and excesses of nitrogen, are you saying these are the factors that do in fact require that magic 1200F temperature to fully ignite? Obviously the hydrogen, methane and carbon monoxide present don’t need those crazy temperatures to burn as we all know.
Preheating secondary air has always seemed a bit odd to me if i’m honest. Take btus away from the fire directly, to heat air which goes right back in the fire, seems a bit redundant. I would understand perhaps if we preheated the air with flue gases which have left the stove already, thats essentially what power plants do, but taking btus directly from the fire to feed the fire seems like a pointless loop. I’ve built quite a few different TLUDs, some with preheated secondary air, some without, and honestly it doesnt seem to make any difference to how clean it burns. Maybe if does though, who knows, without a means to analyze exhaust gases i can only go on visuals alone.
The other thing is heating air is actually extremely difficult, air is a very good insulater. So it seems ridiculous to me that people believe the air coming out the little secondary holes in their stoves is 1000F or more, personally i think thats ludricrous. For a single wall chimney pipe the rule of thumb is that the surface temp of the pipe is roughly 50% the temperature of the flue gases inside. So if we assume the opposite to be true, that would mean that our secondary air pipe would have to be 2000F in order to heat the air inside to 1000 degrees. I know most likely this isnt quite right either, being that the pipe is inside an insualted box, but you see my point. You can put a propane torch to a piece of pipe, get it glowing red hot, blow some air through the pipe, and i guarantee that air isnt gonna be no 1000F, maybe 200-300 degrees if youre lucky.
There you go. You actually do understand the wood stove secondary re-burning purpose.
You are burning up to visible clear, the stoves flue emissions.
Sure. Sure. A true gas analyzer spectrometer can still measure nasties. Some CO. Some tiny particulates. But the visible emissions elimination settles nervous nerves.
S.U.
Oh you added content. So I’ll address that.
Of the different secondary burning wood stoves I’ve had there has been a wide differences in the effectiveness of their secondary burning ability.
The worst had the secondary air upright heating passageways surrounding the front loading door.
The kind’ sorta works ones had the upright secondary air heating channel center in the stoves back wall.
The BEST, the very Best has two cast iron upward secondary air heating channels midway on both sides right-up next to the red/white hot char glowing. Forging temperatures there. Those cast iron channels exposed to the harsh radiant energy do not stay soot covered very long. So, yes I do believe the secondary air is delivered at at least 850F to 1000F in those BEST stoves. Char glow radiant heat can be felt out the front of my glass fronted stoves 13 feet away. S.U.
Hi Steve, do you not agree that it seems like a moot point to be heating the secondary air directly from the firebox though? If i rob 100 btus of heat from the firebox, use that to heat my air, i’m simply putting that 100 btus right back in, whether we take it from the char bed, the top, or the floor of the firebox, we in theory arent gaining anything. Had we simply introduced outside air into the secondary without preheat, that heat exchange will still take place.
I could see if one preheated the secondary air frok the sides of the stove maybe where that heat was going to be radiated away from the firebox anyway, but i have yet to see a system do it this way. Also as i mentiomed earlier, if air was preheated from the flue gases after the stove, that also makes sense, as you are capturing heat that otherwise was going to leave the chimney and not contribute to the combustion process. I’m not saying this is a good idea because it will have its own issues, simply trying to illustrate the point that preheating air woth waste heat makes sense, its recapturing btus that were otherwise not controbuting to the combustion process. But robbing btus from the firebox that if left alone would have contributed doesnt seem to add up. Preheating secondary air takes btus that have to come from somewhere, i would imagine that simply injecting non heated air through the secondaries will still have an end result of heat exchange without the need for extra piping in the firebox.
I guess a weird analogy would be those silly electric ducted fann people would put in their air intakes on their engines to try and gain more power. Watts is power, so if your fan needs 200 watts to operate, your engine will always be wprking 200 watts harder to operate that fan, so any gains from the extra air are used up trying to power it.
So in the last example, they wouldve been better off leaving the engine be to breath naturally. Maybe its not the best example, but if we think of pur btus in our firebox as watts, then its essentially the same principle. You arent gaining heat from preheated air, because that heat had to come from somewhere, its just overcomplicating a system that need not be complicated. The air drawn in from the secondary system will heat exchange whether its in a pipe or not
Not any wood glowing charcoal; but just burning off of pasteboards boxes, junk mail, and sensitive legal papers has turned my secondary air channels sooted black.
The secondary air distribution SS “flute” rods soots blacken too.
The front doors glass soot blackened also.
Just a real wood fire generating a deep bed of real red glowing charcoal will clean all of these up to a light grey coated. The glass doors 90% clear.
Hi Justin,
Here’s a term for you to wrap your mind around; flammable mixture.
The ambient temp woodgas is very ‘rich’. It is easy to mix it with air and get a flammable mixture. On the other hand, the smoke from an ordinary campfire is much ‘leaner’, meaning it has a much lower concentration of flammable gasses, and no amount of ambient temp air is going turn it into a flammable mixture. Such a lean mixture only becomes flammable at the high temperatures you talked about.
Rindert
