Wood Verses Coal Flue Pipe Temps

Hi all At the beginning of this heating season, I started coaching a fellow co worker with burning coal. He is a former wood burner that has the same furnace as mine, the Clayton 1537. I advised him on the small modifications needed, such as sealing the load door better and the blocking of combustion air from going around the coal bed between the fire box liners. Also, to make sure the grate system worked effectively. So far, he is very happy with his results

He noticed that his flue pipe temp runs a lot lower with burning coal compared to wood. Its thought that with wood, you have much more wasted heat going up the chimney, hence the higher flue temps. His question is why? I thought the right answer would be that with wood, you need much more combustion air, which carries heat out the chimney. But in his experience so far, he thinks that he is feeding the coal more air.

I'm not experienced with burning wood, so I'm not really sure what the right answer is. I'd like to hear some thoughts from former wood burners about this dynamic between wood and coal

Wood requires over fire air because it has more volatiles like Bit. The combustion is seen in the flames and you dont see much for flames with Anth. That is why wood burners can't understand how coal heats without a roaring fire.

Kevin

I know many people in maine that hate burning Oak because it doesn't flame up like other woods. The relate the flames with heat. Many people burn junk woods because of this. There are more volatiles in the junk wood so therefore more creosote. Only a problem when smoldered which they do because junk wood will not last very long when allowed to burn hot. It is more difficult to explain that to lifelong wood burners them explaining how to burn coal. I actually know people that buy green wood and claim it is better and reduces creosote. No kidding.

Kevin

KLook wrote:Wood requires over fire air because it has more volatiles like Bit. The combustion is seen in the flames and you dont see much for flames with Anth. That is why wood burners can't understand how coal heats without a roaring fire.

Kevin

Thats what I thought too.. But he is claiming that he is feeding more air to the coal fire than he did with the wood fire But now that I think of it, maybe it just seems that way to him because he had a leaky load door that I advised him to re gasket...

Do wood fires generally take more air than coal fires do?

That is a hard one, I would think that air is the oxidation of the fuel and you can't get something for nothing so you need the same air for the same oxidation. It is just that air goes through the coal bed harder(more restrictive) and wood requires air over the fire to burn the volatiles. I think he means that he is putting more air under the fire with coal as you put non (almost) over. Coal has more carbon so it takes more air to mix with it.
If you had two burners, one oil and one propane, you would add different amounts of air to efficiently burn the fuel based on the carbon chain.

Kevin

I know just enough to be dangerous. But I have burned a lot of wood and now some coal.

KLook wrote:That is a hard one, I would think that air is the oxidation of the fuel and you can't get something for nothing so you need the same air for the same oxidation. It is just that air goes through the coal bed harder(more restrictive) and wood requires air over the fire to burn the volatiles. I think he means that he is putting more air under the fire with coal as you put non (almost) over. Coal has more carbon so it takes more air to mix with it.
If you had two burners, one oil and one propane, you would add different amounts of air to efficiently burn the fuel based on the carbon chain.

Kevin

I know just enough to be dangerous. But I have burned a lot of wood and now some coal.

Me too about the dangerous statement I think Keven is onto it. To set up my stove for coal burning the air restrictor must be removed. This increases the volume of air fed per similar opening by an estimated factor of 5. All of this is under-fire air feeds the combustion of mostly elemental carbon primarily within the deep coal bed. Wood has a higher hydrocarbon content. Wood gassifies into unburned particulates and volatile hydrocarbons that move away from the hot fuel into the space above the fuel. Overfire air and stove design (cat, non cat) influences where, when and if they fully combust.

Moisture content of the fuel is the next limiting factor. Dry wood is quoted at 12% moisture. I've seen figures for anthracite listed in this range an lower (6-<15%). Firewood moisture content can vary from a high of 40 - 60 % for green wood (from above link) to a dry 12%. In either case, to convert a pound of liquid water to steam, it takes about 1000 btu/Lb. You can do a simple calculation to estimate how much heat (btu) from either fuel is being used to evaporate the water from a given load of fuel. The heat in the steam goes out of the flue and is a loss of heating value.

The green wood (@ 40-60% moisture) = less creosote statement is true. The trouble is the fire must burn hot and quick to maintain the wet-wood fire. High heat burns the compounds and particulates. All that moisture must be converted to steam (@ 1,000 BTU/Lb of water) to be carried out of the flue. That heat is lost to the room.

So in HillBilly terms pretty much in a nut shell With wood, there is more moisture which carries more heat up the chimney hence the higher flue temps as compared to coal which has less moisture. Thats pretty much the answer I was looking for

Wood is half gas which comes out fast when heated. If there is enough air it burns causing high stack temps.

I can tell you from experience that wood (at least the 20+ year old OSB I was burning ... ) requires MUCH more air than coal. I had the dial open 4 - 5 turns for a firebox full of that particle board with stack temps of 450°- 600°, vs. 2 turns of the dial, and stack temps of 200°. The wood reached a higher peak temperature - side temps were over 800° ... but only for about 45 minutes tops. Coal maintains a couple hundred degree lower temp for over 20 hours in this same stove. I could easily crank the coal up to 800° but I'll "only" get a 12 hour burn out of it.

Thanks franco b.
I wasn't sure why wood produced higher flue temps but I thought if I read enough responses that I would perhaps find one that at least satsifies the laws of physics. I believe your answer unless proven wrong. You get the pyromania peace prize

The green wood (@ 40-60% moisture) = less creosote statement is true. The trouble is the fire must burn hot and quick to maintain the wet-wood fire. High heat burns the compounds and particulates. All that moisture must be converted to steam (@ 1,000 BTU/Lb of water) to be carried out of the flue. That heat is lost to the room.

I would argue that point however, yes you burn it hotter to even burn it at all, or you try. But when the creosote is running down the flue as the water condenses out in a cold external masonry flue, it is a mess. I would submit that it is difficult to burn it hot enough to "burn" up the creosote with wet wood. Of course the point is moot as it is inefficient to send all that heat up the chimney as steam. I have seen the outdoor boilers with the black goo running out of the door opening as the guys put anything they could get their hands on in them. The insides looked like they had been glazed with roofing tar up to a half inch thick. Don't need that in the chimney.

Kevin

KLook wrote:

The green wood (@ 40-60% moisture) = less creosote statement is true. The trouble is the fire must burn hot and quick to maintain the wet-wood fire. High heat burns the compounds and particulates. All that moisture must be converted to steam (@ 1,000 BTU/Lb of water) to be carried out of the flue. That heat is lost to the room.

I would argue that point however, yes you burn it hotter to even burn it at all, or you try. But when the creosote is running down the flue as the water condenses out in a cold external masonry flue, it is a mess. I would submit that it is difficult to burn it hot enough to "burn" up the creosote with wet wood. Of course the point is moot as it is inefficient to send all that heat up the chimney as steam. I have seen the outdoor boilers with the black goo running out of the door opening as the guys put anything they could get their hands on in them. The insides looked like they had been glazed with roofing tar up to a half inch thick. Don't need that in the chimney.

Kevin

No need to argue that point any more. It difficult to totally burn off those compounds in "normal" stove designs. Condensation in a chimney is not good even if your'e burning dry wood. A condensing chimney will make that weak point ever more obvious. The point was the hotter fire required to keep high moisture wood going vaporizes and combusts as much of the volatiles and the flue gas temperatures will be higher on leaving the stove. A naturally cold chimney never works as well as a naturally warm chimney.

Flue temp is also dictated by the design of the stove as well. All stoves are not created equal and some are more efficient than others. I have found that solid designs like the Harmon, Keystoker, Baker and Hitzer to name a few are more efficient than most. With my Keystoker hand fired I can run the stove at 475 and have a internal stack temp of under 150 and get a burn time of 24 plus hours using 50 lbs of coal. With the old Shenadoah I would burn at 475 and have a stack temp of 300 for a 10 hour burn using twice the coal as the Shenandoah is rather large. My vintage royal oak will run at 475 and have a stack temp of 400 for a 5 hour burn using 25 lbs of coal. Love to freak out friends with the Keystoker H.F. Hopper by putting my hand on the stove pipe and leaving it there as it is around 100 deg external pipe temp with a 400 to 500 deg stove temp. Regarding wood stoves in my opinion the Blaze King line of stoves are the only ones that even come close to coal efficiency. Just my two cents.

My brother gave me some "soft" wood to burn last year I think it was birch or poplar but I just used it for fire starting wood because of the raging fires it always creates and the high flue temps higher than the stove itself. This year I am burning the hickory that I seasoned and it burns more like anthracite than anything at least in my WM. I have fairly low flue temps and good stove heat , it just doesn't last for as long as coal would by far. I can get a few hours on a couple of logs though. The hickory is very steady and stable and I have the manual damper shut . I also treat It like ant and crack the bottom and close the top once a nice bed of hickory coals has set in. I think that what has been mentioned about stove types has a lot to do with it. Hickory and oak by far is premo wood for heating . I'm not a huge wood burner myself but do people usually burn either one of these woods? What is the split stuff you see everyone selling on the sides of the road?

Hickory also has everything beet as far as smell goes. Man do I love the smell around my house when I burn it.

Birch and Poplar are hardwoods. We lived on the coast and the best wood we could get from our own land was Maple. After that it was lots of Birch. Yellow Birch is very good however. Softwoods are various Spruces, Firs, Hemlock, Cedar, Tamarack(Hackmatack or Eastern Larch) and Pines. Different species of each have different characteristics but are all "softer" and less dense then hardwoods with more pitch in particular. Poplar is good after extended drying time and Birch has to be kept dry or it will go punky very quickly. They are in fact "softer" then other hardwoods.

Kevin