Firewood - The Truth

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If a pound of wood is a pound of wood and a pound of wood is worth 6400 btu's, then why is a pound of sawdust worth 8500 btu's?
Why has this thread gone down a qualitative path when the question that is at the core of the inquiry would seem to be available to quantitative analysis?
Is there a way to solve the problem - or is everyone full of shit?
tjwatson1ATcomcastDOTnet (real email) http://home.comcast.net/~tjwatson1 (webpage)
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Tom Watson wrote:

There's average data published for various woods -- the heat content isn't the same for all. For the difference between solid wood and sawdust, the difference has to be specie and, perhaps, combustion process. It's also a possibility one or both numbers is wrong or at least inconsistent in what is/was measured.
I've not taken the time to go look much myself as whatever wood there is here is what I can salvage from trimming the elms around the place and they're lousy for firewood, but since there's no forested areas within 250 miles it isn't cost-effective otherwise.
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I don't know where you get your figures, but perhaps sawdust is dryer than wood.
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If you had read the thread you would know where I got my figures, since the were quoted.
tjwatson1ATcomcastDOTnet (real email) http://home.comcast.net/~tjwatson1 (webpage)
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wrote:

than
toller and I must both have broken newsreaders then Tom, because I don't see any quoted text in your original post either.
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-Mike-
snipped-for-privacy@alltel.net
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Didn't see anything quoted in your message, Tom, and there's no reference header pointing to the thread you may be talking about.
The wreck gets uppwards of 400 posts a day. Maybe you have time to read and follow all the threads.
Some of us have work to do, so we can't.
djb
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efficiency never before possible. People have gotten less competent to
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On Wed, 09 Feb 2005 21:15:39 -0600, Dave Balderstone

(re posted for the benefit of the blind and the overworked)
Since you are a wooddorker, you must make sawdust.
"Pressed sawdust firelogs. These are made from tightly compressed 100% pure wood sawdust, without the addition of waxes, chemicals or other additives. Pound for pound, these give even more heat than natural firewood 8500 BTU per pound in comparison with 6400 BTU for natural wood. They can be used in fireplaces, woodstoves, inserts, and campfires. All in all, these firelogs give all the heat and more of natural wood, and have the convenience of popular wax firelogs."
http://www.worldwise.com/firorfir.html
tjwatson1ATcomcastDOTnet (real email) http://home.comcast.net/~tjwatson1 (webpage)
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wrote:

I think the key is in the "tightly compressed" area. Wood incorporates air and water and other impurities that don't burn. These logs are more nearly pure combustible material and denser than wood so they should yield more heat per cu. ft.
Bob
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Look at t he numbers I posted on Btu and moisture content and you see a big variation from 15% to 20% MC. These guys are using a higher MC to make their product look better. The process of compressing either dries the sawdust further or the dry it ahead of time. The actual caloric content of the fiber structure is not changed, only the amount of water that adds weight to it is regulated. The pressed logs are probably made with sawdust from KD scrap lumber, not just a fallen tree like most firewood.
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Ed
http://pages.cthome.net/edhome/



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Without more detailed information the data points on that web page are worthless.
Wood is wood - if it is ground to dust or one log a pound of wood will have the same heating value. There is no chemical difference that is dependent on form factor of the wood. It may burn faster or slower or more completely depending on the form factor which affects how well oxygen can get to the fuel - but that doesn't change the amount of heat there.
Now, that being said different species will have different heating values for a pound of wood - it has to do with the amount of resin in the wood. Go read a bag of softwood pellets and a bag of hardwood pellets. The softwood pellets are usually much higher BTU content. I found an EPA report once that gave most wood fiber as having close to 8000BTU/lb, but resin being 17000BTU/lb. Real logs being of course a mixture of both.
There is also the moisture content of the material being burned. Materials have different caloric content - usually measured in BTU/lb of "dry matter". This assumes 100% material and 0% moisture. If you have say 8500BTU/lb dry matter wood, and it was at 10% moisture then you have only 90% of the weight in dry matter. You also need to subtract out the energy to vaporise that water. It takes about 1050 BTU/lb to evaporate that water. This gives us this     8500 * 0.9 = 7650 BTU for the dry matter     0.1 lbs water * 1050 = 105 BTU for the water evaporation. Combine this and we get 7545 BTU/lb of usable heat for that 10% moisture wood.
Then if you get the moisture content high enough and the stove temperature low enough you may get incomplete combustion which means that some of the burnable material goes up the chimney as smoke. This would further reduce the usable energy.
Now of course there are all kinds of wild claims by various people about how much heat is in their particular material. I haven't ever researched the wood stuff as much as corn and pellets. But I do know that in that area there are some WILD exaggerations by some manufacturers. Many use the Dry Matter number and don't account for the moisture content and others just plain out lie.
All that being said I am with Charlie and like good hardwood for a wood stove. It banks nicely for the size stove I have used. I know that pine has more BTU/lb, but I can't get half as many pounds into the stove and because of it's low density and high resin content it burns so fast that much of the heat goes up the chimney rather than being radiated into the room. Maybe if I had a bigger stove, or a different type of softwood it would work better for me. Then there is storage - pine in the wood pile goes punky really fast while oak can be several years old and still great.
Dan
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Tom Watson wrote:

Just because there is a web site that says that or a book that says it, it isn't necessarily so. Wood is wood, and if it has the same moisture content, the BTU per pound will be the same.
Arguments about compression are pretty useless since compression has no effect except to change the density. Arguments about volatiles/resins are in the same category since volatiles/resins won't make that much difference as they displace cellulose (by weight) which probably has a higher energy content per pound that resin.
While some of the stuff in the cited article is pretty standard fair, some of the stuff is just plain wrong. If one wants to really find out about wood they go to one of the wood or forests institutes or groups not affiliated with any product but just promote wood use or wood products in general.
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True, but the compressed product is probably very dry compared to the wood they selected for testing. One of the New England pellet makers buys scrap sawdust from my wood supplier, CT hardwood Group. They sell dried wood at about 8% mc and also use it for custom moldings and truck floors. Once compressed, the pellet is probably about the same 8% so if you compare that with typical 3 month old firewood at 30%, the numbers, pound for pound, are probably true.
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Ed
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Edwin Pawlowski wrote:

Yep it probably is very dry, but compression has nothing to do with the moisture content, unless it is so wet that the water is shoved out. It's kind of like comparing a fresh apple with dried slices.
I don't know what 3 month old firewood is? you mean green stuff cut and left for 3 months? I never burned any green cut stuff that sat less than 1 year. Living in an arid climate my firewood was probably 8 % mc or less.
Much of the stuff I burned had been downed from 3-15 years and was very dry.
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On Fri, 11 Feb 2005 04:05:07 GMT, "George E. Cawthon"

http://www.clarkpublicutilities.com/Residential/TheEnergyAdviser/Archives2003/03_01_05
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Well, no. Resins occupy the areas of the wood otherwise filled with air, and they have a higher BTU yield by weight.
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George wrote:

I think you missed the point! And they don't have weight? You are telling me that a pound of sap has more BTU's than a pound of cellulose and lignin? Could be, but it isn't going to be significant in normal tree proportions.
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air,
No, you missed it. They don't displace cellulose by anything. The cube is still the cube, just weighs more because it's got a lot of resin in it, where the reference cube had open channels. Now if you go back to the same weight, it has more calories.
The hydrocarbons composing the resin have a higher heat content than cellulose. Not, as Andy said originally, a "huge" difference, but enough to say confidently that they make a 5-8% more heat per pound over the average wood lacking them.
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George wrote:

You are still missing the point, no one is talking about a cube, they are talking about a pound. If the pound is partially resin then there is less cellulose, the cellulose is "displaced by the resin."
I read the two citations on BTU per pound of different kinds of woods. One simply indicated the difference was primarily attributable to lignin (which is not surprising), not volatiles, which was the point of the study. Although it also said that the volatiles had more heat than the cellulose. That did surprise, as I would not expect pitch to have a higher BTU content.
A criticism of the citation on the cellulose/lignin BTU content was the very small sample size; I believe one was 4 boards (possibly from the same tree?). I would bet that there is nearly as much difference among the same species grown in different locations and altitudes and among subspecies as there is among different species or even between most hardwoods and conifers.
We are still agree that there is not a huge difference in BTU content among species. In the actual heating process, there is probably much greater difference in realize heat from the way stoves are operated based on the statements in one of the citations and based on my own experience.
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On Fri, 11 Feb 2005 04:05:07 GMT, "George E. Cawthon"

No it isn't. The species makes a huge difference.
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wrote:

You've got an uphill climb to prove that here, Andy. Pick any place I've seen, and a pound of hemi/cellulose makes as much heat as any other. You wouldn't have some data to support your statement, would you?
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