a question for you scientific types out there ....

[firegal]

Why is it that wet wood burns faster than dry, seasoned wood?

You would think that having moisture would slow down combustion, and therefore the wood would last longer and more time would elapse between reloading. But such is not the case (as I know so well ...).

I have no background in thermal dynamics, so I'm at a loss as to why this is. Any ideas?

Firegal

 

[wallis54806]

Wet wood should not burn faster than dry wood if all the other variables are equal. Some of the other variables are: type of wood, any rot present, size of pieces, number of pieces, inital temperature of fire box when the wood is loaded and amount of oxygen available (draft).

I know that when I switched to some wood that was not as dry as the wood I had previosly been burning that it burned faster because I split them much smaller to get a good hot burn.

 

[SPED]

Yeah, with wet wood I usually split smaller and give it more air, so I would think that's why it burns faster, you're releasing the BTU's faster, unfortunately many of them are used to dry the wood out before it burns.

 

[stoveguy2esw]

it will burn slower in the beginning as it takes longer to "char" or dry out , once its dried out it tends to burn faster simply because the voided space where the moisture was is still somewhat open. as wood seasons naturally the grain tightens up and the wood actually shrinks as a result , the wood becomes much more dense. this density prevents the wood from being consumed as fast. wood that is quickly dried out through being baked in a fire does not shrink in this manner and the open grained, cooked out wood is less dense and therefore is consumed faster.

 

[SPED]

it will burn slower in the beginning as it takes longer to "char" or dry out , once its dried out it tends to burn faster simply because the voided space where the moisture was is still somewhat open. as wood seasons naturally the grain tightens up and the wood actually shrinks as a result , the wood becomes much more dense. this density prevents the wood from being consumed as fast. wood that is quickly dried out through being baked in a fire does not shrink in this manner and the open grained, cooked out wood is less dense and therefore is consumed faster.

Now that is a scientific explanation, very nice!

 

[firegal]

Hi Mike -

Well, that explanation does make sense, thank you. But does that mean that once the wood is 'dried' from being in the wood stove, it burns hotter than seasoned wood since it burns faster due to all those spaces?

One question begets another ....

Also, GuardGuy & SPED, it's true that I cannot load big splits into the wood stove since they simply do not burn well, so I've split down some of the larger pieces myself. So perhaps it is also partly the size ... I thought it was just my stove that preferred smaller splits (Hearthstone Homestead).

Firegal

 

[stoveguy2esw]

Hi Mike -

Well, that explanation does make sense, thank you. But does that mean that once the wood is 'dried' from being in the wood stove, it burns hotter than seasoned wood since it burns faster due to all those spaces?


Firegal

actually the net return is going to be slightly lower in a lot of cases (depending on the "dryness" of the seasoned charge of wood ,and the "wetness" of the green charge), wood must dry to a certain percentage in order for pyrolisis to begin. in order to do this the existing coal bed or starter medium if you are in a cold start must not only create enough btu's to cook out the moisture (essentially boil it away) but still maintain enough energy to create pyrolisis. as the wood dries and the actual consumption of the wood accellerates the heat output does as well. with dry seasoned wood this accelleration is slower, but the wood being already dried out naturally does not cause the initial waste of energy in the drying process. now , the slower consumption of the wood does not release as much energy as the "peak" output late in the fire at the time when the bulk of this lighter grained wood is being consumed at a fast rate due to lack of density. but if the output of energy from start to finish is measured , there is only a short peak during this cycle where the green wood charge actually exceeds the output of the seasoned charge

here's another little tidbit , off base but interesting , the very act of burning wood actually gives of an interesting by product, water

 

[samandlillie]

Another point to consider is surface area. For example, kerosene fuel will not burn as a liquid but if soaked up in a wick to increase surface area to change to a vapor,it burns. More dense wood(drier) has less surface area to change to vapor. It is the vapor that burns.Icing sugar tastes sweeter than regular sugar because it dissolves faster on the tongue(not vapor this example)because of the greater surface area.
Wayne in NS

 

[firegal]

but if the output of energy from start to finish is measured , there is only a short peak during this cycle where the green wood charge actually exceeds the output of the seasoned charge

So the green wood does release energy faster overall (maybe by a small amount, yes?), though in the beginning, the energy release is slower since it's less efficient because the wood is wet and it is used to boil away the moisture (and therefore we get fewer btu's), and at the end it releases it much faster and burns faster and therefore doesn't last as long (but we get higher output of btus all at once). Overall though, we net more btu's from seasoned wood (obviously), and the btu output curve of seasoned is less steep, more even.

It's slow in the beginning, but fast at the end once it's dry enough to burn well.

Am I understanding this correctly (or at least basically)?

Thanks, Mike, for being so thorough in the explanation.

firegal

 

[jpl1nh]

here’s another little tidbit , off base but interesting , the very act of burning wood actually gives of an interesting by product, water

It' intersting to note that the material that plants (wood) are made up of are like an organic battery. Plants store the energy of sunlight by using it to combine 6 molecules of CO2 and 12 molecules of H20 to form C6H12O6 which is the chemical formula of glucose and they release some left over oxygen molecules in the reaction. This form of storing the energy of sunlight really is the basis of all life energy and all the organic (fossil fuel) energy sources on earth. The act of burning wood is basically introducing enough heat and oxygen to cause the wood to oxidate, that is to absorb oxygen again reverting the glucose back to CO2 andH2O and releasing the stored sunlight energy in the wood. In affect, wood is nothing more than stored solar energy as is oil, coal, etc which all are derived from plant material. Makes the notion of us just tapping solar energy and by-passing the combustion need to get at the stored solar energy of othr sources seem pretty logical to me.

 

[fullbore]

This discussion reminds me of how foolish I was to take a Biochemistry class in college. I still remember drawing glucose molecules...LOL. It is interesting that all the energy leads to back to the Sun.

 

[ketoret]

i figure human beings do the same in reverse - ingest glucose and O2, and release CO2, and water and energy

 

[jtb51b]

i figure human beings do the same in reverse - ingest glucose and O2, and release CO2, and water and energy

dont forget the methane.= u can light a fart but not if its bubbling in water.

Yeah, your wife will get upset if you try to light the hot tub bubbles, especially if you don't really "own" a "hot tub"..


Jason

 

[stoveguy2esw]

This discussion reminds me of how foolish I was to take a Biochemistry class in college. I still remember drawing glucose molecules...LOL. It is interesting that all the energy leads to back to the Sun.

actually my understanding of wood properties comes from 2 sources (as i did not take biochemestry) one , i work in a buisness which it helps me to research the dynamics , i have done so extensively , also , my father was a papermaker for close to 40 years , he knows wood in a way i never will fully understand (though i pester him mercilessly for info when i read somthing , just to get his "take on it") dad's point of view about wood is different than mine is in the use for paper v/s burning , but a lot of the principles do kinda dovetail in. properties in papermaking v/s burning are about opposite as they do not use as much "seasoned" wood , mostly green as its open grain helps it break down easier to its "fiber" stage, for transformation into paper, where a woodburner does not want the rapid breakdown as it uses up the wood too quickly. it can get deeper too , softwoods and hardwoods have different densities , mostly due to their makeup , cellulose/lignin ratios and closeness of grain , straightness and length of grain , there are many variences.

 

[Martin Strand III]

Why is it that wet wood burns faster than dry, seasoned wood?
Firegal

Well, first of all, this is not so.

Take and stack pieces of the same wood the same size, one 'wet', the other 'dry'; light them both. The 'wet' stack is not only harder to ignite and start on fire, it also, once caught, burns longer than the 'dry' pieces. How could you think otherwise? Ever been camping (I wager not)?

Aye,
Marty
Caviat: If by 'wet' you mean wet with gasoline on pine branches, then you win.

 

[thechimneysweep]

Marty, it is so, if you take into account the OP's presumed intent. The original statement probably should have included some language like,

"When using an airtight wood stove to maintain a desired indoor temperature, wet wood burns up faster than dry wood."

That statement, which I believe is what the OP meant to say, is absolutely true, and has been tested and proven several times in various test stoves on our showroom floor over the years.

In order to try to get wet wood to ignite and deliver the same heat as dry wood in a wood stove, you need to give the fire a lot more air, for a lot longer period of time. Depending upon how wet the wood in question is, it often isn't even possible to give the fire enough air to match the heat production of a dry-wood fire. So you pack extra wood in there, crank her wide open, get what heat you can, and reload much more often than usual.

An open campfire, with random-size loads of wood and infinitely available air for combusion would be a different example.

 

[firegal]

Marty, it is so, if you take into account the OP's presumed intent. The original statement probably should have included some language like,

"When using an airtight wood stove to maintain a desired indoor temperature, wet wood burns up faster than dry wood."

Yes, that was certainly my INTENT as I was thinking of my experiences with my own wood stove (nope, Marty, never been camping-- yet). Probably better, too, would have been to use the words 'seasoned' and 'unseasoned' in place of 'dry' and 'wet', since I was not talking about seasoned wood that had been left out in a rain shower or anything.

Thanks, Tom, for the further explanation.

 

[Martin Strand III]

Marty, it is so, if you take into account the OP's presumed intent. The original statement probably should have included some language like,

"When using an airtight wood stove to maintain a desired indoor temperature, wet wood burns up faster than dry wood."

That statement, which I believe is what the OP meant to say, is absolutely true, and has been tested and proven several times in various test stoves on our showroom floor over the years.

In order to try to get wet wood to ignite and deliver the same heat as dry wood in a wood stove, you need to give the fire a lot more air, for a lot longer period of time. Depending upon how wet the wood in question is, it often isn't even possible to give the fire enough air to match the heat production of a dry-wood fire. So you pack extra wood in there, crank her wide open, get what heat you can, and reload much more often than usual.

An open campfire, with random-size loads of wood and infinitely available air for combusion would be a different example.

This is, simply, counterintuitive.

Why is it then, since this forum consists presumably of wood burners in metal stoves, that we air 'dry' (read season - reduce moisture content from about 50+% to around 20%) our wood for months before burning it?

To my mind, adding water to 'dry' wood (with about 20% moisture content) and burning it in a metal stove, or else where, delays the burn since water must be vaporized before pyrolysis of wood can occur.

* "boiling off the water content of he wood into water vapor takes about 2,000 BTU to turn a kg of liquid water at 212* F to a kg of gaseous water at 212* F. This is termed "latent heat loss", is unavoidable and ends up being about 13% heat loss for wood with 20 % moisture content."
- Norbert Senf, All Fuels Expo 96, Burlington, VT, 2 Feb 1996

* "Seasoned wood burns hotter, cuts fuel consumption and reduces the amount of smoke your woodstove produces."
- (broken link removed to http://www.epa.gov/woodstoves)

* "High moisture (in wood) reduces combustion temperature and hence combustion is more incomplete. Alternatively, low moisture (15% - 25%) produces high temperatures which allow volatile organic compounds to be vaporized..."
- Residential Wood Combustion Technical Review, JE Houck and PE Tiegs, OMNI Environmental Services, Beaverton, OR Dec 1998

Please, show me a reference supporting your statement that "wet wood burns up faster than dry wood."

Aye,
Marty

 

[Martin Strand III]

Both Firegal and The Chimney Sweep believe wet wood burns faster than dry wood. The Chimney Sweep says he this is true as has demonstrated it in his shop many times. Really? I would be interested in seeing such a demonstration or at least have it described in more detail, like the "wet wood burned faster" compared to what and under what circumstances?

As stated, this seems so ridiculous to me I struggle to spend more time on it, but alas, here is more:

"The factors affecting the ignition of wood are well known in general: wet wood is difficult to ignite,...
The moisture content of wood has an effect on ignition mainly as a heat sink. Heating-up of the water and especially its vaporization consume heat energy.

*(broken link removed), Burning of Wood

"Moisture content significantly affects burning rate. A difference in moisture content of 10% results in a change of 20–30% in burning rate."

*FIRE AND MATERIALS, VOL. 16, 197-206 {1992), Burning Rate of Solid Wood Measured in a Heat Release Rate Calorimeter,
Hao C. Tran and Robert H. White, USDA Forest Service, Forest Products Laboratory. One Gifford Pinchot Drive, Madison, WI 53705-2398, USA

Thus, absent any references to support their dubious claim that wet wood burns faster than dry wood, I must conclude Firegal and the Chimney Sweep are, in fact, all wet.

Aye,
Marty

 

[billb3]

I don't know what kind of wet wood you all are getting to burn but, the white oak around here, if thrown in green will still be there green as can be in several hours.
I've gotten wet (rained on) dried out wood to burn, but not green fresh cut white oak.
Even in an outdoor firepit with other wood tossed in it goes out.

 

[Todd]

I have to agree with Marty on this one. In my own experience with burning Oak, I can tell you that a full load at 25% moisture burns longer and leaves more coals in my stove than 20% or less. But as far as heat goes it seems the 20% puts out a little more. Then if you burn firewood less than 15% it seems to gas up and burn out very quickly. I think there is a fine line there somewhere finding the best moisture content for your stove for burn times and heat output. My stove seems to be around 20%.

Of course there is not scientific method to my statement, just my real world observations.

 

[thechimneysweep]

Let's not confuse ignition time with combustion time. I'll agree that it takes more time (and effort) to get wet wood burning. My point is, that once it does get going, you have to burn it up faster in order to get the same heat you'd get from dry wood.

absent any references to support their dubious claim that wet wood burns faster than dry wood...

OK, here are a few references, gathered from your posts above:

"The moisture content of wood has an effect on ignition mainly as a heat sink. Heating-up of the water and especially its vaporization consume heat energy."

The loss of heat energy created by the water content translates into cooler stove temperatures. Therefore, in order to produce the same heat output with the same stove while burning wet wood, you need to burn more of it in the same period of time: this requires a higher draft control setting, larger loads and more frequent refueling. The Bottom Line: you're going to burn the wet wood up faster.

[as compared to wet wood] "Seasoned wood burns hotter, CUTS FUEL CONSUMPTION..."

The only way burning dry wood vs wet wood can cut fuel consumption is because you need to burn less dry wood in the same period of time to heat the same area. TBL: the wet wood burns up faster.

"High moisture (in wood) reduces combustion temperature"

Same comment as above; reduced combustion temperature translates to reduced stove temperature and the need to burn more of the wet wood at a higher draft control setting to produce the needed heat output.

This has been our recent experience, burning a Pacific Alderlea T6 on our showroom flue: typically, we fill the firebox and start a fire at 9:00 AM. As soon as the load is kindled and we get secondary burn, we turn the draft conrol all the way down, and let that load burn all day, until we leave at 5:00 PM. The other day, I brought in a load of wood from the wrong side of my woodpile, which we now know is exposed to wind-driven rain. We started the fire at 9:00 AM, but it took a half-dozen firestarter cubes and some vigorous bellows action to get it going, and we couldn't turn the draft control down for several hours, or the fire would go out. By the time the load had dried to the point where it would sustain combustion at a lower draft setting, it was half gone and well into the charcoal stage, and our showroom still wasn't as warm as we normally keep it (we never did see any secondary burn). So, we crammed more wet wood in there, and started the process over. By the end of the day, we had burned about 1-1/2 loads of wood, instead of our typical one load. And the store never did get up to the usual temperature.

 

[Bill]

Tom: From a lawman's opinion, it seems like you had to burn more wood with more draft and because of the wet wood you got less BTU's from the wood and with more draft it burned faster than with dry wood that you damper down.

 

[jpl1nh]

Tom, if you burned dry wood the same way you burned wet wood, lots of fire starters, bellows to get the fire started, wide open air, assuming your stove could withstand the heat assault it would recieve, I'd speculate you could burn at least two, maybe three loads in the same stove in the same time that you burned 1 1/2 of wet wood. Unlike with the wet wood your shop certainly would be warm too. As such I don't buy the premise that wet wood burns faster than dry.

 

[stoveguy2esw]

it will burn slower in the beginning as it takes longer to "char" or dry out , once its dried out it tends to burn faster simply because the voided space where the moisture was is still somewhat open. as wood seasons naturally the grain tightens up and the wood actually shrinks as a result , the wood becomes much more dense. this density prevents the wood from being consumed as fast. wood that is quickly dried out through being baked in a fire does not shrink in this manner and the open grained, cooked out wood is less dense and therefore is consumed faster.

as silly as it seems im going to edit my own statement for a better explanation, watch closely kids i'll bring this to light..

"it will burn slower in the beginning as it takes longer to "char" or dry out "

essentially , the wet wood ( i mean green, as in unseasoned , not rained on but seasoned) is as stated hard to light effectively , and once burning (so to speak) does not burn hot as thermal energy is wasted in the drying (or to be more accurate, "evaporative") cycle which must happen to achieve pyrolisis. the gassification of water takes energy , and evaporation is a "cooling" function during this time the actual gassification of hydrocarbons used to carry out pyrolisis is not as prevalent, so this portion of the fire is very slow moving and does not result in the harvesting of much thermal energy through radiation. (ie. you aint gettin much heat)


"once its dried out it tends to burn faster simply because the voided space where the moisture was is still somewhat open"

after the moisture has been "cooked" from the wood and the gassification process is more completely creating fuel rather than water, the gassification is more rapid as the open grain from rapid release of water promotes a more rapid release of hydrocarbons as the wood is natually less dense. this segment of the fire the wood is consumed more rapidly than the seasoned wood would be. i will explain below.


"as wood seasons naturally the grain tightens up and the wood actually shrinks as a result , the wood becomes much more dense. this density prevents the wood from being consumed as fast".

here's your homework assignment measure the diameter of a round of green wood just cut , first remove the bark as that will not shrink like the internal portion of the round will, let the round fully season naturally then remeasure (naturally this assignment will not be due in class tomorrow) the wood round by the time it has fully seasoned will be of a lesser diameter, obviously lighter as well due to the loss of water weight. now , this wood as it seasons will shrink because the grain of the wood will no longer be seperated by the liquid which has dried up , the individual grains will thus tighten together and the density of the wood will increase. this will have the effect of not releasing hydrocarbons as rapidly causing the wood to be consumed slower during the bulk of the fire. as wood is consumed it becomes less dense naturallly as some of the elements of the wood are consumed faster than others( this is why the coals found after the fire are there , incomplete combustion , the fire did not stay hot enough long enough to break down parts of the wood which contain the larger percentages of the harder to burn elements (or are buried before that can happen cutting off the escape route for the gassification.)


"wood that is quickly dried out through being baked in a fire does not shrink in this manner and the open grained, cooked out wood is less dense and therefore is consumed faster"

the above statement is the key to the arguement, though the overall start to finish episode of a fire from match to ash may actually take longer in some cases, once the wood has "dried" in the fire from that point forward cubic inch for cubic inch the dried green wood will burn faster. however a simple statement that in the overall fire match to ash , this ratio may go either way simply based on the species of wood , its moisture content or density. there is no "clear cut" answer as the comparison has too many variables , i merely was breaking the fire down to describe how the speed of consumption changes in the various stages of a fire. how would the experiment be done in a real world test? would the unit of measure be weight? would it be cubic units of measure? if going by weight , then the green wood would definately not last as long due to the higher water weight which would more rapidly dissipate, much faster than the equal weight (a larger cubic charge) would be consumed. going by cubic units of measure the green wood would last longer due to the longer cooking out process. there are many variables to look at, but in essence its true in one way that green wood burns faster in a portion of the fire and seasoned wood burns faster in another, which one lasts longest is up to interpretation , but i believe it also depends on the species of wood used.