Burning unseasoned wood

[Dalmatian90]

Yup, Bone dry wood is nominally 7 to 8 K buts per pound, we wood can be down in the 4 K range and it barely will burn enough to cover the amount of energy required to vaporize the water in the wood.

I reckon most firewood is around 1/8th of the BTUs to burn off the excess water.

Let's take the example of Red Oak using the figures:
4888# Green
3528# Seasoned
24.6M BTUs

First let's start off with one calculation to show what they consider "seasoned" -- Oven dry wood -- i.e. 0% moisture is 8600 BTUs/pound. 3,528 pounds for seasoned cord is presuming 20% of water weight -- because (3528lbs x 0.8) x 8600 BTUs/lb = 24,272,640. Some rounding by someone somewhere you have the 24.6M BTUs/Cord figure.

4888 - 3528 = 1360 pounds of "excess" moisture between green and seasoned.

By definition, it takes 1 BTU to raise 1 pound of water 1 degree. It also takes 970 BTUs to go from water at 212º to steam.

OWB, so let's presume the wood is frozen --- (212º - 32º) + 970 = 1,150 BTUs to boil off one pound of water.

We have 1,360 excess pounds.

1360lbs x 1,150 BTUs/lb = 1,564,000 BTUs

1,564,000 BTUs to remove the excess moisture / 24,272,640 BTUs in the wood = 0.06

Yeah, that's 6% of your BTUs lost to boiling off the water in the green red oak. That's it.

Now it does slow down the rate of your consumption, all else equal. It keeps temperatures down until the water is boiled off, and that means a lot of incomplete combustion. So the BTUs/hour is lower than if you had seasoned wood even with an unlimited air flow.

Is using wet wood the best way to achieve longer burn times? Heck no -- you're much better off controlling your oxygen. The fire can only burn as fast as the air supply allows it to burn. Throttle it down when you don't need the heat, let it get as hot as it possibly can when you need it -- the hotter the fire the more complete the combustion so the more BTUs you get.

Beyond the initial burning of the wood being less efficient and slower when using green wood, because you're burning at lower temperatures efficiency improving (and pollution reducing) features like secondary burn and catalytic convertors don't function properly. That's where you lose most of your BTUs, not from boiling off the water. Some of this fuel will be lost to creosote build up on the chimney as water in the smoke condenses in the chimney "washing" the bigger, more complex unburned hydrocarbon particles out of the smoke with it. A lot, probably most, of this unburnt fuel will be invisible -- such as carbon monoxide which you won't see even as the steam disappears. (Carbon Monoxide is "city gas" and used to be produced by essentially cooly burning coal or tar in gas plants...and why you used to be able to commit suicide by sticking your head in a gas oven; today's natural gas will just give you a really bad headache.)

 

[Whitespider]

Yeah, that's 6% of your BTUs lost to boiling off the water in the green red oak. That's it.

Ahhhhhh........ no, it's higher than that... much, much higher.

I'll point out just a few of the places where you've gone wrong in your calculations.

1. 8600 BTU's in a pound of wood - Yes, it is possible to get that many BTU's from a pound of wood only in a laboratory bomb calorimeter, only using perfectly dry wood, and only in an atmosphere of pure oxygen. In the best, most efficient, perfectly adjusted, consumer grade appliance, and using perfectly dry wood... maybe 7000 BTU's... maybe... maybe.
2. (212º - 32º) + 970 = 1,150 BTUs to boil off one pound of water - You've forgotten that the steam temperature must be increased to a least equal that of the flue gases or it won't exit the firebox, your calculation does not account for the BTU's required to raise steam temperature to equal flue gas temperature.
3. Burning wet wood requires more oxygen to keep temperatures high enough to sustain the combustion process... meaning the fire is fed excessive air. That air must be heated to sustain combustion, and the non-oxygen part of it (the majority of it, the part not used for combustion) just carries heat out the flue in the same manor as steam does... it's a heat-sink just like water is. Feeding excessive air to a fire robs BTU's whether the wood is wet or dry... but wet wood requires excessive air.

It's not possible to calculate the percentage of BTU' loss when burning "wet" wood with your simple formula... even the humidity level of combustion air comes into play. You would need a lot of information unavailable to you without access to a lab... and the formula would be several lines long on this page.

Depending on the appliance and actual circumstances... burning "wet" wood with a 50% moisture content can rob you of as much as 60% of the available BTU's (according to some calculations). Likely it wouldn't be that high, but a loss of 30-40% would not be out of the ordinary... don't forget, live-cut, "green" Red Oak has a moisture content of 80%‼ Any way you slice it... the BTU loss is a damn site higher than 6%‼

 

[philoshop]

Burning unseasoned/green/wet wood just to control burn times is a really inefficient use of the fuel.
To get the most from your hard work of harvesting the wood, dry it well and control the burn time with the intake air.

 

[fuzz1500]

Burning unseasoned/green/wet wood just to control burn times is a really inefficient use of the fuel.
To get the most from your hard work of harvesting the wood, dry it well and control the burn time with the intake air.

I would agree with that . Its a nice thing to notice that wet wood creates longer burn times...but I think as these fellas are saying.....the risk is not worth the reward . I think its a cool observation though !

 

[Dalmatian90]

I'll point out just a few of the places where you've gone wrong in your calculations.

I'm sorry -- is there somewhere my math is wrong? Show me exactly what is wrong in the calculations?

You may disagree with some of the assumptions, and that's fair. Though for fantastic claims like Red Oak actually being 80% water please cite a reference, because I've hauled loads of freshly cut red oak in my Ranger that would not have been physically possible.

It's not possible to calculate the percentage of BTU' loss when burning "wet" wood with your simple formula...

First, let's step back to the statement I was replying to:
" barely will burn enough to cover the amount of energy required to vaporize the water in the wood."

My math covered, quite well, the amount of energy needed to vaporize the water in the wood.
And it is practical enough to calculate with sufficient accuracy to say the amount is far closer to 6% than your wild ass statement of 60%.

For example, the BTUs it would take to raise the temperature of the steam to match that of the flue is trivial.

The specific heat of steam, in a system like an OWB or woodstove, is 0.47 BTUs/lb.

So after spending 1,150 BTUs to make the steam, to raise the steams temperature from 212º to 350º would take about 64 BTUs. That is not significant to these calculations.

If the facts are on your side, show the math.

This statement though requires a special calling out:

Burning wet wood requires more oxygen to keep temperatures high enough

No -- we're talking basic chemistry here.

All common organic materials, whether we're talking wood, crude oil, plastic, gasoline, etc. release just about 13.1MJ/Kg of O2 consumed (5,587 BTUs per pound of Oxygen).

You're not going to consume any more oxygen no matter how you burn the wood.

It is possible to you may move more air, in some circumstance a lot more air, depending on the setup of the system, and that will rob you of BTUs (which is one reason you're always better off controlling a fire by limiting the oxygen instead of use wet wood to slow it down).

 

[Vermonster]

I'm sorry -- is there somewhere my math is wrong? Show me exactly what is wrong in the calculations?

You may disagree with some of the assumptions, and that's fair. Though for fantastic claims like Red Oak actually being 80% water please cite a reference, because I've hauled loads of freshly cut red oak in my Ranger that would not have been physically possible.



First, let's step back to the statement I was replying to:
" barely will burn enough to cover the amount of energy required to vaporize the water in the wood."

My math covered, quite well, the amount of energy needed to vaporize the water in the wood.
And it is practical enough to calculate with sufficient accuracy to say the amount is far closer to 6% than your wild ass statement of 60%.

For example, the BTUs it would take to raise the temperature of the steam to match that of the flue is trivial.

The specific heat of steam, in a system like an OWB or woodstove, is 0.47 BTUs/lb.

So after spending 1,150 BTUs to make the steam, to raise the steams temperature from 212º to 350º would take about 64 BTUs. That is not significant to these calculations.

If the facts are on your side, show the math.

This statement though requires a special calling out:



No -- we're talking basic chemistry here.

All common organic materials, whether we're talking wood, crude oil, plastic, gasoline, etc. release just about 13.1MJ/Kg of O2 consumed (5,587 BTUs per pound of Oxygen).

You're not going to consume any more oxygen no matter how you burn the wood.

It is possible to you may move more air, in some circumstance a lot more air, depending on the setup of the system, and that will rob you of BTUs (which is one reason you're always better off controlling a fire by limiting the oxygen instead of use wet wood to slow it down).

You have to understand this Spider character. He's pretty similar to "The Topper" from the Dilbert comic strip. http://search.dilbert.com/comic/Topper

 

[Whitespider]

I'm sorry -- is there somewhere my math is wrong? Show me exactly what is wrong in the calculations?

Well, first of all, your assumptions are all “wet” (pun intended).

For example, this calculation… (3528lbs x 0.8) x 8600 BTUs/lb…

The moisture content of wood is not based on total weight; rather it is based on weight relative to the weight of dry (0%) wood. Your calculation of 3528×0.8 equals 2822.4 pounds of dry wood… but in reality, 3528 pounds at 20% moisture would equal 2940 pounds of dry wood (2940×1.20=3528).
Look‼ That one worked out in your favor… that should make it even less than 6%‼

You’re also basing your calculations on 8600 BTU’s, which ain’t possible in any stove… you need to base calculations on available BTU’s. Like I said, maybe… maybe 7000 for dry wood in a consumer stove, in plain air rather than pure oxygen… but that’s before figuring efficiency. So, let’s say the stove tested at 80% efficient, meaning 20% is lost or not available (7000×0.8=5600 available BTU/lb of dry wood). So you see, your 8600 ain’t even close.

You also forgot to allow for the loss of BTU’s to boil the 20% moisture (i.e. you only accounted for the weight difference between seasoned 20% wood and green wood). So if 3528 pounds of 20% wood equals 2940 pounds of dry wood (3528-2940=588) the BTU’s required to boil 588 pounds of water is unaccounted for. In other words, a pound of 20% moisture wood is only equal to 0.85 pounds of dry wood (0.85×1.20=1.02). So, (5600 BTU × 0.85 = 4760)… so you see, each pound of that seasoned 20% moisture Red Oak only makes available, in your 80% efficient stove, 4760 BTU’s/lb. But, that's, just the equivalent dry wood, now we need to subtract the BTU's lost boiling the 20%...
(3528×4760=16,793,280 BTU’s) - (588×1150=676,200) = 16,117,080 BTU/cord.

OK, now you have a cord of seasoned 20% moisture oak weighing 3528 pounds at worth 16,117,080 BTU’s (available for you to use). Well 16.1 MBTU/cord is a long ways from your figure of 24.6 MBTU/cord. There’s a huge difference between what’s possible in a lab, and what happens in your steel box… and I’m thinkin’ my numbers a pretty damn generous.

OK, now we can use your simple calculation, assuming 1360 pounds of excess water at 1150 BTU’s per pound… (1,564,000/16,117,080=0.097). Funny, I come up with 9.7%, not 6%... and that’s after I handed you a gift in the first paragraph.

Like I said, I feel my numbers are generous (damn generous actually)… I don’t believe any stove could actually make 7000 BTU’s per pound of wood, and I don’t believe your 80% efficient stove burns anywhere near 80% in-the-real-world. And we still haven’t accounted for all the other variables… such as the excess air entering the firebox just to keep the fire alive when burning wet wood. But we’d need to know the temperature and humidity of that "excess" air to figure in the heat loss percentage anyway... let's just skip that... OK with you??

Now, if I remember correctly, “they” say 20-30% of the heat generated by these new high efficient stoves is created by burning the smoke… but if the smoke is laden with bunch of steam it flat won’t ignite will it?? Well now, if we add 20-30% to the 10%... well... I’ll... be... damned… we end up with 30-40%‼

Let’s see now, what did I say??
Oh Yeah, I remember… “Depending on the appliance and actual circumstances... burning "wet" wood with a 50% moisture content can rob you of as much as 60% of the available BTU's (according to some calculations). Likely it wouldn't be that high, but a loss of 30-40% would not be out of the ordinary...”

One of those newer (so called) high-efficiency stoves will suffer a lot more BTU percentage loss than the traditional smoke dragon... just following the numbers.
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[flotek]

Sounds like the op already had his mind made up before he asked the question . 6% loss due to moisture is laughable . Even a hobo burning trash in a 55 gallon drum knows that's nonsense .. Try that in a epa unit and you would have a cold smoke filled firebox in no time flat ..All math aside The reality is the higher the moisture content the less heat your going to get. .. Period . As far as your sticking a hand in the firebox test .. You must realize your trying to heat a whole lot of water up .. Not a mans hand of coarse it would be hot to a bare hand even if its smoldering and hissing . If you want the btu of a bic lighter have at it ...Might as well add some water in your gas tank to stretch the miles before you gotta stop at a gas station ..add some water to your beer to stretch between opening cans where does the madness end. ?

 

[philoshop]

dry wood=more heat opcorn:

 

[brenndatomu]

One of those newer (so called) high-efficiency stoves will suffer a lot more BTU percentage loss than the traditional smoke dragon... just following the numbers.
*

WOW! All this math is makin me crosseyed! But giving it all a quick cursory glance, makes sense to me.
But that last line I can vouch for personally. In my furnace, truly dry wood (20% MC MAXIMUM, (15-18% is better) makes a very big difference in how much heat the house actually receives from any one given load of wood. Not to mention the longer burns, and the cleaner chimney, and the fact the furnace is just less "cranky" to deal with.
But after living with my newly installed "EPA" stove in the fireplace for a couple months now (my new "shoulder" season heater) I've found the difference in heat output between a load of, say, 25% VS 18% MC wood, the 25% has half as much! It's like a completely different machine with truly dry wood! Huge difference...huge!
Just follow any of the several "modern" wood burning stove forums for a while, probably better than 80% of the time when someone is complaining that their stove just isn't making any real heat, it's due to wood that isn't really dry. Was sold to them as being seasoned though...

 

[poloman]

To the OP
Clean your chimney frequently. You are going to have much faster creosote build up! I never burn unseasoned wood and clean mine twice a year. It works best when clean and I have peace of mind.


Live a positive life!

 

[Whitespider]

You have to understand this Spider character. He's pretty similar to "The Topper" from the Dilbert comic strip. http://search.dilbert.com/comic/Topper

Vermonster,
I've never seen "Topper" before... but that is the funniest cartoon thing I've ever seen or read‼
Thanks a ton for the link... and yeah, I can laugh at myself.
*

 

[Cantrellc123]

Thanks guys for all the input. I'm a believer in low moisture wood or I wouldn't have a 2 year supply on hand when Fall arrives each year. But when I stick a few sticks of uncured wood in and 12 hours later there is a noticeable amount of unburned wood left over vs dry wood which normally is completely burned 12 hours later-always enough hot coals to easily refire fresh wood then I can't help but post these results in question.
I'd say the biggest variable in this picture is actually daily weather temps. Here in TN the daily high temps can vary greatly day to day. With this its difficult to have a level playing field day to day. Add in a windy day here and there and its further complicated.
As far as the discussion about moisture being burned off, can the time that the fan is off-in this story lets say 40% on/60% off average be a factor of consideration? During the off time the heat from the coals is cooking out the moisture allowing for dryer wood being burnt when the fan kicks on.
Common sense would indicate that dryer wood is better, I easily understand that. It's just that what I'm seeing doesn't match that logic.
After the holidays I'll adjust down the air inlet and see what difference that makes.

 

[brenndatomu]

I have been thinking 'bout this...and you know, it's not completely unlike the evaporative cooling that keeps us humans cool, the moisture carries the heat away...no!?

 

[fuzz1500]

I got lost in the math . My dad taught me that to burn wood....the wood needs to be dried , cured , or seasoned ..to burn efficiently , without starting a chimney fire . Thats all my pea-brain needs to know Sometimes things are the way they are for a reason . I dont like to think backwards . Better to move forward . I try to anyway ! Seriously ......I season my wood like 8 to months usually . Can always tell that its ready . Its all visibly dry-cracked up . Lighter than when I cut it..and I know its been long enough because I remember when I stacked it . The specific numbers of the moisture that was , is , or will be in my firewood dosent mean squat to me . There are too many variables to keep arguing about specific numbers till the end of time !!

 

[fuzz1500]

Sorry to ramble on there !! Its just silly .
Dry your wood like everyone else does..because theres a reason its done that way !!

 

[Whitespider]

I got lost in the math . My dad taught me that to burn wood....the wood needs to be dried , cured , or seasoned ..to burn efficiently... Thats all my pea-brain needs to know... Sometimes things are the way they are for a reason ...

Forget the math... mine ain't correct either.
To be 100% accurate some of it really needs to be applied/subtracted to/from the potential total, some to/from the producible total, and some to/from the actual realized...
Like I said in a previous post... the correct calculation would occupy several lines on this page...
But you can be 100% sure that 6% ain't even close... ain't even in the same parking lot attached to the ballpark...

All you really need to know is... dryer is way more better than wetter... way more‼
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[fuzz1500]

All you really need to know is... dryer is way more better than wetter... way more‼

Agreed !!

 

[Chris-PA]

The core issue is that water has a very high heat capacity. The energy you put into raising the temperature of the water and sending it up the flue is not available to heat your home. The combustion inefficiencies just add to that energy loss by sending uncombusted fuel up with it.

Since you have an OWB you use the firebox to heat water as a transport media. The energy going to heat the water in the wood is comparable to what's needed to heat the system water.

Wet wood could extend the burn time, but at the cost of a large loss of energy.

 

[brenndatomu]

Can always tell that its ready . Its all visibly dry-cracked up.

Random bit of information here (free!) On alot of types of wood, once dry (clear through) those cracks will close back up. Some types take a year to close, some go alot longer. A few years back I cut some live Red Oak in the summer time, took 3 years to dry! I tried to burn a lil bit of it at the one year point, it sizzled out the ends like bacon! Back in the stacks she went...