how do I estimate the efficiency improvement due to burning dry wood?

I understand the creosote-forming danger of burning unseasoned wood, and I understand the performance issues burning unseasoned wood in modern catalytic high-efficiency stoves. But we also talk about the wasted BTUs due to driving off water. Makes sense intuitively, but I did some quick calculations and now I'm questioning how large that effect is.

If we take a model Hearth.com burner running his stove on red oak at 20% moisture content - that cord weighs 3,461 pounds - 2,769 pounds of wood+gas, and 692 pounds of water.

Then we compare him to your "typical" wood burner - cuts that same oak in the spring, burns it in the fall. Let's assume he achieves 30% moisture content. So that same cord would weigh 3,956 pounds - the same 2,769 pounds of wood+gas, plus 1,187 pounds of water. So your typical burner has to drive off an additional 1,187-692 = 495 pounds of water. The energy it takes to do so is wasted heat.

He must raise that water's temperature from room temp (let's say 62F to make the result simpler) to the boiling point, which is 150F away from 212F boiling point at 1 atm pressure. That takes 495 pounds * 150 degrees = 74,250 BTUs.

Then he must evaporate the water as steam. The latent heat of vaporization of water at 1 atm is 970 BTU/lb. So it takes 970 BTU/pound* 495 pounds = 480,150 BTUs to drive off the steam.

So his total wasted heat is 74,250 + 480,150 = 554,400 BTUs. Which sounds significant until you realize that that cord of wood holds 25 MBTU. So the waste heat represents 2.2% of the BTUs in that cord of wood. Hardly anything of practical importance; if I buy a car listing 30 MPG fuel efficiency and I achieve 29.4 MPG I'm a pretty happy camper.

Have I made any errors in my calculations, or is the efficiency improvement of 20% seasoned wood being overstated?

Babaganoosh said:

There are 2 ways to determine the moisture content of wood and I don't think the common way where we come to 20% accurately tells you the true weight of the moisture in the wood.

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If you're referring to the lower heating value vs higher heating value, I assumed LHV since most wood stoves have no way to condense and recover the energy in the steam.

And the 20% moisture content is the percentage of water by weight, no? Is there another definition I should be aware of?

saskwoodburner said:

If it only affected the fire by 2.2%, I don't think a guy would notice at all. While your calculations as they stand might be correct, there has to be more factors not figured here. It's not voodoo, and it's not witchcraft, but it's something.

Have you noticed the difference between a fire with dry wood, and a fire with higher than optimum moisture? That itself will tell you it's more than 2%. I've burned lousy wood just to see if/how to make it work, and it just doesn't get up and go.

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I trust me, I WANT it to help by more than 2%. Otherwise the 10 cords in my side yard are quite a waste of space [emoji23]
I don't believe in voodoo either, I'm an engineer and a BTU is a BTU, regardless of how the fire "behaves". So what am I missing in my analysis?

fox9988 said:

The steam inhibits secondary burn, letting BTU laden smoke go up the chimney, killing efficiency. Woodstock has done efficiency testing with wet wood. Its a good read. They basically say their cat stoves extract the BTU's out of the damp smoke pretty well, but it kills cat life. Secondary burn stove's efficiency falls off fast with moisture, according to their results.

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Is this the report you're referring to?: http://blog.woodstove.com/2012/01/catalytic-combustors-wet-wood.html
This video is linked to it:

If you're looking for justification to burn wet wood (I'm not) - this is it! They are demonstrating how their steel catalyst removes particulate emissions and creates a long burn even at smoldering temperature with wet wood. Unfortunately they didn't measure efficiency which is what I'm most interested in!

rdh1223 said:

I'm no scientist but I would think the "steaming" of of water/combustibles ie black smoke reduces the efficiency of the combustible as well as the appliance.

The carbon that is suspended in the smoke and wisked up the chimney doesn't get used. And because of a higher carbon to air ratio in the firebox the appliance loses efficiency, which in turn would make it hard to get a good burn with the damper closed down a bit. I would think the wetter the wood the more air you would have to allow the stove to pull which means more draft which means more heat up the chimney.

I would tend to think that a secondary burn or cat stove would atleast try to use the carbon before it goes up the flu but for that you have to have very high temps, something that's hard to achieve with wet wood.

I think that the energy in the fuel is constant (for a given type of wood) and that all the energy that can be expended during consumption wI'll be, but in what form? Carbon/heat mix in the form of smoke going up the stack or heat in the room.

Just a guess though, truth be told I failed physics in high school, too busy chasing girls.

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This explanation sounds reasonable. Has any manufacturer done an EPA-approved experiment to measure this effect? If the wasted heat is going up the chimney, it should be easy to measure with a flue thermocouple.

tarzan said:

Saskwoodburner is right! The bottom line is, modern epa stoves do not burn wet wood well. A btu is a btu regardless of how the fire behaves is flawed thinking. If you need 40,000 btu' s to heat your home but your stove is limping along at 20,000 because you can't get it going with wet wood then you have a problem. If you get up, load the stove and then off to work but an hour later your wet wood is still not burning, then you got a problem.

That's without even touching on the creosote build up you would have to deal with.

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I assume you meant 40,000 BTUs per hour, which is the heating RATE, not the total # of BTUs extracted from the wood. If the BTU output rate were 1/2 with wet wood, then the load should burn 2x as long at the same efficiency. I'm interested in fuel efficiency, not heating rate for the present question. I agree that a stove that inadequately heats your house is a practical problem, but it doesn't answer the efficiency question.

rdh1223 said:

Also steam is just hot water, it's takes the BTUs (wood) consumed to make the steam and whatever is held in the steam (residual btus ) up the flue. Just storing energy in a diff way with no mechanism to retrieve said energy.

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Right, that's the mechanism in my calculations. Which is why I added the heat required to get the water to the boiling point, plus the latent heat of vaporization required to flash it to steam, which I assumed was lost up the chimney (not recovered). And those calculations showed 2.2% efficiency loss, unless you can tell me what I did wrong.

fox9988 said:

Correction, from the reply to the comment at the bottom of the page:

"Wet wood should not have an effect the longevity of the steel catalysts, but it will delay light-off and suppress performance somewhat. The biggest problem with wet wood is that it is woefully inefficient (like moisture in your car’s gasoline)."

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I see that conjecture in the comment. And not a single piece of evidence (analytical or experimental) to back it up. They didn't even measure efficiency on their wet wood test - that row of the table is blank!

BrotherBart said:

I want to know how you get red oak down to 30% from Spring to Fall. Been burning red oak for 30 years and sure have never been able to pull that off.

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Fair enough. Let's say your "typical burner" achieves 40% moisture content by weight in one season. Running those numbers through the same calculations, I get 5% loss in total BTUs. The effects we're talking about are a lot bigger than 5%. Where's the mismatch?

Swedishchef said:

Haha..people are getting a little over excited. The OP was trying to hammer out the math and understand the numbers. He is not condoning burning wet wood. We all know it sucks ans doesnt work. Now lets see the numbers to explain it!!

I do think that part of the problem with the numbers is that it is a more complexe system than we think. In order to help burn wet wood, you need to open the primary aor more which significantly reduces the efficiency of the stove.



Andrew

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I have this problem a work all the time as well! Let's understand the phenomenon and look at the data, and leave the emotions out of it. We all "know" that burning wet wood sucks, and that you're wasting BTUs sending steam up the chimney. That should be supported by analytical or experimental evidence. I tried and failed to show it with my calculations, but surely some stove manufacturer has the data using their $100,000 laboratory test setup: http://woodheat.org/emissions-testing.html

If the extra airflow is reducing efficiency, we would see that in the test results. Just like car manufacturers list city and highway MPG, not just the 55 MPH cruise efficiency.