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

[fox9988]

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!

I didn't re-read the entire article (from years ago), just scanned it. Seems that the steel cat cleans up the emissions from wet wood but efficiency is still very low.

 

[begreen]

I've only burned damp wood once in the past 10 yrs. We had a stack of maple that didn't dry well due to where it was located and tarps leaking. That was a miserable winter. We went through at about 25% more wood in order to keep the place warm. The other downside is cooler flue gases. Chimney cap screen plugged after one month. Removed it so that we could keep burning, but that spring cleaning was the dirtiest our chimney has ever been. Normally we get around a cup of soot during cleaning. There was a couple quarts that year.

 

[Doug Morford]

Its interesting to see how little energy is absorbed by the extra moisture. I think a lot of the efficiency loss is due to a lack of secondary combustion while burning wet wood. Unless you have a really tall flue, drawing enough air to maintain those secondaries is impossible with wet wood in most stoves. So that amounts to just smouldering wood, and all that smoke up the flue is a lot of wasted energy. Not to mention, if you are able to move enough air into the stove to keep it lit, there's less energy transfered to the room because it moves up the flue much faster.

Interestingly, garn recommends a mc above 20% in their boilers(I haven't been able to get an actual reason for this from them other than, "it's better"), and they likely are able to get away with it because of how rapidly a garn consumes a load of wood.

 

[Dakotas Dad]

Are they being exaggerated? ANY veteran burner can tell you the answer is "no".

Is the mechanism by which it can be explained not easy to find? Yes.

 

[twd000]

Yes, 40,000 btu's an hour. I get that there is only so many btu's in a given amount of wood but I'm not sure what your trying to accomplish with your posts.

In your first post you stated that the waste heat from burning 30% moisture content wood vs 20% moisture content wood was hardly anything of practical importance. I'm only saying that it is of practical importance in the real world of burning.

Take your car analogy and then add to it that it smokes and takes about an hour to get started, then you would probably not even care that it got close to the average MPG,s that it should.

Also, the title to your thread is "are the efficiency benefits of dry firewood being overstated". Point me to where it is being overstated.

You will never convince veteran wood burners on this sight who have burned <20% wood to ever go backward cause they know better. There is more that factors in to the practical side of wood burning than what you are taking into consideration. What really bothers me is the fact that the veterans on this sight devote so much of there time convincing new burners that they need dry wood (and rightfully so) then you come in here posting something that could easily mislead new burners whether you know better or not.

I'm not advocating burning wet wood; we are all in violent agreement here. A better thread title would have been "how do I estimate the efficiency improvement due to burning dry wood?"
In other words, if I have a stack of 25% moisture wood, how many more BTUs can I expect to get out of it if I let to dry for another YEAR to get down to 20%? What if I'm at 20% and I want to reach 15%? Many new burners have storage space limitations and have a hard time justifying processing and storing that much fuel 2-3 years before they burn it.

I'm coming around to the view that suppressing secondary combustion is the main reason for decreased efficiency, as stated by several in this thread. I ran into this when learning to use my stove - closed the bypass prematurely and sent loads of black smoke up the chimney and very few BTUs into the room. Once I learned how to measure when the catalyst was in the "active" zone, the problem went away. In that case the problem wasn't wet wood (it was 15% MC) - it was operator error, running the stove in a way it was not designed to run.

If we continue to tell new burners that a large % of BTUs are wasting driving off steam, any knucklehead like me can run the #'s and figure out that ain't true. If instead we tout the efficiency improvements of secondary combustion (and creosote reduction), we make a much more justified argument for properly dried wood.

 

[Swedishchef]

Well stated!

I also think that it is difficult to estimate the loss of BTUs from the secondaries not going off. I know that my stove gets HOT when the secondaries are burning but when they are not, the stove is "meh". It would be great to find a way to calculate the difference.

Andrew

 

[begreen]

Even an older stove with a baffle will likely have some secondary combustion occurring and thus some notable efficiency loss. Our old Jotul F602 is an example where I definitely notice the difference in stove top and flue temp. In order to burn more damp wood I need to have the air open up to 25%. With dry wood it needs to be almost completely closed. The heat loss should be calculable. Lower combustion zone temperatures will equal more heat loss in steam, unburnt wood gases and particulate heading up the flue.

Here is a discussion from several years ago on the topic:
https://www.hearth.com/talk/threads/heat-loss-due-to-wet-wood.58445/

For the enthusiasts that want to run the numbers here is a paper on calculating the effect of moisture on heating values. It notes in the tables at the end that the difference between burning 10%mc wood vs 30%mc is not trivial.
http://cta.ornl.gov/bedb/appendix_a/The_Effect_of_Moisture_on_Heating_Values.pdf

 

[twd000]

Even an older stove with a baffle will likely have some secondary combustion occurring and thus some notable efficiency loss. Our old Jotul F602 is an example where I definitely notice the difference in stove top and flue temp. In order to burn more damp wood I need to have the air open up to 25%. With dry wood it needs to be almost completely closed. The heat loss should be calculable. Lower combustion zone temperatures will equal more heat loss in steam, unburnt wood gases and particulate heading up the flue.

Here is a discussion from several years ago on the topic:
https://www.hearth.com/talk/threads/heat-loss-due-to-wet-wood.58445/

For the enthusiasts that want to run the numbers here is a paper on calculating the effect of moisture on heating values. It notes in the tables at the end that the difference between burning 10%mc wood vs 30%mc is not trivial.
http://cta.ornl.gov/bedb/appendix_a/The_Effect_of_Moisture_on_Heating_Values.pdf

Thanks for the links. They support my analysis showing that the "driving off steam" effect is less than 10% loss in BTUs from a wet cord to a dry cord. Since we have all experienced much larger gains in practice, it supports the conclusion that more significant phenomenon are at play - suppressing secondary combustion and excess makeup air.

 

[begreen]

Yes, incomplete combustion due to lower firebox temps is a significant factor.

 

[Sprinter]

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.

Agree with fox and with begreen above. @twd000, your calcs regarding wood combustion efficiency is correct. What is missing is that the secondary mechanism, cat or non-cat requires a certain minimum temperature to do the secondary burning. About 1100F for a non-cat. If this temp cannot be maintained because of the cooling effect of the water, overall efficiency will suffer, but that is due to stove design which depends so much on secondary combustion these days.

I've heard that stove manufacturers specifically design their stoves typically to work best at 20% MC.

 

[Poindexter]

Couple items i havent seen mentioned.

495# of extra water, i burn eight cords a year, carrying 4000# of extra water to the stove every year is gonna cost me extra tylenol and scotxh this year, and likely buy me knee replacements even earlier than i am already on track for.

Emissions. Chwxk your stack next time your stove is limping on 25%MC. If the kid that gets asthma is yours you are going to be spending vacation time in doctors offices and travel money in pharmacies. Maybe go to the lake on a credit card in 2022 instead of paying cash to go to hawaii...

 

[twd000]

Agree with fox and with begreen above. @twd000, your calcs regarding wood combustion efficiency is correct. What is missing is that the secondary mechanism, cat or non-cat requires a certain minimum temperature to do the secondary burning. About 1100F for a non-cat. If this temp cannot be maintained because of the cooling effect of the water, overall efficiency will suffer, but that is due to stove design which depends so much on secondary combustion these days.

I've heard that stove manufacturers specifically design their stoves typically to work best at 20% MC.

Yes it's all about the secondaries; the vaporization's direct effect on efficiency is minimal.

Another way I'm thinking about it- you hardly notice the effect of burning wet wood in an open fireplace. Since the efficiency was so lousy in the first place, with no secondary combustion, burning higher moisture wood isn't much worse than dry.

 

[peakbagger]

If someone really wants to delve into combustion efficiency, chapter 8, Principles of Combustion in the B&W "bible" Steam Its Generation and Use goes into way more detail then most folks would ever want to go. If you go on used book sites there are usually older editions for sale for cheap.

 

[yooper08]

Without any lab equipment to measure output, the easiest/most reasonable way I can think of to test this would be to burn the wet wood and dry wood in very similar conditions and measure the input (wood weight) consumed to maintain similar room temps.

Fuel efficiency in this case is how much of the combustible wood mass does the stove burn. How many BTUs the stove outputs into the room is another story but related. Maybe to give you an idea, consider the pre-EPA stoves burned at 35-55% efficiency. Since these had little to minimal secondary burning, that could be a simple proxy for wet wood in modern stoves. What that won't answer for you is 30% MC vs 20% MC.

 

[twd000]

Without any lab equipment to measure output, the easiest/most reasonable way I can think of to test this would be to burn the wet wood and dry wood in very similar conditions and measure the input (wood weight) consumed to maintain similar room temps.

Fuel efficiency in this case is how much of the combustible wood mass does the stove burn. How many BTUs the stove outputs into the room is another story but related. Maybe to give you an idea, consider the pre-EPA stoves burned at 35-55% efficiency. Since these had little to minimal secondary burning, that could be a simple proxy for wet wood in modern stoves. What that won't answer for you is 30% MC vs 20% MC.

Yes an experimental/empirical method would be the best way to get an answer. I was hoping a woodstove manufacturer would have already performed this test in their EPA-certified laboratory and published the data to emphasize the importance of dry wood, but I can't find anything online. I don't think an amateur can gather quantitative data beyond "burning wet wood sucks!"

 

[begreen]

Every winter we get empirical evidence reported here by first time stove owners that burn through their first cord of seasoned wood and then have to buy some "seasoned" wood in mid-winter that is anything but "seasoned".

 

[Swedishchef]

Every new burner has been shafted...me included!

I would love to see some data from stove manufacturers during their EPA testing.

Andrew

 

[yooper08]

From what I understand from the Oak Ridge paper begreen posted, at least for poplar, moving from 30% to 20% produces 17% more heating value.

 

[Sprinter]

you hardly notice the effect of burning wet wood in an open fireplace.

Some of that lost energy can be reclaimed with a good chimney fire. It's a lousy way of getting that secondary burn, though.

 

[Sprinter]

I've only burned damp wood once in the past 10 yrs. We had a stack of maple that didn't dry well due to where it was located and tarps leaking. That was a miserable winter. We went through at about 25% more wood in order to keep the place warm. The other downside is cooler flue gases. Chimney cap screen plugged after one month. Removed it so that we could keep burning, but that spring cleaning was the dirtiest our chimney has ever been. Normally we get around a cup of soot during cleaning. There was a couple quarts that year.

Same thing here with our first year. What was funny was that our sweep/installer told me that I could expect the screen to get clogged regularly (regardless of wood) and that I should probably remove it when he left. Well, it did get clogged the first month, and I did blow the light stuff out of the screen. But thereafter, I learned better how to get the most out of the stove (thanks largely to this forum...) and the wood improved. I have never once had any buildup on the screen since.

I can only imagine what his other clients are doing that made him think this was the norm.

 

[Swedishchef]

Funny thing: my installer told me that I would not need to sweep for 2 years. Obviously I did not follow his advice during my first couple of years. But I will admit that I inspect regularly. Last fall I inspected my setup and realized there was no need to sweep: it was fine. Now it needs to be swept but 6 cords have passed through since the last sweep. I can't complain about that.

Andrew

 

[DanCorcoran]

If you are posing the question out of idle curiosity, this is probably a great place to ask it. You'll get lots of anecdotes and conjecture. But if you really want the technical answer to an engineering question, you'd probably be better off posting on a chemical or other engineering website. While you may get the technical answer here, there'll be lots of chaff with your wheat.

 

[Blaine]

Now that this discussion has about run its course, I'd like to add my own bit of conjecture. I do not believe it is valid to assume that the amount of energy required to boil 495# of water out of a large pot is equivalent to the amount of energy required to roast the same amount of moisture out of 2 tons of oak splits.

By the time this wood has reached 30% MC, most if not all of what might be called water is long gone; the remaining moisture is not water, per se, but tree sap- a mixture of water with plant sugars, minerals, and whatever. This sap is not on the surface of the wood, it is locked inside cells deep inside the wood.

I suspect that it takes a great deal of energy to get water to separate out of tree sap, penetrate cell walls, travel to the surface of the wood, and then be boiled away.

One way to measure the actual amount of energy required to dry wood from 30%MC to 20%MC would be to place a given amount of 30%MC wood- 10# would be convenient- in an oven and measure the amount of energy consumed while cooking out the extra 1# of water. Our wood-burning experiences indicate that this number will be much higher than the 1120 BTU's indicated in the original example.

 

[begreen]

For the enthusiasts that want to run the numbers here is a paper on calculating the effect of moisture on heating values. It notes in the tables at the end that the difference between burning 10%mc wood vs 30%mc is not trivial.
http://cta.ornl.gov/bedb/appendix_a/The_Effect_of_Moisture_on_Heating_Values.pdf

Agreed. That's why it is referred as moisture content. Technical details were provided here ^.

 

[Hogwildz]

Wow, you guys have far too much free summertime. LOL
Get 3 years ahead, css, moisture should not be a problem, toss splits in as needed, and enjoy the warmth at it's best production.
Screw the numbers...