Heat from firewood...green or air dried

[GeneWengert-WoodDoc]

A recent topic has been firewood.  So, I thought some basics might be interesting.

Question #1.  How much extra heat do you get from a cord of dry firewood (20% MC) compared to the heat from a green cord?  Assume that we have red oak, split and that the cord weighs 4900 pounds green.  Green red oak is 75% MC (oven dry basis).  Assume that a pound of oven dry wood releases 8500 BTU per pound, not counting the efficiency of the stove or fireplace.  Question #2.  What is the gain if you have a green cord and then dry this green cord of firewood to 20% MC?

So, a green cord at 75% MC has (4900 pounds / 1.75 =) 2800 pounds of oven dry wood and 2100 pounds of water.  Evaporating the water in a green cord when we burn the cord of wood will require 2100 pounds times 980 BTU per pound, which is 2,000,000 BTU.  The oven dry wood of 2800 pounds x 8500 BTU/pound means a release of 23,800,000 BTU minus the evaporation energy of 2,000,000 for a final net value of released heat of 21,800,000 BTU per green cord of red oak.

If we dry this green cord to 20% MC, we still have 2800 pounds of oven dry wood, but only 560 pounds of water.  When we burn the wood, we only need to evaporate 20% MC, so the net heat release for air dried is 23,800,000 - (980 x 560 pounds) or 23,250,000 BTU net.

Comparing green cord with 20% MC air dried, the gain by air drying is 1,450,000 BTU or 6.7% more heat.  Note that the value of this energy is around $6.  This is not much.  Decreased weight is another advantage.  We would need to heat the wood throughout the piece to 133 F to kill insects.

Now here is the tricky part.  After drying this green cord, we no longer have a full cord.  Due to shrinkage, we need to add about 12% more wood to bring it up to a full cord of dry firewood.  So, a dry cord has 12% more wood; a total of 3140 pounds of oven dry wood which means 12% more heat or a total of 26,000,000 BTU.  At 20% MC, 628 pounds of water evaporation will require 980 x 628 = 615,000 BTU.  So, an air dried cord will have 26,000,000 - 615,000 = 25,385,000 BTU.  This is a gain of 3,885,000 BTU or 18% by drying and then adding wood to get a full air dried cord.  So, one might incorrectly state that air drying means 18% more energy when burning.

Mainly boring.

[doc henderson]

WOW!  thanks Doc.  not to mention increased fire hazard with creosote build up, and difficulty getting the fire started.  bugs being brought into the house in wetter wood.  i wondered about having wood near the stove to dry for a while.  we like some humidity in the house in the winter, and the heat wood still be released when the water vapor condensed i think!  i guess there could be a fire hazard if too close.  i am hoping to put together a fire wood kiln to dry a bit of wood near the end of the summer.  what would be a good way to compare wood air drying outside vs in a make shift kiln with plastic and a fan?  much easier to get sample wood since they will just go into the stove.  is a before and after weight of some logs similar in size and species ok to compare the two methods?  air dry vs tunnel kiln?  what else should I record daily weight, humidity?  The two ways will sort of control themselves if started on the same date.  i.e. a control group and test group.  i could divide into two containers as I split.

[farmfromkansas]

I heat my shop with wood, using a wood furnace in a tiny separate building 4' from the main shop.  If I had to heat with green wood, don't think I could get a fire hot enough to heat the shop, as it would take most of the day to get a hot fire.  Unless using an accelerant.  Cut most of my firewood during winter or spring, then have some time for it to dry before the next winter. Dry scraps from the shop make excellent starter. 

[GeneWengert-WoodDoc]

If you have drier firewood, you will get more heat per stick.  In warmer weather, especially if you added a bunch of sticks to the fire, you will likely get too much heat, so you will throttle down the fire by limiting the air input.  Less oxygen means more creosote...creosote is unburned wood that, if cooled slightly, condenses into the black gunk or hard gunk if cooler.  Heat softens and eventually ignites the creosote.  So, with dry wood, put in less wood rather than less air.

Because wetter wood burns more slowly, with less heat output as it spends a lot of energy evaporating water, we tend to put more wood on the fire.  With excess oxygen, there is little creosote formed in the flue gasses with wet wood.  But, if there is not enough oxygen, then lots of smoke, which is creosote.

Proper venting (enough oxygen) means no creosote and 100% burning within the fire.

[stavebuyer]

Thanks Doc Gene. I have sold(and in earlier days burned) a world of green firewood. While the "newer" stoves won't much tolerate unseasoned wood; your numbers seem to suggest my "wait until the last minute" firewood customers may not be paying near the penalty I thought they were. 

I would also be curious to know how much the size and shape of a piece of firewood impacts the drying and or burning process. Purely anecdotal but at times like now (with Red Oak lumber being next to worthless) I have 4 slabbed logs to make ties and firewood. The heavy slabs with smooth sawn faces seem to behave more like 12/4 lumber taking years to dry versus months for approximately similar thickness "conventional firewood" run through a splitter and having  ragged/torn faces. 

[Ianab]

Size sure as heck affects the burning and drying properties. 

Dropped a small load of wood off to a friend the other day, this is good dry stuff, but assorted species and random sizes. Mill off-cuts, storm cleanup etc, but it all burns OK, so folks are happy.  Anyway she remarked how she was down to the last few smaller scraps and loaded them in her little fire. After 1/2 an hour, she had to open a window, it was too hot. Now the small pieces don't last long, but how fast wood burns depends a lot on the surface area. Split small there there is a lot more surface area, so as long as you can get oxygen in there, it will burn faster and hotter, just not for as long, as there is only the same number of BTUs. I tell my customers to get the fire going with the smaller pieces and the  softwood, as that will flare up quick and get the fireplace hot, then throw in the bigger pieces of hardwood for a longer slower burn. 

Most woods WILL burn green, as long as you can get a decent fire going, and keep it going. If you don't have some heat in there to drive off that excess moisture you struggle to get a good fire going, and most stoves are designed to burn dry wood, and restrict the air flow to SLOW the burn. So the wet wood is a pain to get and keep burning properly, and hence you have unhappy customers. 

As for drying, wood dries faster through the end grain. So a sawed slab or "sleeper" will dry slow, But lop it into 12" lengths and you have more end grain exposed and much faster drying. Same as "conventional" firewood that's been cut to 12-18" rounds before it's split. It's no so much the split vs sawed, it's the exposed end grain. 

An old neighbour had a different plan, he'd just cut the whole log into ~6" cookies, They would dry in a couple of months lined up in the sun. As long as they would fit through the stove door, he was good. If they didn't, they were easy to split being so short, and usually had a crack 1/2 way though them already. 

[GeneWengert-WoodDoc]

In large commercial burners, there are a variety of ways to add enough oxygen, using fans to bring in the needed air.  Sometimes the burner floor has holes for the air.  Sometimes, the grate has holes.  Sometimes air is blown directly on the fire.  This high air flow allows a lot of heat to be generated in a small space and a fire temperature over 1100 F.

There are three basic parts to burning wood.  All three can be occurring with one stick.  First, the wood heats to 212 F at which point the water boils.  There is no flame or burning.  This is why green wood initially seems to burn slowly, smoke, not start burning easily, etc.  If there is not a clear path from the boiling water to the surface, a miniature explosion will occur...popping sounds and sparks.  Once the water is gone, then the wood heats to around 450 F (but there needs to be a source of heat to get the wood hotter)...at tHis point, we see flame or fire from where the wood turns into flammable gasses which burn actively creating carbon dioxide and water vapor, if there is enough oxygen from outside.  When these gasses are consumed, then the remaining wood is all carbon and if heated to 1100 F or so, glows red and produces carbon monoxide if oxygen is scant or carbon dioxide with more Oxygen.  In a wet stick, you might understand why it seems to burn slowly...212 F has a long way to go before reaching 450 F and 1100 F.

Overall, because of the need for oxygen, and the speed of moving all the water, larger pieces of wood burn much more slowly than little pieces.  Little pieces have more surface area per volume which really helps burning.  As a rough generalization, a 6x6 and larger will have a lot of trouble burning all the way through, especially if the outer coals or ash is not removed.

[doc henderson]

@GeneWengert-WoodDoc .  you have nicely laid out why we joke that wood burning is a "solar" (storage) form of heat.  the energy from the sun, along with water and carbon dioxide, make up the tree.  when the tree is oxidized in a fire or on the forest floor, the water, CO2, and energy from the sun is released.  also, if we put wood in a retort (container the allow gas out, but not air in) and place that in a fire, we start with steam coming out, then volatile gasses.  We are left with lump charcoal of 99% carbon, activated charcoal.  i always thought the water in the smoke of wet wood lowered the smoke temp, and was more likely to condense on the flu pipe and depositing more creosote.  and a later good hot fire could burn it off.  a chimney fire results when this is allowed to build up.  either from not a hot enough routine fire, and or not sweeping the flu.  i use a double wall flu pipe to avoid cooling of the gasses going up the flu.  most fires, i get good and raging before shutting it down a bit.  to heat up the metal parts and get most of the water out.  it is my belief that electric cars run on coal, depending on how the electricity was made.  but that was all originated by the sun as well.  

[GeneWengert-WoodDoc]

Hey Doc...pretty good smarts even for a guy from Kansas. :D

Although I talked about the three stages, all three are going on at once as soon as the fire gets going.

Of course, to reduce the CO2 in the atmosphere, make more things out of wood so that the carbon is locked up.  This is not overwhelming, but every little bit helps.

[Old Greenhorn]

Of course, to reduce the CO2 in the atmosphere, make more things out of wood so that the carbon is locked up.  This is not overwhelming, but every little bit helps.

So as I read this a stupid question popped in my head and I sure can't answer it: When a tree rots, does it release CO2 into the atmosphere? Trees absorb CO2 and release O2, correct? They also release CO and CO2 when they burn, right? SO when they rot and turn to dust, where does the Co2 go? ??? ??? :P

[doc henderson]

when it rots, it is a slow process.  so think about yeast eating sugars in wort.  it is CO2 gas in the beer.  so if fungus is among us in the wood and rotting it, it eats the sugars and puts off gas.  a living tree takes in CO2 via the stomata (pores) in the leaves and water in via the roots, and uses energy from the sun in the chloroplast to make sugars to form the structures of the tree based on a cellular dna map.  It makes sugars and if hooked together makes starches that build the structure.  in nature energy is neither created or destroyed.  so as the tree degrades it gives off energy and carbon.  a live tree takes in the CO2 and after taking the carbon, gives off O2 as a waste product.  lucky for us.  sap i believe is water plus sugars to go throughout the tree.  100% efficient burning gives off mostly CO2, incomplete makes more CO (carbon monoxide).  CO is a poison to humans because it binds to hemaglobin 200 times stronger than O2.  so we can asphyxiate.  normally the gases exchange easily on and off.  the antidote is giving 100% O2 to compete with the CO.  and remove the source of CO.  we all run about 3 to 5% CO from cars.  smokers run 5 to 10% CO.  things that are rotting give off heat.  think of spontaneous combustion or heat in a compost pile.  it is interesting that burning wood gives off heat and light just like the sun, and in fact could be termed radiation.  so if we put all the wood in dry storage it would reduce the CO2, but nature is pretty smart.  if we reduced it too much, in theory plant volume would be less and in theory O2 production would decrease.   

[doc henderson]

@GeneWengert-WoodDoc .  What state is it they say, "so many people, so few teeth!?"   :D :D :D

[Don P]

Of course, to reduce the CO2 in the atmosphere, make more things out of wood so that the carbon is locked up.  This is not overwhelming, but every little bit helps.

So as I read this a stupid question popped in my head and I sure can't answer it: When a tree rots, does it release CO2 into the atmosphere? Trees absorb CO2 and release O2, correct? They also release CO and CO2 when they burn, right? SO when they rot and turn to dust, where does the Co2 go? ??? ??? :P

Back into the atmosphere

Wood, and the plant based CO2 cycle is the shallow carbon loop, labile carbon. It isn't appreciably going anywhere in the long term view.  It's just going round and round, into the plants then back into the air. Or, that is current solar energy. However, when we burn we are releasing other compounds and pollutants into the air that otherwise wouldn't be there. Especially true if burning green wood in conventional domestic wood burning appliances.

If you cover the rotting tree with hundreds of feet of soil or volcanic ash, then it is sequestered and forms stored sunlight within the planet, coal, oil.

When we dig up stored solar energy in the form of coal or oil and release it into the atmosphere, now we are adding to the CO2 load in the atmosphere. It took the planet millions of years to sequester that recalcitrant carbon deep within the bowels of the planet, locked away from the atmosphere.

When we build with wood or use it to make furniture or some other "durable" good and make the claim that we are sequestering carbon, we are telling ourselves a bit of a story. That house, or chair is from the shallow loop and will return to the air in very short order in geologic time. It's not that it makes little difference, it makes absolutely no difference. What we are hoping to convince ourselves of is that it is ok to release permanently stored carbon into the atmosphere as long as we offset that with carbon that is stored for a century. These are not the same thing by any stretch.

If we build with wood rather than concrete, then we are offsetting the sequestered carbon used in the production of concrete with the net zero carbon of current sunlight, a real gain.

Trees capture but do not sequester carbon, but their roots do in a small way. A much more efficient plant at doing that is grass.

[barbender]

Gene, I'm not questioning your knowledge on this subject, but the difference in your math doesn't seem to add up in real world practice. Are you sure those are the right numbers for evaporating a pound of water? It just doesn't seem like a big enough difference between wet and dry wood, almost like why bother with tying wood at all? Again, not questioning your knowledge, just whether the wrong number got plugged in somewhere?

[GeneWengert-WoodDoc]

It takes, in round numbers, 1000 BTU to evaporate one pound of water from wood.

A green cord of oak has 75% MC and weighs about 4900 pounds, so the oven dry weight is 2800 pounds.

The energy from dry wood is about 8000 BTU per oven dry pound. 

So, energy released from wood is 2800 x 8000.  The energy to evaporate 75% MC water is 1000 x 2100.

It all checks out and agrees with published values.  

The reason green wood is hard to burn is that the water keeps the wood at 212 F initially, which is not hot enough to sustain release of flammable gasses and flames.  Once the red hot coals are present, then adding some green wood will be fine.  Add too much wet wood and the fire will try to go out as it cools, especially of oxygen is limited.  Remember that the water evaporated from wet wood has no oxygen...it is all steam.

[barbender]

Thanks for the explanation, Gene👍

[doc henderson]

in reality, it would be even less different if the wood is at 20% instead of the oven dry theoretical 0%.  although that 20% has to be boiled as well.  Gene if I want to test a firewood solar tunnel kiln against air drying, can I just compare the relative % weight loss?  i guess to establish a baseline MC would be informative.

[GeneWengert-WoodDoc]

As I tried to indicate in the OP, it is how much oven dry wood you have that determines the heat minus the heat used to evaporate whatever moisture there is.  So, a green oak cord has about 2800 pounds of oven dry wood, with 2100 pounds of water when green at 75% MC, 560 pounds of water when at 20% MC and 280 pounds at 10% MC, etc.

So what are the advantages for drying to 10% MC?  You will get
* lighter weight sticks- - a 5 pound green stick will weigh about 3 pounds
* 7% more heat as there is less energy used to evaporate water
* no living insects if the wood reached 133 F, which means safer inside storage
* no mold or mildew fungi or decay fungi
* faster burning and easier lighting, as the wood quickly moves from evaporating water to emission of flammable gasses
* hotter fire, as the time at 212 F is shorter
* hotter fire means less chance of creosote formation in typical burning equipment operated correctly (enough oxygen)

Did I forget anything?

[doc henderson]

that is perfect and complete.  I am also asking about methods for comparing passive air drying vs a fan in a solar tunnel.  PM me if that is easier.  i have some relatively fresh cut mulberry and i want to see if the tunnel speeds up drying significantly.  want to set it up in a way that after the fact, you will be satisfied with the methods i use.  thanks!  Bryan

[Stephen1]

Thanks for the post Gene, great info as we move into the firewood time of year. 
I am planning on selling Kiln Dried firewood this fall and this info needs a another read.

[GeneWengert-WoodDoc]

Better than "kiln dried" would be to specify that it is below 10% MC, insect free, etc.  kiln dried firewood does not mean a whole lot to the consumer.