I noticed that as the moisture % climbs up the net heat drops drastically. another great argument for burning the driest wood possible.
****" Wood stoves are a form of Solar heating ? " ***** " Combustion " The fundamental chemical equations for the combustion of wood are very simple: 2H2 + O2 --2H20 C + O2 --CO2 The trick is to supply enough heat and oxygen (air) in a sufficiently turbulent state to persuade the individual atoms of hydrogen and carbon to leave their comfortable quarters in the complex hydrocarbons and to unite with the free atoms of oxygen, and thereby to release, the enery from the sun that was stored during the growth of the wood.
Energy to boil water reduces the BTU: "When wood is alive it consists primarily of water, i.e. most of its weight is actually water. After being cut to length and stacked for a year or two, the average moisture content generally drops to 20% or so. Another way of saying this is that 1.25 pounds of well seasoned wood contains 0.25 pound of water and 1.0 pound of wood fibres. You can easily see that our piece of seasoned wood has 8660 Btu per 1.25 (total) pounds or 6930 Btu/pound of actual total weight."
Yeah I guess there wouldn't be much water in a lump of coal. It is delayed solar heating though. It's definitely noticeable though when you're forced to burn wet wood that you get less heat for the amount burned. Makes the inside of the chimney nice and shiny and volatile too.
I wonder if there is much difference between green wood and wood that has been seasoned that has become wet by rain or snow?
Interestingly if you use that chart to figure work out the math of the heat value per lb of wood you really don't get much (any) extra heat value out of the wood by drying it. Follow this math: Starting with a particular piece of wood that weighs 1 lb with a moisture content of 100% has a btu value = 4015 (low value) Now if we dry that same piece of wood down to 0% moisture content we will reduce it's weight by 50% so it will now weigh 1/2 lb. 1 lb of wood with a moisture content of 0% has a low btu value = 8030 (low value), but our piece of wood now weighs only 1/2 lb so that gives us 1/2 the btus = 4015 btu , 1 lb of wood at 100% MC = 4015 btu 1 lb of wood at 100% MC dried to 0% MC weighs 1/2 lb 1/2 lb of wood at 0% MC = 4015 btu
Besides the heat it takes to boil off the water, when wood is wet there's all the unreleased energy sent up the chimney as smoke because the combustion temperatures are so depressed. Not sure what the source of this quote was, but it's generally wrong. Most species average MC well below 100% even when live and standing.
Lumber-Jack, your math is too simple. The heat content of wood does not change after the wood is dried but the way that heat is used sure does. In wet wood most of that energy is spent drying the wood so that it can burn. In dry wood that same energy is used to heat up your stove.
Yes wetter wood uses energy to evaporating water out of the wood, but this is precisely why the total BTU per lb of wetter wood is less than that of drier wood. What part of the my math below is incorrect? (according to that chart) 1 lb of wood at 100% MC = 4015 btu 1 lb of wood at 100% MC dried to 0% MC weighs 1/2 lb 1/2 lb of wood at 0% MC = 4015 btu
The point is that the wood weight decreases as you dry it, so 1 lb of 100% MC wood dried to 0% MC would only weigh 1/2 lb. So although the wood's btu content per lb goes up as it dries, the weight goes down so it loses overall btu value. So 1 lb of wood at 100% MC = 4015 BTU. The same piece of wood dried to 0% MC will weigh 1/2 lb. That 1/2 lb of wood, now dried to 0% MC, will only have 4015 btu. So according to the math, as outlined on the chart above, that same piece of wood has the same potential btu whether it is 100% MC or 0% MC. So you don't actually gain BTUs by drying wood.
Thats sort of right. You just need to think of the 'wood' as if it was dry. So the one pound piece at 100% was half water and half wood. The half pound of dry wood has potential BTU of 4015. Get rid of the water and you still have the same half pound of wood with a potential of 4015. Nothing changed with respect to the wood.
So you agree then, drying wood is a waste of time. Actually the math logic is not flawed if we were to deal with or were to buy firewood by weight. The weight vs BTU quandary is only really relevant if we deal with firewood by weight, but we don't, rather we generally only deal with it by volume. A square foot (or a cord) of wood dried from 100% MC down to 0% MC would hold a lot more btus, even with the small amount of volume reduction (shrinkage) that occurs.
Yes. But we were headed in a little different direction than others. There's a certain amount of energy in wood regardless of moisture content. I guess you feel the energy when wood is dry and seasoned versus wet simply because it didn't go towards turning liquid water to vapor.
No, a square foot of wood wet or dry will still be a square foot of wood (minus some structural shrinkage). The BTUs in it are exactly the same. You don't get more BTUs by removing water. You just get the ability to utilize them better without the water being there.
