Webmaster said:
Further study...
Wood has 20% moisture, so an extremely efficient stove might output 6,000 BTU per pound
so the cord is 3500 x 6000 or 21,000,000 BTUs
Pellets have less moisture, and a heating value therefore of 7,000 BTU per pound (output)
2000 x 7000= 14,000,000 BTU per ton
21 million is......(drum roll, please)...........50% greater than 14 million.
Hmmm....even more study suggests the following though (I'm a long time wood burner for my wood-fired brick oven so I've done some of these calcs before <grin>):
Seasoned wood is about 20% water so a pound of "dry" wood is actually .83 lbs wood and .17 lbs water.
Every pound of wood has 8,600 BTUs in it.
Every pound of water requires about 1,200 BTUs to vaporize.
So, a pound of dry wood yields (at 100% combusiton efficiency) .83 * 8,600 or 7,138 BTUs. But wood only reaches that level of combustion efficiency at very high temps (1100F+) where it can burn the volatiles that are in the wood (this is why wood fired ovens are white when they're cooking - the soot that forms early in the cycle burns off at these high temps - and why pizza tastes better because the deck of the oven is at 700F). So assuming a combustion efficiency of 85%, that pound of wood releases 6,067 BTUs of which some is lost to flue gas temps, etc. yielding an overall efficiency of perhaps 70% or 5000 BTUs per pound of "dry" wood.
Now a pound of pellets also being wood has the same 8600 BTUs potentially available. However it is far drier - 5% or less so that pound of pellets is .95 lbs wood and .05 pounds water. That will yield .95 * 8600 = 8,170 BTUs at 100% efficiency (very near the 8,200 that's bandied about as the heating value of pellets). Again we're at the mercy of some efficiency loss due to things like flue temps and minor volatile loss but with pellets that's far less - 98.5% according to the EPA so "real" burn efficiency would yield 8047 BTUs. Real-life overall efficiencies of 70% (a pathetic performance if a wood stove can acheive the same) would still yield 5,719 which is 15% better than dry wood and at something like 80% overall efficiency a pound of pellets would yield 6,536 BTUs vs. the dry wood's 5,000 BTUs or 30% better on the pellet side.
So, a pound of pellets is "hotter" than a pound of dry wood. Unfortunately while pellets are sold by weight, cordwood is not. A cord of dry wood contains 128 cu feet of wood & air - the air having bupkis for heating value in the stove since you leave it outside. The actual solid wood content ranges from mid-60s to 90 cu feet. Assuming a quality 80 cu ft/cord you get that magic 3,600 pounds/cord. However, that's for a really tight, straight minimal airspace cord of dry oak. Sound forest management techniques recommend that you want to cut the low-quality crooked many-limbed trees on a woodlot for sustainability (otherwise by cutting all the straight clean trees you end up with a junk woodlot). Those crooked trees yield a very open cord (more on the 60 cu ft end than the 90 cu ft end). If those are the kind of cords you get then you'll get about 2,900 lbs/cord.
According to the calculations above, that would yield 2900 lbs/cord * 5,000 BTUs net/lb = 14.5M BTUs/cord vs. 2,000 lbs/ton pellets * 6,536 net BTU/lb = 13.1M BTUs or only a 9% benefit toward cordwood which in practicality is an almost non-existent advantage.
What we need here is a lab where we can weigh and burn things.
A loaded question.
