stoveguy2esw said:
ok , here's the physics again...
wood in its natural form still conatins approximately 8500 BTU per lb stored energy. the amount of moisture present does not decrease this, however the amount of moisture contained determines what this stored energy is used for in a fire.
before wood can be consumed(as in burnt) the moisture present must be evaporated away. cooking this moisture out of the wood takes thermal energy (BTU's) while this is happening the moisture leaving is simply retarding the building of excess heat (remember evaporation is a cooling function) therefore high heat releasing doesnt happen so the complete burning (which results in lower emmissions) doesnt happen or is seriously degraded. once this green wood has been "dried" by heat it starts being consumed at a rate which allows faster release of stored energy not restricted by evaporation and "clean burning" starts.
the "cooking out" of green wood wastes a large percentage of stored energy. while the "load" may last longer, the usable fraction of the "stored energy" released into the room from radient heat is reduced due to the need for this stored energy to create evaporation before the load can start producing heat used for the intended purpose.
Mike, I'm not a physicist, but I understand the physics and chemistry behind all this pretty well, maybe a little too well. That's why I have such a hard time wrapping my mind around some of the conflicting logic I often read here. So, no disrespect meant, but wood in it's natural form contains water, so it can't possibly contain 8500BTU/lb. Water don't burn. There is also the latent heat loss of water of combustion to subtract. This brings the actual potential for sensible heat output (low heat value or LHV) down to only about 6000 BTU/lb wood at 20% MCwb.
Since about 1050 Btu are necessary to boil or evaporate a pound of water, and 1 Btu additional is necessary to raise the pound's temperature 1°F, it is possible to determine the latent heat fairly easily by knowing the total weight of water vapor given off by the fire. We had the 0.25 pound of moisture content. Add about 0.54 pound of water vapor as products of combustion. If we assume low humidity conditions that contribution is small. We now have 0.79 pounds of water vapor that started at say 60°F average temperature and was heated to say 400°F. The latent heat is then 0.79 times (1050 plus 340 temp rise) or 1098 Btu per 1.25 pound piece, or 880 Btu/pound. Therefore, the (low heat value (LHV) of wood fuel is less than the high heat value (HHV) by this amount. The result is that the available energy in seasoned (20% moisture content) wood used in an actual usage environment (400°F flue gases) is about 6050 Btu/pound. We feel that this is the most realistic number to use for domestic wood burning as it is the number that would apply if the user weighed his wood as part of determining efficiency of his appliance.
http://mb-soft.com/juca/print/311.html
Now, if you were to add another 1/4 pound of water to that 1 1/4 pound split, it would weigh 1 1/2 pounds and would be 1/3 water by weight (BTW such a split would read 50% MC on a meter... if they could read that high). The heat penalty for that extra half pound of water would be about 525 BTU out of a potential 6050 BTU lockup up in the wood fiber... about 8.7%. Significant? Yes, but not a total deal breaker to me.
Now, let's go to that typical set of circumstances that Old Spark was referring to above. A newb can't get a good burn. Now, we all know it
can't be poor air control, load timing, firebox load configuration, poor draft, excess draft, wrong flue size, outside air temp, or even unfamiliarity with what a good burn should look like. It
has to be the wood. Well, his wood was cut and stacked 6 months ago. Immediately, his fuel is brought into question. Wood can't possible dry that fast. Better get a meter. The next day he goes to Lowes, buys a moisture meter and reports a few days later that it's showing 16% on the meter. Immediately, the accuracy of his meter is brought into question. "Is it from inside a freshly-split face?" "Oops. No, it was on the outside."
So the guy grabs his axe, goes out back, and splits the piece right down the middle. Now he gets 28% on the meter. No one questions the meter anymore, the meter's fine, now they say the wood is way too wet. "Burn oil this year instead of ruining your new stove or dying from a chimney fire or maybe the plague. Start getting ahead now for the next 17 seasons, wood just gets better and better with age."
Now for starters, that 28% MC reading on the meter is a dry-basis calculation. After you do the mathematical conversion into a wet-basis calculation (weight of water divided by original weight), you find out that the wood is really only 21.8% water by weight.
This is only 1.8% higher than the maximum MC allowed during the EPA test. Surely, as a stove expert, you must realized the insignificance of 1.8% water to the heat output of the burn. Now add this into the equation.
That split that was 28% MC on the meter on a fresh inner face was also 16% MC on the outside. Since there is a diffusion gradient present throughout the thickness of the wood, the chances are very good that the average MC for the entire split is well below 20% water by weight... but still considered too wet to burn. Yes, that's correct. Wood that is in the same moisture range as the wood used to get those unbelievably low emission rates during the EPA test is simply too wet to burn at home.
Man, it's only 10 AM and already I need a drink. %-P
BTW that quote I took off that website is paraphrased from a great book written almost 40 years ago by Dr. Jay Shelton. It was good info then, it remains so today.
!!
