Wood Has Relatively Poor Heating Value

Technical questions that one would like posed to experts
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Wood Has Relatively Poor Heating Value

Postby Wilberforce » Wed Mar 05, 2008 3:42 pm

Wood Has Relatively Poor Heating Value

Heat Contents of Fuels
http://www.energyadvocate.com/heatvals.htm

SPECIFIC ENERGY/ Calorific Values
http://www.cleavebooks.co.uk/dictunit/notes3.htm

Wood is a carbon-based fuel, and has less energy per kilogram (than coal or oil) simply
because of three properties:

(A) Density. Wood has lower density than some other fuels, but not always.
(wood floats on water, as does gasoline, oil, and alcohol - if it doesn't mix)

(B) Wood's commonly-high moisture content, unless it were dried to free-moisture
extinction in a kiln: Thermodynamically speaking, it does take a considerable quantity
of energy to simply vaporize the free water present in firewood.

(C) Wood is a carbohydrate. Carbs, by weight, contain 60% water, right in the structure
of the molecule. (water does not burn!) This 60% figure is after all free water extinction.
("bone-dry" wood)

Wood is a mixture of mostly cellulose, (with some lignin and resins.) Cellulose is a very
long-chain molecule made up of principally glucose "units" bonded together in strands;
these strands are tightly woven with other similar strands (fibers) not unlike the way a
rope is wound. This accounts for the high strength of wood. These strands are so small
they can only be viewed under an electron microscope.

Under the process of pyrolysis, these strands will break their bonds, snap off from the
base layer, and forcefully blow outwards, to be ignited in the hot conflagration which is
(hopefully) taking place. That is, below 400°C the smoke particles may not necessarily
catch fire - this is the principal problem common to solid fuels.

Calculations
glucose molecular formula: C6H12O6
cellulose molecular formula: (C6H12O6)n [where "n" represents thousands of glucose units]
empirical formula of glucose: CH2O
molar mass: 180.156 g/mol [which figures to 40% carbon + 60% H2O]
wood has about 1-1.5% ash

Other sources
Combustion Science, Principles and Practice, J.C.Jones, pub. Millenium Books
Organic Chemistry, T.W. Graham Solomons, pub. John Wiley and Sons
Enclyclopedia Britannica, 1973, Wood and Wood Products
• The Surgeon General has determined that there is no safe level of exposure to ambient smoke!

• If you smell even a subtle odor of smoke, you are being exposed to poisonous and carcinogenic chemical compounds!

• Even a brief exposure to smoke raises blood pressure, (no matter what your state of health) and can cause blood clotting, stroke, or heart attack in vulnerable people. Even children experience elevated blood pressure when exposed to smoke!

• Since smoke drastically weakens the lungs' immune system, avoiding smoke is one of the best ways to prevent colds, flu, bronchitis, or risk of an even more serious respiratory illness, such as pneumonia or tuberculosis! Does your child have the flu? Chances are they have been exposed to ambient smoke!
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Postby woodburner » Fri Mar 07, 2008 11:42 am

Well yeah, but btu/$$$ is why people use it.
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