Better approximation to biomass...

["Tom Reed" <tombreed@attbi.com>]

Dear Ron and Stovers:

Ron Larson recently said..

>  [Ron Larson]: A simplified chemical formula for much of the dry biomass
>used in small stoves is "CH2O".

For instance, this would give a "biomass combustion equation" of

CH2O + O2 + 3.76 N2 ==> CO2 + H2O + 3.76N2

with an air fuel ratio of  ((32+(3.76*28)/(12+18) = 4.58.

~~~~~~~~~~~~~
This is a good approximation, and since the formula looks like Carbon and
Water, we get the name "carbohydrate" from this formula.

However, in Ray Desrosier's paper on the thermodynamics of biomass he
examined the actual ultimate analyses of wood and other species.  He arrived
at the formula

C H1.4 O0.6

and this is the formula that I recommend you paste on your wall for "typical
biomass".   Most forms of biomass are very close to this (on a DRY ASH FREE
BASIS, DAF) and you can check out the compositions and heats of combustion
of many examples on my website at www.woodgas.com.    The deviation is
primarily due to the presence of lignin which is much more of a hydrocarbon
than the cellulose and hemicellulose.

So the combustion equation is C H1.4 O0.6 + 1.05(O2 + 3.76N2) ==> CO2 + 0.7
H2O + 3.95 N2

and the Air/Fuel ratio is 6.3, significantly different from 4.6, just as a
diet of carbohydrate, protein and fat is healthier than one of pure
carbohydrate. (High quaility ligninless paper probably comes closer to Ron's
approximate formula).

So, use CH2O for back of the envelope calculations and C H 1.4 O 0.6 for
engineering.

~~~~~~~
This is not a criticism of Ron's simplest approximation, since

"there is more joy in heaven over a good approximation than in the most
exact specific data".

Yours truly,                 TOM REED



Dr. Thomas B. Reed
1810 Smith Rd., Golden, CO 80401
tombreed@attbi.com; 303 278 0558 Phone/Fax
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