[Gasification] Proof of Chemical Composition of Biomass

[Thomas Reed]

Dear Biomass Friends:

*Engineers *are usually content to think in mass quantities - burning 
gasoline requires 14 times as much air as gasoline. But thanks to 
Dalton, we know that the world is made mostly of molecules like H2O and 
CO2 that have exact ratios of atoms.  So the *chemist *is forced to 
think first in atoms, and then convert to mass using molecular weights 
(C = 12, H = 1, O = 16 etc.)  I am blessed/cursed by being a chemist 
first and engineer second.
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In 1978 at what is now the National Renewable Energy Laboratory (NREL) 
we formed a team to write a "Survey of Biomass Gasification", (now in 
its third edition under the title "Encyclopedia of Biomass Thermal 
Conversion", available at WoodGas.com). 

A fresh young PhD, Ray Desrosiers and I worked on  chapter 6  "The 
Thermodynamics of Gas-Char Reactions" and the results in this chapter 
(and the others) have been the solid foundation of my thinking ever since. 
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In order to define "BIOMASS" Ray looked at a large number of analyses of 
various species and took an average.  Naive idealists often use the 
formula C6H12O6 (sugar) for biomass, but that is way off the mark.  He 
worked with  a carbon ratio formula
*C H1.4 O0.59 N0.017. *

Being a pragmatist, I rounded this formula to
*C H1.4 O0.6
***

and threw our the N with the ashes and moisture content, so that I could 
store it in my limited (3 kbyte?) number memory.  If you prefer whole 
numbers, this is *C5H8O3*.  This is * *pretty far from C H2 O, sugar or 
C H1.66 O0.83 (cellulose and starch).  I recommend this number to anyone 
wanting to think quantitatively about structural, land based biomass .  
(reproductive biomass, seeds, are and exception because they contain 
starch (C6H10O5) and oil (CH2)
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This morning, 30 years later, I DERIVED these numbers and was so pleased 
with the elegance of chemistry and the universe that I decided to share 
it with the biomass community. 

Because of my limited memory and because it is very important to see the 
forest before you examine the trees, I have simplified fuel chemistry to 
only four categories:

CH4
CH2 (HYDROCARBONS, the average composition of oil)
CH  (AROMATICS, the composition of benzene, coal and lignin, all closely 
related)
CH2O (CARBOHYDRATES, sugars and the original storage of solar energy 
before cellulose and lignin were evolved)

Vegetative Biomass is composed of both Carbohydrates, ~CH2O, (the 
cellulose, starches, sugars) and Lignin (the waterproofing, needed for 
land plants).  Looking at various tables the weight ratio is 
Carbohydrate/lignin ~ 3.  However, the molecular weight of CH2O is 30 
and CH is 13 (~15) so on a molar basis the Carbohydrate/lignin ratio is 
closer to 3/2. 

So
*3CH2O + 2 CH = C5H8O3 ==> CH1.4O0.6*
*
Quod Errat Demonstrandum, QED. 

*Note:  This is "fuzzy math", or the general organizing principles 
behind the more numbers than you can remember if you have a 3 kb brain.)

I hope this pleases you as much as it did me and that you will be able 
to remember *CH1.4O0.6* as more than just an average.
Yours truly, 

TOM REED       THE BIOMASS ENERGY FOUNDATION
*

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