The Mystery of heat variance in biomass

[Carefreeland@aol.com]

 Dear Bio-E friends,
    There seems to be a lot of confusion about how to properly assess the 
heat values of various woods and other biomass energy sources.  This is quite 
understandable when we consider the complexity of the physical organic 
chemistry involved in the production of these fuels.  Allow me to show one 
landscapers view. 
    Woody tissue is not just pure cellulose.  Although cellulose is the 
largest component in most cases, it is anything but alone.  We can take two 
white oak trees for example of similar age and grown in similar locations and 
have quite a bit of measurable variance in wood fuel components.  Some can be 
attributed to cultivar difference as there is a large number of genes which 
can display within the white oak family. 
    I have walked in the native woods locally, and found "classic" Ohio white 
oak tree leaves to be as varied as fingerprints. Most of the time a 
particular tree is identifiable, although the leaves can vary slightly even 
on a single tree.  It is the individual tree that has a given group of genes 
displaying.  Only if a woods has a monoculture of cuttings, would the genes 
displaying be the same, tree to tree. 
    Now lets look at the other side of the equation.  What are the fuels 
present in the wood which we have to judge heat value from?  There is a wide 
variety of tars, resins and oils, both volatile and non, which make up the 
hydrocarbon mix.  There are sugars, starches, and even proteins also involved 
in plant biochemistry.
    Just how a plants roots have grown with relationship to water, and 
localized soil chemistry can alter metabolism.  The presents of sun and 
shadows can dramatically affect the makeup of chemistry in a given part of a 
plant.  Many of these hydrocarbon compounds are building blocks of various 
tissue so what is needed at a given time and place on a plant can affect the 
mix.
      Many plants produce resins and other chemicals as a defense response 
against predatory insects, fungi and bacteria. Other plants produce chemicals 
for frost protection, or to balance less than optimum soil chemistry.  Some 
chemicals are produced to protect plant tissue from excess sun or wind 
drying.  Plants that are exposed to high winds grow completely different in 
structure than the same plants in a calm location.  Fruiting habits change 
from year to year with the weather conditions and can greatly affect growth 
and chemistry of the balance of the plant. 
    I wish I could wave a magic wand and say we could just look at the 
hydrogen to carbon ratios of ash free, moisture free wood, and have a 
formula.  The problem here is how do you predict the bonding of the 
hydrocarbons?  Depending on the energy of a particular pattern of molecular 
bonds in the hydrocarbons, the same hydrogen to carbon ratios can vary in 
bond energy. 
    All is not lost.  Although we can wastefully argue the fine points for 
days, we do know the final result is with in a range.  I am burning some 200 
year old green ash heartwood tonight with a particular hardness, probably 
related to the grain structure which is much more "squiggly" and therefore 
harder than many green ash trees I have burned. I would predict that it still 
has a chemistry within 10% Mj per Kg of most green ash.  The density would 
still be the biggest variance, maybe 30%?              
    When we switch tree species we can count on an even greater variance due 
to the general makeup and quantity of the various hydrocarbons.  Cherry and 
hedge both produce gases which burn like acetylene from the cracking of 
various sap oil components. The hedge however, produces a slightly hotter 
version which seems to burn cleaner with less soot. The cherry needs more 
mixing of the fuel with air to eliminate the soot, but the sap is mostly 
located in the bark. 
    The reason I recommend pellets for testing, is to at least standardize 
the density to a degree.  With pellets from the same producer generally made 
up of a similar mix of wood types grown within a given range of conditions, 
the law of averages will dictate a narrowing variability. 
    When testing stoves side by side, let us at least narrow the variability 
of fuel used in a given set of tests.  Taking dry "clear" wood from the same 
part of the same tree is about as good as it gets.  If such resources are not 
available, than we must allow for variance and use a larger number of tests 
to average the performance. 
    Every fossil fuel has the same problem, it has just been averaged out and 
compensated for by testing and compensating in the refining process.  Why can 
we not do the same?  We are doing the equivalent of inventing a stove or 
gasifier that burns CRUDE oil cleanly and consistently, well to well, field 
to field.  What a challenge.  Have they ever invented such an animal? 
    My hat is off to all the guys who have tirelessly sought the truths in 
natures complex puzzle, living now, or documenting work long ago.  It is on 
this work that we build our devises and attempt to seek standards which can 
be tolerated by a less than perfect society.  Only by adding our work to that 
already documented long ago, can we struggle to find the constants and 
averages within the gift of natural fuel mix. 
    Always Forward, 
                            Daniel Dimiduk                        
    
    

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