[Stoves] Fuel Testing

[psanders at ilstu.edu]

Andrew,

Your clarification is excellent.  I will take it one step further:

Assuming 20% moisture (so that the numbers are both high enough and also
realistic for many fuels in many areas), then the energy would only be 80% of
18.6 MJ/kilo = 14.9, and then the loss of 0.54 MJ from the water results in
14.4 MJ.  That is a significant loss.

However, can we disregard (for the moment, at least) that moist wood is only
very slighly larger in physical size (volume) than is dry wood?  If dry and
moist wood are the same size, then the container of fuel (1.1 liter size in my
case) could have the same full measure of wood chips (or fireballs, etc) AND
include the extra weight of the 20% moisture.  This leads to saying that the
moisture causes only 2.9% reduction in the useful thermal energy, when dealing
by volume, not by weight.

Interpreting this in relation to my method of testing:  I could run two 
tests of
wood chips measured by volume, one with minimal moisture and the other 
with 20%
more moisture.  The total available energy is only 3% less in the heavier
moist-fuel test.  So, IF OTHER THINGS WERE EQUAL, the overall test results of
heating the water should be about the same.

But other things are not equal.  By being dry, the minimal moisture fuel will
pyrolyze differently (as explained by you or another in a previous message
about a moist stick and a dry stick in a fire.).  So therefore I would need to
have VERY GOOD controls on my air flows, etc, and still probably struggle to
prevent a "run-away" fire, or at least I would have a fire that behaves
differently from the way the moist-fuel fire behaves.

For our efforts in the villages, what is discussed above is important to know
and we must live with the difficulties of non-consistent fuels.

But for our efforts in the testing laboratories, this impact is (can be)
devestating!!!!  If stoves are "certified" or tested to meet benchmarks that
are related to the kiln-died or oven-dried moisture levels, those tests do NOT
transfer over to the realities of the villages.  We need to test with
"representative fuels"!!!  even if we must sacrifice some measurement accuracy
because of some fuel variations.

I believe we do not need to use the EXACT fuels of the villages because 
even the
fuels in the villages can have some seasonal variations.  But we had better be
"reasonable and representative."

What we should NOT be doing is to convince ourselves that measured 
emissions and
fuel consumption, as determined in the laboratories using NON-representative
fuels, are accurate or meaningful in relation to the villagers and the
environments we want to assist.

IMO, it is incorrect to tell funding agencies that benchmarks have been 
met when
both the benchmarks and the test results have been determined based fuels that
are known to not representative the fuels available to the target population.

Paul
*******************
Quoting AJH <list at sylva.icuklive.co.uk>:

> On Tue, 03 Oct 2006 23:33:36 -0500, Paul S. Anderson wrote:
>
>> 2.  If a kilo of wood has 10% moisture, then 0.27 MJ must be subtracted.
>> 0.27/18.6 = 0.0145 = 1.45 %.  Seems like a mighty small amount.  Did I miss
>> something?
>
> It's almost insignificant at these low moisture contents but you did
> miss a bit. As the water forms 10% of the mass (given our mc was
> calculated on a wet weight basis) the calorific value of the wood is
> only 18.6 times the remainder after the water is removed so:
>
> (18.6*.9)-(2.7*.1)=16.47 and the percentage of the wood needed to boil
> off and discard the water is 0.27/16.47=1.61% of the net heat
> available.
>
> Of course my rule of thumb figures are a compromise and in most open
> vented stoves the loss will be greater than 2.7MJ/Kg of unnecessary
> water in the wood, bearing in mind in UK it's highly unlikely the wood
> will dry much more than 20% mc (wwb).
>
> The calculation becomes a bit complicated because the specific heat of
> steam changes as its temperature rises  but appears to be about half
> that of water. The calculation should be comprised of the rise in
> temperature from ambient (say 10C) to boiling times 4.1kJ/kg plus the
> latent heat of vapourisation at atmospheric pressure and 100C
> *2258kJ/kg plus the specific heat of steam times the difference
> between 100C and the rejection temperature (say 150C but like as not
> far higher with most stoves)
>
> (4.1*90)+(2258)+(2*100)=2727kJ so my 2.7MJ errs on the low side for
> most cases as it assumes the steam is lost at 150C.
>
> In the greater scheme of things this is no more than an interesting
> calculation and small variations are lost in the noise  but the simple
> calculation to indicate how much water in the wood displaces wood
> available for energy is very significant for wood bought on weight (or
> carried to the stove).
>
> AJH
>



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