Re: Actual heat from the fuel re: African Stove Tests

["New Dawn Engineering /ATEX" <crispin@newdawn.sz>]

Dear Kevin

I am not sure I ma in a position to judge which figures are correct or not.
I can only report on what my eyes have seen and my spreadsheets tell me.  I
have tried to show that the figures reported for things like wet weight hnd
heat content are sensibly described, even if they are not couched using
modern technical forms.

I can forward the view of 100 years ago (for those interested in buying wood
by the cord which certainly happens in the US and probably Canada) that the
volume of solid wood in a stacked cord does depend on the diameter of the
wod stock.  The 'efficiency' as I described it varies from 62 cubic feet of
solid wood for 1 inch diameter material to 85 cubic feet in and about the 5
inch range.  Above that in diameter the solid wood component drops very
slightly.  The table give expectable volumes up to 12 inch stock.

I do not know about 'stove' cut lengths and what the stacking would be.  100
years ago standard ratings of stacking wood were done with 48 long inch
pieces which were then stacked as best could be done in a cord-sized box 4 x
4 x 8 feet.

The efficiency is affected by the species as some grow very straight and
many do not.  All the conifers tend to have very straight 48 inch lengths
and things like beech of maple are bent here and there affecting the stack.

I would expect that short split wood carefully stacked would reach very high
packing.

Your point about the density of the wet oak is well taken.  Perhaps the
authors were refering to 128 cubic feet of green oak, not a cord, but the
figures given were listed as the weight of green wood stacked in cords.  I
know that some woods sink in water when cut, others sink when soaked in
water (even in seawater!) and some never do.  I am not sure the answer will
affect stove list subscribers.  As we have no oak here to measure I will not
be able to verify anything for you.

 >It is unquestionably true that "heavier woods give more heat per
 >CORD" but it is unquestionably wrong that wood of higher
 >specific gravity has more heat per POUND than woods of lower
 >specific gravity.

This is very true and the example you gave was quite helpful to
understanding this.

I cited
>>Thus 100 lb. of green wood (50 per cent. moisture) furnish
>>about 270,000 B.t.u., 100 lb. of air dry wood (10 per
>>cent. moisture) about 580,000 B.t.u. and 100 lb. of kiln-dry
>>wood (2 per cent. moisture) about 630,000 B.t.u."

I feel the figure of 580,000 should be lower by a few percent, and that this
was a stove-based test not a thermal lab evaluation.

> >I would suggest that my 9240 BTU/pound estimate is in very close
> >agreement with the 9153 BTU per pound estimate given in this reference.
>
> That is what I was saying too.
>
So, I was not incorrect after all? :-)

You gave me a figure for a pine with a certain level of resin in it (10%).
Perhaps they used a pine with a different resin level.  I calculated that to
be 9%, and I assumed you were prefectly correct.

>> Most of the things you mention as variables in the wood affect the
>>YIELD not the amount of HEAT per lb dry mass.
>>The cellulose + resin formula is pretty impressive on
>>this account.  It covers the two main fuels in correct
>> proportion.

>I do not understand what you mean by YIELD. Could you please clarify?

Well, things like soil conditions, rainfall, insolation etc affect the
amount of wood the forest yeilds per acre, not the amount of heat one gets
from burning dry cellulose.  I meant yield in the agricultural sense.  If
the wood growing conditions affects the heat produced by the dry mass of the
cellulose I would like to hear about that because it would be important to
species selection for woodlots.

>There are many variables associate with colume measures of wood.
>The "102.4 cubic feet of solid wood per cord" is unrealistically precise.
>This number was undoubtedly calculated from a "face cord", and then
>projected as though it would hold true for a "full cord."

I can't make that assumption.  It is my belief that they based it on a large
number of cords, averaged.  A great deal of timber was loaded into "standard
old" boxcars, whatever that means for transport over long distances.  People
used huge amounts of firewood 100 years ago.

>>...the most important thing to have is a working rule
>>of thumb based on known heat content of dry mass
>>and a factor for moisture content that is good enough
>>to rate stoves.

>I would suggest that a good "base point" is "air dried wood"
>which would typically have in the range of 20% moisture content.

I can be most agreeable with that.  The thing I need is a standard method of
calculating the actual heat available from burning it, compared with burning
oven-dried material.  There is practically no point in us establishing an
internation gentlemen's (or woman's) agreement on testing stove efficiency
if we cannot establish how much heat a fuel charge contains.  This is my
motivation in writing all this.

It would appear from all I have read that our air-dried briquettes have
between 14.1 and 17 MJ/Kg.

I think I remember someone writing in a few months ago giving a method of
calculating the heat loss due to moisture content.  Could they please speak
up?  We should be able to confirm the theory in a stove by doing identical
tests with briquettes of a known moisture content and with zero moisture
content.  The performance difference should gives a close approximation of
the theoretical difference.

Many thanks to everyone.
Crispin


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