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

["Kevin Chisholm" <kchishol@fox.nstn.ca>]

Dear Stovers:
I inadvertently sent Crispins original message to the list, rather than the
following reply. Following is the reply I intendd to send. Sorry about he
confusion.

Kevin Chisholm
*********************************
Dear Crispin

----- Original Message -----
From: "Crispin" <crispin@newdawn.sz>
To: "Stoves" <stoves@crest.org>
Sent: Tuesday, January 01, 2002 7:49 AM
Subject: Actual heat from the fuel re: African Stove Tests


Thanks for your thoughtful ananysis and your "search for thruth" on the
calorific value of woods in general, and resionous woods in particular.
>
> Piet
> >In oven dry state the mix should have a combustion value of around 20
> MJ./kg
>
> Kevin
> >The cellulostic fraction of wood runs about 8,400 BTU per pound, and the
> >resins from pine are similar in heating value to oil. Very aproximately,
> >consider 16,800 BTU per pound. Pine that may be as much as 10% resin
> >would then be:
> >                8400 x .9 + 16,800 x .1 =  9,240 BTU/Lb
>
> I have found the following from_Mechanical Engineer's Handbook_ McGraw
Hill,
> N.Y. First Edition, "Materials of Engineering" section/Wood/General
> Properties of Wood, by Hermann Von Schrenk, p.579
>
> "Calorific Value.  The specific heat of practically all kinds of wood when
> oven dry is 0.327 (Dunlap, _Forest Service Bulletin_ No. 110, 1912).

This is probably reasonably accurate. However, the precision is probably a
bit excessive.

 The
> calorific value of wood depends on its specific gravity (dry), heavier
woods
> giving more heat than light woods.

I hate it when I have to say "The book is wrong." :It makes me look
arrogant. :-) However, in this case, it is wrong. Firstly, the statement was
made 90 years ago, when wood was generally bought and sold by the CORD, a
volume measure, and not by the POUND (or kG). 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 CORD than woods of lower s
pecific gravity.

According to Schrenck, 1 cord of green
> wood contains 250 gal. of water

Going by memory, a cord of "green softwood, as cut" weighs about 3000
pounds, and a cord of "green hardwood weighs about 4000 pounds.  250 gallons
of water, if Imperial Gallons, would weigh 2,500 pounds. This would suggest
that "green softwood" has a moisture content wet basis of:
2500/3000 = 83.3%

With all due respect, Schrenk is quite wrong here.

and the heat required to evaporate this into
> steam is not available for other purposes.

This statement was correct when he made it, but with condensing boilers it
is wrong.

According to German experiments,
> wood with 45 per cent. moisture gives only 50% as much heat as dry wood.

This is indeed approximately correct in that wood with 45% moisture has
approximately only 1/2 as much dry wood per pound.

> Rosin increases the heating power by about 12 per cent.

This is wrong. He makes no reference to the quantity of rosin or resin
present. Taking his statement to a silly extreme.... if I purchased a cord
of "non-resinous wood" and added 1 pound of resin, I could increase its
heating value by 12%. If he said "Resinous woods containing typically 10%
reson have a hearing value about 12% greater than non-resinous woods." then
he would be approximately correct.

According to Roth,
> 100 lb. of wood, as sold in woodyards, contains 25 lb. of water, 74 lb. of
> (dry) wood and 1 lb. of ashes.

This is about 25% moisture wet basis. This is close to reasonably well
seasoned fuel wood.

  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."
>
> ========
> Although the type of wood Roth used is not stated and the figures do not
> give a straight line, they give a total of only 6500 BTU per lb (oven-dry)
> as a realizeable maximum which perhaps means an efficiency of only 70% in
> burning.

There a number of "unclarities" in the Roth statment which can lead to this
confusion:

1: He doesn't state if this is "moisture content wet basis" or "moisture
content dry basis.

2: He does not state whether this is the "true heating value" of the fuel,
in the sense of the total heating energy contained, OR if htis is the "net
heat available from burning the wood in a typical boiler or furnace.

Based on Kevin's figure of 9240 I get 389813, 837375 and 909563
> BTU respectively per 100 lbs based on their 'curve'. The experiment is
> correct in principle but had serious errors. - CPP
> ========

Would you care to "revisit that statement? :-)
>
> "Relative Values of Woods as Fuels
> Best:    Hickory, beech, hornbeam, locust, heart pine.
> Good:    Oak, ash, birch, maple.
> Moderate:    Spruce, fir, chestnut, hemlock, sap pine.
> Poor:    White pine, alder, linden, cottonwood."
>
> Under "Fuels" by Ozni P. Hood, Table 7, p. 608, shows the ash content of
> various woods from "Slippery Elm" (1.69%) to Tamarack (0.09%).  Pine is
> around 0.3%.
>
> Part of Table 8, p.609, "Analyses of Various Woods (Dry)" are two columns
of
> interest:
>
> "Name       Calories    B.t.u.
> Oak            4620        8316
> Ash            4711        8480
> Elm            4728        8510
> Beech        4774        8590
> Birch          4771        8586
> Fir              5035        9063
> Pine           5085        9153"
>
> ========

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. My estimate is
in error by about

(9240-9153)/9240 = 0.9%

Note, however, my calculated estimate was based on ash free cellulose while
his wood had .3% ash. If my estimate is corrected for ash presence, then my
estimate is off by only about 0.6%

On this point, I would say that were quite close to being in violent
agreement. :-)

> Based on the figure 9153 for pine I can calculate that the resin content
of
> their sample must be about 9%, according to Kevin's formula. - CPP
> ========
>
I would suggest that my approximation for the heating value of pine wasn't
really all that bad.

> Under "Other Solid Fuels", p. 609:
> "Sawmill Refuse, consisting of saw dust, "hogged" or shredded wood chips,
> etc., containing from 40 to 60 per cent. moisture.  The calorific value of
> redwood, pine fir, hemlock, spruce and cedar refuse is practically 9000
> B.t.u. per lb."
>
This is a very generalized statement. There is a huge difference in the
heating value of a given wood when it contains 40% or 60% moisture. He would
not be far off howevr, if his figures were "corrected for moisture content
present."

> Lastly, charcoal (allowed access to atmospheric moisture after cooling) is
> considered to be "84 per cent. carbon, 12 per cent. water, 3 per cent. ash
> and 1 per cent. hydrogen." with a heating value "of about 12,850 B.t.u.
per
> lb."
>
> ========
>
> Piet's 20 MJ/Kg divided by Kevin's 9250 BTU/Lb = 981 Joules / BTU.  That's
> pretty close.
> Piet's 20 MJ/Kg divided by the olden days figure of 9153 = 991 Joules/BTU.
> Even closer.
>
> Going with a bone dry 9153 BTU/Lb and 1054 Joules/BTU I get 21.27 MJ/Kg.
> This is a far cry from my estimated actual 15 MJ/Kg yield from the fuel.
>
Would this be the explanation for your unexpectedly high indication of your
stove efficiency?

> My interpolation from the Roth slope and the 9153 BTU figure is about 17.2
> MJ/Kg for 15% moisture content and 14.1 KMJ/Kg for 25% moisture for pine.
>
> Can anyone give us soemthing more accurate?
>
Piet and I seem to be in reasonably close agreement. Perhaps others could
bring some additional facts to the matter to confirm whether we are closer
to the truth or if Marks and Schrenk are closer.

> New Year's Greetings from
> Crispin
>
Thanks!! And the same to you.

Kindest regards,

Kevin Chisholm



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