Actual heat from the fuel re: African Stove Tests

["Crispin" <crispin@newdawn.sz>]

Dear Piet and Kevin

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).  The
calorific value of wood depends on its specific gravity (dry), heavier woods
giving more heat than light woods.  According to Schenck, 1 cord of green
wood contains 250 gal. of water and the heat required to evaporate this into
steam is not available for other purposes. According to German experiments,
wood with 45 per cent. moisture gives only 50% as much heat as dry wood.
Rosin increases the heating power by about 12 per cent.  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.  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.  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
========

"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"

========
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
========

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."

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.

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?

New Year's Greetings from
Crispin


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