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

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

----- 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


> 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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