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

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

Dear Kevin

>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
>specific gravity.

When I have lots of time I will copy out the comparison of heat content of
wood purchased by cord.  It is going to be useful to find out what to burn
if you are buying wood, but it does not really get to the nitty gritty of
the math of real fires burning non-dry wood..

>>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%

Your memory may have let you down here as obviously 83% is impossible.  On
page 454 the density of apple wood (for instance) is 44 lb/ft^3 = 5632
lbs/cord at avg 17.5% moisture.  Long leafed yellow pine is the same.

Wet oak is 7552 lbs/cord.  Air dried it is 5120 for a difference of 2432 lbs
of water.  It does not have a figure for wet pine.  I think the 250 gallons,
Imperial or not, is correct.

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

>He makes no reference to the quantity of rosin or resin present.

IT is taken to be 'ordinary' meaning what an average buyer would find.

>> 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."

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

I went over the numbers and it is a moisture content wet basis.

>2: He does not state ...if this is the "net heat available from
>burning the wood in a typical boiler or furnace.

It is my belief that it is the yield from a particular or average furnace
because it is well below the actual heat content and these guys were no
slouches about something this important.

>>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? :-)

The working out of the numbers shows correctly that wet basis wood burns
with a higher heat content when there is less water in it and more dry
matter per unit input mass.  The error was that the lowest moisture content
wood (2%) yielded less total heat per kg of dry matter than the 10% moisture
wood (again, per kg of dry matter).  If you plot the experiment there is a
curve, not a straight line which 'physics math' would say has to be seen.

>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.

>> 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.

The figures are for BTU/lb dry mass.

Daniel wrote
>The plain fact is that there is more variance in wood
>than we can easily quantify here.

I have no problem understanding variations, but we can't have stoves tested
with a pelletized wood as a standard test, or rather THE standard test
because some stoves will not work with that fuel.  It packs too densely for
enough air to travel through it to feed the flames properly.

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.

For example"
>The base of the tree certainly has more ash and
>density than the top branches.

It makes no real difference if you are measuring fuel consumption by unit
mass.

It is probably worth our while to find out what the heat content is for the
pellets as it surely varies regionally.

To get an idea of how much these old guys knew about wood, listen to this
description of "Southern Yellow Pine" by H Von Schrenk (Ibid. p.577-8)

"Southern yellow pines (all pines of the Southern States manufactured into
lumber including longleaf, shortleaf, loblolly and Cuban pines.  The lumber
is divided according quality into "dense southern yellow pine" and "sound
southern yellow pine;" dense southern yellow pine should show on either end
an average of at least six annual rings per inch and at least one-third
summerwood, or else the greater number of rings should show at least
one-third summerwood, all as measured over the third, fourth and fifth
inches on a radial line from the pith; wide-ringed material excluded by this
rule is acceptable, provided the amount of summerwood, as above measured, is
at least one-half; the contrast in colour between summerwood and springwood
should be sharp, and the summerwood should be dark in colour, except in
pieces having considerably above the minimum requirement for summerwood;
sound southern yellow pine includes pieces of southern yellow pine without
any ring or summerwood requirement);..."

Summerwood is about twice as strong as springwood and so nature demanded
this appropriate categorization.

The section on strength of timber includes calculations for all these sorts
of wood because most structures were mostly built of wood in those days.
They knew what to put where.  I recall that Shaker chairs have 13 different
types of wood in them.

For your interest, conifers contain on average more wood per cord that
hardwoods because of better straightness.  Black Oak with an average
diameter of 5 inches is 85 cubic feet per cord (66% volumetric efficiency).
Eight inch chestnut gives 95 cu ft/cord.

Ibid p.584 reads "One cord of first-class split wood obtained from sound
pieces 12 in. in diam. contains 102.4 cu. ft. of solid wood."

Wet chestnut with a density of 0.96 kiln dries to only 0.47.  All in all
there is a lot of variability but 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.

As to the question of how much the cordwood will shrink when it dries out, I
can quote from p. 589:

"
The volumetric shrinkage of wood from the green to the oven-dry state,
expressed in percentage of volume, is 26.5 times the specific gravity as
based on green volume.  Thus, for white oak, shrinkage = 0.46 x 26.5 = 12.2
per cent., and consists almost entirely of lateral contraction."

The Basintuthu test showed an overall efficiency (wood to water) of 35%
based on 15 MJ/Kg.  If it was only 14.1 because of the water content, then
the efficiency was 37%.  If it was 18, then it was only 29%.  That is a
significant difference and if I did not know what the fuel was giving me, I
might make a wrong decision about whether a modification to the stove was an
improvement or not.

Thanks for the feedback!
Crispin

PS  In 1912 white pine structural timber sold for $27.70 per 1000 board feet
in New York.


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