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

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

Dear Crispin
----- Original Message -----
From: "Crispin" <crispin@newdawn.sz>
To: "Stoves" <stoves@crest.org>
Sent: Tuesday, January 01, 2002 3:20 PM
Subject: RE: Actual heat from the fuel re: African Stove Tests


> 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.

The above is the statement I made with a typo error... the correct statement
should have been:

> >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.

>
> 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..

There is no question that when you buy by the CORD, you get the best deal
with wood that has the higher specific gravity, simply because you are
getting more pounds per cord with the more dense woods.
>
> >>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.

Another possibility is that green wood does not contain 250 gallons of water
per cordand that Schrenk was wrong!! :-)
On
> page 454 the density of apple wood (for instance) is 44 lb/ft^3 = 5632
> lbs/cord at avg 17.5% moisture.

This would be the correct number if you had 128 cubic feet of this wood in a
cord. To do this, you would need planed wood. The reality is that typical
"as stacked round wood" contains about 85 actual cubic feet of wood per
cord. This calculates out to 3740 pounds per cord. Not far off my 4000 pound
estimate.

 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.
>
I would respectfully suggest that you are wrong here. If we assume that the
average cord of wet oak is in the round form, and that it is carefully
stacked to maximize the wood content, then it is unlikely that it would
contain more than 100 cubic feet of solid wood per cord. This would give a
wood density of 75.52 pounds per cubic foot. This wood would not float.

If you take a piece of wet oak and drop it in the water, does it sink or
float?

If the "bone dry" wood has a specific gravity of less than 1.000, then it is
inherently impossible to add enough water to make the wood sink in water.
Some tropical woods such as purple heart, and lignum vitae, are "sinkers."

> >> 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.
>
With all due respect, he was a slouch. He left too many areas for ambiguity.
Thats what slouches do. What he said may have been appropriate for a given
context, but it is quite inappropriate to take his work out of context, and
to rely seriously on it, to deduce what he did not intend to convey. What
you quoted of his work was definitely sloppy.

> >>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 don't know if you answered the question or not..... I maintain that what I
stated was indeed correct. Would you please clearly state whether you feel I
am correct or incorrect. If you feel I am wrong, please show me specifically
where I am wrong.

> >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? :-)

> >> 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.
>
OK... if it is "dry wood, then this is quite close to my estimate of 9,240
BTU/pound assuming "as much as 10% resin." However, as originally presented,
the statement was anything but clear.

...del...
> 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?
>
> For example"
> >The base of the tree certainly has more ash and
> >density than the top branches.
>
This is questionable, for the simple reason that the ramial wood has a
higher percentage of bark, and bark generally tends to have more ash than
wood. Alternatively, if the wood was harvested by a skidder rather than a
porter, it would have MORE ash due to the mud content picked up in skidding.

> It makes no real difference if you are measuring fuel consumption by unit
> mass.
>
Agreed. This is where Schrenk went wrong, with his contention that density
was an important factor in determination of heating values.

> It is probably worth our while to find out what the heat content is for
the
> pellets as it surely varies regionally.
>
If we go on a "per pound basis" and correct for resin content, it should not
be much of a problem. All you have to do is ask the pellet manufacturer
"What is the typical heat content of your pellets, and how much do you
expect the heat content to vary from batch to batch?"

> 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.
>
With all due respect to the good Mr. Schrenk, this is a lot of fluff and is
not at all relevant to the present discussion.

> 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.
>
Thats a Carpentry consideration, not a biomass energy consideration.

> 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.
>
With all due respect, this data is meaningless because of its generality.
Firstly, the diameter is totally irrelevant: Secondly, the species is
totally irrelevant. Thirdly, it is irrelevant whether the wood is coniferous
or deciduous. Fourthly, the "physical size of the actual cord" is important.
("Whle tree harvesting " of trees, giving a measure of 128 cubic feet of
stacked wood volume wil give a very much different "stacking efficiency"
than if the wood was blocked up as firewood in stove lengths.The art of
bucking a tree into logs determines how much yield a sawyer will get from a
"cord" of logs. Well bucked logs will stack well, and will give a relatively
high stacking efficiency because they are straight. Wood that is cut for
firewood has little to no concern for straightness, and as a consequence,
the "stacking efficiency" of wood for firewood is invariably less than for
wood that was cut for logs. Indeed, it is to the direct advantage of the
firewood seller to produce "less efficiently stacked wood"

> 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."
>
The key thing here is the skill employed in stacking the split wood. Well
stacked wood will have few gaps or voids.

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." One problem is butting the wood
together.... if the wood was cut into stove lengths of approximately 16" and
split, it would be extremely unlikely the "three sticks per 4' width" would
butt perfectly. Three "face cords" would thus appear in reality to be
significantly more than one "full cord."

Hexagonal close packed wood of uniform size would have a maximum solid wood
content of approximately 116.0865 cubic feet per cord. However, that assumes
perfectly straight cylindrical stciks, with no knot lumps, sweep, bend,
hollow center, crooks or taper, which never occurs in real life. Especially
so, for wood cut for the heating trade.

> Wet chestnut with a density of 0.96 kiln dries to only 0.47.

I presume you mean "specific gravity."

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.
>
I would suggest that a good "base point" is "air dried wood" which would
typically have in the range of 20% moisture content.. However, even this can
be a big variable. When accuracy is important, then the "general rules" or
"rules of thumb" are not necessarily dependable. When accuracy is important,
then the best way by far is to dry representative samples, and correct for
moisture on a "bone dry basis."

I would indeed appreciate a confirmation of where specifically you feel I am
wrong and an explanation of why you feel I am in error, so that, in the
event I am actually in error, I can learn from it.

I look forward to your helpful reply.

Kindest regards,

Kevin Chisholm


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