April 1996 Gasification Archive

For more information to help people develop better stoves for cooking with biomass fuels in developing regions, please see our web site: http://www.bioenergylists.org

From 73002.1213 at compuserve.com Fri Apr 5 10:10:32 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:53 2004
Subject: Clay stoves
Message-ID: <960405150914_73002.1213_FHM62-4@CompuServe.COM>

Piet - no need to apologize for levity here, loved your proem.

Ron's wife is an exceellent potter, and I have charged him with finding an
"insulating clay". The heat capacitty and deensity of ordinary clay makes it
inappropriate for stoves - more heat to the pot more than the food!

Can we mix vermiculite with clay to make it more insulating and lower deensisty?
(Reegularly done with plaster)?

On the other hand, metal cans have a very low heeat capacity - but high heeat
loss by conduction-convection. About 4 Btu/(ft2-hr-F) (sorry I ddon't have this
in metric and no calculator handy, any commeents?).

So far, I like the riser sleeves best for high temperature resistance, no
cracking, easy working, good insulation. I can put my hand on the outside of
the gasifier (but not the burner) 15 minutes into the cooking cycle. But I
realize that they won't be available locally in all places (but should be
cheeapaer than U.S. in China and India, both big targets for biomass stovees.

Looking forward to Piet's contributions

TTOM REED on his busted laptop.

From E.Moerman at stud.tue.nl Fri Apr 5 11:13:06 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: Clay stoves
Message-ID: <65913.s335192@popserver.tue.nl>

Tom Reed writes:
> Ron's wife is an exceellent potter, and I have charged him with finding an
> "insulating clay". The heat capacitty and deensity of ordinary clay makes
> it inappropriate for stoves - more heat to the pot more than the food!
>
> Can we mix vermiculite with clay to make it more insulating and lower
> deensisty? (Reegularly done with plaster)?

----------

Etienne:
What about mixing clay with (burnt) straw. This is regularly used at present
and apparently quite satisfying.

--------

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From verhaarp at janus.cqu.edu.au Sat Apr 6 00:16:12 1996
From: verhaarp at janus.cqu.edu.au (Peter Verhaart)
Date: Tue Aug 31 21:34:54 2004
Subject: Stove Test and comment
Message-ID: <9604060519.AA31367@janus.cqu.edu.au>

To All Stovers
>From Piet Verhaart

WPWin 5/1 File: tst60401

Stove Test 960401

8.24 780 g of wood long 30 cm, air dry. Split from an 80 mm diameter
piece of trunk. Not very dense as
Australian timbers go. Could barely squeeze them all in, they
protrude about 1 cm from the top of the
nether stove half.
8.49 Lit the stove with a petrol soaked rag. Did not work.
8.53 Lighting with propane torch through adjustable slit.
9.03 Relit. Was only producing smoke. Primary air hole 10 mm open
9.05 Mix of yellow flames and smoke, the usual. They do seem quite
lively, lateral airflow is doing good
things to the flames but not enough.
9.08 Good fire going, flames, yellow, of course, sometimes coming out
of the top of the insert. Bottom
valve open 2.5 mm.
9.15 By playing around with the bottom valve I can maintain a
reasonable kind of combustion. It is quite
critical. If we are going to use this principle we need a very
good bottom valve.
9.19 It is possible to relight the flames by opening the bottom valve
wide for a short period and closing it
carefully. If closed too far the flames will go out and be
replaced by thick smoke. There are times
when I could have put on a pan without it getting black but if
that were the object, we would have to
withdraw the pan quite a number of times to wait for good
combustion to reappear. The combustion
is quite sensitive to the opening of the bottom valve, fractions
of a millimetre make a difference.
9.23 There is a spell of good combustion, meaning I can't smell
anything and not because my olfactory
organ is burnt. Bottom valve about 4 mm open. It is quite a fire,
flames extending about 150 mm from
the insert. In any case the heat output rate is certainly not
constant, it seems to attain a maximum
toward the end of the process, that is if you wish to keep the
flames alive.
9.32 Closed the bottom valve, most of the volatiles have been driven off.

Some hours later it transpired we have produced 125 g of charcoal e.g.
125/780 = 0.16 or 16 % charcoal.
There is some uncarbonised wood left, two sticks only partly charred.

First we have to find out whether the volatiles coming out of the bottom
half of the stove are at all times
sufficiently combustible. If they are, we have to do a lot of work on a
burner which produces a premixed flame.
Does anyone think we could improve things by introducing a swirl by having
angled blades on the bottom
flange of the insert?

I suppose we could do experiments first to see of we can vary the charring
time, and between what limits. If
we have neighbours we might have to rig a propane burner to the top of the
stove to burn the smoke. In this
way we can vary the settings of the bottom valve dispassionately. Having
thus determined the range of charring
speed we can find out the corresponding heat output rate of the volatiles if
they could be burned and see if that
conforms to the specs (from 100 % down to 15 % for domestic gas burners if I
remember correctly).
Independently those of us with ideas could work on premixed burners of a gas
with variable composition with
a high percentage of tar droplets, having to mix with ambient air, both
fluids with minimal kinetic energy or
dynamic pressure. The term "pressure differential" e.g. dp is almost
appropriate here, a vanishingly small
quantity.

Be of good cheer,

Piet Verhaart

The above has been Selected, Copied to Clipboard from WP 5/1 and copied in a
Eudora Message. I will comment shortly on Etienne's message about attaching
files. I have seen that JPEG files (drawings and photographs) need the least
number of bites, the same scanned photograph takes up 3.21 Mb in BMP format
and 87.2 kb in JPG, while showing no difference in resulution when printed
at 300 dpi.

From verhaarp at janus.cqu.edu.au Sat Apr 6 07:26:39 1996
From: verhaarp at janus.cqu.edu.au (Peter Verhaart)
Date: Tue Aug 31 21:34:54 2004
Subject: The Open Fire
Message-ID: <9604061229.AA04805@janus.cqu.edu.au>

>From Piet Verhaart
What I was looking for in the old files the other day was not the recipe for
conventional stoves but something on a much less popular subject viz The
Open Fire (shudder). Today I found it: File MDOF, format Wordstar 3, which
could be translated by my WordPerfect for Windows 5/1 5/2. Here comes.

MAKING DO WITH THE OPEN FIRE
By P.Verhaart (revised 10 Feb. 1986)
Dept. Mechanical Engineering
Eindhoven University of Technology
P.O. Box 513
5600 MB Eindhoven.

1 Introduction
In recent years the "Appropriate Technology" scene has been flooded
with reports on "Improved"
woodstoves, usually claiming to reduce wood consumption to half of what it
was. Unfortunately it is not easy
to make a really efficient woodburning cookstove and the flamboyantly
advertised stoves don`t usually fulfill
their builder`s claims. Quite often the inventor did not take any
measurements to ascertain its performance and
a very rare animal indeed went so far as to actually compare performances
between old and new. As a matter
of fact there is no consensus of opinion among the world`s woodstove big
brass yet on how to do measurements
on a stove to prove its wood saving properties. One point all parties could
agree on is that the ultimate merit
figure for a stove must have some relation to the amount of food processed
on a standard amount of wood.
One reason for the poor progress is that the problem is in many quarters
underestimated and indeed, not
considered to offer any challenge to professional engineers. Another reason
is that wood is a very difficult fuel,
it starts out as a solid but when heated gives off a mixture of gases,
vapours and finely divided droplets,
collectively called volatiles, which under as yet unspecified conditions can
burn, usually with luminescent and
sooting flames. Now, designing a burner for a liquid fuel of constant
specifications such as kerosene is much
easier, yet it took nearly 50 years to come up with a good kerosene burner
such as the Primus stove.
We feel confident that in time very good woodburning cookstoves will be
developed by one or more of the
groups involved. However, introducing new stoves will take time, during
which many square kilometers of forest
are threatened with extinction. For that reason we have written up a number
of directions, some of them
practicable, others perhaps merely academic, that can lead to savings in
firewood when applied to the open fire.
The simplest woodburning cookstove conceivable is the open- or three-stone
fire. Since a great majority of the
third world`s population uses some version of the open fire it seemed to
make sense to investigate ways in
which this device may be used more effectively. (Indeed, it remains to be
seen whether the open fire, as used
by the traditional users, is such an inefficient device as some publications
would have us believe.)
Although the author shares the conviction that the open fire as a cooking
device has a number of serious
drawbacks, he would like to swim against the current in making a case for
it. In this article some measures are
discussed which can make the open fire perform more efficiently, making it
fully competitive, in fuel
consumption per cooking task, with many of the more elaborate wood burning
cooking devices, presently being
promoted.

2. Pros and cons of the open fire
The open fire is the oldest woodburning kind of stove in existence.
Compared to more recent and more
elaborate wood-burning cookstoves it comes out a winner on most of the
following points:

1. It is quickly built up with a minimum expenditure of materials and time.

2. While in operation it provides light as well as heat.

3. As a combined space heater and cooking stove it has an
efficiency close to 100 %.

4. The state and condition of the fire is easily checked.

5. The heat output rate can be varied over a wide range.

6. It is easily adapted to any size pan.

7. Efficiency-wise it is competitive with most of the more
elaborate wood burning cookstoves.

The above statements are based exclusively on laboratory experiments. We
cannot, as it were, "compare notes"
with the traditional users since neither they, nor any visiting development
workers took any. It is therefore quite
probable that in this article we restate common knowledge that could have
been obtained at the source if only
one had thought of it at the time (and spoken the language).

Disadvantages of the open fire when compared to the presently available
(closed) cookstoves are the following.

1. When a chimney is used there is less nuisance from smoke to the user
even though the combustion
may be worse (the effect on the environment may be quite another matter).

2. Some stoves have a higher heat transfer efficiency. When this is not
offset by lesser range of control
the same amount of food can be prepared using less fuel.

3. The adverse effect of wind on the heat transfer efficiency of some
closed stoves is less than on the
open fire.

4. Some closed stoves lose less heat to their immediate environment
making things more comfortable
for the operator.

5. Accidental touching of hot parts causing burns is less easy in some
closed stoves, the open fire at
ground level is reputed to be a cause of burns , particularly in small
children.

3 About Efficiency
Efficiency is more than the great catchword in debates on the relative
merits of woodstoves. Properly,
efficiency is an engineering concept denoting how much of what we want comes
out of what we built as a
percentage of what we put in.
In "woodstovery" it is common practice to take the heat absorbed by the pan
as the desired output and the
calorific value of the used fuel as the input. As such it is a healthy
concept. The efficiency defined above can
be shown to be a necessary quantity in this respect. It is, however, not the
only one quantity which can decide
what stove can cook a certain amount of food using the least amount of fuel,
while that should ultimately be
our criterion.

3.1 Turndown ratio
A second property that, together with the efficiency, determines the fuel
economy of a cookstove is the measure
in which the heat output rate can be diminished, we call it
"turndownability", or in more solemn moments "turn
down ratio".
It is quite easy to see that we can waste fuel even in a very efficient
stove if the heat output rate far exceeds
the demand. We will enlarge on this in an example on boiling.

The temperature of boiling water is the same irrespective of whether it
boils gently or vigorously. The only
difference is in the rate at which steam is evolved and consequently the
amount of heat absorbed by the pan
per unit time.
As the process of food boiling depends on the temperature and on the
duration, the rate of boiling makes no
difference to the processing time of a particular foodstuff. It does however
make a lot of difference in the fuel
consumption and in the amount of operator attention that is required. If a
cookstove cannot be turned down to
any degree (has a low turndownability) and if, as often seems to be the the
case, the user insists on completing
the whole process of boiling on the stove, we can meet with two extreme cases.

1) The stove has a very low heat output rate. Here we can put on the
pan with the food and no more than
the water required for the boiling process. (In the case of rice, for
example, the usual ratio is one part by weight
of rice to one and a half part of water producing two and a half parts of
boiled rice). Trouble starts right at the
beginning when the small fire takes ages to bring the pan and its contents
to boil. All this time fuel is being
consumed. The stove may have a very high efficiency but the heat flow that
is transferred to the pan is only
marginally more than the losses of the pan to the environment at boiling
temperature. After the pan has reached
boiling point the process proceeds under optimal conditions. The simmering
time will be somewhat less than
after a rapid heating up since part of the final warming up period
contributes to the processing time. Altogether,
however, the fire has been on for a very long time, using fuel all the while.

2) The stove has a high heat output rate and also a good efficiency. It
can not be turned down to give a
lower rate of heat output. Three undesirable things occur here. First, we
have to start with a large excess of
water in the pan, which is going to be evaporated during the period the pan
has to remain at boiling temperature.
Second, getting this water to boil will take more time and more fuel than
heating only the process water. Third,
because of the high heating rate the contents of the pan have to be
continuously stirred to prevent burning of
the food.

>From the above we see that high efficiency of a stove in itself does not
necessarily guarantee fuel economy.
Fuel economy can only result from a combination of two factors, one being
the efficiency, the other being the
"turndownability", the extent to which the heat output rate can be decreased.

3.2 Turndownability at home
Western society is often accused of being an energy squandering one. In the
light of this observation it may
seem surprising that the gas ranges in the kitchens of the Industrialised
part of the world score very high in
efficiency as well as in turndown ratio. Their development was probably
stimulated for reasons of operator
comfort and speed rather than from any thoughts of fuel economy, the result
is something we could well wish
future generations of woodstoves to posses.
What our domestic gas ranges can do, in brief, is give high heat to rapidly
bring the pan and its contents to
boiling point and, at the turn of a valve, low heat to bring the process to
its conclusion with a minimal
expenditure of fuel. Gas ranges can be turned down to 15 % of their full
power to give just about enough heat
to compensate the heat losses from the pan to the environment.
We don't have to stir the contents during the warming up so we can leave the
lid on, preventing evaporation
losses. After boiling point has been reached the heat is turned down. The
contents simmer, generating a minimal
amount of steam and we do not have to stir as there is little danger of
burning. The lid can stay on the pan also
during this stage, preventing evaporation losses which would otherwise have
required a higher heat input rate.

4 About Lids and Evaporation
In the foregoing paragraph we have, in passing, mentioned having a lid
on the pan to prevent evaporation
losses. It may be good to say a little more on that subject here.
Having a lid on the pan is important only in operations with low heat.
When the pan is on a large fire, steam escapes in any case and at the same
rate, either from under the lid or
directly from the surface of the boiling water itself. Obviously in this
case the presence of a lid does not have
any effect.
A different situation occurs at low heat. If a lid is used the space between
the lid and the liquid in the pan will,
as the pan warms, fill up with a mixture of air and a steadily increasing
concentration of water vapour.
Thermodynamically the liquid and the air-watervapour mixture above it are in
equilibrium. As the temperature
rises, the vapour concentration also rises. This represents a certain but
very slight amount of heat.
If, however, the pan is left open, the water vapour above the liquid in the
pan can not reach the equilibrium
concentration, the vapour diffuses into the surrounding air as soon as it is
formed. Thus a continuous flow of
water vapour is given off. This represents a serious loss of heat starting
well below boiling point. For this reason
higher heat is required for maintaining the state of boiling in an open pan.
If the aim is to drive off water as,
for example, in small scale sugar manufacture, boiling without a lid makes
good sense.

5 Operating the Open Fire
There are several stages of sophistication in the operation of the open
fire as we will see in the following
sections.

5.1 Control
In the open fire the air has free access to the fuel, control therefore is
achieved through manipulating the fuel.
The output can be controlled primarily through dosage of fuel and, to a
lesser extent, through control of the size
of the fuel pieces. Many small pieces will give a higher rate of heat output
for a shorter time than the same
mass of wood in larger pieces. Being open the effect of manupulating the
fuel pieces can be directly observed,
affording the operator a very direct method of control.

5.2 Heat output rate and wood consumption
The heat of combustion of oven dried wood varies between 18 and 20 MJ/kg,
depending on the kind of wood.
In practice wood always contains moisture. If we assume a moisture content
of 15 % on a dry wood basis
(meaning 1.15 kg of this wood contains 1 kg of dry wood), the heat content
is around 15 MJ/kg. A heat output
rate of 1 Kilowatt is then equivalent to a wood consumption of 4 gram per
minute.

5.3 Pan position
Laboratory tests have indicated that roughly 2/3 of the heat output of a
wood fire comes from the combustion
of the volatiles. The volatiles are liberated in the pile of fuel and
ignite, under favorable conditions, on leaving
the fuel bed. To best utilise this heat, the position of the pan relative to
the fuelbed must be such that access
of air to the space above the fuelbed is assured to enable the volatiles to
burn. Laboratory experiments,
following theoretical study, have indicated that the optimal distance z (in
m) between top of fuelbed and
panbottom is found from the expression:

0.4
z = 0.056 * P

Where P is the power of the fire in kilowatt.
In table 1 some values for the pan height as a function of the rate of heat
output are given.

----------------------------------------------------------------------------
-------------
TABLE 1. Fire power vs height of pan above fuelbed
----------------------------------------------------------------------------
--------------
| Fire power in kW / Pan height in mm |
----------------------------------------------------------------------------
---------------
| 1/56 | 2/73 | 5/106 | 10/140 | 12/151 | 14/161 | 16/170 | 20/186 |
----------------------------------------------------------------------------
---------------

To find the height of the pan above the grate, the thickness of the fuelbed
is added to the figure in table 1. We
can assume the fuelbed to consist of one layer of fuel pieces.
When the wood in the fuelbed has all turned into char, production of
volatiles stops. At this instant the flames
die down and the contents of the pan slowly stop boiling. Particularly when
no grate was used, the volatiles
enveloping the fuelbed, tended to protect the charring wood from the ambient
air, preventing it from catching
fire. In this way some measure of two stage burning occurs, the char only
seriously commencing burning after
the volatiles have disappeared. When presently char burning picks up,
boiling recommences. Observation and
measurements by Visser have shown that, particularly when no grate is used,
extensive burning of char only
begins after the volatiles have all escaped from the fuelbed.

5.4 Using a grate
When a grate is used the two burning processes take place more
simultaneously and it can be expected that the
burning char contributes to a higher extent than before toward heat input
into the pan. A more sophisticated
setup could make use of two pan positions, one for making the utmost use of
the heat output of the flames, the
other for using the heat from the char.
The specific fire power that can be achieved on a grate in an open fire
configuration is between 17 and 22 Watt
per square centimeter.

5.5 Efficient Operation
We can here give some general recommendations that can result in decreased
use of firewood when using an
open fire. Following these pointers will also result in less smoke
production and faster response of the fire to
changing power output demands.

1) Use wood in small pieces, not larger than say 20 * 20 * 60 mm

2) Dry the wood before use, small pieces dry much more rapidly than
larger pieces.

---------------------------------------------------------
TABLE 2. Pan sizes, food quantities and optimal fire power for use
with the open fire.
---------------------------------------------------------
|Panbottom |Mass of |Height of |Relative |Open fire |
|diameter |(food and)|(food and)|height of |heat |
| |water |water in |contents |output |
| | |pan |in pan |rate (*) |
| | | | | |
| cm | kg | cm | --- | kW |
---------------------------------------------------------
| 12 | 0,50 | 4,4 | 0,51 | 1,58 |
| 16 | 1,31 | 6,5 | 0,60 | 2,80 |
| 20 | 2,71 | 8,6 | 0,65 | 4,38 |
| 24 | 4,86 | 10,7 | 0,70 | 6,30 |
| 28 | 7,93 | 12,9 | 0,72 | 8,60 |
| 30 | 9,85 | 13,9 | 0,74 | 9,86 |
| 36 | 17,5 | 17,2 | 0,77 | 14,22 |
| 40 | 24,3 | 19,3 | 0,79 | 17,54 |
| 44 | 32,7 | 21,5 | 0,80 | 21,22 |
| 50 | 48,3 | 24,6 | 0,81 | 27,46 |
---------------------------------------------------------
| (*) Heat transfer efficiency assumed 25 % |
---------------------------------------------------------

3) Use a grate

4) Use high power to bring the pan to boil (see table 2 for optimal
combination of pan, food quantity
and fire power output).

5) Use a pan and a heat output rate that will give optimal performance
according to table 2.

6) Add one piece of wood at a time to the fire to get a constant output.

7) When contents of pan have warmed throughout (see table 3 for larger
food particles), transfer pan
to a haybox for the remainder of the processing time.

8. Place the fire on an elevated flat surface if there are small
children around.

It is appreciated that all the above recommendations are not always equally
practicable.
Table 2 has been adapted from the Netherlands VEG standards for domestic gas
ranges.

5.6 Feeding the open fire.
How can we feed an open fire so that a specified rate of heat output is
achieved? We can give a rule of thumb
here. Assuming an average calorific value of wood of 16 MJ/kg at air dried
condition we can conclude that we
need a feed rate of about 4 gram per minute for a fire of 1 kW gross heat
output rate. A 3 kW fire is therefore
fed at the rate of 12 g/min.

6 Foods
What we sorely miss in most woodstove reports is any mention of the
foods that are processed. Micuta, we
hasten to add, makes a praiseworthy exception. We think one of the important
operating variables of a cookstove
is the food that is to be processed on it. When we know that, we can try to
find out how it can be made edible
at the least expenditure of fuel.
In the above we have shown that stirring food in an open pot over a big fire
is not usually necessary. If we can
turn the fire down to a sufficient degree, we can put a lid on the pan and
the contents will not burn.

6.1 Inefficient processing.
There is evidence that the ways of cooking, practised in various parts of
the world are not always optimal in
so far as energy use is concerned (neither for that matter from other points
of view such as ease of operation).
One example is the cooking of pulses. Micuta reports that in Kenya dried
pulses are cooked for several hours
until they are soft. He himself has shown that a large amount of fuel can be
saved when the beans are soaked
in water overnight prior to boiling, the latter process in that case not
taking longer than half an hour. Cooking
processes, he adds, do not have to take more than one hour at the most.
Islam reports on several ways of
cooking rice in Bangladesh where excess water is used. In some cases part of
the water is discarded at some
stage of the process, in others it is boiled away. It can be shown that in
this way an amount of heat (and
therefore fuel) can go to waste corresponding to ten times the minimum
process heat required by the rice.
Inspired by these examples we feel there are probably several more instances
where much fuel can be saved by
turning to a more fuel economic process.

6.2 Heat penetrating time.
For larger food particles such as potatoes or other tubers we have to allow
time for the heat to penetrate. This
is done by continuing to boil (at suitably reduced heat if possible) for a
certain amount of time before turning
the fire down to simmering level or placing the pan into a haybox. The time
required is a function of the size,
specific heat, the density, the heat conductivity and of the shape.

In table 3 warming through times for potatoes as a function of size have
been tabulated, for simplicity's sake
the shape is assumed to be spherical. If we further simplify the problem by
assuming that the particles are
dropped into boiling water, the time necessary for the centre of a particle
to reach a temperature very close to
boiling point can be calculated using methods that can be found in standard
texts on heat transfer. The times
in the table were calculated for the centre of the food particle in question
to reach a temperature of 97 C from
an initial uniform temperature of 25 C. Actual cooking differs in that the
potatoes are usually heated
simultaneously with the water in the pan. The table does, however, give a
good indication of the importance
of cutting up the tubers into small and equal pieces if we want fuel economy.

-------------------------------------------
TABLE 3. Warming through times for tubers
as a function of size.
------------------------------------------
|Number of |Averaged |Warming |
|pieces |diameter |through |
|per kg |of pieces |time |
| |mm |minutes |
------------------------------------------
| 5 | 80,5 | 34 |
| 6 | 76 | 30 |
| 7 | 72 | 27 |
| 8 | 69 | 25 |
| 9 | 66 | 23 |
| 10 | 64 | 21 |
| 12 | 60 | 19 |
| 14 | 57 | 17 |
| 16 | 55 | 16 |
| 18 | 53 | 14 |
| 20 | 51 | 14 |
| 24 | 48 | 12 |
| 30 | 44 | 10 |
| 40 | 40 | 9 |
| 50 | 37 | 7 |
| 60 | 35 | 6 |
| 100 | 30 | 5 |
------------------------------------------

6.3 Classes of basic foods.
We rejoice with other gourmets in the vast variety of delightful dishes
human civilisation has developed the
world over. The existing variation is enough for a lifetime of enjoyable
dining (for those who can afford it).
Nonetheless we contend that from a detached thermal processing point of view
there are only few really
different foods. The requirements for efficient processing would fill only a
modest number of pages in a book.
This book, we think, should be compiled soon, it would be a very useful
undertaking.
To briefly illustrate this point we will mention that for staple foods there
is very little besides cereals, tubers
and pulses. Using a good cookbook, common sense and some arithmetical skills
one would not be far off the
mark when computing the processing time for any unknown type of food from a
known type that is found in
the cookbook. As regards foods made from flour (any kind), the processing
times, taking into account the size
of the products, should be fairly easy to compute.
We must remember that foods are prepared through physical and chemical
processes, not by magic, which
means that it is amenable to logic and common sense.

7 References

Jacobs, M. (1967)
Heat Transfer Vol 1.
John Wiley and Sons London, Sydney.

Anon (1968)
Keuringseisen huishoudelijke gasverbruikstoestelllen (Standards for domestic
gas appliances).
VEG-Gasinstituut, Apeldoorn.

Tsuchiya, Y.; Sumi, K. (1971)
Computation of the behavior of fire in an enclosure.
Combustion & Flame 16, (1971) pp 131...139.

Islam, M.N. (1980)
Study of the problems and prospects of biogas technology as a mechanism for
rural development.
(Village resources survey for the assessment of alternative energy
technology)
Report for the International Development Research Centre. Ottawa Canada.

Polley, S.L.; Snyder, O.P. ; Kotnour, P. (1980)
A compilation of thermal properties of food
Food Technology November 1980, pp 76...94.

Micuta, W. (1982)(a)
Report on the Bellerive Foundation's work in the village of Ruthigiti
(Kenya), from 23 rd May to 20 th June 1982.
Bellerive Foundation
Geneva 1982.

Micuta, W. (1982)(b)
Personal Communication.

Visser, P. (1982)
A test stand for Woodburning cookstoves
Final Report.
Department of Mechanical Engineering,
Eindhoven University of Technology, April 1982.

Verhaart, P. (1982)
On Designing Woodstoves
Proc. Indian Inst. of Sciences.
Vol.5, part 4, December 1982, pp 287...326
Bangalore 1983.

There is a lot I will have to reply to and I will do so, later. Tom Reed,
could you pass the stove recipe on to Fred Hottenroth, I am sure it will
remind him of the Eighties?
That's all for now, ever onward, with open minds,

Piet Verhaart

From E.Moerman at stud.tue.nl Sat Apr 6 07:37:00 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: Stove Test and comment
Message-ID: <52947.s335192@popserver.tue.nl>

Piet wrote:

> Does anyone think we could improve things by introducing a swirl by having
> angled blades on the bottom
> flange of the insert?

----
Etienne:
I have been thinking about this, but as you know I cannot weld and have
virtually no experience with metal working at all. As a result I could not
do any experiments on this configuration, but I am very interested in the
results. I hope you will be able to test this configuration.

----
Piet:
> Independently those of us with ideas could work on premixed burners of a gas
> with variable composition with
> a high percentage of tar droplets, having to mix with ambient air, both
> fluids with minimal kinetic energy or
> dynamic pressure. The term "pressure differential" e.g. dp is almost
> appropriate here, a vanishingly small
> quantity.
----
Etienne:
I have been asked to work on this or suggest methods to avoid using this. I
will probably be working on it for the next 2 or 3 months. I will forward
results to this list.
----
Piet:

> files. I have seen that JPEG files (drawings and photographs) need the least
> number of bites, the same scanned photograph takes up 3.21 Mb in BMP format
> and 87.2 kb in JPG, while showing no difference in resulution when printed
> at 300 dpi.
----
Etienne:
Still there is a 40,000 byte limit imposed by the list server. Note 40,000
bytes is less then 40kB, it is 40,000/1,024 kB.
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From E.Moerman at stud.tue.nl Sat Apr 6 09:38:16 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: A new stove design: staircase stove.
Message-ID: <60223.s335192@popserver.tue.nl>

In view of the information exchange I would ideally like to get on this list
and in view of the fact that things on the list are a bit slow at the moment
I would like to mention a new stove.

A few years back I thought of a stove I will call staircase stove. It was
intended to solve the problem of pan placement for the downdraft stove. It
can support 2 pans. In principle it can support more pans, would the more
pans means very low efficiencies and probably too little power for the pans
after pan no.2. Although it is developed to solve problems with the
downdraft stove, it can also be adapted for other chimney stoves.

At the time I thought of this concept I built the stove using fired bricks.
I pile up the bricks without the use of mortar, as a result the construction
was very feeble and leaked air. The biggest problem however was the
placement of the pans. Since I did not have the proper materials available I
was not able to expose more than a fraction of the pan bottoms to the flue
gases. This proved insufficient to get the water to the boil.

The past few months we had a student testing this stove again, this time
metal plates were used to cover the ducts and holes were cut into it for the
pans. As a result a far greater part of the pan was exposed to the hot flue
gases and this time it was sufficient to get both pans to the boil. In fact
she managed to get the pans to the boil in about 10 min., which is
comparable to most other wood stoves. The efficiency she measured was
roughly between 19 and 24% for the 2 pans combined. Though these figure are
still not great, I have to point out that it was only a very crude
prototype. Further experimenting and a few small changes to the pan supports
should easily push up the efficiency to 30%, while using far less material
for the stove (metal or thinner walls) can further improve the efficiency.
It is important to realise that this stove is not that wonderful, it is only
useful in a few cases. For example if you want to bring the water for rice
or beans to the boil quickly (put the pan in the first hole) than want to
let it simmer (place it in the second hole) and at the same time start
preparing the sauce (place the new pan in the first hole).

Anybody interested in a sketch of this stove can drop me a line and I will
send a sketch enclosed with an email message.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From 73002.1213 at compuserve.com Sun Apr 7 07:58:10 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:54 2004
Subject: NEW TEST, NEW MODEL
Message-ID: <960407115944_73002.1213_FHM33-2@CompuServe.COM>

Stovers all:

Here is a digest of a recent top-ignited gasifier test that I ran and a simple
model constructed from it. I would appreciate comments on its utility and what
you believe should be added to a final model. (Because of my double letter
probleem, it was coomposed on MSWORD aand EEXCEL, then transferred.)
*****
MASS ENERGY BALANCE ON TOP-IGNITED GASIFIER

4/1/96: A very careful experiment was performed to determine an accurate mass
and energy balance and the air/fuel ratio for a top-ignited gasifier (TIG).
Original data in notebook Biiomass Gasification 1= p. 97.

GASIFIER: The gasifier consisted of a closed-bottom can 10 cm diameter, 13.5 cm
high with an open mesh grate 1.5 cm above the bottom, so that fuel could fill 12
cm. A type K (chromel-alumel) sheathed 3 mm diameter thermocouple was mounted 7
cm from the top at the axis. Air flowed under the grate from a compressor,
through a needle valve, through a flowmeter (Dwyer, 0-10 scfh), through a gas
meter (Singer, 0.1 ft3/rev). The pressure was measured at the grate.

The gasifier was held steady on a ring stand with a 10 cm diameter circular
clamp (large hose clamp style). The gasifier, air supply and thermocouple and
ring stand stood on a digital balance and the weight was monitored continuously.

FUEL: 135 g of ~2X2X0.5 cm aspen chips filled the can to the rim. The moisture
content was measured as 7.8%, (wet basis). The fuel is therefore 124.5 g and
the H2O is 10.5 g. The volume of the fuel was 942.5 cm3. The apparent density
was therefor 0.143 g/cm3. If the true density of the chips is taken as 0.3
g/cm3, the void space is then about 50%.

CALIBRATIONS: The flowmeter was calibrated with the gas meter. It was found
that 10 scfh setting on the flowmeter corresponded to a flow of 11.6 Denver
ft3. Since Denver (Golden) is at 6,000 ft above sea level, it was decided to
accept the flowmeter calibration as sufficiently accurate.

BED RESISTANCE: The pressure drop between the Dwyer draft gauge and the empty
gasifier was 0.010 in water column; with a 12 cm layer of aspen chips it was
0.014. Therefore the pressure drop through the chips was 0.004 in water column
or 0.1 mm water column. (This is very approximate since it is the limit of
observation of the meter. We need a draft gauge at least ten times as
sensitive.)

THE GASIFICATION EXPERIMENT: The air flow was set to 10 scfh or 4.72 liter/min.
The bed pressure, thermocouple temperature, weight and total air flow were
monitored every minute or half minute, when things got exciting. The experiment
was terminated at 30 min when the volatiles stopped burning and charcoal burning
started reheating. Air flow was terminated, a plate was put across the top and
the outside of the can was sprayed with water to quench the charcoal.

The top of the bed was ignited with a propane torch. In 3 minutes flames
covered the top of the can. From 4 to 30 min a 10-15 cm conical diffusion flame
burned at the top of the can. There was no smoke and no odor in the closed
room.
A graph of the weight vs time showed a very steady decline of 3.1 g/min. Since
the bed was 12 cm deep, this corresponded to a propagation rate of 0.4 cm/min
through the bed.

TEMPERATURE: The temperature remained at 24 C for 12 minutes; it then rose at
a rate of 33 C/min, reaching a peak of 397C at 24 min. It then declined to 325
C at 28 minutes. It then began to rise again to 346C(as charcoal began to burn -
this is a good indication that volatilization is complete). The reaction front
reached the thermocouple at 17.5 min. (This description leaves out the short
transition from constant temperature to constant heating rate.)

MASS AND ENERGY BALANCE: The charcoal yield was 26.3 g, 19.5% of the moist
weight, 24.2% of the dry weight fuel.
Total air flow was 5.43 Denver ft3 or 4.68 standard cubic feet. The mass of air
was 4.68 ft3 (X 28.32 l/ft3 X 28.9 g/mol/24.04 l/mol) = 160 g air. (Taking 20C
for standard conditions). So mass balance is:

WOOD WATER AIR ===> VOLATILE GAS WATER CHARCOAL
124.5 10.5 160.0 230.7 10.5
26.3 grams
2614.5 1680.8
933.6 kJ
The volatile gas contained 64% of the original wood energy (as chemical and
sensible energy).
The average power in the flame was 0.93 kW (1/2 hour burn).

AIR FUEL RATIO: The air/fuel ratio was (160/124.5) 1.285, dry basis.

*****
SIMPLE MODEL

Here is a simple model of the top ignited stove, based on the above experiment.
To operate the model one needs to measure or know the following, as for instance
meeasured in the above experiment::

DRYING
Fuel as received 100 g
Oven dried fuel 92.2 g
Moisture content: 7.8%

FUEL DENSITY
Fuel Volume as recd 757 cm3
Apparent Density (dry) 0.13 g/cm3

HEAT CONTENTS
Fuel Heat content 21.0 kJ/g
Charcoal Ht Content 33.5 kJ/g
Charcoal Yield 24%
Volatile Energy/g fuel 12.9 kJ/g

(The heat content of various forms of biomass vary very little if expressed on a
DRY, ASH FREE basis.)
Knowing the above information, on needs to decide on the stove size, burn times
and powers.

DESIDERATA - SMALL STOVE DESIDERATA - LARGE STOVE

Burn Time-hi 10 min Burn Time-hi 20 min
Burn Time-lo 20 min Burn Time-lo 60 min
Power-hi 2.0 kW Power-hi 2.5 kW
Power-lo 1.0 kW Power-lo 1.2 kW
Gasifier Height 10 cm Gasifier Height 20 cm

>From these assumptions, one can calculate the following:

OUTPUTS - SMALL STOVE OUTPUTS - LARGE STOVE
Energy Required 2400 kJ Energy Required 7320 kJ
Fuel Required (WB) 202 g Fuel Required (WB) 525 g
Fuel Volume 1528 cm3 Fuel Volume 3971 cm3
Fuel bed height 10.0 cm Fuel bed height 20.0 cm
Fuel bed diameter 13.9 cm Fuel bed diameter 15.9
cm

Charcoal produced 48.8 g Charcoal produced 127.0 g
Heat loss - bare can 111 kJ Heat loss - bare can 672
kJ

Note: Assumes 25 kJ/m2-hr-C heat loss coefficient (4 Btu/hr-ft2-F)

Again, let me say that I would appreciate receiving comments on what's here -
and what's missing from all you thoughtful people. This model is on an Excel
spreadsheet and I can readily predict sizes for other forms of fuel or other
stove sizes.

THANKS TOM REED

From 73002.1213 at compuserve.com Sun Apr 7 07:58:12 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:54 2004
Subject: Clay stoves
Message-ID: <960407115949_73002.1213_FHM33-3@CompuServe.COM>

Etienne:

I don't think sstraw will makee clay more insulation. It has been used for
5000 years to make clay bricks sstsronger, but I presume when fired to brick
temperatures no strength remains, only the charcoal - or voids. VVoids of
course would make it more insulating. I'm thinking that you would need to add
so much straw to reduce density that all strength would be lost.

On the other hand, veermiculite (a) maintains its platelike structure to
600-700C, (b) would be claylike in nature (c) is probably available oon all
continents, since it is made by steam explosion of low grade mica and used by
plasterers and shippers and (d) wouldd be very low density.

Can onee havee a very porous intterior and then add a "skim" coat of solid clay
as cement makers do?

Do you agree that high insulating, low density firedd clay would be a boon in
all stove manufacture?

In any case, I don't know the answeer yet and hope that someone will take the
challenge to make an insulating plaster for stoves since I'm not a potter and my
high-firing capabilities are limited.

From 73002.1213 at compuserve.com Sun Apr 7 07:58:11 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:54 2004
Subject: Comments on Piet's Stove
Message-ID: <960407120018_73002.1213_FHM33-4@CompuServe.COM>

Piet and all stovers:

(UPPER CASE IS TBREED SPEAKING. PLEASE EXCUSE THE DOUBLEE LEETTERS THIS LAPTOP
MAKES VOLUNTARILY.)
WPWin 5/1 File: tst60401

Stove Test 960401

GOOD TO IDENTIFY NOTEBOOK WHERE ENTERED.

I HAVE MENTIONED THAT I BELIEVE "TINDER" OR "FIRST STARTER" MAY BE NEECEESSARY
FOR OUR FINAL STOVE. IT CAN: 1) ASSURE IMMEDIATE, SIMPLE STARTS, NO SMOKE; 2)
LIGHT UNIFORMLY ACROSS THE FUEL BED SO THAT THE REACTION ZONE TRAVELS DOWN
HORIZONTALLY, RATHER THAN TIPPED; 3)HEAT UP THE MIXER/BURNER TO CREATE A GOOD
DRAFT THAT WILL KEEP THE REGULAR FUEL BURNING BRIGHTLY; 4) DISTRIBUTE THE GASES
ACROSS THE MIXER/BURNER ENTRANCE UNIFORMLY.

I HAVE EXPERIMENTED WITH PINE NEEDLES SOAKED IN BACON FAAT. GOOD, BUT I'M
SSTILL LOOKING FOR BETTER. IN ORDER TO KEEP THE REMAINING CHARCOAL THE SAMEE
SIZE AS THE LIGHTER I AM GOING TO TRY ADDING SOME ZNCL2 WHICH PROMOTES CHARRIING
OF THE ORGANICS RATHER THAN VOLATALIZATION. ANY COMMENTS ON OTHER CHAR-MAKERS?

8.53 Lighting with propane torch through adjustable slit. (LARSON WOULD BE
HORRIFIED, BUT THAT'S WHAT I DO FOR NOW.
9.03 Relit. Was only producing smoke. Primary air hole 10 mm open
9.05 Mix of yellow flames and smoke, the usual. They do seem quite
lively, lateral airflow is doing good
things to the flames but not enough.
9.08 Good fire going, flames, yellow, of course, sometimes coming out
of the top of the insert. Bottom
valve open 2.5 mm.
9.15 By playing around with the bottom valve I can maintain a
reasonable kind of combustion. It is quite
critical. If we are going to use this principle we need a very
good bottom valve.

YES, NEED GOOD BOTTOM VALVE. I USE A LARGE METAL CIRCLE WITH A SMALLER (2 CM D)
HOLE CUT ALONG A RADIUS, SIMILAR ON BOTTOM OF CAN, RIVET TOGETHER, ROTATE TO
EXPOSE FULL TO ZERO CIRCLE. CIRCLE PROBABLY NOT BEST SHAPE, AND WE'LL IMPROVE
WHEN WE KNOW WHAT WE WANT. I HAVE ALSO USED A SLIDING VALVE. CAN BE MADE
LINEEAR OR TAPERED EITHER WAY. GLAD PIET VERIFIES IMPORTANCE OF GOOD SETTING.
9.19 It is possible to relight the flames by opening the bottom valve
wide for a short period and closing it
carefully. If closed too far the flames will go out and be
replaced by thick smoke. There are times
when I could have put on a pan without it getting black but if
that were the object, we would have to
withdraw the pan quite a number of times to wait for good
combustion to reappear. The combustion
is quite sensitive to the opening of the bottom valve, fractions
of a millimetre make a difference.

WE NEEED A POSITIVE FEEDBACK SITUATION IN WHICH MORE DRAFT FROM MIXER-BURNER
CREATES MORE FUEL FROM LOWER GASIFIER AND VICA-VERSA. THINK OF THE ANALOGY TO
CHOKE (TO CHANGE MIX) AND THROTTLE (TO INCREASE/DECREASE MIXED AIR-FUEL) IN YOUR
CARBURRETOR.

9.23 There is a spell of good combustion, meaning I can't smell
anything and not because my olfactory
organ is burnt. Bottom valve about 4 mm open. It is quite a fire,
flames extending about 150 mm from
the insert. In any case the heat output rate is certainly not
constant, it seems to attain a maximum
toward the end of the process, that is if you wish to keep the
flames alive.

I ONLY GENERALLY UNDERSTAND GEOOMETRY OF STTOVE, SO CAN'T APPRECIATE THE
SIGNIFICANCE OF 4 MM. WAS THIS STOVE ACCURATELY DEESCRIBED BEFORE? SHOULD I
FIND IT IN MY FILES?

9.32 Closed the bottom valve, most of the volatiles have been driven off.

Some hours later it transpired we have produced 125 g of charcoal e.g.
125/780 = 0.16 or 16 % charcoal.
There is some uncarbonised wood left, two sticks only partly charred.
I HAVE ALSO HAD TROUBLE WITH THE "VERTICAL STICK" STOVE LOADING IN GETTING
COMPLETE CHARRING BEFORE I GET CHARCOAL COMBUSTION (DOUBLY UNDESIRABLE).

I HAVE PURSUED SWIRLS AND MIXERS A LONG TIME AND CURRENTLY BELIEVE NO AMOUNT OF
NATURAL DRAFT WILL GIVE GOOD MIXING. NEED 20-100 MM WATER COLUMN TO GET ENOUGH
VELOCITY (AS IN GAS STOVEES). I BELIEVE THE MIXING WILL HAVE TO BE CONTROLLED
DIFFUSION (AS IN ALLADIN KEROSENE LAMPS. I BELIEVE I HAVE SEEN THIS WORK VERY
WELL, BUT NOT YET CONTROLLED ENOUGH.

IF YOU ALL DON'T HAVE A CLEAR PICTURE IN YOUR MIND OF THE WORKINGS OF AN ALLADIN
KEROSENE MANTLE LAMP, PLEASE CHECK IT OUT ASAP. THE AIR FLOW PASSAGES ARE QUITE
INGENEOUS AND THERE ARE THOUSANDS OF SMALL HOLES THAT ALLOW DIFFUSIONAL MIXING.
I PRESUME (A) THAT THIS IS CLOSE TO AN OPTIMUM DESIGN (HASN'T CHANAGED IN 50
YEARS)AND (B) COULD NOT BE ARRIVED AT WITHOUT EXTENSIVE EXPERIMENT (AND
MODELLING BASED ON THE EXPERIMENTS).

SEE MY NEXT POSTING ON MY LAST EXPERIMENT. WE CAN EXPERIMENT ON THE GASIFIER
INDEPENDENTLY OF THE MIXER-BURNER.

Be of good cheer,

THIS IS NECESSARY FOR STOVE RESEARCH. BE WILLING TO ACCEPT ONE STEP BACK FOR
EACH TWO FORWARD.

Piet Verhaart

THANKS FOR COMMENTS ON JPEG TYPE FILES. WHERE DO I GET A READER AND WRITER.
.... HOW DO I FIND THE RELATIVE VIRTUURES OF EUDORA VS COMPUSERVEE WITHOUT
SPENDING MUCH TIMEE ON BOTH? HOW DO MY COMPUSERVE MESSAGES LOOK ON THE EUDORA
READEER? (THE EUDORA WRITTEN MESSAGES ARE SOMEEWHAT CHOPPY ON THE COMPUSERVE
READER, BUT OK. (DON'T BLAME THE DOUBLE LETTERS ON COMPUSERVE.)

EE--MAIL IS GREAT - BUT HAS A WAY TO GO STILL. (SIGH, SIGH).

From E.Moerman at stud.tue.nl Sun Apr 7 09:14:40 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: Fw:
Message-ID: <55208.s335192@popserver.tue.nl>

I received a mail from jvhenry@...... Below I copied a part that might be
interesting.

Etienne

------

I have used a MUCH simpler version of that tin-can stove on backpack
trips in the
wilderness to minimize impacts from fire. Boy Scouts call it a hobo
stove. It is lighter than carrying even the smallest Primus. They work
great, but are very easy to tip over (top-heavy).
I have seen advertised a version with a little battery operated blower
which seems stupid. It claims to work faster, but who cares in the
wilderness? Perhaps those marathon hikers who do
50 km/day carrying 30 kg?
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From E.Moerman at stud.tue.nl Sun Apr 7 16:08:56 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: Comments on Piet's Stove
Message-ID: <80057.s335192@popserver.tue.nl>

A few comments on Tom Reed's comments on Piet Verhaart's experiment.

-----

Tom R:
> I HAVE MENTIONED THAT I BELIEVE "TINDER" OR "FIRST STARTER" MAY BE
> NEECEESSARY FOR OUR FINAL STOVE. IT CAN: 1) ASSURE IMMEDIATE, SIMPLE
> STARTS, NO SMOKE; 2) LIGHT UNIFORMLY ACROSS THE FUEL BED SO THAT THE
> REACTION ZONE TRAVELS DOWN HORIZONTALLY, RATHER THAN TIPPED; 3)HEAT UP THE
> MIXER/BURNER TO CREATE A GOOD DRAFT THAT WILL KEEP THE REGULAR FUEL BURNING
> BRIGHTLY; 4) DISTRIBUTE THE GASES ACROSS THE MIXER/BURNER ENTRANCE
> UNIFORMLY.
> I HAVE EXPERIMENTED WITH PINE NEEDLES SOAKED IN BACON FAAT. GOOD, BUT I'M

--------

Etienne:
I think it was Ron who pointed out that lighting is not likely to be a
serious problem. The stove users usually have a daily experience for years
with this
and contrary to us with primitive means. So I wouldn't spend too much time
on lighting.

------

Tom R:
> SSTILL LOOKING FOR BETTER. IN ORDER TO KEEP THE REMAINING CHARCOAL THE
> SAMEE SIZE AS THE LIGHTER I AM GOING TO TRY ADDING SOME ZNCL2 WHICH
> PROMOTES CHARRIING OF THE ORGANICS RATHER THAN VOLATALIZATION. ANY
> COMMENTS ON OTHER CHAR-MAKERS?

-------

Etienne:
Have a look at:
Heterogeneous kinetics of coal char gasification and combustion.
Laurendeau
Prog. Energy Combust. Sci. Vol.4, pp. 221-270, 1978.

>From that article pp. 226-227 it follows that for charring you should avoid
the use of Fe, Ca and Mg salts.

I hope this is just for testing and that you are not seriously considering
the use of this by stove users in developing countries.

A better option might still be the moisture content. Although the char yield
is hardly influenced by it in an isothermal environment in a stove it
probably is. This is due to extra cooling of the pyrolysis zone due to the
slower reaction rates and this probably leads to a higher char yield.
However I also would like to point out that the calorific value of char is
usually not 33MJ/kg, but between 24 and 31 MJ/kg. See Emmons and Atreya in
Woodheat for cooking, edited by Prasad and Piet Verhaart.

Good luck,

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From E.Moerman at stud.tue.nl Sun Apr 7 16:08:57 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: Clay stoves
Message-ID: <80053.s335192@popserver.tue.nl>

Tom Reed:

You wrote:
> I don't think sstraw will makee clay more insulation. It has been used for
> 5000 years to make clay bricks sstsronger, but I presume when fired to brick
> temperatures no strength remains, only the charcoal - or voids. VVoids of
> course would make it more insulating. I'm thinking that you would need to
> add so much straw to reduce density that all strength would be lost.

------

Etienne: I just reported what I heard. Apparently it is used and quite
useful. Perhaphs other have more info on its use. Piet Verhaart? Prasad?
Piet Visser?

------

Tom R.:
> Can onee havee a very porous intterior and then add a "skim" coat of solid
> clay as cement makers do?

-----

Etienne:
I suppose so. I don't see why not.

-----

Tom R.:
> Do you agree that high insulating, low density firedd clay would be a boon
> in all stove manufacture?

-------

Etienne:
For affordable prices certainly. If you come up with a solution here please
do.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From larcon at csn.net Sun Apr 7 23:48:45 1996
From: larcon at csn.net (Ronal Larson)
Date: Tue Aug 31 21:34:54 2004
Subject: Modeling and chimneys
Message-ID: <Pine.3.89.9604072122.A26679-0100000@teal.csn.net>

I. Modeling

A. To Ken Bryden - thanks - I enjoyed your papers ("Numerical Modeling
of a Deep, Fixed Bed Combustor" and "Combustion of Thermally Thick Woody
Biomass"), which show excellent agreement with experiment. Although the
papers do not relate directly to stoves, I think others in the stoves
group will enjoy them and get some new ideas.

B. To other stovers:

I took advantage of Ken Bryden's offer of reprints, received
these two a few days ago and thought I would reply publicly because there
are some modeling and pyrolysis topics here of interest to stove
research.

Ken's work concerns the whole tree combustor - generating many MW
per square meter of plan area - a very different regime fron that of
stoves. My reading leads to these questions for Ken:

a) Can you tell us more about the early development of this
geometry. It produces a fair amount of gas but is a combustor - not a
gasifier. (There is secondary or "overfire" air - but apparently less
than half of the total?) Are there any notable weaknesses for this geometry?

b) Might a pyrolyzer (charcoal by-product) make technical and economic
sense? I am especially thinking of developing countries with a stronger
need for charcoal than here.

c) You describe gasification updraft modeling by Kayal of
vertical sticks. Might his work be of use in the pyrolyzing stove? (I
haven't yet found Bioresource Technology 1994, 49, pp61-73.) Was this
top-lit or bottom lit? You don't mention this vertical orientation for
the whole tree combustor - is there a reason?

d) You state (p E) these Kayal sticks "are parallel to the air
flow vice in cross flow as might be expected." Could you explain this
use of the word "vice"?

e) Your model assumes a constant-with-height (65%) void
fraction. This seems high and even difficult to achieve; any comment?
Do you have any data for Kayal's void fraction and the relative air-flow
resistances (or pressure drops) of the two fuel orientations?

f. You report that the ash is blown upward. Might this be true
for a much lower primary airflow (pyrolysis) situation as well? (where
the air flow is much less). I have presumed that not seeing ash meant
that I was not consuming much charcoal; when do you think that might not
be true?

g. Your seven-gas analysis seems amenable to being used for a
charcoal-making stove geometry also, with reasonably small
modifications. Among them are
1) top lighting with charcoal on top rather than the bottom
2) a need for a very different variation in particle size with
height (being only a small decrease as the pyrolysis zone moves past).

I hope that you will find time and/or funding to do such modeling.

II. Chimneys

Some of you will recall that I have felt it important to focus on
the internal pressures, flows, and velocities due to the several thermal
buoyancies and the chimney. Etienne recommended that chimneys might be
in the book by Gebhart entitled "Buoyancy Induced Flows". I need to go
back, but think I found something closer to my needs in Chapter 26
(Chimneys) of the 1988 (probably other editions also) ASHRAE handbook on
"Equipment". There are several nice nomograms to accompany the usual
equations. I am in the process of trying to simplify this for the
charcoal-making stove - but will be busy this next week. I hope others
will look up this reference and see if it helps you also. My problem
now is not yet knowing how to specify the resistances of the fuel bed and
of the secondary air inlet.

I also found an 1899 book on chimneys - not very useful, since it
started with 50 footers and went larger. But this was based mostly on
papers in the ASME Transactions - mostly vol XI from a few years
earlier. These I found quite fascinating - as the chimney paper
reviewers then held nothing back in their dislike of each other and the
others' theories. These review comments and author responses were much
more interesting than the original papers. Because everything was based
on several feet of coal fuel bed and only primary air - I don't think I
will go back to these. There is a lot more generality in the ASHRAE
work. I later found much of the same heuristic 19th century chimney
tabulations in several old ME handbooks.

More in about a week - as this week is devoted to local solar politics.
Sorry I have been so quiet.

Ron Larson

From E.Moerman at stud.tue.nl Mon Apr 8 09:58:50 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: Modeling and chimneys
Message-ID: <57849.s335192@popserver.tue.nl>

Ron Larson wrote:

> will look up this reference and see if it helps you also. My problem
> now is not yet knowing how to specify the resistances of the fuel bed and
> of the secondary air inlet.

-------

Etienne:
I will make a copy of my paper on fuelbed resistance and send it to you. As
far as the secondary air inlet is concerned I recommend you check fluid flow
text books and journals, especially the fluid flow engineering text books
(eg. student editions) should contain pressure drops over different shaped
and sized holes. You can also find a little on this in Paul Bussmann's
thesis.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From prasad at tn7.phys.tue.nl Tue Apr 9 06:07:30 1996
From: prasad at tn7.phys.tue.nl (prasad)
Date: Tue Aug 31 21:34:54 2004
Subject: Clay stoves
In-Reply-To: <960405150914_73002.1213_FHM62-4@CompuServe.COM>
Message-ID: <9604091002.AA29729@tn7.phys.tue.nl>

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From 73002.1213 at compuserve.com Tue Apr 9 08:28:19 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:54 2004
Subject: Thanks - and good modelling
Message-ID: <960409122410_73002.1213_FHM75-1@CompuServe.COM>

Etienne:

Yes, very much appreciated. I hope we will be able to SEARCH on a large body of
this type information. The digests are a first step in this very dynamic
process.

Thanks again TOM REED

Then, after your comments on Boyles and Charles etc. ....

There is an enormous amount of literature on pyrolysis, both 1880-1950 on SLOW
PYROLYSIS (Nbiot <0.1; typical yield 20-30%) and FAST PYROLYSIS (Nb>10, typical
yield 5-15%). In almost none of it is there an attempt to predict charcoal
yields. Given the yield at one heating rate, however, Diebold's thesis at CSM
about 1985 did some of this (from NREL/SERI). Then there is the most recent
work of Antal at elevated pressure (<10 atm) with yields of 40-45%.

The word "charcoal" covers a multitude of compositions, from torrefied wood,
brands, ... with 50% volatiles still remaining to activated charcoal with less
than 10% volatiles remaining. This range covers chemically the loss of water of
composition. I have promoted the use of "XCOAL" as a name to cover this wide
range.

I believe there is a homeostatic composition describing primative cooking
charcoal in which self-heating takes over at about 280C and carries the reaction
to about 450C, no matter what the conditions.

I would be interested in whatever basis you think you might have for modelling
charcoal yields.

I do think that there is a homeostatic

From 73002.1213 at compuserve.com Wed Apr 10 09:59:10 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:54 2004
Subject: "Whole Stick Stoves"etc.
Message-ID: <960410140049_73002.1213_FHM55-6@CompuServe.COM>

Ron, Ken Bryben Etienne et al:

Let me second Ron's interest in Ken's modelling whole tree combustion; received
and noted. Interesting use of whole trees. (Aren't they routinely too wet?)

Concerning the "three stone" stove and Etienne's comments: Is it possible that
we have been overlooking the possibility of making significant improvements in
the TSS? Feeding the sticks in supplies the volatiles and makes charcoal. The
charcoal then sustains further pyrolysis. This could be called the "whole
stick" stove.

How about a small pit under the burning area to collect the charcoal (which
burns much slower than the volatiles). If the pit is deep, the charcoal would
extinguish itself for later use.

How about an air tunnel under a shallow pit and a crude wire grate? How about
always using <20% moisture, uniform sticks. Then as the sticks are pushed into
the fire zone they are able to jet the pyrolysis gases into the burning char
gases for added combustion - and give high heat rates - less wood feed gives
low heat rates.

Top burning of charcoal: See my earlier comments that I couldn't get a top
burning charcoal fire lit! I was very surprised. Any experience out there?

Comments? TOM REED

(These are Comments on Ron's comments on Ken's modelling papers.
I. Modeling

B. To other stovers:

Ken's work concerns the whole tree combustor - generating many MW
per square meter of plan area - a very different regime fron that of
stoves. My reading leads to these questions for Ken:

b) Might a pyrolyzer (charcoal by-product) make technical and economic
sense? I am especially thinking of developing countries with a stronger
need for charcoal than here.

d) You state (p E) these Kayal sticks "are parallel to the air
flow vice in cross flow as might be expected." Could you explain this
use of the word "vice"?

II. Chimneys

More in about a week - as this week is devoted to local solar politics.
Sorry I have been so quiet.

Ron Larson

From 73002.1213 at compuserve.com Wed Apr 10 09:59:47 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:54 2004
Subject: Charcoal, Xcoal, ...
Message-ID: <960410140032_73002.1213_FHM55-2@CompuServe.COM>

Etienne:

I stand corrected on charcoal heat of combustion. I took it from graphite,
/\Hc=96 kcal/mol, being on the road. Obviously the difference between 24 and 33
is the amount of volatile material left in the charcoal.

However, I think your statement that a 25% yield being due to unpyrolysed or
less pyrolysed wood is too strong. The Lambiotte (Belgian? still manufactured?)
charcoal process regularly gave yields over 30% from hardwoods. As mentioned
before, the presence of minerals greatly influences the Xcoal yield. Also,
pressure can take us up to 45% cooking charcoal and I hope to see the pilot
plant in Hawaii this June. More later. It is in part a question of how much of
the volatiles can be caused to char before they exit the wood - or redeposit on
colder portions.

Do you agree that there is room here for a few basic theses? I'd love to come
to Twente for a few years, but my wife would veto. Visit?

Your 24-31% charcoal yield sounds fine, but I'd like to re-fine it further. I
made comments earlier on Xcoal, X being the degree of charring (water removal)
and I also suggested that "cooking charcoal" production is homeostatic. Once
you reach about 300 C the reaction becomes exothermic and carries itself to 450C
and end point that is hard to exceed. Do you agree? And what is our best heat
content at that point? Probably your 24 kJ/g.

Regarding kindling/tinder: I don't think the African continent should be our
only focus. China and India have a high level of culture - and still cook on
biomass. If it takes a preformed "fire starter" costing a penny to make a
working fire, they'll distribute it. (I''m not saying it does.)

Too much preoccupation with ZERO COST too early limits our thinking too much.

Weds:

I just received "WOOD HEAT FOR COOKING". Thanks so much. Clearly this is the
"Old Testament of Stoves". Let's hope we may write a New Testament here, though
so far we are probably only apocryphal.

I'm confused by the relationships between Twente University (Nederlands) and TU,
Eindhoven (Germany?). None? Nearby? Also between Moerman, Prasad and Verhaart
- all old friends? What's the history here?

Forward.. TOM REED

From 73002.1213 at compuserve.com Wed Apr 10 09:59:54 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:54 2004
Subject: Hobo Stove
Message-ID: <960410140037_73002.1213_FHM55-4@CompuServe.COM>

Etienne:

Your reference to the "Homo Stove" is very interesting, but didn't have his
complete address or any description. Top lighting?

The battery operated stove if VERY superior to the hobo stove, made by Fred
Hottenroth and has inspired Ron and me. It is truly elegant and a 10c battery
(in India) lasts 30 hours. A variation is being commercialized in Nepal. (We
don't have many Hobos in the US any more, but lots of backpackers.)

The Girl Scouts also had an interesting sawdust stove/candle which provided a
steady flow of pyrolysis gas for 15-30 minutes. I'll try and reproduce it.

The real problem of hobos, Hottenroth and most others is not making gas, but
getting it to mix with air in suitable proportions and burn steadily, cleanly
and controllable.

It is very useful for us stovers to have a very wide view of all previous
manifestations.
Note that I am back on my main PC and no more double letters.

TOM (I'm running out of cheery greetings.)

From tduke at igc.apc.org Wed Apr 10 10:33:35 1996
From: tduke at igc.apc.org (Thomas Duke)
Date: Tue Aug 31 21:34:54 2004
Subject: NEW TEST, NEW MODEL
Message-ID: <199604101435.HAA00567@igc3.igc.apc.org>

> Date: 07 Apr 96 07:59:44 EDT
> From: Thomas Reed <73002.1213@compuserve.com>
> To: STOVES <stoves@crest.org>
> Subject: NEW TEST, NEW MODEL
> Reply-to: stoves@crest.org

> Stovers all:
>
> Here is a digest of a recent top-ignited gasifier test that I ran and a simple
> model constructed from it. I would appreciate comments on its utility and what
> you believe should be added to a final model. (Because of my double letter
> probleem, it was coomposed on MSWORD aand EEXCEL, then transferred.)
> *****

Hi Tom,

Thrilled to see you are modeling in excel. I have excel so can
probably open a file from you. Have you posted an excel file? I would
like to open an excel file from you. So if you havn't posted one
please do if you can. You can send it as an attached file to your
message. I am able to open attached files.

I have been to Walt Disney World doing some imagineering. So I am a
little behind on my e-mail. However I am benefiting from exposure to
Disney's approach to inovation. So expect to be able to make more
significant contribution in the days to come.

I model in excel also. Charts work good. Can change input by draging
chart bars. See output visually. Also can draw sketches in excel.

Tom Duke
Tom Duke
4363 Hunt Road
Burlington IA 52601-8917
The Renewable Energy Research Center & Farm (319)754-7384

From E.Moerman at stud.tue.nl Wed Apr 10 12:06:21 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: Charcoal, Xcoal, ...
Message-ID: <65504.s335192@popserver.tue.nl>

Tom Reed wrote:

> Do you agree that there is room here for a few basic theses? I'd love to
> come to Twente for a few years, but my wife would veto. Visit?

----

Etienne:
For Twente you have to contact other people (Huub Stassen, Piet Visser), but
as far as I am concerned you are welcome at Eindhoven. You better check with
Prasad though, he might be able to arrange a temporary office and lab.

-----

Tom R.:

> homeostatic. Once you reach about 300 C the reaction becomes exothermic
> and carries itself to 450C and end point that is hard to exceed. Do you
> agree? And what is our best heat content at that point? Probably your 24

-----

Etienne:
Yes, very likely.

------

Tom R.:

> I'm confused by the relationships between Twente University (Nederlands)
> and TU, Eindhoven (Germany?). None? Nearby? Also between Moerman, Prasad
> and Verhaart - all old friends? What's the history here?

-----

Etienne:
TU Eindhoven is also in The Netherlands. We have contacts with Twente.
Distance about 250km. Piet Verhaart, Piet Visser, Prasad and I all worked at
WSG Eindhoven (TU) at one time or another.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From bryden at cae.wisc.edu Fri Apr 12 00:47:07 1996
From: bryden at cae.wisc.edu (Mark Bryden)
Date: Tue Aug 31 21:34:54 2004
Subject: Modeling and chimneys
Message-ID: <199604120450.XAA79384@audumla.students.wisc.edu>

Hello Stovers all,

At 09:51 PM 4/7/96 -0600, Ron wrote:
>To Ken Bryden - thanks - I enjoyed your papers

Thanks for reading them. I am always eager for comments and suggestions.
>
[snip]
>Ken's work concerns the whole tree combustor - generating many MW
>per square meter of plan area - a very different regime from that of
>stoves. My reading leads to these questions for Ken:

comment:
In this paper I work on WTE (whole tree energy) and a portion of my research
has been funded to work on this model, but the core of my research is the
development of engineering models of woody biomass combustion. Because of
this I am very interested all types of wood combustion.
>
>question
> a) Can you tell us more about the early development of this
>geometry. It produces a fair amount of gas but is a combustor - not a
>gasifier. (There is secondary or "overfire" air - but apparently less
>than half of the total?) Are there any notable weaknesses for this geometry?

answer
With the caveat that no one has yet built a Whole Tree combustor, I am not
aware of any particular weakness. Yes, the overfire is generally less then
half of the total air, rather than thinking of it as a gasifier I would
think of it as deep fixed bed combustor. As a comment most stoves are also
"deep" combustors or gasifiers and most are both to some degree.
>
>question
>b) Might a pyrolyzer (charcoal by-product) make technical and economic
>sense? I am especially thinking of developing countries with a stronger
>need for charcoal than here.

answer
probably not. The WTE combustor is designed to be operated in steady state
and as a consequence trying to get the charcoal out from a 3 or 4 m deep
pile of wood would be very difficult. think of the weight of the wood above
the charcoal.
>
>question
> c) You describe gasification updraft modeling by Kayal of
>vertical sticks. Might his work be of use in the pyrolyzing stove? (I
>haven't yet found Bioresource Technology 1994, 49, pp61-73.) Was this
>top-lit or bottom lit? You don't mention this vertical orientation for
>the whole tree combustor - is there a reason?

answer
It may be but it was steady state not transient. His experiment was similar
to the WTE combustor ... steady state, up draft, with char combustion on the
bottom and drying and gasification on the top.
>
>question
> d) You state (p E) these Kayal sticks "are parallel to the air
>flow vice in cross flow as might be expected." Could you explain this
>use of the word "vice"?

answer
In most of the work I am familar with the fuel is placed in the combustor in
cross flow not in parallel with the flow. To place the fuel in cross flow
all one has to do is fling it in. To place it in parallel flow it has to be
gathered or bundled and carefully put in and even then it seems that in a
high volume high heat release operation jamming could occur. Even in the
small combustor of Kayal he reports jamming and clogging of air passages by
tar which had to be relieved by tapping on the unit.
>
>question
> e) Your model assumes a constant-with-height (65%) void
>fraction. This seems high and even difficult to achieve; any comment?
>Do you have any data for Kayal's void fraction and the relative air-flow
>resistances (or pressure drops) of the two fuel orientations?

answer
A constant void fraction was chosen for two reasons 1) there was no
experimental evidence to support any assumption 2) if the original fuel
geometry is retained ie. cylinders vice breakup, it is easy enough to show
that the void fraction remains constant. I agree that the void fraction
seems high but it is a measured value for whole trees, not logs, stacked up
over 100 ft high also void fractions in the 50-60% range are common. Also
it gave good agreement with the single data point.

Kayal doesn't report the pressure drop. As a starting place for this I
would suggest looking at non-reacting packed beds. There is a large body of
work wich addresses pressure drop for this case. I suspect the reacting
part doesn't add much to the pressure drop in the bed.

>question
> f. You report that the ash is blown upward. Might this be true
>for a much lower primary airflow (pyrolysis) situation as well? (where
>the air flow is much less). I have presumed that not seeing ash meant
>that I was not consuming much charcoal; when do you think that might not
>be true?

I'm not sure, but you can measure you flow rate and then perform a force
balance and determine the maximum size that will blow out. I would bet that
you will find the ash that is released (and there won't be much) will blow
out. If you need help with this let me know I have a simple EES (this is a
commercial solver...like math cad) routine that solves this problem given
particle shape and flow rate.
>
>comment
> g. Your seven-gas analysis seems amenable to being used for a
>charcoal-making stove geometry also, with reasonably small
>modifications. Among them are
> 1) top lighting with charcoal on top rather than the bottom
> 2) a need for a very different variation in particle size with
>height (being only a small decrease as the pyrolysis zone moves past)

answer
both of these are easy to incorporate, a larger issue is the for transient
analysis instead of steady state analysis. it may be possible to assume
quasi-steady operation and then step in time but right now the solver finds
the steady state solution. I suspect that this is a couple of months of
work. As a comment WTE has good hope that a pilot plant will be built, If
this is the case, I'll probably have to build the transient model could then
be applied to this stove.
>
> I hope that you will find time and/or funding to do such modeling.

As you both are always in short supply! but I will eventually (maybe not /
probably not this year) get around to it.

As a final comment in support of combustion modelling let me quote Prasad
and Verhaart from the forward to "Wood Heat for Cooking" 1983.

"Another feature of the woodstove for domestic cooking is its small size.
This is its strength as well as its weakness -- strength because testing can
be carried out with relative ease and at a small cost on full scale
prototypes, and weakness because modelling -- the forte of engineering
science -- is presumed unnecessary for design and development of small
devices. It is the editors' contention that modelling is the precisely what
is required if one is to contemplate the availability, in a period of twenty
years, of stoves with superior performance to a billion people or so living
in diverse places with varying local resource situations."

WELL SAID!

I would be interested to hear from Prasad and Verhaart how they think we
(the engineering world) is doing at making the twenty year goal mentioned
above. I wonder if we are "behind schedule" and need to catch up.

Keep burning!

Mark Bryden
UW-Madison

From bryden at cae.wisc.edu Fri Apr 12 00:47:14 1996
From: bryden at cae.wisc.edu (Mark Bryden)
Date: Tue Aug 31 21:34:54 2004
Subject: "Whole Stick Stoves"etc.
Message-ID: <199604120450.XAA19530@audumla.students.wisc.edu>

Tom,

At 10:00 AM 4/10/96 EDT, you wrote:
>
>Let me second Ron's interest in Ken's modelling whole tree combustion;
received
>and noted. Interesting use of whole trees. (Aren't they routinely too wet?)
>[major snip]

Thanks for the interest. They aren't too wet to burn but if left wet the
efficiency drops dramatically and the bed height becomes too high ... hence
the heat output drops. Because of this plan is to use waste heat from the
combustor to dry the trees to ~23% moisture (as rec'd) prior to use as fuel.

Mark Bryden
UW-Madison

From prasad at tn7.phys.tue.nl Fri Apr 12 04:24:27 1996
From: prasad at tn7.phys.tue.nl (prasad)
Date: Tue Aug 31 21:34:54 2004
Subject: Stoves of the Eighties
In-Reply-To: <9604010239.AA12885@janus.cqu.edu.au>
Message-ID: <9604120819.AA03643@tn7.phys.tue.nl>

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From E.Moerman at stud.tue.nl Sat Apr 13 09:26:15 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: Modeling and chimneys
Message-ID: <55897.s335192@popserver.tue.nl>

Mark Bryden wrote:

> Kayal doesn't report the pressure drop. As a starting place for this I
> would suggest looking at non-reacting packed beds. There is a large body of
> work wich addresses pressure drop for this case. I suspect the reacting
> part doesn't add much to the pressure drop in the bed.
>
----------

Etienne:
This is not true. The reacting part is causing the greatest pressure drop in
the fuelbed. This can be concluded from measurements I did on this (I will
send you a copy of the article). The large contribution of the reacting part
of the fuelbed is most likely to be caused by the increase in air
temperature in the zone. This results in a lower air density and a larger
gas velocity. The larger velocity means probably more drag. For very large
combustors the situation might be a bit different due to a transition from a
laminar to a turbulent flow.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From 73002.1213 at compuserve.com Sat Apr 13 09:38:36 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:54 2004
Subject: The Whole Stick Stove
Message-ID: <960413133953_73002.1213_FHM36-6@CompuServe.COM>

Hi Mark and all:

The other day I speculated on an improved "3 stick stove" (after reading good
things about three stone stoves in Prasad et al). As you say, easy to test
small scale ideas, easier than whole tree combustors.

APPARATUS: I have a sheet of insulating refractory board 2 inches thick from
which I cut a 5 inch circle. At the center I drilled a 1 inch hole 1 1/2 in
deep to form an insulated furnace/crucible. At the periphery I drilled 3 1 inch
holes 1 1/4 below the top intersecting the central hole so that I could feed
sticks into the central crucible at any rate I pleased. I drilled 4 3/16 in
holes in the bottom of the crucible to let in limited air. (Construction time <
10 minutes).

EXPERIMENT: I placed three irregular, very dry, nearly 1 inch sticks in the
horizontal feed holes. I ignited the intersection with a propane torch.
Initially they burned quite brightly and steadily with a luminous flame
(volatiles burning). After about 5 minutes the flame began to waver and dim but
I could revive it by rotating or feeding the sticks to the center. After about
10 minutes I got a beautiful violet (CO) flame, quite transparent, for a minute
or so. Then the fire went out and no amount of torch or feed or fiddling would
relite.

CONCLUSIONS: Charcoal formed at the ends of the sticks and charcoal burns so
much more slowly than volatiles in wood that this geometry would never be
practical with any charcoal forming wood. I had hoped that the charcoal would
fall into the crucible-pit and add its heat to the fire. Not this time. Maybe
someone can suggest a modification.

From bryden at cae.wisc.edu Sat Apr 13 11:35:10 1996
From: bryden at cae.wisc.edu (Mark Bryden)
Date: Tue Aug 31 21:34:54 2004
Subject: Modeling and chimneys
Message-ID: <199604131538.KAA73860@audumla.students.wisc.edu>

At 03:31 PM 4/13/96 +0100, you wrote:
>Mark Bryden wrote:
>
>
>> Kayal doesn't report the pressure drop. As a starting place for this I
>> would suggest looking at non-reacting packed beds. There is a large body of
>> work wich addresses pressure drop for this case. I suspect the reacting
>> part doesn't add much to the pressure drop in the bed.
>>
>----------
>
>Etienne:
>This is not true. The reacting part is causing the greatest pressure drop in
>the fuelbed. This can be concluded from measurements I did on this (I will
>send you a copy of the article). The large contribution of the reacting part
>of the fuelbed is most likely to be caused by the increase in air
>temperature in the zone. This results in a lower air density and a larger
>gas velocity. The larger velocity means probably more drag. For very large
>combustors the situation might be a bit different due to a transition from a
>laminar to a turbulent flow.
>
Mark:
I look forward to reading the article.

I agree with Etienne to a point. What he has written is true but can be
accomodated by using the nonreacting data and correlations as local data
with quasi-steady state assumptions and integrating top to bottom knowing
the temperature and mass flux as a function of height. This is what makes
the natural convection numerical model yet more difficult to achieve
convergence on. The old proverb that everything we want to use is atttached
to everything else and is this case numerically stiff and tightly coupled.

Purnomo and several others used this in their investigations of reacting
packed beds (wood and other) with forced convection.

Mark Bryden

From E.Moerman at stud.tue.nl Sun Apr 14 19:08:05 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:54 2004
Subject: Modeling and chimneys
Message-ID: <4406.s335192@popserver.tue.nl>

Mark Bryden wrote:

> I agree with Etienne to a point. What he has written is true but can be
> accomodated by using the nonreacting data and correlations as local data
> with quasi-steady state assumptions and integrating top to bottom knowing
> the temperature and mass flux as a function of height. This is what makes
> the natural convection numerical model yet more difficult to achieve
> convergence on. The old proverb that everything we want to use is atttached
> to everything else and is this case numerically stiff and tightly coupled.
>
> Purnomo and several others used this in their investigations of reacting
> packed beds (wood and other) with forced convection.

-------

Etienne:
For a cold fixed fuelbed the resulting airflow is surpisingly simple. It can
be represented as an exponential or an inverse linear function of the weight
of the fuelbed. For a burning bed the situation is more complicated, but for
a large cold section and a small reaction zone this dependence still holds.
For larger reaction zones you have to resort to temperature profiles and
mass production profiles in the fuelbed as described by Mark.

Etienne
>
> Mark Bryden
>
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From 73002.1213 at compuserve.com Mon Apr 15 09:48:02 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:55 2004
Subject: Draft in top burning
Message-ID: <960415134213_73002.1213_FHM78-2@CompuServe.COM>

Moerman, Bryden et al:

Having made many natural draft runs on the top burning (inverted downdraft)
gasifier using balance to follow the rate of fuel consumption, I can say it is
quite linear.

This is at first surprising, since initially the only resistance is that of the
cold fuel, while toward the end the gases pass through many inches of hot
charcoal.

However, there is about a 50% shrinkage in the fuel-charcoal conversion, so I
suppose that compensates for the higher resistance of hot vs cold.

OK? TOM REED

From E.Moerman at stud.tue.nl Tue Apr 16 07:26:59 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:55 2004
Subject: Draft in top burning
Message-ID: <48734.s335192@popserver.tue.nl>

Tom Reed wrote:

> Having made many natural draft runs on the top burning (inverted downdraft)
> gasifier using balance to follow the rate of fuel consumption, I can say it
> is quite linear.
>
> This is at first surprising, since initially the only resistance is that of
> the cold fuel, while toward the end the gases pass through many inches of
> hot charcoal.

------

Etienne:
I thought there was only a small layer of hot and burning char. If there was
a large layer the char would burn away. I understand from you, Ron and Tom
Duke that this is not the case. Also the sticks have the same direction as
the airflow. Under these circumstances the exponential and the inverse
linear dependance reduces to a linear dependance.

----
Tom R.
> However, there is about a 50% shrinkage in the fuel-charcoal conversion, so
> I suppose that compensates for the higher resistance of hot vs cold.

----

Etienne:
This is only one of the reasons.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From 73002.1213 at compuserve.com Wed Apr 17 08:43:46 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:55 2004
Subject: Draft in top burning
Message-ID: <960417124539_73002.1213_FHM36-5@CompuServe.COM>

Etienne:

Well, in the run I reported last week in the small, un-insulated, 4 in diameter
can the reaction only reached 400C and declined quite rapidly in the layer of
charcoal above the zone as it grew from 0 to final dept, 3-4 in.

In the run I made last night I inserted a riser sleeve (3 in ID) and it made a
world of difference giving a 6 in fuel bed.. The reaction peaked at 492C at
thermocouple # 1, 3 in below the top of the can (and starting point of fuel) and
at 538C at thermocouple #2 5 in below the top. The TC at 3 in had only cooled
to 448C (from 492) at the end of the experiment.

So with good insulation there is very little cooling of the charcoal; without
it cools very rapidly. (This is a very small diameter - it would be less
dramatic in larger diameters, but similar near edges.) INSULATIN IS IMPORTANT.

Tom

From E.Moerman at stud.tue.nl Thu Apr 18 10:08:09 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
Message-ID: <58403.s335192@popserver.tue.nl>

Yesterday I spent an afternoon of testing. I tested a stove originally
thought up and constructed by Piet Verhaart, both as intended in normal mode
and in inverted downdraft mode. In its original mode it worked quite nice.
In inverted downdraft mode it didn't work very well, although the charcoal
yield looked substantial. However I didn't like the lack of control and the
combustion quality was worse than in normal mode. I suppose I should check
the inverted mode with another stove.

Also I did a cooking test with the shielded fire. It was my first cooking
test, so I used far more wood than necessary. Here a description of the
test:

Stove: Simple cilindric shield. Pan supports mounted on the shield.
-inner diameter 295mm
-shield height 95mm
-charge hole 120x120mm
-4 air supply holes 30x40mm

Pan: Flat bottom
-diameter 280mm
-height 170mm
-no lid

Content:
-2 litre of water
-after getting the water to the boil I added 1 kg of rice.

Rice:
-Oryza. Surinam rice. White rice. Whole grains.

Fuel:
-White Fir
-moisture content 10% on a dry matter basis
-blocks with dimensions 35x30x20mm

Temperature:
-water and ambient temperature: 295K

Ambient conditions:
-due to the use of a strong fan necessary for the smoke evacuation in the
closed test cell a strong side wind was simulated.

Observations:
-pan is too large for this amount of rice
-due to the fan sometimes a bit of smoke escaped through the charge hole

Events:
14:45:45 Lighted fire with 3 blocks of wood that were put in kerosene for a
few minutes. Total stack of wood at the start 0.181kg
14:46:50 Pan placed on the stove.
14:51:00 Suddenly fierce burning occurs, some black smoke through the pan
shield gap.
14:56:30 Charge added 0.115kg.
15:00:00 Charge added 0.101kg.
15:00:30 Water boils.
15:01:00 Rice added 1kg.
15:02:30 Water boils again, large flames which sometimes come out of the
charge hole.
15:06:30 Rice grains can easily be split using a finger nail. When tasted
the kernel of the grain is still a little hard.
15:11:00 Kernel of the grain is done too.
Charcoal left at least 16 g.

Due to the the oversized pan and the inefficient charging the fuel use can
be further reduced. I expect to be able to reduce the fuel use to below
0.3kg/kg of rice.

Specific fuel consumption:
0.397kg of moist wood/kg of rice
0.361kg of dry wood/kg of rice

Using an appropriate pan size, adding the rice to the water at the start
instead of waiting for the water to boil and more optimal charging I
expect that the specific fuel consumption can easily be reduced to
0.25kg of dry wood/kg of rice.
Using a more efficient version of the shielded fire can further reduce the
specific fuel consumption probably to around 0.2kg of dry wood/kg of rice
(moisture content of wood around 10%).

1kg of White Fir with 10% moisture has a combustion value of 16.7MJ/kg wet
wood.

The geometry of the tested stove is very simple, therefore the construction
costs of tis configuration of the shielded fire will be very low. Possibly
around 2US$ on the local market.

I am looking forward to other results of cooking tests.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From larcon at csn.net Thu Apr 18 17:38:08 1996
From: larcon at csn.net (Ronal Larson)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
In-Reply-To: <58403.s335192@popserver.tue.nl>
Message-ID: <Pine.3.89.9604181536.A5845-0100000@teal.csn.net>

Thanks to Etienne for getting into the cooking business. I just saw a
little snow, but in general the weather is much better and it is time to
get back to this sort of research. Just a few questions below, (and I
have deleted most of the message):

On Thu, 18 Apr 1996, E.Moerman wrote:

> Yesterday I spent an afternoon of testing. I tested a stove originally
> thought up and constructed by Piet Verhaart, both as intended in normal mode
> and in inverted downdraft mode. In its original mode it worked quite nice.
> In inverted downdraft mode it didn't work very well, although the charcoal
> yield looked substantial.

Has this been described in any literature I may have?

However I didn't like the lack of control and the
> combustion quality was worse than in normal mode. I suppose I should check
> the inverted mode with another stove.
>
Why was there no possibility of control? I encoiurage just
starting with a couple of old cans.

> Also I did a cooking test with the shielded fire. It was my first cooking
> test, so I used far more wood than necessary. Here a description of the
> test:
>
> Stove: Simple cilindric shield. Pan supports mounted on the shield.
> -inner diameter 295mm
> -shield height 95mm
> -charge hole 120x120mm

It would seem that the one of the charge hole dimensions should
be less than the 95 mm shield height. Or is the 95 mm dimension perhaps
wrong?

> -4 air supply holes 30x40mm

> 15:00:30 Water boils.
> 15:01:00 Rice added 1kg.
> 15:02:30 Water boils again, large flames which sometimes come out of the
> charge hole.
> 15:06:30 Rice grains can easily be split using a finger nail. When tasted
> the kernel of the grain is still a little hard.
> 15:11:00 Kernel of the grain is done too.

I will try to get as close as possible to this rice - but I'm
surprised at the 10 minute cook time. Anyone else have experrience with
this? I'm afraid that we should standardize the cook time and
not worry about whether the rice is cooked. But then we might as well do
a boiling water test. But I leave at almost a 2 km altitude. Oh well.

> I am looking forward to other results of cooking tests.
>
Me too.

Regards Ron

From E.Moerman at stud.tue.nl Sat Apr 20 06:22:55 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
Message-ID: <44891.s335192@popserver.tue.nl>

Ron Larson had a few questions and comments regarding my cooking tests. I
will copy the relevant passages and try to answer them below.

--------
Etienne, original:
>> Yesterday I spent an afternoon of testing. I tested a stove originally
>> thought up and constructed by Piet Verhaart, both as intended in normal
>> mode and in inverted downdraft mode. In its original mode it worked
>> quite nice. In inverted downdraft mode it didn't work very well,
>> although the charcoal yield looked substantial.
>>
------
Ron:
Has this been described in any literature I may have?
------
Etienne, reply:
I don't think so, Piet Verhaart should know. It looks a little like figure 5
at page 56 in woodheat for cooking. Except that the internal diameter of
the tube is constant over the whole length and secondary air is supplied
over the whole lenght of the fuelbed. The stove is also filled almost to the
the top.
-------
Ron:
> Why was there no possibility of control? I encoiurage just
> starting with a couple of old cans.

-----
Etienne, reply:
There are valves for primary and secondary air, but their influence on the
combustion rate was minimal. Their influence on the smoke levels were
enormous. I did not intended this as a serious atttempt of an inverted
downdraft, I just had some time left and wanted to get an idea of the
processes occurring.
--------
Etienne, original:

>> Stove: Simple cilindric shield. Pan supports mounted on the shield.
>> -inner diameter 295mm
>> -shield height 95mm
>> -charge hole 120x120mm
>>
----------
Ron:
>
> It would seem that the one of the charge hole dimensions should
> be less than the 95 mm shield height. Or is the 95 mm dimension perhaps
> wrong?

--------
Etienne:

With shield height I denoted the height of the shield along the pan, the
protecting part. The total height of the stove I forgot to measure, but it
is about 300mm.

--------

Ron:

> I will try to get as close as possible to this rice - but I'm
> surprised at the 10 minute cook time. Anyone else have experrience with
> this? I'm afraid that we should standardize the cook time and
> not worry about whether the rice is cooked. But then we might as well do
> a boiling water test. But I leave at almost a 2 km altitude. Oh well.

-------
Etienne:

Usually you don't have to cook white rice very long, unless you want
porridge. Standardising the cook time is probably a good idea, but it is
influenced by the amount of rice cooked. If I do another test within the
near future I will probably use 2 kg of rice. This is forced by the size of
the available material. You can correct for the altitude using a pressure
cooker or by making a small calculation.

Keep cooking.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From verhaarp at janus.cqu.edu.au Sat Apr 20 08:26:24 1996
From: verhaarp at janus.cqu.edu.au (Peter Verhaart)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
Message-ID: <9604201229.AA31516@janus.cqu.edu.au>

To Etienne from Piet Verhaart

Very glad to hear you did a test but puzzled as to which stove you
used. From your description it best fits the "Tubular Stove", built in 1979
or thereabouts. What does not fit is that secondary air is supplied over the
whole length of the fuelbed.
Do you mean the inner cylinder has holes in the cylindrical wall over the
whole height?
The Tubular stove had a number of rows of small holes ( about 5 mm) starting
about 100 mm down from the top and extending to about 150 mm below the top.
Below that, the wall of the pipe (around 90 mm diameter) is whole. At the
bottom of this inner cylinder there is a perforated bottom which admits
primary air.
If this was the stove you tested I can add my experiences (from memory).
This stove was the first that gave me an indication that it is impossible to
control the production rate of volatiles by varying the primary air. We
supposed that the amount of heat needed to pyrolyse wood is so small that
the range of control we have over the heat production rate of the char on
the grate way exceeds these needs.

I am glad you did a test with the Shielded fire, the WSG's piece de
resistance, signifying to our sponsors a welcome end to the need for further
research (being a dirty word) and the beginning for large scale
dissemination activities as well as keeping us in blissful ignorance of the
art and science of biomass combustion. I remember one guy (who later became
a professor in the Faculty of Industrial Engineering) officially stating
that, since the smoke of woodburning cookstoves constituted only a few
percent of the combustion value of wood, it was not worth our trouble
improving the combustion. Any more arse I can kick? Not at the moment, it
seems, back to 1996.

Rice cooking.
>From here only silence. By now I must have cooked at least a ton of great
long grain Australian rice, but always in a pressure cooker, 750 ml of water
with a little salt and a spoonful of vegetable oil. Bring to the boil and
add 500 g of rice, stir until the mix boils, close and simmer under pressure
for 5 minutes. The result is perfect dry rice. The maximum amount our 4.5
liter pressure cooker will take is 750 g rice. There is room for more but
heat transfer apparently suffers, leading to burning of the bottom layer of
rice. Once I attempted to cook rice in an ordinary pan, this took place more
or less in public. The result of both attempts was glue and I still don't
know what I did wrong, very humiliating. Since that time I take a pressure
cooker wherever there is the slightest chance of my having to cook rice.

The shielded fire was our first step up from the open fire, which was the
first cookstove we tested. Paul Bussmann had made a bet with Piet Visser for
a carton of stubbies (I mean a box with 24 bottles of beer) that he couldn't
achieve a heat transfer efficiency, with his shielded fire, of 50 %. After
some modifications Piet Visser had his carton of stubbies!

Etienne, I am still looking through old diskettes to see if there are usuful
numerical test results. The trouble is that most reports of that era are in
Wordstar 3 or 4. WordPerfect can translate these files, but only if they are
immaculate, else the computer locks up, making me for a while the only
active cursor around. These imperfect files can be read by less pernickety
wordprocessors but the result is that the last letter of every word is the
character corresponding to the ASCI value increased by 128. The solution is
in making a little QBasic program that transforms these characters back to
their original value.

That is all for now, I realise there is a lot I still have to reply to and I
will.

Anybody out there got any ideas on how to interest the Central Quensland
University in our project?

Death to smoke

Piet Verhaart

From 73002.1213 at compuserve.com Sun Apr 21 08:29:03 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
Message-ID: <960421123050_73002.1213_FHM39-1@CompuServe.COM>

Etienne, Piet et al:

Enjoyed the latest posting and Piet's discussion of rice cooking, since I had
just finished reading what he said in 1983 (Wood Heat for Cooking), pp 36-40.

1) There seems to be an understandable confusion about the heat for pyrolysis
(not heat OF pyrolysis), ie the heat required to get the biomass to pyrolysing
temperature, the heat (positive or negative) to pyrolyse, and the heat to get
the gases and vapors out to the surface of the particle and away. I am just
finishing a poster paper (with Sid Gaur) for Banff in which I have measured this
heat at moderate rates of heating (such as during cooking). 2800-3200 kJ/g,
depending on particle size. I'll put the abstract here in a day or two. This
is about 15% of the heat of combustion, so is not trivial, and Ron and I and
others find that controlling primary air is a very effective throttle on
volatile fuel production.

2) I understand the problem of bringing up old files - never as easy as one
hopes. Does it help to convert it to an ASCII text file and lose the
formatting? A pox on complex word processors that keep making last years
software obsolete - and unreadable. I suppose one should re-translate all ones
old files every time one makes a change.

I just bought a neat machine - an HP Officejet 350 ($600). It is a
combination printer, fax, copier and scanner, and includes optical character
recognition software that works pretty well. For a home office it is a lot in a
little package.

Piet, I'd like to try some "stubbies'" someday. Did I bring up the subject of
Neville Shute and his books on Australia, Tasmania etc? Do you have any great
gasifiers in Australia that would justify a stop there? Did you know Jack
Humphries who manufactured gasifiers? Lived here ten years, then went back
there? A good friend of Tom Miles who has a sister (?) in New Zealand.

Cheers TOM REED

From prasad at tn7.phys.tue.nl Mon Apr 22 05:06:03 1996
From: prasad at tn7.phys.tue.nl (prasad)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
In-Reply-To: <9604201229.AA31516@janus.cqu.edu.au>
Message-ID: <9604220900.AA14615@tn7.phys.tue.nl>

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From E.Moerman at stud.tue.nl Mon Apr 22 09:39:09 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
Message-ID: <56657.s335192@popserver.tue.nl>

To Tom Reed et al from Etienne:

Tom:
> 1) There seems to be an understandable confusion about the heat for
> pyrolysis (not heat OF pyrolysis), ie the heat required to get the biomass
> to pyrolysing temperature, the heat (positive or negative) to pyrolyse, and
> the heat to get the gases and vapors out to the surface of the particle and
> away. I am just finishing a poster paper (with Sid Gaur) for Banff in
> which I have measured this heat at moderate rates of heating (such as
> during cooking). 2800-3200 kJ/g, depending on particle size. I'll put the

----------
Etienne:

I suppose this should be 2800-3200 kJ/kg.

---------
Tom:

> abstract here in a day or two. This is about 15% of the heat of
> combustion, so is not trivial, and Ron and I and others find that
> controlling primary air is a very effective throttle on volatile fuel
> production.

---------
Etienne:

Perhaps for very low heating rates, but for the heating rates I have in my
stove tests I find it very difficult to control the power output with the
air supply. Virtually every time I try this I get lots of smoke and it takes
at least 10 min. for the power output to adjust a little to the new
situation.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From E.Moerman at stud.tue.nl Mon Apr 22 09:39:01 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
Message-ID: <56649.s335192@popserver.tue.nl>

To Piet Verhaart from Etienne

Piet, I had planned to send you a message privately too about the stove I
used. However I couldn't find any drawings yet. Since others might be
interested too I decided to try and answer your questions on the list.

--------
Piet:

> used. From your description it best fits the "Tubular Stove", built in 1979
> or thereabouts. What does not fit is that secondary air is supplied over the
> whole length of the fuelbed.
> Do you mean the inner cylinder has holes in the cylindrical wall over the
> whole height?

---------
Etienne:

Yes. There is an inner cilinder and an outer cilinder. In between these
there is a flow of secondary air. The inner cilinder has small holes in it
over about the whole height. I am not sure about the size of the holes
though, 5mm sounds a bit large.

----------
Piet:

> The Tubular stove had a number of rows of small holes ( about 5 mm) starting
> about 100 mm down from the top and extending to about 150 mm below the top.
> Below that, the wall of the pipe (around 90 mm diameter) is whole. At the
> bottom of this inner cylinder there is a perforated bottom which admits
> primary air.

----------
Etienne:

This is roughly the description of the stove I used, but as far as I
remember the tubular stove has thermocouples soldered to it and a tube
attached to the side to add fuel. Also I think the area of holes is
different. The small holes started 10 or 20 mm below the top and extended
almost to the bottom (about 300mm?). Perhaps you know which stove I used if
I tell you that the stove is very heavy (thick stainless steel plates?) and
packed in a thick blanket of insulating material. The ports for primary and
secondary air are right above each other, both with a round metal plate
that can be used to regulate the airflow. I am not quite sure anymore, but
I think that the primary air supply flows through a kind of cyclone channel
before it gets to the fuelbed.

----------
Piet:

> supposed that the amount of heat needed to pyrolyse wood is so small that
> the range of control we have over the heat production rate of the char on
> the grate way exceeds these needs.

---------
Etienne:

This is my impression too.

---------
Piet:

> art and science of biomass combustion. I remember one guy (who later became
> a professor in the Faculty of Industrial Engineering) officially stating
> that, since the smoke of woodburning cookstoves constituted only a few
> percent of the combustion value of wood, it was not worth our trouble
> improving the combustion. Any more arse I can kick? Not at the moment, it

--------
Etienne:

Every now and than you come across some quite absurd ideas indeed.

-------
Piet:

> rice. Once I attempted to cook rice in an ordinary pan, this took place more
> or less in public. The result of both attempts was glue and I still don't
> know what I did wrong, very humiliating. Since that time I take a pressure
> cooker wherever there is the slightest chance of my having to cook rice.

--------
Etienne:

After adding the rice too much heat transfer to the pan. Once or twice I
have cooked some rice with sufficient water and a high power output and I
got glue (rijstebreipap) too.

---------
Piet:

> Etienne, I am still looking through old diskettes to see if there are usuful
> numerical test results. The trouble is that most reports of that era are in
> Wordstar 3 or 4. WordPerfect can translate these files, but only if they are
> immaculate, else the computer locks up, making me for a while the only
> active cursor around. These imperfect files can be read by less pernickety
> wordprocessors but the result is that the last letter of every word is the
> character corresponding to the ASCI value increased by 128. The solution is
> in making a little QBasic program that transforms these characters back to
> their original value.

--------
Etienne:

I am very interested in any other information you might have. Can you export
ASCII files with wordstar? If so I can read it easily. I also have
wordperfect 5.1, so if you can read the files with your wp without problems
I expect I can do so too.

------------
Piet:

> Anybody out there got any ideas on how to interest the Central Quensland
> University in our project?

---------
Etienne:

I think it is more a matter of finding the right person and try to interest
him/her. Perhaps a small article in the local university paper will result
in some reactions.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From verhaarp at janus.cqu.edu.au Wed Apr 24 03:49:25 1996
From: verhaarp at janus.cqu.edu.au (Peter Verhaart)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
Message-ID: <9604240752.AA06918@janus.cqu.edu.au>

Dear Prasad

>On The Professor of Industrial Engineering and cooking rice
>
>I do not remember this worthy gentleman. I am sure it could not be Jan
>Claus. Attwood didn't get to be a professor. Remains a mystery.
>
Couldn't it?

And anyway, isn't that exactly the kind of sweeping, no nonsense statement
one would expect from a prospective professor in the Faculty of Industrial
Engineering? An energy conscious one? Keeping in mind that we weren't
talking about OUR environment.

That statement gave rise to our statement on smoke, "Nuissance value >
combustion value".

On the outings of our local Field Naturalists, one of the members boils the
water for our morning tea ('Smoko' in OZ) and he uses an enlarged version of
the water boiler Piet Visser's parents brought from their trip to New
Zealand, a vessel built around the chimney. Like palefaces, gringo's , orang
bule, etc the world over they stuff fuel into the combustion chamber until
it gags and I notice that the non smokers live in much greater fear of the
smoke of my cigar than of the smoke of the wood fire and don't believe me
when I tell them both smokes are, from a health point of view, identical.

I know you cook your rice on a chula.

I love E-mail (when it works)
Yours
Piet

From prasad at tn7.phys.tue.nl Wed Apr 24 03:56:47 1996
From: prasad at tn7.phys.tue.nl (prasad)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
In-Reply-To: <9604240752.AA06918@janus.cqu.edu.au>
Message-ID: <9604240751.AA17493@tn7.phys.tue.nl>

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From verhaarp at janus.cqu.edu.au Thu Apr 25 07:23:42 1996
From: verhaarp at janus.cqu.edu.au (Peter Verhaart)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
Message-ID: <9604251126.AA11795@janus.cqu.edu.au>

>From Piet Verhaart

To Etienne:
Etienne, a while ago you mentioned an idea of yours you called "Staircase
stove". From your description the construction is not clear to me. You
mentioned it was intended as an improvement on the downdraft stove. Tell me
more about it.

In response to my comment on the stove you used in your experiment, it does
sound like the 'Tubular Stove', except for the holes in the cylinder wall.
The holes, less than 5 mm diameter could be for inserting thermocouples in
which case they would probably be in one vertical row. You could block them
with stove cement.

What you describe as the 'Tubular stove' is the "Aldestove" Supposed to work
like Alladin kerosene lamp Tom Reed is so justifiably lyrical about. There
is some description in an old file which I will attach to a personal E-mail.
You could see it as a further (and equally unseccesful) step from the
tubular stove. The idea is to burn the volatiles in a separate location viz
in the region where the secondary air is injected. It does work some of the
time.

As you have noticed, at least I hope they came over, I sent you a bunch of
attachments to your personal E-mail address. Tell me if you can read them.
Soon I will dig into a likely pile of papers in the hope of finding drawings
(I am sure there should be at least a sketch of each stove) and scan them
into WordPerfect documents, see how that works.

To Dave Beedie:
First of all, welcome to our club!
Some comment on the Summary in your message dated 21 March 1996
>at about 200 kW thermal, with integral gas-air heat exchanger. The
>use of a heat exchanger to transfer the energy of the combustion

This suggests to me the combustion was not good enough to enable direct use
of the gaseous combustion products for drying.

Clearly the batch mode of operation caused variable combustion quality.

It would be good if somebody came up with an arrangement or system where a
steady state is created in the combustion chamber, irrespective of the
method of feeding fuel

There is a reference to NOx formation. Is NOx formation ever a problem in
wood burning? I always thought the temperatures wouldn't be high enough.

To Tom Reed

I see, Etienne has beaten me to it, the heat for pyrolysis, but what is a
factor of 1000 among friends?
About 15 % of the heat of combustion of wood.
Say the combustion value of wood is 21 MJ/kg and 25 % of the wood ends up as
charcoal. Charcoal having a combustion value of 28 MJ/kg then contributes
about 1/3 of the total heat of combustion e.g. 7 MJ/(kg wood). So, according
to you, around half the heat of combustion of the charcoal formed is needed
to pyrolyse wood.
If I remember correctly we did some experiments to find out how small a
charcoal fire we could maintain. It would be interesting to see the weight
loss rate of a load of fuel pyrolysing on the heat of the char at the bottom
of the cylindrical stove being fed metered air from a compressor.
Simultaneously the combustibility of the volatiles could be monitored.
Interesting to think about. You could close the top of the pyrolysing vessel
with a perforated plate with holes smaller than the quenching diameter.
Anybody got any figures?

A few hours earlier I read some old files and the computer locked up only
once. I translated them into WP5.1 and sent them to Etienne.

Sounds like a great machine, that HP Officejet 350, almost as universal as
the Lorena Stove. OCR is good to reduce scanned text back to a reasonably
sized file. Have you any idea of the cost of consumables? My HP Desjet 660C
has so far produced reams of printed paper from its first ink cartridge and
the print is very clear

Just you come over here and I'll shout you some stubbies, Tom. Yes, you
mentioned Nevil Shute. I think I read all his books, usually instead of
works on Calculus or Thermodynamics. Great gasifiers in Australia, you mean
on TV, yes, those earnest enlighteners on 60 Minutes, for instance.
Seriously, not that I know of. Do you need an excuse to come down. You are
welcome in Gracemere, I described the accomodation to Ronal in some detail.
Come to think of it, I wonder if he got it. It was Re: Sunrayce, quite some
time ago.

That is all for now, be of good cheer

Piet Verhaart

From SCEDWJB at cardiff.ac.uk Thu Apr 25 11:49:11 1996
From: SCEDWJB at cardiff.ac.uk (David Beedie)
Date: Tue Aug 31 21:34:55 2004
Subject: Piet's observations on my 200kW project
Message-ID: <14C171472C8@nrd1s.cf.ac.uk>

My comments are interspersed with Piet's originals:

> To Dave Beedie:
> First of all, welcome to our club!
Thank you, thank you.

> Some comment on the Summary in your message dated 21 March 1996
> >at about 200 kW thermal, with integral gas-air heat exchanger. The
> >use of a heat exchanger to transfer the energy of the combustion
>
> This suggests to me the combustion was not good enough to enable
direct use > of the gaseous combustion products for drying.

Not ALWAYS good enough. However the aim of my work was to eliminate
or at least minimise the occurrences of incomplete combustion, for
pollution control. I made much progress towards this aim by
implementing an air supply control system, which stabilised the
Excess Air Value of combustion, and created excellent combustion for
most of the time. Thermal input was thereby partially stabilised. I
studied the relations between temperatures, air supplies and thermal
input rates extensively...

> Clearly the batch mode of operation caused variable combustion
quality. >

It is likely with a batch-loaded thing that there will be
incomplete combustion during the worst cases of transient conditions
(e.g. start- up, reload and shutdown). This is both because open
loading doors allow unconstrained air influx, upsetting air:fuel
ratios, and a fresh load of fuel gives off much water vapour,
lowering combustion temperature. Use of a heat exchanger eliminates
any need to divert the combustion product stream in the event of
incomplete combustion.

I wanted to experiment with sealed lockhopper-type reloading systems,
because they could help by both preventing unconstrained air influx
and perhaps spreading out moisture evolution.

> It would be good if somebody came up with an arrangement or system
> where a steady state is created in the combustion chamber,
> irrespective of the method of feeding fuel.

That's what the control system did, except for the aforementioned
transient conditions. If continuous feeding, then it is perfectly
feasible to come up with an arrangement for a steady state in the
combustion chamber. If you want a steady state, I think it is
imperative not to batch-load, or to batch-load in small batches.

> There is a reference to NOx formation. Is NOx formation ever a problem in
> wood burning? I always thought the temperatures wouldn't be high enough.

The adiabatic flame temperature for dry wood with no excess air is in
the vicinity of 2000K, so it is possible to generate high NOx levels,
given a highly insulated low excess air combustor and dry fuel.

The system I developed recorded peak combustor temperatures of
around 1200-1300 celsius, and NOx levels typically between 100 and
200 ppm at ~6% oxygen - varying during the fuel cycle.

> Just you come over here and I'll shout you some stubbies, Tom. Yes, you
> mentioned Nevil Shute. I think I read all his books, usually instead of
> works on Calculus or Thermodynamics. Great gasifiers in Australia, you mean
> on TV, yes, those earnest enlighteners on 60 Minutes, for instance.
> Seriously, not that I know of. Do you need an excuse to come down. You are
> welcome in Gracemere, I described the accomodation to Ronal in some detail.
> Come to think of it, I wonder if he got it. It was Re: Sunrayce, quite some
> time ago.
>
> That is all for now, be of good cheer
>
> Piet Verhaart
>
>

I currently live in Cardiff but I lived in Melbourne for a couple of
years, and regularly envy you Aussies all that sun and open space -
and the brilliant bird calls. What's Sunrayce??

Regards, Dave.

*****************************************************
(Dr) David Beedie
Division of Mechanical Engineering and Energy Studies
University of Wales, Cardiff
FAX: 01222 874317
Tel. 01222 874000 ext.6876
762197 (home)
*****************************************************

From E.Moerman at stud.tue.nl Fri Apr 26 06:17:35 1996
From: E.Moerman at stud.tue.nl (E.Moerman)
Date: Tue Aug 31 21:34:55 2004
Subject: Cooking test: shielded fire.
Message-ID: <44569.s335192@popserver.tue.nl>

>From Etienne to Piet Verhaart et al.

>From Piet:
> Etienne, a while ago you mentioned an idea of yours you called "Staircase
> stove". From your description the construction is not clear to me. You
> mentioned it was intended as an improvement on the downdraft stove. Tell me
> more about it.

--------------
Etienne:

I will send you a sketch attached as a wp file. Anybody else interested?

----------------
Piet:

> In response to my comment on the stove you used in your experiment, it does
> sound like the 'Tubular Stove', except for the holes in the cylinder wall.

-----------
Etienne:

You might be right.

-------------
Piet:

> The holes, less than 5 mm diameter could be for inserting thermocouples in
> which case they would probably be in one vertical row. You could block them
> with stove cement.

------------
Etienne:

No way. They were cleary meant for the air supply, I just don't know the
exact sizes. I had the impression they had a diameter of 2-3mm.

-----------
Piet:

> What you describe as the 'Tubular stove' is the "Aldestove" Supposed to work
> like Alladin kerosene lamp Tom Reed is so justifiably lyrical about. There
> is some description in an old file which I will attach to a personal E-mail.

----------
Etienne:

You might be right about the Aldestove. I will try and find a few sketches.
Thanks for the files. They are ok, I just have not had time to read them
yet.

---------
Piet:

> You could see it as a further (and equally unseccesful) step from the
> tubular stove. The idea is to burn the volatiles in a separate location viz
> in the region where the secondary air is injected. It does work some of the
> time.

------------
Etienne:

I hope I can find the time to test this one too.

------------
Piet:

> As you have noticed, at least I hope they came over, I sent you a bunch of
> attachments to your personal E-mail address. Tell me if you can read them.
> Soon I will dig into a likely pile of papers in the hope of finding drawings
> (I am sure there should be at least a sketch of each stove) and scan them
> into WordPerfect documents, see how that works.
>
-------------
Etienne:

I received 3 attachements. No problems reading them with wp.

Thank again.

Etienne
---------------------------------------------
Etienne Moerman E.Moerman@stud.tue.nl
Joh. Buyslaan 71 tel. +31-40-2571491
5652 NJ EINDHOVEN The Netherlands

From 73002.1213 at compuserve.com Fri Apr 26 08:25:53 1996
From: 73002.1213 at compuserve.com (Thomas Reed)
Date: Tue Aug 31 21:34:55 2004
Subject: Piet's comments
Message-ID: <960426122629_73002.1213_FHM21-4@CompuServe.COM>

Piet:

Bubble jet (ink jet) technology is so close in quality to laser printer
technology (but slower - 3 pp/min) that I'm surprised at how many lasers are
still being sold. You mentioned cost of consumables on ink jets. Are you aware
that you can re-ink the cartridges (probably once, twice at most) and cut costs
further?

I just discovered how to get rid of the routing garbage to print up the odd
letter to show my wife at breakfast. If I pretend I want to reply I get your
note in editable form, then edit out the routing and print the rest. Then I can
delete the whole, since the original is filed. The only penalty is those little
> signs at the beginning of each line.

In a well ordered day I read the paper and put out the dog from 5-5:30AM, then
read and answer E-mail (typically 20 messages) from 5:30 - 6:00. Your tripple
note came up first.

> (Your note to Etienne): As you have noticed, at least I hope they came over,
I sent you a bunch of
> attachments to your personal E-mail address. Tell me if you can read them.
> Soon I will dig into a likely pile of papers in the hope of finding drawings
> (I am sure there should be at least a sketch of each stove) and scan them
> into WordPerfect documents, see how that works.

Looking forward to seeing sketches.
> To Dave Beedie:
> Clearly the batch mode of operation caused variable combustion quality.
>
> It would be good if somebody came up with an arrangement or system where a
> steady state is created in the combustion chamber, irrespective of the
> method of feeding fuel

INDEED: Several times I have thought that I had the perfect stove, only to find
that it was sometimes perfect, more often not. Last night I had a third run on
perfect stove No. 123. The reaction zone got tipped, and reached the grate on
one side at 25 min while there was still half the charge unburnt on the opposite
side. Never did THIS before! Cause for thought, and I think I can fix.

> There is a reference to NOx formation. Is NOx formation ever a problem in
> wood burning? I always thought the temperatures wouldn't be high enough.

I KNOW you don't get NOX from conventional pyrolysis; IF you could make a
great, blue, stoichiometric flame, maybe you could get NOX during combustion.
We should be so lucky.
>
> To Tom Reed
>
> About 15 % of the heat of combustion of wood.
> Say the combustion value of wood is 21 MJ/kg and 25 % of the wood ends up as
> charcoal. Charcoal having a combustion value of 28 MJ/kg then contributes
> about 1/3 of the total heat of combustion e.g. 7 MJ/(kg wood). So, according
> to you, around half the heat of combustion of the charcoal formed is needed
> to pyrolyse wood.
>
Note that my figures were for relatively rapid heating of birch dowells so that
the outer surface reached 700C before the center even began to heat. In slower
pyrolysis it is not necessary to drive heat in so fast, so max surface
temperature could be 450 or 500 C. Then the heat FOR pyrolysis would probably
drop to 10% of the heat for combustion. With "autopyrolysis", perfect
insulation, infinite time available, bone dry wood, it is only necessary to get
up to about 280C before the exothermic heat of combustion of the charcoal
carries the reaction to 450C. MAYBE not heat required at all for pyrolysis.
Also, a thermodynamic calculation of Desrosiers shows that wood releases heat
going to charcoal plus equilibrium gases. (Flawed by the fact that pyrolysis
does not give equilibrium gases.) So last year I did an experiment to see if I
could achieve "autopyrolysis.

I used oven dry wood chips, sealed them in a well insulated cylindrical
container, ignited them thoroughly, then put on a cap with an insulated vent.
At first I thought the reaction had stopped. No. It stayed warm for 12 hours
and gave a good yield of charcoal. So I believe that "autopyrolysis" is
possible without any energy input. However, add a few % water and you can
forget it.

> If I remember correctly we did some experiments to find out how small a
> charcoal fire we could maintain. It would be interesting to see the weight
> loss rate of a load of fuel pyrolysing on the heat of the char at the bottom
> of the cylindrical stove being fed metered air from a compressor.
> Simultaneously the combustibility of the volatiles could be monitored.
> Interesting to think about. You could close the top of the pyrolysing vessel
> with a perforated plate with holes smaller than the quenching diameter.
> Anybody got any figures?

I think this would work - it is the principle of the updraft gasifier. Burn
char on the bottom and let it roast the fuel above. If I understand your
proposition, by metering in the air to the charcoal, you can artificially
provide heat to release pyrolysis gases at any rate you please.

However, I doubt is this is useful in practice with a less than bone dry fuel.
I tried an updraft operation of our stove the other day. For the first few
minutes I got mostly steam, incombustible. Then when the wood was dry, I got
great quantities of pyrolysis gas, more than I could supply air to for about 5
minutes. Then the charcoal burned in conventional manner for a long time.

INK JETS:

Australian Trip? You mentioned gasifiers on 60 minutes. I missed it, please
explain. A visit would be a long shot in the short term today, but long shots
often hit the mark. There is a round the world ticket on United that permits
lots of stops for about $3,000. I'm considering it.

Keep the faith,
TOM REED

BioEnergy Lists: Improved Biomass Cooking Stoves

April 1996 Biomass Cooking Stoves Archive