[Stoves] Energy loss into stove walls

[Dean Still]

Dear Paul,

When I started experimenting in a more controlled way with stoves in about
1993 we began again looking at the options for combustion chambers. At that
time and still today a lot of combustion chambers were made from mud and
sand. So the stove could weigh a lot! 

One of the wild things we did was to boil and simmer water and then throw
the whole stove (or a combustion chamber) in a barrel of water. Stir the
water, until it was all the same temperature, and see how much energy had
been absorbed. Heavier stoves absorbed more but this doesn't account for
losses during the cooking event.

We did tests that showed that a lighter weight, well insulated combustion
chamber reduced fuel used and emissions of CO and PM. Today the options for
high temperature combustion chambers that we use are:

310 or 309 stainless steel
High nickel cast iron
Refractory ceramic of varying densities

Not a long list, unfortunately! Normal metals do not last at high
temperatures, normal ceramic cracks, etc.

For ceramic combustion chambers we use extruded cylinders, cast flat pieces
that fit together, found flat tiles (baldosa), etc. These are usually around
1.2 or more grams per cc. They sink in water. Then we fill around these
materials with loose insulation. Because they are denser, they can resist
abrasion well. There are stoves in Central America that use this approach
that have been in use for more than 4 years.

Or, we also cast lighter ceramic into shapes. These materials range from .4
to .9 grams per cc. This floats in water. We add materials like sawdust that
burn out or add light weight materials like vermiculite to clay.

There are coatings that help protect the lighter, less durable ceramic
materials. Or, as you saw at Stove Camp, some of the mixes are durable
enough to last for a while without a coating. The Uganda Two Pot stove that
Stove Campers used to make lentils has a clay/sawdust combustion chamber
that has been in use for more than 5 years. There is a denser tile at the
back of this combustion chamber.

Damon's Institutional stove uses 310 surrounded by vermiculite. It is
amazing how this light weight combustion chamber is easy to light, gets hot
very fast, which cuts CO, etc.

Generally, in my experience, the heavier combustion chambers add a bit to
fuel used and have considerably higher CO but not necessarily PM. They are
harder to start. But, all three options can work well.

The three types of combustion chambers, when used in a stove with optimized
heat transfer, result in a heating system that meets our benchmarks for fuel
used, CO and PM made when bringing 5 liters of water to a boil and simmering
it for 45 minutes. Changing the combustion chamber material does not have to
have a big effect on stove performance. Optimizing heat transfer is so
powerful that it tends to mask the variable of material used.

Best,

Dean







-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Paul S. Anderson
Sent: Saturday, August 04, 2007 10:39 PM
To: crispin at newdawn.sz; Discussion of biomass cooking stoves; Crispin
Pemberton-Pigott
Cc: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] Energy loss into stove walls

Dear Crispin and Penn and all,

Thank you to both of you for your assistance.  Crispin has stimulated 
discussion
about ceramic vs metal stoves.  I hope that Tom Reed will state his
pro-metal
arguements.

About Crispin's message and my specific stove question, I was aware of
several
of your key points, but it was instructive to read and think about them
again,
plus the additional concepts.  I was assuming that the heat lost through the
sides of the ceramic gasifier was about the same as from a metal one.  Big
assumptions, but I am just after some preliminary findings.  Also, I did not
mention before:  The 6 insulative bricks were held together inside a very
thin-walled metal can (the kind in which you get 3-flavors of popcorn in the
USA at Christmas time.)

Crispin wrote:
> I welcome an open investigation and debate of this question.  That is what
> science is about.  We don't have to come to a firm conclusion, or even
> agree, but we can't over-simply the stove and then invest large amounts of
> money in multiplying the products based on an inaccurate theoretical
> representation of a partially understood truth.
>
>> How much heat energy (and wood equivalent) is needed to raise 5 kg of
>> material 700 deg C?
>
> Well, it is about 200 gm of wood (assuming no losses), but you forgot to
> divide by two at one point.

Thanks for the approx number.  I did not forget to divide by two.  The
inside
temperature is well over 1000 deg C and the outside temp was much more than
30
C, so that is why I used the 700 deg C number.

I note and agree with your use of 0.84 because of what Wikipedia says is the
specific heat capacity of many construction materials (cement, soil, sand)
as
being in the 0.8 to 0.9 with the units expressed as  J * 1/g * 1/K    or
J
g-1 K-1  or as Penn wrote:  J/(kg*K) .   That gives the heat at about 3000
J,
which is 200 gms of wood because we get about 15 MJ per kg of wood.

Nordica has not yet sent me the data sheet for my test of this stove, but
from
the Report on Stove Camp recently posted by Tom Miles, the stove used 
231 gm of
wood per liter, time 5 liters = 1155 gms.  That was above the 850 gm
benchmark
for 5 liters.  Subtracting 200 g would put it at 955 gms.  And there were
several other factors.

A.  I used about 50% of the fuel as Illinois woodchips that have a higher
moisture content (MC) estimated to be about 15 to 20 % compared to the kiln
dried wood (MC = 5 %) that Apro uses in the official testing.  An increase
of
10% of MC for half of my fuel means 75 gm of water (meaning less fuel value)
and that the additional water must be raised from 30 C to 100 C and boiled
off
from the fuel (consuming energy that could not heat the pot).

B.  Plus issues of the pot being too high from the fire and

C.  having a pot-skirt that was with a large gap, promoting a rather 
rapid fire
during the simmer stage.

Remember that this was done at Stove Camp and the stove configuration was
only
finalize while at camp.  I had only tested other aspects (primary air inlet,
etc.) of the ceramic AVUD gasifier (or continuous feed TLUD) with 2
lightings
prior to arrival at Stove Camp.

I only had one chance to run a test of this stove under the emissions 
hood, so I
was not able to get all the variables fully controlled nor maximized with
repeated testing.  I was glad that I got the one test run of this stove and
that the CO and PM were wonderfully low!!! ( CO  =  1.2 g per liter,   and
PM = 63 g per liter.)   But fuel usage was high, and I am trying to
understand
it and correct the situation.  I hope to drop the CO and PM even lower when
I
am using only half the amount of fuel.

In conclusion, the updraft gasifier (continuous feed TLUD that becomes an
updraft AVUD gasifier for continuous feeding) is extremely hot, and I
believe
that ceramic protection is important.  However, Crispin has help me realize
that a relatively thin but dense ceramic liner inside a sturdy steel
cylinder
might be just fine, especially if it can be easily patched if cracks cause
parts to fall off.  That could be more appropriate than getting heavily
involved with the rather fragile insulative ceramics and the need to fire
them
beforehand.

[NOTE:  If you are going to shift to a discussion of ceramics (and not 
continue
about the energy loss into stove wall and/or my stove test), please change
the
Subject of your reply message.  Personally, I am annoyed when we have weeks
of
discussion with one Subject line that has long been forgotten.  
Maintaining the
"threads" can be important, but so is having a Subject that reflects 
the content
of the message/thread.]

I love this Stoves Listserv!!!!!!!!!  Thanks for being so helpful.

Paul

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