Re: Refractories and Insulation

["Ron Larson" <ronallarson@qwest.net>]

Harmon and stovers:

    First, apologies for not responding to several earlier letters.  I am
getting closer to getting out from other (have to deliver a proposal on
Friday - but this looks timely and relatively easy to answer.

See more below.

----- Original Message -----
From: Harmon Seaver <hseaver@cybershamanix.com>
To: stoves <stoves@crest.org>
Sent: Wednesday, October 31, 2001 12:30 AM
Subject: Re: Refractories and Insulation


>    Reading more in the archives, I've found a bunch of posts about using
> insulation like duroboard and cerablanket in gasifiers around the fire
> chamber itself to increase the temps.

    (RWL):  Certainly, the result is to increase the temperatures - but I
think it better to say that one is trying to reduce radial heat loss in the
combustion chamber region.

>     And then looking more at Tom and Ron's stove design, I'm seeing what
> looks like an insulation layer higher up, after the secondary air inlets
> -- I'm thinking this doesn't serve the same purpose.

    (RWL):   I wouldn't look too deeply at any single design - either
nothing or little is on the market.  Most eveything is limited to materials
on hand.  In my work I have mostly concentrated on getting the dimensions
right and getting something to work - and to keep the price down (working
only with cans that are on the way to the dump - as with Richard Boyt's
ten-can design).  As I am sure you know, multiple layers of metal can
provide good insulative value.  I checked out the price of a ceramic "glass
filament" flexible blanket last week - and (again) decided against buying it
despite its nice properties.  The cost was $4.00 per square foot (about $4
per square meter).  This cost could come down considerably with bulk
purchases - but I think is not yet what we can recommend in most developing
countries.   I think a preferred approach is ceramics - which seem to be
made locally almost everywhere - with emphasis on lightening them (as Dean
Still has been doing with bricks).   A thin layer of metal on each side
would make this a formidable competitor.

    The secondary air inlet design needs a lot of work also.  Preheating
could be from the top or bottom - but we need better ways to either make it
demountable or not (depending on the use of a separate removale fuel can,
etc)

>      Also -- am I correct in thinking that if there was no secondary
> air, there would be no ignition of the producer gas.

    (RWL):  The simple answer is yes.  Only a very small portion of the
total air needs to be used in the first or primary pyrolysis stage (some of
the required oxygen is obtained from the fuel itself).  But most of us have
not been controlling the secondary air, so I can conceive of designs where
one controls total air - so that it becomes harder to distinguish one from
the other.

>I'm wondering how
> far away from the actual flame front can the secondary air be introduced
> and still have auto ignition of the gases?
>
    (RWL)  This is what I really wanted to explore today - a very good
question that I do not recall being discussed at all on this list.  I do not
have the answer, and hope that others can jump in who may have studied this
phenomenon..  It also gets at the issue of maintaining a flame and the whole
issue of mixing before lighting - or settling for a diffusion flame (and the
consequences on price of the stove).

    My first answer is that in the simplest designs where the secondary air
is introduced through a narrow slit (maybe 1 to 3 mm - doesn't seem
critical) or a series of "nail holes" (of similar diameter), the flame seems
to attach right at the edge of the metal.  I have had problems with holes of
say 1 cm diameter - and believe that smaller is better, but can't prove that
quantitatively.
    My experience is that one is going to have trouble with the whole
process if ignition is higher and not maintained close to the secondary air
source.  I used to think that it was advantageous to have flaming sticks
above the level of the secondary air inlets to serve as a "match" to keep
the flame lit.   It is very necessary (and not necessarily shown in the
simple schematics) to have a wind screen near the secondary air inlets - a
top lit, charcoal making, IDD design is pretty easy to blow out - and
especially when the primary air supply is cut back to a minimum.  What works
in a laboratory might not work outdoors.  But fixing this problem is
relatively easy and cheap and will help in the preheating process.

    Now if one wishes, one can concentrate on premixing the air - and this
is the approach of Tom Reed and some others.  An example in addition to the
ones Tom Reed recently mentioned is the Bunsen Burner.  This works because
the (laboratory) gas is under pressure so that a Venturi effect can draw in
the correct (controllable) amount of total air (no need to separate into
primary and secondary when the gas is totally combustable).   Then mixing
occurs over a height of many diameters (6-10?) in what I remember, and the
flame is a lovely blue color that we all want.  I hope someone can tell of
experiments or theory that says what that minimum height has to be for the
laboratory-type Bunsen burner - that will help get at the question you are
asking.  I think Alex English may have studied this some.  A lot must have
to do also with the turbulence that one can achieve when the air and gas
first start mixing and the angle at which the two flows meet and progress up
to the point wher the flame begins.

    In the Bunsen burner, that point is where one has a "screen".  There is
something about flame propagation speed that dictates how small the screen
pores must be to not have the flame propagate down the "Bunsen burner"
mixing tube.  I hope someone on the list can point us to that literature -
it could help answer something about mixing. (The reason for thinking all
this through well is that we want to keep the excess air down so as to keep
the temperatures and therefore the efficiency high.)

    So this brings us back to the issue of preferring diffusion vs pre-mixed
flames - which I believe is part of your question.   If the flame is not
near the secondary air inlets, then you have a chance for pre-mixing and a
better flame.  I don't know how to specify that height to achieve the
pre-mixing - but the height will dictate the economics and complexity of the
final cooker - which we are trying to keep in the $5-$20 range.

    I found experimentally that I had a workable stove with the diffusion
flame approach when the combustion chamber height was about 1.5 times the
diameter.  I have also seen (but haven't any  hope of now finding)
statements that open flames (such as oil on water) have about this same
relationship.  This argues I believe for smaller throats (but I have seen no
reports on experimental work along this line) or inserts in the combustion
region.  Fortunately tin cans have about this ratio.  If it is taller, you
have less chance of blackening the pot, but more cost and more heat
radiating surface.  If it is smaller, there will be more blackening and soot
production.

    The flame will be much shorter when using premixed air, but one still
has the height problem to accomplish the premixing.  This is partially
compensated by having this "mixing height" operate at a (much) lower
temperature, so losses are less.  But one has to pay for the blower and
energy source to achieve the pre-mixing (Venturi or Koanda effect or
wahtever). So far no one has offered a low cost method of doing the
premixing without an external power source.   Even if one had such an
approach, one probably would have some operating difficulties (getting lit
one way and then switching over?)  This is a great challenge for everyone on
the list!!   (Which remind me that I started some conversations with Andrew
Heggie about alternatives to electricity for supplying higher pressure air.
Andrew reminded me that the "brake" for spring wound toys is a "blower".)

    We also should be talking at the same time of ways to maintain a high
temperature internally to assist in auto ignition - as you have implied in
your question.  Because catalysis is important in flame ignition
temperature, we should be asking something about materials as well - maybe
even a sacrificial material (if low cost).  There has been zero discussion
on this list about this topic.  (note that the early movie projectors used
lime ("limelight") where the gas flame hit in order to get an intense white
light.

    I don't think I have helped answer your question (on how far) - but you
did get a partial data dump on what I know and what I thnk is involved in
getting the right decision for different uses (with emphasis on this list
for low cost) .  Perhaps other on the list can chime in on this important
topic you have raised.

> --
> Harmon Seaver, MLIS
> CyberShamanix
> Work 920-203-9633
> Home 920-233-5820
> hseaver@cybershamanix.com
> http://www.cybershamanix.com/resume.html



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