Re: Gasifier fundamental question

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

Dear Paul and All

First:
I wrote to Paul:  "I am still waiting to hear ANY reasonable explanation for
not smoking off the wood in any pattern or position."

This followed the discussions and experiments we had together a few weeks
ago in Swaziland.  Paul brought a few small gasifying stoves with him and we
lit a couple on my (cement) veranda.

We tried one with chopped wood and it flamed very nicely. Clean, hardly any
visible emissions after the flame was established (don't think it doesn't
smoke at all - it smokes at the beginning and a lot at the end).

When it burned down to charcoal, at which time it smoked profusely, I added
more chopped wood on top.  It went out.  Next time I added it just before
the pyrolysis stopped and it re-established itself as a gasifying stove.
This gassing and flaming continued as the new fuel (added on top of the hot
charcoal which remained at the bottom) pyrolysed to produce gas in the
regular way, though it no longer had a top-down flame front.  It was hot
here and there.

It was top-lit to begin with and bottom lit for refueling and middle lit
during the second burn.

I have been reading for 6 months on this list that this cannot be done and
doesn't work and gives off uncontrollable amounts of gas.  Paul told me just
before we did it that it wouldn't work, but Paul also taught me to try and
see just for the heck of it, so I did.

Refuelling works very nicely with regular wood.  Perhaps it won't work with
pelletized wood.

Second:
I feel there are some apples mixed in with the oranges.  Ron wrote,
"Charcoal is indeed made in the way Crispin describes. Unfortunately, it is
one of the most abominable practices on the face of the earth..."

Well...yes it is when done to produce charcoal from wood in a forest, but we
are not doing that.  The significant difference is that in the stove we are
not burning wet wood.  This is an important consideration.  A number of the
complaints about the process and the chemical analyses discussed for
emissions and flamability of the gasses are made with reference to wet wood
in a forest, not dry wood in a stove.  In other words some of the arguments
presented against bottom lighting a gassifying stove are based on an
analysis of wet wood charcoal production.

Tom recommends that the fuel be "...0-25% moisture content...".  That is not
the condition in a forest-based charcoal production so to be useful we
should limit the discussion of charcoal making stoves to forseeable
conditions in a stove.

On Apr 03 Tom wrote, "...gasifiers burn at a VERY constant rate until the
volatiles are gone...".

This is a very important point and one that can be considered essential in
certain cooking applications.  However constancy is not a good thing if you
want to vary the output of heat because once it is going I have found
gassifying stoves to be quite difficult to control vis-a-vis the heat
output.  Getting high heat is difficult.  It is very important to keep in
mind that there is zero hope of us in the field getting the fuel Tom is
using to achieve this constant heat output.  The experiment, if I may call
it that, is burning unobtanium.

The heat requirement in our area is for a lot of heat at the beginning
followed by a significant turndown and a long simmer.  This can be achieved
by lighting up a relatively large amount of gasses followed by a relatively
small amount after 20-30 minutes.  This is approximately what happens if you
bottom light a charcoaling stove, according to Tom recent message with the
Camp Stove data attached.  Tom's statement that the gas cannot be burned
effectively because of a lack of air being available immediately after
adding fuel to an operating stove is conditioned upon a certain stove
design, not something inherent in the making of gasses and charcoal.
Perhaps this observation could be rephrased as, "If you add fuel to an
operating IDD stove, there will initially be a large amount of gas
produced."  The ability of the stove to burn it effectively is a different
matter and does not limit the type of stove you might construct.

I have not observed the gassing phenomenon Tom describes using chopped wood.
One reason for that may be the smaller surface-to-volume ratio of the wood
compared with the pellets.

Third:
It is important to me to know if Ron's statement, "The problem is that the
valuable combustible gases that are released are so diluted by carbon
dioxide and water content that it is not combustible when emitted from
traditional (or even improved) charcoal kilns." holds true for stove-based
gas production.  Am I right in observing that this is only a problem when
working with wet wood in a forest?  By forest I am thinking of a
batch-process stacked log charcoaling operation.

I have a chemistry question.  If I were to charcoal a small amount of dry
wood in a stove, and produced the aforementioned high CO2 gasses, is that
not OK in that all I want is the heat, after all I am trying to cook here.
I am not particularly after CO.

I cannot yet grasp the difference between having, on the one hand, the
gasses mostly burned lower in the stove to CO2 and then burning off the
remaining CO with preheated secondary air and on the other hand, carefully
tinkering with the pyrolyzing zone to produce more CO and then burning it to
CO2 slightly higher in the stove.  The end result is (hopefully) CO2 in both
cases.  The amount of heat released is the same.  Why all this concern for
intermediate gas quality?

Fourth:
Ron mentioned "...the period when Elsen Karstad was first trying to convert
sawdust.  He was only able to flare with downdraft and the equivalent of
bottom lighting."  This is interesting but might have been for a number of
reasons not applicable to our stoves.  Was he using preheated secondary air?
If not, I am not surprised he had problems.  Was he using preheated primary
air?  That also might have helped a great deal.

Ron very reasonable asks, "With persons all over the world selling charcoal
as a fuel to be preferred over wood (mainly because it is cleaner burning) -
I fail to understand the logic on why charcoal production is a problem."

I don't see it as a problem per se, but I see it more as a non-issue, in
other words, not something worth chasing as a 'needed element' of a wood
stove.  It does seem to be chucked in as an end in itself or an essential
feature.  I can't imagine a better use for the charcoal than to leave it
where it is in the stove and burn it before it needs to be reheated to get
going.  If the gassifying process is as clean as is claimed, and the burning
of charcoal is as clean as is claimed, why not just burn both the gasses and
the charcoal at the same time?

I fully agree with Ron that, "There is nothing inherently requiring any
precision (ie high cost) that I am aware of."  Agreed.  The 'stoves' Paul
has been making cost a few cents.

Fifth:
I am very interested in this: "In the top-lit, updraft charcoal-making
stove, the upper layer of charcoal converts any uprising CO2 back to the
desired combustible CO." I understand from previous submissions that this
process is highly endothermic and also that it reduced charcoal production.
True?  Or perhaps is the charcoal carbon being liberated.  To be very clear,
is the uprising CO2 knocking C off the charcoal to make CO, or is the CO2
being 'converted' back to CO leaving the charcoal unmodified?

Sixth:
Ron wrote, "In an updraft gasifier, the flow of material is also downwards,
and the pyrolysis front is stationary, but the resulting pyrolysis gases
(drawn off of course at the top) now have considerable CO2 content along
with the still valuable pyrolysis gases driven out of the wood
by the rising hot combustion (CO2) gases."

I understand this, but as with the forest charcoal production, this is not
directly applicable.  We are not actually trying to produce wood-gas, we are
trying to cook or produce heat.  Whether the heat is produced lower in the
stove or higher is a 'clean blue flame' is not important as the net effect
(if final gasses and heat) is the same.

The arguments proffered for the various combinations of stove layouts have
not satisified me on this point.  Yes, one can tinker with the relative gas
ratios, and yes there are analogies in other gas and fire processes, but
they are not 100% related to the task at hand.

How can I say that?  Well, one of the widely discussed limitations on stove
design, based on these theoretical and analagous arguments, is that you
cannot make a really clean fire using wood with bottom firing.  Yet David
Hancock did it years ago with a production stove.

Another conclusion is that top-lit updraft gassifying stoves cannot be
refuelled.  Now this is a very important claim because now we have Paul
building stoves that have slide-out containers so he can try to get the
charcoal out and reload the stove to continue cooking his porridge.  Yet the
first time I tried refuelling with Paul's cheesy 10 cent top-lighting
updraft stove it worked very well.

Gentlemen and women, this is serious stuff.  Why are workable designs being
ruled out?  What am I missing here?  It seems to me that features and
effects particular to individual stove layouts and fuels are being projected
onto stoves in general.

Seventh
I want to highlight the following by Ron in his 7th comment
>(RWL7):  I hate to be picky, but there is no "burning that
>comes downward from the top".  The pyrolysis front has
>chemical reactions that I feel should not be called "burning".

In our discussions in Swaziland, Paul and I had a problem trying to describe
this because if we call the pyrolyzing zone 'primary combustion' (there is
in fact some combustion) then we have above it a 'secondary combustion' zone
where the gasses are reacted with oxygen.  Those unburned or partially
reacted gasses like CO which remain, can be burned in a zone above that and
we settled on 'tertiary combustion' for obvious reasons.

In an open wood fire these three combustions are in fact going on.  The
gasses sprout from the wood (all wood fires are gas-producing), they are
burned and above that (in a Basintuthu stove for example) the combustion
products are reacted with hot 'secondary' air.  There seems to be at least
some confusion about the terms 'primary combustion' and 'secondary
combustion' which are worthy of resolution.  Air is provided to make the
charcoal. That is primary air.  Air provided to burn the gasses higher up
will properly be called secondary air.  Pre-heated air provided to react the
still remaining fuel gasses should then be called 'tertiary air'.  While
unconventional, I don't see any way around this.  The fact that people seem
not to be providing for tertiary combustion does not mean that it is not
required or would not clean up the emissions even further.

Eighth:
I fully believe Ron's statement, "The downdraft design gives a higher
quality gas."  The thing is, we are not after producing gas, we are
producing heat for cooking and the quality of the gas is not important
unless it will not react with hot secondary air.

I do not agree with the statement, "...bottom lighting still entails
pyrolysis reactions and eliminates the batch-loading restriction, but allows
output control only through fuel
loading and complete combustion is essentially impossible."(RWL8)

Bottom lit stoves can be controlled by restricting the primary air which in
the extreme position, turns it into a gassifying stove.  I don't think there
is any essential or net difference between burning wood to make a gas to be
burned nearby and burning wood to make gasses to be burned in the immediate
vincinity.  If it all goes to CO2 and water, heat is heat.

"...the secondary air supply for top-lit stoves has not seemed to need a
separate control..."

That is something Paul and I are investigating.

Many thanks for all the energy contributors spend adding fuel to this fire!

Regards
Crispin


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