Re: Gasifier fundamental question

[AJH <andrew.heggie@dtn.ntl.com>]

On Sat, 13 Apr 2002 08:21:28 +0200, "Crispin" <crispin@newdawn.sz>
wrote:

>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."

Well for my first I will say I have now met or talked to 5 posters to
the list and one lurker, all but one have been both helpful and
honourable, so I would say contributions are likely to be designed to
be helpful. Ambiguities and wrong interpretation can arise. I think
Ronal, though correct about venting of pyrolysis products, got the
wrong end of the stick.

Second I agree with Peter Verhaart.

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

Which is fine, top lighting conveys an advantage in itself, that is
making the pyrolysis products all pass through the flame, this gives
both an opportunity for them to reach a high temperature and an
ignition source.
>
>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.

I must have missed this, we have discussed updraught burning and
adding fresh fuel to top lit stoves in the past, the problems arise
when too much is added causing the local fuel:air ratio to take an
excursion beyond that which will hold a flame, this extinguishes the
flame even though the stove continues to generate pyrolysis gas.
>
>Refuelling works very nicely with regular wood.  Perhaps it won't work with
>pelletized wood.

It works fine with pelletised wood in a top stoked pellet stove as
well as in an idd stove, subject to amounts.
>
>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..."

Ronal's misunderstanding of your statement I believe.
>
>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.  

It is possible though, with top lighting and stoking if dry kindling
is used as a starter. A number of the list can vouch for this,
including the lurker who has scaled up and implemented a lightly
forced
draught device based on my design, CO down to 150ppm at >50kW(t)
burning freshly cut hazel chips.

>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.

I do not believe you are correct to draw this inference.
>
>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.

Let's leave charcoal making out of this for the moment, I did not
think you meant this in your apparently private e-mail which was
quoted on list, poor netiquette that.
>
>On Apr 03 Tom wrote, "...gasifiers burn at a VERY constant rate until the
>volatiles are gone...".
>
>This is a very important point 

Indeed, to my mind it is a very important point, it allows the air
supplies to be treated in a simple manner, akin to controlling fuel
and air together, I think however it is but one special case of top
down burning. 

>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. 

I can see this, there is little positive feedback as in a bottom lit
design, high heat is available with forced convection though.

> 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.

Very good point, pellet fuel is a developed world waste disposal
solution at present.
>
>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 can live with that idea, there are a number of ways of adding fuel
to a top lit device without causing snuffing problems even without
resorting to under stoking.
>
>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.

Pass on that, I may not have noticed the post.
>
>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.

It certainly is a fact that if the cv of the offgas is diluted it will
reach a point where it will not support combustion, this dilution
could be by other combustion products, water or excess air, this is
why a fire will just smoulder, the air:carbon reaction will slowly
continue without strictures of air:fuel ratios, the offgas will not
ignite in either the absence of a flame, wrong air:gas ratio or low
temperature (from low cv of the total mass flow).
>
>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.

Yes in retaining the carbon unburnt you are not releasing all the fuel
value of the wood, but did you mean high CO2 or high 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?

I once saw a list of benefits of the gasification on one of the CRESt
list, I cannot remember it, but controllability is one of them, In any
case I think you are surmising the enthalpy of the gases in the two
systems will be the same, in practice this will not be the case
because of losses low down in the stove, CO generation will take place
at 1100C, CO2 generation ~2000C, with both columns of gas rising which
one has the highest propensity to lose heat?

In any case Alex English posted figures to suggest that the idd
pyrolysis front is probably not a good CO generator, the major
advantage of the idd offgas is that primary air is kept very low, heat
losses at the front are low and the offgas has not been diluted by
much nitrogen. The offgas is of similar cv to that from a retort
system and hence burns cleanly when secondary air is added.
>
>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.

I do not see any references to preheating air, ELK will answer this.
My understanding is that it is primarily a pragmatic solution to a
waste problem in the presence of a market for charcoal briquettes, ELK
has not suggested it is efficient use of fuel.
>
>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."

The point being that wood can be cleanly burnt, in the face of this
people ought to be able to cook with wood. If cultural needs dictate
that cooking is done with charcoal then the charcoal should and can be
made in an environmentally sound manner, hopefully using the waste
energy in the offgas at the same time.
>
>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. 

I agree, I do not see charcoal production from a cooking stove to be
an advantage per se.

> 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?

Yes, however even commercial sized gasification units seem to discard
char with the ash, it's inevitable really as chemical reactions tend
to equilibriums, if you supply just the 1kg of air needed to gasify
1kg of dry biomass then some will react all the way to produce H2O and
CO2, hence some char must be left. In a simple stove we can add excess
air above the 6kg needed for stoichiomentric conditions, this allows
unreacted O2 to leave the system rather than unreacted C.
>
>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." 

Who said that? Tom Reed has previously said that to get to an
equilibriom where most of the carbon has reacted to CO requires a
temperature of >700C maintained through a bed of coals at least 20
particle diameters deep, I do not believe this equilibrium is
approached in a natural draught idd stove.

>I understand from previous submissions that this
>process is highly endothermic and also that it reduced charcoal production.

Intuitlivel if you increase production of CO or CO2 you must reduce
yield of char.

>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?

My understanding which I have posted a number of times inviting
responses is:

1) At fairly low temperature ~200C an oxygen molecule dissaciates by
surface adsorption onto the carbon. O2 disocaition very mildly
endothermic.

2) CO is generated, strongly exothermic.

3) This CO immediately reacts with incoming oxygen to CO2, stongly
exothermic

4) in passing away from the reaction site if this CO2 has both high
enough temperature and residence time in contact with carbon it
reduces to CO, strongly endothermic.

3 and 4 cancel out, 2 exceeds 1 so overal CO production is exothermic.
>
>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.

True if the overall combustion is equally clean. I have tinkered with
all sorts of burning, toplit top down burning has given me the
cleanest results, demonstrably so in the device recently built and
tested by one of this list to my initial design.
>
>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.

Again I do not believe this has been claimed, Dean Still has a clean
burning updraught, bottom lit device. IDD and top lighting and top
down lighting have been put forward as tools to get a result, no one
has claimed they are the only way. They are relatively new concepts in
relation to humanities use of wood cooking, and as a result are novel
to many.
>
>Another conclusion is that top-lit updraft gassifying stoves cannot be
>refuelled.  

Not so at all, I can remember posting about this way back as well as
in one of the coal cooking threads, I recall Ronal posting on it also.

>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.

Good.
>
>Gentlemen and women, this is serious stuff.  Why are workable designs being
>ruled out?

They are not.

>  What am I missing here?  

Pass, I just take the view you can take the horse to water but you
cannot make it drink ;-).

>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".

I think this is mistaken, I take burning to be an oxydation type
reaction, I do believe combustion can take place without flame, by
simple defintion.
>
>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)

Yes, but you can also have pyrolysis without combustion, as in a
retort.

> 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.

As have many other combustion engineers in the past, in fact in large
furnaces many later stages of air addition are used between heat
removal stages to reduce formation of NOx.
>
>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. 

Not quite, these terms originated in coal burning, coal is low in
volatiles and high in C, primary air refers to the air to gasify the
carbon, wheter it be to CO2 or CO, pyrolysis products are produced
when the heat of primary combustiion releases them.

> Air provided to burn the gasses higher up
>will properly be called secondary air.

OK

>  Pre-heated air provided to react the
>still remaining fuel gasses should then be called 'tertiary air'. 

Fair enough

> 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.

Technically it would still be secondary combustion though.
>
>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.

Yes

>
>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)

It's true up to the stage that secondary combustion is established.
>
>Bottom lit stoves can be controlled by restricting the primary air which in
>the extreme position, turns it into a gassifying stove.

This can be true of bottom, mid or toplit.

> 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.

Yes as long as the conditions are such that combustion is complete and
the heat is liberated where it can be utilised.
>
>"...the secondary air supply for top-lit stoves has not seemed to need a
>separate control..."
>
>That is something Paul and I are investigating.

It is also a function of the earlier statement that offgas from an idd
stove is realtively constant, it is not true when a toplit stove is
allowed to turn into an updraught stove. I think there is some
confusion between top lit and idd.

AJH

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