Re: flash carbonization of biomass

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

On Sat, 5 Jan 2002 12:16:18 -0700, "Ron Larson"
<ronallarson@qwest.net> wrote:

Sorry if this is out of sequence with my last posts, I inadvertently
replied to Ronal. Since then Tom has posted in another thread
something that makes me think I should revisit the large scale idd.

>1.  You said (after quoting me on Antal's new discovery):
>    ".... I thought the major achievement of his process was the
>high yield of carbon as opposed to high volatiles charcoal.  This
>should have good implications for low ash charcoal for use in high
>grade metal refining......"
>    (RWL1):  I need help on this from both yourself and Mike.   Are you
>suggesting 1) that Mike does not have high volatiles and

Mike would have to answer this, I imagine his process is fairly
controllable, the thing is there is only a limited number of carbon
atoms in the biomass, you either retain them as solid char or remove
them as offgas or pyroligneous acids (bio-oil).

> 2) that high carbon
>means low ash, and/or 

Intuitively if the  rawstock biomass has 0.5% ash and the char is 40%
of the dry weight with the ash retained in the char matrix then the
char contains 1.25%, if the char yield drops to 25% and the ash is
still in the char matrix then the char contains 2% ash.

>3) that high grade metal refining requires low ash.

Yes this is my understanding, I was referring also to the production
of high grade silicon metal for semi-conductor production.


>My confusion comes from not knowing enough about different grades of
>charcoal and their uses - which I am sure both you and Mike do.

I believe the recent post from Tom Reed illustrates this.

>    I once gave some stove-produced charcoal to a woman using it for burning
>incense - she didn't like it as it (I think) had too much "high volatiles".
>On the other hand, I have heard others on this list say that high volatiles
>was an advantage in lighting charcoal for cooking.   Could you give us a
>tutorial on charcoal - especially as desired in rural areas for cooking?

Again Tom Reed has re-iterated this point, the higher you heat the
biomass at atmospheric pressure the lower the yield of char and the
more pure the carbon.


>
>2.  After another quote from me asking Mike about small scale pressurization
>for cooking, you said:  "
>    "High pressure is very limiting as the vessel size increases, which has
>implications for mechanised loading. What a lot of people seem to fail
>to appreciate is that charring even at low pressures can be a quick
>process."
>
>    (RWL2):  Your response is (naturally) dictated by the very impressive
>large scale (one person operation but megascale compared to a cook stove)
>work that I had the privilege to see in operation in October - including
>mechanised loading.  But my view of a "pressurized cook stove" is one that
>is of course hand-loaded and is smaller than the usual pressure cookers I
>see.  While agreeing with you completely, I'd like your opinion on whether
>there is some fundamental reason to forget working on pressurized
>small-scale cook stoves.

Yes my comments were largely about dedicated production of char. I do
believe there could be a use for presurising a retort forming part of
a cook stove, with the aim to utilise the stream of offgas produced to
power air movement. There is potentially a safety issue here. If a
retort at pressure ruptures whilst pyrolising there will be a large
volume of hot offgas released into the stove body. There will be
insufficient air supply to combust it and a cloud of gas at above its
spontaneous temperature of ignition will be released into the area
above the stove. So the pressures I am thinking of are measured in a
few centimeters of water gauge rather than the 10bar mentioned in
flash pyrolysis.

>    On your second point, I guess I agree - but need clarification.  First,
>we are generally not interested in speed of charcoal-making with a CM stove.
>My interest in pressurization is more in being able to get a superior and
>higher quantity of charcoal.   Maybe to also use the pressure productively
>(perhaps).

See above, I cannot see how the high yield charcoal expounded by Mile
Antal can be attained in a cook stove, similarly, apart from better
air control and insulation, I do not think you can better the yield of
char in the idd configuration. 
>
>3a.  The third quote of mine you used was directed to "waste" heat
>utilization.  You referred to our "cocoa tin" discussion  and said:
>    "The drawback of this retort heated from outside approach is that the
>heat
>transfer can only take place through the walls of the vessel and
>thence from char particle to unreacted biomass, but it does make use
>of the offgas being flared in a supporting fire. There is likely to be
>gradation of the extent of charring from outside to middle."
>    (RWL3a)  This also was the case with the similar CM stove developed by
>Professor Grover in India.  It was not rapidly controllable either.  The
>retort's principal advantage is that it effectively prevents air entry.
>(but not so easy to obtain complete closure.)

I can see how controllability is an issue, as Tom Reed has often
pointed out pyrolysis between 270C and 440C is reckoned to be slightly
exothermic. So, in the absence of heat removal from the process, once
up to 270C the biomass will continue pyrolysing without further heat
input. So cutting off primary air or support fuel will not stop the
evolution of offgas. The offgas then continues to burn in the flare
contributing further positive feedback.


>
>3b.  You continued:  " Traditionally char was made in kilns, basically an
>air starved fire in
>a container, I believe the idd stove is a kiln in this respect. The
>kiln has a heat transfer advantage in that the offgas and combustion
>products circulate around as yet unreacted biomass. The disadvantage
>is that valuable char is consumed preferentially to offgas to achieve
>this."
>    (RWL3b):  Again, a need for further discussion.

Yep, except precious few of us seem to join in, the knowledge is out
there and currently implemented by commercial organisations, the
interest for me is seeing how it can be adapted for use at the
intermediated technology level.

> 1.) I don't see the
>"heat transfer advantage".  The CM UD (or should I say UD CM? 

The what? up draught charcoal making? for it to make sense to me you
need to add top lit. I still prefer idd but will accept descending
pyrolysis front.

> I choose the
>former only because it is eawier to say) stove puts about the same about of
>hot gas past about the same amount of unreacted wood, it would seem (albeit
>a much longer path in a large kiln.

I fail to see this, in a plain air starved updraught device the bottom
lit fire burns biomass, if it is a small fire then the fire bed is not
hot and the sequence of combustion is :

1) Initial heat from ignition source pyrolyses biomass
2) Oxygen from the air reacts with freshly made char
3) if there is sufficient oxygen left this combines with offgas to
produce a flame, otherwise *any* oxygen reaching the glowing biomass
reacts with nascent carbon. 
4) biomass above this region is "cooked" by the mass flow of products
of combustion.
5) Once this offgas, which is heavily diluted by CO2 and N2, reaches a
temperature sufficient to maintain combination of these gases with
secondary air then a flame holds instead of smoke being evolved.

The idd or descending pyrolysis front behaves differently

1) again heat from the ignition source pyrolyses the top of the
biomass
2) pyrolysis products are evolved and rise from the surface, unless
there is copious primary air this offgas is swept upwards by narueal
covection.
3) If thesre is sufficient turbulence and primary air we get Tom's
micro mixing and some combustion of offgas, otherwise the offgas
passes unreacted. 
4) if the descending pyrolysis front becomes hot enough and the char
depth is great then toltal gasification of the char ensues, this is
the aim, I believe, of the turbo stove. I have posted a worry
(completely theoretical as yet and hopefully irrelevant) that the
superficial velicities involved from forced draught could be enough to
fluidise fine particles of wood ash into a sol which may have health
implications,
5) If the minimum amount of primary air is used, such that enough high
cv offgas is generated to allow a self supporting secondary flame and
continue to supply sufficient heat to maintain a descending pyrolysis
front by igniting and pyrolising succeddin layers below, then up to
25% by weight of high volatiles char will be produced. This low
conversion of char produced at a relatively low temperautre indicates
to me that a good proportion of the fixed carbon is gasifiied in the
process.

>   In the CM UD stove, there is a
>continued similar parallel gas flow path  whereas in most kilns they seem to
>be mostly bottom lit (my reading of the charcoal kiln literature) - and the
>air flow is dictated either by moving a number of chimneys or by air holes
>that appear as the dirt-covered pit kiln collapses.
>    [Oops - after rereading, I realized your comparison was not between a
>kiln and a CM UD, but rather between a retort and a kiln.  But I decided to
>leave my comment, since it might help someone.)

In fact it was you that explained to me what was supposed to be
happening in the ring kilns I was using up till 97. The thing about
biomass combustion is that nothing is black or white, there are
combinations of things happening. I know that in our use of these
kilns with poorly dried wood (even though it had seasoned in our
summer) we accepted that that wood which lay at the centre bottom of
the lin woiuld be burned to ash, that at th outside top would be brown
wnds (brands). So gash wood of poor species was sacrifieced in the
middle and smaller pieces were laid around the outside. There is a lot
of scope for convection of offgas and combustion products in a ring
kiln so loss of char was probably mitigated by some combustion of
offgas.
>
>    2)  There is a charcoal loss in both kilns and the CM UD by combustion -
>but I don't believe this is intentional.  Rather, I believe that a
>well-managed kiln (and CM UD) continues in operation by virtue of the
>pyrolysis reaction being exothermal.  My "proof" of lack of combustion is
>that the charcoal from a CM UD is rarely white (except where near the
>secondary air source) - no ash.

Yes I agree there is little visible ash but how would you explain the
low yield of carbon?
>
>3c.  You said:  "So one needs to combine the attributes of a kiln's good
>heat transfer,
>with those of a retort's high char yield. This is the approach Lurgi
>used in coal retorts and I believe the Simcoa plant. Mike appears to
>be taking the same route and it is also my preferred approach.
>....."
>    (RWL3c):  I think we probably have a lot to learn from the world of coke
>making.  Can you provide some references to the Lurgi/Simcoa processes? 


I believe I have a schematic to which I have added notes of my
interpretation of what is happening, however I am unsure if a
technical explanation is yet in the public domain other than in
lurgi's patents.

> As
>I interpret this, you are saying that you prefer the retort to the kiln -
>and probably even at the small CM UD stove level.

No, the Reed-larson stove, and its progeny, has demonstrated it has a
place in clean cooking, my prefernce for a hybrid kiln/retort is for
reasons of maximising char yield and content by controlled combustion
of offgas, preferably with further use of the offgas heat for a
downstream process or power genration.

>  My reaction is that I
>don't know enough about the retort approach to make a choice at the large
>scale level (except that most charcoal kilns are venting only - no flaring -
>horrible!).  I am bothered by Mkie Antal's response that his approach does
>not release a combustible gas - presumably too moist, maybe because he (like
>many charcoal makers, including yourself) wants to utilize material of any
>moisture content.   

Pass

>I am not willing to accept that the retort approach is
>better for cookstove operations - it has a very slow response time.  Do you
>agree? 

Yes

> Should I be taking most of your comments in this message as not
>applying to cookstoves?

Yes
>
>3d.   You said next:  "This is why I am not keen on promoting IDD as a means
>of making
>charcoal, though I am happy to accept its by product is charcoal that
>is produced with little pollution and as such is better than much
>charcoal making. I think Tom Reed agrees with me the IDD pyrolysis
>front is powered by charcoal burning, plainly this detracts from
>charcoal yield. With the advanced clean charcoal making I, and Mike
>Antal, are playing with any heat necessary for maintaining pyrolysis
>comes from burning some of the offgas and recirculating this hot gas
>stream through the char. There are a number of ways of configuring
>this, none patentable IMO but nonetheless the actual designs will be
>proprietary and subject to confidentiality."
>    (RWL3d):  I like your first sentence.  I also believe that rapid
>controllability of power output is of high value - as is having a saleable
>co-product.
>    I invite Tom and others  to comment on the second, which I don't believe
>is true - 

I am waiting also, in the absence of other comment we should take this
off list.

>but
>I am of course willing to give up some charcoal, even if true.  The CM UD
>stove charcoal yield (25%?) is about equal to that from the best of (heavily
>polluting) rural charcoal makers.

Agreed on both points, (I cannot bring myself to say CM UD) I just am
unable to envisage the mechanism for the marketing of this small
amount of char to be viable.

>    There is no single optimization demand on processes with co-products.
>So far, the only big drawback I hear is that education is required.  You,
>Mike, and I (but not everyone) agree that we should be trying to get rid of
>the traditional kiln approach.  Which approach will end up being successful
>will depend on faactors we can now probably not even guess.  We have to
>fight thousands of years of traditional charcoal making  (and my concerns
>are not only that the gases are un-healthy, but also global warming
>concerns, and desertifications and deforestation from something that is so
>wasteful.
>    I understand your and Mike's concerns about confidentiality - and
>appreciate all that you do help us with good ideas.

I cannot speak for Mike, for myself there is little to be confidential
about, all I have worked on is in the public domain as it has been
discussed on this list. What is proprietry is the actual nitty gritty
of how to achiev this. For my part the only reason for not putting
this up as an Aunt Sally is that the only way I can see to take it
forward is if it is aided by an entity with funds. If this is to be a
private venture they will want to own, at least, a shear in the
intelectual property, if it is a charitable or government fund it will
fail if it is not considered novel.
>
>(3e)  The implementation I am looking at uses quite basic technology and
>should be possible to deploy in the sort of situations envisaged by
>the Karves' project. A benefit to my mind is that this method can
>control the temperature and cook time of the biomass to vary the
>resulting char from post torrefied wood through Tom's seasweep and up
>to metallurgical grades. My interest is in high volatiles charcoal and
>I have made material which is friable like charcoal but retains 45% of
>the mass of the original dry matter.
>    (RWL3e):  Having seen your work up close, I can attest that your steam
>drying technology is quite exciting to me - and I understand you  know how
>to control temperature - which I don't think we will get to for the rural
>stove market.  I would like to hear more about your reason for interest in
>the high volatiles market - and whether there is a lesson there for the
>rural stove area - both for those producing charcoal briquettes and for
>those of us interested in CM stoves (either UD or DD).

The steam drying is a bit of a red herring, I assume for the most of
the world using charcoal that air/solar drying would be adequate. Once
"Denver dry" (15%wwb) biomass is available then the next step is to
maximise the yield of a friable energy dense fuel that can be formed
into a briquette suitable for the market and of an energy density
maxing the local transport system.

AJH

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