Re: Gasifier fundamental question

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

Dear Crispin (cc stoves):

    Today you said:   <snip header>

> 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).
>
    (RWL1):  To those who haven't tried this, the early smoking is about
what you see with any stove start-up.  That at the end (or if there is an
inadvertent flameout due to a breeze) is awful.  This is one of the major
problems with charcoal-making stoves - and one that needs a lot more work.
I try to quickly snuff or transport the hot charcoal to another location.
Others open the primary supply a lot and try to consume it in situ.  (In my
last paragraph, I discuss this in situ point further.)

> 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.
>
    (RWL2):  a.  "the regular way, though it no longer had a top-down flame
front." perhaps needs a bit more discussion.  I presume that the newly added
pieces were somewhat near the secondary air supply - but I am guessing that
they were profusely outgassing due to the relatively high temperature gases
coming from below and not due to surface reactions (as were occuring at the
pyrolysis front below).  There is also considerable heat input from the
radiation coming from above.  To those who haven't seen this phenomenon - it
is very satisfying to see the rapid increase in flaming, with the top-added
fuel.  I agree that you can't do it late in the process.  I should also note
that I try to use longish twigs and small branches - which do not shrink
greatly longitudinally as they are turned to charcoal.  There is much
greater shrinkage radially.  Thus the top of the charcoal in my tests is
probably closer to the secondary air supply in my tests than those Crispin
is describing - and certainly in those of Tom.
    b.  re "It was hot here and there." - I presume you are saying that you
did not have the relative uniformity I have been claiming.  Can you describe
this more fully?   I often get hot spots near the primary air holes - was
that the case?  Was it a problem?

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

    (RWL3):  Sorry - not understanding the "middle lit during the second
burn".  Can you describe your control of primary air - and where the flame
appeared to originate?  Could this be ignition of pyrolysis gases produced
as I described above?  That is - where was the necessary oxygen coming
from - the primary air ports or the secondary  air "ring".

>
> 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.
>
    (RWL4):  If you go back in the archives (not that I recommend the
search) - I believe you will find other discussions of adding extra fuel.  I
don't remember any denial of that possibility.  But if your aim is to
produce charcoal (my aim), this is not particularly helpful.  I would say
that what you did was rather like the upper fuel container in Paul's Juntos
stove - and one that can be very helpful in long burns, requiring refueling

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

    (RWL5):  I think it should - if you also increase air flow openings to
compensate for the added air flow resistance .
>
> 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.

    (RWL6):  a.  I contend that the copious smoke in traditional rural
charcoaling could often be flared - and that flaring should be mandated.
The first step is to use drier wood.  Apparently this is usually not done,
for fear that the downed, piled (generally wet) wood will be stolen.  But
the use of wet wood not only prevents flaring, but also requires
considerable combustion of produced charcoal to dry the wood out.
    b.  Several on the list have been exploring the use of recirculated
steam for drying the incoming wet wood - quite a fascinating way to use the
"waste" heat - and not having a problem with the flaring.  I haven't seen a
cheap easy way to do this with a stove.
    c.  I lived for a few months in Sweden (partly working on charcoaling
stoves) in 1995.  It seemed like every home had a huge pile of wood near
it - and I believe it was "seasoned" for two years.  You obviously favor
using dry wood and I concur.  My argument against bottom lighting is not
based on moisture content alone.  I still don't believe one can get
adequately clean combustion or controllability with bottom lighting.

>
> 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.
>
    (RWL7):   One reason for my bringing in the "forest-based" approach was
to say that it seems to be based on bottom lighting.  I have tried to read
everything I could on forest-based approaches - and never have found a
description of "top-lighting".  Some of the Swedish charcoaling literature
seems to imply that something equivalent was happening (but in 3-D).  The
standard (smoky, non flared) approaches to charcoaling are bottom lit -
presumably because this is so common for every other form of biomass
conversion.

> 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.
>
    (RWL8):  a. There has been considerable discussion on this list of
different ways to control the primary air - which seems to have a linear
relationship to output. We have talked about various plugs (my favorite),
and rotating and sliding flaps and vanes.  A big topic of discussion has
been "turn-down" ratio - with "3" being a common value (Max power maybe 9
kW, min power maybe 3 - depending mostly on the diameter).   But it appears
that you were doing your tests without being able to vary the primary air
supply - and that is a (probably "THE") key element of the top-lighting
approach.  So when we brag about "constancy" - that is a statement of
constancy after setting the desired power level (within the turn-down ratio
limits).  It is not meant at all to imply that only a single power level is
possible.  I presume most of us also open the primary air port to the
maiximum during startup.   The fuel I always try to use is picked up off the
ground - definitely not "unobtainium."  And to repeat, the power output
variation is quite independent of the fuel - it is directly related to stove
size and primary air supply.
    b.  I am very confused about why your tests seemed to have no control
over primary air supply.  If you can supply the data on weight of fuel and
duration of the pyrolysis phase, I can estimate your power output and I
assert that with some modifications you should be able to vary the power
(and/or time) over about a 3:1 ratio.  Below are some comments on Tom's data
that are along this line - ie 10 grams per minute being about 3 kW.

> 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.

(RWL9):  I think you have read Tom's message of Thursday correctly - but one
should not assume that his stove (or any) should be used that way to get a
lot of early power.  I think Tom was also saying that there were way too
many emissions (unburned gases) because of the bottom-lighting.  I am not
sure that Tom would agree that one can design a bottom lighting stove
(pyrolyzing or not) that will give the desired range of power output (It
sounds like you want a turndown ratio of perhaps 5 or more).   The standard
US gas stove doesn't do that well with one burner I believe (I think Tom has
reported that electric ranges (like mine) have about 3 - anyone able to
report a number for a modern gas range?

    In his Thursday message, Tom said:  "(10 g/hr = 3 kW for our stoves on
high)".  Tom obviously meant 10 g per minute or 600 g per hour.  However,
since I saw this small typo, I looked more closely at Tom's data and can say
that a more accurate pair of values would be 7.5 g/minute and 450 g per
hour.  Three other comments seem appropriate on Tom's nice data and graph
(which some may not have seen - as you have to change pages on his Excel
spread sheet).
    a.  First, the linearity of the power output (or g/min) data is well
apparent over a 37 minute period.  I doubt anyone doing a non-pyrolyzing
stove can show anything similar over even a one minute period.  This was all
done on a single (high) setting - but I am sure Tom could show us a similar
plot with a high and a low setting - both with similar linearity
    b.  The ratio of water evaporated to fuel converted (both in g/min - the
slopes) is very nearly 3 in Tom's test.  I'd like to hear from others as to
whether they have a better Figure of Merit number to report.   I do not know
whether Tom had a "convection/radiation shield" around the pot - but his
ratio is a very good one.
    c.  I want to argue that Tom's efficiency computation (in cell B20)
given by Tom is overly simplified - as it gives no credit to the charcoal
being produced  (As indicated above, I would use a weight ratio slightly
less than the 220/70 given by Tom - but that is not my point).  This
equation would be correct if no charcoal was being produced (assuming Tom's
energy density value of 18.9 MJ per kg is correct) - but we who like
charcoal have to be picky on this point.  In some of the stove literature I
have seen, the residual charcoal weight (15.5%) is subtracted from the wood
weight for the efficiency computation.  This would raise the efficiency
value by 1/(1-.155) or about 38%*1.18=45%.  If one gives credit to the
different energy contents of wood (18.9 MJ/kg) and charcoal (28? MJ/kg), one
should subtract not 15.5% but something more like 15.5*(28/18.9) = 23%.
Thus Tom's energy efficiency might be revised from 45% to
38%/(1-(.145/.845)*(28/18.9)) = 38/(.75) = 50.7%.
    [We have to come back to this later - but I have a further quibble with
even this adjustment,  I don't like the subtraction of terms in a
denominator.  I would rather be messing with two numerator co-product terms
that are additive. If we were only interested in making charcoal, Tom's data
would give a conversion efficiency of 15.5%*28/18.9 = 23%.  I would add this
to Tom's computation of 38% (during this phase only, not overall) and get
61% efficiency.  Who disagrees and why? ]

>
> 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.
>

    (RWL10):  Not sure how to respond - I guess you are saying you never saw
too much gas when you added extra fuel.  And I would say the same.  Tom
would probably also agree if he added less fuel - or he might say that you
can always add too much.

> 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.
>
    (RWL11)  I say that the issue relates greatly to where the secondary air
is coming from.  In stoves like the Rocket stove, both the primary air and
secondary air are introduced from below.  If you are producing lots of
uncombusted gases along with the CO2 and steam, and the secondary air is not
mixed in soon enough, then you will have excessive emissions.  My reading of
the limited lab measurements we have is that 5% CO in the exit stream is
common for usual (bottom lit) stoves. I don't think that the best are that
bad, but they are nowhere near zero either.

> 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.
>
(RWL12):  As I am getting to this point - I have been out for the evening
and now pooped.  Since I also find that Andrew Heggie has answered a lot of
these questions, and I have written too much, and am busy in the AM, I am
going to stop here.  I will try to pick up again at this point - as Crispin
has asked some excellent questions.   On this one question above, I would
say that it is very difficult - if not impossible - to produce charcoal and
CO2 at the same time.  If you want charcoal, you are well off getting the CO
(and CH4 and H2) - which are better off burned as close to the pot as
possible.   If one starts combusting the charcoal at the end of the
pyrolysis phase, that production of heat and CO2 is way at the bottom of the
pile of charcoal (we don't see a flame above the charcoal typically), and
the plots of energy output (as seen by achievable temperature in the pot)
are very discouraging.  Although the charcoal is disappearing, essentially
no useful work is being accomplished - just because the charcoal and air
supply are in the wrong places.  I feel it much better to salvage the
charcoal as soon as the pyrolysis front reaches the bottom - and then sell
it and/or use it in a combuster designed for that job.  There is not a great
deal of money to be made this way, but for a woman making zero income, even
a small income can be quite liberating.

More promised.    Ron

<long snip>


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