[Gasification] Perfect Hearths

Wed Apr 2 22:35:38 CDT 2008

On Wed, Apr 2, 2008 at 6:21 PM, Toby Seiler <seilertechco at yahoo.com> wrote:
> Greg and Jim,
>
>   Greg, please help me understand your "perfect hearth".  I understand your desire is to preheat the incoming air, but it seems that your suggesting taking the heat from the bottom of the reduction zone area with a tubular grate.  If I understand Jim, I agree with him that no heat should be removed there because it reduces the temperature at precisely the area that it is needed most.  Seems that by reducing the temp there, you will be more likely to make tars rather than tar free gas.  After that grate, in the gas collection area, would be fair game as the reactions are complete.  I think that is what Jim is saying (but not exactly sure).  Were you working on a feeder unit (as opposed to batch feed)?  If so, is it airtight?  Have you thought about preheating (indirect) the incoming fuel, rather than air?  DTU did this with a screw feed and the gas in a shell around it, longer would seem better for more heat transfer.
>
--------------------------------------------------

yes, you understand what i was saying.  but maybe we misunderstood
what greg was saying.  it is difficult to understand textual drawings
of things that are visual.  he can clarify.  greg understands all this
very well and certainly he has created an impressive low tar unit.

the DTU is also very impressive in its fuel preheating.  there is much
to learn considering their system and how it performs.  but see notes
at end below.

>   Jim, my compliments, that's a nice paint job for demonstration and a well thought out unit. I think I understand the flow of air/gas in your "kit" but do not see what is happening with the center pipe that comes up out of the hopper and also the bottom center pipe.  Is there more air entry coming in than comes through the nozzles?
---------------------------------------------------------------------------------------

i apologize for not explaining all this as promised.  i'm behind and
trying to catch up.

the tube toby is asking about that comes up from the grate is in the
picture here http://theshipyard.org/gek/large-24.html.  this is one of
the modular options so one can model the gasifier types which bring
air up from the bottom, through the reduction tube for preheating.  it
is only used when running in this particular mode.

when running a typical constriction and side nozzle set up this tube
is removed and a plug put in the grate hole.  see here:
http://theshipyard.org/gek/large-18.html.  the small hole in the plug
is for poking a thermocouple up from the bottom to measure reduction
tube temps along the whole length.

similarly, air can also be brought down from the top using this add on:
http://theshipyard.org/gek/large-14.html.  this is only used when
running in this mode.  when running in others, the top nozzle tube is
removed so you can fill fuel through the top opening.

this flexibility is motivated to create the conditions to test the
very question behind the current discussion and other related ones.
several historic designs, and some current bring air up from the
bottom in a manner which mines heat from the reduction zone.  some do
it by a tube right up the center.  others do it by air cowlings around
the reduction tube/bell.  i'm curious what the realities are of doing
this.  one can argue it is a zero sum problem as the heat needs to
come from somewhere in the combustion and reduction process.  one can
argue otherwise along the lines i did previously.  ultimately, it has
not been well tested.

there are similar unknowns for side and top air delivery.  as well as
preheated and not preheated.  i've attempted to make a system where
all these different delivery scenarios can be tested on one base, with
easy change out between them.

no claims are being made about what is better or worse.  no guaranteed
performance claims are being made.  i'm not trying to deliver a
turnkey gasifier for civilians.  rather, i'm trying to create the
conditions where we each can easily set up the physical forms of our
experimental speculations and record the results.  or just change
things as the realities of fuel supplies change.  a basic truth of
fixed bed gasifiers is their extreme fuel sensitivities and need for
purpose configuration as fuel specifics change.

> Your using the gas collection area with the J pipe to preheat the incoming air and do some cooling...correct?
---------------------------------
yes.  cooling the product gas is a nice indirect benefit of preheating
the air.  the radiator has to do much less work.  i am surprised how
much cooler the syngas is with this J tube arrangement than typical no
preheat scenarios.  a bit of energy is conserved in the process too.
note that you have to increase the nozzle and air delivery tube size
accordingly, as heated air expands.

> If gas is coming up around the fuel hopper, why do you insulate between them?
--------------------------------------------------------------------

i am insulating because most of the product gas rise path is along the
reduction and combustion zones.  as i do not want to be mining heat
form these zones, i insulate as best as possible.

also, what is in the pictures is considered the hearth only.  a proper
hopper would add to the top.  flanges are sized to easily take a 55gal
drum.  i like to be able to break the unit near the top of the the
hearth area.  this is what doug and greg do.  it helps much for
inspection.  wwii units are usually a single cabinet from top to
bottom i believe.  experimenters seemed better served with a breaking
type.

but i also wanted to get enough space in the basic hearth so very
different constriction sizes and nozzle heights could be supported.
thus i put the break point a ways above the nozzle line.  6" or so.
greg's is about 2" above.  the total height of the core reactor is
motivated by the range of nozzle placement i wanted to support.   more
info on these ranges later.

with small constriction configuration, this gives enough room in the
top of the basic hearth to hold fuel for a short run.  i was trying to
get enough extra hearth to allow batch runnings for testings, without
a proper hopper.

the basic hearth size is a motivated compromise between these concerns.

> Does preheating the air do something that I'm not considering (air becoming less dense with heat) that would not be accomplished by heating the fuel?
>

preheating fuel can evolve more tar smoke and at times water vapor.
if pyrolysis is too fast you will overwhelm the combustion needs for
tar smoke, as well as likely evolve tars at higher temp which are the
tars which are harder to break.

the monorator designs purposely keep fuel cool until the hearth so
that passive convection currents are set up, with condensing on the
top/sides.  the DTU unit is doing the opposite, but with a different
architecture, towards a similar high moisture tolerance end.  each
works to the same end, but through opposite mechanisms.  it is an
interesting puzzle to figure out.

j

>   Best regards,  Toby Seiler, STC
>
>
>  ---------------------------------
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>
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-- 
--------------------------------------------------------------------------------
jim mason
website: www.whatiamupto.com
current project: mechabolic (http://www.mechabolic.org)
announce list: http://lists.spaceship.com/listinfo.cgi/icp-spaceship.com