GAS-L: Small Scale Gasifiers Defined

["Tom Reed" <tombreed@attbi.com>]

Dear Doug and All:

Doug's opinions below form the basis for an important discussion - "when is
gasification really just combustion".  There is in fact a continuum of
operation between pyrolysis, gasification and combustion at both the
microscopic and macroscopic level. The exact boundary can't be nailed down
exactly, but it is important for all of us to understand the issues. I have
seen drag racers recently with flames coming out their exhaust pipes.
However, I don't think I want to call them "gasifiers".

Doug seems to be saying below that if you pipe the gas created by partial
combustion (in a downdraft or inverted downdraft or fluidized bed or updraft
"gasifier") to a separate device (engine, burner, chemical process) you have
a "true" gasifier; if you burn it immediately in close coupled mode it is
not really a gasifier at all.  While I see some basis for this claim I don't
agree.

In our WoodGas stoves partial combustion gas is generated by flaming
pyrolysis in a zone 2-3 cm thick down inside the bed.  The gas then passes
through a layer of characol which converts more of the gas to CO and H2.
The gas that issues from the charcoal many cm above the flaming pyrolysis
zone is then burned immediately.  However, we could easily add a 6 inch or 6
foot chimney and inject air at the top for very similar combustion.  By
Doug's thinking the first would be a combustor and the 2nd and 3d would be
gasifiers.

In the U.S. the difference between combustion and gasification  has become
formalized in the laws relating to incentive credits for gasifiers that
don't apply to combustion.  I have been involved in helping write the legal
definitions.  Something like.."If you can establish a level BETWEEN the
gasifier zone and the combustion zone at which a gas sample will have an
energy value of more than 2 MJ/scm (50 Btu/scf) it is a close coupled
gasifier/combustor combination".  If not, not.  Since $$$ are involved, this
is obviously an important definition.

The close coupled gasifier/combustor combination has a number of advantages.
Optimal gasification of bone dry wood puts 17.8 MJ/kg into chemical energy,
but still leaves 3.3 MJ/kg in the sensible heat of the ~750 C gas coming off
the charcoal pile.  If this is burned immediately you can approach 100%
efficiency in the gas conversion.  If you have to cool the gas to room
temperature you lose 16% of the energy to coolant.  Furthermore, close
coupled combustion burns all the volatile tar vapors up.  Finally close
coupling saves space and our 1.5 V WoodGas CampStove is only 7 inches tall
and 5 inches in diameter.

So, I will restate that if you have producer gas in a section of the unit,
it is a close coupled gasifier/combustor.

Yours truly,                                TOM REED            BEF GASWORKS
----- Original Message -----
From: "Graeme Williams" <graeme@powerlink.co.nz>
To: <gasification@crest.org>
Sent: Wednesday, June 26, 2002 2:36 AM
Subject: GAS-L: Small Scale Gasifiers


> Dear Gasification Colleagues,
>
> It is easy to understand the confusion that exists about gasification in a
> general sense as the phenomenon can be found anywhere you create fire.
> However, for this interest group to include gasifying stoves is in my
> opinion incorrect, as they should be identified by their correct name of
> double burning or smokeless stoves which commercial manufacturers call
them,
> and as such stay in the stoves forum.
>
> For people researching gasification and interested in using gas to use
> separately for whatever end use, this is the one group who can offer the
> most assistance and information on the subject, particularly if it is for
> engine applications.  Those of us who actually work in this field of
> technology have made a commitment to see gasification fulfill an important
> role as a source of renewable energy and chemical feedstock.  To cross
> pollinate gasification with close coupled combustion, then discuss
adapting
> these stoves for engines is less than helpful to those who seek accurate
> information about gasification.
>
> While it might be important for individuals to buy a small gasifier, it is
> equally important for the manufacturer to find enough individuals to
create
> a market.  Then, instead of saying how much are small gasifiers, ask
> yourself - how much am I prepared to pay for one.  This is a valid
question,
> so state your financial ability to pay for your commitment to renewable
> energy.
>
> Alternatively, decide on a size (discuss it with a manufacturer first),
get
> a quote and order 50.  Then onsell to those who just want one unit.  If
this
> isn't a solution for you specifically, then you have to accept that the
only
> way to own one, is to build it yourself.  Since I posted the design for a
> small gasifier on the Fluidyne Archive last year,nobody on this list has
> written to me saying "I'm ready to go, what's next?"
>
> Last August in Northern Ireland, two French engineering students working
at
> ITI (Innovation Technologies Ireland) built one out of salvaged scrap
steel
> and I shared their excitement of having it flaring gas within 3 minutes of
> ignition.  I'll see if I can find the photographs and ask Graeme to put
them
> up on the Fluidyne Archive - www.fluidynenz.250x.com.  This will take a
> couple of days.
>
> Gasification is very addictive and a lot of fun . . . so let's discuss
> making gas and not heat, unless it's burning gas!
>
> Doug Williams
>
>
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>



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www.webpan.com/BEF
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Other Gasification Events and Information:
http://www.bioenergy2002.org
http://www.crest.org/articles/static/1/1010424940_7.html Bioenergy
http://www.crest.org/articles/static/1/1011975339_7.html Gasification
http://www.crest.org/articles/static/1/1011975672_7.html Carbon