[Gasification] Gasification Operational Principles

Thu Feb 28 04:29:31 CST 2008

Tom and list,

With my experience from a handful of traditional gasifiers both for mobile and
for stationary use I agree with the presented “operational principles”. 

Fuel flow is a challenge -especially at heavy loads.

Normal wooden chips made for space heating do not work well even in a small “car
size Imbert style” gasifier with constriction plate and nozzles. By sieving out
most of the fines, usual heating chips can be used.

Chips made with a conical screw chipper (Laimet or Kopo) with a suitable screw
do not need sieving to work well.

Air dry chips of both deciduous tree and conifer tree species can be gasified
without tar problems.

The flat restriction plate is the part that by far is worn out the fastest in
the traditional design. If the unit is built of stainless steel and well
designed it does as such not get worn. After some 10 000 km in mobile use, the
flat restriction plate starts to crack along its radius due to shrinking of the
centre of the plate while the periphery does not change. The flat restriction
plate shall not be welded to the hearth tube in order to make it easy to
change.

Can somebody on the list with experience from both closed top nozzle type
gasifiers with a small reduction pipe a´la Doug Williams Pioneer class gasifier
and closed top nozzle type gasifiers with the traditional widening cone style
reduction zone come up with some comparative information on the differences? 

With the traditional style, there is at least a build-up of fine char dust above
the grate when a grate scraper is not used. This forms especially with fine
chips a high pressure drop over the gasifier.

Are there differences in the temperature of the gas leaving the grate between
these designs?

Are there differences in the soot content of the gas, and if, why?  

Are there differences in the heating value of the gas?

What is the importance of the shape and height of the oxidation zone in Williams
design?

Does the recommended Superficial Velocity for this design differ from the ~2,5
m/s of the “Imbert type” design?

Best regards,

Fredrik Ek

Lainaus Thomas Reed <tombreed at comcast.net>:

> Dear Ferruccio:
> 
> Thank you so much for sharing your very inciteful list of biomass 
> gasification principles with us.  I have long admired the Ankur gasifier 
> as the best (and cheapest) WWII (Nozzle air, Imbert) gasifier. I am 
> forwarding your note to the STOVE list as well, since gasification of 
> biomass makes much cleaner and more intense cooking. 
> ------------------------------------------------------------------------
> We have also been intensively developing the low tar version of the 
> stratified downdraft gasifier.  Coming along nicely. 
> 
> I'd like to add to your list:
> 
>     * The fuel size needs to be large enough to permit free air passage
>       through the fuel magazine and char beds. 
>     * The fuel size limits the diameter of the nozzle type gasifier,
>       since air must pass from the nozzles to the center.  (The UCDavis
>       1998 scaleup of the 100 hp WWII gasifier to a 1000 hp unit was a
>       monstrous tar producer.  The Brazilian 1000 hp gasifier using fist
>       size chunks was OK.)
>     * Fuel flowability or gasifier shaking is essential
>     * Nuts and shells are often great gasifier fuels. 
> 
> I hope others will add to this list.  We'd best hang together, or we'll 
> all hang (out to dry) separately...
> 
> Yours truly,
> 
> TOM REED         BEF/BEC
> ------------------------------------------------------------------------
> 
> Ferruccio Pittaluga wrote:
> > Dear all,
> > we are coming up from a very heavy, one year long, experimentation campaign
> on our small (12 kwel) Ankur downdraft gasifier, properly instrumented by us,
> fueled by woodchips and by mixtures of woodchips and "coal" (both charcoal
> and clean coke). Our skin, our nose and our lungs have not enjoyed it,
> particularly when all the plant had to be dismantled, cleaned and re-mounted,
> which has happened about 10 times. The scientific results, as of now still
> proprietary (we have been funded by the technological agency of our "county"
> Regione Liguria) will be officially published later this year.  Some,
> preliminary, have been published at a national level. We have learnt a lot,
> in the good and in the bad. It seems to us that there is ground for laying
> down some very basic "operational principles", not of deep thermo-dynamical
> nor chemical nature, and, quite importantly, almost technology-independent,
> at least for fixed bed gasifiers. They are not really new, but what is given
> for granted (or trivial), so many times is overlooked or not always
> considered in its impact. At least this happened to us, so, having been
> helped quite a lot by this forum in the past, we now like to share our
> outcome in terms of general, maybe generic, operational principles, some of
> prospective character. Any positive or negative feedbacks would, of course,
> be most welcome.
> > They are listed below:
> > 1. Wood gasification, at the contrary of coal gasification, should be
> considered an "appropriate", not an "advanced", technology in order to really
> penetrate the global market
> > 2. Many times "gasifiers" are not actually such, they are just "pyrolysers"
> (The simple presence of one thermocouple in the gasifying bed can tell the
> truth) 
> > 3. The first process (gasification) is a clean one, the second one
> (pyrolysis) is necessary, but it is a dirty tar producer. 
> > 4. In order to have a clean outgoing gas, it is mandatory that
> "gasification" actually takes place, and that all the pyrolysis products
> undergo it, without any bypass.
> > 5. Not all the "black" which is transported by the product gas is tar. If
> hearth temperatures are adequate (in order to reach gasification), most
> "black" content is just non-sticky, pulverised charcoal. 
> > 6. Independently from technology, what most makes the difference is the
> fuel energy content (i.e. its calorific value less the fuel humidity latent
> heat) provided the oxidant flow rate (i.e. the superficial velocity) is
> sufficient to allow the said "content" to be released.  
> > 7. The fuel energy content is increased (energy nobilitation) by efficient
> drying processes, extremely important to be always performed at the highest
> possible degree
> > 8. The energy nobilitation of the fuel, achieved by proper mixing of
> woodchips with charcoal (say, 30% of the latter), easily succeeds in shifting
> the process from pyrolysis to gasification, improving product gas quantity
> and quality. However, charcoal is a dirty material, and is produced by
> intrinsically inefficient processes (charcoal makers) with heavy
> environmental impacts
> > 9. The energy nobilitation of the fuel, achieved by proper mixing of
> woodchips with clean coke (quality coke, not metallurgical coke) gives
> excellent gasification results, under any respects, even better than
> expected, but coke is not a renewable fuel. Non the less, coke is a "noble"
> fuel, clean and energy rich. Its production is of course extremely impacting,
> and "quality" industrial coke ovens are very few in the world. One, is
> extremely near our site.
> > 10. The energy nobilitation of the fuel achieved by wood torrefaction
> appears (we have got still no results, we are working after it) extremely
> interesting and highly promising under any aspects. Torrefaction is a very
> efficient, clean process. In our opinion, it is going to be a "strategic"
> process for the wood energy field.
> >
> > Thank you for reading.
> > Ferruccio Pittaluga
> > --------------------------------------
> > DIMSET/SCL - Savona Combustion Lab - University of Genoa
> > http://proxy.sv.inge.unige.it/SCL/
> >
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-- 

-- Fredrik