[Gasification] Gasification Operational Principles

[Thomas Reed]

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. 
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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
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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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