[Gasification] Design Parameters based on Superficial Velocity

[Steve Carroll]

Design Parameters based on Superficial Velocity
   
  In short, my question is: How do I determine the most effective cross sectional area of each zone in a down draft gasifier to obtain a desired superficial velocity?  Actually, I have many questions that extend to many more aspects of the gas production process that I can’t fit into a neat little sentence.  I know that many people’s first response will be to say, “go build it and let us know” but I am looking to see if the data already exists either in formula or empirically.  If it doesn’t, it sure seems like that would be a project that the DOE should be more than happy to fund.
   
  I have read a few papers on the internet that seem to give some clue as to what the needs are for the most efficient design and operation of small test gasifiers.  I have been assuming that these factors are at least somewhat scalable but I don’t really have any idea.  The two papers that come to mind for me are: Superficial Velocity – The Key to Downdraft Gasification and Biomass Gasification: Clean Residential Stoves, Commercial Power Generation, and Global Impacts.  I realize that those papers weren’t intended to be design manuals for downdraft gasifiers but I used them anyway to make assumptions about air volume, air ratios, rate of fuel consumption, and throat diameter in a heating gasifier that I am still trying to design.
   
  I’m really interested in how all of the following factors would affect a gasifier’s design and operation for a gasifier in which the desired total heat output is 100,000 BTUs and nearly all gasses, tars, and charcoal are consumed.
   
  Size and shape of fuel:  Does the degree to which the fuel impedes flow through the gasifier affect the actual SV?
   
  Density of fuel:  Does this affect the rate of gas production and hence optimal SV?
   
  Fuel’s ratio of surface area to mass:  Again, probably affects rate of gas production and desired SV in a measurable way.
   
  Volume of primary air:  Kind of a side question here but; does the volume of air control how much fuel can be pyrolysized or can pyrolysis take place in a complete vacuum?  Same question for gasification.
   
  Velocity of primary air:  How closely related will air velocity through a gasifier be to it’s superficial velocity?
   
  Temperature of primary air:  It seems that a higher primary air temperature would lead to a larger volume of pyrolysizing fuel.  Is it likely that a poor design would pyrolysize more fuel than it could gasify?
   
  If the rate at which fuel is pyrolysizing changes does the desired SV through the reduction zone change?
   
  Gasifier shape:  Should the cross sectional area of each zone change to achieve optimal SV for each stage?  Assuming a round gasifier, can the depth of each zone be predicited when the diameter and air volume are known?  Or can the volume and shape of each zone be predicted or controlled?
   
  A proposed test if it has not already been done or if the results cannot be calculated through existing formulas:
   
  Build four or five small and simple Imbert style close-coupled gasifiers with varying throat diamters.  Obtain wooden spheres in three different sizes to use as fuel.  For one of the sizes, get more wooden spheres of the same size but different species (density).  Have controls to vary primary and secondary air flow.  Conduct a series of tests that show what happens as one of the variables is changed for each set up.  Record a matrix of data that shows, temperatures, total BTU output, depth of zones, tar collection, left over charcoal, etc., etc.
   
  Anyway, sorry for the long winded email.  I know that I asked a lot of questions but it was only to make clear what it is that I am trying to understand.  I do not expect an answer to every question.  In fact, I would be pleased to discover the existence of some “Gasifier Design for Dummies” equivalent or perhaps just the Cliff Notes.  I realize that most of the people on this list have a far deeper understanding of gasification than I do so I really appreciate the time spent answering my amateurish questions.
   
  Thanks,
  Steve

 			
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