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Most of my earlier prototype gasifier/combustor systems were based on a rectilinear side-draft gasification flow through a narrow fuel column into a close-coupled combustor surrounded by a ceramic heat exchanger that  preheated gasification air to at least 800F, even using fuels with up to 2/3 their weight in water. For more details on my previous work

 

 

OMNIFUELED GASIFIER

 

I have a design for a gasifier that I want to share with the world. It is the culmination of my 35 years of work in the field, and I think it will solve many of the problems that now plague those modeled after the old WWII Imbert gasifiers.

The design is based on the following observations and discoveries:

  • Fuel in a hopper gasifies best and most evenly, without bridging, when it is vaporized/gasified evenly at the base of a vertical-sided hopper. Whenever there is constriction without size-reduction from burning or gasification, as is often the case in the upper throat of an Imbert or constricting throat gasifier, where combustion doesn’t consume the fuel to the contour of the throat, bridging and uneven gasification occurs.
  • Whenever there is bridging of the fuel, air supports combustion beneath the bridge, creating hot flames and a spent gas with excess oxygen and little if any energy value.  Then the bridge collapses, quenching the flames and heat with cool damp fuel and steam, creating a burst of sooty gas, followed by diminished cool gas production.
  • Providing the first feed conditions are addressed, preheating the incoming air can almost entirely solve this problem, if it is hot enough (800-1200F), because the endothermic gasification is sustained in a deeper coal bed by the heat of a lesser volume of air.  This is quite different from conditions created by a larger volume of cool air, with its oxygen content creating combustion to supply the heat along with lots of diluting nitrogen and CO2.
  • Of the three fundamental thermodynamic ingredients of Time, Temperature and Turbulence, Time is too often neglected in favor of (old school) Turbulence, as in the jet of speedy air shooting from the tuyers of a downdraft gasifier. I have found it better to let the air slowly permeate the fuel, heating up a large mass of fuel slowly, evenly, creating a large hot coal-bed. letting the gas become saturated with CO and Hydrogen over time and temperature. I think we can forget turbulence for most applications, and instead rely on laminar mixing of self-propelled convection flow.  Steam and CO2 are primary combustion enhauncers, speeding heat transfer significantly by their bipolar molecular property of absorbing and radiating radiant energy.  Radiation is the primary mode of heat transfer at the temperatures of gasification. N2, O2, CO and H2 do not absorb radiant energy, so heat transfer must come from neighboring bipolar molecules or be delayed.
  • Contrary to expectations, adding all this heat and insulation does not deteriorate the materials of construction as much as allowing local hot spots of 2500F combustion, which is far above the required gasification temperatures of 1300 - 1600F. Temperatures above 1500F rapidly oxidize metals and thermal-shock ceramic when cool fuel suddenly lands on orange-hot refractory,
 
 
  DOBSON GASIFIER DRAWING
 

This new design is superior in several ways:

    • The hopper and entire construction is cylindrical, creating more even feed and flow, less thermal stress, simpler construction, more reliable seals.
    • A small fraction of the generated gas is burned in a concentric combustion shell, which feeds heat to the incoming air and fuel in the hopper, augmenting the calorific value of the produced gas without diluting it with exhaust, thereby requiring less air for more gasification of a higher quality gas.
    • Highly preheated gasification air is introduced to the preheated fuel through large openings between conical slats that create the desired steady-state fuel feed without disruption of the simmering coal bed.
    • Any fuel that will fit in the hopper and produce a combustable gas can be gasified ~ logs, chips from the tree trimmers, bark, sawdust, corn cobs, leaves, wood-, straw-, municipal solid waste-pellets, green and wet biomass (up to 2/3 water), household and farm waste, etc.
    • A conical grate at the base rotates to dump ashes and break up any bridging.
    • Ideally, a microprocessor controller monitors temperature, gas quality, changing conditions at the grate base of fuel column, air preheating, etc. to optimize gas generation quality and quantity, dramatically reduce thermal shocking & metal deterioration, and can also be adjusted to create maximum biochar production instead of CO + H2 if desired.

The technical drawings associated with this design only show the basic concepts, leaving details such as interpenetrating gas and air ducting, lid, seals, dampers, etc. up to the innovation and skills of the fabricator. Initial prototypes may most easily be built of 1/8” 304 stainless interior (cheapest high chromium), including the combustion shell, shown in the drawings as ceramic.  Many variations and degree of refinement are possible, beginning with a simple insulated inner shell and grate/air-intake/ash cone configuration. Adding an air-preheating/cas-cooling shell will substantially improve performance.
There will be areas of thermal deterioration and other preliminary details to be refined down the road, but I hope this gets the show on the road!

    I’m trying out a more direct approach to materialize my inventions.  Rather than involve corporations, grants, entrepreneurs, banks, taxes, patents, and protection in all the ways sanctioned accountability reaps its share and retards the process, I am going as direct as possible, which is to fully share my gifts wherever I dare give up security, in this case these drawings.  Can the people manifest directly, as a government of the people, in a new creative spontaneous sharing, in a far more satisfying directness?  Ah.....life is such a gamble and discovery these days!

    This is no longer a patentable design, as I hereby release it to the public domain, in the hopes that others will implement these new design features and do the R&D work necessary to bring biomass gasification to a new level of practicality, offering a greater contribution to decentralized alternative energy technologies.

    If you get inspired to build a gasifier incorporating some of these new features, please share your experience and photos with me. I am available for consulting and further design work.

     

    .Warm regards,
     

    Larry Dobson
    dancer@stiltman.com
    360-579-1763
    7118 Fiske Rd
    Clinton, WA  98236