GEK Wiki / PyroCoil
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Page history last edited by jim mason 10 years, 11 months ago




The GEK PyroCoil is a gas circulating heat exchanger used to introduce external heat sources to the pyrolysis zone of a downdraft gasifier.  An external heat source for pyrolysis can significantly improve pyrolysis dynamics, resulting in tars which are easier to crack, as well as eliminating the thermal drag of pyrolysis from the combustion zone.  The external heat source of greatest interest to return to the gasifier is the exhaust gas from an internal combustion engine.  The PyroCoil can also be used to return hot flue gas from an external tar burning circuit.


Using external heat sources to assist pyrolysis is an old idea in gasification.  What is new here is a method to implement it easily, as part of the modular GEK gasifier system.  The PyroCoil heat exchanger is a double-jacketed shell which inserts into the standard GEK downdraft reactor, and bolts to the reactor top flange.  It can be used with or without the fuel feed Auger.  When used with the Auger, the PyroCoil replaces the doghouse lid that usually joins the Auger to the Reactor. 


The auger attaches to the large circular hole on the side.  The two square flanges and mamifolds are for hot gas in and less hot gas out (no, the cyclone does not go here, is just the same flange shape).  Internal baffles are used to circulate this gas down and around the double jacketed shell.  The small holes through the sides of the shell are for tar recirculation systems in the hearth zone.





Click here for gallery of bPyroCoil pictures




             Full Res of above left                                             Full Res of above middle                                                        Full Res of above right




The Details:


An external heat source for pyrolysis can significantly improve the pyrolysis dynamics of a downdraft gasifier.   A typical downdraft has a very shallow pyrolysis zone right above the nozzles of the combustion zone.   The pyrolysis zone is often only a couple inches deep.  The result is the raw biomass has a very short residence time to finish pyrolysis, and the pyrolysis temps are higher than ideal.  The short residence times means that fuel can often pass into the formal combustion and reduction zones before fully finishing pyrolysis.  In fact, it has been proposed that pyrolysis rate is the real determinate of max fuel flow rate in a downdraft.   And of course things only get worse the larger the fuel chunks get, given the insulative effects of both the biomass and char solids.


The result of this short and late pyrolysis is that tar gasses are evolving at very high temps, and thus recombining into larger chains and more refractory forms.  These secondary and tertiary tar forms are more difficult to crack downstream.  Tars produced at lower temps are easier to crack, requiring less energy and lower max temps to do so.  Thus it is to our advantage to keep pyrolysis temps as low as possible, and increase the time for pyrolysis completion as much as possible.



The IC exhaust is by far the largest source of "waste heat" in a gasifier/engine system.  There is more than 3x the heat available in IC exhaust as there is in the output gas from the gasifier.  (see energy balance work for details).  This heat is also at a higher temp than the gasifier output gas, thus better suited to run higher temp processes.   The external heat source can also be the flue gas from an external tar burning circuit on the gasifier.    Burning off some portion of our excess tar gas and returning the heat to the reactor (but not the gas), can give us significant leverage over the perennial problem of too much tar gas vs char produced from biomass pyrolysis (see mass flow work for details). 


Using IC exhaust gas as the heat source is easy, as it is self-regulating (the gasifier and engine flow rates vary in sync).  Using an external tar burning circuit is more difficult as active control of the tar pull/burn rate is needed.   Either/or and/or both can be used together, as one finds of interest.




Here's the .dxf file of the above for cutting or deriving specific dimensions









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