[Gasification] The New New Fuel

[Toby Seiler]

Bob,
   
  Go to http://vtwoodsmoke.org/pdf/Hill-79.pdf  and look at page 22.  The values given are on a dry basis.  Compare a cord of oak to a cord of white cedar.  Since the btu per pound is near the same (about 8,500-9,000 btu/lb), the difference is in the water content of the fuel (that is removed).  A cord of oak is twice the weight of a cord of cedar, thus cedar is carrying more lbs water until it is dried.  I point this out because one has to handle twice the volume of cedar to get equal heat value with oak.  And the weight then of the water removed is correspondingly higher in the cedar, placing a much greater load on any drying system for the cedar.  
   
  Fuel moisture content is the single most important factor in pyrolysis.  But since woods are harvested in the nearly saturated state (about 80% m.c.), there is a lot of water that must be dealt with.  I see no reason to think that grasses or hay would be any different.  This then seems to make a case for growing the highest density woods so less water will be handled.  It seems unrealistic to intentionally ignore the water content, because of the significant burden in drying it.  
   
  But then the issue of growth rate comes into account.  I don't have those numbers at hand but will be looking for them.  Growth rates in your location are based on the availability of sun, water and nutrients.  Some high water content woods may grow more biomass (even after drying).  In my example, Osage orange is the highest density wood that I know grows in North America, thus less handling of water would occur in the harvest for energy.  If the drying of a fuel is natural or air dried, these differences are less of a factor, but if one is using energy from the system, drying of less dense fuel will rob the energy away from productive uses (because the volume of water to be gotten rid of, is greater).  
   
  Note the difference in yellow pine and white pine.  Yellow pine has nearly twice the btu in a dry cord than white pine.  Is this a case for growing yellow pine over white pine?  The answer would appear to be in the growth rates and water removal system, including handling. 
   
  This then comes back to my point that a planned approach to fuel growth will take into account both growth rates and the amount of water that must be removed.  Just because a fuels annual growth appears high, the net heat gain should take into account the method of drying and that less dense woody plants will require twice or more volume (handling) to achieve the same heat output.
   
  As for your area, look to the density of woods, hay, etc and determine what weight is water.  If you purchase the material "wet", extract the cost to dry it and look at the "net" energy gain.  My belief is that the denser woody plants are a better feedstock unless your well equipped for solar drying (giving field dried hay an advantage if it does not get rewetted).
   
  Sincerely,  Toby Seiler
     
   
   

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