[Bioconversion] Retting and Binders

[Jeff Davis]

Dear list,

RETTING AND PRESSING
  
 Partially decayed and processed cellulosic materials give a much
 higher heating value than if the materials are simply dried.  For
 example, dried rice straw (10 percent moisture content) has a
 heat value of only 3,000 BTU/pounds (7 million joules/kilogram
 [J/kg] or 0.0698 gigajoules/kilogram [GJ/kg]), but this will
 increase to between 7,500 (17.4 million J/kg or 0.0174 GJ/kg) and
 12,000 (28 million J/kg or 0.0279 GJ/kg) when the material has
 partially rotted before it is dried.   In the Philippines, the
 MAPECON research group has set up a pilot plant producing such
 fuel, with 25 percent moisture content and an average of 10,000
 BTU/pounds (23 million J/kg or 0.0232 GJ/kg) which they call
 `green charcoal,' at the rate of one ton per hour.   The group
 reports that it is very competitive with other types of fuel.
  
 Retting--soaking in water for several days or longer at normal
 air temperatures--allows chopped, moistened woody residues to be
 biodegraded (partially decayed).   This process is used to produce
 mats that can be pressed into fiberboard, but a simple hand press
 can also be used to make briquettes from retted agricultural
 residue or wood wastes.  The lever is made from steel pipe and the
 timber mold has holes on each side to allow water to escape
 during pressing.





Binders
  
 To produce satisfactory briquettes economically, the binding
 substance must meet certain requirements.   It must produce a
 briquette strong enough to withstand damage during transport,
 storage, and stoking.  Exposure to weather must not cause crumbling
 or softening and, during use, the heat must not cause disintegration
 and loss of fine pieces through the grates.   It must
 burn without smoke and unpleasant smell and not be too dusty.
 Ideally the binder should have as high a heat value as the charcoal.
  
 Binders fall into three categories:   inorganic materials, organic
 materials, and fibers.
  
 o    Inorganic materials, such as cement and silicate of soda
      are appropriate for wood fuel.  These substances are poor
      because they give more ash, reduce the heat value, and fall
      apart while burning.
  
 o    Organic materials such as tar, pitch, resin, and glue
      usually increase the heat value and create no extra ash.
  
 o    Various types of fibrous material may serve as binding
      agents.   The cheapest is hydrated wood fiber-wood waste--ground, 
      pulped wood waste, which, when dry, binds together
      in the same way as paper.
  
 Some binders permeate the material to be briquetted; others coat
 the surface.  Starch binders, such as cassava, corn, and others
 are smokeless, but not moisture resistant.   They are normally used
 in the proportions of four percent (dry basis).   Tar, pitch,
 asphalt, and sugar cane molasses are used in less than 30 percent
 of the cases.  They are moisture resistant but not smokeless.  This
 is no drawback in industrial uses, such as smelting and heating,
 but would be inappropriate for home fuel or cooking.
  
 Secondary distillation (heating a second time) can drive off the
 smokey gases, but increases cost and does not completely remove
 objectionable smells during burning.   A good smokeless charcoal is
 one that contains at least 75 percent fixed carbon and not more
 than 24 percent "volatile" (able to be emitted as gases) matter


http://sleekfreak.ath.cx:81/3wdev/VITAHTML/SUBLEV/EN1/WOODFUEL.HTM

-- 
Jeff Davis
Somewhere 20 miles south of Lake Erie, USA
http://www.velocity.net/~jeff0124