Chemistry of Bioenergy

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Chemistry of Bioenergy

• Don Wells
• dwells_at_ppvir.org
• Talk prepared for
• Biomass Energy in the Home
• 2009-02-11

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A high-level view
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topics

• Gasification Fischer-Tropsch
• Cellulosic Ethanol via enzymes
• Pyrolysis
• Torrefaction

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Pyrolysis vs Gasification
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Gasification
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Gasification Fischer-Tropsch

• Biomass is confined with air (oxygen) and water
  (as needed), and is partially burned to raise
  temperature high enough to break essentially all
  molecular bonds
• The carbon, hydrogen and oxygen atoms recombine
  to H2 plus CO this mixture is called synthesis
  gas, aka syngas
• Ratio of H2 to CO set by biomass/air/water input
  ratios

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Gasification Fischer-Tropsch

• Syngas at high temperature pressure plus
  catalysts (iron, cobalt, nickel, ruthenium, etc)
  forms target chemicals e.g., ethanol, diesel,
  dimethyl ether (DME)
• (2n1)H2 nCO ? CnH(2n2) nH2O
• Choice of H2 to CO ratio, temperature, pressure
  catalyst determines the target produced

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Other uses of syngas

• Syngas can fuel gas turbines (IGCC)
• Syngas can be processed by bacteria to yield
  target chemicals, such as ethanol

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Gasification
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Syngas Gas Turbine
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Coskata syngas schematic
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A surprise (water _at_ 400-650C)
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Cellulosic Ethanol Enzymes
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Pyrolysis
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Pyrolysis

• Biomass is heated rapidly with little or no
  oxygen (air) added anaerobic
• Many molecular bonds are broken, gases and
  liquids are released, char remains

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Pyrolysis vs Gasification
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Pyrolysis Oil Char
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Torrefaction
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Torrefaction

• Torrefaction is a thermochemical treatment of
  biomass at 200 to 300 C. It is carried out under
  atmospheric conditions and in the absence of
  oxygen. In addition, the process is characterised
  by low particle heating rates (lt 50 C/min).
  During the process the biomass partly decomposes
  giving off various types of volatiles. The final
  product is the remaining solid, which is often
  referred to as torrefied biomass, or torrefied
  wood when produced from woody biomass.

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Advantages of torrefaction

• Get 70 of the mass retained as a solid product,
  containing 90 of the initial energy content,
  whereas pyrolysis yields only 55-65 of the
  energy content.
• Product is hydrophobic (store in open like coal)
• Product is superior cofiring fuel
• Torrefied wood can be used for metallurgical
  applications (biomass becomes more like coal ?
  steel from biomass?!)

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Torrefaction process reactions

• Drying 200C
• Torrefaction reactions 200-300C
• Devolatilization and carbonization of
  hemicellulose
• Depolymerization and devolatilization/softening
  of lignin
• Depolymerization and devolatilization of
  cellulose

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