RL Stevenson Presentation Biological Fuels

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RL Stevenson PresentationBiological Fuels

• Daniel M. Jenkins
• University of Hawaii, Manoa
• April 27, 2007

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Why Use Biologically Derived Fuels?

• Finite fossil fuel reserves (energy crisis)

• Environmental impacts of fossil fuel combustion
• -release of sequestered CO2
• -climate change
• -volatile organic compounds, aromatic compounds,
  hydrocarbon spills

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What are Biological Fuels?

• Fuels derived from biological materials (e.g.
  plants) or processes (e.g. methanogenic
  bacteria)

• Examples
• -Wood (undegraded cellulosic material)
• -Methane
• -Hydrogen
• -Ethanol
• -Oils (triglycerides)

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Biological Fuels

• Wood (and other undegraded plant materials)

-High energy content, low processing
requirements -Often used for heating energy, and
sometimes to power the boiler for steam
turbine, but
-High ash content, high NOx -Rate of combustion
difficult to control
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Biological Fuels

• Wood (and other undegraded plant materials)

-Can be processed to yield pure carbon (e.g. UH
flash carbonization process -Carbon (e.g. coal)
burns hotter, thermodynamically more efficient
for generating electricity, but still
-High ash content -Rate of combustion difficult
to control
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Biological Fuels

• Methane (often results from anaerobic
  decomposition of organic matter)

-Can be recovered as off-gas from landfill and
waste treatment operations, or from dedicated
methane generating fermentation processes
-Some coevolved gases may be corrosive (e.g.
H2S) further processing/ purification may be
required
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Biological Fuels

• Hydrogen

-Under certain conditions, may be derived from
photosynthetic bacteria or algae (e.g., see
Juanita Matthews thesis defense, Agr. Sci 219,
300 PM today)
-Hydrogen production confers no biological
benefit to organism, so difficult to sustain
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Biological Fuels

• Ethanol

-Produced by anaerobic fermentation of sugars by
yeast
-Controvertial are energy inputs into
cultivation and fermentation processes recovered?
Should we process material that people can eat?
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Biological Fuels

• Ethanol

-Usually only small portion of plant has sugars
directly available for fermentation
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Biological Fuels

• Ethanol

-Alternative is to use cultivate plants with
higher fermentable sugar content
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Biological Fuels

• Ethanol

Amylose (starch)- very easy to degrade to glucose
Cellulose (problem- very difficult to hydrolyze
bonds)
-Better alternative is to convert cellulosic
materials to fermentable sugars- to enable
utilization of all manner of crop residues
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Biological Fuels

• Oils

-Typically concentrated in seeds of grains,
legumes, and trees (e.g., corn, sunflower, soy,
peanut, olive, etc)- so quantity is limited,
but
-Vegetable oils are already extracted for food
industry, and waste vegetable oil is readily
available!
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Biological Fuels

• Oils

-Waste vegetable oil (after filtering, separating
from water, free fatty acids, etc) can be burned
directly in modified engines
-Waste vegetable oil can be chemically converted
to biodiesel, which can run an unmodified
diesel engine
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Biological Fuels

• Making Biodiesel

-vegetable oil is composed of triglycerides-
groups of three fatty acids esterified to glycerol
Triglyceride
Fatty acids
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Biological Fuels

• Making Biodiesel

-transesterification of tryglyceride with
methoxide results in biodiesel and glycerol

(Potassium methoxide, prepared in advance by
addition of KOH to methanol)
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Biological Fuels

• Making Biodiesel

-biodiesel (hydrophobic) and glycerol
(hydrophilic) are immiscible- easy to separate at
completion of transesterification
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Biological Fuels

• Making Biodiesel

-Reactor for transesterification have controlled
temperature and agitation
Temperature control provides activation energy to
accelerate the reaction
Agitation improves contact area between
immiscible reactants, and improves mass transport
at phase boundaries
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Biological Fuels

• Making Biodiesel

-Process considerations
Stoichiometry of reactants must be close
-excess hydroxide (KOH) causes saponification
and increases amount of free fatty acids
(corrosive) -excess triglycerides result in mono
and diglycerides which are difficult to burn
cleanly
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Biological Fuels

• Making Biodiesel

-Safety considerations
Methanol vapor is hazardous- need vapor trap?
Use ethanol as an alternative?
Pressure vessel?
Combustible materials?