RL Stevenson Presentation
Biological Fuels
Daniel M. JenkinsUniversity of Hawai‘i, MānoaApril 27, 2007
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’
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)
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
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
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
Biological Fuels•Hydrogen
-Under certain conditions, may be derived from photosynthetic bacteria or algae (e.g., see Juanita Matthews thesis defense, Agr. Sci 219, 3:00 PM today)
-Hydrogen production confers no biological benefit to organism, so difficult to sustain
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?
Biological Fuels•Ethanol
-Usually only small portion of plant has sugars directly available for fermentation
Biological Fuels•Ethanol
-Alternative is to use cultivate plants with higher fermentable sugar content
Biological Fuels•Ethanol
-Better alternative is to convert cellulosic materials to fermentable sugars- to enable utilization of all manner of crop residues
Cellulose (problem- very difficult to hydrolyze bonds)
Amylose (starch)- very easy to degrade to glucose
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!
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
Biological Fuels•Making Biodiesel
-vegetable oil is composed of triglycerides- groups of three fatty acids esterified to glycerol
Fatty acids:
Triglyceride:
Biological Fuels•Making Biodiesel-transesterification of tryglyceride with methoxide results in biodiesel and glycerol
CH3 O-
K+
+
(Potassium methoxide,
prepared in advance by addition of KOH
to methanol)
O
...
O
CH3
Fatty acid methyl ester (biodiesel)
+
OHOH
OHglycerol
Biological Fuels•Making Biodiesel-biodiesel (hydrophobic) and glycerol (hydrophilic) are immiscible- easy to separate at completion of transesterification
O
...
O
CH3
Fatty acid methyl ester (biodiesel)
+
OHOH
OHglycerol
Biological Fuels
•Making Biodiesel
-Reactor for transesterification have controlled temperature and agitation
O
...
O
CH3
Fatty acid methyl ester (biodiesel)
+
OHOH
OHglycerol
Temperature control provides activation energy to accelerate the reaction
Agitation improves contact area between immiscible reactants, and improves mass transport at phase boundaries
Biological Fuels
•Making Biodiesel
-Process considerationsO
...
O
CH3
Fatty acid methyl ester (biodiesel)
+
OHOH
OHglycerol
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