rl stevenson presentation biological fuels daniel m. jenkins university of hawai‘i, mānoa april...
TRANSCRIPT
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
Biological Fuels
•Making Biodiesel
-Safety considerationsO
...
O
CH3
Fatty acid methyl ester (biodiesel)
+
OHOH
OHglycerol
Methanol vapor is hazardous- need vapor trap? Use ethanol as an alternative?
Pressure vessel?
Combustible materials?