Biomass and Biofuels
MECH 5970
25 April 2011
Background
• Biomass: material of recent biological origin.
• Provides (directly or via processing) HC fuel for
combustion applications.
• Very long history
– Wine, beer making: fermentation methods
– Wood (a biofuel) was the original fuel.
– Whaling: provided the original lamp oil.
Current status
2005-2008: ethanol is gasoline increased from 3.8 to 5.5%,
and biodiesel in diesel increased from 0.9 to 1.5%
Critical issues
• Competition of resources:
– Water
– Agricultural land
– Agricultural products: food vs. fuel.
• Effect of extensive energy crop farming on
ecosystem and climate.
• Net energy return: are fossil fuels saved?
• CO2 neutrality of biofuels is not always
obvious.
Types of biofuels
• Plant oil based (sunflower, palm, waste oil,…)
– Straight use of oil with minor modifications (water
removal, viscosity reduction) in diesel cycles.
– Biodiesel: transesterfication of plant oil to provide
fatty acid methyl ester (FAME).
– “Green diesel”: traditional fractional distillation of
plant oil.
Alcohol based fuels
• Ethanol from agricultural crops (carbohydrate
feed stock): corn, cereals, sugar cane, beets,
cassava,…
– Fermentation to ethanol/water mix.
– Distillation to ethanol.
Alcohol based fuels
• Ethanol from herbaceous and woody crops
(cellulosic biomass): grasses, corn stover,
wood,…
– Breakdown (digestion) of cellulosic biomass in
several steps to form sugars.
– Fermentation of sugars to ethanol/water mix.
– Distillation to ethanol.
• Methanol: chemical, rather than biochemical,
process.
Gas fuels
• Biogas: primarily CH4, CO2
– Produced by anerobic digestion and/or
fermentation of biodegradable materials.
– Swamp gas, landfill gas.
• Syngas: H2 and CO
– Produced by partial oxidation (pyrolosis) of
biomass.
Cellulosic ethanol
• Biological approach:
– Cellulose hydrolysis: pretreatment and digestion
of cellulose into sugars
– Fermentation to ethanol, followed by distillation,
purification
• Thermochemical approach:
– Partial oxidation of cellulose to CO, CO2, H2
– Fermentation using Clostridium ljungdahlii to
ethanol
Algae—based fuels
• Advantages over crop—based fuels:
– Better use of water resources (can use salt, waste
water)
– Theoretically yield 10-100 times more energy per
unit area than croplands.
– Simplified process: algae consume CO2, produce
fuel (oils or alcohols).