Biomass and Biofuels

MECH 597025 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%

• 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.

Critical issues

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).