Biofuels are liquid fuels made from recently living material such as corn, sugarcane, soybeans, algae, used cooking oil, and organic waste. They matter because they can power engines, trucks, farm machines, and some generators using carbon that was recently taken from the atmosphere by plants. Ethanol and biodiesel are two major biofuels used today, often blended with gasoline or diesel fuel.
A biorefinery is the machine system that turns messy biological feedstocks into cleaner, usable fuel.
Key Facts
- Photosynthesis stores solar energy in biomass: 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
- Ethanol fermentation converts sugar into alcohol: C6H12O6 -> 2C2H5OH + 2CO2
- Distillation separates ethanol from water because ethanol boils at about 78 °C and water boils at 100 °C at 1 atm.
- Biodiesel is commonly made by transesterification: triglyceride + methanol -> biodiesel + glycerol.
- Energy efficiency can be estimated by efficiency = useful energy output / total energy input.
- Biofuels are renewable only when feedstocks are regrown or waste streams are replaced at about the same rate they are used.
Vocabulary
- Biofuel
- A biofuel is a fuel made from biological material such as crops, algae, plant waste, or animal fats.
- Feedstock
- A feedstock is the raw material that enters a processing system, such as corn, sugarcane, soybean oil, or used cooking oil.
- Fermentation
- Fermentation is a process in which microorganisms break down sugars and produce ethanol and carbon dioxide.
- Transesterification
- Transesterification is a chemical reaction that converts oils or fats into biodiesel using an alcohol and a catalyst.
- Biorefinery
- A biorefinery is a facility or machine system that converts biomass into fuels, chemicals, heat, or electricity.
Common Mistakes to Avoid
- Assuming all biofuels are automatically carbon neutral, which is wrong because farming, fertilizer production, transport, and processing can release greenhouse gases.
- Confusing ethanol with biodiesel, which is wrong because ethanol is an alcohol usually blended with gasoline while biodiesel is made from oils or fats and used in diesel engines.
- Ignoring water removal in ethanol production, which is wrong because fermentation makes a dilute ethanol mixture that must be distilled or dehydrated before use as fuel.
- Treating waste oil as ready-to-use biodiesel, which is wrong because oils must be cleaned and chemically converted so they have proper viscosity and combustion properties.
Practice Questions
- 1 A fermentation tank starts with 180 kg of glucose. Using C6H12O6 -> 2C2H5OH + 2CO2, estimate the theoretical mass of ethanol produced. Molar masses: glucose = 180 g/mol, ethanol = 46 g/mol.
- 2 A small biodiesel processor uses 50 L of vegetable oil and produces 45 L of biodiesel. What is the percent volume yield?
- 3 Explain why a biofuel made from used cooking oil can have a different environmental impact than a biofuel made from crops grown only for fuel.