Ethanol as biofuel A biofuel is a fuel produced from plants Ethanol produced from fermentation is a biofuel. It can be argued that ethanol produced from this method is classed as carbon–neutral as any carbon dioxide given off when the biofuel is burnt would have been extracted from the air by photosynthesis when the plant grew. There would be no net CO2 emission into the atmosphere. This does not take into account any energy needed to irrigate plants, fractionally distil the ethanol from the reaction mixture or process the fuel. If the energy for this process comes from fossil fuels then the ethanol produced is not carbon neutral Removal of CO2 by photosynthesis 6CO2 + 6H2O C6H12O6 + 6O2 Production of CO2 by fermentation and combustion C6H12O6 2 CH3CH2OH + 2 CO2 2 CH3CH2OH + 6O2 4CO2 + 6H2O Equations to show no net contribution to CO2 6CO2 molecules are removed from the atmosphere when the plants grow by photosynthesis to produce one molecule of glucose When 1 molecule of glucose is fermented 2 molecules of CO2 is emitted. The two ethanol molecules produced will then produce 4 molecules of CO2 when they are combusted Overall for every 6 molecules of CO2 absorbed , 6 molecules of CO2 are emitted. There is no net contribution of CO2 to the atmosphere The term carbon neutral refers to “an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”
188.8.131.52 Alcohol production
Students should be able to:
• explain the meaning of the term biofuel
• write equations to support the statement that ethanol produced by fermentation is a carbon-neutral fuel and give reasons why this statement is not valid
• discuss the environmental (including ethical) issues linked to decision making about biofuel use.