A similar energy diagram is shown in figure 2 (below) for the production and combustion of biodiesel. For details of the chemistry and chemical structures involved click here.
Significantly, the figures take no account of the inputs of fossil fuel energy required in the production of these biofuels, including:
1. Agricultural inputs such as fertilisers, pesticides and farm machinery in growing and harvesting the crops.
2. The extraction and processing of starch and sugar solutions for fermentation.
3. Distillation of the aqueous solution of ethanol that is the product of fermentation followed by drying to obtain pure ethanol.
4. Extraction of oils and subsequent chemical processing to produce biodiesel.
There is disagreement about whether biofuel replacement of petroleum-derived fuels leads to CO2 emission reduction when the above factors are accounted for. A recent study indicates that bioethanol yields 25% more energy than that required to produce it—i.e. a positive net energy balance. For biodiesel the figure is 93%. Replacement also leads to reductions in total greenhouse gas emissions (CO2, methane and nitrous oxide)—12% in the case of bioethanol and 41% for biodiesel. The study also refers to earlier studies, most of which come to similar conclusions, although a minority conclude that more energy is required for production of the biofuels, especially ethanol, than is yielded by their combustion, thus questioning the sustainability of this strategy. Improvements in crop yields and efficiencies of biofuel production processes are likely to increase net energy balances and reductions in greenhouse gas emissions. In a recent innovation, corn has been genetically modified so that it is able to synthesise an enzyme that breaks down starch to simple sugars, thus reducing energy and financial costs of making ethanol from starch.
However, the competition between food and energy use is a more serious drawback to the use of the food crops in first generation biofuels. It has been argued by some as making a significant contribution to instability in food prices. To avoid the food-energy competition, alternative crops, which are inedible, have therefore been identified or inedible parts rather than the starchy parts of those crops used for first generation fuels have been explored to produce what are referred to as second generation biofuels.
Second Generation Biofuels
Certain non-food plants, e.g. the grasses switchgrass and Miscanthus, are particularly useful sources of biofuel since they grow on marginal farmland with low agricultural inputs. They provide cellulose, which can be converted to ethanol. Cellulose, the most abundant organic chemical on earth, is chemically bound up with lignin in lignocellulose, the complex matrix that largely comprises the cell walls of all plants. The cell walls of husks, leaves and stalks of cereal grain crops can also be used as sources of lignocellulose, which is broken down into lignin and cellulose by pre-treatment with steam or acid followed by degradation with enzymes. Like starch, cellulose is made up of molecules consisting of very long chains of linked glucose units. Significantly however, the glucose units are linked up in cellulose in a different way compared to that in starch. As a consequence, while starch molecules can be broken down by an enzyme in our digestive system, cellulose is indigestible (although some cellulose in the cereals that we eat is considered to be good for the flow of digestion products through our digestive system). To a large extent therefore, using cellulose as feedstock for bioethanol avoids food-energy competition.