Compared to conventional fossil fuels, CO, HCs and particulates are generally reduced for the E85 mix (85% ethanol and 15% petrol), M85 methanol blends and pure-alcohol fuels. Air-quality emissions for biodiesel are also reduced when compared to mineral diesel – comparative tests suggest that particulate emissions are 10–15% lower than with ultra-low sulfur mineral diesel. Biodiesel's low sulfur content also allows the use of advanced emission control systems, which can further reduce particulates. However, some pollutants are increased when using biofuels, including higher NOx for biodiesel.
The great promise of biofuels is their potential to be carbon neutral, as all the CO2 emitted during the processing and use of the fuel is theoretically balanced by CO2 absorption from the atmosphere during the fuel crop's growth. However, in practice this is rarely the case, as the process of growing the biomass requires the input of fossil fuels for fertilizers, harvesting, crop processing and fuel distribution. The actual extent of greenhouse gas emissions is therefore strongly dependent on the type of energy crop grown and the fuel processing used.
For example, in Brazil – where sugar cane is used as the feedstock for ethanol production (Figure 11) – large amounts of bagasse (the woody fibres that remain after the juice is extracted from the cane) are used to provide the process heat energy. As a result, the average energy ratio of ethanol output to fossil fuel input is of the order of six, i.e. six units of energy are produced for each unit input. Therefore, on a fuel life cycle basis, carbon emissions are significantly reduced – by up to 90%. This contrasts with the net energy ratio for corn-derived ethanol from the USA, which in some cases can be negative (i.e. the fossil fuel required to produce the ethanol is greater than the energy value of the final product).
Similarly, the results of life-cycle analysis of greenhouse gas emissions for biodiesel depend on the production processes employed. For biodiesel produced from waste oil, there is a substantial CO2 reducing effect – often by as much as 85%. For rapeseed biodiesel, the carbon benefits are around 40%, taking into account upstream emissions from the production of fertilizer (Concawe, 2007).
Biogas produced from a food waste anaerobic digester also has clear environmental benefits. This is set to be an increasing source of biogas, as there is a UK programme to build anaerobic digesters to reduce waste sent to landfill sites (see TheBioenergySite, 2009).
Overall, it appears that if the right sort of biofuel and production system is used then this is a potentially sustainable transport technology.
Activity 7 (self-assessment)
What is the carbon-reduction potential of biofuels?
Biofuels present a complex picture. If produced in the right way, some biofuels can be very carbon efficient; others are not.
Even for fuels that have a good carbon-reducing potential, there is an issue around the amount of biofuel produced (e.g. the level of CO2 emitted from waste oil or biogas is very good, but only a limited amount of these fuels is available).