Transport and sustainability
Transport and sustainability

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Transport and sustainability

Environmental impacts

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).

Figure 11 Harvesting sugar cane

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?

To use this interactive functionality a free OU account is required. Sign in or register.
Interactive feature not available in single page view (see it in standard view).


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).


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371