6.2 Biofuels
Liquid biofuels, derived from plant or animal sources, have the potential to be carbon neutral. In theory, the CO2 emitted during the processing and use of the fuel could be balanced by CO2 absorption from the atmosphere during the fuel crop’s growth. However, in practice this is rarely the case as growing the biomass currently requires the input of fossil fuels for fertilisers, harvesting, crop processing and fuel distribution.
Such biofuels can be produced by the fermentation of energy crops or from vegetable oils or animal fats.
Biofuels can come from a wide number of sources, and include:
- bioethanol , an alcohol that can be produced from virtually any fermentable source of sugar.
- methanol, an alcohol that is normally produced from natural gas, but can also be produced from biomass. It has the disadvantage of being poisonous and requires careful handling.
- biodiesel, which is most commonly produced from energy crops such as oilseed rape or recycled vegetable oils. Although many vegetable oils can be burned directly in road diesel engines, most are chemically modified to produce biodiesel by a process called transesterification. This turns the oils into chemicals called fatty acid methyl esters (FAME).
- biogas, which is a mixture of methane and carbon dioxide gases. This can be produced from farm or food waste or sewage sludge by anaerobic digestion in the absence of oxygen.
Practical use
Transport biofuels have been widely used in many countries, primarily to reduce dependence on imported oil. They have been promoted in the USA ever since the oil crises of the 1970s and the USA is now the world’s largest consumer of them.
Although ethanol may be used in conventional petrol engines, it is not a complete ‘drop-in’ replacement. It is normally sold blended with petrol. In the UK, since 2021, petrol may contain up to 10% ethanol (this blend is called E10).
The effect of this is to reduce the CO2 emission factor of pump petrol. In 2022, the average UK emission factor for petrol was 2.18 kg CO2 per litre compared to 2.3 kg CO2 for petrol with no added biofuel (BEIS, 2022).
Much higher proportions of ethanol are used in other countries. For example, 85% ethanol (E85) is used in Brazil.
Since the ethanol molecule contains some oxygen, blends of petrol and ethanol require a different fuel/air mixture and the ignition timing may need modifying. ‘Flexible fuel vehicles’ can change their engine settings automatically according to the fuel used.
Biodiesel may be used blended with conventional DERV or in a pure form (B100 = 100% biodiesel). It has a naturally low sulfur content which allows its use with catalytic clean-up systems to cut NOx and particulate emissions.
The actual extent of greenhouse gas emissions is strongly dependent on the type of energy crop grown and the amount of fossil fuel used for processing.
Indirect environmental impacts of biofuels
Although using the right sort of biofuel could yield a significant reduction in transport CO2 emissions, there have been serious criticisms in particular over the diversion of productive land from growing food and the destruction of rainforest for biofuel production.
As a result, transport biofuels from energy crops have fallen from favour in the EU and the UK. Given the alternatives of battery electric or hydrogen powered transport, the UK Committee on Climate Change has recommended that the use of biofuels in UK surface transport should be phased out in the 2030s (CCC, 2019). They suggest that its use in the UK should be restricted to the area most difficult to decarbonise, air transport.
However, the International Energy Agency sees a continuing expanding role for biofuels in transport. In its ‘Net Zero Emissions by 2050’ scenario, it projects world consumption of transport biofuels to more than double from 5 million tonnes in 2023 to 12 million tonnes by 2040 (IEA, 2024).
If you wish to explore this issue further, a useful online review of biofuel issues is provided on the explainthatstuff [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] website.