Conventional petrol and diesel vehicles have less impact on the environment if they are highly fuel-efficient – in other words, if they have high fuel economy, 'miles-per-gallon' or 'mpg'.
Bioethanol is an alcohol which is made from plants (biomass). Sugar cane, sugarbeet and cereals (wheat and barley) are the most common sources of the fuel. The production first uses enzyme amylases to convert a feedstock crop into fermentable sugars. Yeast is then added to the 'mash' to ferment the sugars to alcohol and carbon dioxide, the liquid fraction being distilled to produce ethanol.
Being a liquid at room temperature, bioethanol can be handled in a similar way to conventional petrol. Bioethanol can be used in spark-ignition engines with little or no modification as a low percentage alcohol-petrol blend ('E10' is 10% ethanol) or as pure alcohol fuel in modified vehicles.
To convert a conventional spark-ignition engine vehicle to run on pure bioethanol requires adjustment of the ignition timing, and the fitting of a larger fuel tank due to the fuel's low energy density. As alcohol fuels degrade certain types of rubber and metals, some engine components may also need to be replaced.
Pure bioethanol is difficult to vaporise at low temperatures, so it is usually blended with a small amount of petrol to improve ignition (E85 is a common high percentage blend). Several manufacturers now offer 'Flex-Fuel Vehicles' or 'FFVs', which are able to run on any percentage of bioethanol blend up to E85 (see below).
Biodiesel is commercially produced by the 'esterification' of energy crops such as oil seed rape or from waste vegetable and animal oils (from the food industry). The oils are first filtered to remove water and contaminants and are then mixed with an alcohol (usually methanol) and a catalyst. This breaks up the oil molecules before they are separated and purified.
Low percentage biodiesel blends (B5) can be used in place of mineral diesel without any engine modification in many diesel engines (a 'B5' blend is 5% biodiesel mixed with 95% mineral diesel).
While some diesel cars will also run on higher percentage biodiesel blends, their use can degrade rubber products (such as fuel pipes) and clog fuel injectors in certain conditions. To reduce the risk of these problems, users of ester-based biodiesels should ensure the fuel's compliance with EN14214.
Important notice: use of biodiesel blends over 5% usually invalidates a vehicle's warranty, so you should check with your vehicle manufacturer/supplier before using biodiesel at more than 5% concentration.
Both biodiesel and bioethanol are liquid at room temperature, and so can be dispensed from fuel pumps in the same way as conventional liquid fuels. It is likely that you have already driven on ethanol as it is routinely added to petrol (as a 5% blend) to improve octane ratings and as an oxygenate additive (to reduce carbon monoxide emissions).
Commercially produced higher percentage blends are still relatively new to the UK, and the distribution of biofuel pumps (at standard fuel stations) is therefore not as established as conventional mineral fuels.
Of the two fuels, biodiesel is more widely available across the UK (see below). A small network of bioethanol stations were being established by the supermarket chain Morrisons, however, a review of Government subsidies reduced the economic viability of these stations which were eventually withdrawn. As a result, bioethanol filling station are virtually non-existent.
The great promise of biofuel is its potential to be 'carbon-neutral' with all the carbon dioxide emitted during use of the fuel being balanced by the absorption from the atmosphere during the fuel crop's growth. However, in practice the process of growing the crop requires the input of fossil fuels for fertilisers, harvesting, processing and fuel distribution.
Taking into account carbon dioxide and nitrous oxide emissions (associated with agriculture) for oil seed rape sourced biodiesel and sugar cane bioethanol, studies show that lifecycle greenhouse gas emissions can be reduced by around 60% and 90% respectively.
This means a 5% biofuel blend would result in a carbon reduction of around 2.5% (biodiesel) and 4% (bioethanol). Much greater emission reductions are possible for biodiesels if waste oils are used, as the sources material would otherwise be thrown away.
Regarding regulated emissions, tests show that biodiesel particulate emissions are lower than from mineral diesel. Its low sulphur content also increases efficiency of exhaust control systems, reducing carbon monoxide and hydrocarbon emissions.
For high percentage bioethanol fuels, tailpipe carbon monoxide, particulates and hydrocarbons are generally reduced – however, some unregulated emissions such as aldehydes are actually increased.
Car purchase costs are unaffected by using low percentage biofuel blends as no engine modifications are required. Indeed, standard petrol and diesel fuels often contain up to 5% biofuel as part of their specification. The only caveat is that, as already mentioned, use of biodiesel blends over 5% can invalidate a vehicle's warranty, so you need to check with your vehicle manufacturer/supplier before using biofuels at more than 5% concentration.
While petrol cars can be modified to use high percentage bioethanol, which costs several hundred pounds, most bioethanol cars are Flex-Fuel designed to operate on any percentage of bioethanol up to 85% (E85). Where available, these tend to be similarly priced to conventional (petrol) models.
Due to economies of scale, forecourt prices of biofuel blends tend to be higher than those of conventional mineral fuels, depending on the strength of the blend. Another drawback is that both biodiesel and bioethanol have lower energy content than petrol or diesel, so more fuel by volume is required per mile. The result is that using commercially produced high percentage biofuels can increase fuel costs pert mile.
Biodiesel is the most widely available biofuel in the UK – the website Biodiesel Filling Stations provides a list of UK biodiesel outlets (for all higher percentage blends). In contrast, there are almost no bioethanol stations in the UK since the supermarket chain Morrisons stopped supplying E85 in 2010.
For bulk biofuel buyers, a number of UK companies are now supplying blended and pure biofuels to order. One of these is ATX Fuels who offer B100 complying to the EN14214 standard.