Fuelling the debate01 December 2005

With the price of fuel soaring in the UK - and diesel hovering around the £1 per litre mark - the time may well be right to look at viable alternatives. Biofuels seem to be attracting the headlines, so Plant Engineer felt it would be worth finding out exactly what they have to offer.

The overall focus of this article is on biodiesel, the most widely used of the biofuels in the UK, with the objective of guiding readers through some of the current happenings in this particular field.

Terminology: the terms differ relative to their end uses, whether that be for transportation, electric power or heat, and products such as chemicals and materials.

Bioproduct: short for biomass products and can be used to describe a chemical, material or other product derived from renewable biomass resources.

Biofuel: short for biomass fuel, namely liquid fuels for transportation, such as ethanol and biodiesel. These can be purely from biomass, such as B100 or, in part, such as E10 (the number after the letter represents the percentage of biodiesel or ethanol in the fuel).

Biomass: any sort of vegetation from which energy can be extracted. Biomass can produce electricity, heat, liquid fuels, gaseous fuels and a variety of chemicals, including those currently manufactured from conventional fossil fuels. In the UK, The Energy White Paper covers the use of biomass for generating heat/power or conversion in to road fuels.

Renewable: as plants produce oils from sunlight and air, and can do so year after year, these oils are renewable. Animal fats are produced when the animal consumes plant oils and other fats, and they are also renewable. Used cooking oils are both recycled and renewable.

Bioethanol and Biodiesel

Bioethanol: widely used in North America and Brazil. Ethanol can be produced from any source containing sugar or materials that can be converted into it by:
- the fermentation of sugar derived from grain starches (wheat & corn), sugar beets or sugar crops using micro organisms
- the fermentation of the non-sugar lignocellulosic fractions of crops (grasses & trees)
- using waste biomass, such as crop residue, forestry waste, municipal waste and food processing waste.

In the UK, negligible quantities of bioethanol were used in transport until the introduction of a duty incentive in January 2005, but since then growth has been faster than that of biodiesel (graphs). New purpose-built facilities are planned to overcome the problem relating to bioethanol's affinity for water. This requires blending at specialist terminals (unless converted to ETBE, as in France) and cannot be shipped through the UK's existing multi-product pipeline system.

Biodiesel (fatty acid alkyl esters) is made from natural, renewable sources, such as new and used vegetable oils, animal fats and recycled cooking oils, the most common sources being soybeans (US) & rapeseed (Europe).

Blends: biodiesels can be used in neat form (B100), or blended (up to 20%) with diesel. The vehicle and vehicle component manufacturers, together with the oil and biofuels producers, have developed a British/European standard (BSEN14214) for 100% biodiesel and as a blending component with ultra-low sulphur diesel (ULSD). The standard requires that at least 96.5% of the oil is converted to methyl esters.

The British/European Standard for diesel, BSEN590, now allows up to 5% biodiesel (meeting the above biodiesel standard BSEN14214) to be mixed with ULSD without affecting the manufacturer's warranty. When greater than 5% biodiesel is used, vehicle modifications are necessary. Existing vehicle fuel systems have not been designed to operate on biodiesel, which can be aggressive to certain seal materials.

The quality of biodiesel used in a blend is important, as research shows that unprocessed raw or refined vegetable oil or greases (at low levels) can cause long-term engine deposits, ring sticking, lube oil gelling and can reduce engine life.

To encourage the use of biodiesel, the UK Government currently has a 20p/l duty incentive on biodiesel (and bioethanol) - this is based on a 100% biodiesel content, reduced to around 1p/l for a 5% blend.

Manufacture: the most economic production process is base (sodium or potassium) catalysed transesterification of the oil with alcohol (usually methanol).

The manufacturing cost of biodiesel (and bioethanol) is estimated to be two to three times that for diesel, depending on: cost of feedstock, by-product values and other costs, including the costs incurred in their transportation (both feedstock and biofuel transportation is more expensive than for conventional diesel).

Although it is widely agreed that, due to their renewable nature, these fuels contribute to carbon dioxide reductions in their cycle, this argument can be offset by the additional energy required in their processing. Consideration should be given to the energy consumed in the manufacture of fertilisers and pesticides to produce the crops, plus the energy used in the cultivating, harvesting, transporting and processing of that particular crop. The research and figures vary, depending on the feedstock, processes and methodologies used.

Biofuels in Europe and the USA are promoted independently from their potential air quality benefits, because they are considered as a means to promote energy sustainability and reduce the dependence on energy imports.

Emissions: various studies on exhaust emissions have shown that, despite the lower life cycle CO2 emissions, biodiesel does not provide actual exhaust emission benefits. This is due to the advances in engine technologies that have already taken place, by way of compliance with the strict emissions requirements in the EU and USA.

In the graph below, it is possible to see that the emissions of Particulate Matter (PM), CO and HC decrease as the percentage of biodiesel increases. However, the emissions of NOx increase as biodiesel increases. Biodiesel reduces PM emissions due to its oxygen content, but increases NOx for the same reason.

In most cases, the use of pure biodiesel requires engine modifications to handle the lower energy content and material compatibility associated with its use. The current use of 5% biodiesel blends is not expected to bring any emissions difference.

Biodiesel benefits

Cetane: biodiesel has a higher cetane number (typically 45-60) than conventional diesel, thus helping to improve ignition quality and enabling quick starts for vehicles.

Lubricity: significant lubricity improvements help engine performance. Biodiesel tends to be the lubricity improver added to low sulphur fuels

Sulphur content: in neat biodiesel, there is low or no sulphur content. Sulphates are reduced as the percentage of biodiesel increases. This helps make oxidation catalysts more efficient in vehicles.

Other physical properties:
- No aromatics contents (and low PAHs)
- Higher viscosity
- Higher flashpoint makes them safer to handle
- No toxicity or low toxicity (in neat biodiesel fuels)

Biodegradable: The biodegradability of biodiesel is an obvious advantage, from the environmental point of view (fuel spills), but is also a drawback for everyday engine use. The higher the concentration of biodiesel in a fuel blend, the more susceptible the fuel is to degradation and water absorption

Biodiesel problems

NOx increase: using biodiesel results in an increase of NOx.

Fuel economy reduction: biodiesel will generally reduce the number of miles per gallon a vehicle can be driven, due to its lower energy consumption in comparison to conventional diesel fuel. Using B20, a reduction in fuel economy of 1 -2% can be expected. The greater the percentage of biodiesel, the higher the reduction in fuel economy.

Cold flow properties: the use of biodiesel increases cold flow properties, where the fuel can cloud or even gel in colder temperatures.

Material compatibility: biodiesel has been found to soften and degrade certain types of elastomers and natural rubber compounds over time.

Storage and handling: Biodiesel is hygroscopic. The humidity/water absorption then forms an emulsion and develops microbial growth. Even a moderate amount of water uptake can take the material or a subsequent blend out of specification. However, it has been found that adding the AV100 treatment to blends of up to 20% biodiesel prolongs the life of the fuel by slowing down the water absorption process.

Oxidation stability: biodiesel has lower oxidation stability than conventional diesel and is prone to oxidation, resulting in gum formation due to long-term or overventilated storage conditions.

Fuel filter plugging: The excellent solvency characteristics of biodiesel means it will dissolve existing gums and sediments in fuel storage tanks, which may cause fuel filter plugging.

- A recent study from CONCAWE, EUCAR and JRC (2004) provided some interesting comments about the emissions benefit of using biofuels that add to the overall debate. Certainly, there are so many options still being investigated on the cost/benefits, by-product usage and emissions, as well as on land use and biodiversity of biofuels, that this is only the beginning of the story.

SOE

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