Forecourt Trader - 30 years at the heart of the fuel retailing community
A scientist's view of biodiesel
Published:  01 May, 2007

Biodiesel in some form or other is here to stay - if only to reduce the UK's dependency on petroleum - so we might as well take the time to understand the pros and cons of using this fuel and the cautions that apply.

== What is biodiesel? ==

Biodiesel is fuel made by chemically processing vegetable or animal fats. In Europe it is predominantly based on rapeseed oil chemically processed to give the fatty acid methyl ester, or FAME for short. Sunflower oil is also used. In the US biodiesel has been mostly based on the oil from soybeans which has a chemical structure leading to greater application problems, while in Newfoundland, for example, biodiesel has been based on fish oils.

== What are its advantages? ==

Obviously, for governments, the reduced dependency on imported crude oil as a source of transport fuel is paramount. For the environmentalist, the prime benefit is the recycling of carbon dioxide, reputedly 80% reduction in net carbon dioxide, according to some sources, and 60% reduction in others. This is just one of the environmental benefits.

Diesel engines are by design lean-burn engines, but with biofuels which contain oxygen within the chemical structure they are even more lean burn. That means reduced hydrocarbon, carbon monoxide and particulate emissions. Also, the fuel has no aromatics and no sulphur, so local air quality benefits. Biodiesel auto-ignites easily so its cetane number (the measure of ignition quality) is naturally higher than that of petroleum-derived diesel. This means less diesel knock and quieter engines on biofuels. Because of the fatty nature of the fuel it has good lubricity. In blends with petroleum-derived diesel it compensates for the removal of sulphur- and oxygen-containing compounds during processing, and provides anti-wear protection for pump and injector components.

These are all very positive aspects of biodiesel to which may be added the politico-economic benefits of oilseed growth for rural areas. Biofuels provide a way that governments can, productively, put more money into rural economies. A factor which is particularly important in countries such as France.

== What are its disadvantages? ==

It is at this stage that a little more science is called for. Like me, many in the motor trade will remember the early aroma of Castrol R based on castor oil. As a crankcase oil, this had terrific lubrication properties for protecting highly stressed racing engines but had to be changed very frequently because of its poor thermal and oxidation stability. Biofuels have the same inherent problem. This, plus the fact that they are hygroscopic, and so absorb water, makes for many of the difficulties experienced.

The fats/oils which form the basis of biofuel are called triglycerides. The fat molecules themselves are too large and too viscous to be used effectively as biodiesel. They are, therefore, converted into esters such as rapeseed methyl ester by a process called transesterification using methyl alcohol (methanol) and a catalyst which could be sodium or potassium hydroxide. By this means three smaller molecules of ester and one molecule of glycerine are obtained from one molecule of fat. The smaller molecules have a far better viscosity for diesel fuel. Unfortunately, unless the unreacted methanol, the catalyst material, the glycerine and other impurities are completely removed all sorts of engine and fuel system problems can occur, and these are compounded by the thermal instability, the reactivity and the absorbed water of biodiesel.

Here are a few of the problems experienced:

? corrosion of fuel injection equipment and non-ferrous metals;

? elastomeric seal failures;

? low-pressure fuel system blockage;

? fuel injector spray hole blockage;

? pump seizures due to high fuel viscosity at low temperature;

? increased injection pressure;

? fuel dilution and polymerisation of crankcase oil;

? formation of corrosive formic and acetic acid on ageing.

The corrosive nature of biodiesel can stem from impurities remaining after processing but also from the reversibility of the chemical reaction which produces the ester. So, under the right conditions, absorbed water can convert some of the ester back to fatty acid plus methanol. The acid can then react with metals. Two recent examples are the corrosion of a silver component of the in-tank fuel gauge mechanism, and the removal of the zinc protective layer from galvanised pipework carrying the fuel from storage. Bear in mind that the recirculating fuel from the modern common rail injection systems can be returned to the fuel tank at temperatures well over 100°C so the fuel tank itself becomes a reactor.

I didn't mention that the environmentalists also extol the use of biodiesel because it is non-toxic and biodegradeable. So petrol retailers and motorists can be proud that they will be giving a home to all sorts of micro-organisms that will happily feed on this fuel.

== The future for biodiesel ==

The current British/European Specification BS EN 590 already permits the incorporation of 5% FAME in the diesel as an extender. FAME must meet the European Specification EN 14214. The blend is termed B5. In the US, B20 biodiesel is already being used - in out-of-warranty vehicles, it must be assumed. Europe, with France and Germany in the lead, is considering a standard for B10.

This traps fuel injection equipment suppliers and motor manufacturers between a rock and a hard place - having to meet strict emission standards for their engines yet, at the same time, under pressure to allow higher proportions of biofuel in blends without invalidating warrantees. They are disturbed by the mechanical and chemical problems being caused by impurities and ageing of biodiesel and are seeking higher purity and higher thermal stability for the biofuel components of blends.

== Know what you are buying! ==

B5 can be sold in the UK as meeting the BSEN 590 specification without any labelling showing that the fuel contains a biofuel extender.

So the motorist, commercial customer, or petrol retailer is unaware that the fuel can have different properties from those expected.

At a recent BSI fuel standards committee meeting I raised the issue of unlabelled biofuel-containing blends in stand-by generators. Normally, non-biodiesel can be in tanks for years before use without being rotated. Emergency generators need to operate reliably and only by knowing what is being delivered can operators take the steps necessary to ensure that stand-by power is available when needed.

Petrol retailers, also, need to be informed if they are being supplied with a biofuel blend so that they can take steps to maintain clean and water-free storage tanks to avoid biological growths and corrosion. Another downside of biodiesel is that because it contains oxygen its energy content is not as great as that of conventional diesel. This may not matter much with B5 but will be more significant in higher biodiesel proportions and should be reflected in the price.

There is a long way to go in the biodiesel saga and many problems to solve. There is also considerable innovation in the pipeline. Remember the scheme for taking carbon dioxide from power station flues, liquefying it and injecting it into deep saline aquifers offshore?

A Massachusetts rocket scientist has come up with another solution, he concocted a green algae soup which was placed in clear tubes in a power station flue stream.

The algae grew happily, consuming by photosythesis 40% of the carbon dioxide and, as a bonus, 86% of the nitrous oxide as well. The algae is harvested daily and its oil extracted to make biodiesel leaving a green dry flake which can be converted to bioethanol. Algae could produce 15,000 gallons of biodiesel per acre compared to just 60 gallons from a soybean crop.

Rudolf Diesel demonstrated the first engine to run on peanut oil at the World Exhibition in Paris in 1900.

In 1912 he said: "The use of vegetable oil may seem insignificant today. But such oils may become, in the course of time, as important as the petroleum and coal tar products of the present time." Insight indeed!

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Weekly retail fuel prices: 15 January 2018
RegionDieselLPGSuper ULUL
East Midlands124.34132.31121.54
North East123.94133.63121.07
North West124.1658.50132.51121.18
Northern Ireland123.4169.90128.40120.85
South East125.1561.40132.52122.48
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West Midlands123.7465.23132.27121.20
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