Biodiesel is a fuel for conventional Diesel engines made from plant or animal oils or fats that have been chemically transformed into alkyl esters.
Lets take each part of this definition and break it down.
Biodiesel is a fuel for conventional Diesel engines…
Diesel engines are compression-ignition engines of the type most successfully developed by Rudolph Diesel. The fuel which we are used to calling ‘diesel fuel’ is just one of the possible fuels for this type of engine. In this manual we will call this fuel ‘petrodiesel’ or ‘mineral diesel’. It is also called ‘dinodiesel’ in the biodiesel community, although it is made not from dinosaurs but from ancient plants, mostly algae. Diesel engines have been made which will run on milk powder, coal dust, or straight vegetable oil. However the most common form of Diesel engine is designed to run on petrodiesel. This is the type of engine in most cars and commercial vehicles. When we make biodiesel, we must make a fuel that is suitable for these engines. Note that it is not possible to run petrol engines on petrodiesel or biodiesel.
…made from plant or animal oils or fats…
Biodiesel can be made from a very wide range of raw materials or ‘feedstocks’. They range from animal fats or tallows to vegetable oils including the common rapeseed, sunflower and peanut oils. Palm oil and coconut oil can also be used, as can tropical oil-bearing shrubs like jatropha. You can even make biodiesel from oil extracted from algae (cutting out millions of years of underground processing). To maximise the green advantage of biodiesel we make it from waste vegetable oil which has been used for deep-frying. In this manual we refer to this as WVO (waste vegetable oil). It may contain small amounts of animal fats, and many of the techniques we describe can be adapted to work with pure animal fats.
… that have been chemically transformed into alkyl esters.
It is possible to make a Diesel engine which will run directly on many plant oils. In fact one of Rudolf Diesel’s first engines was exhibited at the Paris exhibition of 1902 running on pure peanut oil. Even today, Elsbett of Germany make pure vegetable oil burning engines which are often used by German taxi drivers. However, conventional Diesel engines will generally not run well on pure vegetable oil. The reason for this can be summed up in one word: viscosity.
Viscosity: The “gloopiness” or “stickiness” of a fluid; its resistance to flow
Vegetable oil is much more viscous than petrodiesel, and conventional Diesel engines simply aren’t designed to deal with this. There are two ways around this problem:
- modify the engine to accept more viscous fuel.
This is a good option in its way, but it has some major downsides. On many engines, particularly recent ones, it is impractical. On older engines it may be possible but costly, in the region of £2000. This option is not one which we cover in this book.
- modify the fuel to reduce its viscosity
This is a more practical option for nearly all situations. Once we have decided to take this route, there are three major options:
- heating the oil: in this option we modify the fuel system of the vehicle so that the pure vegetable oil is heated on its way to the engine. This reduces its viscosity to nearer that of the petrodiesel the engine was designed for. This option is far more realistic than actually modifying the engine. However, it can still involve quite a large investment in one vehicle, around £500. It is also impractical for many more recent Diesels. This fuel is generally known as SVO (straight vegetable oil). It is not what we call biodiesel.
- mixing the oil with a thinner: there are a number of variations on this technique, which involves making a ‘micro-emulsion’ between vegetable oil and a solvent – for example white spirit, methanol, kerosene etc. This has the virtue of simplicity, however there is little research into the effects of this sort of blend on environmental impacts and engine life. This sort of fuel is known by a number of terms including emulsion, and MWVF (modified waste vegetable fat). In press reports it is sometimes called biodiesel. However it is not what we call biodiesel
- chemically transforming the oil to make it thinner: vegetable oil is viscous because its molecules are built around glycerol. If you chemically alter the vegetable oil to replace the glycerol with something less viscous you get a fuel which fits the specifications of conventional Diesel engines. The chemical process is known as transesterification. The glycerol is replaced by a simple alcohol such as methanol or ethanol to produce a product known as alkyl esters. This process is the subject of this manual and produces what we call biodiesel.
Remembering the key issue of viscosity will make it easier to remember the distinction between the various ways of running a Diesel engine on vegetable oils, which are often confused by beginners and journalists.
Biodiesel is sometimes called RME (rape methyl ester) when it is made from new rapeseed oil (the most common feedstock in large European biodiesel factories). A more general term is FAME (fatty acid methyl esters), which covers any feedstock. Both terms assume that methanol is used as the replacement for glycerol in the reaction, though ethanol and isopropyl alcohol can also be used in theory.
Different types of plant oils
Many types of plant oil can be used to make biodiesel, including coconut, peanut, sunflower, rapeseed, oil palm, soya bean, linseed, olive and hemp.
Rapeseed is called canola in the US. It’s what’s growing in the bright yellow fields you see in the summer. Rapeseed oil has no saturated fat, which is very good, as this is what makes oil solidify (sunflower oil has 8% saturated fat). So rapeseed oil is probably best for making biodiesel. Waste cooking oil from takeaways, or oil whose label just says ‘vegetable oil’ or ‘cooking oil’ is often rapeseed oil (other oils such as sunflower or olive will state which plants they are from). It has an edible version and an industrial version, sometimes called HEAR (high erucic acid rape).
Rape yields around 2 tonnes of oil per hectare (pressed from 4 tonnes of harvested seed), but also produces 4 tonnes of straw per hectare, which many boilers can now use as a fuel.
This is a good yield, but the champion is the African Palm. Palm oil can yield 5 tonnes of oil per hectare – but not in the European climate. Also, palm oil is solid at room temperature.
Rape is the clear winner in temperate regions. It grows well in a temperate climate; it has double the yield of most of the competition, and is liquid at room temperature.
The future of biodiesel production probably lies with algae though. Algae could produce (in ponds) up to ten times more oil per hectare than rapeseed. And algae love carbon dioxide; they gobble it up so fast that they could be of huge assistance in reducing global warming. They could even utilize the carbon dioxide emissions from industry. Each factory could have an algae pond to eat up its carbon emissions and produce oil. Algae can even grow in salt water or wastewater, so they don’t need to compete with agriculture for fresh water.
Waste vegetable oil
Throughout this guide we are focusing on making biodiesel from waste vegetable oil (WVO). This is oil that has previously been used for deep-frying and is disposed of as a waste product by catering outlets. The techniques we describe for making biodiesel apply in most cases to both WVO and fresh vegetable oils.
In the UK, using WVO is the best way to maximise the potential of biodiesel as a greener and cheaper fuel. Utilizing a waste resource saves energy and reduces pressure on landfill.
Biofuels and fossil fuels
Both biofuels and fossil fuels are ultimately derived from living things. The distinction between biofuels and fossil fuels is essentially one of time. Fossil fuels come from plants (making coal) and tiny sea creatures (making oil and gas) that died millions of years ago, and were laid down in coal deposits, and in oil and gas fields. The key difference is that because of the time scale involved in the creation of fossil fuels, they are in effect finite – i.e. not renewable in our lifetimes (or indeed a million lifetimes). When they’re gone, they’re gone.
Biofuels on the other hand, are renewable. We can just plant more, and if we plant them as fast as we use them, they are a sustainable resource.
Biofuels can be in liquid form, like biodiesel, or can be burnt solid, like wood. Boilers have been developed that can take straw, woodchips or pellets, or miscanthus (a species of tall, heavy grass). In this form they are known as biomass. Plant matter can also be gasified, and biogas (methane) from landfill, or decomposing organic matter, is also a biofuel.
Bioethanol (100%) has been used successfully in vehicles, notably in Brazil, where it is made from sugar cane. It is produced in liquid form for use in petrol vehicles. In the long term, this may prove more efficient than producing biodiesel, but equipment will have to be changed – modifications are needed to petrol engines, and it can’t be made very cheaply in the UK. This is not the case with biodiesel, and so production of a biofuel that can be used in existing equipment is very useful, even if it turns out to be short term. It also gets people used to the idea of biofuel, which involves a completely different way of thinking (cyclical, renewable) from fossil fuels (linear, non-renewable).