A Renewable Alternative to Petro-diesel
|Bio-diesel is not only a good substitute for Petro-diesel but some think it smells nice too.
Biodiesel, a green and renewable fuel for motor vehicles.
As its name suggests biodiesel is a fuel oil derived from biological sources. In 2008 it was mainly derived from vegetable oils and recycled cooking greases or oils, and it is a viable alternative to traditional petro-diesel for fuelling diesel engines. It has other uses, for example in heating boilers, but our main concern is its use in cars and other motor vehicles. As a motor fuel, it may be used 100% pure (in compatible engines) or combined with petro-diesel in proportions as low as 2%. Although on combustion it releases greenhouse gases (mainly, but not only, CO2) it is described as carbon-neutral, since it is derived indirectly from living plant sources. It is argued that the plants absorb essentially the same amount of these gases during the growing stage and this is in stark contrast to fossil-based fuels.
There are two generic sources of biofuels (for both biodiesel and methanol) referred to as biomass feedstock . They are 'first generation' which comprise grain and other vegetable crops grown for their sugar, starch and oil content. Then there are the 'next generation' sources which comprise cellulose rich sources including wood, tall grasses, crop residues and municipal waste organics. The latter are dependent on developing technology to improve yields. Two technologies are favoured: gasification and hydrolysis using enzymes. These methods should enable next-generation to become competitively priced with first-generation sources but with the big advantages of offering the potential to dramatically increase the amounts of biofuels produced and with far less impact on the environment and ecology.
The benefits of biodiesel exceed its carbon-neutral property and some of the other advantages are identified below. Biodiesel can also be manufactured from animal oils and greases for which the arguments are different. However, if these oils and greases are normally discarded then taking them out of the waste stream and converting them to a fuel has obvious, substantial environmental benefits. First let's briefly see how it is manufactured then identify some limitations before identifying its main advantages.
|How is it manufactured, its byproducts and what does it cost?
Put simply, vegetable and/or animal oils, fats or greases are combined with methanol in the presence of sodium or potassium hydroxide (as a catalyst). The chemical reaction is called transesterification, and the products are fatty acid methyl esters (the biodiesel) plus glycerine. The latter is an alcohol with many useful properties for example it is sweet, a lubricant and a humectant which means it retains moisture. These properties give rise to many commercial applications in commodities such as drugs, paints, personal-care products, foods, flexible plastics etc. Popular raw sources for biodiesel processing include rapeseed oil (Europe), soybean oil (USA), palm oil (SE Asia). Various cooking oils and fats are suitable (new and recovered). Some keen individuals make their own biodiesel, especially in the States, and there are sites describing the methods, but the chemicals produce toxic fumes and the liquids are corrosive (eg caustic soda) so great care is needed. It is possible for individuals to buy small and medium sized purpose-made plant for production. Reading some articles on the net we see examples where experimenters are pouring neat untreated oils into their wagons and claiming to get away without processing, however, there are serious drawbacks to such a method which can result in expensive damage to the engine. The cost of manufacturing biodiesel currently is greater than petro-diesel although with the volatility of conventional oil prices we are not giving a figure. Even so, at the point of sale the difference may be negligible due to the imposition of tax. In the UK, for example, where taxes are high and biodiesel gets some preference (Ref: UK Budget, 2002, duty 20p/litre less) the prices of commercially produced diesels are similar. If the biodiesel is home produced it can be significantly cheaper in the tank.
There are some limitations to its green credentials.
The carbon-neutral feature of biodiesel, in practice, can be reduced to some extent because fossil fuels may be used in general transportation and in the manufacturing process. That is a variable depending on the detailed procedures but in any case should not be as much as a third.
Where specialised crops are grown on a vast commercial scale there is a distinct threat to diversity, natural forestation and the provision of foods.
Frequently biodiesel is combined with petro-diesel to ensure greater compatibility with unmodified engines and to make it operate within the temperature specs of the motors. The precise proportions are usually signified with a code which includes the percentage. For example, one method is to refer to it as B20 or E20 when there is 20% of biodiesel in the mixture, and so on. If the mixture is not B100 the full potential benefits will not be realised.
When used in standard compression-ignition engines biodiesel can increase nitrogen oxide emissions. This phenomenon is not due to nitrogen in the fuel but comes from the intake air as a result of the particular combustion process. The order of the increase appears to be fairly linearly related to the proportion, reaching a maximum of around 10% with B100. This phenomenon is not replicated in some other applications such as oil fired boilers and we believe that the effect can be reduced in vehicle engines by refining the tuning. Now let's review some of the additional benefits of biodiesel.
Biodiesel produces less noxious emissions than petro-diesel.
Biodiesel reduces tailpipe particulate matter, hydrocarbon, and carbon monoxide emissions from most modern four-stroke compression ignition engines. The reason is mainly associated with the presence of oxygen in the fuel which allows more complete combustion. Particulates are mainly carbon dust, commonly described as soot, and contain very fine particles that can be dangerous in the atmosphere since they can get into the lungs by bypassing the body's natural filter mechanisms. Together with the unburned hydrocarbons, particulates are suspected of being carcinogens and can cause other serious toxic effects. Carbon monoxide is, of course, well known for its lethal action in the bloodstream. Fairly obviously, the higher the proportion of biodiesel in a mixture, the more these noxious exhausts are reduced.
Biodiesel has a very low concentration of sulphur, which can for example lead to SO2 and acidic precipitation. The amount is very much less than traditional diesel oils and is comparable with the amount in Ultra Low Sulphur Diesel (ULSD).
And there are other fringe benefits of biodiesel.
The overarching benefit of biodiesel has already been stated namely that as a renewable fuel it is nominally carbon-neutral. An associated factor is that the global petroleum resources are being depleted rapidly and the term renewable is more significant than just reducing emissions. When petroleum runs out that's it!
Biodiesel improves the lubricating power of the fuel and therefore ensures correct operation of key components in the engine system; this can be a significant advantage in ULSD blends, even where the Bxx proportion is small.
Generally the fuel can be used in existing diesel engines especially at low proportions. Some modifications may be necessary, especially at higher concentrations of bio, and vehicle purchasers should be aware of engine specifications; also possible insurance exclusions. The modifications, important though they can be, are usually only minor and do not represent a basic change in the technology.
The fact that one possible raw material is used cooking oils etc gives a special advantage. Large caterers (think McDonald's) will have residues contaminated with meat. One of the outlets for this has been animal feed but in the wake of the BSE debacle that avenue has been barred and instead the waste is likely to go into landfill. To be able to use this product for biodiesel is an opportunity to be welcomed, the only downside being that the contaminated materials would need to be filtered and 'washed' first.
There seems to be a general view that the exhaust smell is pleasanter than from standard diesel.
But some disadvantages too.
Biodiesel has a lower energy density* than petro-diesel and because it is a denser fuel, the statistics vary slightly depending on whether you want a figure of energy per gallon or per pound. Roughly speaking the reductions are in the vicinity of 10%.
Viscosity at low temperatures is important with diesel, sometimes making cold-weather starting difficult; biodiesel can be at a disadvantage in this respect. We have read of occasional use of dual tank vehicles where petro-diesel is used to start and shut down but switched to biodiesel for main running. This is a technique for the over zealous, not for the typical motorist. A more practical solution is to introduce some special additive or a proportion of petro-diesel. Ideally, we suspect, engine manufacturers who have been shown to be resourceful in the past, should be able to find some technical solution.
The properties of biodiesel mean that rubber pipes may have to be replaced by plastic ones and it may remove engine deposits hence filters may need changing more frequently; not serious drawbacks.
If biodiesel-from-crops was widely adopted as a motor fuel then there would be a knock-on effect with serious problems caused by reduced biodiversity, displaced forests and the disrupted supply of other vital agricultural products. There is no reasonable expectation that petro-diesel could be completely replaced by bio-diesel. There are already signs that over development of crops for bio fuels can, paradoxically, be damaging to the environment.
Biodiesel is potentially a green fuel with considerable prospects. It scores well in its contribution to reducing vehicle, carbon emissions hence its global impact on climate change can be low when compared with petro-diesel. It also reduces noxious pollution levels on a local basis. The main drawback is in the production of suitable crops.
Critically, it can be used with current engine technology, albeit with a few minor engine modifications. It is also quite compatible with existing storage and distribution networks and the energy density is quite acceptable.
Although currently more expensive to produce than petro-diesel, we think that large scale production and acceptance would be bound to reduce costs, and with the prospect of inflated petroleum prices always on the horizon, we guess that the price differential may not be a major issue.
If car manufacturers could work with the producers and distributors with international and national ideals in mind, to provide suitable vehicles, and if the tax regime was made friendly then there is a great potential to be realised. This needs political vision and leadership. Perhaps the main hurdle would be the likely resistance of the wealthy oil magnates and crude suppliers. However, the entrepreneurs should be able to adapt to an alternative cash-cow and any independence we can gain from monopolistic overseas producers, who often hold the economic whip-hand, should be welcome.
The danger, to the environment, of excessive expansion of fuel crops is real and there should be monitoring and control internationally. Nevertheless if biodiesel could replace a proportion of petro-diesel as a vehicle fuel then that could be good for our environment.
our estimates of energy densities are derived from a pdf article: "Fuels of the Future for Cars and Trucks", Dr. James J. Eberhardt, Energy Efficiency and Renewable Energy, U.S. Department of Energy, 2002 Diesel Engine Emissions, Reduction (DEER) Workshop, San Diego, California, August 25 - 29, 2002. That pdf link no longer works but some comparable material may be found here on Energy Density: Wikipedia.
: "Fuelling the Dilemma", Mark Venables, Engineering & Technology, 10-23 May 2008.