Interesting article: The Future of Fuels – 2nd Generation Alternatives

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Despite all of Audi's efforts to produce even more efficient cars, another decisive factor must always be borne in mind: fuel. Fuel bears an enormous potential for reducing CO2 emissions, and only a minute fraction of that potential has been exploited to date. Within the framework of Group Research, Audi is working energetically on this issue.

The fact that ever fewer sources of oil are being discovered all around the world highlights a basic truth. Fossil fuel resources are limited. The effect of falling crude oil reserves can be seen ever more clearly in the price of fuel at the pumps. This makes research into alternative or complementary fuels all the more important. Audi has already developed a concept for the use of CNG (compressed natural gas). This concept greatly improves the CO2 balance without compromising on drivability.

A glance at a map of the world shows how diversified the range of fuels is today, and how diversified it will be in the future. In South America pure bioethanol (E100) is often used, and gas is increasingly being used in China. These fuels can almost be referred to as modern classics. They are accompanied by a host of new types, synthetically produced fuels from gas (Gas to Liquid, GTL), biomass (Biomass to Liquid, BTL) or coal (Coal to Liquid).

These specially tailored fuels improve the combustion process inside the engine and thus allow a significantly improved energy balance. Here, so-called SunDiesel, made from biomass, seems to be particularly attractive from the point of view of its CO2 balance. During combustion, this fuel releases no more carbon dioxide than the plants that were processed to make the fuel previously drawn from the atmosphere by photosynthesis.

If we look in detail at existing and future alternative fuels, we can identify a strategy that clearly leads to the so-called second generation biogenic fuels. These avoid the drawbacks of the first generation biological fuels that are on the market today. Because these can only be used to a limited degree in today's vehicles, and because they require special measures, for example on the fuel tank and fuel lines, Audi does not aim to achieve general use for them on the German market. What appears sensible is a mixture within the framework of applicable standards.

Biodiesel and ethanol
Best known among today's biological fuels is biodiesel, which became widespread across Europe and in Germany in particular in the early 1990s. Biodiesel is produced by etherising, that is converting, rapeseed oil or sunflower oil into rapeseed methyl ester (RME) by adding methanol. This process is relatively straightforward and easy to perform. It can even be done by agricultural businesses.

Since 1996, Audi has approved its entire TDI fleet for 100 percent RME operation to promote its use in the interest of reducing CO2 emissions. However, as engines with a secondary particulate filter cannot be run purely on RME for various reasons, this approval was withdrawn in 2003. Today, only the addition of up to 5 percent RME is approved. Higher rates should not be achieved with esterised vegetable oils, but rather with hydrated vegetable oils, which offer a far higher degree of stability.

The second classic among biogenic fuels is ethanol, which is an alternative for use in spark-ignition engines. Sugary plants such as sugar beet or cane provide the raw material, which is processed into ethanol by alcoholic fermentation and subsequent distillation. Another source of raw materials is the starchy elements in cereals or maize. However, these can only be processed after an initial enzyme treatment.

Ethanol has a number of benefits as a fuel, in particular in hot countries. However, its wide availability, especially in Brazil, is primarily intended to reduce reliance on oil imports. Fundamentally, first generation biogenic fuels have the serious drawback that they directly compete with the growing of materials for food. Only the fruits or shoots are used for their production, which means that they only achieve relatively low substitution rates in their CO2 balance.

The yield is also insufficient with respect to the area under cultivation – an area of one hectare only produces 1183 litres of biodiesel per year. Moreover, such monoculture constitutes a massive interference in the natural flora and fauna, which contradicts the very idea of environmentally sensitive, sustainable agriculture.

SunFuel
Future second generation BtL fuels are superior in all of the critical points mentioned. They do not compete with the food industry, they have a high yield for a given area and they have a very high potential (approx. 90 percent) for reducing CO2 emissions. BtL fuels are specially "tailored" during production. This means that they need absolutely no modifications on the cars that they are to power. They are perfectly adapted to the combustion process.

One highly attractive second generation biological fuel is ethanol produced by the Iogen process. The Iogen Corporation, based in Ottawa, Canada, has been working for 20 years on developing a process for producing alcohol. A disused hangar at the Canadian capital's airport houses a demonstration plant.

The basic material is straw – a material that contains around 32 percent glucose. Firstly, the straw is treated with hot steam and acids to open up its fibres. Specially grown bacteria produce enzymes in a digestive fluid. These split the softened straw into a solid waste product and a viscous liquid with a very high sugar content. This is fermented and then distilled to produce ethanol.

About 320 litres of ethanol can be produced from one tonne of straw. Cellulose-based ethanol reduces the overall balance of CO2 emissions by about 90 percent. The waste product, so-called ligno-cellulose, can also be used as a fuel for producing energy. Within the framework of the Volkswagen Group, Audi supports Iogen's integrated approach to a wide-ranging degree.
The brand is also active in a similar way for the production of so-called SunDiesel. Choren Industries GmbH operates a plant in the city of Freiberg in Saxony that processes wood, straw or cereals. This combines various steps in its Carbo-V process. Firstly, biomass is separated into a solid and a gaseous component by low temperature gasification. After dust extraction and desulphurisation, the result is a gas that is then transformed into fuel by a process known as Fischer-Tropsch synthesis, which was developed in Germany during the 1920s.

Today, an Audi A4 TDI running on conventional mineral oil diesel emits 149 g/km CO2. Running on SunDiesel it emits a mere 22 g/km CO2, and this figure is likely to be reduced further when the process is running on an industrial scale. In addition, SunDiesel allows a 2.5-fold increase in yield for a given area compared to biodiesel – 3101 litres/ha.

SynFuel
Besides the second generation biological fuels, there are also other alternative fuels at the ready. These are so-called SynFuels, synthetic fuels that are already used in small quantities today. These are also known under the terms Gas to Liquid (GtL) and Coal to Liquid (CtL). They are produced using a largely homogenous process based on Fischer-Tropsch synthesis. However, large volumes of CO2 are released in the CtL process.

In the first step, the synthetic gas, which is made up of hydrogen and carbon monoxide, is generated by adding air and water. In the second step, hydrocarbons and steam are generated using Fischer-Tropsch synthesis.

The end product, basically comprising approximately 60 percent raw diesel, 20 percent raw petrol and other paraffinic chemicals (its precise composition depends on the details of the processing) is produced in the third stage of the process. Just like fossil fuels, GtL fuels have to be refined with additives.

The great benefit of GtL fuels is that they are free of sulphur and aromatics. This means that in a combustion engine, emissions are greatly reduced, especially particulates and sulphur compounds, which have a negative effect on the exhaust gas aftertreatment. Depending on your perspective, the potential for reduction is up to 80 percent. In field trials, drastic reductions in carbon monoxide and hydrocarbon emissions were measured with unchanged engine tuning.

GtL fuels can be mixed with mineral oil diesel. Their benefits become significant once a proportion of 50 percent has been exceeded. If the engine is optimally configured for GtL fuel, there is a further reduction of about 50 percent in nitric oxides. The direct reduction in CO2 emissions is about 10 percent.

Various mineral oil companies are currently building large-scale plants using this process in the Arabian Emirate of Qatar. Even today, GtL is added to some fuels, for example to Shell's V-Power Diesel, which contains a five percent proportion. The Audi R10 diesel racing car won the Le Mans 24 hour race twice in succession running on V-Power, serving both Audi and Shell as a racing laboratory for gathering new experience values under extreme conditions.

Hydrogen
Hydrogen, known by its chemical formula H2, has long been seen as a potential energy source. As early as 1874, the French novelist Jules Verne wrote "Water will be the coal of the future". H2, the most common element in the universe, can be manufactured as a biogenic fuel on the basis of biomass, similar to BtL, except that Fischer-Tropsch synthesis is not necessary. Hydrogen could be a CO2-neutral route to the future.

The alternative possibility of electrolysis is better known (many people will remember the reverse reaction with hydrogen gas at school), but it is also more problematic. This process is only beneficial from an environmental point of view if the necessary electrical current is provided regeneratively or by CO2-neutral means. In Germany, renewable energies currently only account for about 11 percent of power generated, although this proportion is continually rising.

Hydrogen can be used in two ways as an energy source in cars. It can be burned in a traditional manner in a piston engine or it can be used in a fuel cell. Classic combustion carries a number of drawbacks. Hydrogen is extremely light and must be cooled to -253 degrees Celsius to achieve the necessary liquidity. It needs a large and complex fuel tank, which greatly reduces the volume of the luggage compartment, but still only allows short ranges. In addition, hydrogen volatises through warming and subsequent evaporation.

For these reasons, apart from the problem of technical maturity, there are a lot of arguments for using hydrogen in a fuel cell. The efficiency of this system is significantly greater. The drive system works extremely quietly and produces absolutely no pollutants. And because there are no moving parts, no mechanical wear is to be expected.

Audi tested the potential of fuel cell technology as early as 2004 in its A2H2 concept car. The brand is continuing its work in this field in the framework of the California Fuel Cell Partnership, in which major carmakers and various other partners are involved. And even if fuel tank technology and filling station infrastructure today still leaves some questions unanswered, Audi continues to work on the development and testing of hydrogen-powered fuel cell cars.

The path to the future
The transition from fossil to regenerative fuel will be a slow and gradual one. Old and new types will coexist for a long time. CO2 emissions are continuously declining. A scenario for the future could be:
Today: Mainly spark-ignition and diesel combustion engines, hydrogen only for fleet operations with certain vehicles.
In 10 years: SynFuels are becoming more commonplace, and hybrid-supported systems are available in large numbers on the market.

In 20 years: SunFuel and H2 are also available at filling stations. Fuel cells are starting to enter small-scale production.
In more than 20 years: Hydrogen is becoming ever more important and the fuel cell is in series production.

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