Unconventional or alternative fuels.
Keywords: alternative fuels, fuels, alternative, oil, pollution, depollution, environment
GNC (Natural gas-fuel)
The use of CNG in the gaseous state and compressed under 200 bars is a technological solution that has already been tested since more than 500 vehicles are involved worldwide. On dedicated and optimized engines, CNG provides notable advantages which outweigh a more expensive energy supply. Driving pleasure, acceleration performance, recovery, maximum speed are very satisfactory.
Fuel efficiency exceeds that of gasoline engines by around 10% (with the exception of lean-burn gasoline engines like those recently offered by Japanese manufacturers), but falls short of that of a direct injection diesel engine. Emissions from CNG engines consist almost exclusively of methane, therefore of low toxicity.
Methane is, however, an important greenhouse gas. But, if we consider greenhouse gas emissions over the entire use chain, CNG provides savings of around 20 to 25% compared to the gasoline sector and 10 to
15% with respect to diesel.
The main handicap of GNc concerns storage which is very penalizing in terms of weight and size. New materials such as resin composites and glass or carbon fibers, currently under study, should make it possible to divide by four the weight of the tank at constant capacity.
CNG therefore appears to be a substitute fuel, the penetration of which is certain without being able to assess its extent at present. It should materialize first in urban uses (especially buses) where pollution is a concern.
Many studies have been conducted in the 1970 years on the development of fuels containing 85 100% methanol, designated by the initials M85, M90 or M100 according to their composition.
Currently, this theme has lost much of its interest. Methanol is indeed intrinsically toxic and it provides very little benefit in terms of air pollution. In particular, the risks of tropospheric ozone formation are hardly modified for vehicles adopting M85 or M100.
Methanol is maintained indirectly on the fuel market as a basic player in the synthesis of MTBE. This ether is an excellent constituent of gasolines, highly sought after for its high octane number, its perfect compatibility with hydrocarbons and
benefits it can provide to reduce air pollution.
Today, MTBE concentrations of 5-10% are very common in gasolines. However, there are problems related to the low biodegradability of MTBE.
Ethanol is potentially a good quality fuel capable of supplying spark ignition type engines. It can be used pure or mixed in small proportion (up to 20%) in a conventional gasoline. In the first case, the engine must be adapted to this specific use (modification of the fuel system and higher compression ratio); in the
second case, the ethanol-gasoline mixture is completely commonplace and interchangeable in the distribution network with products of strictly petroleum origin.
However, even Brazil, which had embarked on a proactive policy in favor of the ethanol-fuel sector, is reviewing its strategy. The reasons for this turnaround in Brazil and the slow economic take-off in the rest of the world are due to a few technical obstacles which, without being prohibitive, provoke reluctance from the oil and automotive industries.
Ethanol-gasoline blends are less stable in the presence of water, more volatile and sometimes more corrosive than products of exclusively petroleum origin.
This is why, like methanol, the ethanol-fuel sector is preferably oriented towards the production of ETBE from ethanol and isobutene.
European regulations set a maximum content of 15% (volume) of ETBE in gasolines, i.e. around 7% (weight)
ethanol. This legislative framework therefore leaves sufficient room for the penetration of ethanol at significant rates into the fuel market.
Derivatives of vegetable oils
Although diesel engines can operate with crude vegetable oils, this approach does not appear realistic for vehicles that have become very efficient. On the other hand, the transformation of vegetable oils into methyl esters offers considerable advantages on the technical level.
The methyl esters of vegetable oils have physicochemical properties close to those of gas oil in which it is perfectly miscible. The types of oilseeds concerned are mainly rapeseed and sunflower. The agronomic data are as follows: it is
possible to obtain 30 to 35 quintals / year of colza seeds per hectare, or 1,2 to 1,4 tonnes of methyl esters per hectare and per year.
On the regulatory front, a decree authorizes, in France, the unmarked distribution of rapeseed methyl ester up to 5% mixed in diesel.
Ultimately, the energy balances of biofuel production sectors are favorable. The ratio between the energy contained in the biofuel and that which was necessary to produce it, is always greater than 1. But, from an economic point of view, with the current costs of access to crude oil and without tax incentives , biofuels are not competitive.
Finally, the conclusions of studies concerning the contribution of biofuels in terms of impact on atmospheric pollution are very nuanced. Depending on the type of pollutant considered, the fuels
of plant origin may be sometimes slightly beneficial, sometimes slightly unfavorable. With the exception of protection against the greenhouse effect, for which the use of biofuels undoubtedly brings a significant improvement.
Synthetic fuels are traditional gasolines and gas oils, but derived from raw materials other than petroleum, mainly coal and natural gas.
The corresponding processes make use of cumbersome and expensive technologies. They consist in producing, in an intermediate step, synthesis gas (CO and H2), from which, two routes are possible: the direct obtaining of hydrocarbons according to the Fischer-Tropsch technique or the passage through methanol which will be then turned into gasoline.
The yield of these sectors is a major handicap: between 35 and 55% for the Fischer-Tropsch process of essences depending on the characteristics of the raw material and the quality requirements of the finished products; between 60 and 65% for the synthetic gasoline sector via methanol developed in 1986 by the company Mobil in New Zealand. These low yields go hand in hand with high CO2 emissions.
Consequently, the significant production of synthetic fuels is conditioned by a high price of oil (at least 30 $ / bbl) and a strong demand for very low polluting products.
In the medium term, it is for hydrogen to properly manage an announced shortage. Highly consuming refining units (hydrodesulfurizations, hydrotreatments and hydroconversions)
will multiply to improve the quality of petroleum products and to adapt to demand increasingly oriented towards light products.
Apart from reforming which will quickly reach its limits, the production of hydrogen can be envisaged by methane vapor reforming, by oxyvapogasification of residues or by electrolysis. The first two paths lead to self-consumption and significant CO2 emissions. The path to electrolysis would require a revival of investment in nuclear power and acceptance by the general public of this
technology and its risks.
If we arbitrarily avoid these questions of availability of raw materials, the use of hydrogen as motor fuel still comes up against great difficulties: storage on board the vehicle is a real technological bottleneck.
If we assume, moreover, that the storage on board vehicles is technically solved and that the basic safety conditions are met, two possibilities are then possible: the hydrogen can first be used, pure or in mixture with CNG, in engines specially designed for this type of fuel. Engine efficiency is then limited by the laws of thermodynamics and NOx emissions are inevitable. Second, hydrogen can be consumed in fuel cells.
But problems of technological development then appear. The electrodes are made of precious metals (platinum and palladium) and the power density is low. Despite recent commitments
large industrialists to develop fuel cell vehicles, this way does not seem, in the face of the competition of more conventional converters but near zero pollution promised a great future.
Tensions are foreseeable on the hydrogen market and the fuel route remains very prospective. It is certain that the use of hydrogen to improve the qualities of traditional fuels will remain the most technically and economically efficient route for a long time to come.
As a result, the fuel cell and hydrogen engine do not seem likely to emerge in the medium term.