Alcohol fuel

Although fossil fuels have become the dominant energy resource for the modern world, alcohol has been used as a fuel throughout history. The first four aliphatic alcohols (methanol, ethanol, propanol, and butanol) are of interest as fuels because they can be synthesized biologically, and they have characteristics which allow them to be used in current engines. One advantage shared by all four alcohols is octane rating. Biobutanol has the advantage that its energy density is closer to gasoline than the other alcohols (while still retaining over 25% higher octane rating) - however, these advantages are outweighed by disadvantages (compared to ethanol and methanol) concerning production, for instance. Generally speaking, the chemical formula for alcohol fuel is C_nH_2n+1OH. The larger n is, the higher the energy density.

Alcohol fuels are usually of biological rather than petroleum sources. When obtained from biological sources, they are known as bioalcohols (e.g. bioethanol). It is important to note that there is no chemical difference between biologically produced alcohols and those obtained from other sources. However, ethanol that is derived from petroleum should not be considered safe for consumption as this alcohol contains about 5% methanol and may cause blindness or death. This mixture may also not be purified by simple distillation, as it forms an azeotropic mixture.

Methanol and Ethanol

Methanol and ethanol can both be derived from fossil fuels or from biomass. Ethanol is produced through fermentation of sugars and methanol from synthesis gas.

As a fuel methanol and ethanol both have advantages and disadvantages over fuels such as petrol and diesel. In spark ignition engines both alcohols can run at a much higher EGR rates and with higher compression ratios. Both alcohols have a high octane rating, with ethanol at 129 RON, 102 MON, (which equates to 116 AKI) and methanol at 123 RON, 103 MON (which equates to 113 AKI) ^[1]. Ordinary European petrol is typically 95 RON, 85 MON, equal to 90 AKI. Note that AKI refers to 'Anti-Knock Index' which averages the RON and MON ratings (RON+MON)/2, and is used on U.S. gas station pumps. As a compression ignition engine fuel, both alcohols create very little particulates, but their low cetane number means that an ignition improver like glycol must be mixed into the fuel with approx. 5%.

With SI engines alcohols have the potential to reduce NOx, CO, HC and particulates. A test with E85 fueled Chevrolet Luminas showed that NMHC went down by 20-22%, NOx by 25-32% and CO by 12-24% compared to reformulated gasoline^[2]. Toxic emissions of benzene and 1,3 Butadiene also decreased while aldehyde emissions increased (acetaldehyde in particular).

Tailpipe emissions of CO₂ also decrease due to the lower carbon-to-hydrogen ratio of these alcohols, and the improved engine efficiency.

Methanol and ethanol contain soluble and insoluble contaminants ^[3]. Halide ions, which are soluble contaminants, such as chloride ions, have a large effect on the corrosivity of alcohol fuels. Halide ions increase corrosion in two ways: they chemically attack passivating oxide films on several metals causing piting corrosion, and they increase the conductivity of the fuel. Increased electrical conductivity promotes electrical, galvanic and ordinary corrosion in the fuel system. Soluble contaminants such as aluminum hydroxide, itself a product of corrosion by halide ions, clogs the fuel system over time. To prevent corrosion the fuel system must be made of suitable materials, electrical wires must be properly insulated and the fuel level sensor must be of pulse and hold type (or similar). In addition, high quality alcohol should have a low concentration of contaminants and have a suitable corrosion inhibitor added.

Methanol and ethanol are also incompatible with some polymers. The alcohol is solved by the polymers causing swelling, and over time the oxygen breaks down the carbon-carbon bonds in the polymer causing a reduction in tensile strength. For the past few decades though, most cars have been designed to tolerate up to 10% ethanol (E10) without problem. This include both fuel system compatibility and lambda compensation of fuel delivery with fuel injection engines featuring closed loop lambda control. In some engines ethanol may degrade some compositions of plastic or rubber fuel delivery components designed for conventional petrol, and also be unable to lambda compensate the fuel properly.^{[citation needed]}

"FlexFuel" vehicles have upgraded fuel system and engine components which are designed for long life using E85 or M85, and the ECU can adapt to any fuel blend between gasoline and E85 or M85. Typical upgrades include modifications to: fuel tanks, fuel tank electrical wiring, fuel pumps, fuel filters, fuel lines, filler tubes, fuel level sensors, fuel injectors, seals, fuel rails, fuel pressure regulators, valve seats and inlet valves. The cost of this E85 upgrade to a modern engine is inexpensive and is less than $100^{[citation needed]}. "Total Flex" Autos destined for the Brazilian market can use E100 (100% Ethanol).

One liter of ethanol contain 21.1 MJ, a liter of methanol 15.8 MJ and a liter of gasoline approximately 32.6 MJ. In other words, for the same energy content as one liter or one gallon of gasoline, one needs 1.6 liters/gallons of ethanol and 2.1 liters/gallons of methanol. Although actual fuel consumption doesn't increase as much as energy content numbers indicate.

Methanol has been proposed as a future biofuel. Methanol has a long history as a racing fuel. Early Grand Prix Racing used blended mixtures as well as pure methanol. The use of the fuel was primarily used in North America after the war.{What war?} However, methanol for racing purposes has largely been based on natural gas and therefore would not be considered as biofuel. Methanol is an excellent biofuel and compared to ethanol its primary advantage is its much greater well-to-wheel efficiency when produced from syngas.

Ethanol is already being used extensively as a fuel additive, and the use of ethanol fuel alone or as part of a mix with gasoline is increasing. Compared to methanol its primary advantage is that the fuel is non-toxic, although the fuel will produce some toxic exhaust emissions. From 2007, the Indy Racing League will use ethanol as its exclusive fuel, after 40 years of using methanol.^[4]. Since September 2007 petrol stations in NSW, Australia are mandated to supply all their petrol with 2% Ethanol content^[5]

Methanol combustion is: 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O + heat

Ethanol combustion is: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat

Butanol

Propanol and butanol are considerably less toxic and less volatile than methanol. In particular, butanol has a high flashpoint of 35 °C, which is a benefit for fire safety, but may be a difficulty for starting engines in cold weather. The concept of flash point is however not directly applicable to engines as the compression of the air in the cylinder means that the temperature is several hundred degrees Celsius before ignition takes place.

The fermentation processes to produce propanol and butanol from cellulose are fairly tricky to execute, and the Weizmann organism (Clostridium acetobutylicum) currently used to perform these conversions produces an extremely unpleasant smell, and this must be taken into consideration when designing and locating a fermentation plant. This organism also dies when the butanol content of whatever it is fermenting rises to 7%. For comparison, yeast dies when the ethanol content of its feedstock hits 14%. Specialized strains can tolerate even greater ethanol concentrations - so-called turbo yeast can withstand up to 16% ethanol ^[6]. However, if ordinary Saccharomyces yeast can be modified to improve its ethanol resistance, scientists may yet one day produce a strain of the Weizmann organism with a butanol resistance higher than the natural boundary of 7%. This would be useful because butanol has a higher energy density than ethanol, and because waste fibre left over from sugar crops used to make ethanol could be made into butanol, raising the alcohol yield of fuel crops without there being a need for more crops to be planted.

Despite these drawbacks, DuPont and British Petroleum have recently announced that they are jointly to build a small scale butanol fuel demonstration plant ^[7] alongside the large bioethanol plant they are jointly developing with Associated British Foods.

Energy Environment International developed a method for producing butanol from biomass, which involves the use of two separate micro-organisms in sequence to minimize production of acetone and ethanol byproducts.^[8]

The Swiss company Butalco GmbH uses a special technology to modify yeasts in order to produce butanol instead of ethanol. Yeasts as production organisms for butanol have decisive advantages compared to bacteria^[9].

Butanol combustion is: C₄H₉OH + 6O₂ → 4CO₂ + 5H₂O + heat

The 3-carbon alcohol, propanol (C₃H₇OH), is not used as a direct fuel source for petrol engines that often (unlike ethanol, methanol and butanol), with most being directed into use as a solvent. However, it is used as a source of hydrogen in some types of fuel cell; it can generate a higher voltage than methanol, which is the fuel of choice for most alcohol-based fuel cells. However, since propanol is harder to produce than methanol (biologically OR from oil), methanol fuel cells are still used a lot more often than those that utilise propanol.