What are fuel oxygenates ?

Oxygenates are compounds containing oxygen in a chain of carbon and hydrogen atoms. Today, oxygenates are blended into gasoline in two forms: alcohols or ethers.

In alcohols, each oxygen atom is linked to a carbon atom and a hydrogen atom, forming a carbon-oxygen-hydrogen sequence. Ethanol is the most commonly used alcohol oxygenate. In ethers, each oxygen atom is linked to two carbon atoms, forming a carbon-oxygen-carbon sequence. Methyl-tertiary-butyl-ether, or MTBE, is the most commonly used ether oxygenate, followed by the biofuel ethyl-tertiary-butyl-ether, or ETBE, and tertiary-amyl-methyl-ether, or TAME.

Alcohols vs. ethers

When added to gasoline, alcohols by themselves tend to be very volatile - due to the formation of an azeotrope with light hydrocarbons - and water soluble, which can create problems in the fuel distribution system and vehicle engine as well as in the environment. These problems of volatility and water solubility can be overcome by "stabilising" the alcohols with various petroleum-derived components through a process known as etherification. Ethers retain all the benefits of their alcohol feedstocks, without their shortcomings.

MTBE is the most widely used fuel oxygenate, due to its combination of technical advantages and supply availability. As an octane enhancer, MTBE is an effective replacement for aromatics, because it delivers high octane numbers at relatively low cost. By comparison with alcohols, MTBE offers low water solubility, low reactivity and low volatility - characteristics that enable refiners to avoid the handling problems associated with alcohol oxygenates.

The key properties of the most commonly used oxygenates, when blended at 5% in gasoline, are found in the table below:

Substance	Motor Octane Number	Research Octane Number	Reid Vapour Pressure (kPa)	Boiling point (°C)	Water tolerance
Ethers
MTBE	101	118	55	55	Excellent
ETBE	102	118	28	72	Excellent
TAME	99	109	10	86	Excellent
Alcohols
Methanol	92	125	522	65	Very poor
Ethanol	96	130	222	78	Very poor
TBA	95	105	62	71	Poor
Gasoline	82-88	92-98	70-100	26-230

Fuel ethers properties

The following table summarises fuel ethers (MTBE, ETBE, TAME) key physical and chemical properties:

Properties	MTBE	ETBE	TAME
CAS Number	1634-04-4	637-92-3	994-05-8
Molecular Wt. (g/mol)	88.2	102.2	10.2
Boiling Point (Deg C)	55.3	73.1	86.3
Melting Point (Deg C)	-108	-94	-80
Oxygen Content (% wt.)	18.2	15.7	15.7
Research Octane Number	118	118	109
Motor Octane Number	101	102	99
Vapour Pressure (hPa @ 20 Deg C)	270	128	90
Water Solubility (g/l)	42	23.7	11
Log Pow	1.06	1.28	1.56
Odour Detection in Water (average, mg/l)	95	49	194
Taste Detection in Water	134	47	128
Henry's Law Constant (Pa.m3/mol)	43.8	140	83

Biofuels

As explained above, fuel ethers are already an alternative component which makes the production of modern, cleaner-burning gasoline possible. High octane, easy-to-blend and cost-effective they are essentially drop-in blending components for the refiner. There are a few other non-aromatic octane options such as alkylates, isomerates and ethanol, but they are limited in terms of both octane contribution and supply availability. In addition, they provide fewer air quality benefits.

The net octane contribution of a blending component is related to both its octane number and the amount of the component used in petrol. Since the content of aromatics in petrol is being reduced by another 7% in Europe (from 42% max. today to 35% max. in 2005), their octane contribution must be replaced to maintain the quality of the petrol.

Fuel ethers and ethanol are more effective octane components since they deliver higher octane numbers than aromatics and therefore require less volume to deliver the same amount of octane. However, ethanol cannot be utilised without difficulties by the refiners in petrol because of its high water solubility, which effectively prevents its use in the distribution system for petrol. Ethanol has also a high, non-ideal blending vapour pressure which tends to knock off some other, light, low-boiling hydrocarbons from the gasoline blend in order to meet the volatility regulations. Technically, and because of its higher cost, ethanol is not really a viable alternative.

In 2003, the European Union adopted directive 2003/30, which aims at promoting the use of biofuels or other renewable fuels to replace diesel or petrol for transport purposes in each Member State, with a view to contributing to objectives such as meeting climate change commitments, environmentally friendly security of supply and promoting renewable energy sources.  Allowed biofuels include alcohols such as bio-ethanol and ethers such as bio-ETBE and bio-MTBE.  The directive sets no obligation, but indicates that Member States should set national indicative targets for a minimum proportion of biofuels and other renewable fuels to be placed on their markets, these targets being 2 % in 2005 and 5.75 % in 2010 in terms of energy content. A review will be carried out in 2006 based on progress reports by each individual Member State in EU 25.

Today, the high cost of production of biofuels requires that they benefit from fiscal incentives to become economically viable.  Directive 2003/30 has therefore been implemented by Directive 2003/96 on the taxation of energy products, which allows Member States to apply a total or partial tax exemption to biofuels.  As the various Member States prepare their national plans to implement the biofuels directive, specific care should be taken to ensure that the implementation of the fiscal directive does not hinder the free movement of products or creates distortions in the market. 

Due to their relatively low octane numbers, alkylates and isomerates are much less effective suppliers of octane, which means that larger amount must be used to deliver the same amount of octane. However, the future supply available for these two components is very low due to the limited amounts of feedstocks available in the refinery. The cost of their octane is also much higher than that of fuel ethers. 

For these reasons, fuel ethers have the potential to replace all octane loss from aromatic reductions in petrol.