Fuel ethers and air quality

Fuel ethers have been playing an important role in improving air quality in Europe. Alongside the improvements in car technology over the past years, the use of fuel ethers as petrol components have, both directly and indirectly, contributed to cleaner air in Europe.

Direct improvement of air quality is obtained thanks to the oxygen content of fuel ethers. Adding oxygen to petrol enables a more complete combustion of the fuel and therefore reduces exhaust emissions of carbon monoxide (CO). When used as part of the petrol formulation, fuel ethers lead to a reduction in emissions of exhaust pollutants such as volatile organic compounds (VOCs) and particulates (PM).

In comparison to the alcohols used in their production, fuel ethers reduce the overall vapour pressure of petrol and the related evaporative emissions to air, which causes ozone. In fact, fuel ethers have the lowest ozone forming potential of all the oxygen containing octane boosters. They contribute to lower evaporation and permeation of high volatile organic carbons in the finished petrol, which in turn means less ozone formation. Therefore, fuel ethers help indirectly to reduce emissions of toxic substances in the air.

MTBE Reduces (1) Emissions of:

  • Benzene by 37%
  • Toxics by 23%
  • Carbon monoxide by 14%
  • Particulate matter (PM) by 1%

Another characteristic of fuel ethers is their capacity to raise the octane number in petrol, replacing less desirable components. Using ethers allows refiners to substitute other components, which are more energy intensive to produce, and/or have less desirable environmental properties e.g. aromatics and olefins. Raising the octane number of petrol has allowed the development of more efficient engine technologies with lower emissions. Fuel ethers also provide the possibility for this trend to continue.

A good practice standard when handling fuel ethers and petrol containing fuel ethers is necessary to minimize emissions of fuel ethers and petrol in the air.

Fuel ethers, including ETBE and MTBE fuel compare favourably with the fuel containing 10% volume ethanol (E10), which actually increases certain toxic vehicle emissions when compared to a non-oxygenated fuel blend, in particular:

 E10 Increases (2) Emissions of:

  • Toxics by 6%
  • Volatile organic compounds (VOCs) by 17%
  • PM 2.5 by 29%

Direct and indirect effects of adding fuel ethers to petrol

As a rule of thumb, exhaust emissions (VOCs) are reduced by the same percentage as the MTBE content in petrol.

Effect How? How big is the reduction?
Direct Oxygen enables a more complete fuel combustion Fuel ethers reduce CO emissions by the same percentage as their content in petrol.
1-2% of fuel ether in petrol typically leads to reduction of hydrocarbon emissions by 1%.
Indirect High octane and other properties of fuel ethers diminishes the use of other, more polluting components of petrol. Fuel ethers have a significantly lower ozone forming potential compared to other conventional petrol blending components such as for instance aromatics or olefins.

Fuel Ethers reduce Greenhouse Gas Emissions (GHG)

The bio-ethers ETBE and TAEE have been shown to offer GHG additional savings compared to the bio-ethanol used in their manufacture. Indeed, the energy saved thanks to fuel ethers’ high octane level and reduced volatility counterbalances the extra processing step required to convert bio-ethanol into bio-ether.

Studies carried out by Hart Energy Consulting and CE Delft (1,2), using their own in-house modelling systems showed that ETBE offered an additional CO2 benefit over direct ethanol blending.

The studies support the following conclusions:

  • Blending ethers into petrol is more energy efficient (and therefore reduces CO2 emissions) than blending ethanol into petrol. Pure ethanol blending requires a base petrol that has a lower RVP compared to a base petrol in which ETBE is blended.  Producing a lower RVP base petrol increases energy consumption in a refinery because the base petrol requires more processing in order to compensate for the higher RVP of the ethanol.
  • Ethanol in the form of ETBE allows for more efficient transport from the source to the pump compared to ethanol, which reduces CO2 emissions.
  • ETBE decreases crude oil and reduces refinery energy consumption for crude oil processing and octane production, thus reducing CO2 emissions.
  • ETBE typically offers an additional saving of 24kg of CO2-equivalent/GJ of ethanol.

While the CO2 reduction is small compared to the total refinery footprint, it is significant when considering the greenhouse savings obtained by using ETBE instead of ethanol.

Hart Energy Consulting has also completed a follow-up study on TAEE (3), which shows that this is a general benefit for bio-ethers.

(1) Study on relative CO2 savings comparing ethanol and ETBE as a petrol component, Hart Energy Consulting, July 2007 Link

(2)ETBE and ethanol: a comparison of CO2 savings, CE Delft, October 2007 Link

(3) CO2 reduction of TAEE vs Ethanol, Hart Energy Consulting, October 2010 Link