Whenever environmental considerations come to the forefront of the fuel quality debate, just as it’s happening in some countries across the world, ethers emerge as an effective solution. Mexico, for example, just renewed its commitment to the use of high-octane, clean burning ethers, mandating the use of ethers in metropolitan areas, where reducing ozone levels has become a priority.
Increasing relevance “emissions efficiency” in global conversation
Protecting air quality is certainly not a new concept in the fuels conversation. In fact, the increasing need to protect air quality is what drove the initial environmental regulations wave that started in the 1970s, resulting in, among other things, the elimination of lead in gasoline and the introduction of catalytic converters. This same wave mandated the use of oxygenates in fuel blends across the world. Subsequent ones from the fuel quality perspective would focus, in general, on, stricter sulfur, olefin, benzene, and aromatic limits.
As air quality in metropolitan areas improved dramatically, however, climate change became the primary concern related to the combustion of oil and gas products. Air quality, though still the object attention from regulations, became relegated from the top of the agenda of the mainstream energy debate. As an example, during the last decade, the International Energy Agency (IEA) has produced over 50 reports that either focus, or include significant discussions, on climate change. Up until early 2016, no report from the IEA focused on the relationship between energy use and air quality.
But that’s changing, fast. In fact, as the IEA acknowledged with the release of its first-ever “Energy and Air Pollution” special report, the scale of the positive impact that progress on air quality would have is putting the issue back on the top of the global agenda. “To stay relevant,” Fatih Birol, the executive director of the IEA has declared recently “we need to work much closer with new emerging energy economies (on air quality issues).”
In the Americas, the trend is clear. Following results from a 2014 World Health Organization (WHO) report, several countries, including the US and Mexico, have adopted stricter air quality regulations. Ozone and particulate matter, among others, are being more intensely regulated. In tandem with this regulations, countries—including the US and Mexico—have continued to make fuel standards stricter, in an effort to improve and quality and combat climate change at the same time.
In other words, emissions efficiency has become a crucial topic in the Americas–for governments, consumers, and refiners alike.
The use of ethers, such as methyl-tert-buthyl-ether (MTBE) and ethyl-tert-buthyl-ether (ETBE) enables a more complete fuel combustion, resulting in direct air quality benefits that include the reduction of specific pollutants limited by law, such as carbon monoxide (CO) and unburned hydrocarbons (HCs), as well as other serious pollutants such as particulate matter (PM) and ground-level ozone (O3).
Studies commissioned by the California Air Resources Board (CARB) and the U.S. Environmental Protection Agency (EPA), two of the world’s leading regulatory agencies, have demonstrated that the use of MTBE, in particular, reduces permeation—a key component of evaporative emissions from a vehicle fleet—compared to both pure gasoline and gasoline blended with ethanol. As a result, the use of MTBE reduces the formation potential of, among other criteria pollutants, ground-level ozone.
Recent studies have confirmed these laboratory findings, showing that lower ethanol use in gasoline has resulted in lower ozone levels in highly-polluted cities such as Rio de Janeiro. Gasoline blended with ethers entails an even lower ozone formation potential than gasoline itself.
Mexico: air quality concerns drive fuel quality considerations (click to download)
The use of ethers also entails positive indirect effects, arising from the dilution of other, less desirable, gasoline pool components—such as olefins, aromatics, sulfur, and benzene levels.
The extent of MTBE’s air quality benefits depends on various parameters, such as the percentage of blended MTBE, the presence of catalyst devices, the type and age of engine and the driving cycle. Nevertheless, there is general agreement in the industrial and scientific communities on broad values, as reported in the chart below.
Ethers and alcohols
Studies commissioned by leading regulatory agencies across the world have demonstrated that the use of MTBE reduces permeation—a key contributor to evaporative emissions. The use of MTBE, thus, reduces ground-level ozone and airborne particulate matter formation. The Coordinating Research Council E-65 study (CRC E-65) that was commissioned by the California Air Resources Board, for example, shows that fugitive emissions with 11% MTBE gasoline were 12% lower than with un-oxygenated gasoline.
In contrast, ethanol use significantly increases hydrocarbon permeation from the fuel tank and gasoline distribution system. In its 2014 Evaporative Emissions from On-road Vehicles in MOVES2014 report, the United States Environmental Protection Agency concludes that “ethanol was then seen to have a significant effect compared to E0 fuel (that does not contain ethanol)”. The table below, taken directly from the study, summarizes those findings, according to the type of vehicle being used:
U.S. EPA study shows increase in emissions from gas containing Ethanol
In addition to the detrimental effect of ethanol on permeation, the table also suggests that this effect has worsened as the US vehicle fleet modernized. Studies that looked at MTBE and ethanol simultaneously, such as the CRC E-65 study, show that gasoline blended with 6% ethanol increased VOC permeation by 70-74% in pre-2001 vehicles compared to gasoline containing 11% MTBE and that those fugitive VOC emissions could increase ground level ozone by as much as 55% compared to MTBE-blended gasoline.
Finally, international studies whose methodology is appropriate for analyzing tailpipe emissions (not fugitive ones), have shown that MTBE use also provides tailpipe emissions benefits compared to ethanol. For example, a 2011 study from the Mexican Petroleum Institute (IMP) showed that, with 11% MTBE volume in fuels, formaldehyde emissions and acetaldehyde emissions decreased by as much as 17 and 29 percent, compared to fuels oxygenated with 6% ethanol by volume. Aldehydes, besides being carcinogenic air toxics, are also potent ozone precursors.
Ethers also vaporize more readily during the cold-start cycle, during which 80% of PM is formed.
In addition to reports that look directly at air quality components, recent studies have challenged the notion that the use of ethanol, compared to unoxygenated gasoline, reduces GHG emissions—even when looking at US ethanol production, which is widely acknowledged as leading in terms of agricultural efficiency. For example, a 2016 study from the University of Michigan concluded that «when it comes to the emissions that cause global warming, it turns out that biofuels are worse than gasoline». This represents one more data point that supports the fact that ethers are more “emissions efficient” than all other octane sources, including ethanol.