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Cleaner gasoline
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Gasoline is a mixture of many different hydrocarbons, which is obtained from refining crude petroleum. Being highly volatile and flammable and having vapors denser than air, it has a high risk of fire and explosion ( w1 ). It is the most common type of fuel used in road vehicles.

The pollutants of greatest concern from non catalyst gasoline-fueled vehicles are carbon monoxide (CO), hydrocarbons (HC), Nitrogen Oxides (NOx), Lead and certain toxic hydrocarbons such as benzene. Each of these can be influenced by the composition of the gasoline used by the vehicle. Usually combustion of gasoline without exhaust gas aftertreatment leads to much higher emissions of hydrocarbons (including benzene) and carbon monoxide than diesel. NOx emissions are similar (slightly less in two stroke engines and more in four stroke engines). Particulate emissions are generally far lower than from diesel except in two stroke engines commonly used in motorcycles and scooters where PM emissions and smoke often are unacceptably high.

The number of countries using lead in gasoline as an octane enhancer is rapidly declining.

In the United States, three stages of "cleaning up" gasoline have been identified, which apply similarly to other industrialized countries ( 1 ). These are:

  • 1. Phasing out leaded gasoline
  • 2. Introducing reformulated gasoline (RFG)
  • 3. Reducing the sulfur content of gasoline

These are briefly described below.

Lead-Free Gasoline

Lead alkyl compounds are artificially added to gasoline as a low cost octane enhancer. More than 50 % of the lead in the fuel is emitted with the exhaust gas. Lead also pollutes the engine oil and large amounts are trapped in the exhaust system causing health problems during repairs, scrappage and recycling. It is highly toxic to humans, particularly with regard to the early development of nervous systems in children and fetuses ( 2 ).). Some lead alkyl compounds are evaporated from the gasoline and emitted in the exhausts too. Lead alkyls are extremely toxic to man. In addition to its toxicity, lead also seriously impairs the effectiveness of catalytic converters ( 1 ). Therefore it is considered to be the first substance that should be removed from gasoline.

There are three options to achieve the same effect without lead, of which most countries use a mixture of the first two approaches ( 2 ):

  1. Modify the refinery process to raise the octane level of the unleaded gasoline pool,
  2. Add alternative octane enhancing additives, or
  3. Reduce vehicle octane requirements.

Japan was the first country to completely ban lead in gasoline. Lead in gasoline is banned throughout North America and Europe and many other countries. Detailed information on this can be found in the report "Phasing Lead out of Gasoline" ( 2 ).

Reformulated Gasoline (RFG)

There are hundreds of different formulations for making gasoline. The ingredients used to make RFG are no different from ingredients used to make conventional gasoline, but the levels at which these ingredients are used differ to the effect that RFG emits lower emissions, has reduced evaporative tendencies, and contains fewer impurities ( w2 ).

For gasoline, there are many quality constraints, including octane, RVP (Reid vapor pressure,) olefins, benzene, aromatics, polyaromatics (PAH), , sulfur, and distillation properties. Blending is a complex operation, with any number of constraints to be met simultaneously. Table 1 presents a listing of quality tradeoffs for key gasoline blendstocks.

Quality Tradeoffs for Key Gasoline Blendstocks


[>89.5 (R+M)/2]
High Octane?
[<7 RVP]
Low RVP?
[<14 vol %] Low Olefins? [< 1 vol %] Low Benzene? [<35 vol %]
Low Aromatics?
Butanes YES NO YES YES YES
Alkylate YES YES YES YES YES
Isopentane YES NO YES YES YES
C6 Isomerate NO YES YES YES YES
Lt FCC Naphtha YES NO NO NO YES
HV FCC Naptha NO YES (Varies) YES NO
Reformate YES YES YES NO NO
HDC Naptha NO NO YES YES YES
Thermal Naptha (Varies) (Varies) NO (Varies) YES
MTBE YES NO YES YES YES
LSR Gasoline NO (Varies) YES YES YES
Low Sulfur Gasoline

Besides leading to SOx and increased particulate emissions, sulfur impedes the performance of catalytic converters in cars ( 3 ), which are used to reduce tailpipe emission. In general, if catalytic converters are used, sulfur levels should be constrained to 500 PPM maximum; reducing these levels to near zero will further improve catalyst performance.

As noted by the US Auto-Oil study, "The regression analysis showed that the sulfur effect (lowered emissions) was significant for HC on all ten cars, for CO on five cars, and for NOx on 8 cars. There were no instances of a statistically significant increase in emissions." Based on the auto/oil study, it appears that NOx would go down about 3% per 100-PPM sulfur reduction for a typical catalyst equipped car.

The situation is even more critical for advanced low pollution catalyst vehicles. Operation on typical US conventional gasoline containing 330-ppm sulfur will increase exhaust VOC and NOx emissions from current and future new US vehicles (on average) by 40 percent and 150 percent, respectively, relative to their emissions with fuel containing roughly 30-ppm sulfur.

A recent study (Sulfur - The Lead of the New Century:The Need For and Benefits of Reducing Sulfur in Gasoline & Diesel Fuel, Katherine O. Blumberg, Michael P. Walsh , Charlotte Pera) assessed the impact of gasoline sulfur levels on emissions and its principle results are summarized below.

Sulfur levels in gasoline relate only to direct SO2 emissions in gasoline vehicles that do not have functioning emissions control equipment. Most gasoline vehicles currently in use, however, even in many Latin American countries are equipped with catalysts for the control of CO, HC, and NOx, which are impacted by sulfur levels in the fuel. The sulfur impact increases in severity as vehicles are designed to meet stricter standards. Current sulfur levels in fuel are the primary obstacle in bringing advanced emission control technologies to market, technologies that will dramatically reduce conventional pollutants and also enable more fuel efficient engine designs.

The major issues related to sulfur levels in gasoline include:

  • Sulfur increases emissions from the three-way catalyst, the most common type of emission control technology in gasoline vehicles.
  • The negative impact of sulfur increases with more efficient and advanced catalytic controls.
  • Current sulfur levels present a significant barrier to the introduction of more efficient gasoline engine designs, which will require advanced emission control technologies that are severely impacted by sulfur.
  • High sulfur levels contribute to increased Particulate emissions.
Current Three-Way Catalysts


Worldwide 85% of new gasoline vehicles are equipped with a three-way catalyst (TWC) to simultaneously control emissions of CO, HC, and NOx. Sulfur levels in fuel impact TWC functioning in several ways:

  • Fuel sulfur reduces conversion efficiency for CO, HC and NOx. The TWC catalyst stores SO2 during normal driving conditions and releases it during periods of fuel-rich, high-temperature operation, such as high acceleration. In this way the sulfur competes with other gaseous emissions for reaction space on the catalyst, demonstrated by emissions of HC, CO and NOx decreasing steadily with lower sulfur content. Reductions in sulfur levels in gasoline from highs of 200-600 ppm to lows of 18-50 ppm have resulted in 9-55% reductions in HC and CO emissions and 8-77% reductions in NOx emissions, depending on vehicle technologies and driving conditions. Greater reductions have been demonstrated for low emissions vehicles and under high-speed driving conditions. Pollutant emissions appear to drop sharply as sulfur is reduced below 200 ppm, with emissions dropping steeper still below 100 ppm.

  • Sulfur inhibition in catalysts is not completely reversible. Although conversion efficiency will always improve with use of low sulfur fuel, the efficiency of the catalyst does not always return to its original state after desulfurization. In tests using 60 ppm sulfur fuel followed by a single use of 930 ppm sulfur fuel, HC emissions tripled from 0.04 g/mile to 0.12 g/mile. With a return to low sulfur fuel, emissions dropped again to 0.07 g/mile but, in order to achieve original emissions levels, fuel-rich operation resulting in high exhaust temperatures was required to fully regenerate the catalyst.

  • Sulfur content in fuel contributes to catalyst aging. Higher sulfur levels cause more serious degradation over time and, even with elevated exhaust temperatures, less complete recovery of catalyst functioning. And high regeneration temperatures contribute to thermal aging of the catalyst. Sulfur also raises the light-off temperature, the temperature at which catalytic conversion to take place, resulting in increased cold-start emissions.

  • Regeneration requirements add to overall emissions and reduce fuel efficiency. Fuel-rich operation, required to reach regeneration temperatures, results in significant increases in CO and HC emissions, and increased PM emissions that can rival diesel PM emissions. In addition, fuel-rich combustion requires increased fuel use. Vehicles that tend to operate at low speed and low load will have lower exhaust temperatures and less opportunities for desulfurization and catalyst regeneration.
Emerging Three-Way Catalysts


The benefits of reducing sulfur levels in fuels increase as vehicles are designed to meet stricter standards. These highly tuned engines practically eliminate fuel-rich operation, which is required for desulfurization. The U.S. EPA tests new cars on a standardized 30 ppm sulfur fuel and estimates variable emissions increases with use of higher sulfur commercial fuels, depending on the vehicle's emissions standards. Emissions increases can be up to 13 times higher for certified "low emissions vehicles".

One goal for the improvement of modern TWCs is to have the ability to store oxygen in order to continue efficient conversion of CO and HC during fuel-rich swings, including idle or acceleration periods. Effective storage materials that can withstand the high temperatures of close-coupled catalysts (placed for rapid warm-up to improve cold-start conversion), require use of low sulfur fuels.

Increasingly strict emissions standards require extremely efficient catalysts over a long lifetime. Recent regulations in Europe and the U.S. will require the warmed-up catalyst to have over 98% HC control even after 100,000 miles of use. Reductions in conversion efficiency, additional fuel-rich operation requirements, increased catalyst light-off time, reduced ability to store oxygen, and other inefficiencies imposed by fuel sulfur would jeopardize the ability of vehicles to meet these new stringent standards.

NOx Storage Traps


Gasoline vehicles are under increasing pressure to raise fuel economy, and thus decrease CO2 emissions. Lean-burn gasoline engine designs offer an enormous increase in efficiency, with the potential to reduce fuel consumption by 15-20%. In order to not trade off higher fuel efficiency for increased pollutant emissions, lean burn engines will require new aftertreatment technology for control of NOx emissions. NOx storage traps, the most efficient existing NOx control technology for lean burn engines, are much more dramatically impacted by fuel sulfur than TWCs. Because high sulfur levels reduce the effectiveness of the traps and necessitate increased fuel consumption, low sulfur gasoline is the key enabler for this promising technology to increase the efficiency of gasoline vehicles.

The lean burn engine increases the ratio of air to fuel, thus reducing fuel use. Lean burn engines provide an automatic benefit for CO and HC control, which are formed in smaller amounts and can be more easily oxidized in the oxygen rich exhaust. The challenge comes with control of NOx in an oxygen abundant environment. NOx storage trap technology is currently being developed for diesel NOx control and faces even less technical challenges when applied to gasoline engines because combustion temperatures can be more easily controlled. NOx storage traps demonstrate over 90% efficiency in storage and conversion of NOx to N2 but require virtually sulfur-free fuels for efficient use.

Storage traps operate by incorporating basic oxides into the catalyst, which will react with the oxidized NO2 in the presence of excess O2 to form fairly stable nitrates. NOx can be stored in this way during lean combustion (excess oxygen) conditions. As the storage medium approaches saturation, or whenever acceleration occurs, the engine will burn fuel-rich, generating CO and HC gases. This triggers the release of NO2, which reacts, as in a TWC, to oxidize CO and HC to CO2 and H2O while simultaneously being reduced to N2.

NOx storage traps are highly efficient with sulfur-free fuels, but sulfur dramatically reduces their the efficiency, lifetime, and durability. Sulfur dioxide competes with NOx for storage space on the catalyst. The sulfates stored on the catalyst become more tightly bonded than nitrates and can only be removed with prolonged fuel-rich, high temperature combustion. The original efficiency of the catalyst may never be fully recovered following sulfur poisoning, especially in more highly tuned, low-emission vehicles, which may not reach high enough temperatures for desulfurization. Sulfur also precludes the use of more efficient catalyst materials that do not require such frequent regeneration for NOx but are highly sensitive to sulfur.

The fuel-rich, high temperature combustion, required to purge the catalyst of sulfur, also significantly negates the fuel efficiency benefits of the lean burn engine. When the efficiency of the catalyst drops from 95% to 90%, tailpipe NOx emissions double and desulfurization occurs. With virtually sulfur-free fuel the NOx storage trap will retain close to 95% storage efficiency without regeneration for over 5,000 miles. At 7 ppm sulfur, a drop to 90% efficiency occurs after 1,695 miles, necessitating desulfurization. As demonstrated in figure 2.4, at over 40 ppm the distance between desulfurization events drops to a little over 100 miles. This distance between desulfurization events relates directly to fuel consumption, with increased NOx conversion efficiency translating into better fuel efficiency.

In Japan commercially available NOx absorber catalysts require use of premium grade gasoline, which has an average sulfur content of 6 ppm. The full potential of this emissions control technology, enabling dramatically increased fuel efficiency, will only be possible with ultralow sulfur gasoline.

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The types of gasoline described above require no modifications to vehicles. However, their production requires alterations to the conventional fuel manufacturing process, and their distribution in each case necessitates a separation from the conventional fuel type in order to avoid contamination ( w3 ).

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With regard to lead, it has been successfully phased out in many countries at low cost: "less than two US cents per liter for most countries", according to ( 2 ). "The benefits, however, have been substantial. According to the World Bank, countries can save five to ten times the cost of converting to unleaded gasoline in health and economic savings. These facts should encourage policy-makers that lead can be removed from gasoline without harm and with net economic benefits" ( 2 ).

RFG is also slightly more expensive than regular gasoline: "the market established a price premium for RFG currently amounting to about 5 cents per gallon above the wholesale price of unleaded regular gasoline. Markets continually fluctuate, and the 5-cent differential will likely vary over time. " ( w3 ).

The European oil industry has estimated that the cost of upgrading refineries (to produce <10 ppm sulfur gasoline) would be over 10 billion euros over 15 years" ( 4 ). For the US, the cost of reducing gasoline sulfur to 30 ppm average has been estimated to be (in 1998 dollars) US$ 8 billion, or 4.5 ¢/gal ( w4 ).

A more recent study of China's refineries concluded that reducing gasoline sulfur levels to 10 ppm maximum from current levels of 800 ppm would cost less than 2 cents per gallon. ( 5 )

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In 1999, unleaded gasoline accounted for 80 percent of total worldwide sales ( 2 ). Many Latin American countries including Brazil, Mexico, Argentina and Costa Rica have completely banned leaded gasoline. Similarly, other developing countries such as China, India and Thailand have eliminated leaded gasoline with resulting improvements in air quality and public health.

According to ( 6 ), in the United States in 1999 and the beginning of 2000, 33% of gasoline was RFG.

In the US, gasoline sulfur standards of 30 parts per million on average, and 80 parts per million maximum, will take effect in 2004 ( 1 ). European Union member states are required to make 10-ppm sulfur gasoline available from 2005 ( 4 ).

In 1998, the vehicle industry presented a proposal for fuel specifications worldwide, related to different levels of emission requirements . The "World-Wide Fuel Charter" is updated periodically.
A common problem, especially in Latin American countries, is adulteration of fuel. The fuel is mixed with kerosene, solvents or other components. This can be done at production, during distribution, at the sales station or by the final user, usually in order to reduce cost per liter fuel. Adulteration can have drastic effects on emissions.

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In Europe and in North America, lead has been largely removed from gasoline. However, according to ( 2 ), millions of people in Asian, Latin American and African countries are still exposed to unacceptable levels of airborne lead. Removing lead from gasoline in countries where it is still used should be straightforward and inexpensive as has been demonstrated in a wide variety of countries which have completed the process - see ( 2 ).
RFG with its increased combustion efficiency and lower tailpipe emissions has been introduced relatively successfully in the United States but is not yet ubiquitous.

Low sulfur gasoline will penetrate the US and EU markets in line with governmental regulations. The main problem in this regard is the capital investment necessary for refinery upgrading.

Finally, one problem that applies to all of the above is that of contamination with cheaper fuels. For example while leaded gasoline was still available in the US, it cost less at the pump than unleaded gasoline, which motivated some individuals to use it in cars equipped with catalysts, which impaired their effectiveness ( 1 ). This could also happen with regard to sulfur. One measure to counteract this would be to offer lower tax rates for cleaner fuels, as is done for example in Germany ( 4 ). Another problem is that frequently too much lubricating oil is mixed with gasoline for 2 stroke engines; premixing the fuel and lube oil can solve this problem.

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