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Cleaner Diesel
Cleaner Diesel can reduce emissions from existing diesel vehicles and some parameters such as low sulfur are required to enable an effective operation of advanced emission control devices.
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Diesel fuel is a complex mixture of hydrocarbons composed primarily of paraffins and aromatics, with olefin content amounting to only a few percent by volume. The combustibility of a diesel fuel in diesel engines is characterized by its cetane number, which is a measure of its ability to undergo compression ignition under standard test conditions. Fuels with a higher cetane number more readily ignite in compression ignition engines.

The most important pollutants of concern from diesel engines are particulate matter and NOx. A number of diesel fuel properties have been linked to emissions of these pollutants from diesel engines. The constituents of diesel that are of particular interest are distillation control, density, sulfur, PAHs, and cetane. Distillation curve, density and to some extent PAH control go hand in hand, as diesel with a lower final boiling point tends to have lower density and lower PAHs. In addition, studies have been conducted examining the effects on Diesel engine emissions as a result of adding oxygenated compounds to diesel fuels.

The impact of a given change in fuel composition can be different depending on the relative emissions level of the engines. Emissions from engines with high base emission rates (generally older designs) tend to be more sensitive to changes in fuel composition than those from engines with lower base emissions rates (which tend to be newer designs). It is important to note that changes in all of the fuel properties have been found to have, at most, small impacts on emissions from engines with low base emission rates. One exception is that reducing S from 0.30% to 0.05% gives relatively large benefits; reducing S from 0.05% to lower levels has minimal direct benefit but as discussed below is necessary to enable advanced technologies which can bring about very substantial reductions in diesel NOx and PM.

In addition to studies of fuel composition effects on emissions in heavy-duty diesel engines, there have also been numerous studies performed on engines used in passenger cars and light-duty trucks. A comprehensive study of fuel composition impacts on light-duty diesel emissions was performed as part of the European Programs on Emissions, Fuels and Engine Technologies (EPEFE) ( 1 ). This study investigated the impact of the density, cetane number, T95, and polycyclic aromatic content of diesel fuels on emissions from light-duty vehicles sold in Europe. The program involved 19 vehicles, 14 of which had indirect-injection engines. Indirect-injection engines currently dominate the market, but are expected to be supplanted over the next several years by direct-injection engines due to the latter's greater fuel efficiency.

Although there are some differences in terms of the magnitude of fuel composition effects on emissions from vehicles with indirect- and direct-injection engines, the directional impact on emissions is usually the same.

A comparison of the impact on heavy duty and light duty vehicles indicates that there are some instances where changing a given diesel fuel property is expected to have the opposite directional impact on emissions depending on whether the fuel is being used in a heavy-duty engine or light-duty vehicle. The most notable are the increase in NOx emissions from light-duty direct-injection engines in response to a decrease in fuel density and the increases in NOx emissions from both light-duty indirect- and direct-injection engines in response to a decrease in the T95 temperature. ( 2 ) . On the other hand experiences from Swedish EC1 (Environmental class 1) diesel fuel (ultra low sulfur, PAH and aromatic concentrations) in spite of low T95 shows a reduction of NOx emissions and also very low emissions of PAH and aromatics.

Sulfur

Sulfur occurs naturally in crude oil, and the amount of sulfur in "straight-run" diesel (diesel obtained from fractionating crude oil without further processing) is correlated with the crude sulfur content. Sulfate particulate and SOx emissions, both of which are harmful pollutants, are emitted in direct proportion to the amount of sulfur in diesel fuel. Sulfate PM contributes to PM10, and PM2.5 emissions directly with their associated adverse health and environmental effects. SO2, one fraction of the SOx, is a criteria pollutant with associated adverse effects. Further some of the SOx are also transformed in the atmosphere to sulfate PM with the associated adverse effects noted for PM.

Diesel PM, which has been found to be a human carcinogen by the California Air Resources Board, consists of three primary constituents - a carbonaceous core, a soluble organic fraction (SOF) which sits on the surface of this core and a mixture of SOx and water, which also sits on the surface of the core. Lowering the sulfur in the fuel lowers the SOx fraction of PM thus lowering the overall mass of PM emitted.

In the literature, the term low sulfur diesel (LSD) is sometimes used by different sources for sulfur levels ranging from 15 ppm to 50 ppm. Within the Info Pool, the term low sulfur diesel (LSD) will not be used except in quotations, and will refer to sulfur levels of 50 ppm or less. Ultra low sulfur diesel (ULSD) contains 15 ppm or less ( 3 ) ( 4 ). Diesel containing 10 ppm or less is called "sulfur free" ( 1 ).

The EU and North America are planning to move towards 10 and 15 wt ppm sulfur limits (also called "sulfur-free") in diesel, respectively, during the mid to latter half of this decade, in order to be able to take advantage of the advanced diesel emission reduction technologies that have extremely low tolerance for sulfur such as NOx and particulate traps and lean de-NOx catalysts. These devices can make diesel vehicles as clean as or cleaner than CNG vehicles.

A recent study summarized the impacts of sulfur in diesel fuel on diesel vehicle emissions (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). Its principle findings are summarized below.

Without Emissions Controls

In diesel vehicles, reducing sulfur not only reduces SO2 emissions in diesel vehicles, it can also significantly reduce particle emissions. In the oxygen rich exhaust of diesel vehicles several percent of the SO2 formed during combustion is oxidized to SO3, which dissolves in the water vapor present to form sulfuric acid (H2SO4) vapor. H2SO4 is one of the few substances that is capable of homogenous nucleation, which, aside from soot formation, appears to be the primary mechanism for initiation of ultrafine particle formation in diesel exhaust, producing newly formed particles of around 1 nm. Even though sulfate particles account for only a small fraction of particle volume or mass, it can account for a large fraction of particle numbers. And sulfate nanoparticles provide a relatively large surface area onto which HC species condense, resulting in particle growth and increasing particle toxicity.

Even without the benefit of additional emissions controls, reducing sulfur levels in diesel fuel leads to lower total PM emissions and a substantial decrease in the mutagenicity and toxicity of the particulate matter formed. A variety of testing opportunities have supported this conclusion. In Denmark, a reduction in fuel sulfur levels from 440 to 70 ppm led to a 56% reduction in numbers of particles emitted from diesel vehicles. Testing on Japanese diesel trucks demonstrated that a reduction in fuel sulfur from 400 to 2 ppm cut the mass of PM emissions in half. In heavy duty diesel truck testing in the U.S., a decrease in fuel sulfur from 368 to 54 ppm yielded a 14% reduction in PM emissions.

In addition to these primary PM emissions, SO2 emissions can lead to secondary particle formation, as particles form in the ambient air. EPA models estimate that over 12% of SO2 emitted in urban areas is converted in the atmosphere to sulfate PM.

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Although not nearly so widespread as the TWC in gasoline vehicles, diesel oxidation catalysts (DOCs) are the most common aftertreatment emissions control technology found in current diesel vehicles. Oxidation catalysts can increase the oxidation rate of SO2, leading to dramatic increases in sulfate nanoparticle emissions. Sulfate conversion also depends on overall catalyst efficiency, with more efficient catalysts capable of converting nearly 100% of the SO2 in the exhaust to sulfate. At high temperature operation, generated under high speed or load operating conditions, the DOC accelerates the oxidation of SO2, increasing the formation of sulfate particles. A DOC, when used with higher sulfur fuel, can dramatically increase emission of the smallest, potentially most damaging particles. Sulfur also delays the DOC light-off, increasing cold-start emissions.

Diesel Particulate Filters

The Continuously Regenerating Diesel Particulate Filter (CR-DPF) and the Catalyzed Diesel Particulate Filter (CDPF) are two examples of PM control with passive regeneration. The CR-DPF and CDPF devices can achieve 95% efficiency for control of PM emissions with 3 ppm sulfur fuel. But efficiency drops to zero with 150 ppm sulfur fuel and PM emissions actually more than double over the baseline with 350 ppm sulfur fuel. The increase in PM mass with fuel sulfur comes mostly from water bound to sulfuric acid. Soot emissions also increase with higher sulfur fuel but even with the 350 ppm sulfur fuel DPFs maintain around 50% efficiency for non-sulfate PM. The systems eventually can recover to original PM control efficiency with return to use of low sulfur fuels, but recovery takes some time due to sulfate storage on the catalyst.

Sulfur also increases the required temperature for regeneration of the filter to take place. When going from 3 to 30 ppm sulfur fuel, the exhaust temperatures required for regeneration of both filter types increased by roughly 25°C. The CDPF required consistently higher temperatures but held stable above 30 ppm, while the required temperature continued to increase for the CR-DPF.

With low sulfur fuel use, diesel particulate filters provide effective control of particles across the particle size range, even for the smallest particles, considered most dangerous for human health. Diesels with particulate filters generate comparable PM emissions to Compressed Natural Gas (CNG) vehicles, which are often promoted as a low emission alternative to diesel. A vehicle using low sulfur fuel and a DPF may in fact reduce total PM emissions below a CNG vehicle.

In addition, the elemental carbon, also known as black carbon, is virtually eliminated by a DPF. Elemental carbon appears to be responsible for more than 90% of light absorption by atmospheric aerosols and has recently been implicated as a significant factor in global warming, which highlights the potential for DPFs, with use of low sulfur fuel, to greatly reduce the warming impact of diesel vehicles.

In addition, particulate filters provide effective control of CO and HC emissions, which is less impacted by fuel sulfur. DPFs greatly reduce emissions of benzene, polycyclic aromatic hydrocarbons, alkenes such as 1,3-butadiene, and other unregulated but harmful gaseous pollutants. Measured efficiency for control of CO was between 90 and 99% and for HC was between 58 and 82%, resulting in 1 to 2 orders of magnitude lower emissions for these pollutants than for gasoline and CNG vehicles.

NOx Control Systems

Two very different technologies, NOx Adsorbers and Selective Catalytic Reduction (SCR) systems, are the prominent alternatives for further NOx control.

NOx adsorbers are also known as NOx Storage Catalysts or Lean NOx Traps (LNT). NOx traps very efficiently store sulfur. SO2 emissions, stored as solid sulfates, are much more tightly bound to the substrate and require higher temperatures for removal. Over a period of time fuel sulfur, even at low levels, will fill the capacity of the trap, causing NOx storage and conversion efficiency to decline significantly.

The regeneration of the trap and release of stored sulfur presents an additional technical hurdle for diesel engines because exhaust temperatures are lower and less controllable than in gasoline engines. And turbocharged diesel engines, in particular, cannot achieve temperatures required for desulfurization under urban driving conditions. In addition to the fuel penalty of increasing the fuel-to-air ratio, diesel engines have a limited ability to operate fuel-rich due to unacceptable levels of smoke and HC emissions which can result.

Regeneration and desulfation control logics under transient conditions are the major technical challenge facing NOx adsorbers. Optimizing the system for maximum efficiency and minimum fuel penalty, already a challenge, is complicated further by variability in operation and durability concerns.

Selective Catalytic Reduction (SCR, where a solution of urea is injected before the catalyst, is emerging as the leading NOx reduction technology in Europe to meet Euro IV and Euro V heavy-duty diesel standards. The engine is tuned for low PM and high fuel economy. This eliminates the need for a DPF, due to less stringent PM standards in Europe compared to the US, but increases the NOx emissions above standards. The regulations can be met with an SCR system with 65-80% NOx conversion efficiency. In addition, the fuel economy benefits, even accounting for the reductant, can be as high as 7%.

While high levels of efficiency can be achieved for mobile sources, SCR use in vehicles presents several obstacles. With the variable power required by vehicle systems it can be difficult to achieve precise dosing of urea, necessitating use of a downstream oxidation catalyst to prevent unreacted urea from being emitted as ammonia, which has chronic and acute human health impacts. Fuel sulfur will increase the PM emissions from the downstream oxidation catalyst. Sulfur reactions in urea-based SCR systems can also form ammonium bi-sulfate, a severe respiratory irritant.

Retrofit Technologies

Where low sulfur fuel is available, retrofit technologies can dramatically reduce emissions from existing vehicles. Oxidation catalysts and particulate filters for heavy-duty diesel vehicles are the most common retrofit technologies. Recent projects have also included EGR in combination with a DPF, for simultaneous control of NOx and PM. While DOCs are less sensitive to sulfur than particulate filter technology, increasing fuel sulfur levels do result in reduced conversion efficiency of the catalyst and can lead to increased sulfate formation, resulting in even higher particle emissions than operation without a catalyst. DPFs are also an easy, and much more effective retrofit option but require at least low sulfur fuel use.

Sweden started a tax incentive program ten years ago to promote the introduction of ultra low sulfur diesel fuel. Sweden's <10 ppm (in practice 1-3 ppm) sulfur diesel fuel has now a market share of 98-99 %. This has facilitated the introduction of over 6,500 buses and trucks equipped with DPFs.

Volatility

Diesel fuel consists of a mixture of hydrocarbons having different molecular weights and boiling points. As a result, as some of it boils away on heating, the boiling point of the remainder increases. This fact is used to characterize the range of hydrocarbons in the fuel in the form of a "distillation curve" specifying the temperature at which 10%, 20%, etc. of the hydrocarbons have boiled away. A low 10% boiling point is associated with a significant content of relatively volatile hydrocarbons. Fuels with this characteristic tend to exhibit somewhat higher HC emissions than others.
Fuels with a lower end point distillation temperature tend to give lower particulate mass emissions as does reduced aromatic and polyaromatic contents.

Aromatic Hydrocarbon Content

Aromatic hydrocarbons are hydrocarbon compounds containing one or more "benzene-like" ring structures. They are distinguished from paraffins and napthenes, the other major hydrocarbon constituents of diesel fuel, which lack such structures. Compared to these other components, aromatic hydrocarbons are denser, have poorer self-ignition qualities, and produce more soot in burning. Ordinarily, "straight run" diesel fuel produced by simple distillation of crude oil is fairly low in aromatic hydrocarbons. Catalytic cracking of residual oil to increase gasoline and diesel production results in increased aromatic content, however. A typical straight run diesel might contain 20 to 25% aromatics by volume, while a diesel blended from catalytically cracked stocks could have 40-50% aromatics.

Aromatic hydrocarbons have poor self-ignition qualities, so that diesel fuels containing a high fraction of aromatics tend to have low Cetane numbers. Typical Cetane values for straight run diesel are in the range of 50-55; those for highly aromatic diesel fuels are typically 40 to 45, and may be even lower. This produces more difficulty in cold starting, and increased combustion noise, HC, and NOx due to the increased ignition delay.

Increased aromatic content is also correlated with higher particulate emissions. Aromatic hydrocarbons have a greater tendency to form soot in burning, and the poorer combustion quality also appears to increase particulate SOF emissions. Increased aromatic content may also be correlated with increased SOF mutagenicity, possibly due to increased PNA and nitro-PNA emissions. There is also some evidence that more highly aromatic fuels have a greater tendency to form deposits on fuel injectors and other critical components. Such deposits can interfere with proper fuel/air mixing, greatly increasing PM and HC emissions.

Polycyclic aromatic hydrocarbons (PAH) are included in the great number of compounds present in the group of unregulated pollutants emitted from vehicles. Exhaust emissions of PAH (here defined as three ringed and larger) are distributed between particulate and semi-volatile phases. Some of these compounds in the group of PAH are mutagenic in the Ames test and even in some cases causes cancer in animals after skin painting experiments. Because of this fact, it is of importance to limit the emissions of PAH from vehicles especially in densely populated high traffic urban areas. An important factor affecting the emissions of PAH from vehicles is selection of fuel and fuel components. A linear relationship exists between fuel PAH input and emissions of PAH. The PAH emission in the exhaust consists of uncombusted through fuel input PAH and PAH formed in the combustion process. By selection of diesel fuel quality with low PAH contents (# 4 mg/l, sum of PAH (i.e. individual PAH (phenanthrene to coronene (amounts added together) the PAH exhaust emissions will be reduced by up to approximately 80% compared to diesel fuel with PAH contents larger than 1 g/l (sum of PAH). By reducing fuel PAH contents in commercial available diesel fuel the emissions of PAH to the environment will be reduced. . For example the Swedish diesel fuel has a aromatic content of less than 5% and PAH "not measurable".

Other fuel properties

Other fuel properties may also have an effect on emissions. Fuel density, for instance, may affect the mass of fuel injected into the combustion chamber, and thus the air/fuel ratio. This is because fuel injection pumps meter fuel by volume, not by mass, and the denser fuel contains a greater mass in the same volume. Fuel viscosity can also affect the fuel injection characteristics, and thus the mixing rate. The corrosiveness, cleanliness, and lubricating properties of the fuel can all affect the service life of the fuel injection equipment-- possibly contributing to excessive in-use emissions if the equipment is worn out prematurely.

Fuel additives

Several generic types of diesel fuel additives can have a significant effect on emissions. These include Cetane enhancers, smoke suppressants, and detergent additives. In addition, some additive research has been directed specifically at emissions reduction in recent years.

Cetane enhancers are used to enhance the self-ignition qualities of diesel fuel. These compounds (generally organic nitrates) are generally added to reduce the adverse impact of high aromatic fuels on cold starting and combustion noise. These compounds also appear to reduce the aromatic hydrocarbons' adverse impacts on HC and PM emissions, although PM emissions with the Cetane improver are generally still somewhat higher than those from a higher quality fuel able to attain the same Cetane rating without the additive. In the Dutch study cited earlier, no significant effect of ashless Cetane improving additives could be detected on NOx or particulates.

Smoke suppressing additives are organic compounds of calcium, barium, or (sometimes) magnesium. Added to diesel fuel, these compounds inhibit soot formation during the combustion process, and thus greatly reduce emissions of visible smoke. Their effects on the particulate SOF are not fully documented, but one study has shown a significant increase in the PAH content and mutagenicity of the SOF with a barium additive. Particulate sulfate emissions are greatly increased with these additives, since all of them readily form stable solid metal sulfates, which are emitted in the exhaust. The overall effect of reducing soot and increasing metal sulfate emissions may be either an increase or decrease in the total particulate mass, depending on the soot emissions level at the beginning and the amount of additive used.

Detergent additives (often packaged in combination with a Cetane enhancer) help to prevent and remove coke deposits on fuel injector tips and other vulnerable locations. By thus maintaining new engine injection and mixing characteristics, these deposits can help to decrease in-use PM and HC emissions. A study for the California Air Resources Board estimated the increase in PM emissions due to fuel injector problems from trucks in use as being more than 50% of new-vehicle emissions levels. A significant fraction of this excess is unquestionably due to fuel injector deposits.
There is an absolute need to add lubricating additives to low sulfur diesel. These can be specific additives from the additive industry or a few of percent of for example RME.

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Unless special processing is done, the distillate fractions will contain sulfur in proportion to the sulfur content of the crude oil (i.e. high sulfur content crude oil will yield high sulfur content diesel fuel) (5). In order to produce low sulfur fuel, refineries must be adapted to extract the sulfur, e.g. hydrogen must be available for the sulfur removal process. In addition, an appropriate distribution infrastructure must be installed, which keeps LSD separate from regular diesel in order to avoid sulfur contamination of LSD (w2).

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A recent study summarized the costs associated with lowering sulfur levels in diesel fuel (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). Its principle findings are summarized below.

A report prepared for the Asia Development Bank estimated the costs associated with lowering sulfur levels in diesel fuel throughout Asia. The sulfur standards in the countries included range from 500 to 10,000 ppm for diesel, with an average standard of 2-3,000 ppm. The study found that costs are roughly constant, at around 4 cents/gallon (~1 cent/l), , for product levels ranging from 1,000 to 250 ppm sulfur. Costs more than double when making the jump from 250 to a 50 ppm product, but remain roughly constant at just over 10 cents/gallon for 10 and 50 ppm sulfur diesel fuel (Enstrat 2002).

The estimates in the Asia Development Bank study are roughly double the near-term costs reported in a Trans-Energy study for China.

The Trans-Energy study modeled the costs of improving transportation fuel quality in China. Starting from a baseline of 800 to 1,000 ppm sulfur gasoline and 2,000 ppm sulfur diesel, the costs varied depending upon the final fuel quality achieved and the length of time over which the investment was stretched.

  • Over the near-term, to meet 2005 goals, total increases in diesel fuel costs ranged from 1.9 cents/gallon (~0.5 cent/l), , to achieve nationwide supply of 500 ppm sulfur diesel, to 2.1 cents/gallon (~0.6 cent/l) for 350 ppm sulfur diesel. From a 500 ppm sulfur baseline, the cost rose to only 2.0 cents/gallon (~0.55 cent/l) to provide higher quality 50 ppm sulfur fuel for polluted urban areas, up to 35% of total supply. All the long-term 2010 scenarios were priced at 3.2 cents/gallon (~0.8 cent/l) to achieve a baseline of 350 ppm sulfur diesel, with up to 40% of the country receiving 50 ppm sulfur diesel.
  • Costs associated with gasoline desulfurization were roughly half the costs for diesel in all scenarios. Over the near-term, increases in gasoline costs ranged from 0.8 cents/gallon (~0.2 cent/l), to achieve nationwide supply of 500 ppm sulfur gasoline, to 1.1 cents/gallon (~0.3 cent/l) to reduce sulfur levels to 150 ppm. All of the longer term scenarios for gasoline cost 1.5 cents/gallon (~0.4 cent/l) for a 150 ppm sulfur gasoline as the baseline and up to 40% of the country receiving 50 ppm sulfur gasoline (Trans-Energy 2002).

The U.S. EPA found that desulfurization costs associated with meeting the new sulfur standard for diesel will be roughly twice the cost of meeting the standard for gasoline. In the U.S., current sulfur levels range from 300 to 350 ppm in gasoline (with the exception of California). The EPA found that average costs to meet the new, phased-in 30 ppm sulfur standard for gasoline will be over 1.9 cents/gallon in 2004 but will decline to less than 1.7 cents/gallon in 2010, as lower cost technology becomes more viable (EPA 1999). On the other hand, the costs of meeting the new 15 ppm standard for on-road diesel, from the baseline of a 500 ppm sulfur standard, are expected to start at a national average of 4.3 cents/gallon and increase to 5 cents/gallon in 2010 (EPA 2000b).

Current non-road fuels in the U.S. can have sulfur levels as high as 3,000 ppm, giving refiners some flexibility in how they meet the on-road diesel standards. The EPA has recently announced plans to extend the 15 ppm sulfur cap to non-road diesel fuels, which could increase incremental costs for all diesel fuel. A MathPro study estimated that an extension of the 15 ppm sulfur cap to non-road diesel fuel would raise the estimated range of incremental costs for all diesel fuel to 4.7-7.8 cents/gallon (MathPro 2000).

In a study to assess the costs of a phased-in lowering of sulfur content from a maximum of 50 ppm to a maximum of 10 ppm for Europe, lower average costs were reported to achieve the 10 ppm cap once the initial reductions to 50 ppm had been made. Costs were expected to range from 0.4 to 1.1 cents/gallon for gasoline and from 1.1 to 2.3 cents/gallon for diesel, with a possible price premium for diesel that could reach as high as 3.4 cents/gallon.

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Very low sulfur diesel fuel has been available in Scandinavian countries for many years and is beginning to spread rapidly also in other industrialized countries. By 2005, diesel fuel with a maximum of 10 ppm sulfur will be widely available across the EU. By 2006, most on road diesel fuel in the US must have less than 15 ppm sulfur. In Hong Kong, diesel fuel currently is under 50 ppm sulfur.

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, although it tends to lower the sulfur levels through its dilution effects (kerosene).

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As the use of clean-diesel grows in the USA and Europe during the period from 2005-2010, world-wide supplies should increase. Sulfur reduction technologies will become more available as well. But the capital investment to upgrade refineries may prevent many developing countries from upgrading their refineries. keep prices for low-sulfur diesel fuel very high and unreasonable for developing countries. In ( w2 ) the difficulties of infrastructure development are discussed.

See Also
Cleaner fuels - Cleaner diesel
General topics
Vehicular air pollution - Cleaner fuels and energy sources
Conventional fuels - Diesel

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