SULPHUR occurs naturally in crude oil and the Sulphur content can range anywhere between 100 and 33,000 parts per million (ppm).
If the Sulphur is not removed during the refining process it will contaminate the diesel fuel. Depending on the crude oil used, as well as the refinery configuration, sulphur levels in diesel fuel it can be as low as 10ppm or higher than 10,000ppm.
In South Africa, since Jan 2006, 2 grades of Diesel fuel have been available:
- Standard Grade diesel – 500ppm Sulphur (max) Standard Diesel 500ppm
- Low Sulphur grade diesel – 50ppm Sulphur (max) Low-Sulphur diesel 50ppm
The major concerns with regard to the presence of Sulphur in fuels are:
Sulphur is the most important fuel parameter affecting exhaust emissions and it contributes significantly to fine particulate matter (PM) emissions, through the formation of sulphates both in the exhaust stream and later in the atmosphere. Low-Sulphur fuels burn cleaner and reduce engine particulate emissions.
Low-Sulphur diesel is thus required primarily to reduce engine particulate emissions. Reductions in diesel fuel Sulphur will provide particulate emission reductions in all engines, regardless of the emission hardware installed. The latest generation of common rail engines emit 60% less particulate matter than their immediate pre-chamber predecessors, and when combined with a DPF system, reduce the number of particulates in the exhaust gases to the level of ambient air, and completely eliminates black smoke.
The Sulphur contained in Diesel fuel is likely to be transformed into gaseous Sulphur compounds in the oxidation catalyst contained in the DPF system, and may be transformed through secondary sulphate particulates in the atmosphere. Therefore the use of Sulphur-free fuels in vehicles with DPF systems is highly recommended to avoid this phenomenon.
Another reason for introducing lower Sulphur fuels is to allow for the introduction of emissions control devises that can significantly reduce vehicle emissions. These technologies are already in place and are continuous being improved to further control vehicle emissions. However, these technologies generally require specific fuel qualities, often including low Sulphur levels.
The presence of Sulphur in fuels restricts the possibilities to use or introduce emissions control technologies. Furthermore the efficiency of some exhaust after-treatment systems is reduced as fuel Sulphur content increases, while others are rendered permanently ineffective through Sulphur poisoning.
Oxidation catalysts are widely used on diesel passenger cars and light commercial vehicles in South Africa since 2006 to help meet emission requirements. They are effective in reducing unburnt hydrocarbons and carbon monoxide from the exhaust, but they are not able to burn soot particles in the exhaust.
The Diesel Particulate Filter (DPF) allows vehicles to achieve extremely low values of particulate emissions through the filtration of solid particles from the exhaust gas. They can be very efficient, removing more than 90% of the PM from the exhaust, and have been successfully used both for HD diesel vehicles and for diesel passenger cars.
The latest generation of common rail diesel engines have electronic injection strategies for increasing exhaust gas temperatures, so combusting the trapped particulate and thus making it possible to regenerate the DPF while in service in the vehicle.
The presence of Sulphur in diesel fuel can lead to the corrosion and wear of engine components and this can have a significant effect on engine life. As the Sulphur level decreases, the relative engine life increases.
During combustion, the formation of Sulphur oxides and water vapour combine resulting in Sulphur acids (sulphuric and sulphurous acids). During combustion, nitrogen from the air is also oxidised, forming nitrogen oxides (NOx) which in turn become nitric and nitrous acids. These acids are extremely corrosive to engine components, so good quality engine oil fortified with appropriate levels of neutralising agents are necessary to protect the engine components from damage.
Exhaust Gas Re-circulation (EGR), is now widely used in Light Duty and Passenger vehicles and to a lesser extent also in heavy duty engines. EGR effectively returns a portion of the exhaust gas back to the engine under part-load operating conditions.
Since NOx formation is extremely sensitive to temperature, by replacing some of the intake air with inert exhaust gas, the flame temperature can be reduced by a small amount and the NOx emissions can be reduced without seriously affecting the combustion efficiency.
The exhaust gas is usually cooled before returning it to the engine via the intake manifold and the small amount of sulfuric acid in the exhaust may condense, and this presents a risk of internal engine corrosion.
The lower the Sulphur level in the diesel fuel, the lower the risk of internal engine corrosion.