Green Alternatives - Low Sulphur Diesel

The emissions of most interest in relation to diesel engines are oxides of nitrogen (NOx), hydrocarbons and fine particles.

NOx are a precursor to the formation of photochemical smog and there is also evidence that NOx react with other pollutants to form particles. Fine particles have been identified as a major health risk and the smaller the particle, the greater is that risk.

Motor vehicles powered by diesel engines are a significantly disproportionate contributor of fine particle pollution and oxides of nitrogen in urban areas.

Diesel fuel is derived from light virgin gas oil that is produced from the distillation of crude oil.

Low Sulphur Diesel (LSD) is produced in the refineries with a hydro-desulphurisation unit and has a sulphur content of 350 ppm. High levels of sulphur are undesirable as during combustion these are converted into volatile sulphur oxides, which lead to increased engine wear.

They also contribute directly to acid rain and form solid sulphates, which add to the particulate matter in the exhaust gas.

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Ultra Low Sulphur Diesel

Ultra low sulphur diesel is produced in much the same way as diesel but requires a two-stage high severity hydro-desulphurisation unit using cobalt-molybdenum catalyst in the first stage and nickel ^molybdenum catalyst in the second stage.

Hydrogenation of diesel over the Co-Mo catalyst removes mostly sulphur associated with aliphatic hydrocarbons, while more active Ni-Mo catalyst facilitates hydrogenation of aromatic sulphur as well as saturation of aromatic hydrocarbons thus increasing the cetane number and changing the physical properties of resulting diesel such as cloud point and viscosity.