Cetane improvers bridge the quality gap for cracked streams, but at up to 5,000 mg/kg, they are an expensive last resort.

Gas oil as diesel fuel: the blending component options

All gas oils fractions produced from refining crude oil are, in principle, suitable for diesel fuel blending, but of course some are better than others. Straight-run light gas oil is generally paraffinic and therefore has a good ignition quality, so: higher cetane number ( 50 – 55). The cracked gas oils from e.g. the cat.cracker or thermal cracker have a higher olefinic and aromatic content and are around the 30 – 35 cetane range.

EU Automotive diesel fuel (EN 590) has a high cetane number

Gas oil from hydrocracking however, adding a lot of hydrogen into the gas oil, tends to be more paraffinic too and will make a good diesel fuel blending component. In fact, imported gas oils are blended with hydrocracked gas oil to improve the quality to the EU automotive diesel fuel standard. Another blending component is kerosene, often added to improve the cold flow properties of gas oil/diesel fuel to meet winter quality requirements. In addition, kerosene enjoys an acceptable cetane number as well (40 – 50). 

Managing Cetane Levels in EN 590 Automotive Diesel

In other words: in order to produce automotive diesel fuel according the EU EN 590 standard, refineries would ideally make the EN 590 (Cetane: min. 51) quality from straight-run and hydrocracked gas oil fractions, and do without the cat.cracked or thermal cracked gas oils. But in reality the blender would, or is sometimes forced to, add some of the cracked gas oil into the diesel fuel. This addition will lower the cetane number and consequently the blender may require the use of ignition improver additives. This addition of the ignition improver would lift the cetane number up by 4 – 7 points! However, adding the improver, typically around 1.000 – 5.000 mg/kg, is expensive and blenders will try to avoid using them in the first place.