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Cetane
Also known as: Cetane number, Cetane index
Cetane is one of the most important product qualities for diesel. Specifically, cetane is a measure of the tendency of diesel fuel to auto-ignite when injected into the combustion chamber of a diesel engine that has been heated by compression of the air in the chamber.
In a diesel engine, air is compressed in the engine's combustion chamber in the absence of any fuel, resulting in very hot compressed air. Fuel is then injected, using the heat of the compressed air to trigger combustion. For efficient engine operation, it is important that the fuel ignites quickly and uniformly when injected. Some hydrocarbons will have a tendency to auto-ignite easily while others will resist ignition. Cetane is a measure of a hydrocarbons ability to readily ignite. So in diesel, a higher cetane number is more desirable.
Cetane is valuable to refiners as it allows them to make higher-valued grades of diesel and/or to blend cheap low-cetane material into finished diesel and still meet the cetane specification. The refiner's options for increasing cetane are through hydroprocessing (hydrocracking, hydrotreating) or using cetane enhancers.
Measurement of cetane number
Cetane number is measured using a scale reflecting the degree to which a fuel will auto-ignite under specific operating conditions. The scale is defined based on the auto-ignition characteristics of two reference fuels. Cetane (C16H36) auto-ignites quite readily and is assigned a cetane rating of 100. Heptamethylnonane (C16H34) has a much lower tendency to auto-ignite and is assigned a rating of 15. Other fuels are assigned a cetane rating based on the mixture of cetane and alpha-methyl-naphthalene that results in the same level of auto-ignition as the fuel being considered. For example, a fuel that auto-ignites like a mixture of 55% cetane mixed with 45% alpha-methyl-naphthalene would have a cetane number of 55.
Cetane index
Cetane index is an estimation of the cetane number made using bulk properties (density and distillation profile) without having to employ a test engine. Since cetane index cannot account for the presence of cetane improvers, its value is always equal to or less than the true cetane number.
High and low cetane materials
Certain chemical structures are associated with high and low cetane numbers. Generally speaking, aromatics, olefins, and iso-paraffins (branched paraffins) will have lower cetane numbers. Straight-chain saturated paraffins will have higher cetane numbers and therefore make very good blendstocks for diesel.
Some of the higher cetane diesel blendstocks are:
- Straight run distillate and kerosene - Straight chain, saturated paraffins
- Hydrotreated distillates - Saturated paraffins
- Hydrocracked distillate and kerosene - Saturated paraffins
The most common lower-cetane diesel-range blendstocks are cycle oil from the FCC and coker gasoil, both of which tend to have a high concentration of aromatics and need to be processed through a diesel hydrotreater before they can be blended into diesel.
