Diesel fuel also called Gas oil is a liquid fuel used in internal combustion engines. It is a mixture of hydrocarbons (C11~C22) obtained by Crude Oil Distillation. Diesel is used for transportation, marine shipping, manufacturing, power generation, construction, and farming to strengthen the world’s economy. In many countries, diesel fuel is standardized and most of the countries follow the European Union standard EN 590 for diesel with some minor changes. The purpose of euro fuel standards is to reduce the levels of harmful exhaust emissions from combustion engines.
European Union has systematically improved fossil fuel quality and introduced more stringent emission standards. The EU’s fuel quality improvement initiatives have resulted in the supply of both gasoline and diesel fuel with near-zero sulfur content. The EN 590 standard covers automotive diesel fuel quality and describes the physical properties that must be met to be sold in the European Union and several other European countries.
The major applicable fuel properties include cetane number and sulfur content for diesel. The CEN (European Committee for Standardization) standards are periodically updated to reflect changes in specifications, such as the mandatory reductions in sulfur content. The diesel produced according to the Euro 5 specifications with sulfur less than 10 ppm is called Ultra Low Sulfur Diesel.
The first set of standards by the European Committee for Standardization (CEN) was voluntarily introduced in 1993 called Euro 1 for automotive fuels but was observed by all fuel suppliers in Europe. Some of the important revisions of the EN 590 standard are as follows;
- EN 590, Revision 1, 1996 directed a new sulfur limit of 500 ppm. The cetane number remained at 49 and was referred to as Euro 2 diesel fuel.
- EN 590, Revision 2, 1998, directs sulfur limit of 350 ppm and cetane number at 51 specified by Directive 98/70/EC, called Euro 3 diesel fuel.
- EN 590, Revision 3, 2003, directs sulfur limit of 50 ppm and cetane number at 51 specified by Directive 2003/17/EC, called Euro 4 diesel fuel.
- EN 590, Revision 4, 2009, sulfur limits of 10 ppm and cetane number at 51 specifications by Directive 2009/30/EC, called Euro 5 diesel fuel.
These revisions in euro diesel fuel specifications are summarized in the table below;
To meet the specifications of diesel fuel, the hydrotreating process is being applied worldwide by oil refineries. Cetane number, flash point, pour point, cloud point, and sulfur are the most important properties of diesel fuel. The fuel volatility requirements depend upon the engine design and applications. For automotive diesel fuel, more volatile fuels are better while for rail cars, ships, and power stations, heavier fuel is more economical.
1. Cetane Number
It is the measurement of the ignition quality of diesel fuel the same as the Octane number for Gasoline. A Higher Cetane number means shorter ignition delay periods. The reference fuel is Cetane (n-hexadecane, n-C16H34) with a Cetane number of 100 and Alpha-Methyl Naphthalene having a Cetane number of 0.
High Cetane number of diesel fuel benefit short ignition delay, improved cold start, reduced white smoke during start-up, low emissions, low engine noise, and low fuel consumption.
- Cetane number improves with paraffin and naphthene contents.
- Operating conditions can improve Cetane number due to high saturation of PNA (Parrafins, Naphthenes, Aromatics) high hydrogen partial pressure, high system pressure, and low LHSV.
- Catalyst selection as NiMo show higher rates of PNA saturation at reaction conditions as compared to CoMo
- Use of additives to improve the Cetane number.
2. Wear Scar or Lubricity
It is the ability to reduce friction between moving parts (fuel pumps and injectors) of a diesel engine. There must be a minimum level of lubricity in the fuel to avoid excessive wear and to save the engines from being damaged. Very small amounts of oxygen- and nitrogen-containing compounds and certain classes of aromatic compounds in diesel fuel play the role of lubricant but after hydrotreatment, these compounds are removed leaving the diesel dry.
- A lubricity additive has to be injected into diesel to control the lubricity.
- In addition, controlling the sulfur content at the maximum limit will also reduce the chemical requirement because of natural lubricating agents.
3. Flash Point
It is the lowest temperature at which diesel produces sufficient vapours that result in the inflammable mixture in the air and ignites when the ignition source is provided. It is an important property from a storage and handling point of view. A lower flash point means it will ignite at a lower temperature. High flash point diesel is safe to handle.
- It is improved by increasing striping steam in strippers or reboilers at crude distillation and hydrotreating units.
- The blending of kerosene with diesel reduces the flashpoint which has to be compensated with stripping steam as well.
Both cloud point and pour point described here are not mentioned in the standards but are season or weather-based i.e. winter or summer specs.
4. Cloud Point
It indicates the stability of the fuel at low-temperature operations. It is the temperature below which wax crystals are formed due to the precipitation of paraffin and shows the cloudy appearance of diesel. This is an important parameter because precipitates of wax can clog the fuel filters and atomizers, resulting in poor performance of the engine.
In the winter season, produce the diesel with specifications according to the region’s cold temperature.
- Producing lighter diesel at Crude Distillation Unit or Blend lighter products like kerosene or jet fuel to improve the cloud point.
- Use of additives, solvent dewaxing, and catalytic dewaxing of diesel fuel.
5. Cold Filter Plugging Point
Paraffins are an important constituent of all petroleum fuels. When the fuel is cooled, normal paraffin solubility decreases, and some paraffins separate out as wax crystals. The highest temperature at which the fuel when cooled under defined conditions will not flow through a defined wire mesh within a certain time is called the cold filter plugging point (CFPP). CFPP indicates the low-temperature operability of fuel when cooled below cloud point temperature.
5. Pour Point
Pour Point is also a low-temperature performance indicator of diesel fuel. It is the lowest temperature at which fuel ceases to flow. It is normally, 8 °C below the cloud point of diesel. In a temperate climate, the diesel pour point generally varies between +6 and -6 °C.
Ultra-low sulfur diesel (<10 ppm, sulfur) is being produced in most parts of the world due to the strong negative environmental impacts of sulfur. In Europe, there is a 10 ppm sulfur limit in diesel since 2003. Sulfur has a strong negative impact on the environment and there have been continuous efforts to reduce sulfur content in diesel. To control the sulfur in diesel, the hydrotreating process is applied across the globe. The performance of the hydrotreating and hydrocracking catalyst plays a vital role in controlling the sulfur content. Temperature increase across the Hydrotreating reactor is the key parameter to be applied for reducing sulfur in the product.
- Hydroprocessing for Clean Energy by Frank (Xin X.) ZHU, Richard Hoehn, Vasant Thakkar and Edwin Yuh
- Handbook of Petroleum Product Analysis by James G. Speight
- Petroleum fuels manufacturing handbook by Surinder Parkash