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Heavy oil

Heavy Oil

The term "heavy oil" describes various mixtures of mineral oil with high viscosity. Depending on the country and industry sector, heavy oil is also referred to as "Heavy Fuel Oil (HFO)", "Marine Fuel Oil (MFO)", "Residual Fuel Oil (RFO)", and in Germany "Schweres Heizöl" or "Bunkeröl". It is in every case an especially cost-effective fuel, which is used primarily on ships today. Depending on the country and industry sector, it consists mostly of high boiling residues of crude oil distillation. In contrast to light oil it has a significantly higher viscosity. The quality of the individual types also fluctuates considerably and therefore places high demands on the firing plant.

Due to the high sulfur and nitrogen content in the fuel, the emission level of heavy oil firing systems is significantly higher than for light oil plants. Therefore heavy oil is used less and less frequently in Western Europe. The tank and fuel lines must also be heated to be able to pump this fuel and atomize it. In Germany heavy oil is standardized by DIN 51603, with ISO 8217 for international shipping.

The essential basis of heavy oils is crude oil, which is extracted worldwide, in the offshore industry for example and also directly for energy production. Often they consist of a wide range of low and high boiling oil fractions. The bandwidth depends greatly on the origin. Arabian oils exhibit good qualities overall with high contents of light fractions which are very well suited to form the basis for diesel and benzene. On the other hand, some Chinese or Canadian "crudes" have considerably more heavy fractions and the proportion of high-quality benzene and diesel qualities during distillation is therefore significantly lower.

Heavy oil is a combustible mixture of various hydrocarbon compounds. It consists of primarily of the residues of crude oil distillation, often mixed with lighter fractions for technical applications. Heavy oils are distinguished above all by the specification viscosity and the amount and composition of accompanying substances. The heat value of fuel oil S is between 39 and 41 MJ/kg, depending on the chemical composition and thus generally lower than that of extra light fuel oil (LFO). The specific density depends on the proportion of the various oil fractions and fluctuates between 0.94 and 1.05 kg/l. The flashpoint of heavy oil is around 120°C.

The quality and numerous test processes for determining the quality of heavy oil are recorded for example in DIN 51603-3 or ISO 8217:2005.


Viscosity is a determining feature for the specification of heavy oil. It varies depending on the batch and is usually specified for two temperatures to describe how the fuel can be handled during preheating. Usually this property can be found in the name: For example, the viscosity of marine fuel RMG 380 is approximately 380 cSt at a temperature of 50°C. As the temperature increases, viscosity decreases, reaching a value of approximately 25 cSt at 100°C for the same type of oil.

The viscosity depends on both the composition of the residual oil (the basic material) and the amount of thinner in the finished mixture. The thinner generally has a low viscosity and a low boiling point. If the ratio is not selected with great care, very heavy oil may crack even if the viscosity in the preheater is correct because the highly volatile components boil off.

Because oils can generally only be pumped to a viscosity of 350 cSt, heavy oil must always be heated before it can be moved. Furthermore, all atomization principles have different requirements for viscosity: For example, steam pressure atomizers generally require a more thin-bodied oil than rotary cup atomizer burners, which can also atomize more thick-bodied fuels with a viscosity of up to 45 cSt with no problems.

Asphaltenes, sulfur and accompanying substances

Depending on the origin and production process, heavy oil may also contain a whole series of accompanying substances, some of them undesirable, which can only be exactly determined by a chemical analysis. These precise data points are needed for a qualified evaluation of combustion emission value, for example, or to isolate causes of poor combustion. The group of undesirable substances consists mainly of water, sediments, asphaltenes, nitrogen and sulfur as well as a few metals.

A high water content in the oil draws energy from combustion and lowers the heat value. In the most unfavorable case this water can result in formation of steam bubbles in the preheater or burner and cause firing to become unstable.

Sediments are solid contents of various grain sizes originating from the crude oil. They can precipitate as sludge in the tank, clog filters and cause significant abrasion in pumps, valves and nozzles. Because sediments do not combust, they remain in the boiler as deposits or are discharged with the flue gas.

Asphaltenes are the "heaviest" components of heavy oil. They consist of very long-chain hydrocarbon compounds and tend to crack at high temperatures. In unfavorable cases this process leads to coke residues on the atomizing nozzle, rotary cup or heating surfaces.

The nitrogen content in heavy oil, measuring about 0.2 - 0.4%, is responsible for a considerable portion of nitrogen emissions of the firing plant. The ratio of fuel nitrogen to NOx in the flue gas does fall as the nitrogen content in the fuel rises. However, the values are already well above typical emission requirements, so that downstream flue gas cleaning measures are often necessary. 

The amount of elementary sulfur permitted in heavy oil has been continuously lowered in recent years. The upper limit of DIN 51603-3 is therefore 2.5% by weight, while up to 4.5% is still permitted in very heavy oils for ship operation in accordance with ISO 8217. This sulfur combusts to produce primarily sulfur dioxide. It forms sulfurous acid in the flue gas and can be responsible for considerable corrosion damage to the flue gas plant. Emission requirements generally force operators to replace fuels with sulfur contents of considerably less than 1% by weight unless extensive cleaning measures are undertaken.