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When #6 Oil Does and Doesn’t Make Sense

By October 8, 2014August 9th, 2019Technical Resources

In geographic locations where the cost of natural gas remains relatively high, using #6 oil can be a good strategy for reducing energy costs in industrial furnaces.

However, #6 oil also presents considerable operational challenges. And in many places, the cost of oil is rising at a consistently higher rate than natural gas, undermining the traditional cost advantage of #6 oil.

Extra heat needed

Oil_BarrelTo create #6 oil, the residual oil at the end of the refining process is blended with a better grade of oil to produce a product with the certified specifications. However, to make #6 oil pumpable, it usually needs to be heated. Then it needs to be heated even more to reduce its viscosity at the burner otherwise the oil won’t atomize into droplets fine enough to burn efficiently.

If the droplets aren’t small enough, the more-volatile components in the droplet tend to burn off, leaving behind carbon particles that don’t release energy, as well as produce smoke.

Because of these heating requirements for #6 oil, the system needs a recirculating ring main of oil steam-traced and insulated continuously moving back to the tank to keep the temperature constant. This recirculating also helps to maintain the necessary viscosity and keep additives mixed.

Much maintenance required

Upon shutdown, the line downstream of the return (which is located as close as possible to the burner) must be completely cleaned of oil to avoid freezing and clogging.

Additionally, the sulfur and vanadium in residual oil can lead to corrosion in the storage tank and the boiler. Oil treatment chemicals often have to be used to protect the equipment.

Burning #6 oil also produces carbon ash, and this soot reduces the efficiency of heat transfer. Therefore, the soot must be constantly blown off the fireside surface of the boiler.

Savings depend on location

Historically, hydrocarbon energy prices have moved relative to each other. However, in recent years as political and social pressure to reduce greenhouse gas emissions has increased the long-standing price relationship between natural gas and oil has been altered. In many locations including most of North America the price of natural gas has dropped lower than usual relative to oil, which releases more carbon dioxide into the atmosphere than natural gas.

In those locations, using #6 oil as a fuel in industrial combustion processes makes little sense at the moment. While it used to be a viable cost-saving alternative, with slightly lower cost than natural gas, the current lower cost of natural gas almost invariably makes it a better option now. When or if this situation will reverse is an open question.

However, many areas of the world don’t have ready access to oil through natural gas pipelines, and transportation costs drive natural gas prices higher in those regions. In these locations, #6 oil remains the more efficient option in some cases.


In locations where natural gas prices have not significantly dropped, the use of #6 oil for industrial combustion processes can reduce energy costs. However, installing, running, and maintaining the necessary system is expensive and time-consuming. If you’re going to use #6 oil, intelligent engineering solutions are needed to achieve the best-possible energy efficiency increase.

Image courtesy of Amiralis, Creative Commons.  


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