How Can Mixing Two On Spec Bunker Cargos Lead to Unusable Fuel?

Blending, excessive handling and treatment, and mixing fuels can all destabilise bunkers.

Mixing two bunker cargos that are perfectly within ISO8217 specification can lead to unusable fuel, Steve Bee, Global Business Director, Intertek (Lintec) ShipCare Services told delegates last week who had gathered for the Connecticut Maritime Association (CMA)'s Shipping 2014 annual conference.

Bee explained that problems with fuel quality tend to fall into two catagories: Those that are covered by ISO8217 and those that are not.

The key concern in relation to the quality of residual fuels is the inclusion of blend or cutter stocks to lower sulfur content, with density, viscosity, cat fines, chemicals, and sulfur being the parameters most affected by the blending operations.

While in most cases density and viscosity issues can be treated onboard, Bee explained they could indicate underlying issues that are not so easily handled.

"Low levels of chemicals aren't uncommon and at low levels probably don't cause any problems at all. It's when those concentrations increase that the problems are likely to be seen," he said.

And while those chemicals can lead to the formation of plastics, the real concern is with the inherent stability of the fuel, which can lead to combustion problems and other slow burning characteristics of the bunkers.

"In a number of cases where problems are seen the actual fuel has met the ISO8217 criteria, but physically still couldn't be used," Bee said.

Fuel Stability

A key issue in such cases is the stability of the fuel, which from a technical perspective is its ability to retain the long chain asphaltene molecules in the fuel solution.

A stable fuel holds the asphaltenes in suspension dispersed uniformly throughout the fuel without issue, while in a fuel with low stability the asphaltenes fall out of suspension causing sludge formation.

"When the oil medium is of an aromatic, or ring structure in nature, the polar asphaltenes repel each other and are, as such, evenly distributed throughout the fuel matrix," Bee explained.

"But if the oil blend is paraffinic, or alkane chain in nature, then the asphaltenes will tend to colonise together and the fuel becomes unstable and the asphaltenes form sludge.

"So effectively the asphaltenes are soluble in an aromatic medium, and insoluble in linear chain environment."

Mixing, Blending, Excessive Handling

When bunker suppliers are looking to reduce density, viscosity, or sulfur content within the fuel it is common to blend it with various cuter stocks, and that, Bee explained, can affect the stability of the fuel as it actually changes the chemistry of the oil medium.

"Mixing with other fuels can also destabilise the fuel, as can excessive handling and treatment. You can get two perfectly stable fuels, put them together, and in certain cases it all goes to hell in a handcart," he said.

"In one month towards the end of 2013 we noticed 8 cases where poor fuel stability was exhibited."

CMA's next event, CMA Shipping 2015, will be held on March 23-25, 2015 at the Hilton Stamford Hotel, Connecticut.