Low sulfur fuel raises new issues for diesel operators

Ever since the first professional mariners went to sea, they have learned to adapt to changes. The introduction of the internal combustion engine and more advanced control systems brought both benefits and challenges. The latest challenge is the impact of emissions regulations and subsequent changes in fuels needed to meet those emission standards. Mariners who need to deal with these dramatic changes may have very little knowledge of how they can impact operations, costs and equipment.

The Clean Air Act has mandated that sulfur levels be reduced to less than 15 parts-per-million (ppm) in distillate fuels. A good source for information on the implementation dates of these changes can be found at www.clean-diesel.org. The Northeast and Middle Atlantic region are already at 500 ppm in marine diesel fuels. The remaining regions will switch to 500 ppm sulfur fuel by June 2010. All regions are scheduled to switch to Ultra Low Sulfur Diesel (ULSD), with a sulfur content of 15 ppm or less, in June 2012. One exception to these rules is California, which has already switched to ULSD fuel in many non-road applications. Although some of these dates are still well into the future, the reality is that refiners are now making ULSD which can be downgraded to LSD.

The process of removing sulfur has caused changes in many other properties of the fuel. On-road fuels have already made the switch to ULSD, and the problems caused by the changes in the fuel characteristics are well known. Fortunately, solutions are known and already proven. Bringing the experience and lessons learned from on-road fuels to marine applications requires knowing what problems to look for. Some of these changes have already arrived in the marine market. For example, fuels with 19 ppm sulfur can be downgraded and sold as an LSD (500 ppm) fuel. This fuel would have the properties and problems associated with ULSD.

The methods used in most refineries in North America to remove the sulfur are known as hydrotreating. These methods are very efficient at removing sulfur, but also make other physical changes to the fuel’s chemical composition. The consequences of sulfur reduction include removal of natural lubricity compounds, natural conductivity properties and changes in combustion properties.

Fuel serves three major functions in a diesel engine. It is the lubrication for fuel pumps and injectors, a coolant for the injectors and the energy source to drive the engine. The reduction in natural lubricity compounds leaves no protective barrier between metal surfaces, resulting in damage to vital engine components. This would be similar to, but not quite as drastic, as replacing lube oil with a solvent. Imagine the damage. For this reason ASTM, an international standards organization, put a lubricity specification on all D975 middle distillate fuels in 2005 in order to keep the fuels fit for purpose and protect consumers. A special fact to keep in mind is that the current lubricity test was developed to protect rotary fuel pumps and may not be the best indicator for modern fuel injectors or barrel and piston type pumps where clearances are smaller.

Without the use of lubricity additives, most fuel would not have the lubricity required to protect the fuel injection systems. The ASTM specification requires that the wear scar from the lubricity test be 520 microns or less. There are many original equipment manufacturers that recommend a wear scar of 460 microns or even lower, and that is the European specification. The only way to determine if enough lubricity additive has been added to the fuel is having the fuel tested using ASTM D6079. If it is determined that more lubricity is needed, there are several chemistries of lubricity improvers that have been used. With the higher treat rates needed for ULSD fuels, some of these additives have been found to cause problems. Mono acids, amides and dimer acids additives may react with water, caustic carryover or other additive packages that may be present in the fuel or lube oil. Fully synthetic esters do not cause these problems.

The removal of sulfur and other compounds has decreased the fuel’s conductivity, creating a potentially dangerous situation. A static charge can readily build up in the fuel while it is being pumped and/or going through fine filtration. Unfortunately, ultra low sulfur fuels no longer have the ability to dissipate the charge without an additive. This can lead to sudden arcing or sparking in the vapor phase, often with catastrophic results. Static dissipater additives are required for ULSD fuels and most LSD fuel. These are added most often at the terminal. The ASTM specification for conductivity of a D975 fuel is 25 picosiemens/meter at time and temperature of loading. This specification is not found in marine fuels, which may lead to more dangerous incidents occurring.

Another important word of caution when selecting a lubricity additive is that many lubricity additives have been proven to disarm the static dissipater additive over time. This can cause the conductivity of a fuel to drop to far riskier levels and result in the danger of ignition from static electricity.

Fully synthetic esters have the least effect on static dissipater additives.

Sulfur is antagonistic towards microbial growth, meaning that its presence helps control the reproduction and growth in fuel storage tanks and systems. The removal of the sulfur and phenolic compounds makes an ideal situation for microbial growth to occur. Biocides have been necessary in diesel fuel for years when the fuel is stored in certain regions. With the addition of B2 and B5 biodiesel blends into the fuel distribution network, the use of biocides may become a necessity. The Bxx blends can hold more water molecules in suspension, which spreads out where microbial, fungal, and yeast growth will occur. Proper housekeeping and the use of a biocide that works in both the water phase and the fuel phase is the only way to ensure that microbial problems are kept at a minimum.

If light cycle oil (LCO) is added to the marine distillate fuel supply there can be serious stability issues and other problems. LCO tends to degrade rapidly and turn to sludge. It may look like diesel fuel when it is new, but without stabilizers, it can clog fuel filters and lines. LCO also has a low cetane number between 20 and 25, so it can contribute to rough starting, rough idling, poor combustion, deposit formation and very high emissions. Additives containing a cetane improver can help to avoid these combustion problems and help to reduce emissions.

The switch to LSD and then to ULSD can be a smooth transition if the proper steps are taken. Knowing what problems may occur and how to treat the fuel to avoid these problems is an important step. The maritime industry is under pressure to reduce emissions and improve efficiency. Maintaining engine reliability, while switching to these cleaner fuels is imperative for the clean and safe operation of vessels. The use of fuel treatment is present at the refineries and terminals, but to ensure vessel reliability it may be necessary to take additional steps.

Brian Rhoades is Eastern regional marine manager for Innospec Fuel Specialties and holds a chief engineer license from the U.S. Coast Guard. Dr. David Daniels is a director of research and development for Innospec and has almost 30 years of experience with middle distillate fuels. Innospec is the largest dedicated fuel additive company in the world.

By Professional Mariner Staff