Global Monitoring of Marine Fuels and Machine Condition
Hélène S. Gutman, Ph.D., CTO - Xenemetrix Ltd

The new sulfur emission regulations for ocean going vessels present a serious challenge for the maritime industry. Lower sulfur contents in the fuel oil is likely to cause increased machine wear. Therefore timely, exact, and frequent measurements of the sulfur and wear metal concentrations on each vessel is desirable in order to avoid legal infractions, and unnecessary machine wear. This article presents the use of Energy Dispersive X-Ray Fluorescence (EDXRF) for this purpose, combined with a GPS-based global reporting system that enables the fleet management to optimize also for bunkering and machine maintenance.

Scientific studies have shown that vessel exhaust emissions contains harmful air pollutants, like sulfur oxides, which are associated with a broad array of adverse impacts that harm human health and the environment. To reduce pollution coming from vessels the International Maritime Organization (IMO) revised the MARPOL Annex VI in 2008 and established new limits on emissions of sulfur oxides and other pollutants. The new limit in areas other than designated Emission Control Areas was set to 3.5% sulfur from 2012, and will be reduced to 0.5% sulfur from 2015. In special designated Emission Control Areas (ECA) the limit was set to 1% from 2010, but will be reduced to 0.1% from 2015.

The ECA included the Baltic Sea from 2006, and the North Sea from 2007. In 2012 the North America ECA was defined as within 200 nautical miles from the coasts of North America and Canada. The Caribbean sea ECA will join by 2014. The European Commission directive 2005/33/EC from 2010 mandates the use of fuel oil containing maximum 0.1% of sulfur for ships berthed for two or more hours at any port in the European Union.

The viscosity of low sulfur fuel oil is extremely low compared to the viscosity of heavy fuel oil and marine diesel oil. Internal leakage in the fuel oil pumps, and increase in flowability of fuel from the injection nozzle are likely to occur. The resulting effect on various equipment is a cause of concern. Since the lubricity of fuel oil depends on the viscosity, the lubricity of low-sulfur fuel oil is low. Consequently, abnormal wear is likely to occur in the sliding/contact parts of e.g. pumps.

The International Council on Combustion engines (CIMAC)1 listed the following concerns associated with switching between heavy fuel oil and distillate fuels with low sulfur content in coastal waters. Low lubricity: shore side distillate fuels have specific requirements for minimum lubricity which is usually met by inclusion of additives. Marine distillate fuels have no such requirements. Delivery-side thermal issues: Heavy fuel oil must be heated to 150 0C to flow due to its high viscosity. Marine distillate fuel if introduced too fast at ambient temperature could cause the fuel pumps to seize due to a combination of thermal contraction and low lubricity.

Fuel compatibility: When switching from heavy fuel to a low aromatic distillate fuel, some of the heavier asphaltic materials could be precipitated from the heavy fuel. If this happens, fuel filters could clog and fuel pumps could stick, causing sudden loss of power. CIMAC cited the last issue as the most important as it relates to the potential mixing of two fuels in a common tank which can occur during fuel switching.

Oil analysis spectrometers have become the primary analytical tool for the analysis of wear metals, contaminants and additives in lubricating oils during the last 50 years.

The principle behind spectrometry is that atoms, e.g. sulfur or iron, when properly excited by light or X-rays, emit or absorb unique spectral radiation whose intensity is proportional to their amount. The main building blocks of a spectrometer is the excitation source, the optical system, and the readout system. Spectrometers for oil analysis fall into two categories: atomic absorption spectroscopy (AAS)2 and atomic emission spectroscopy (AES )3.

AAS uses a flame to dissociate molecules into atoms, and a primary light source from which the atoms absorb specific spectral lines. The disadvantage of AAS is the need for primary source radiation for each element to be analysed.

AES uses inductively coupled plasma (ICP)4 which is created by the flow of ionised inert gas, e.g. argon that reaches temperatures of 8,000-10,0000C. The oil sample is aspirated into the plasma where the atoms present are totally dissociated and excited sufficiently to emit radiant energy.

The main advantage of the ICP technique is performance: accuracy, precision and detection limits are excellent. Secondary excitation effects are minimal since the samples are diluted prior to aspiration.

The ICP also lends itself to analysis of almost any material that is dissolvable into a fine spray. Automation of the sample introduction is possible.

The primary limitation of ICP is the sample preparation by dilution. This timeconsuming step can lead to accuracy errors. A well-trained person with experience in basic laboratory procedures is required. Used oil samples are not ideally suited for the sample introduction system, and a certain amount of care and cleaning is required.

AES and AAS suffer from a well-documented particle size limitation in being unable to analyse particles larger than 10 µm that are important indicators of wear. In conclusion atomic spectroscopy requires a laboratory environment with trained personnel to function properly.

X-ray Fluorescence spectroscopy5 is a simple analytical method for direct measurements of atomic elements in a wide range of materials, solids, powder and liquids. The great advantage of XRF is that the method is fast, accurate and reliable, anyone can operate it, and no sample preparation or special laboratory environment is required.

Commercial EDXRF instruments have been around since the 1970's. With the PC as its platform, X-ray fluorescence spectroscopy became a simpler and low cost alternative to atomic spectroscopy.

In X-ray fluorescence an electron is ejected from its atomic orbital by the absorption of an X-ray photon whose energy is greater than the energy with which the electron is bound to the atom’s nucleus. When an inner orbital electron is ejected, an electron from a higher energy level orbital falls to the lower energy level orbital, and simultaneously a photon, the X-ray fluorescence, is emitted. The energy of the emitted photon equals the difference between the energies of the two orbitals. Each element has its own set of energy levels so that the X-ray fluorescence spectrum is characteristic of it. Hence it is possible to determine the identities of the elements present. The number of photons per unit time is proportional to the amount of the corresponding element.

EDXRF can be used for almost all elements of interest to the petroleum industry, and in particular for marine oil analysis in the shipping industry . Sulfur, wear elements, additives, and contaminants can be analyzed down to single parts per million (ppm) level. E.g. by periodically monitoring the trends of wear element concentrations, EDXRF analysis can help to determine whether parts are failing. The monitoring of what remains of certain oil additives may indicate the remaining service life of the oil.