Bunker fuel, also known as heavy oil, number 6 oil, residual fuel (resid) or Bunker C is a low value product typically used aboard ships. Bunker fuel comes from the refining process and is the liquid residue from crude oil fractionation after the the lighter fractions have been removed by distillation. The residue is comprised of large molecules such as asphaltenes, waxes and contaminants from the crude that become concentrated in the residue. After removal of the heavy insoluble materials, the remaining liquid may be cut back with lighter fractions, (often high value kerosine or diesel fractions) in order to meet relevant specifications such as sulphur and viscosity. The solids removed from the residue commonly go to road and roof tar.
Tighter specifications on air quality have largely ignored the bunker fuel market because of the difficulty legislating for oil products used at sea. In October 2008, the Marine Environment Protection Committee of the International Marine Organisation (IMO) adopted the revised Annex VI, Prevention of Air Pollution from Ships to the MARPOL 73/78 Convention, which sets limits on nitrogen oxide and sulphur dioxide emissions from ship exhausts, with consequent control of bunker fuel sulphur content being imposed. Annex VI does not specify standard test methods to be used, but the test method of preference for sulphur is currently ISO 8754:2003, 'Petroleum Products - Determination of Sulphur Content - Energy Dispersive X-ray Fluorescence Spectrometry', which is stipulated in marine fuel specification ISO 8217.
The IMO action applies to vessels in emission control areas (ECAs), which are currently in the Baltic and North Seas with proposals for North America and Canada. California has its own state-wide fuel restrictions in place that will operate until the North America ECA comes into force. In the ECA areas, ships must either switch to using a fuel with a sulphur level of <1.5 per cent or fit an exhaust scrubber system that will achieve equivalent reductions. Under the IMO agreement, MARPOL Annex VI (2008), the maximum sulphur level in fuel will be progressively lowered for an ECA to 1.00per cent m/m on or after 1 July 2010 and 0.10 per cent m/m on or after 1 January 2015.
In addition to the sulphur dioxide reductions, new engines operating in these areas must use emission controls that also achieve an 80 per cent reduction in nitrogen oxide (NOx) emissions, starting from 2016. The majority of NOx emissions are as a result of atmospheric nitrogen being exposed to the high temperatures and oxidative environment within the engine during the combustion process itself. However, fuel borne nitrogen contributes to NOx emissions and nitrogen-containing species in the form of contaminants or additives can be key markers of potential fuel quality issues. The measurement of sulphur and nitrogen in bunker fuel is therefore becoming increasingly important as legislative limits decrease.
Traditional techniques
Bomb combustion has traditionally been used for the analysis of sulphur in fuel. However, the method does not have a low detection limit and is only applicable to low volatility samples containing more than 0.1 per cent (1000ppm) of sulphur. In addition, it is a time consuming gravimetric method.
Inductively coupled plasma optical emission spectrometry (ICP-OES) is a sensitive and accurate technique applicable to the measurement of a large proportion of the elements in the periodic table. It has low detection limits for sulphur at approximately 10-15 µg/kg in petroleum products. However, there is some evidence that the efficiency of the method for the detection of sulphur in the plasma is dependent upon the species of the sulphur compounds in the sample. This may be caused by the different transportation efficiencies of the various species. In addition, the method is not currently capable of measuring nitrogen in fuels due to the large background associated with atmospheric nitrogen in the plasma.
In order to address these shortcomings, a more accurate and reliable analytical technique is required capable of minimising safety concerns whilst maximising precision. X-ray fluorescence (XRF) is a powerful technique for sulphur measurement in fuels and is very quick and simple, particularly at the high levels of sulphur traditionally found in bunker fuel.
XRF achieves fast multi-elemental analysis of a wide variety of materials in a non-destructive way with little or no sample preparation but requires matrix matching for optimum accuracy. The method is flexible and offers sequential and/or simultaneous analytical capabilities, although it is generally a more expensive solution. There are two types of XRF technology; energy dispersive X-ray fluorescence spectrometry (EDXRF) and wavelength dispersive X-ray fluorescence spectrometry (WDXRF). The principal difference is in the achievable energy (spectral) resolution. WDXRF typically provides working resolutions between 5 and 20eV whereas EDXRF systems typically provide resolutions between 150 and 300eV or more.
While X-ray spectrometry, ICP-OES and bomb combustion are all excellent techniques in their own right, they all demonstrate certain limitations for this type of analysis. The analysis of total nitrogen and total sulphur in bunker fuel by combustion and subsequent chemiluminescence/UV fluorescence detection is a highly competent alternative. The total sample is combusted and analysed making sample homogeneity and matrix matching trivial considerations. In addition, repeatability and reproducibility are excellent with typical RSDs less than 2 per cent even at low concentrations. These instruments are also regulated by ASTM, DIN and ISO methodology for a wide range of sample types.
Experimental
A total nitrogen (TN)/total sulphur (TS) analyser (TN/TS 3000, Thermo Scientific) was used to measure total nitrogen and total sulphur content in a bunker fuel sample. The system performs high temperature combustion analysis according to ASTM D5453 principles. A bunker fuel sample was received from a multinational oil company to assess the capability of the method to accurately measure both sulphur and nitrogen in the fuel. The instrument was calibrated using nitrogen and sulphur standards prepared from pyridine and thiophene diluted in xylene over the range 0 to 6000mg/l (where the density of xylene is taken to be 0.86g/cm3).
The heavy viscous sample was weighed directly into a boat and introduced into the combustion tube using automatic boat injection. Sample combustion occurred at 1000°C under controlled conditions ensuring complete combustion of sulphur to sulphur dioxide and nitrogen to nitrogen oxide.
The dry gas sulphur stream entered the reaction chamber of an ultra violet (UV) fluorescence detector, where the SO2 molecules were excited by a pulsed UV light source. A condensing lens focused the pulsed UV light onto a mirror assembly comprising eight high precision mirrors which also functioned as a bandpass filter ensuring only wavelengths corresponding to ground state transitions in SO2 pass through.
The photomultiplier tube (PMT) subsequently detected the emitted UV light from the decaying SO2 molecules. A photo detector located at the back of the fluorescence chamber continuously monitored the pulsed UV light source and was connected to a circuit that compensated for fluctuations in the UV light. This ensured ultimate stability of the system.
A PMT detector measured the light intensity and converted it electronically, with reference to stored calibration information, to display the analytical result as the mass of nitrogen or as nitrogen concentration in the sample.Results. Fig. 1 and 2 display examples of the sulphur and nitrogen calibration lines used.
The sulphur and nitrogen results are detailed in Tables 1 and 2, whereas Table 3 provides a summary of results. The extremely low relative standard deviation (RSD) demonstrates the suitability of the method used for the analysis of bunker fuel samples.
Conclusion
Sulphur and nitrogen content in bunker fuel can be regularly monitored using high temperature combustion with chemiluminescence and UV fluorescence detection to ensure compliance with new legislative requirements. High temperature combustion offers a reliable, fast and accurate technique for the simultaneous analysis of sulphur and nitrogen in bunker fuel samples.
Debbie Batt is Senior Applications Specialist, TN/TS/TX/TOC Analysers, Thermo Fisher Scientific, Cambridge UK. www.thermofisher.com
