The Challenge of Sulfur Analysis in the Fuels of the Future
by R. A. Kishore Nadkarni
Sulfur and its compounds are present in virtually all petroleum products and lubricants from crude oils to the ultra-low sulfur fuels of the future. The sulfur levels in these products vary from the highest amounts in the crude oils and some lubricants to trace levels in the currently proposed road fuels. Sulfur oxides formed during the combustion of gasolines or diesel fuels in internal combustion engines are undesirable because of the damage they do to the engine and to the environment. Acids of sulfur oxides increase rusting and corrosion of engine parts, piston rings, and cylinder walls. In the atmosphere, sulfur oxides convert to sulfuric acid by reaction with moisture, and harm vegetation, aquatic, animal and human life. They also corrode man-made buildings and monuments.
Because of the deleterious effects of sulfur emissions from motor vehicles, a number of government regulatory agencies, principally in North America and Europe, have been vigorously controlling and gradually reducing the sulfur content of the fuels used in automobiles, aviation, marine vessels, off-road vehicles, power generating utilities, and home heating. The permitted sulfur levels are steadily being lowered from a current level of about 330 mg/kg in gasoline and 500 mg/kg in diesel fuels to 30 and 15 mg/kg in gasoline and diesel, respectively, by 2006, and near zero by 2010. When the U.S. Environmental Protection Agency published its proposed Tier 2 emission standards for vehicles and gasoline sulfur standards for refineries, (1) they mandated a modified ASTM D 2622, Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-Ray Fluorescence Spectrometry, and in later announcements D 6428, Test Method for Total Sulfur in Liquid Aromatic Hydrocarbons and Their Derivatives by Oxidative Combustion and Electrochemical Detection, as the mandatory methods for sulfur determination in diesel fuels.
Over the years, through litigation and discussions with the industry, these rules have been somewhat changed to the point where alternate methods can be used instead of only the mandated methods. The alternate methods allowed would be 1) those developed by voluntary consensus-based standards bodies such as ASTM International, and/or 2) oil company methods for which the data will have to be provided to EPA to establish their provenance under the performance based measurement system. Discussions between EPA and the industry are ongoing to define further the statistical basis for judging the available methods acceptance and other related issues.
There are numerous methods available for the determination of sulfur in diverse petroleum product and lubricant matrices from sub-mg/kg to m% concentrations. One of the earliest test methods issued by Committee D02 on Petroleum Products and Lubricants was D 129, Test Method for Sulfur in Petroleum Products (General Bomb Method). The analytical techniques used for sulfur determination include classical wet chemistry, X-ray fluorescence, inductively coupled plasma atomic emission spectrometry, and various micro-elemental combustion methods using various detection techniques such as micro-coulometry, ultraviolet fluorescence, electrochemistry, etc. A list of such available methods is given in Table 1.
Several of these ASTM methods have also been separately issued by other international standards-writing bodies such as the International Organization for Standardization (ISO), the Institute of Petroleum, Deutsches Institut für Normung (Germany), Japan Industrial Standards, and the Association Francaise de Normalisation (France), etc. These are listed in Table 2. In an interactive global market, it is useful in commerce to recognize the analytical methodology used in other industrialized nations.
Although nearly 20 ASTM test methods are listed in Table 1, the appropriate selection of a method for analysis would depend on the matrix, sulfur concentration, and the desired precision of the analysis. Thus, out of about 20 methods listed in Table 1, only four qualify to be considered as potentially useful methods for low levels of sulfur in ultra-low sulfur fuels.
ASTM Interlaboratory Study
In 2000, the U.S. EPA published requirements for Tier 2 fuels and also mandated test methods that can be used for sulfur determination in these fuels. At that time, ASTM method D 6428 was chosen as the mandatory method for diesel fuels with D 2622; D 3120, Test Method for Trace Quantities of Sulfur in Light Liquid Petroleum Hydrocarbons by Oxidative Microcoulometry; and D 5453, Test Method for Determination of Total Sulfur in Light Hydrocarbons, Motor Fuels and Oils by Ultraviolet Fluorescence, permitted as alternates, if the results were shown to be equivalent to those obtained by D 6428 test method.
The petroleum industry widely uses the last three methods mentioned, but had virtually no experience with the mandatory D 6428 test method, which was under the jurisdiction of Committee D16 on Aromatic Hydrocarbons and Related Chemicals. The scope of the method was for the analysis of liquid aromatic hydrocarbons and related chemicals. Most important, its precision was too uncertain to be used for regulatory compliance purposes. Hence, Subcommittee D02.03 on Elemental Analysis undertook a major interlaboratory study to define clearly the precision obtainable by these three established, and one new, test method. Eventually this study would involve over 6,000 data points from about 70 laboratories worldwide, four test methods, and 16 samples each of gasoline and diesel with less than 100 mg/kg sulfur concentrations. It has been called truly a mother of all ASTM crosschecks.
The results of this study summarized in Table 3 clearly showed that the mandated D 6428 (and its D02 equivalent, D 6920, Test Method for Total Sulfur in Naphthas, Distillates, Reformulated Gasolines, Diesels, Biodiesels, and Motor Fuels by Oxidative Combustion and Electrochemical Detection) was least precise both in repeatability and reproducibility for both gasoline and diesel at the sulfur levels of compliance interest, and was not useful for the precise determination of low levels of sulfur in the fuels of the future. None of these four test methods could reproduce alleged ultra-low sulfur levels in samples supposed to have less than 1 to 2 mg/kg sulfur. As an outcome of this major interlaboratory study, the three existing sulfur methods were updated to reflect the precision data obtained and a revised D 6920 was issued. In reality, out of this study only D 2622 and D 5453 would be appropriate for industry and regulatory use. Both are widely used in industry laboratories and are also approved by the California Air Resources Board for regulatory compliance.
Emerging New Technology for Sulfur Analysis
Given the increasing importance of the ability to measure precisely lower and lower quantities of sulfur in future fuels, it is not surprising that a number of organizations are working to develop better methods for such analysis. A few promising approaches are briefly discussed here.
Monochromatic Wavelength Dispersive X-Ray Fluorescence Traditional wavelength dispersive XRF uses polychromatic excitation. This new technology uses monochromatic, focused excitation. Preliminary work showed a reproducibility of about 2 mg/kg at 10 mg/kg sulfur levels in gasolines and diesels.
Polarization Energy Dispersive X-Ray Fluorescence Polarized excitation geometry used with traditional EDXRF employing low power tubes and an X-ray end window tube is claimed to lower the detection limit to 1 mg/kg sulfur.
EDXRF with Proportional Counter A low background proportional counter is the key in a new EDXRF-based method to measure lower levels of sulfur. This low background proportional counter suppresses the noise resulting in a lower detection limit compared to traditional proportional counters used in EDXRF.
Online Measurements For many operations, online measurement is a preferred choice rather than waiting for the off-line laboratory analysis. Many proposed online methods are extensions of the established laboratory test methods such as ASTM D 2622; D 4045, Test Method for Sulfur in Petroleum Products by Hydrogenolysis and Rateometric Colorimetry; D 4294, Test Method for Sulfur in Petroleum and Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectrometry; D 5453, etc. There are no ASTM standard test methods issued yet for online sulfur determination, but several are in the works. A method based on the D 5453 combustion ultraviolet-fluorescence technique is being widely used in the industry. Another based on the oxidative combustion of the sample, followed by gas chromatographic separation of sulfur species, and finally flame photometric detection of the sulfur dioxide produced is also being widely used in the industry. Other techniques suggested for online analysis include hydrogenolysis and rateometric colorimetry based on the D 4045 lab method, an EDXRF method based on the D 4294 lab method, and a pyrolysis chemiluminescence method. Many of these online measurement proposals are being progressed through the ASTM consensus process for standardizing these methods.
ASTM ILCP Programs
For a decade, Committee D02 has been involved in ongoing proficiency testing programs for 18 products. This has proved quite popular, with nearly 1,900 laboratories participating worldwide, about 45 percent of them non-U.S. labs. In many of these programs, the determination of sulfur is included. However, in January, a new program was launched specifically for ultra-low sulfur in diesel. Only four test methods may be used (D 2622, D 3120, D 5453, and D 6920). We expect that this will prove to be a useful program for laboratories involved in analyzing fuels.
There is little doubt that, given the increasing importance of low-level sulfur determinations in petroleum products, more and more methods are going to be developed to satisfy the needs of both the petroleum company laboratories and the regulatory agencies. We do indeed live in interesting times. //
1 U.S. EPA Federal Register 62(133), 40 CFR Part 80 Part II, page 37337 (July 11, 1997); 64(92), 40 CFR Parts 80, 85 and 86, Page 26055 (May 13, 1999); 65(28), 40 CFR Part 80, Pages 6752 6774 (February 10, 2000); and 66 (12), 40 CFR Part 80, Pages 5002-5141 (January 18, 2001).
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