Fuel Property Testing: Sulfur

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Abstract: Sulfur test methods are important for compliance with mandated fuel sulfur regulations. Measurement techniques for fuel sulfur include wet chemistry, X-ray fluorescence, atomic spectroscopy and various thermal combustion methods. The most common ASTM test methods include D2622, D5453 and D7039.

Overview

Sulfur test methods have always been important for compliance with mandated fuel sulfur regulations. As fuel sulfur regulations have tightened, for example to the 15 mg/kg for ultra low sulfur diesel (ULSD) in North America and 10 mg/kg in Europe, the test method limits have been pushed further. It is very challenging to accurately and precisely quantify fuel sulfur at these levels. Many test methods cannot be used because of poor precision and/or accuracy.

Techniques for fuel sulfur include wet chemistry, X-ray fluorescence, atomic spectroscopy and various thermal combustion methods with different detection techniques such as microcoulometry, UV-fluorescence, and electrochemistry.

Table 1 outlines some test methods used to quantify sulfur in middle distillate fuels. Also shown is the range of sulfur concentrations and ASTM D975 diesel fuel grades they are applicable to.

**Table 1**
Common sulfur test methods, range of applicability and ASTM D975 fuel grade applicability
Method	Title	Range	Grades
D1552	Standard Test Method for Sulfur in Petroleum Products by High Temperature Combustion and Infrared (IR) Detection or Thermal Conductivity Detection (TCD)	> 0.071% mass	No. 1-D S5000 No. 2-D S5000 No. 4-D
D2622	Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry	0.0003 to 4.6% mass 3 to 46000 mg/kg	All grades
D3120	Standard Test Method for Trace Quantities of Sulfur in Light Liquid Petroleum Hydrocarbons by Oxidative Microcoulometry	3.0 to 1000 mg/kg	No. 1-D S15 No. 2-D S15 No. 1-D S500 No. 2-D S500
D4294	Standard Test Method for Sulfur in Petroleum and Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectrometry	0.0017 to 4.6% mass 17 to 46000 mg/kg	No. 1-D S500 No. 2-D S500 No. 1-D S5000 No. 2-D S5000 No. 4-D
D5453	Standard Test Method for Determination of Total Sulfur in Light Hydrocarbons, Spark Ignition Engine Fuel, Diesel Engine Fuel, and Engine Oil by Ultraviolet Fluorescence	0.0001 to 0.8% mass 1.0 to 8000 mg/kg	All grades
D7039	Standard Test Method for Sulfur in Gasoline, Diesel Fuel, Jet Fuel, Kerosine, Biodiesel, Biodiesel Blends, and Gasoline-Ethanol Blends by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry	3.2 to 2822 mg/kg	No. 1-D S15 No. 2-D S15 No. 1-D S500 No. 2-D S500
D7220	Standard Test Method for Sulfur in Automotive, Heating, and Jet Fuels by Monochromatic Energy Dispersive X-ray Fluorescence Spectrometry	3 to 942 mg/kg	No. 1-D S15 No. 2-D S15 No. 1-D S500 No. 2-D S500

When the US EPA originally published regulations requiring 15 mg/kg sulfur in on-road diesel fuel in 2001, the designated test method was ASTM D6428. This test method was later found to be inadequate because it was originally developed for aromatic hydrocarbons and not petroleum products. Its precision did not conform to the statistical protocols used for petroleum products. D6428 was later revised with new precision data and issued separately as D6920. After legal action, the EPA changed the test method requirements in 2004. Instead of requiring a specific test method, a performance-based criteria was adopted. Any test method could qualify as an EPA approved test method provided it met the minimum performance requirements outlined in 40 CFR 80.584.

In 2001 the EPA initially allowed a 2 mg/kg sulfur compliance margin to account for test method reproducibility. This was increased to 3 mg/kg early in 2006 for the first 2 years of the program (until October 14, 2008) to allow refiners, distributors and marketers time to improve their testing procedures.

The EPA measurement procedures for fuel properties have been updated several times since then. In 2020, they were relocated to 40 CFR 1065 (1065.1350 to 1065.1375). In these updated procedures, ASTM D2622 is designated as the referee method for sulfur in ULSD, 500 ppm diesel fuel, ECA marine fuel and gasoline. Any laboratory may use the specified referee procedure without qualification testing. Alternative procedures require testing to demonstrate compliance with precision and accuracy requirements. Ongoing quality testing requirements apply to labs using either referee procedures or alternative procedures.

Three test methods are commonly used for ULSD type fuels and will be discussed in more detail: D2622, D5453 and D7039. While other test methods are used, their availability is relatively small. Of the instruments used in a 2005 EPA round robin study, 93% were one of D2622, D5453 or D7039 [1261].

For diesel fuels with a maximum of 10 mg/kg sulfur, the European diesel fuel specification, EN 590, requires the use of:

While the European test methods are similar to some ASTM methods (EN ISO 20846 is comparable to ASTM D5453 and EN ISO 20884 is comparable to ASTM D2622) there are some differences. Two important differences are in the scope of applicability and the determinations of method precision. The precision of comparable ISO and ASTM methods are often determined with different testing programs using different fuel samples, instruments and laboratories.