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 D 2622, D 5453 and D 7039.

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, 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 of the more common 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
D 129	Standard Test Method for Sulfur in Petroleum Products (General Bomb Method)	> 0.1% mass	No. 1-D S5000 No. 2-D S5000 No. 4-D
D 1266	Standard Test Method for Sulfur in Petroleum Products (Lamp Method)	0.0005 to 0.4% mass 5 to 4000 mg/kg	No. 1-D S500 No. 2-D S500
D 1552	Standard Test Method for Sulfur in Petroleum Products (High-Temperature Method)	> 0.06% mass	No. 1-D S5000 No. 2-D S5000 No. 4-D
D 2622	Standard Test Method for Sulfur in Petroleum Products by Wavelength Dispersive X-ray Fluorescence Spectrometry	0.0003 to 5.3% mass 3 to 53 000 mg/kg	All grades
D 3120	Standard Test Method for Trace Quantities of Sulfur in Light Liquid Petroleum Hydrocarbons by Oxidative Microcoulometry	3.0 to 100 mg/kg	No. 1-D S15 No. 2-D S15 No. 1-D S500 No. 2-D S500
D 4294	Standard Test Method for Sulfur in Petroleum and Petroleum Products by Energy-Dispersive X-Ray Fluorescence Spectrometry	0.0150 to 5.00% mass 150 to 50 000 mg/kg	No. 1-D S5000 No. 2-D S5000 No. 4-D
D 5453	Standard Test Method for Determination of Total Sulfur in Light Hydrocarbons, Motor Fuels and Oils by Ultraviolet Fluorescence	0.0001 to 0.8% mass 1.0 to 8000 mg/kg	All grades
D 6920	Standard Test Method for Total Sulfur in Naphthas, Distillates, Reformulated Gasolines, Diesels, Biodiesels, and Motor Fuels by Oxidative Combustion and Electrochemical Detection	1 to 40 mg/kg	No. 1-D S15 No. 2-D S15
D 7039	Standard Test Method for Sulfur in Gasoline and Diesel Fuel by Monochromatic Wavelength Dispersive X-ray Fluorescence Spectrometry	2 to 500 mg/kg	No. 1-D S15 No. 2-D S15 No. 1-D S500 No. 2-D S500

When EPA originally published the regulations requiring 15 mg/kg sulfur in on-road diesel fuel in 2001, the designated test method was ASTM D 6428. This test method was later found to be inadequate because it was originally developed for aromatic hydrocarbons and not petroleum products and its precision did not conform to the statistical protocols used for petroleum products (D 6428 was later revised with new precision data and issued separately as D 6920). After legal action, EPA changed the test method requirements in 2004. Instead of requiring a specific test method, a performance based criteria was adopted. Any test method can qualify as an EPA approved test method provided it meets the minimum performance requirements outlined in 40 CFR 80.584.

In order for a test method to qualify for EPA approval, each test facility seeking approval must show that the precision and accuracy of their test facility meets EPA requirements. These requirements for facilities measuring sulfur in ULSD are outlined in Table 2.

**Table 2**
EPA Performance Requirements for Facilities Measuring Sulfur in ULSD
Precision	The standard deviation of a minimum of 20 repeat tests made over 20 days on a single homogeneous commercially available diesel fuel with a sulfur content in the range of 5-15 mg/kg must be less than 0.72 mg/kg.
Accuracy	The arithmetic average of a continuous series of a minimum of 10 tests on a commercially available gravimetric sulfur standard in the range of 1-10 mg/kg sulfur shall not differ from the accepted reference value by more than 0.54 mg/kg.
Accuracy	The arithmetic average of a continuous series of a minimum of 10 tests on a commercially available gravimetric sulfur standard in the range of 10-20 mg/kg sulfur shall not differ from the accepted reference value by more than 0.54 mg/kg.