Wilhelm, S. M., Liang, L., Cussen, D. and D. Kirchgessner, “Mercury in Crude Oil Processed in the United States”, Environmental Science and Technology, American Chemical Society, 41 (13), 4509, 2007.

The major sources of anthropogenic mercury emissions in the United States are fossil fuel and waste combustion. Mercury in coal has been studied extensively and it contributes the largest amount of atmospheric emissions of any fuel source (1, 2). An obvious analogy exists between mercury in coal and mercury in petroleum, however, the amount of mercury in crude oil, and thus potentially liberated by liquid fuel combustion or other pathways, has remained uncertain because of a paucity of accurate mercury concentration data across the huge range of crude oils entering U.S. refineries.

Studies of mercury in crude oil published before the year 2000 reveal a range of concentrations of total mercury (THg) in crude oil between approximately 0.1 and 20,000 µg/kg (3). The breadth of the range reflects the wide variability of natural geological conditions. Calculations of a mean concentration from measured concentration data published before year 2000 were biased high because of poor analytical detection limits and because analysts tended to select oils with higher concentrations for investigations (3). A consistent and broadly focused study of mercury in crude oil has not been available to allow comparison to coal and other fuels.

In 2003, the United States Environmental Protection Agency’s Office of Research and Development initiated a study directed to determining the mean and range of concentrations of mercury in crude oil processed in the U.S. for the point in time accessible. The research program was organized into three parts, all now complete. Part 1 examined analytical procedures (4). Part 2 examined sampling and species effects (5, 6). In Part 3 and reported here, total mercury concentrations were measured for a wide variety of crudes made available to the study by the U.S. oil refining industry.

Mercury in Crude Oil. A number of mercury species have been identified in crude oil (7 - 9). The list includes elemental mercury (typically the major component) and dialkylmercury (typically a trace component), both volatile and reactive and mercuric sulfide and asphaltene mercury (major component in some oils), both non-volatile and relatively non-reactive. Complexed ionic mercury species are also present in some oils as a minor component.

In measuring total mercury concentration (THg) in crude oil matrix, samples must reflect the homogenous amounts of all mercury species that are present in the stream being sampled and analytical methods must avoid losses of volatile species in aliquot removal procedures. Because some mercury compounds are associated with oil sediment, some loss of mercury is expected in stagnant tanks that allow settling or stratification of suspended material. Elemental and organic mercury are volatile and losses are also expected when volatile components escape heated tanks or in transportation. Elemental mercury also reacts with steel surfaces irreversibly and some losses likely occur in pipelines. In the present study such possible losses were not investigated. The focus of the study was not the wellhead concentration of mercury but rather the “as delivered” concentration in crude entering the refining process.

Crude Oil processed in the United States. A wide variety of materials are processed by U.S. refineries to make liquid fuels and other products derived from crude oil (asphalt, coke, wax, lubricants). The raw material categories include condensates, crude oil, heavy crude oil, and synthetic crudes. Samples in all crude oil categories were donated and analyzed in the project. Oils having trade names are blends of oils that come from several or many individual wells and typically consistent in composition and characteristics. The conscious desire to maintain consistency of oil properties likely constrains the range of mercury concentration in trade-named oils as well. The variation of measured mercury concentrations of  trade-named crude oil streams over time is not well studied, but it was postulated that the range of concentrations exhibited by a single trade-named oil was much less than the range of mercury concentrations found in crude oils generally.

This study relied on industry volunteers to donate samples of crude to the project for analysis. Considerable efforts were made to ensure that the sample ensemble reflected both domestic production and imports in proportion to what is actually processed. No major crude sources are known that were left out of the group of oils analyzed, but certainly not all of the oil streams processed in U.S. refineries were included.