Mercury Technology Services (MTS), specifically its principal scientist - S. Mark Wilhelm, Ph.D., was asked to comment on the concentration of mercury in fuel oils and the amount of mercury that may be emitted to the atmosphere when fuel oils are burned.

Background

The discussion to follow adopts the ASTM D396-04 definitions and specifications for grades of fuel oil used in oil-burning equipment. These general grade descriptions are as follows:

• Grade #1, #1 (Low Sulfur), #2, and #2 (Low Sulfur) are middle distillate fuels for use in domestic and small industrial burners.
• Grades #4 (Light) and #4 are heavy distillate fuels or distillate/residual fuel blends used in commercial/industrial burners equipped for this viscosity range.
• Grades #5 (Light), #5 (Heavy), and #6 are residual fuels of increasing viscosity and boiling range, used in industrial burners.

Mercury in Fuel Oils

Wilhelm [1] reviewed mercury in fuel oil data in 2001. Essentially all of the measurements of mercury in fuel oil prior to 2001 were conducted on oils for utility and industrial boilers. At that time, a considerable disagreement existed between data acquired prior to roughly 1995 and more recent data. The older data were much higher on average than more recent data. The disagreement was attributed to the evolution of analytical methods for mercury in oil [2 - 4]. In the 2001 review, data for mercury in fuel oils collected between 1995 and 2001suggested that the earlier U.S. EPA data compilations, and estimates of emissions based on them, were biased high.

Origin of EPA Estimates for Mercury in Fuel Oil and Associated Emissions

The U.S. EPA's opinion on mercury in fuel oil is stated in its recent report on mercury flows in the United States [5]. Excerpt from [5]:

Data are available quantifying the amount of fuel oil used in utility, non-utility, and residential applications. For the utility and non-utility sectors, the quantity of fuel oil used in 1996 is as follows (USEPA 1999d):
• Utility, residual oil: 3,900 million gallons
• Utility, distillate oil: 684 million gallons
• Non-utility, residual oil: 3,100 million gallons
• Non-utility distillate oil: 5,500 million gallons

For the residential sector, the quantity of oil is not directly available. However, the heating content of oil used in the non-utility and residential sectors is reported as 2,180 and 880 trillion Btu, respectively, in 1994. Assuming that the heating value of oil used in these two sectors is similar, and that only distillate oil is used for residential applications, the quantity of oil estimated to be used in residential applications is 3,500 million gallons of distillate oil.

The mercury content of these fuels is expected to be variable. Minnesota (1999) [6] provides estimates of the mercury content of product oils, reporting that residual oil has 0.004 ppm mercury and distillate oil has 0.001 ppm mercury. Using the concentration data together with the above volume data provides the following estimates for the mercury content of raw materials in 1996:
• Utility: 0.06 tons of mercury (corresponding to 0.06 tons from residual oil and 0.003 tons from distillate oil)
• Non-utility: 0.07 tons of mercury (corresponding to 0.05 tons from residual oil and 0.02 tons from distillate oil)
• Residential: 0.01 tons of mercury (corresponding to 0.01 tons from distillate oil).

Air Releases - The estimates of mercury releases from utility, non-utility, and residential combustion of oil vary depending on the mercury emission factors that were used in the calculations. These estimates are as follows:
• Utility: 0.2 tons/year
• Non-utility: 5.0 - 7.7 tons
• Residential: 2.8 - 3.2 tons

For utility boilers, mercury emissions were estimated using emissions data available from 58 emission tests conducted by USEPA, the Electric Power Research Institute (EPRI), the Department of Energy (USDOE), and individual utilities. Boiler-specific emission estimates were then calculated by multiplying the calculated inlet mercury concentration by the appropriate emission factor for each boiler configuration and control device. For non-utility and residential boilers, the ranges account for the different mercury emission factors for oil that were used in the Mercury Study Report to Congress (USEPA 1997a) and in Locating and Estimating Air Emissions from Sources of Mercury and Mercury Compounds (USEPA 1997b) [8]. The following factors were used in the Report to Congress:
• Residual Oil (No. 6): 2.9 kg/10¹⁵ J
• Distillate Oil (No. 2): 3.0 kg/10¹⁵ J

The following emission factors were used in the Locating and Estimating Air Emissions document:
• Residual Oil (No. 6): 2.7 kg/10¹⁵ J
• Distillate Oil (No. 2): 0.02 kg/10¹⁵ J

The mercury emission factors for residual oil and distillate oil were multiplied by oil consumption estimates in order to estimate the amount of mercury released to air.

The estimates of air releases of mercury that EPA promulgated in [5] are inconsistent. When based on actual measurements, utility mercury discharges are 25 times less than estimates for non-utility discharges that are based on mercury concentration in oil data. It should be noted that the amounts of fuel oil consumed by utility and non-utility sources are roughly the same.

Emission factors for residual and distillate fuel oil (non-utility sources) were based on the mean of published values for mercury concentrations in fuel oil compiled in [8] and available at that time (1989). Excerpt from [8]:

Concentrations of mercury in fuel oil depend upon the type of oil used. No comprehensive oil characterization studies have been done, but data in the literature report mercury concentrations in crude oil ranging from 0.023 to 30 ppmwt, while the range of concentrations in residual oil is 0.007 to 0.17 ppm wt. Because only a single mean value was found in the literature for mercury concentration in distillate oil, no conclusions can be drawn about the range of mercury in distillate oil. Table 6-11 lists typical values for mercury in oils, which were obtained by taking the average of the mean values found in the literature. The value for distillate oil is the single data point found in the literature and may not be as representative as the values for residual and crude oils.

Table 6-11 - Mercury Concentration in Oil By Oil Type

Mercury concentration, ppm wt.
Fuel oil type	-	Range	Typical value
Residual No. 6	-	0.002-0.006	0.004a
Distillate No. 2	-	-	<0.12b
Crude	46	0.007-30	3.5c

Source: References 40 [9], 50, 56.
^aMidpoint of the range of values.
^bAverage of data from three sites.
^cAverage of 46 data points was 6.86; if the single point value of 23.1 is eliminated, average based on 45 remaining data points is 1.75. However, the largest study with 43 data points had an average of 3.2 ppmwt. A compromise value of 3.5 ppmwt was selected as the best typical value.

The Brooks report [9] compiled published values of mercury in fuel oil concentration available in 1989.

More Recent Data and Estimates

The principal studies of mercury in fuel oils conducted since 1995 are as follows:

• Bloom [10] measured mercury concentrations in 32 samples of utility fuel oil and found a mean concentration of 0.67 ?g/kg (standard deviation 0.96). Mercury concentrations in lighter distillates (gasoline, diesel) were 1.32 µg/kg (SD = 2.81).
• Liang [11] measured the mercury concentration in one sample of heating oil and found 0.59 µg/kg.
• Rising [12] examined metals in fuel oil from 13 sites in the U.S. and found mercury below detection limits (reported as 0.2 ppb).

From [12]:
Mercury in Crude Oil

Wilhelm [1] reviewed mercury in crude oil data in 2001. The data available at that time showed a similar pattern to the fuel oil data. The data acquired in the 1970's and 1980's were biased high due to the focus of analysts on oils that were problematic due to their mercury content. The mean concentration of mercury in crude oil that was calculated in 2001, based on studies published between roughly 1995 and 2001, was estimated to be less than 5 ppb (see [1] for a detailed discussion).

U.S. EPA, in cooperation with the American Petroleum Institute (API) and the National Petrochemical and Refiners Association (NPRA), is presently involved in a project to determine the mercury content of crude oil processed in the United States [13]. The focus of the project is to determine the mean concentration and range of concentrations of total mercury in crude oil accurately and with statistical justification. Data generated in the project will be used to calculate the maximum potential contribution of crude oil mercury to anthropogenic mercury emissions in the United States. The sampling and analytical methods that have been developed in the course of this project [13, 14] have been applied to fuels only recently.

Recent data for average mercury content in crude oil (THg<5 ppb) and average mercury content of fuel oil (THg ~ 1 ppb) are in general agreement relative to one another whereas the older data cited in [8] are not.

Conclusions

The average concentration of mercury in fuel oils that feed combustion devices in the U.S. is close to 1 (+/- 1) ?g/kg based on the recently available data. It is expected that fuel oil products will exhibit mercury concentrations that reflect the concentration of mercury in the crude oils processed by the refinery. An understanding of the variability of mercury in fuel oil concentrations, both regionally and nationally, awaits focused studies. There is no reason to believe, based on recent data for mercury in crude oils and fuel oils, that refineries produce fuel oils with mercury concentrations that exceed the mercury content of the crude oils they process.

Mercury emissions from combustion sources can be no greater that that amount expected based on mercury in fuels. As opposed to the time frame of early EPA estimates, newly developed methods for sampling and analysis of mercury in liquid hydrocarbons [11, 13, 14] are capable to measure mercury concentrations with good accuracy and precision. Actual emission monitoring for mercury content have also evolved positively. Admittedly, it would be helpful to have a more statistical estimate of mercury in fuel oil, especially home heating oil but the tools to construct such a database are only recently available.

The early data of U.S. EPA, and the methods that are used to calculate emission factors from them, need to be re-evaluated and amended in light of the evolution of analytical methods and the revelations of recently acquired data on mercury in fuel oils.

References

1. Wilhelm, S. M., "An Estimate Mercury Emissions to the Atmosphere from Petroleum", Environ. Sci. Tech., Vol. 35, 24, p. 4704, 2001.
2. Kelly, W.R., Long, S.E., Mann, J.L., "Determination of mercury in SRM crude oils and refined products by isotope dilution cold vapor ICP-MS using closed-system combustion", Anal. Bioanal. Chem., Vol. 376, pp. 753-758, 2003.
3. Liang, L., Horvat, M., Fajon, V., Prosenc, N., Li, H., Pang, P., "Comparison of Improved Combustion/Trap Technique to Wet Extraction Methods for Determination of Mercury in Crude Oil and Related Products by Atomic Fluorescence", Energy & Fuels, Vol. 17, 5, pp. 1175-1179, 2003.
4. Liang, L., Lazoff, S., Horvat, M., Swain, E., Gilkeson, J., "One Step Determination of Mercury in Crude Oil and Related Products Using a Simple Lab Built Thermal Decomposition System", Fresenius' J. of Anal. Chem., Vol. 367, pp. 8-11, 2000.
5. U.S. EPA, Use and Release of Mercury in the United States, EPA/600/R-02/104, December 2002.
6. Minnesota Dept. Environmental Quality, Mercury in Petroleum Refining; Crude Oil and Refined Products. Final Report to Legislative Commission on Minnesota Resources, August 20, 1999.
7. Wilhelm, S.M. and Kirchgessner, D.A., "Mercury in U.S. Crude Oil: A Study by U.S. EPA, API and NPRA", SPE/EPA/DOE Exploration and Production Environmental Conference, San Antonio, Texas, Society of Petroleum Engineers Paper 80573, 2003.
8. U.S. EPA, Locating And Estimating Air Emissions From Sources Of Mercury And Mercury Compounds; United States Office Of Air Quality, Planning And Standards; December 1997, Research Triangle Park, NC 27711; EPA-454/R-97-012.
9. G. Brooks, 1989. Brooks, G. Estimating Air Toxic Emissions from Coal and Oil Combustion, EPA-450/2-89-001, Prepared by Radian Corporation for U. S. Environmental Protection Agency, Research Triangle Park, NC. April 1989.
10. Bloom, N.S., "Analysis and Stability of Mercury Speciation in Petroleum Hydrocarbons", Fresenius' J. Anal. Chem., Vol. 366(5), p. 438, 2000.
11. Liang, L., Horvat, M. and Danilchik, P., "A Novel Analytical Method for Determination of Picogram Levels of Total Mercury in Gasoline and Other Petroleum- based Products", Sci. Tot. Environ., 187, 57 (1996).
12. Rising, B., Sorurbakhsh, P., Wu, J., "Survey Of Ultra-Trace Metals In Gas Turbine Fuels", Proceedings 11th Annual International Petroleum Environmental Conference, Albuquerque, NM, October 2004.
13. Wilhelm, S.M. and Kirchgessner, D.A., "Mercury in U.S. Crude Oil: A Study by U.S. EPA, API and NPRA", SPE/EPA/DOE Exploration and Production Environmental Conference, San Antonio, Texas, Society of Petroleum Engineers Paper 80573, 2003.
14. Wilhelm, S. M., Kirchgessner, D. A., Liang, L., and Kariher, P., "Sampling and Analysis of Mercury in Crude Oil", Journal of ASTM International, in press, Paper 12985, 2005.

S. Mark Wilhelm, Ph.D.
President and Principal Scientist
Mercury Technology Services