I got an email from a reader. They wanted to know why coal has so much mercury in it. In reality coal doesn’t have that much mercury in it. The problem is that you are burning the coal and releasing the mercury into the atmosphere.

Interestingly enough, the smarty-pants at the U.S. Geological Survey have been analyzing coal for years. They have an entire database of coal samples from all over the country. Admittedly, it is somewhat skewed to samples of coal from the eastern United States, but it is the best collection out there.

Mercury is one of the elements for which the Geological Survey has been analyzing the coal samples. Their database includes about 7000 samples of coal. The ranges for mercury in coal are from .003 parts per million to 1.753 parts per million. Eighty percent of coal samples are less than .25 parts per million. The mean coal content is .17 parts per million. (Remember mean is the average – all the mercury values for samples in the database are added up and divided by the number of samples.) Parts per million are milligrams per kilogram. These are very tiny numbers. And that is not even the coal that is going to the power plants. A lot of coal is “cleaned” before it goes to the power plant. Utilities want a good burning coal without a lot of ash and sulfur, so after coal is mined it is often sent through a cleaning process to reduce these unwanted attributes. Ash and sulfur are unwanted because the United States Environmental Protection Agency has regulations to reduce particulates (ash) and sulfur. You don’t want to breathe particulates because they are linked to respiratory problems and you don’t want to have sulfur in the atmosphere. Remember acid rain? Yes sulfur is the big contributor to that problem. The Geological Survey says that 37 percent of mercury is removed by coal cleaning.

When the Environmental Protection Agency wrote their regulations on reducing mercury at power plants, they didn’t rely on the Geological Survey’s database. They conducted a study back in 2010 on mercury content of coals delivered to power plants. I didn’t go looking for this study. The Geological Survey says that the Environmental Protection Agency’s mean mercury content for coal delivered to power plants is .12 parts per million. A bit less than the .17 parts per million the Geological Survey had for their database. The Geological Survey attributed the difference primarily to coal washing and the fact their database had more eastern coal which is higher in mercury than many western coals. Interestingly, the Geological Survey analysis of their database showed Northern Appalachian and Southern Appalachian eastern coals to have the highest mercury content of U.S. coals.

The Geological Survey is doing a lot of research on where mercury resides in coal. I personally thought this was interesting because it was something new to me, so I wanted to share it with you. Pyrite is the source of most of the mercury in coal. You may know pyrite by the term fool’s gold. It is an iron sulfide and often has trace metals like mercury associated with it. Mercury in coal can also be bound to the organic carbon in coal, it can be sort of diffuse as elemental mercury, and it can be part of other minerals found in coal.

Here’s another interesting fact from the Geological Survey. Their database also includes about 1600 coal samples from 57 other countries around the world including China. They looked at the mean mercury content of those samples and found it was 0.16 parts per million, about the same as the mean mercury content of coals in the United States.

Anyway I digress from the main point of my post here which is coal is not teeming with mercury, in fact not any more so than say soils. The same Geological Survey has analyzed soils from all over the country. One of the elements they have analyzed for is mercury. They looked at three different depths of soil. Just looking at the top 5 centimeters of soil, mercury levels ranged from non detectable by current analytic methods to 3.55 parts per million. Soils in the middle part of the U.S. were lower than the soils on the east and west coasts. Again these are really tiny numbers. But we don’t burn soil.