Radioactive coal ash
A coal seam in New South Wales
Mum says that you found an article in Scientific American about radioactive coal ash. This is something that has been discussed for many years.
Do you remember going down the State Coal Mine at Wonthaggi? You will remember from that trip and the Red Australia article that coal is a sedimentary rock. It is formed when organic matter - mostly trees and other plants - are buried, compressed and heated for thousands of years. There is a good overview of the process here. This metamorphoses the plants into coal which contains more than 50% by weight and 70% by volume of carbon-rich material.
Coal occurs in different types or 'ranks'
Anthracite is the hardest and most energy dense form of coal. It has a very high carbon content, around 86%, and therefore a low ash. It looks shiny, almost metallic, and has the highest energy density.
Bituminous and sub-bituminous
Bituminous coal is not as hard as anthracite and has a lower energy density. It is the main type of coal mined around the globe, and has a dull black appearance. This is because of the tar-like bitumen that is included in the mineral. This type of coal is typically 70% to 85% carbon, and burns well because of the included hydrocarbons.
Lignite is also known as 'brown coal' and has the lowest proportion of carbon (60% - 70%.) As an energy source it is quite poor, but it is often used because it is easy to mine.
Coal burning in a blacksmith's furnace
There are lots of compounds that occur naturally in coal. The biggest non-carbon element is water. This is followed closely by hydrogen, and then sulphur. There are also lots of 'volatile hydrocarbons' in coal. These are chemical compounds that have carbon atoms connected to hydrogen atoms, and are similar to compounds found in crude oil. In fact, many products that are now derived from oil were once made from coal-tar. The volatile hydrocarbons in coal will be driven off as it is heated to about 950 °C, then the carbon burns at about 1,000–1,200 °C. When burning, carbon produces both carbon monoxide (which is toxic to humans) and carbon dioxide (which is a greenhouse gas.) Because coal can also contain impurities such as sulphur and nitrogen compounds, burning coal can also produce 'acid rain' which forms when sulphur dioxide and nitrogen dioxide are washed from the atmosphere by rainfall.
Bulldozer moving ash at a Welsh power station
What remains after burning is 'ash.' Better quality coals leave less ash, but all of the ashes are similar. They are basically the things that could not be burned at the temperature of the fire. An ABC article says that Australia produces 12 million tones of ash every year from burning coal at power stations. In some countries this is mixed into concrete for building, or asphalt for roads. In Australia it accumulates at the power plants.
If you look at the carbon percentages above, you will see that 'other things' make up between 12% and 30% of the coal, depending on type. Some of the other things that occur in tiny amounts are radioactive elements such as uranium and thorium.
Uranium in coal
In the 1950s and 1960s the United States government geological service studied coal seams for uranium content. They found that the coal had between 0.005% to 0.02% uranium by weight. This is between 50 and 200 grammes of uranium per tonne of coal. A tonne of solid anthracite is slightly less than a cubic metre, and 50 grammes of uranium is about 2.5 ml, or half a teaspoon.
If all of the carbon is burned, then just the impurities are left behind as ash. Think of evaporating sea water to make salt. When the water is gone, the salt is concentrated as crystals. In the same way, the uranium is now mixed in a smaller volume of ash. This means that the uranium is more concentrated than it was in the coal. Even so, the United States Environmental Protection Agency says that "these wastes are only slightly more radioactive than the average soil in the United States."
There is more to the Scientific American article than just a headline. Its main claim is that "the waste produced by coal plants is actually more radioactive than that generated by their nuclear counterparts." This is immediately qualified by looking at fly-ash emissions to the environment compared to nuclear plant emissions to the environment.
As you read through the article you will see that many naturally occurring rocks emit more radiation than fly ash; and that the radiation risk from coal is more of an occupational hazard to miners than an environmental threat to the public. The article's final paragraph talks about other environmental harm from coal, such as greenhouse gas emissions and acid rain, and asks whether nuclear power is more environmentally appropriate.
Ideally we would use less energy and therefore not need to produce as much. The current focus is on renewable energy sources. Hydro has also been around for many years, producing 'clean' energy in many countries. Wind and solar have the highest profile of the newer technologies.
However, renewable sources are imperfect. If there is no wind or sun, there is no electricity generated. Even with advances in battery technology we can not yet store enough renewable energy to cover our night time and calm-wind needs.
You should also think about energy security and risk, which takes us away from science and to economics, sociology and politics.
There is good science about the climate; there are long-term studies of health risks from mining and radiation; particulate pollution has become a concern in recent years. All of these things are affected by people's daily lifestyle choices, which inform political choices and therefore government action.
As a young adult, you must understand that your choices affect everyone around you, even if only slightly.
Do you want nuclear power? Or fossil fuel power from coal or gas? Or renewable, if it can be guaranteed to run your air conditioning on the hottest days? Each of these choices contains risk that science can help you navigate so that when you make a choice it is properly informed.