Coal Combustion

Another interesting topic came about in my radiation class. We discussed harmful radiation contamination with coal burning. I never even thought that coal contained radioactive material, but I learn something everyday. According to the EPA, coal contains trace quantities of the naturally occurring radionuclides uranium and thorium as well as potassium-40. The EPA suggests that when coal is burned, minerals, including most of the radionuclides, do not burn an concentrate in the ash. Most of the material is collected, but tiny particles known as “fly ash, along with some radionuclides escape from the boiler and is released into the atmosphere. Moreover, trace quantities of uranium in coal range from less than 1 part per million (ppm) in some samples to around 10 ppm in others. Generally, the amount of thorium contained in coal is about 2.5 times greater than the amount of uranium. For a large number of coal samples, according to Environmental Protection Agency figures released in 1984, average values of uranium and thorium content have been determined to be 1.3 ppm and 3.2 ppm. Using these values along with reported consumption and projected consumption of coal by utilities provides a means of calculating the amounts of potentially recoverable breedable and fissionable elements. The concentration of fissionable uranium-235 (the current fuel for nuclear power plants) has been established to be 0.71% of uranium content. Some other sources that have been determined to be released from coal combustion includes daughter products of the decaying isotopes of uranium and thorium known as radium, polonium, bismuth, and lead. According to the National Council on Radiation Protection and Measurements, the average radioactivity per short ton of coal is 17,100 millicuries/4,000,000 tons. A good explanation was presented by the Department of energy on how the radioactive particles get into the atmosphere. During the combustion process, the volume of the initial coal will be reduced down to almost 85%, which in turn, increases the concentration of the metals originally in the coal. (coal being one of the most impure substances) Heavy metals such as uranium tend to concentrate on the tiny glass spheres that make up the majority of this “fly ash.” This uranium is then released into the atmosphere, at about 1% of the original amount according to the NCRP data. The retained ash is now enriched in the remaining uranium several times over the original uranium concentration in the coal. “All studies of potential health hazards associated with the release of radioactive elements form coal combustion conclude that the perturbation of natural background dose levels is almost negligible. However, because the half-lives of radioactive potassium-40, uranium and thorium are practically infinite in terms of human lifetimes, the accumulation of these species in the biosphere is directly proportional to the length of the time that a quality of coal is burned.