Thermal Conductivity of Coal
Coal is a naturally occurring combustible material consisting of primarily carbon, with other elements such as hydrogen, sulfur, oxygen and nitrogen, found in varying amounts. Due to the vast array of available coal sources and its diverse range of uses, it is well known for its large and complicated physical structure which is composed of many small particles. This irregular and highly branched particle structure, coupled with the large variability in coal composition, makes an accurate description of coal properties difficult. Of the many properties found in coal, one of the most important is its thermal conductivity.
Thermal conductivity is a material property which is used to describe how well heat can travel through a material. In general, materials with low thermal conductivity are insulators and do not allow heat to pass through them easily, while materials with higher thermal conductivity are good conductors of heat. As coal formation relies on a number of environmental and other factors, its thermal conductivity can vary greatly depending on the age, rank and type of the coal.
Coal of different ranks, or types, have different thermal conductivity ratings. Anthracite, the highest rank and hardest of coals, has the highest thermal conductivity rating and is known as the best conductor of heat, with a value of 0.17 W/mK. Also known as metallurgical coal, it is largely composed of carbon molecules and is used primarily for the production of metals, plastics, synthetic materials and combustible fuels.
At the other end of the thermal conductivity scale is lignite, the lowest ranked coal and softest in consistency. It has a much lower conductivity rating with values ranging from 0.030 to 0.041 W/mK, depending on the type and age of the coal. It is a much leaner and less efficient energy source than anthracite, but is still widely used in power plants, as well as in the production of synthetic materials, fertilizers and combustible fuels.
In between these two extremes are bituminous and sub-bituminous coals. Commonly used in the U.S. for thermal energy production, bituminous coal is intermediate in rank between anthracite and lignite, and is the most common type found in the world. Its thermal conductivity ranges from 0.059 to 0.077 W/mK. Sub-bituminous coal, which is also widely used in electricity generation, has a thermal conductivity of 0.041 to 0.065 W/mK.
In order to accurately predict the thermal conductivity of coal, it is necessary to take into account the coal type, i.e. the organic content, the ash content, the carbon dioxide content and the sulfur content; as well as the density and moisture content. Factors such as the particle size of the coal, and its thermal history, can also have an impact on the thermal conductivity of the material, as they influence its porosity and permeability, as well as the surface area and airvoid ratio of the material.
The thermal conductivity of coal can also be affected by the surrounding environment, as several factors can play a role in enhancing or dampening the thermal conductivity of the material on a micro level. For example, the presence of volatile petroleum and other organic compounds can decrease the thermal conductivity of coal. Conversely, the presence of water and other ionic compounds can increase the thermal conductivity of coal.
In conclusion, the thermal conductivity of coal is affected by a number of factors, both natural and man-made. As such, it is important to take all of these factors into account in order to accurately determine the thermal conductivity of coal. By understanding the thermal properties of coal, it is possible to exploit its potential as an efficient energy source and to develop more effective ways of utilizing its many advantages.
