Is coal a good conductor of heat?

Answer 1

Coal is usually not a very good conductor of heat, but it depends a lot on the type of coal as to how poor a conductor it is.

In a study done at the University of Oklahoma, conductivities of 55 different types of US coals were measured at 22°C in the laboratory. Their thermal conductivities ranged from 0.22 to 0.55 W/m°K. Other, more limited studies get the same types of values.

For comparison, air at room temperature and pressure (which is considered a pretty fair insulator) has a conductivity of about 0.026 W/m°K (about 1/10 of coal). Copper (which is considered a really good conductor) at the same conditions is around 360 W/m°K (roughly 1000 times that of coal).

By that comparison, coal would be considered a fairly poor conductor of heat. This is kind of interesting since graphite and diamond (pure forms of carbon - which is what coal is mostly made up of) are rather good conductors of heat. The difference is mostly due to the fact that coal is not PURE carbon, that it has a fairly heterogeneous structure (as opposed to crystalline), that it varies in how wet or dry it is, and that it may actually be somewhat porous.