Thermal Conductivity Diffusivity and Expansion

Coal is a low-to-medium conductor of heat, with thermal conductivity values ranging from about 3 x 10-4 to 9 x 10 4 cal/s cm °C (Speight, 1994, and references cited therein). The thermal conductivity X of coal is related to thermal diffusivity (a) and the heat capacity (Cp) by 

Steady-state periodic heating and unsteady-state methods can be applied to measure the thermal conductivity and diffusivity of coal. Methods such as the compound bar method and calorimetry have been replaced by transient hot-wire/line heat source, and transient hot plate methods that allow very rapid and independent measurements of a and X. In fact, such methods offer the additional advantage of measuring these properties not only for monolithic samples but also for coal aggregates and powders under conditions similar to those encountered in coal utilization systems. 

Due to contributions from mineral matter (pyrite being of great importance), no systematic correlation exists between coal rank and thermal 

Thermal conductivity increases with increasing apparent density, volatile matter, ash, and mineral matter content. Due to the high porosity of coal, thermal conductivity is also strongly dependent on the nature of gas, vapor, or fluid in the pores, even for monolithic samples (van Krevelen, 1961). Moisture has a similar effect and increases the thermal conductivity of coal since its thermal conductivity value is approximately three times higher than that of dry coal (Speight, 1994, and references cited therein). However, the thermal diffusivity of coal is practically unaffected by moisture since the /Cp value is not essentially changed by moisture. 

No overall model applicable to the prediction of thermal conductivity/diffusivity values is available, but assuming the presence of additive contributions from the elements in coal, the following correlation has been proposed  

---

PH Klein, WJ Croft. Thermal conductivity, diffusivity and expansion of Y2O3, Y3AI5O12 and LaF, in the range of 77-300 K. J Appl Phys 38 1603-1607 (1967). 

---