| Heat transfer within a char bed of a recovery boiler used in pulp and paper mills is important in determining the cooling rate of a char bed following an emergency shutdown. The rate of heat transfer depends on the thermal conductivities of frozen smelt and char, but there is little reliable data available on these thermal properties.; An experimental apparatus was thus built to measure the thermal conductivity of various char samples made in the laboratory and collected from operating recovery boilers, under normal operation and following an emergency shutdown procedure. The reliability of the apparatus and measurement method was first verified using firebricks with known thermal conductivity values and a numerical simulation.; The thermal conductivity of char bed materials with different porosity and structure was found to be constant up to 500°C, and then increase linearly with the temperature. Based on the data, a two-part correlation is recommended to predict the effective thermal conductivity of char bed materials with various porosity (between 55% and 80%) and structure: ForT≤500°C keff&parl0;W/m° C&parr0;=0.21 ForT>500°C keff&parl0;W/m°C&parr0;=0.0035 T&parl0;°C&parr0;-1.54. |
