Smouldering fires in large storage facilities may be caused by glowing nests or embedded hot bodies and remain undiscovered for very long times. Due to large temperature gradients near the glowing nest in a deposit of bulk material the detection of a smouldering fire is difficult. This paper reports experimental investigations on thermal conditions, which may cause or promote an ongoing smouldering process, e.g. critical initial temperatures of embedded hot bodies or critical initial sizes of glowing nests. Propagation velocities of smouldering fires were found to be independent of calorific properties of three combustible dusts.