A vertical furnace into which a portion of a dust/air mixture is blown, was used to study dust cloud ignition. A dust/air cloud was formed in the working chamber space, with an average dust concentration of 4.8.10(-2) kg/m(3). Ignition studies were conducted for one separated particle fraction of 63-90 mu m. The purpose of the paper is to determine the parameters characterising the ignition (minimum ignition temperature, i.e. critical temperature of the furnace, where ignition takes place, and ignition time) and to describe the mechanism of ignition of coals with various rank (from brown coal then bituminous coal to anthracite) under conditions of particle interaction (cloud ignition). In addition, for comparison purposes, tests of ignition of various types of biomass, sewage sludge, segregated municipal waste and sewage sludge were carried out under the same experimental conditions. The moment of flame appearance (ignition) was recorded using two optical detectors. Ignition delay time was determined at different furnace temperatures ranging from the ignition temperature to 1100 degrees C. The tests performed show that the values of minimum ignition temperatures and the ignition delay times for the analysed coals depend to a large extent on the coal rank. The ignition temperature and ignition time decreased when the volatile matter content in the coal increased and the calorific value of the fuel decreased, i.e. when the rank of coal decreased. For the remaining category of the fuels tested, it was observed that, unlike for coals, the ignition temperature and the ignition delay time increases with the increase of volatile matter content of fuel and the decrease of the calorific value of fuel. Based on the research, it was concluded that under experimental conditions, the ignition of dust clouds of fuels with a volatile matter content below 65-70% daf (anthracites, bituminous coals and some brown coal) occurred according to the heterogeneous mechanism. However, for the content of volatile matters in the fuel above this value, volatile matter are ignited in the gaseous environment (homogeneous ignition). On the basis of the experimentally determined ignition characteristics, an attempt is presented in the paper to interpret the test results based on a simplified model of a heterogeneous ignition of a dust cloud and a model of homogeneous ignition of volatile matters in the vicinity of a fuel particle.