Packed beds of solid particles are widely used in solid fuel combustion, waste incineration, drying and filtration processes etc. where gases flow through the beds at a certain superficial velocity. Non-uniformity in the bed porosity causes uneven distribution of the gas flow rate across the bed, and in extreme cases free-flowing channels or passages can be formed in the beds, downgrading the performance of the bed processes. In this paper, experimental observations of channelling in the burning beds of municipal solid waste incinerators are described and detailed numerical simulation of the growth of an individual burning channel is carried out.