A method of operating a fluid bed combustor

A fluid bed combustor comprising a generally rectangular housing defining a freeboard space. A plurality of vertically spaced and horizontally staggered beds are provided in the housing and extend inwardly into the space from a rear wall and from a front wall of the housing. A serpentine flue gas path is defined between the staggered beds. Each bed is supplied with combustion air and fluidizable fuel with the freeboard space of each bed including gases from that bed plus gases from all previous beds in the flue gas path. In this way an increased residence time is established without substantially increasing the dimensions of the combustor. A simplified recycle system can also be provided since materials from each bed cascade down into a lower bed so that all materials are eventually accumulated in the lowest beds from which they can be removed.