The combustion gases which leave the bed carry with them material from the bed. This material may contain unburnt fuel which has not been burnt during its dwell time in the bed. Particularly at a high load with a high bed level and a large gas flow, coarse bed material--both fuel particles and absorbent particles--are thrown up high in the freeboard, and a certain amount of these coarse particles is carried away from the bed vessel together with the combustion gases. It is known to return material from dust separators for burning such material and hence increase the degree of combustion.