Abstract: There is provided a process and reaction system for the highly efficient utilization of solid fuels with a low content of organic matter, which may have a high sulfur content and a high mineral carbonate content. Carbonate decomposition is minimized and most of the sulfur is removed from the resulting ash which can be used to make construction materials. The process is effected in 3 stages, the first two of these comprising fluidized bed operation. In the first stage pyrolysis, gasification and only partial oxidation of organic matter takes place, in the second stage combustion of residual carbon is effected in the presence of an excess of air. In the third stage the streams of combustible material coming from the first two stages are combusted. Among suitable dilute solid fuels there are low grade oil shales. The operating conditions have to be adjusted according to the nature of the solid fuel type.