Abstract: A method of combusting oil shale in a circulating fluidized bed boiler. Fuel, including oil shale, is introduced into a furnace of the circulating fluidized bed boiler, primary oxygenous gas is introduced through a bottom grid of the furnace, and secondary oxygenous gas is introduced to the furnace at a first level above the level of the bottom grid. The primary oxygenous gas is introduced to the furnace at a rate providing below the first level a fluidizing velocity of less than 2.5 m/s, and the primary and secondary oxygenous gases are introduced to the furnace in such a way that a circulating fluidized bed is formed, and the fluidizing velocity at the first level is less than 70% of the fluidizing velocity in the upper portion of the furnace.

Abstract: A method of combusting oil shale or fuel having similar properties as oil shale in a circulating fluidized bed boiler. The method includes the steps of (a) introducing fuel into a furnace of the circulating fluidized bed boiler, (b) introducing primary oxygenous gas through a bottom grid of the furance, and (c) introducing secondary oxygenous gas to the furnace at a first level above the level of the bottom grid. The primary oxygenous gas is introduced to the furnace at a rate providing below the first level a fluidizing velocity of less than 2.5 m/s, and the primary and secondary oxygenous gases are introduced to the furnace in such a way that the fluidizing velocity below the first level is less than 70% of the fluidizing velocity in the upper portion of the furnace.

Abstract: In a fast fluidized bed reactor having, in a lower region, downwardly and inwardly inclined walls, a change in the direction of the wall surface or a ledge or protrusion is provided to change the direction of the particles flowing downwardly adjacent the wall for directing the particles inwardly away from the wall and into the region acted on by the nozzles providing the fluidizing gas. The change of direction, abutment or projection is preferably provided along the refractory wall surface at an elevation 200-1100 mm above the grid plate. In this manner, clogging of the nozzles is prevented and more efficient and effective fluidization and combustion processes occur.

Abstract: A simple yet effective method and apparatus control the temperature within a fluidized bed reactor within a desired range (e.g. about 800.degree.-950.degree. C.). Particulate solids in the off-gases from the combustion chamber of the reactor are separated out, and pass through one or more of a plurality of return conduits back to the combustion chamber. The temperature of the returning solids is controlled by positively cooling them, utilizing heat exchangers, within one or more of the conduits, and by controlling which of the conduits the solids flow through. Valves at the top of the conduits can control the flow of solids into the conduits. Alternatively U-shaped passageways at the bottom of the conduits between them and the combustion chamber can have fluidizing air injected therein, the injected air controlling the flow of solids out of the conduits into the combustion chamber.