Combustor high heat release

The fluidized bed reactor is a relatively new approach to the design of high heat release combustors. The primary functions of the air-fluidized inert bed material are to promote dispersion of incoming solid fuel particles, heat them rapidly to ignition temperature, and to promote sufficient residence time for their complete combustion within the reactor. Secondary functions include the uniform heating of excess air, the generation of favorable conditions for residue removal, and the ability to reduce gaseous emissions by control of temperature or by use as a gas-solids reactor simultaneously with thermal conversion. 

The values of laminar flame speeds for hydrocarbon fuels in air are rarely greater than 45cm/s. Hydrogen is unique in its flame velocity, which approaches 240cm/s. If one could attribute a turbulent flame speed to hydrocarbon mixtures, it would be at most a few hundred centimeters per second. However, in many practical devices, such as ramjet and turbojet combustors in which high volumetric heat release rates are necessary, the flow velocities of the fuel-air mixture are of the order of 50m/s. Furthermore, for such velocities, the boundary layers are too thin in comparison to the quenching distance for stabilization to occur by the same means as that obtained in Bunsen burners. Thus, some other means for stabilization is necessary. In practice, stabilization... 

In circulating fluidized bed combustors, the volatile release and combustion are important with regard to pollutants emissions, heat release patterns and the subsequent reactivity of the char particles. For coals with high volatiles content, the release of volatiles is important in the selection of the feed point location, since the time constant for volatiles release and combustion is much shorter than the char burnout time. In addition, solid mixing and rate of devolatilization determine the region of volatile release. 

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