Ash agglomeration

A. van der Drift and A. Olsen Conversion of biomass, prediction and solution methods for ash agglomeration and related problems. Final Report., Pettcn, ECN, ECN-C-99-090, 62 p. (1999). 

A good example of continuous particle separation is the industrial application in the ash-agglomerating fluidized bed gasifier. The ash remained from gasification of coal is agglomerated into bigger and denser agglomerates and is continuously removed from the fluidized... 

All the discussion so far is on the particle separation in a gas fluidized bed based on velocities very close to the minimum fluidization velocity of the jetsam. Particle separation can also be accomplished by employing velocities close to that of the terminal velocity of the particles. The U-Gas coal gasification process developed by the Institute of Gas Technology utilized the principle of terminal velocity for separating ash agglomerates from char in the coal gasifier. A comprehensive study on this subject using a fluidized bed similar to that of the U-Gas process has been published by Leppin and Sahay (1980). Some experiments were also carried out by Chen and Keairns (1978). 

In the ash/char separation zone, if it is desirable to incorporate such a zone, controlled fluidization results in efficient segregation of the larger ash agglomerates from the char particles, acting as a mechanism to... 

Steam and recycle fuel gas distribution for local fluidization to avoid dead zones in the bed and to minimize local hot spots resulting in uncontrolled ash agglomeration. 

The pyrolysis gasifier temperature (greater than 800° C for most coals) is controlled by the char fueled circulating bed combustor temperature (greater than 900° C) and the solids circulation rate, and the gasifier bed temperature may be limited to lower than optimum values by ash agglomeration limitations in the circulating bed combustor. 

The bed temperature is 760-870°C depending on the biomass fuel ash agglomeration tendencies. 

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