Comminution of Carbons In Fluidized-bed Combustion

The study surveys fifteen years of research on carbon comminution in bubbling fluidized bed combustors. It is carried out on the premise that comminution can be seen as the result of at least four phenomena occurring in series-parallel with each other and with combustion, namely: the primary fragmentation of coals or other carbonaceous materials, the secondary fragmentation, the fragmentation by uniform percolation and the attrition of chars, cokes and graphite. In combination with combustion, these phenomena control size reduction of feed carbon particles into fines of elutriable size and, as a consequence, combustion efficiency and particulate emissions. Information on fragmentation and attrition behaviour is conveniently obtained by means of laboratory and pre-pilot scale combustors when carbon is charged batchwise to the bed. Some of the literatiure data separately reported in the paper for each of the four phenomena taken into consideration can be used, as they are, or after appropriate modification, in the design of larger units. To this end, submodels directed to extrapolate comminution data beyond the ranges of experimental conditions in which they have been determined are thoroughly examined. The paper also discusses how comminution parameters are embodied into the equation model of bubbling fluidized bed combustors. The starting point is the carbon particle population balance. Depending on whether secondary fragmentation is or is not relevant, the population balance is expressed by means of an integro-differential or an ordinary differential equation. Reference is also made to a simplified model which contains essential comminution effects and adequately describes the performance of combustors charged with coals of different rank.