OCR Text |
Show - 7 - MODELLING OF CHAR COMBUSTION Five char combustion models have been programmed for computation: model 1, the char is oxidized directly to C0 2 - model 2, reaction C02+C=2CO at the surface, reaction CO+!02=C02 in a thin flame - model 3, the char is oxidized to CO, no flame, the CO produced escapes the boundary layer to the bulk flow without combustion - model 4, the same as 1, but the diffusion resistance of the ash layer is included - model 5, both CO and C02 are produced at the surface, no flame In these models for small particles it is assumed that the spatial temperature variations inside the particle are insignificant. The char particle is assumed to be burning with reducing diameter and constant density. If the combustion is limited by the diffusion, the instantaneous particle radius is given by the equation - ~ dR = 1/2SH In (l+B) PgDg dt (8 ) where the parameter B = YOoof c {1+fg ). The char/oxidizer mass ratio fc depends upon whether the combustion product is CO or C02. f c =MC/M02 =0.375 when the product is C02, then there is no flame and fg=O (model 1). fc=MC/MC0 2 =12/44 and f g= 2MCO/M02=56/32 for model 2. fc=0.75 and fg=O for model 3. For model 5 the value of fc depends on the ratio of the amounts of the products, usually the correlation of Arthur is used for the ratio CO/C02 = 10 3 . 4 e-12400/RT The mass transfer coefficient is calculated from the well-known correlation Sh = 2 + 0.6iRe sc 1/ 3 ( 9 ) ( 10) |