| OCR Text |
Show - 8 - The gas density and diffusivity depend on the temperature PgDg = (T /T o)0.75 P oDo g ( 1 1 ) The particle diameter decreases during the combustion and Sh is not constant. The integration of the equation (8) gives the relation _ _ 'P R5 - R2 t - td - PgD g In(1+B) f(a,R/R o) ( 1 2 ) where the correction function that takes into account the relative velocity between the particle and the gas is 2 1 f ( a, R/ R 0) = 1 _ (R/ R ) 2 J o R/Ro 1 +aIX x dx ( 1 3 ) where the parameter a = 0.3/Reo pr1/3 and Reo corresponds to the initial diameter, Reo=wdo/v. The correction function is determined by assuming the correlation (10) for She The correction function f(a,R/Ro)=1 with relative velocity w=O or with value a=O. The correction coefficient for the total char combustion time is f(a,R/Ro=O). The correction coefficient for the total diffusion limited char combustion time due to relative velocity is presented in Fig. 4. Values of the function f(a,R/R o ) are presented in Fig. 5. It can be seen that the values are almost constant with reducing diameter. The effect of the slip velocity on the total combustion time including the pyrolysis stage is the same due to the analogy between the heat and mass transfer. The relation between time and the instantaneous particle diameter becomes ( 14) td = {til-B) IR~-R3/Ro) + HIR~-R2)} DgPginll+B) when the effect of the diffusion resistance of the ash layer is taken into account. It is assumed that Sh=2. The solution is analogous to the drying problem presented in/9/. |
