| OCR Text |
Show where f\ is an effectiveness factor depending on pore diffusivrty. In the pure kinetic regime , the effectiveness factor is close to unity ( ^ *J : V = V .. S. = S..k .. P* (3) c si 1 1 si O and V should not bear a dependency on the particle size. In the kinetic-Hiffusional regime (or transitional regime), the effectiveness factor is a function of the particle radius (r), the rate constant and the ,10, oxygen pressure at the surface of the particle (-) : 1 - n : 1 .05 77 * ~ (p^ ) 2 * . (4) r °2 S1 and thence : 1+n c r si v O v„ <* 7 £ 5 <?n) * (5) In particular, V should be inversely proportional to the particle radius and characterized by an overall order with respect to oxygen equal to n = (1 + n)/2 (where n is the true reaction order) and by an overall activation temperature (d ) which is about half the true one ( 9 ). For external combustion on the external surface of the particle (pure diffusional regime , constant density combustion) V is related to the external surface rate V (expressed in grams per unit time and se per unit external surface) by the expression : V = 3 V / p r (6) c se where p is the density of the solid; V should then also be inversely proportional to the particle radius . The experimental dependency of V on the particle radius is shown c on figure 12 for the case of a lignite coal burning in air. Being close to unity for particles larger than 150 microns, the dependency of V with respect to l/r drops fastly and even seems to tend to zero for smaller 7-23 |
