OCR Text |
Show and 0.70 m, respectively. These values are compared with the gas sample analyses in Fig. 6. The distributions in Figs. 4 and 5 appear to have a single dominant mode (other than the one ascribed to equilibration, discussed above), which suggests that the bed might be approximately treated as a single phase with distributed gas-particle contact times analogous to those used by Gilliland and Knudsen (1970). Suppose that the reaction of 0? with char is controlled by extra-particle diffusion, with rapid oxidation of the intermediate CO to CO : C(char) + 02 = C02 (R2) For a parcel of gas in a region of voidage e, the rate of change of the oxygen mole fraction (neglecting changes in gas density and mass transfer between adjacent parcels) is approximately: ^. = - kX (7) dt ShrD fi 1 where k = -±- - ^ - Yr p£ - ^ (8) dc *cpcdc C ^ e and X = X at t = 0 o Some of the parameters in Eq. 8 (especially voidage) are variable, therefore the mole fraction of 0 in the sample under consideration is: X = XQexp(-/ kdt) (9) Since the reference concentration for the cell is approximately the same as the initial oxygen concentration at the distributor (X _ = ref X = 0.21 at P = 100 kPa), we can combine Eqs. 2 and 9 to obtain a relationship of the cell potential, the rate coefficient, and time: •g- = / kdt (10) o o 6 |