OCR Text |
Show - 5 - Furthermore, we assume that the pyrolysis is increased due to the temperature increase in the familiar exponential manner. The contribution of the interval i can be presented as . -E*/R(T-(T . - Tref )) (li p ._ Pl' = k* e 1 u 6 p. ) el 1 The relation T - (T.-6T f) can be justified for the 1 re ( 3 ) reason that the driving temperature difference is T-T. 1 rather than T - OK, since the pyrolysis of the interval i is not started until T>Ti . If we use T-OK as the driving force, then the reactions of the higher temperature intervals would be faster. The use of T-T. makes the driving 1 temperature difference the same for all reactions. The total rate of devolatilization is obtained by summing up the contributions of each interval p = L 6p. 1 ( 4 ) The devolatilization model requires two constants, the preexponential constant k* and the effective activation energy E*. 6Tre f is ~hosen to give good correspondence. The additional 6Tref is requi:ed, since exp(-E*/(T-Ti )) ~ 0, when T ~ Ti' which gives Pi=O. However, for large particles the reactions are fast compared to the heat transport, the term 6Tref makes it possible to include the diffusion regime in the model. The values of the constants E* and k* can be found from measurements in the entrained-flow, isothermal furnace in nitrogen flow and by comparison with model calculations. The pyrolysis is schematically described in Fig. 3. At the time instant t the particle temperature is T, and the driving temperature difference for the reactions of the interval i is T-Ti . The particle contains still some of the volatile matter of the interval i and the driving force is 6p .-6p,. el 1 |