OCR Text |
Show 2.1 .5 The following simplifying assumptions are made: - The moisture transfer in the form of liquid water is neglected. The drying front is assumed to be the receding surface of a wet core. The phase change from liquid water to steam takes place at the constant temperature 373 K. In reality the temperature may be somewhat higher due to the incensing pressure caused by the storage of steam and volatiles in the pores. - Steam and volatiles are regarded as flowing out of the particle immediately after their generation due to the great change in the specific volumes that increases the pressure - Secondary char reactions are neglected - The effect of shrinkage or swelling is neglected Usually the Arrhenius equation of the form 0 = A e~E/RT(p-pf) (12) is applied for the local generation of volatiles or for the local disappearence of the solid matter. However, this formula has several drawbacks, when the particles are large. In the previously reported models the final volumetric mass (p^, char density) is a given constant that does not depend on temperature. It is obvious that this cannot be physically correct, since a local point inside the particle cannot "know" in advance its final density that depends on the final temperature. The final temperature is determined by the boundary conditions and the temperature of the atmosphere surrounding the particle. A local point cannot have any effect on these conditions that may even change with time. The reported values for the activation energy E and the pre-exponential factor A greatly vary, which may partly be due to inclusion of heat transfer effects. The final volumetric mass (equiliblium mass, char density) is a strong function of temperature. If the dependence of p_ on |