| OCR Text |
Show -9- / *> m = a k m , (24) n n n uc v J here ra is the mass of the unreacted coal in the particle. Volatile production UC r r continues until the non-carbon constituents of each of the volatiles has been driven off or until m =0, whichever comes first. In the absence of strong uc 6 evidence to the contrary, we assume that the heat of reaction for devolatilization is zero. The char-oxygen heterogeneous reaction is included simply as 2C + 0£ + 2C0 , (25) with a reaction rate due to Skinner and Smoot [141, Rate (g/s) = k P A , (26) C 02 p where P is the partial pressure of oxygen, A is the surface area of the parti- °2 P cle, and 8.6 x 10"3 exp (-1.8 x 104/T ) , T < 1625 K K - 7 in P ? (27) 1 - 4.84 x 10 + 3.8 x 10 T , T > 1625 K . P P ~ The energy change rate due to char combustion is then '% = kc AHf,C02 /MWC02 - (28) where AHf is the heat of formation of CO at 0 K (in Kcal/mole) and MW t , CU L 9 is its molecular weight. The temperature of the particle is therefore increased by 6T = 6tq /mc"~ , (29) P c p where 6t is the numerical time increment and rac is the mass-weighted specific P heat of the particle. D. Coal Particles In modeling an experimental study of pulverized coal devolatilization, it would be prohibitively expensive to compute the trajectory of individual particles. Instead, we study the motion and the interaction with the environment of groups of particles, which we call parcels. The number of particles in each parcel is chosen such that there are sufficient parcels in the system to adequately ^ |
