| OCR Text |
Show Here J.t is the kinematic fluid viscosity, Rep == IV I - Vpl ,dRe, and IT == Tp/TI == IPIJ(Re/18) , the ratio of the aerodynamic response time (pp~/18J.t) to the time scale for large turbulent structures (~d/Uo). IP == pp/ PI and Id == dp/d, thus, IT characterizes the effectiveness of the large-scale structures for particles moving laterally in the mixing region. Here, subscripts p and f designate particle and fluid, respectively. d and dp denote the diameter of the combustor and the particle. This formulation is based on drag coefficient 2 Cd == (24/ Rep) (1 + O.15ReJ) as given in [4]. The particle energy equation is written as (6) where he is the heat transfer coefficient, h char is the heat of char reaction, and r char the time rate of change of unburned fraction, and == dUb/ dt, is given in the following section. 2.3 Reaction Kinetics Particle reaction comprises two steps: pyrolysis of the coal particles, followed by char reaction including external and internal reaction. This two-step model allows for predictions of pyrolysis time and reaction efficiency for individual particle sizes and input locations. Pyrolysis of the coal particles is modeled by the following single-step, first-order rate equation: (7) The volatile release for each particle is obtained by integrating this expression point by point as the particle temperature Tp varies along the path. Reaction of the char is modeled using the "Extended Resistance Equation" [6] for calculation of the specific reaction rate R, : 1 1 1 1 -=--+--+R& kDYa f.kaYa f.kd 6 (8) |
