OCR Text |
Show The particulate phase is treated as a dispersed, dilute solid phase. The particle-particle interactions are 'neglected. A given size distribution of particles is subdivided into size ranges. Each range is then represented by a Lagrangian trajectory with a constant number flux of spherical particles all with the same diameter. The conservation of mass, momentum and energy equations in their Lagrangian fonn for a single, representative particle in each group are integrated using a finite difference scheme suitable for stiff ordinary differential equations. The influence of turbulence on the particle motion is taken into account by a dispersion velocity calculated from a Fikian-type diffusion model for the particle number density (see Celik, 1988 for a review). The heat of reaction of coal is assigned entirely to the gas phase. The cumulative properties of the particles are obtained by appropriately averaging over finite difference computational cells. The gas/solid equations are coupled using the "Particle Source In-Cell (PSIC)" model (Crowe et al., 1977). For modeling purposes, a coal particle is idealized as being composed of raw coal, char, moisture and ash. For devolatilization modeling, the frrst order, irreversible, twocompeting reaction model of Kobayashi et al. (1976) is used. The heteregeneous char reactions are modeled based on only three char (assumed as pure solid carbon) reactions with oxygen, carbon dioxide and steam. The reaction rates are prescribed empirically (see Celik and Chattree, 1988, for details). The homogeneous gas reactions are assumed to be in chemical equilibrium on the basis that micromixing is the limiting factor rather than the kinetics. The mixture properties are calculated by minimizing the Gibbs free energy for a given pressure and enthalpy level. 3.0 SIMULATED REACTOR CONDITIONS The procedure described above was used to simulate a steam-coal test series conducted (Bissett, 1986,1988) in the Morgantown Energy Technology Center (METC) advanced gasification facilities entrained flow reactor. The reactor was designed to provide rapid turbulent mixing in the quarl section to achieve rapid coal particle heating. The geometry of the reactor is depicted in Fig. 1. Three quenching locations were available along the reaction tube, and, along with total gas flow rates, were used to control gas residence times. The inlet quarl had a 15 degree divergence angle. The i.d. of the coal conveying (primary) tube was 3.04mm with a 1.02mm wall thickness; the i.d. of the secondary tube -4- |