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Show process of developing and implementing a biomass particle combustion model in the commercial C F D code F L U E N T to examine fuel sizing, shape, and injection location on unburned carbon in a full-sized boiler. The computer simulations of coal fired combustors are an economically efficient tool for evaluating the design and control strategies to improve energy (fuel) efficiency, process stability and emissions control. The current state-of-the-art technology is now capable of solving the complex interdependent processes like fluid flow, turbulence, particle trajectories, heat transfer, soot generation and hetrogeneous and homogeneous chemical reactions involved in the fossil fuel combustion. However, the complete description of the chemistry of devolatilization and char oxidation is still based on kinetically simple empirical models that do not account for the chemical structure and complicated physics of the process. A recent sensitivity study of a CFD-based coal combustion model has shown that uncertainty in the devolatilization/oxidation parameters has a dominant effect on unburned carbon in model predictions (Jones et al., 1999). The purpose of this article is to perform some exploratory simulations in a lab/pilot scale combustor which examines the effects of fuel shape, size and injection location on unburned carbon and N O x emissions. The next section describes the gas and dispersed phase models used in the present simulations. In section 3, we compare the predicted species and carbon burnout data with the experimentally measured values on M F C . The parameters obtained from the M F C simulations are used for performing some exploratory simulations to predict the carbon burnout and N O x emissions for different co-firing rates in the C E R F combustor. The results from these exploratory simulations are also discussed in section 3. Finally, the conclusions from this study are presented in section 4. 2.0 MATHEMATICAL MODELING The mathematical model is based on the commercial CFD code, FLUENT, where the gas flow is described by the time averaged equations of global mass, momentum, enthalpy and species mass fractions. The particle-phase equations formulated in Lagrangian form, and the coupling between phases is introduced through particle sources in the Eulerian gas-phase equations. The standard k-e turbulence model, two-mixture-fraction probability density function (PDF), and the Discrete Ordinate radiation models are used in the present simulations. The coal devolatilization is simulated using the two-competing-rates Kobayashi model, and the char oxidation is modeled as the kinetics controlled surface reaction. The biomass devolatilization is incorporated using an Arrhenius-type. first order kinetic rate model. The biomass char oxidation is controlled by diffusion-limited surface reaction, and it is modeled as a constant density process. The standard F L U E N T code is updated with the modified char oxidation submodels for coal and biomass via an externally defined user function. 2.1 Fluid Phase The fluid flow model is based on the time averaged Eulerian equations for conservation of mass and momentum. The mean fluid motion represented as the time averaged equations for the conservation of momentum is written as: 3 Xj ( _ Puju, + pS,rM ___. + ___. _ d"* g dxj d x, 3dXk d -• + Pg, + ^-(Pu-jU;) = 0 (D °x, where u< is the velocity component in the direction of coordinate Xj, p is the fluid density, g; is the magnitude of the gravitational acceleration in the i-direction, p is the pressure, u is the laminar viscosity, and the operator 5^ is unity for i=j and zero when i*j. The last term (Reynolds stresses) is modeled using the "two-equation" k-e model: •( Pu-,u;)= M, ___ + __! _ _i_ x d x, o x, 3 d Xk (2) where \i{ is the turbulent viscosity that may be related to k and e by dimensional arguments. |
