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Show The importance of the interaction between a coal-laden primary air jet and the swirl induced IRZ is demonstrated in this article using the Acrodynamically Air Staged Burner (AASB) shown in Figure 2 (Smart and Weber, 1989). The burner is equipped with a movable-block swirlcr producing a constant tangential velocity vortex of around 2 0 % turbulence intensity. The near burner zone properties and consequently N O x emissions can be reduced by optimizing trajectories of coal particles (Smart et al., 1988). Figure 2 shows a range of coal-particle trajectories within the AASB. The near burner flow field, momentum of the primary air and position of the fuel injector can be optimized to promote trajectories 3 and 4. By far the simplest way of achieving this is by inserting the coal injector into the burner quarl. THE MATHEMATICAL MODEL Fluid Flow Equations The computation of two-phase turbulent flows requires that the continuity equation. dxt (p^/) = ^/w, (D Global Combustion Chemistry. Gas phase combustion is modeled using a simplified, two-step, chemical reaction scheme (Visscr, 1991). A n important parameter in this scheme is the local stoichiometry defined as: X = mo, suol mvol +sCO mCO (4) and the momentum balance equation, where mQ2. mvol an<* m C 0 are m c mass fractions of oxygen, volatilcs and carbon oxide while sVO| and HQQ arc the stoichiometric oxygen requirements (kg/kg) for combustion of volatilcs and carbon oxide. Under fuel rich conditions (X<1). volatiles burn to C O and H 2 O while under fuel lean conditions (X>1) to C O 2 and H2O. Everywhere in the furnace, C O can burn further to CO2. Under fuel rich conditions the avaiablc oxygen can react with both volatiles and C O depending on the CO/volatilcs molar ratio. The primary product of char oxidation is carbon monoxide. Similarly to the volatile combustion, it is assumed that the C O released from char reactions is instantaneously converted to C O 2 when the stoichiometry is above 1.0. When the stoichiometry is below 1.0 the C O is added to fuel-rich flow parcels which have to be mixed with oxygen to react further to C O 2. _d A:, ' ( T,TT\ dp d ( (dU, MA ' (2) + S Par*,) be accompanied by a model of turbulence that relates the Reynolds stresses pu/u- to known or calculable quantities. In the above equations U,. uj stand for time-mean and fluctuating velocities, p and u. for fluid density and viscosity while p is the static pressure. Spart an(* SparU are source terms representing interactions between the solid and gaseous phase. Three turbulence models are used in this study; the Reynolds Stress Model (RSM), the Algebraic Stress Model (ASM) and the k-e model. Their formulation and constants are given in Weber et al. (1990). Chemically Reactive Gaseous Species Four gaseous species are considered: oxygen, volatile matter, carbon monoxide and final combustion products consisting of carbon dioxide and water vapour. The mass balance equation for species J is as follows: &»«>>"£{%%)** (3) with mi being the mass fraction of specie J, Sc is the Schmidt number. Si is a source/sink term due to gaseous combustion and generation of gaseous species (volatilcs, C O ) from the coal phase. Turbulent Combustion. The various source and sink terms entering the mass balance equations for gaseous species are modeled by means of an eddy-break-up model which is very similar to the model of Magnussen and Hjertager (1976). This model relates the rate of combustion to the mixing rate between oxidant and reactant: R0l = Ap- min[m02 ,s^i m w / + scomco J (5) In this equation, R Q J is the consumption rate of oxygen. The term Ae/k has the dimension of 1/s, as it relates the mixing rate to locally existing turbulent properties. Enthalpy A n d Radiation The energy balance equation is as follows: ^-(PM= B dx *l + Scorn + Srad + $ part (6) The right hand side contains source/sink terms due to combustion of volatiles, radiation and energy exchange between combusting char particles and gas phase. The radiative heat transfer is calculated using a four-flux model of Gosman and Lockwood (1972). Nowadays, there are certainly more accurate radiation sub-models available which can be coupled into the enthalpy equation. The discrete-ordinate methods of Adams and Smith (1992), Fiveland (1987), Truelove (1988), the discrete-transfer method of Lockwood and Shah (1981) belong to this category. 3 11-11 |
