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Show IFRF Doc. No. K 70/a/12 August 1989 2.3 CHAR COMBUSTION - 4 - Mathematical Modelling of Pulverised Coal Flames For predicting the near burner zone properties of a high volatile coal flame, the char combustion model is less critical than the model for devolatilisation. Char combustion occurs mainly downstream of the zone of primary interest with respect to fuel NOx chemistry. The pseudo reaction model of char combustion is based on oxygen diffusion to the particle surface and an Arhenius expression which includes both the pore diffusion and chemical reaction rates. The model constants applicable to the considered coal char can be found in Reference 8. 2.4 GLOBAL COMBUSTION CHEMISTRY Gas phase combustion of volatiles is modelled assuming a simple scheme: C ===) CO ===) C02 (products) volatiles (2) H,O,S etc. ===) H20 etc (products) while the char combustion is modelled as follows char ===) C ===) CO ===) C02 (products) (3 ) For each numerical cell, the local stoichiometry defined as mO/(Svolxmvol+Scoxmco) is calculated; where mx is the mass concentration of specie x and Sx is the oxygen requirement to burn 1 kg of specie x. It is assumed that carbon in the volatiles and the char reacts to CO when the local stoichiometry is below 1.0, otherwise the volatiles and the char react to "products" consisting of C02 and H20. Thus, when a fluid eddy containing volatiles mixes in an oxygen rich environment, the volatiles react to final products, but when such an eddy mixes in an oxygen deficient environment, the volatiles only partially combusts. New eddies are formed containing partially reacted intermediates like CO. 2.5 MIXING In semi-industrial flames, the gas-phase combustion rate is controlled· by the mixing rate of oxygen with combustibles. Three different mixing mechanisms may be distinguished: convective mixing, (turbulent) diffusive mixing and micro-mixing. Both convective and diffusive mixing are transport processes and these are modelled by the general transport equation: (Ui mJ) = ] + (4 ) Xl convection diffusion source term |
