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Show Thus, a second extension to CFD codes may include a priori calculations of swelling and its consequences for char structure. 1.2. Char reactivity and burn-out The fourth step, combustion of the char, depends upon many parameters such as temperature, oxygen concentration, residence time etc. as well as char reactivity, and is much slower than devolatilisation. It therefore determines the bum-out time of pulverised coal particles in the furnace. The char oxidation is either modelled as a first order global reaction, or in a way that incorporates the diffusion of oxygen within the pores of the char particle. It is well known that char combustion occurs at different rates in three regimes: Chemical control (Zone I, low temperature), where the oxygen concentration within the pores is equivalent to that in the bulk phase; internal diffusion control (Zone II), where the oxygen concentration decreases to zero within the particle; and external diffusion control (Zone III, high temperature), where the oxygen concentration is zero at the particle surface. In some CFD models, the Baum and Street6 model is used directly or indirectly which includes both chemical and diffusional reaction rate terms7. In such expressions the rate of mass loss by combustion depends on particle density, diameter, and the ratio of reacting surface to external surface area of the particle which is usually assumed to be spherical: ^=-7tD2ppRT^"(R^+R;l)"' (7) '2 Where the reaction order is assumed to be unity here but a value of 0.6 is commonly used, and Rc=Af(pexp(-Ea/RT) (8) and 25D0MC R _ o c diff ~ rT +T ^°'75 P "g RT0Dp v (9) Other models include the unreacted core-shrinking model1,7, which can incorporate ash diffusional resistances to oxygen. Catalytic effects of the ash have received less attention. An alternative approach is the use of empirical correlations which relate char reactivity to experimentally measurable char or coal properties8"10. This approach is often limited to a particular suite of coals/chars and so lacks accurate predictive applicability. It is desirable to be able to model porous char formation and char structure, since its combustion properties. Thus, areas for potential refinement within char burn-out models include coal structure and maceral effects on porosity, fragmentation, and reactivity of chars produced under different temperature - time histories. Such a model might incorporate a statistical distribution of char particle reactivities such as that developed by Hurt". In addition, 3 |