| OCR Text |
Show conductivity. The output of the total heat balance of the surface zones is the distribution of surface temperatures of the deposits. Any wet slag spot due to local attainment of ash fusion temperature can be identified. This is also possible for eyery superheater and reheater surface area. The total heat balance for volume zones is based on decoupled prescription of mass flux vectors at each zone boundary obtained indirectly or directly from physical flow modeling (16_). An "intelligent' automatic program coupled to the furnace model corrects-the flow field for continuity. The correction program is based on a stochastic tracking procedure of fluid lumps and yields, for a given zone arrangement, a flow field free of mass sources which is as similar as possible to the uncorrected specified flow field. The automatic continuity correction subroutine is also used to generate, from a given flow pattern, approximate mass flow fields for furnace operating conditions in which burners are unbalanced or taken out of service in order to reduce load. It was found that, even for a coarse zone arrangement, turbulent transport of energy between zone pairs must be accounted for. Therefore, a turbulent field is superimposed on the mean mass flow field by assigning additional mutual mass flow vectors at non-wall boundary surfaces with help of a simple model of turbulence. The resulting turbulent flow field, can be verified with isothermal physical flow models by comparing measured and predicted tracer concentrations and, specially, by comparing the dispersion of photographed and numerically generated steaklines (16) (see also Fig. 12). The heat release distribution for a given zone arrangement is evaluated in the following way. Heat release due to burning of gaseous fuels or volatile matter is either prescribed a priori or, alternatively, calculated from a simple transport model. It is assumed that lumps of gaseous fuel or instantaneously released volatile matter are transported with the main turbulent flow. These lumps are tracked within the furnace zones and the lifetime and associated heat release from the individual lumps is calculated statistically from weighted random numbers taking into account the fact that life expectancies follow certain exponential decay functions. Heat release and burnout of residual char particles is calculated from mass balances. It is assumed that the particles will follow the main flow without slip and that devolatilization is completed in the first downstream burner zones. The mass balances are set up for 10 different particle size classes and O2 concentration. The particles burn with decreasing diameter under combined kinetic and diffusion control. In order to compute more accurately the influence of water evaporation during CWF combustion the burn-out model was recently extended to account for droplet tracking and evaporation. The burnout model which is directly coupled to the iterative solution of the total heat balance allows also the calculation of zonal concentrations of gaseous and solid species from which local radiative properties of the combustion products are determined. In particular, local soot concentrations are assumed to be proportional to the local amount of unburnt volatile matter. The constant of proportionality is related to the carbon content of volatile matter obtained from the proximate analysis and to an empirical factor. 6 |
