Computer simulation of gas flow and heat transfer in waste-heat boilers of the outokumpu copper flash smelting process

Gas flow and heat transfer phenomena in an old and a modified industrial-scale waste-heat boiler of the Outokumpu copper flash smelting process were simulated with a commercial computational fluid dynamics code Phoenics. The standard k-epsilon model was adopted for representing gas turbulence, and the six-flux model for thermal radiation. Various operating conditions were studied, including different types of off-gas circulation from the electrostatic precipitator, and air leakage from the boiler dust-hoppers. The computed flow patterns were compared with the results from laboratory scale physical models, and the heat transfer simulation of the old boiler was compared with temperature measurements in the smelter. Good agreement was reached between the computed results and the laboratory models and industrial measurements. It was found that gas flow in both boilers is clearly three-dimensional, especially with off-gas circulation. The off-gas circulation may change the flow pattern significantly, while the air leakage does not affect the main flow pattern. The computational results indicate that radiative heat transfer accounts for 80-95% of the total heat transported through the heat transfer surfaces in the radiation section under different operating conditions. Radiation screens in the modified boiler play an important sole in the gas cooling, accounting for about 40 - 50% of the total heat transferred in the radiation section. The additional gas cooling due to the circulated off-gas is significant in the old boiler, but minor in the modified boiler. The simulation results indicate that the modified boiler can efficiently cool the off-gases with increased volume flow rate and higher temperature.