OCR Text |
Show tempering ports. Inlet velocity vectors at the burners are not perpendicular to the wall due to the swirling flow. The gas flow is relatively quiescent in the furnace hopper, has uniform upward velocity over the furnace cross section above the burners, and is mildly disturbed by the penetration of the tempering flue gas in the upper furnace. The flow bends around the nose of the furnace and exits the furnace through the secondary superheater tube bank. The complex three-dimensional flow patterns exhibited in these results demonstrate that numerical flow modeling provides valuable detailed information for combustion and heat transfer calculations. It is impractical and unlikely that enough experimental data could be collected to provide the detailed information required for combustion modeling. Model predictions of temperature distribution at furnace centerline planes are shown in Figure 5. Combustion causes gas temperature to increase from a mean inlet value of 517°K at the burner throat to a maximum temperature exceeding 2000°K at the center of the furnace. Radiant heat loss from the flue gas to the furnace walls causes temperatures to decrease as the gas flows upward within the furnace. Additional cooling occurs as the tempering flue gas enters the upper furnace at 613°K and mixes with the hotter combustion gases, thus producing depressions in the front-view, centerline temperature distribution. The mixture exits the furnace at a mean temperature of 1484°K. Predictions of species concentrations include oxygen, nitrogen, carbon dioxide, carbon monoxide, water vapor, fuel species, and particulate - coal, char, and ash. Nearly all of the fuel species are consumed within one or two meters of the burner throats due to the rapid mixing of the fuel and swirling air effected by the circular burners. Carbon monoxide reaches a maximum of 2400 ppm near the burner throat and rapidly decreases to zero outside the flame. The char burns at a slower rate, and is not consumed until it reaches the center of the furnace. The combustion pattern is illustrated by the distribution of oxygen concentrations shown in Figure 6. Oxygen concentrations are 21% at the burner throats and rapidly decrease to 3.5% after complete combustion of the fuel. The steepest gradients in oxygen concentration are near the burners where the fuel volatile species are more rapidly consumed. The distribution of carbon dioxide is approximately the reverse of the oxygen concentration - zero at the burner throats increasing to 14% after complete combustion of the fuel. -18- |