OCR Text |
Show For the CFPF furnace configuration, the combustion occurs in two stages. In the first stage there is insufficient air to completely oxidize the fuel and intermediate combustion products, thus large amounts of CO, char, soot, etc., are present in the lower furnace. This is apparent by the large yellow flame that is observed (see Figure 2). In the second stage, an excess amount of air is provided through the air ports in the upper furnace. The intermediate combustion products mix with the air and combustion is completed before the flow exits the furnace. The varying conditions under which CO oxidation occurs in this configuration make this case ideal for evaluating CO reaction mechanisms. Combustion Predictions and Probing Measurements Predictions for furnace combustion and heat transfer include flow velocity distribution, fuel mixture fraction, fuel remaining, gas temperature, and major species concentrations (unburned carbon, 02' CO, CO2 , H20, etc.). Detailed comparisons were made between model predictions and furnace probing measurements. Predicted velocity vectors are presented in Figure 4 for a vertical plane through the center of the furnace and for a horizontal plane through the center of the overfire air ports. The arrows indicate the direction of the flow and their length is proportional to the magnitude of the velocity. At the bottom throat of each burner, a swirling flow of coal and air is injected into the furnace. The opposing swirl directions cause the flow to converge on the right side of the furnace and to diverge on the left. At the upper throat of each burner, air is introduced through flow vanes tilted upward 10°. The gas exiting the burners impinges on the back wall and turns upward. This causes large recirculation patterns to form in the upper furnace and hopper region. The air introduced into the furnace at the overfire air ports is well mixed with the flow from the lower furnace. The presence of recirculation in the upper furnace causes some of the air from the overfire air ports to mix with flow at the burners, reducing the effect of combustion staging. Predicted distributions of fuel mixture fraction and fuel remaining are shown in Figures 5 and 6. The (gaseous) fuel concentration is zero where the coal and air enter the furnace, but increases almost instantaneously as devolatilization occurs a short distance from the burner throats. Fuel mixture fraction reaches a maximum of 12%, then decreases as the fuel mixes and becomes diluted with air from the upper burner nozzles and later with air from the ports in the upper furnace. The fuel remaining, rnfr' shown in Figure 6, is illustrative of the flame shape that was observed in the furnace. It also represents where fluctuations in fuel mixture fraction occur. The fuel remaining decreases rapidly to zero outside the flame zone. Since the approximation for the PDF is only dependent upon turbulent fluctuations in the fuel mixture fraction, then fluctuations in species and temperature predicted by the model also decrease as shown in Figure 6. This has a large impact on CO predictions as discussed in the following paragraphs. The heat release produced by the low-NOx cell burner is illustrated by the complex temperature distribution shown in Figure 7. The highest temperatures (approximately 22000 K) are within 2 feet of the burner throats where the combustion is nearly stoichiometric and heat release rates are the greatest. Near the upper nozzle of each burner, the temperatures are much lower because the combustion in these regions is fuel-lean. Large temperature gradients ex- 17 |