OCR Text |
Show NOx emissions below 20 vppm were demonstrated over a large range of firing rates, as shown in Figure 7. Especially notable, as low as 10 vppm NOx emissions were achieved at lower and higher firing rates. This is the result of the interaction of combustion and heat transfer in the combustion zone. Greater heat removal from the grate at lower firing rates and from the first row of tubes at higher firing rates results in further reduction of NOx emissions. In summary, the experimental data have demonstrated high combustion intensity and high heat-transfer rate achieved in the test surface combustor-heater. Particularly, because the combustor and heat exchanger are combined in a single unit, density of energy conversion - defined as amount of energy transferred from fuel combustion to working media within a unit of volume - is extremely high, up to 8 MW/m3 in the present test unit. The data also demonstrate that combustion emissions of both NOx and CO less than 15 vppm and THC less than 3 vppm have been achieved over a large range of operating parameters. The experimental data show a strong interaction between combustion and heat transfer. Combustion stability and emissions are strongly affected by heat extraction by the embedded tubes. Simultaneously, the combined convective-radiative heat-transfer mode and its intensities are also affected by combustion characteristics. Effect of Vertical Spacing of Tubes (HID) To evaluate the effect of heater configuration on performance, three combustor-heater sections with different vertical spacings of the two rows of tubes have been tested at the same operating conditions. The relative ratios of the vertical spacing of tubes are 1.0 (as the base configuration), 1.33, and 2.0. Figure 8 shows overall thermal efficiency of the three combustorheater configurations versus excess air at a 30-kW firing rate. For the configuration with closest tube coils, the heat-exchange surface of the second row of tubes is less effectively used. The overall thermal efficiency is therefore relatively low, as shown in Figure 8. For the heater section with the tube coils quite far apart, however, the heat-transfer rate is also decreased. An apparent reason is that enhancement of flow turbulence by the tube array is depressed. For the combustor-heater with a medium vertical spacing of tubes, the overall heat transfer can increase by 10% to 15%. The effect of vertical spacing of tube coils on NOx emissions is shown in Figure 9. NOx emissions are constantly decreased with an increase in excess air, as expected. It is also noted that NOx emissions are Significantly decreased with an increase in vertical spacing of tubes. However, CO emissions from the burner with the largest HID are much higher than others, particularly at higher excess air, as shown in Figure 10. This is evidence that flame stability and location are strongly affected by vertical configuration of the combustor-heater. In the combustor-heater with largest HID, the flame is relatively closer to the grate. That results in lower NOx, but higher CO emissions. Effect of Horizontal Spacing of Tubes (SID) Three combustor-heater sections with different horizontal spacings of the first row of tubes were tested to evaluate the effect of the combustor -heater configuration on its performance. Figure 11 shows the overall thermal efficiency of the unit obtained from the three different configurations at different excess air, but at the same firing rate, which was 45 kW. It is obvious that the heat-transfer rate from the tubes with the largest SID is lowest because of the least number of tubes installed. Heat transfer to the tubes with the smallest SID is highest at low excess air operation, as expected, but significantly decreased at relatively high excess air, as shown in Figure 11. This is because combustion is partially completed above the first row of tubes, which are too closely arranged. The effects of horizontal spacing of tubes on combustion emissions were also evaluated. It has been found that NOx emissions do not significantly depend on horizontal spacing, but CO emissions do. As shown in Figure 12, CO 10 |