OCR Text |
Show Results Because only one row of water-cooled tubes was installed for the purpose of preliminary investigation, the range of the firing rate was limited to between 57 X 103 and 110 X 103 Btu/h, and the excess air ratio was varied from 5% to 26% in the testing. Combustion was stabilized and easily controlled within the porous matrix. To evaluate flow distribution through the porous matrix and uniformity of the burner performance, gas analyzing was conducted at different positions across the porous bed. Figure 5 shows the general agreement in NOx emissions data taken at the right side and the left side of the porous bed. However, some deviation was found at relatively high levels of excess air. It also shows significant NOx reduction (up to 14 ppm at 0% 02) with excess air increased to about 20%. The effects of the presence or the absence of the porous matrix on combustion emissions, heat transfer, and pressure drop across the burner-heater were also explored. Figure 6 shows the effect of excess air ratio at a given firing rate on CO and NOx emissions. Placement of the porous matrix in the combustor resulted in a small reduction in NOx emissions and a noticeable reduction in CO emissions at excess air of less than 12%. This effect was even more pronounced at the lower firing rate. However, at a relatively high level of excess air ratio, that is, above 20%, CO emissions in the surface combustor-heater were higher. Visual observation revealed that combustion took place within the bed at the lower excess air levels and gradually moved upward with higher excess air. At the extreme, combustion was observed above the bed - producing a long, semi-transparent flame. The sampling probe, positioned approximately 3-3/4 inches above the bed, was therefore in a region of high CO emissions. Lowering the excess air caused the flame to subside back into the bed and reduced the formation of CO emissions. The mixture pressure was also measured with and without the porous matrix. As shown in Figure 7, the pressure drop of the porous matrix with one row of tubes was approximately 0.4 in. wc. The greatest part (65%) of the pressure drop across the present unit was caused by the distribution grate. The effect of firing rate on combustion emissions and pressure drop across the porous matrix was explored at low excess air levels. As presented in Figure 8, NOx emissions were quite flat, at the very low level of between 15 ppm and 30 ppm corrected to 0% of oxygen, with the firing rate at a given level of excess air ratio. However, CO emissions at various firing rates appeared to be saddled. At the lower firing rate, gas samples taken from different locations across the porous matrix showed nonuniform CO emissions. Higher CO emissions could be 12 INSTITUTE o F GAS TEe H N 0 LOG Y |