| OCR Text |
Show -6- combustion air temperature and excess oxygen levels were set and the furnace temperature was then allowed to stabilize again. The water cooled furnace tubes were then adjusted to achieve the desired furnace temperature. Emissions data were collected and the burner performance was analyzed. After selecting the best prototype and finalizing the burner design, data were collected for various mixtures of hydrogen and natural gas, various combustion air preheat temperatures, various excess oxygen levels and a range of furnace temperatures. Some of the most interesting data involves the variation of NOx with excess oxygen in the flue gases and with hydrogen content in the fuel gas. Figure 3 shows the variation of NOx with excess oxygen when firing natural gas. The air preheat was 350 of for this test. The NOx levels increase as the excess oxygen concentration increases, however, even up to the maximum level tested, 4.9 % excess oxygen, the NOx level is still well below the 40 PPM limit for this application. The emissions are below 30 PPM over the range of excess oxygen that is typical for this type of furnace. While low, the emissions are seen to increase by a factor of about 35% over the range from 2% to 3% excess oxygen. The NOx emission variation with excess oxygen when firing 100 mole percent hydrogen fuel gas are shown in Figure 4. The air preheat was also about 350 of for this test. Somewhat suprisingly, the NOx emissions for natural gas and 100% hydrogen are similar in magnitude. When firing hydrogen the rate of increase of NOx with increased excess oxygen is not as great as seen with natural gas. Over the range from 20/0 to 3% excess oxygen, the NOx only increased by a factor of about 15% . For excess oxygen levels below 2.15% the PPM NOx emissions with hydrogen fuel gas are greater than that with natural gas and above 2.15% excess oxygen the converse is true. |
