OCR Text |
Show - 3 - injection of ammonia was delayed about one minute after the initiation of the firing cycle to allow the flue gas temperature to reach the minimum value. Injection of ammonia before this point would not achieve any appreciable NOx reduction and would result in all the ammonia going through unreacted. As can be seen in Figure 6, the effect of the ammonia injection is an immediate reduction in NOx • The treated NOx concentration continues to decline until the end of the cycle, indicating that the optimum temperature had not been reached. The maximum instantaneous NOx reduction is over 80 %, with a time averaged reduction of over 60 %. If a suitable location for injection into a flue is not available there are two possibilities: hydrogen addition and injection into the checkers. As mentioned earlier, hydrogen addition can extend the useful temperature range for the THERMAL DeNOx process down to 1300 degrees F. The effect of hydrogen is to shift the NOx reduction vs. temperature curve to a lower temperature. The ratio of hydrogen to ammonia controls the magnitude of this shift. The hydrogen can be supplied by an electrically heated ammonia dissociator, as shown in Figure 7, thus eliminating the need for separate purchase and storage of hydrogen gas. This technique was used in 1986 for the second THERMAL DeNOx process application to a glass plant. This plant is similar in construction to that shown in Figure 2 in that there is only one large checker on each side of the furnace. Again the ammonia injection was delayed, this time for two minutes, to allow the flue gas temperature to reach the useful range. At this point, the flow of ammonia and hydrogen was initiated, with the hydrogen to ammonia ratio at the maximum value. As the cycle progressed the hydrogen flow was gradually reduced to reflect the increasing temperature. In this manner high NOx reductions could be obtained throughout the operating temperature cycle. A time averaged reduction of 65 % was obtained in this plant. Injection into the checkers has also been used in a THERMAL DeNOx process application. In this case the flue gas temperature at the checkers outlet was too low for effective NOx reduction even with hydrogen addition. Injection into the checkers is normally very difficult because there are no open spaces to inject into. The bricks are typically arranged in a "basket weave" pattern so that the longest open distance is about half the length of one brick. However, this installation was to a grassroots furnace where special openings were made in the checkers as they were built. There are four levels of staggered injectors, with injection from both sides of both checkers, resulting in a total of nearly 500 injection nozzles. |