OCR Text |
Show air applications with air preheat up to 1100°F (e.g., paint fume incinerators) with N O x emissions as low as 9 ppm (corr. to 3 % 02) that would not be possible without the air preheat. This fact can be explained by the fact that the extend of recirculation necessary for the reliable ignition is reduced with high combustion air temperature. By 1996, COEN had accumulated substantial experience with low NOx burners utilizing premixed and partially premixed air and fuel gas concepts. It became obvious that major difficulties with ultra low N O x burners were not with simply reducing NOx, but achieving this reduction without pulsation over the turn down and with reasonable requirements to the control system accuracy. As studying the pulsation phenomena with C F D packages computationally is still a task of the future, the main approach to the burner development became experimental. The main test facility used during the development is described in the Appendix. A new type of a simulated premix burner with a strong recirculation area created by the flow with weak swirl in the wake of a large axially positioned bluff body was used in the next step of the development process. Initial tests showed the ability to operate with N O x as low as 2 ppm and F G R as high as 4 5 % without significant C O emissions (i.e., C O , less than 25 ppm). The tests also showed that the amount of F G R necessary to reduce N O x to a benchmark level of 9 ppm with the prototype burner required similar levels compared with other ultra low NOx burner designs. There were difficulties controlling combustion induced pulsation when operating below 3 0 % of the burner design capacity. Even in the range of 30 to 5 0 % firing without pulsation, the burner required very high accuracy combustion controls to prevent blow out. Reductions in the excess air or in the amount of FGR, as well as attempts to operate below 3 0 % capacity, resulted in pulsation. Hence, further developments of the burner itself were aimed on extending the range of combustion stability with respect to the amount of F G R and excess air over the turndown. Pulsation Phenomena: The location of the front of the flame generated by premixed type burner is dependent primarily on the ratio of the flame propagation speed and the speed of the mixture of fuel and air flowing near the source of ignition. The pulsation problems in the flow field with high velocity gradients affected by combustion arise when the velocity of the mixture in the ignition region is comparable with the flame front propagation speed. Flame front propagation speed in the premixed flow of gaseous fuel FGR and air is heavily dependent on the amount of FGR, excess air and turbulent energy in the flow. In the region of fully developed turbulent flow, the latter is proportional only to about the cubic root of the Reynolds Number while the local velocity is simply proportional to the Reynolds Number [Isserlin, 1980]. Thus, with high velocity through the burner, the ratio between flow velocity and flame front propagation speed is high and combustion pulsation is not likely to develop 4 |