| OCR Text |
Show The flame structure of a QLN burner firing gaseous fuels is extremely complex a~d to som~ with degree depends on the adjustments made to the burner. Typically the combustion proces QLN burners can be described as foJlows. . e as it The mixture of primary gas and air is very lean. This mixture gradually burners In, the fu~ac mixes with combustion products of the core, hot furnace gases and secondary fuel. "f!11S f b th combustion process is mostly taking place at the lean limits of flammability, thus formation 0 0 prompt and thermal NOx are greatly reduced. The secondary fuel ignition point is also delayed as the furnace gases in the area around the burner has very low oxygen content. Gradual burning starts as the secondary fuel engages oxygen from the combustion air both prior and after the combustion of primary gas. The core fuel is a small percentage of the overall fuel flow. It provides a reliable ignition source for the primary and secondary fuel. ' An overall interleaving pattern of air and fuel distribution creates conditions for a sufficiently compact overall flame and low carbon monoxide emissions with low excess air. As the mixing of primary gas and air takes place inside the burner, flash back can happen at low loads. This problem was addressed during the early development stage by simply allowing flash back to occur. This typically happens with one or two slots of air and primary gas flow at the loads below 200/0. At these reduced loads flash back is continuous, but does not cause any damage to the burner. Each small flash back flame is surrounded by considerable amount of combustion air and is not in contact with the burner parts. The overall turn down of the burner is 1.2 to 1.3 times of the turn down in the combustion air flow. Typically turn down of 8:1 to 10:1 can be met with the basic QLN design without a significant increase in CO emissions. For special applications requiring wider turn downs a special modified QLN burner was developed that is capable of a turn down up to 2.5 times that of the turn down of the combustion air. With the adequate fuel controls this easily translates into the burner turn downs over 15: 1. This higher tum down modification, however, shows slightly higher NOx. The above description is based on the observations of the flame patterns generated by burners of different sizes operating under different conditions. CFD modeling was only moderately used in the burner design, as accurate predictions of ignition points flame fronts, etc. are still very difficult and not reliable for this complex three dimensional problem even with modem CFD codes like Fluent. Some useful results came from the mixing model between primary gas and air that helped in the optimization process. Most of the work, however, was done experimentally. The burner also shows good NOx performance when firing oil. This is attributed to the effect of enhanced recirculation of combustion products in the part of the furnace adjacent to the burner and the effect of air staging. As the major portion of combustion air discharges into the furnace in a spoke star shaped pattern the entrainment of rate of furnace gas by combustion air is greatly enhanced, if compared with the entrainment by a single round jet. This creates reduction in the flame temperature and reduced thermal NOx production. At the same time, diverting a certain portion of air flow to th~ ports around the burner throat creates air staged combustion, when most of the fuel burns in reduced oxygen environment, thus reducing NOx formation from fuel bound nitrogen as well as thermal NOx. BURNER PERFORMANCE AND OPERATING PARAMETERS The first prototype of the burner was built with the throat diameter of 20", The second prototype had 26" diameter throat and was rated t.o 60 .MMBt~/Hr. Test firings were performed in an eight feet diameter water cooled furnace partially lined ~Ith refractory. The furnace had numerous observation ports and P,orts for the flue ~a~ sa~pllng . The verv. first prototypes of the burner had limited stability range With :espect ~? vanatlon.s In the ~xc~ss air. After the improvements to th design, however, combustion stability was reliably maintained up to 9-100/ 0 02. e The test data shown in Fig. 2 were plotted versus excess air as it appeared to be the m . parameter determining the NOx for any given burner modification within the wide range ~~~oads. 3 |
