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Show The scientific analyses which are now being applied at Peabody Engineering Corporation has been driven by the need to reduce N O x , C O , VOCs, and particulates while simultaneously improving flame "fit" inside the furnace in order to provide long evaporator tube life and superheater life as well as temperature control. Measurement of flame sizes has had to rely less on the traditional human eye peering through limited view ports and more on the actual emissions measurements themselves. INTRODUCTION The common feature which was fundamental to all mathematical solutions was the correlation of individual fuel jet trajectories with furnace size and with the aerodynamics and gas dynamics of the reaction zone mixing. In order to minimize thermal N O x it is necessary to maximize the surface area of the flame and its radiant view factor to the furnace water walls. It is also necessary to carefully preclude every pocket of higher intensity combustion reaction [l]. This requires that two phenomenon be satisfied simultaneously. Despite the obvious practical problems, the individual fuel jets must not intersect, even in multiple burner furnaces, if low N O x is required. It also requires that the fundamental principle of diffusion flames be maintained, that is, the combustion reaction rate must be dominated by limiting the fuel and air fluid dynamic mixing rate in order to slow its local exothermic heat release rate and, also, to allow the burning to be spatially widely separated. This is opposed to premixed and perfectly stirred fuel and air mixtures which usually have much greater exothermic heat release rates due to the chemical reaction rate being so much faster than the fluid dynamic mixing. It is interesting to note that, while reducing both fluid dynamic and aerodynamic turbulence in a mixing reaction rate limited burner, which is the class of burners of interest here, there is a limit to the paucity of turbulence permitted. Turbulence occurs when fuel has a velocity gradient which differs from the combustion air. It is caused by the dynamic shear stresses created between the viscous boundary layers shared by the fuel and air, or by fuel and fuel, or by air and air. The latter two turbulence generators are to be avoided everywhere. The turbulence generated from shearing fuel with an oxidant, air in most burners, is necessary or the flame will not be sustained. This mixing requirement establishes the minimum level of turbulence, minimum draft losses and fuel pressure losses across the burner nozzles. Most present diffusion flame burners have excess turbulence in their reaction zones [2.3.4]. Reduced turbulence of all kinds [5], facilitates lowering N O x in low excess air burners. In practice, the detail design of the flame shape is accomplished individually, one fuel jet at a time. This individual jet theory is applied for both gaseous and for liquid fuels. The aggregate of all fuel jets in one burner is what is meant by flame shape in this paper. Individual jet trajectories must end within close wall proximity without reflecting due to impingement. In the case of radiant furnaces followed immediately by a superheater, the length of each fuel jet trajectory is even more critical because superheat temperature is dependent on it. Sites with superheaters have provided the most accurate validation of fuel jet trajectory analysis because of the presence of accurate superheater temperature instrumentation which has been recorded during commissioning. NOx YIELD Fuels which have high quantities of fuel bound nitrogen can, and in these low NOx, low excess air burners usually do, yield more N O x from F B N than from atmospheric nitrogen sources. This occurs when the overall flame temperature is below 2800 F, as is the case for these flame shape designs. In the reaction zone, free nitrogen radicals scavenge free oxygen radicals to first form N O [6]. The oxygen radicals are freed from the atmospheric oxygen molecules as hydroxyl ions or hydrogen atoms chemically react with them [7]. The nitrogen atoms, which are weakly bound to the fuel molecules, become free radicals as the carbon and hydrogen are progressively oxidized. Both reactions occur in an inseparable manner. In a low excess air burner the availability of free oxygen radicals is limited. Because the nitrogen radicals are freed from the fuel beginning at much lower temperatures than the freeing of atmospheric nitrogen radicals, the freed F B N radicals scavenge the reaction zone radical pool of oxygen radicals before the atmospheric nitrogen radicals exist in significant quantities. The presence of large quantities of FBN, therefore, inhibits the formation of thermal N O x because the atmospheric nitrogen cannot find the necessary oxygen radicals. A potentially exploitable feature of free nitrogen radicals is that they will react with other radicals than oxygen. One of the other important reactions is that of two nitrogen free radicals finding each other in the turbulent mixing taking place in the radical pool, before colliding with an oxygen radical, to exothermally form a stable nitrogen molecule. Each nitrogen molecule formed in this way potentially precludes two N O x molecules. The best mechanism for further reducing N Ox from these high nitrogen fuels is to create a reaction zone with even fewer free oxygen radicals. Therefore, instead |