| OCR Text |
Show of spreading the fuel jets as widely as for low nitrogen oil, it has been shown that fewer jets which are closer to each other, initially mix with less combustion air, hence less oxygen is available yielding fewer free oxygen radicals to react with nitrogen radicals before the comparatively immense quantities of carbon and hydrogen bind to the oxygen radicals. This means of forming a substoichiometric reaction zone reduces total N O x formation as well as the yield percentage of fuel nitrogen going to N O x in lieu of N2. Thermal N O x is the predominate formation mechanism in diffusion flame burners when the fuels contain little or no nitrogen. Therefore, the greatest N O x reduction is realized by minimizing thermal N O x and not F B N NOx. It would be beneficial to minimize both, but this is not possible because the thermal mechanism is minimized by multiple staging of the fuel spray both axially and radially. All of this staging exposes the fuel to more oxygen radicals, which is deleterious for fuel bound nitrogen in forming NOx. Prompt N O x can be formed by any hydrocarbon fuel. One of the radicals freed in the chain of chemical reactions which take place in breaking down a hydrocarbon fuel molecule is C H . CH2 and other hydrocarbon radicals can also lead to N O x , but are not discussed further here. C H does chemically react with atmospheric N 2 molecules and combines with some of the nitrogen radicals to form H C N [6,8]. Some of the H C N further reacts with oxygen radicals to form N O , some N O 2 and other less stable compounds. The H C N reaction is not dependent on high temperatures to dissociate nitrogen molecules, which is similar to FBN. Prompt N O x can be formed in any flame where carbon and hydrogen are in the presence of molecular nitrogen, e.g. in air. These constituents are present in all of the burners of interest here. Prompt N O x cannot be practically measured outside the laboratory, separately from the other sources of N O x , and is of necessity included in the field measurements. Combining and summarizing these N Ox mechanisms clarifies key temperature differences. Thermal N O x is formed from molecular nitrogen, N2, dissociating at high temperatures into free radicals. The dissociation rate is strongly temperature dependent as is the thermal N O x formation rate. F B N N O x is formed when nitrogen radicals are chemically freed from the fuel. This N O x is not thermal N O x because it is able to form at lower temperatures, and also because it is always derived from the nitrogen in the fuel molecules themselves. Prompt N O x is formed when C H reacts chemically with molecular nitrogen. The temperature at which this reaction occurs is dependent on the temperature at which C H is formed in the flame. The chemical freeing of nitrogen atoms bound in the fuel occurs at whatever temperature the fuel is being oxidized, which is highly variable. Similarly, the chemical reaction creating C H occurs at the temperature at which the fuel is being oxidized, also. Both the fuel derived nitrogen reactions and the C H reactions begin to occur at low temperatures where the mixing fuel and air streams first become just lean enough in fuel rich zones, and just rich enough in fuel lean zones, to become sufficiently flammable to sustain the reaction. Whereas, the dissociation of molecular nitrogen requires the nitrogen molecules to be heated to approximately 2800 F to cause even small concentrations of nitrogen radicals to be formed. Even though the concentration of N 2 derived nitrogen radicals increases rapidly with increasing temperature, additional N O x is formed only with available oxygen radicals which are in extremely limited concentrations in an analytically designed flame. The low excess air design and the two lower temperature N O x formation mechanisms severely limit the yield of thermal N O x. The jet trajectories are analytically arranged to exploit the differences in N O x formation temperatures to, both, reduce overall N O x and to make higher F B N content fuels insensitive to changes in overall N O x yields from a given flame design. SIZING FLAMES Figure 1 is a representation of the aggregate flame issuing from one burner. In the previous art of predicting flame shapes, this shape was treated as a basic characteristic of the burner and its operating conditions. The analytical method now being applied, shapes the flame for optimum fit in the selected furnace and for minimum emissions. Figure 1. ONE BURNER AGGREGATE FLAME. In previous applications, even though the flame shape was presumed to be a given size, the burner producing the flame shape was not limited to installation in only one size furnace. Allowing for the supposition that a burner produces one characteristic flame size implies that there is a furnace shape into which that flame fits precisely. Figure 2 is a representation of exactly that furnace. The flame has the precise clearance along the walls to simultaneously yield the lowest C O and the lowest N O x . It also has the precise clearance to |
