| OCR Text |
Show are sufficiently reactive to be oxidized directly. In addition to saturated hydrocarbons, which are not significantly reactive with O, C O is similarly unreactive regarding O. In boiler burner flames, the primary mechanism is C O + O H -> C 0 2 + H, and not C O + O -> CO,. The latter would only occur at temperatures greater than is typical of boiler flames surrounded by non-adiabatic walls and using air, with its high inert mass content, as the source of the oxidant. Furthermore, molecular nitrogen, N2, which has a triple covalent bond, does not readily react with H, and therefore certainly not O, at temperatures as low as does molecular oxygen 02. At low flame temperatures, N 2 is the most stable diatomic molecule known. In high temperature flames, above 1900°C which is atypical of low N O x boiler flames, oxygen atoms will react directly with nitrogen molecules, O + N 2 <-> N O + N. Also, nitrogen atoms will react directly with oxygen molecules, N + 0 2 <-» N O + O. In addition, the hydroxyl ion reacts with nitrogen atoms, O H + N <-> N O + H, all three of which yield nitric oxide, N O . Unfortunately, nitric oxide is very stable against decomposition to molecular nitrogen. Unless some form of post flue gas treatment is employed, such as S C R [8], or reburn, or SNCR, or combinations thereof, it is essential that N O be precluded everywhere, as far as possible, in the flame in order to yield N O x sufficiently low to comply with the intent of M A C T . The temperature dependency of the molecular nitrogen reactions is the feature which enables very low N O x in the flame itself. Boiler burner flame temperatures are generally too low for either of the atomic oxygen reactions written above to be significant. It is the third of the three reaction mechanisms that dominates the rate of N O formation in nearly all boiler flames. Therefore, the rate of N O formation, in non-adiabatic boiler flames which burn in air, is dependent on the availability of hydroxyl ions. Ignition energy is the initial source of a zone of locally concentrated H atoms. The mechanisms which yield a rapidly increasing pool of O H are; H + 0 2 <-> O H + O, O + H 2 <+ O H + H, and O H + H2<-> H 2 0 + H. By contrast, even in boiler burners whose flames are highly non-adiabatic, and the rate of free radical formation is retarded for most species, the hydroxyl ion formation continues with quite sufficient rapidity even with very cool flames, as low as 700°C in the combustion process. Therefore, the burner flame can be stably operated continuously at temperatures which do not yield N O but which do produce sufficient H and O H to bum-out the high C O and toxic organic species. The burner is simultaneously capable of yielding low particulate matter; including total suspended particulates, P M 1 0 and P M 2 5 only when the flame does not burn too close to the cool boiler furnace walls, otherwise reactions such as the H and O H mechanism above are terminated, leaving thermally quenched toxic substances including CO, U H C s and other V O C s in the exhaust products. In addition, it is imperative that the flame be close to all cold boiler furnace walls in order to produce a sufficiently low flame temperature to preclude NO formation. Precision flame shaping permits exactly that, the coolest flame and the cleanest flame, with simultaneously low CO, N O and entrained fine particulate matter without substantial direct O reactions with either C O or N O , and with O H dominating both [9]. Precisely fitting, therefore low temperature, burner flames within evaporator wall boilers have no N 2 0 , and less than one ppmv N 0 2 as the products leave the visible burning zone in an otherwise dark furnace. It is therefore valid to realize that the total N O x produced in a cool flame which contains no C x H y N is N O . Much care is required in handling emissions gas samples to entirely preclude further oxidation of N O to N 0 2 inside of the extraction and measurement system [10]. When N 0 2 is reported in samples leaving low temperature flames, it is usually quite small, only a few ppmv, and likely due to contaminant oxygen is the sampling flow circuit and not as a result of formation in the flame. Nitrogen chemically bound to the CxHy molecules presents an additional N O source, and most unfortunately, a N O source which can be, and often is, formed at lower flame temperatures than those discussed above which yield thermal N O . Further, N O formation from CxHyN occurs in an otherwise ultra-low N Ox burner operating with cool flame temperatures of an evaporator wall steam boiler. Flame temperature minimization does not preclude these N O x formation mechanisms. Paucity of O H and to a lesser extent, O, does. N O formed from chemical bond annihilation of C x H y N is commonly referred to as fuel-bound-NOx. There is also a third N O formation mechanism in boiler burner flames. Molecular nitrogen, while thermally stable below 2800°F and not subject to the three reactions which yield N O above, is subject to chemical attack. While the rate of the chemical attack is related to temperature, the chemical bond of the N 2 molecule is broken chemically at significantly lower temperatures than required for dissociation of the bond by energetic thermal excitation. The reaction mechanisms involve intermediate radicals which are near the low mass end of the reaction chain. C H and C H 2 are two of these. T w o of the reactions are C H 2 + N <-» H C N + H and C H + N 2 <-> H C N + N , with both followed by O + H C N <-> N O + CH. In addition to these reactions, there is the limit case of the nitrogen bearing hydrocarbon, C x H y N , having only one atom of C and H, i.e. H C N . Hydrogen cyanide can be imported with a gaseous fuel into a flame, or be present as an intermediate, where the same result occurs as above, O + |
