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Show substances without increasing any of the other pollutants is an even greater challenge. Affordably fulfilling the intent of cleaner air requires expending the least incremental cost for the greatest incremental emissions benefit [4]. The most cost effective method in the past was to improve the burner, and this remains true today [5]. The burner is less than fifteen percent of the cost of a new boiler system, but it has the greatest impact on N O x generation of any single item in the system and it has great potential to further reduce solid particulate matter from leaving the furnace. Flame shape and individual fuel boundary flame sheets can simultaneously satisfy both ozone and fine particulate matter requirements by being specifically designed to precisely fit the boiler [6]. Concomitantly, improvements in the size and shape of the boiler furnace can enable the optimization of least emissions in assisting the burner in precision flame fit and temperature optimization. A m o n g the most costly methods of emissions reduction is treating the flue gas after it leaves the furnace. The mass of flue gas is typically more than sixteen times that of the fuel being burned making post treatment equipment much larger than the burner which creates it. In addition, employing post treatment systems which consume things such as electric power or ammonia, burdens the boiler operator with continually recurring expense. The burner rarely, if ever, requires added consumables, e.g. ammonia or electricity. This paper discusses the above in the context of the latest analytical flame shaping science and flame chemistry, which has enabled cutting edge N O x and P M 2 5 minimization simultaneously. Key reaction mechanisms are presented which identify critical differences between nearly adiabatic combustors in gas turbine engines, and operating boilers in which highly nonadiabatic flames, designed by fluid dynamic and high temperature chemical analysis, provide the flame shaping accuracy required to minimize N O x. FLAME CHEMISTRY NOx formation in boiler burners is the single pollutant precursor most responsible for emissions performance guarantee uncertainty. The reason for much of the uncertainty is inexorably tied to the combustion reaction chemistry which takes place in the flame. The chemistry involved in N O , N 0 2 , N 2 0 and other species being formed in hydrocarbon flames has certain important similarities. The higher molecular weight hydrocarbons typical of oil, as well as methane which is predominant in natural gas, experience chains of chemical reactions on their way to high levels of carbon dioxide and water vapor as the combustion products approach equilibrium. The desired result of the hydrocarbon oxidation to C 0 2 and H 2 0 is exothermic chemical heat release. The chemical reaction chains proceed exponentially more rapidly with increasing flame temperature. Stable, high activation energy molecules react more slowly than low activation energy molecules. The chemical reactions required to proceed from reactant molecules to product molecules through various multi-step reactions, involve both stable molecules and many radical intermediates. The rates of each of these reactions, and the very chain of reactions followed, are highly temperature dependent. A high temperature path will yield more N 0 2 , for example, than a lower temperature reaction chain, and the sequence of reactions will not be the same. Many chains of reactions are plausible, and extensive analysis has been carried out during the past fifty years in the quest to precisely predict the behavior of real burner flames. Verification of the results from these, thirty or more step branching reactions, depends on the experimental techniques developed to measure radicals which may only be present in the flame for nanoseconds and in concentrations which are within the uncertainty thresholds of the instruments. Mass spectrometry, chemiluminescence and classical resonant absorption spectroscopy have recently been augmented with the application of additional optical "spectro" physics measurements using frequency modulated laser spectrometry and molecular beam sampling mass spectrometry. With the orders of magnitude increase in absorption sensitivity, more accurate rate constants are now known, as well as time dependent concentrations of intermediate radicals previously known only by approximate techniques. Minor specie concentrations, especially pollutants, now have the promise of improved prediction accuracy. Most intermediate reactions require free radicals with at least one free valence to proceed. Before molecular oxygen can be consumed from the atmosphere, it must be broken down to atomic oxygen. H, derived from a comparatively weak chemical bond in the fuel is essential to breaking the strong, double covalent 0 2 bond, and other high activation energy molecular bonds. H + 0 2 <-> O + O H . In addition, H and O H play substantial roles in breaking down the stable saturated hydrocarbons; C H 4 , C2H6, and C3H8. For example, C H 4 requires higher activation energy than CH3. The later, in addition to requiring less activation energy to free an H, is also not saturated. While O does not contribute significantly to the annihilation of saturated hydrocarbons, O does destroy the weaker, heavy, and non-saturated hydrocarbons like ethylene and acetylene, by first attacking a C in the C x H y molecule [7]. Free radicals, such as CH3 , which are produced from C H 4 + O H -)• C H 3 + H 2 0 or C H 4 + H -> C H 3 + H 2 |
