OCR Text |
Show level for a furnace operating with a 1600°F bridgewall temperature, ambient air temperature, 1 5 % excess air and firing natural gas. The baseline level is corrected using a set of individual correction curves for various parameters such as furnace temperature, excess air, combustion air temperature and fuel composition. While this method of prediction has many limitations that will become obvious, a major limitation, with respect to the internal furnace gas recirculation burners, is the data base used for construction of the existing prediction curves was collected using primarily raw gas and staged fuel burners. To fully understand the problem of using existing correction curves for predicting N O x levels produced by internal flue gas recirculation burners, it is necessary to understand the different burner designs and the mechanisms of N O x formation. NOx Formation NOx formation in the combustion process is the result of the oxidation of atmospheric nitrogen and the oxidation of nitrogen chemically bound in fuel molecules. The three oxides of nitrogen that are normally discussed are N O , N O 2 and more recently N2O. Normally, N O represents at least 9 0 % of the nitrogen oxides formed in the furnace during the combustion process. N O is produced in the high temperature regions of the flame. At high temperatures, equilibrium shifts towards N O while at lower temperatures the equilibrium shifts toward N O 2 . However, the reaction rate is so slow at lower temperatures, that very little N O 2 has an opportunity to form in the furnace. The third oxide of nitrogen that is of concern is N 2 O . Essentially, no N 2 O is produced in the flames of process heater burners because of the "high" combustion temperatures. Thus, the major oxide of nitrogen that is of concern and must be controlled if low N 0 X levels are to be produced during the combustion process is NO. Formation of NO occurs via the fuel N0X, prompt NOx and thermal N 0X mechanisms. Fuel N0X is produced when nitrogen is a part of a fuel molecule. Fuel N 0 X is normally a concern when firing either liquid or solid fuels; however, in some cases, gaseous fuels may contain compounds such as N H 3. The prompt N0X mechanism, if it exists, is the least understood of the three. Prompt N 0 X is the N 0 X formed from molecular nitrogen that is in excess of the N 0 X that would normally be predicted to be formed by the thermal N O x mechanism. Of course, this definition is based on an assumption that thermal N O x can accurately be predicted. Several reaction paths have been proposed for the formation of prompt N 0 X . One of the more accepted involves the reaction of molecular nitrogen with hydrocarbon radicals during the decomposition of the fuel in the initial reaction zone. Normally, prompt N 0 X , if it exists, is a very small component of the total N 0 X . Thus, the major concern for gaseous fuel firing is thermal N O x . Figure I1- shows the impact of temperature on the formation of thermal N 0 X . The figure gives equilibrium IV-21 |