| OCR Text |
Show One of the fundamental technical questions that came out of these studies was "what is the lowest oxygen concentration of oxidant to react fuel in practical combustion systems?" This question led to the development of the Dilute Oxygen Combustion (DOC) method described in this paper. The process has been patented and recently commercialized as Praxair Type L Burner (Ref 5). DILUTE OXYGEN COMBUSTION The ultimate design goal of the DOC method is to react fuel with an oxidant stream containing the lowest possible oxygen concentration. In figure 1 and 2 the adiabatic flame temperatures of methane-oxygen and methane-air combustion are shown as a function of oxygen concentration and preheat temperature of oxidant at a stoichiometric ratio corresponding to 2% O2 in flue gas on a wet basis. It is assumed that the wet flue gas is recirculated to dilute pure oxygen or air to provide the oxidant with different oxygen concentrations. The lowest possible oxygen concentration in this case is 2% which is attained only with an infinite amount of flue gas recirculation. Without oxidant preheating, the calculated adiabatic flame temperature is 50300F at 100% O2 and decreases gradually at high oxygen concentrations. At low oxygen concentrations the adiabatic flame temperature decreases sharply from 31SgoF at 25% Oz, to 21000F at 15% O2 and to 642°F at 5% O2 for the methane-oxygen combustion (figure 1). Similar temperature decreases are shown for the air case in Figure 2. It should be noted that the adiabatic flame temperature at 21 % O2 for the methane-oxygen case is approximately 27000F as compared with 336goF for the methane-air combustion with 2% excess O2 in flue gas. The difference is attnbutable to the higher heat capacities of CO2 and H20 as compared with N2 and the endothermic water-gas shift reaction. In order to establish a stable flame in a practical furnace a minimum adiabatic flame temperature of 1800 to 2()()()oF is considered to be required. Oxygen concentration of 12 to 13% may provide the lowest limit of DOC for the oxygen case and 10 to 11% for the air case based on this criterion. This limitation can be overcome by preheating of oxidant. The effects of oxidant preheating on adiabatic flame temperature are also shown in Figure 1 and 2 for oxidant preheat temperatures of 1500 to 30000F. At high oxygen concentrations flame temperature increases only by 100 to 2000F even with 30000F preheat due to dissociation of various species and the small volume of oxidant available for preheating. At low oxygen concentrations the increases in flame temperature become close to the increases in oxidant temperature. At 5% O2 adiabatic flame temperatures are only 300 to 5000F above the preheat temperatures. Therefore, the use of a preheated very dilute oxygen stream offers potential for stable low NO. combustion. 2 t |
