OCR Text |
Show temperature processes, especially those using highly preheated air, cannot meet limits of NOx emissions set by current and pending NOx regulations in many parts of the U.S. and in many other countries. Indugas, Inc. has developed, studied, and tested a new approach which offers a promise of reducing NOx levels below 10 PPM in high temperature (>870"C (1600*F)) furnaces while using highly preheated combustion air. This approach uses two well established NOx reaction fundamentals to prevent formation of NOx within industrial size flames. These are: (i) Lowering of flame temperatures. (ii) Reactions involving reducing gases such as hydrogen and carbon monoxide to retard formation of nitrogen oxides. Rich or sub-stoichiometric combustion of natural gas can be used to achieve the above. Low flame temperatures may, however, limit the process temperature to levels which are unacceptable for many industrial applications. Highly preheated combustion air must then be used to attain high flame temperatures even with sub-stoichiometric combustion of natural gas. Subsequent controlled combustion (reburning) of the combustion products of sub-stoichiometric combustion can be used to maintain high process thermal efficiency and to meet environmental regulations limiting NOx and unburned hydrocarbons. A research and development program was initiated at Indugas with the support of The Southern California Gas Company to study the proposed approach. The program scope includes theoretical analysis and experimental verification of the proposed method of NOx reduction in high temperature processes. THEORETICAL STUDIES: EFFECTS OF RICH COMBUSTION It is well recognized that NOx in reaction products is reduced significantly as the reaction temperature is lowered. However, it is difficult to achieve low flame temperatures when the process calls for high process temperatures (on the order of 870*C to 1300*C (1600'F to 2400*F)) and the burner is operated near the stoichiometric air-fuel ratio using highly preheated air. Various methods such as use of excess air, flue gas recirculation, or flame quenching have very little effect on NOx emissions when the process temperatures are too high. Sub-stoichiometric or rich combustion of natural gas produces H2 and IV-20 |