OCR Text |
Show temperature dependence but a strong dependence on oxygen availability4,5,6,7. A third mechanism, prompt NOx, is relatively insignificant for high temperature applications and only contributes noticeably to NOx emissions in low temperature processes.8 The development of an oxy-fuel burner that could fire high-nitrogen liquid and gaseous fuels with low NOx emissions, in combination with all available oxygen sources, requires a twofold approach. First, fuel NOx must be addressed by reducing the available oxygen atoms that can contact fuel bound nitrogen in the root portion of the flame. Second, the flame temperature must be lowered to reduce the formation of thermal NOx and thus address the troublesome effects of other nitrogen sources. Staged oxygen combustion reduces both thermal and fuel NOx (Fig. 1). Staged oxygen combustion consists of burning fuel initially with a substoichiometric quantity of oxygen and then with the addition of more oxygen, completing combustion under lean conditions. In the fuel rich first stage, high temperatures convert most of the fuel-bound nitrogen found in natural gas and fuel oils to molecular nitrogen. Fuel NOx formation is suppressed in this zone due to the limited availability of oxygen atoms. The hotter the fuel-rich first stage, the faster the fuel nitrogen compounds are vaporized to become available for conversion to molecular nitrogen and the greater the optimum oxygen deficiency9. The use of oxygen for atomization of fuel oils and combustion of both oils and gases is highly effective in the creation of this high temperature first stage. After heat is extracted through radiation and dilution, combustion is completed in the second stage as the balance of the required oxygen is introduced into the flame. Consequently, second stage flame temperatures are lowered and overall thermal NOx formation is suppressed. Properly applied, staged oxygen combustion goes beyond conventional oxy-fuel combustion methods and addresses the nuisance sources of nitrogen -- fuels, noncryogenic oxygen and melter leakage. Laboratory Results To establish a comparison, laboratory tests were run on a conventional oxygen burner versus a staged oxygen burner at identical firing rates and furnace oxygen levels. The conventional burner tested was identical in design to those supplied by Maxon to Coming for the 1991 conversion of Gallo Glass's No.1 melter. On this melter, Gallo reported NOx emissions of 0.8 lbs. per ton of glass lO in 1992. The results of the comparison testing using natural gas are illustrated in Fig. 2. NOx reductions of greater than 90% were measured, depending upon the volume ratio of oxygen injected into the second stage. Additional tests using liquid fuels provided similar results. Tests on #6 fuel oil yielded NOx reductions of 50% (Fig. 3), while tests on #2 fuel oil showed NOx reductions of 60% when compared to conventional oxy-fuel firing (Fig. 4). In addition to the reduction in NOx emissions using staged oxygen combustion, a variety of positive results were documented at the optimized level of staged oxygen. At a fIring rate of 3 MMBtuIhr, the flame length increased 500/0 compared to conventional oxyfuel combustion (Fig. 5). Later comparison tests on a float glass furnace compared 10 4 |