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Show IT Low NOx with Oxygen-Enhanced Combustion in High N2 Environments C.E. Baukal, V.Y. Gershtein and P.B. Eleazer Copyright 1998, Air Products and Chemicals, Inc. Allentown, PA, U.S.A. ABSTRACT It has been well-established that oxygen can be used to enhance the combustion processes. Typical benefits include increased productivity and thermal efficiency and lower flue gas volumes and pollutant emissions. In particular, it has been shown in the glass industry that replacing preheated air with ambient temperature oxygen can reduce N O x emissions by an order of magnitude. However, one of the challenges of using oxygen in combustion processes is minimizing N O x emissions when a substantial amount of N 2 is present in the combustor. This N 2 can come from air infiltration, from the incoming batch materials, or from the fuel and/or the oxidizer. Recent developments in oxygen production have shown that vacuum swing adsorption (VSA) can produce oxygen much less expensively than traditional cryogenic distillation but at the cost of the 0 2 purity which is typically around 90-93% with much of the impurity being N2. It has been shown in the secondary aluminum smelting industry that using an oxidizer consisting of a combination of air and 0 2 often has better economics compared to using high purity 02. In the case of using either V S A oxygen or a combination of air and 02, N O x emissions are often much higher than using either air or high purity 02. This paper first shows the theory of why it is often preferable economically to operate with oxygen-enriched combustion processes in high N 2 environment. The paper then shows lab data, computational fluid dynamics modeling, and field measurements for several methods of reducing N O x emissions under the conditions of oxygen-enriched combustion in high N 2 environments. INTRODUCTION The conventional air/fuel combustion system uses air, which consists of approximately 21% 0 2 and 7 9 % N 2 by volume, as the oxidizer. For example, if the fuel is methane and no excess air is used for combustion, then the following global reaction can be written: CH4 + 2(02 + 3.76N2) -> C02, 2H20, 7.52N2, trace species + heat (1) where (02 + 3.76N2) is the approximate composition of ambient air. Oxygen-enhanced combustion (OEC) refers to using an oxidizer that has a higher concentration of oxygen than is commonly contained in air which is approximately 2 1 % 0 2 by volume [1]. This basically means eliminating some or all of the N 2 in the oxidizer. There are three common operating regimes, depending on the overall oxygen concentration in the oxidizer. L o w level O E C refers to oxidizers containing from 21 to about 3 0 % 02. Medium level O E C is generally in the range from 30 to about 9 0 % 02. High level O E C is from 90 to 1 0 0 % 02. The l |