A numerical study on the effect of hydrogen/reformate gas addition on flame temperature and NO formation in strained methane/air diffusion flames
Introduction
Hydrogen enrichment is a promising concept for reducing fuel consumption and pollutant emissions. It has been shown that hydrogen enrichment can improve flame stability and thus reduce NOx formation in premixed flames [1], [2], [3], [4], [5], as well as increase burning velocity [6], [7], [8]. For diffusion combustion, hydrogen enrichment can suppress the formation of soot particles [9], [10] and shorten ignition delay [11], [12].
Relatively, not enough attention has been paid to the effect of hydrogen enrichment on NOx formation in diffusion flames. When hydrogen is added to a hydrocarbon diffusion flame, it is expected that the NO formation rate by the prompt route is reduced. On the other hand, the addition of hydrogen may modify flame temperature, which in turn may change NO formation rate by the thermal route. The net effect of hydrogen enrichment on NO formation in a hydrocarbon diffusion flame depends on the relative variations of the thermal and prompt routes. Naha and Aggarwal [13] investigated the effect of hydrogen addition on NOx formation in strained nonpremixed methane and n-heptane flames at a fixed strain rate (100 s−1). They found that the addition of hydrogen had a minor effect on NO formation in methane flames and reduced the formation of NO in n-heptane flames. The variation in strain rate modifies the residence time of reactants in the reaction zone of a flame, and thus affects NOx formation mechanism [14]. The effect of hydrogen enrichment on NOx formation depends on strain rates. Therefore, it is of great interest to further investigate the effect of hydrogen addition on NOx formation in diffusion flames at various strain rates.
Besides, hydrogen is an energy carrier, and has to be obtained from other hydrocarbon fuels or water. One way to generate hydrogen is fuel reforming. The product of fuel reforming, known as reformate gas (RG), contains not only hydrogen, but also carbon monoxide and some other components. Instead of using hydrogen, it is more practical and economical to directly use RG for fuel enrichment. In addition, the study of RG enrichment combustion is directly related to the application of syngas that is an important alternative fuel. Therefore, it is also of practical importance to investigate the effect of RG addition.
This paper presents a detailed numerical study on the effect of hydrogen and RG addition to fuel on NO formation in laminar methane/air diffusion flames at different strain rates. Since NOx formation is closely related to flame temperature, the effect of hydrogen and RG addition on temperature is also examined. Methane was selected as the base fuel, because its reaction scheme is relatively well known. The study is limited to NO formation, the main component of NOx.
Section snippets
Numerical model
The flame configuration studied is a traditional axisymmetric laminar counterflow diffusion flame, as shown in Fig. 1. By assuming the stagnation point flow approximation [15], the governing equations are written as where ; t is the time; x is the axial coordinate; V is the axial mass flow rate and a is the strain rate. Quantity G is a combined function of
Results and discussion
The fraction of hydrogen or RG in the fuel covers a range from 0.0 to 1.0 for completeness purpose.
Conclusions
A detailed numerical study on the effect of hydrogen/RG enrichment on flame temperature and NO formation in strained CH4/air diffusion flames has been conducted. The results indicate that when hydrogen or RG is added, the variations in both the adiabatic flame temperature and the fuel Lewis number significantly affect the flame temperature. Because of the Lewis number effect, the flame temperature increases much faster than the adiabatic equilibrium temperature, and a super-adiabatic flame
Acknowledgment
The financial support of the Government of Canada's PERD/AFTER program is gratefully acknowledged.
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