Elsevier

Combustion and Flame

Volume 156, Issue 2, February 2009, Pages 477-483
Combustion and Flame

A numerical study on the effect of hydrogen/reformate gas addition on flame temperature and NO formation in strained methane/air diffusion flames

https://doi.org/10.1016/j.combustflame.2008.07.009Get rights and content

Abstract

This paper investigates the effects of hydrogen/reformate gas addition on flame temperature and NO formation in strained methane/air diffusion flames by numerical simulation. The results reveal that flame temperature changes due to the combined effects of adiabatic temperature, fuel Lewis number and radiation heat loss, when hydrogen/reformate gas is added to the fuel of a methane/air diffusion flame. The effect of Lewis number causes the flame temperature to increase much faster than the corresponding adiabatic equilibrium temperature when hydrogen is added, and results in a qualitatively different variation from the adiabatic equilibrium temperature as reformate gas is added. At some conditions, the addition of hydrogen results in a super-adiabatic flame temperature. The addition of hydrogen/reformate gas causes NO formation to change because of the variations in flame temperature, structure and NO formation mechanism, and the effect becomes more significant with increasing strain rate. The addition of a small amount of hydrogen or reformate gas has little effect on NO formation at low strain rates, and results in an increase in NO formation at moderate or high strain rates. However, the addition of a large amount of hydrogen increases NO formation at all strain rates, except near pure hydrogen condition. Conversely, the addition of a large amount of reformate gas results in a reduction in NO formation.

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 asdρdt+dVdx=2ρG,L(G)=ddx(μdGdx)ρG2+ρ(dadt+a2),CpL(T)=ddx(λdTdx)k=1KKρYkVkCpkdTdxk=1KKhkωkMk+qr,L(Yk)=ddx(ρYkVk)+ωkMk, where L(ϕ)=dϕ/dt+V(dϕ/dx); 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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