Elsevier

Combustion and Flame

Volume 154, Issue 3, August 2008, Pages 601-623
Combustion and Flame

Experimental and numerical investigation of low-pressure laminar premixed synthetic natural gas/O2/N2 and natural gas/H2/O2/N2 flames

https://doi.org/10.1016/j.combustflame.2008.04.018Get rights and content

Abstract

The oxidation of laminar premixed natural gas flames has been studied experimentally and computationally with variable mole fractions of hydrogen (0, 20, and 60%) present in the fuel mixture. All flames were operated at low pressure (0.079 atm) and at variable overall equivalence ratios (0.74<ϕ<1.0) with constant cold gas velocity. At the same global equivalence ratio, there is no significant effect of the replacement of natural gas by 20% of H2. The small differences recorded for the intermediate species and combustion products are directly due to the decrease of the amount of initial carbon. However, in 60% H2 flame, the reduction of hydrocarbon species is due both to kinetic effects and to the decrease of initial carbon mole fraction. The investigation of natural gas and natural gas/hydrogen flames at similar C/O enabled identification of the real effects of hydrogen. It was shown that the presence of hydrogen under lean conditions activated the H-abstraction reactions with H atoms rather than OH and O, as is customary in rich flames of neat hydrocarbons. It was also demonstrated that the presence of H2 favors CO formation.

Introduction

Vehicles fueled by natural gas/hydrogen blends are a first step toward a hydrogen economy. These natural gas/hydrogen blends have the potential for environmental improvement with immediate reduction in emissions. Hydrogen is a clean fuel with no carbon emissions; the combustion of hydrogen produces only water and a reduced amount of nitrogen oxides. Conversely, combustion products from fossil fuels, such as CO, CO2, nitrogen oxides, or other air pollutants, cause health and environmental problems. Hydrogen will help reduce CO2 emissions as soon as it can be produced in a clean way either from fossil fuels, in combination with processes involving CO2 capture and storage technologies, or from renewable energy.

Even if hydrogen is seen as a key future energy carrier, it has several drawbacks. First, new technologies for production, storage, and transportation will have to be developed in order that we may begin to consider using hydrogen as an everyday fuel. Second, the flammable range of hydrogen is very wide (4–75%), and will require several safety standards to be redefined. Mixtures of natural gas and hydrogen can be part of a transitional pathway toward a hydrogen economy. These blends will allow reductions in emissions during which science can continue to develop technologies.

A limited number of publications have been written about natural gas in contrast to those on methane, the principal component of natural gas. Nevertheless, natural gas combustion cannot be fully represented by methane combustion. Natural gas combustion has been studied extensively for 10 years in our laboratory in partnership with “Gaz de France.” Previous studies have effectively demonstrated that higher hydrocarbons, even at low concentrations, significantly enhanced methane reactivity, leading to smaller ignition delay times and faster burning velocities [1]. Another study concerning the oxidation of synthetic natural gas has identified a methane/ethane/propane mixture as the best model for natural gas oxidation [1], [2].

Experimental data on natural gas combustion have been acquired to develop and optimize a kinetic mechanism named GDF-Kin. The second release, GDF-Kin 2 [1], has been validated against a large variety of experiments: ignition delays, species mole fraction profiles in premixed laminar flames, and jet-stirred reactors, burning velocities, and shock tubes. This C1–C6 mechanism includes 671 reversible reactions and 99 species. The last release, GDF-Kin 3.0, includes NOx chemistry, 121 species, and 874 reactions [3]. The GDF-Kin 3.0 mechanism was also validated using CH and NO mole fraction profiles measured in laminar premixed flames representative of natural gas combustion [4].

Finally, the latest version, used in the present work, has been validated against a large range of combustion conditions including detailed kinetic data, obtained both in jet-stirred reactors and flames, and global data in the form of ignition delays and laminar burning velocities. Regarding flames, the mechanism was validated solely at low pressure and equivalence ratios near stoichiometry (0.5ϕ1.3).

The next step is to capture the effect of additives on natural gas/air mixtures via our mechanism predictions. In this study we focus on hydrogen addition.

The influence of hydrogen addition on methane oxidation has been studied under a wide range of experimental conditions by measuring ignition delay behind reflected shock waves [5], [6], laminar and turbulent burning velocities [7], [8], [9], [10], [11], [12], concentration profiles of stable species in jet-stirred reactors [13], [14], pollutant emissions, and performance characteristics in spark-ignited engines [15], [16]. Results show that hydrogen can improve combustion stability [17], reduce pollutant emissions, and extend flammability limits [18]. Furthermore, hydrogen/methane combustion results in high power/energy output and resolves some of the safety and storage issues associated with hydrogen.

Some studies have investigated the influence of hydrogen blending on CO emissions in various combustion applications. Numerical and experimental studies on lean premixed swirl-stabilized flames found that a significant reduction in CO emissions is obtained by hydrogen addition [17], [19]. The authors postulated that this reduction with hydrogen addition is a direct result of a higher OH radical concentration, which increases the rate of the CO+OHCO2+H reaction, which is the loss route of CO. Other studies postulated that the reduction of CO is due to the reduction of carbon content [20]. Soot precursors, such as acetylene, were found to be reduced considerably in modeled counterflow nonpremixed and partially premixed flames [20].

The main objective of this work is to investigate the effect of using natural gas/hydrogen blends on pollutant emissions and soot precursors such as acetylene. To do this, we provide detailed and accurate experimental data on mole fractions of stable species in laminar premixed CH4/C2H6/C3H8/O2/N2 and CH4/C2H6/C3H8/H2/O2/N2 flames at low pressure. To our knowledge, there has been no previous experimental investigation of natural gas/hydrogen oxidation under premixed flame conditions. This configuration allows us to examine the effects of fuel blending on both flame structure and emissions characteristics by stabilizing flames either at the same global equivalence ratio or at the same carbon and oxygen content. These results have been compared to the predictions obtained with our previously published mechanism of natural gas combustion (GDF-Kin 3.0). Furthermore, modeling has been used to identify the dominant pathways associated with the various pollutant species for different fuel blends.

Section snippets

Experimental setup

The chemical structure of laminar premixed CH4/C2H6/C3H8/H2/O2/N2 flames has been investigated. The oxidation of the synthetic natural gas methane–ethane–propane (NG) was found to be representative of natural gas combustion. The initial conditions of all flames studied in the present work are shown in Table 1. The flow rates of the natural gas blend (2.00±0.04% C3H8 and 9.00±0.18% C2H6 in methane), hydrogen (99.999% pure), nitrogen and oxygen (99.996% pure) were measured and regulated by

Modeling

We used PREMIX code for the flames modeling [21]. Our detailed kinetic reaction, GDF-Kin 3.0 [3], optimized for natural gas oxidation, is used to simulate the present experimental results. Note that GDF-Kin 3.0 has been validated over a wide range of conditions. It is able to reproduce correctly global data (burning velocities, ignition delays) and species mole fraction profiles obtained in different configurations such as premixed laminar flames, jet-stirred reactors, and shock tubes [1], [2],

Results and discussion

Mole fraction profiles of reactants (CH4, C2H6, C3H8, H2, O2), final products (CO2, CO, H2O), and stable intermediate species (C2H2, C2H4, C3H6) have been measured. Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6 exhibit experimental and computed mole fraction profiles for the species obtained in natural gas (NG)/O2/N2, (80%NG/ 20%H2)/O2/N2, and (40%NG/60%H2)/O2/N2 flames for an overall stoichiometric composition. The overall equivalence ratio is calculated byϕ=2XCH4XO2+72XC2H6XO2+5XC3H8XO2+12XH2

Conclusions

A recently developed kinetic model for natural gas combustion was used to identify major reaction pathways in low-pressure laminar premixed CH4/C2H6/C3H8/O2/N2 flames with and without the addition of hydrogen (0–60% in the fuel) operating at global stoichiometric composition. Experimental mole fraction profiles for stable species were measured by means of gas chromatographic analysis (GC-TCD-FID) and FTIR. Good agreement between model predictions and measured mole fraction profiles was obtained

Acknowledgments

This work is supported by Gaz de France, the Nord/Pas de Calais Region, and European Funds (FEDER).

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