Laminar burning velocities and flame stability analysis of H2/CO/air mixtures with dilution of N2 and CO2

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Abstract

Experimental measurement of the laminar burning velocities of H2/CO/air mixtures and equimolar H2/CO mixtures diluted with N2 and CO2 up to 60% and 20% by volume, respectively, were conducted at different equivalence ratios and conditions near to the sea level, 0.95 atm and 303 ± 2 K. Flames were generated using contoured slot-type nozzle burners and Schlieren images were used to determine the laminar burning velocity with the angle method. Numerical calculations were also conducted using the most recent detailed reaction mechanisms for comparison with the present experimental results. Additionally, a study was conducted to analyze the flame stability phenomenology that was found in the present experiments. The increase in the N2 and CO2 dilution fractions considerably reduced the laminar burning velocity due to the decrease in heat release and increase in heat capacity. At the same dilution fractions this effect was higher for the case of CO2 due to its higher heat capacity and dissociation effects during combustion. Flame instabilities were observed at lean conditions. While the presence of CO in the fuel mixture tends to stabilize the flame, H2 has a destabilizing effect which is the most dominant. A higher N2 and CO2 dilution fraction increased the range of equivalence ratios where unstable flames were obtained due to the increase in the thermal-diffusive instabilities.

Introduction

An increase in the world energy demand and environmental concerns about pollutant emissions have generated the search for more convenient and reliable energy sources. Synthetic gas (syngas) is considered as one of the most promising alternative fuels that will play an important role in the diversification of the energetic sources since it can be produced from the gasification of coal, whose reserves are abundant worldwide, and multiple solid feedstocks such as organic waste and biomass [1], [2], [3], [4], [5]. Moreover, carbon dioxide and other pollutant emissions are considerably reduced compared to conventional fuels, due to the presence of hydrogen in the syngas mixture and the use of cleanup methods after the gasification process [1], [4], [6], [7], [8].

Syngas composition varies considerably depending on the gasification process and the feedstock type. Hydrogen and carbon monoxide are the main components of syngas; the volumetric H2/CO ratio usually varies from 0.33 to 40. The presence of diluent gases, such as nitrogen, carbon dioxide and water, is significant and represents from 4 to 60% of the final composition [1], [4], [5], [9]. Variability in syngas composition significantly modifies the combustion behavior in terms of pollutant emissions, chemical kinetics, and flame structure and instabilities; especially due to the presence of high amounts of hydrogen. Therefore, it is necessary to design flexible combustion systems capable of stable operation when burning a broad range of syngas compositions, and at the same time keeping high efficiency and low pollutant emissions. Designing these systems requires the determination of the influence of variations in syngas compositions in the most important combustion properties, such as flammability limits, ignition delay, adiabatic flame temperature and laminar burning velocity [1], [3], [4], [5], [6], [10], [11], [12], [13].

One of the most important characteristic parameters of a fuel mixture is the laminar burning velocity, SL. Information on SL is fundamental for the analysis of the combustion phenomena such as the structure and stability of premixed flames, flashback, blowoff and extinction; turbulent premixed combustion; and the validation of reaction mechanisms in the presence of diffusive transport at high temperatures. Previous studies have reported a high sensitivity of SL to the presence of hydrogen in the fuel mixture, because it has the highest laminar burning velocity when burned with air compared to conventional gaseous fuels [14], [15], [16], [17], [18], [19]. Besides, addition of diluents such as nitrogen and carbon dioxide to the fuel mixture decreases SL considerably due to the inhibiting nature of these gases [16], [20], [21], [22], [23]. Consequently, significant variations are expected in the SL of syngas mixtures due to the multiple compositions they can have. Therefore, it is necessary to assess experimentally the behavior of SL over a wide range of H2/CO ratios and diluent additions.

SL is defined as the flame propagating velocity of one-dimensional flat flame. However, such kind of flame is an idealization since actual flames are affected by heat losses, curvature and stretch; then other types of flames have to be used for measurements. Burning velocities of syngas–air premixed flames have been studied in the past using two measurement methods, the spherical bomb method and the burner stabilized flame method [24], [25], [26], [27], [28], [29], [30]. In the former, problems related with pressure change during flame propagation, influence of the ignition energy and heat loss in the early state of flame propagation have been corrected in recent measurements [24], [25], [26], [27], [28], [31]. Nevertheless, among researchers there has not been consensus in the flame radii range considered for extrapolations to non-stretch conditions. On the other hand, in the burner stabilized flame method SL can be determined either by the flame area or by the flame angle, but only average values can be obtained since local burning velocities vary along the flame front due to effects of flame stretch and curvature at the flame tip and heat losses near the burner walls. However, a technique using particle tracking velocimetry (PTV) combined with Schlieren photography determines SL using the mean value of local burning velocities along the flame front in the region of non-stretch [32], [33], [34], [35]. Recently, Pareja et al. [35] used the angle method and generated the flames with a contoured slot-type nozzle to reduce stretch and curvature effects, which yielded experimental results in very close agreement with the PTV technique for the case of H2–air mixtures. Therefore, this experimental methodology was implemented to measure SL in the present study.

The equimolar mixture of H2 and CO has been the most common syngas composition studied to determine SL at atmospheric conditions, and most of the researchers have used the spherical bomb method [24], [25], [26], [27], [28]. An exception is the study by Natarajan et al. [29], [30] where the burner stabilized flame method was used to determine SL through the flame area; their results were in close agreement with those of the bomb method. However, Natarajan et al. [29], [30] only reported SL data for equivalence ratios (φ) lower than 1.0. Thus in the present study measurements were conducted up to φ = 4.3 at 0.95 atm and 303 ± 2 K. The effect of varying the H2/CO ratio in SL has been studied using the spherical bomb method at atmospheric conditions; a high sensitivity to the fraction of H2 and chemical kinetics dominated by H2 were reported [24], [25], [26], [27], [28], [29], [30]. In addition, concerning flame stability, Hassan et al. [28] and Brown et al. [36] reported that H2 was the dominant specie that causes flame instabilities. In the present study, SL corresponding to the mixture 25%H2–75%CO was measured for a wide range of φ and a flame stability analysis of H2/CO mixtures is presented.

Prathap et al. [25] used the spherical bomb method to study the effect of N2 addition from 0 to 60% in the SL of an equimolar mixture of H2 and CO at atmospheric conditions. A decrease in SL and an increase in flame stretch and preferential diffusion effects at lean conditions were found when the dilution was increased. To the best of the authors’ knowledge the effect of N2 dilution in syngas mixtures has not been studied using the burner stabilized flame method. Therefore, measurements at atmospheric conditions, 0.95 atm and 303 ± 2 K, of SL for 20, 40 and 60% of N2 dilution by volume in an equimolar mixture of H2 and CO were conducted in the present study at lean and rich equivalence ratios. In addition, results on flame stability are presented and discussed. Concerning CO2 addition effects, the study by Natarajan et al. [30] is the only one available in literature. The burner stabilized flame method was used to measure SL at 10 and 20% of dilution by volume in an equimolar mixture of H2 and CO at atmospheric conditions; a significant reduction in SL was reported. However, this data is only available for lean conditions. In the present study the same syngas compositions were considered but measurements were conducted for rich conditions as well. The effects of dilution with both N2 and CO2 were studied up to dilution fractions that simulated real syngas compositions. Finally, measurements of SL for all the syngas compositions previously mentioned are compared with numerical calculations using the three most recent reaction mechanisms developed for H2–CO combustion [37], [38], [39].

Section snippets

Experimental setup

Fig. 1 shows a schematic diagram of the experimental setup implemented. Flames were generated using two burners with contoured slot-type nozzles; 5 × 14 mm and 10 × 30 mm for the measurement of high and low values of SL, respectively. These burners allowed to keep laminar Reynolds numbers at every equivalence ratio studied, as well as to reduce the effects of flame stretch along the flame front and curvature at the flame tip. Additionally, a refrigeration circuit with water was implemented at

Numerical methodology

Numerical calculations of SL were conducted using the one-dimensional premixed flame code PREMIX of the CHEMKIN-PRO package. For comparison, present simulations considered three detailed reaction mechanisms, the mechanism of Frassoldati et al. [37], Davis et al. [38], and the H2/CO/O2 reactions of Li et al. [39]. For an accurate prediction of SL, recommendations of Bongers and De Goey [41] were followed; transport properties were evaluated using the multi component diffusion model, and thermal

H2/CO laminar burning velocities

Fig. 4 shows the experimental and numerical results of the laminar burning velocity of the 50%H2–50%CO and 25%H2–75%CO mixtures with air at different equivalence ratios, 0.7–4.3, along with data from previous studies at standard pressure and room temperature. For the former mixture, at lean conditions the experimental data obtained is in good agreement with those of previous studies, as well as at very rich conditions, above φ = 3.0. However, there are significant differences at equivalence

Conclusions

Measurements of the laminar burning velocities with the angle method of H2/CO mixtures, and equimolar H2/CO mixtures diluted with N2 and CO2 were conducted. Flames were generated with slot-type nozzle burners at conditions near to the sea level, 0.95 atm and 303 ± 2 K. Numerical calculations were also conducted using existing detailed reaction mechanisms for comparison with the present experimental results. Additionally, a flame stability analysis was carried out. The following results were

Acknowledgments

The authors acknowledge the support of University of Antioquia through the “Sostenibilidad 2009–2010” program and COLCIENCIAS through the “Jóvenes Investigadores 2008” program under contract number 8703-139-2009. The authors also acknowledge the valuable support of Mr. Julian D. Carvajal during the experimental measurements.

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