Control of combustion instabilities by local injection of hydrogen

doi:10.1016/j.proci.2006.07.085

Proceedings of the Combustion Institute

Volume 31, Issue 2, January 2007, Pages 3207-3214

https://doi.org/10.1016/j.proci.2006.07.085 Get rights and content

Abstract

New generation combustion chambers are designed to reduce pollutant emissions and combustion instabilities. One consequence of these instabilities can be large amplitude pressure oscillations, which are often coupled with heat release oscillations. As this coupling leads to performance degradations or acoustic noise, various instability control techniques have been developed. The purpose of the present paper is to propose an original instability control technique acting on thermo-acoustic combustion instabilities appearing in an experimental lean premixed burner. The actuation system consists in injecting an hydrogen jet in a premixed hydrocarbon flame. Several diagnostic techniques have been used to characterize the burner behavior and to evaluate the effects of hydrogen injection on both combustion instabilities and pollutant emissions. It is shown that hydrogen injection induces a significant decrease of the amplitude of pressure oscillations while the heat release oscillations remain at the same level. To increase the understanding of the phenomena observed, phase locked images of OH* and CH* emission fluctuations have been obtained. These images show that the hydrogen jet breaks the coupling between acoustic and heat release oscillations. The transient period following the hydrogen injection has also been studied, using a solenoid valve to command the hydrogen injection. This study shows that the hydrogen jet acts quickly on pressure oscillations (<50 ms) without inducing any oscillatory transitory regime in the combustion chamber. Lastly, it has been shown that the hydrogen injection has no negative effects on NO emission but induces a strong increase of CO emission. However, as the hydrogen injection modifies the properties of the gaseous mixture that fed the burner, a new emission index was defined, to compare the different cases studied in terms of pollutant emission.

Introduction

As environmental standards become more and more stringent, many studies have been carried out to develop low pollutant emission burners. One interesting solution to reduce pollutant emission is to enhance air/fuel mixing [1]. This can be achieved by using lean premixed burners where the local equivalence ratio is reduced. One consequence is a flame temperature decrease inducing a decrease of nitric oxides (NO_x) formation.

However, combustion instabilities due to a coupling between pressure and heat release oscillations are often observed in these devices [2], [3]. Instabilities remain a major drawback of these burners as they lead to performance degradation or unacceptable noise levels. Several instability control techniques have already been developed and complete reviews can be found in [4], [5]. Passive control techniques consist in a clever modification of the combustion device to change the resonance of the system [6]. In recent years, much attention has been focused on active control techniques that consist in actively injecting perturbations into the combustor using an actuator to decouple the physical processes responsible for the combustion oscillations. The reduction of combustion instabilities can be achieved by using open or closed loop control strategies. Various actuators, from loudspeakers to pulsed air [7] and fuel jets [8] or more recently plasma actuation, have already been tested. The purpose of the present work is to evaluate an original instability control technique for lean premixed burners, which consists in a local injection of a more energetic fuel in an unstable lean premixed hydrocarbon/air flame.

Previous studies [9], [10], [11] have shown that one possibility to improve the overall premixed hydrocarbon flame efficiency and stability is to add a high heating value fuel, like hydrogen, to the main fuel. These studies have indicated that the lean stability limit of a burner can be lowered by hydrogen addition to the fuel. A study concerning a mixture of natural gas, hydrogen, and air [10] has also shown that a significant hydrogen addition to the fuel has no negative effect on pollutant emissions as the lean stability of the natural gas limit is approached.

Another study concerning pollutant emissions in a non-premixed burner fed with hydrogen and hydrocarbon [12] was carried out to analyze the effect of hydrocarbon addition to hydrogen regarding carbon monoxide (CO) and nitric monoxide (NO) emissions. It was shown that the CO emission increased while the NO emission decreased as the content of hydrocarbon in the hydrogen increased.

A large number of studies have been carried out on premixed [9], [10], [11] and non-premixed [12] hydrogen/hydrocarbons combustion (known as “blended fuel” combustion). However, the number of studies related to non-premixed hydrogen injection in lean premixed hydrocarbon flames used as actuation to control combustion instabilities is much reduced. This is why the present work was undertaken to evaluate this possibility.

As pure hydrogen is added, the properties of the gaseous mixture are modified and the composition of burnt gases is expected to change. The second objective of this study is then to quantify the influence of hydrogen injection on NO and CO emissions.

The experimental burner and diagnostics are first described. Then experimental results are presented. The acoustic behavior of the burner is studied for a lean premixed propane/air flame. Then we focus on hydrogen jet effects using acoustic measurements and free radical emission imaging. Using a solenoid valve, the response delay of the actuation is estimated. Simultaneously, CO and NO emissions in the pure premixed propane/air flame and in the propane/hydrogen/air flame are measured and compared. Lastly, an emission index that enables the comparison of burners using different fuel blends is proposed.

Section snippets

Experimental setup

Figure 1 presents a schematic view of the experimental setup. It is composed of an annular duct (0.27 m long) supplied with lean propane/air mixture. No swirling motion is induced in the premixed flow. The propane/air mixture is injected into a cylindrical combustion chamber (0.15 m in diameter and 0.5 m in length). This length was chosen to enhance the flame coupling with longitudinal acoustic modes. The combustion chamber is made of fused silica to enable optical diagnostics in the flame. The

Analysis of propane–air flame combustion instabilities

Pressure oscillations and OH* emissions are simultaneously recorded for the propane/air case. OH* emissions can be divided into two parts, a mean part and a fluctuating part, which is proportional to the heat release oscillations, q′. Only this second part is considered and analyzed here. Results concerning OH* emissions are given in arbitrary units (AU), since chemiluminescence is a relative signal. This first recording (not shown here) indicates that pressure oscillations are around 200 Pa,

Effects of hydrogen actuation on combustion instabilities

To quantify the effects of hydrogen injection on combustion instabilities, pressure oscillations and OH* emission have been simultaneously recorded for both cases. Figure 4 presents a microphone recording during 150 ms. The recording begins when the burner is supplied with lean propane/air mixture only (left part of the figure) and stops when the burner is continuously fed with propane, hydrogen and air (right part). Hydrogen is supplied through the solenoid valve at t = 0 s. These measurements

Pollutant emission measurements

Figure 8 presents radial concentration profiles (dry gases) of CO₂ (diamonds) and O₂ (triangles) at the outlet of the burner fed with the propane-air mixture (dashed line) or with propane/hydrogen/air mixture (solid line). The radial position of the probe is noted r. For the premixed propane/air regime (respectively, propane/hydrogen/air), the average measured value of CO₂ and O₂ concentrations are respectively, 11.8% and 2.48% (respectively 11.5% and 1.1%). Values analytically computed for

Conclusion

The use of hydrogen injection as a combustion control technique was analyzed in the present paper. The burner studied here exhibits strong combustion instabilities due to a thermo-acoustic coupling while fed with a lean propane–air mixture. The unstable mode was identified as the quarter wave mode of the combustion chamber.

It is shown that swirling hydrogen injection acts on the coupling of p′ and q′ so that a reduction by a factor of 2 of the amplitude of p′ was measured and that this control

Acknowledgments

Authors thank ADEME for the grant of M. de La Cruz Garcia. V. Faivre and IMFT are gratefully acknowledged for the help in developing the actuator system.

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Control of combustion instabilities by local injection of hydrogen

Abstract

Introduction

Section snippets

Experimental setup

Analysis of propane–air flame combustion instabilities

Effects of hydrogen actuation on combustion instabilities

Pollutant emission measurements

Conclusion

Acknowledgments

Prog. Energy Combust. Sci.

Proc. Combust. Inst.

Prog. Energy Combust. Sci.

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