Turbulent flame speed for syngas at gas turbine relevant conditions

doi:10.1016/j.proci.2010.05.057

Proceedings of the Combustion Institute

Volume 33, Issue 2, 2011, Pages 2937-2944

https://doi.org/10.1016/j.proci.2010.05.057 Get rights and content

Abstract

Modifications of conventional natural-gas-fired burners for operation with syngas fuels using lean premixed combustion is challenging due to the different physicochemical properties of the two fuels. A key differentiating parameter is the turbulent flame velocity, S_T, commonly expressed as its ratio to the laminar flame speed, S_L. This paper reports an experimental investigation of premixed syngas combustion at gas turbine like conditions, with emphasis on the determination of S_T/S_L derived as global fuel consumption per unit time. Experiments at pressures up to 2.0 MPa, inlet temperatures and velocities up to 773 K and 150 m/s, respectively, and turbulence intensity to laminar flame speed ratios, u′/S_L, exceeding 100 are presented for the first time. Comparisons between different syngas mixtures and methane clearly show much higher S_T/S_L for the former fuel. It is shown that S_T/S_L is strongly dependent on preferential diffusive-thermal (PDT) effects, co-acting with hydrodynamic effects, even for very high u′/S_L. S_T/S_L increases with rising hydrogen content in the fuel mixture and with increasing pressure. A correlation for S_T/S_L valid for all investigated fuel mixtures, including methane, is proposed in terms of turbulence properties (turbulence intensity and integral length scale), combustion properties (laminar flame speed and laminar flame thickness) and operating conditions (pressure and inlet temperature). The correlation captures effects of preferential diffusive-thermal and hydrodynamic instabilities.

Introduction

Lean premixed combustion represents the state-of-the-art gas turbine combustion technology for high efficiency and low emissions. This technology has been optimized for natural gas combustion and has demonstrated single-digit nitrogen oxides (NO_X) and very low carbon monoxide (CO) emissions (<20 ppm).

In recent times, integrated gasification combined cycle (IGCC) power plants have attracted increased attention, predominantly due to two reasons. The main driving force is the availability of diverse fuels, like biomass, coal, tars, etc. (sources of syngas), and the increasing concern for fuel supply security and flexibility, which prompt the use of syngas for high efficiency gas turbine-based power generation. A second reason is that syngas-based power plants facilitate cost-effective carbon dioxide (CO₂) emissions reduction, when combined with fuel decarbonization.

The gasification of the aforementioned diverse fuels leads to a mixture, termed syngas, whose composition varies according to the gasification process and the original feedstock. Main fuel components in syngas are hydrogen (H₂) and CO, while diluents such as water (H₂O), nitrogen (N₂) or CO₂ can also be present in various amounts. These low-to-medium heating value gases are characterized by chemical and physical properties that can be drastically different from the ones of natural gas. High values of laminar flame speed (S_L), low density, high mass diffusivity and consequently low Lewis numbers (thermal over mass diffusivity), require different engine operating conditions for lean syngas combustion (so as to achieve a similar flame temperature with methane). The resulting effects on the turbulence-chemistry interactions portray a quite different combustion system when employing syngas fuels.

One of the most important parameters in premixed turbulent combustion is the turbulent flame speed (S_T), with its knowledge being of paramount importance for combustor design. The significance of mixture composition has been exemplified by Lipatnikov and Chomiak in their review [1], concluding that the turbulent flame speed is strongly affected by molecular transport: in fuel-lean syngas/air mixtures, transport effects are dominated by the H₂ component.

As reviewed in [2], in contrast to the extensive knowledge gained in the lasts decades for hydrocarbons (especially CH₄), very few experimental data are available in the literature for syngas fuels [3], [4], [5]. Moreover, the aforementioned data are not directly comparable due to the different adopted definitions for S_T, and the different experimental methodologies used. The lack of syngas data becomes very obvious especially when considering gas turbine relevant conditions, involving high pressures (1–3 MPa), high preheat temperatures (600–700 K) and high turbulence intensities (u′/S_L > 50).

The objective of this work is to describe turbulent combustion characteristics and in particular turbulent flame speeds for a variety of syngas-based fuel mixtures as function of equivalence ratio, pressure, inlet temperature and velocity, and turbulence intensity at gas turbine pertinent conditions.

Section snippets

Experimental setup, measuring techniques and data processing

Experiments are performed in a high pressure, optically accessible cylindrical combustion chamber (see Fig. 1) delivering a maximum thermal power of 400 kW. The chamber has a length of 320 mm with an inside diameter of 75 mm, and it is made of an air-cooled double-wall quartz tube (for more details, see [6]). The fuel/air mixture is delivered via a 25 mm diameter tube, coaxial to the reactor. The fuel is injected and mixed with electrically-preheated air 400 mm upstream of the combustion chamber.

Turbulent and chemical kinetic regimes of data set

Figure 2, positions the data of the present work in the Borghi diagram, as modified by Peters [13]. The bulk of the data fall in the thin reaction zones regime, a major part characterized by Damkholer numbers (Da) <1, with only a few data points in the broken reaction zones, characterized by Karlovitz number (Ka_δ) >1. For CH₄ no data could be acquired at Da < 1 as these flames encountered lean blow-out in this regime. In Fig. 2, l_m denotes the Zimont mixing scale [14].

Figure 3 provides OH-PLIF

Conclusions

Lean premixed turbulent flames were investigated at gas turbine relevant conditions. Extensive measurements were performed for a variety of syngas mixtures and for methane. Measurements were mainly conducted in the thin reaction zones regime. For the first time, experimental turbulent flame speed data were reported for syngas at pressures up to 2.0 MPa, preheat temperatures up to 773 K and u′/S_L > 100. In contrast to many other works, S_T in terms of global consumption has been related to the

Acknowledgments

We gratefully acknowledge financial support of this research by the Swiss Federal Office of Energy (BFE) and Alstom Power Switzerland, and thank Dr. Rolf Bombach, Dr. Wolfgang Kreutner, Dr. Alexey Denisov and Mr. Daniel Erne for supporting the experimental campaign.

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Turbulent flame speed for syngas at gas turbine relevant conditions

Abstract

Introduction

Section snippets

Experimental setup, measuring techniques and data processing

Turbulent and chemical kinetic regimes of data set

Conclusions

Acknowledgments

Prog. Energy Combust. Sci.

Proc. Combust. Inst.

Proc. Combust. Inst.

Prog. Energy Combust. Sci.

Prog. Energy Combust. Sci.

Combust. Flame

Proc. Combust. Inst.

Proc. Combust. Inst.

Prog. Energy Combust. Sci.

Proc. Combust. Inst.

Combust. Flame

Proc. Combust. Inst.

Synthesis Gas Combustion – Fundamentals and Applications