Flashback Mechanisms in Lean Premixed Gas Turbine Combustion

Ali Cemal Benim, Khawar Jamil Syed

• Publisher: Academic Press
• Year: 2014
• Print ISBN:
  9780128007556
• eBook ISBN:
  9780128008263
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Blending fuels with hydrogen offers the potential to reduce NOx and CO2 emissions in gas turbines, but doing so introduces potential new problems such as flashback. Flashback can lead to thermal overload and destruction of hardware in the turbine engine, with potentially expensive consequences. The little research on flashback that is available is fragmented. Flashback Mechanisms in Lean Premixed Gas Turbine Combustion by Ali Cemal Benim will address not only the overall issue of the flashback phenomenon, but also the issue of fragmented and incomplete research. - Presents a coherent review of flame flashback (a classic problem in premixed combustion) and its connection with the growing trend of popularity of more-efficient hydrogen-blend fuels - Begins with a brief review of industrial gas turbine combustion technology - Covers current environmental and economic motivations for replacing natural gas with hydrogen-blend fuels

Chapter 1

Introduction

Abstract

This introductory chapter emphasizes the necessity for lean premixed combustion within land-based gas turbines in order to meet the strict emission regulations that are in place over much of the world. One of the key challenges faced in the design of lean premixed systems is that of flame “flashback” into the burner. Although in principle, flashback margins may be extended by increasing burner velocities, this increases the burner pressure drop, which is limited by both engine performance and combustor component life and operational constraints. Additionally, there is a trend toward increasing firing temperatures, as this leads to higher plant efficiency. The risk of flashback therefore increases, as there is greater heat feedback from the combustor into the burner, and, typically, higher pressure drop is required for the combustor cooling, leaving less pressure drop available for the burner. Furthermore, there is an increasing interest for broader fuel flexibility, including the introduction of hydrogen and/or hydrogen-containing fuel blends. The flashback risk therefore further increases, due to the high reactivity and flame speeds associated with such fuels.

Keywords

Flashback

gas turbine combustors

hydrogen-containing fuels

lean premixed combustion

ultralow NO_X emissions

In industrial gas turbines, to achieve low pollutant emissions, a variety of preformation and postformation control technologies are employed. Among the former, “lean premixed combustion” is now the standard for achieving the low NO_X emission targets that are in place for natural gas applications (Eroglu et al., 2001; Krebs et al., 2010; Sattelmayer, 2010).

The control principle utilized by lean premixed technology is the strong temperature dependence of the NO formation rate. The low flame temperatures attained under lean conditions effectively limit NO formation. For nonpremixed combustion, however, even if the fuel and air are fed “globally” in a lean ratio into the combustion chamber, the whole range of equivalence ratios occur “locally” within the combustion zone, leading to reactions occurring around stoichiometric conditions, and, thus, to locally high temperatures and high NO formation rates. Only after complete and uniform mixing of fuel with air, locally lean conditions can be achieved throughout the combustion zone, and the potential of lean combustion can be fully exploited. On the other hand, to accomplish a satisfactory degree of mixing within the limited space and residence time available in the premixing section of a gas turbine burner is not straightforward. Difficulties arise because of conflicting requirements from different design criteria such as mixing, pressure drop, robustness, reliability, and flashback safety, the latter being the focus of the present treatise.

Flashback is the undesirable penetration of flame into the burner, resulting in combustion within the premixing section. Indeed, a tendency toward flashback is an intrinsic feature of all premixed combustion systems, including gas turbine combustors (Lefebvre, 1983), as, in such systems, a burnable mixture is always available upstream the combustor. Flashback can take place, for example, when the local flame speed exceeds the approach flow velocity, in all regions, where fuel/air mixture exists in flammable proportions.

Burners for land-based gas turbines are designed to predominantly accept natural gas as fuel. There is also a requirement for the same hardware to accept liquid fuel, typically diesel oil. Depending on the burner technology, low NO_x emissions may be achieved without water injection, where the liquid burns in a premix mode or NO_x may be controlled by injecting water to reduce flame zone temperatures.

However, interest in utilizing other energy resources, due to concerns about the environment and energy security, has stimulated the investigation of alternative fuels, such as coal-derived syngas, biomass, landfill gas, or process gas (Lieuwen et al., 2008a) possibly in combination with technologies such as integrated gasification combined cycle (IGCC). Such fuels can contain large amounts of hydrogen, which is significantly more reactive than natural gas, and therefore have greater flashback propensity. Hydrogen has a laminar flame speed, which is about four times that of typical natural gases. Its turbulent flame speed and its resistance to hydrodynamic strain are also greater. Consequently, the risk of flashback grows and its abatement becomes a significant challenge.

Even if a burner is designed to operate safely for a design point, flashback can still occur if the flow field of the burner temporarily experiences deviations from the design, which can be caused by some kind of disturbance, for example, rapid deloading of the gas turbine. This transience may lead to momentarily very low flow velocities and therefore gives the opportunity for the flame to enter deep into the burner and ignite fuel within the premixing zone, for example, in the wakes of the fuel injection jets. In these wakes, the fuel concentration exhibits values around stoichiometric, implying much higher laminar flame speeds compared with the well-mixed downstream region. The comparably high local turbulence intensities may additionally increase the local turbulent flame speed. These features favor an anchoring of the flame after such a transience has occurred. If the burner is robust, when this transience is over, the flame should be extinguished in these zones and move into the combustor. If the burner is not robust, when the transience is over, the flame does not extinguish and remains in the burner. In this case NO_X emissions will be high and burner damage may occur. This propensity for flame anchoring in low-velocity regions in the burner, after a temporary flashback, is usually referred to as the “flameholding” behavior of the burner.

Thus, flashback can be viewed from two perspectives, both of which must be addressed to ensure a flashback-safe design. One is to ensure that, under steady conditions, the flame speed nowhere exceeds flow velocities. The critical phenomenon in this case is flame propagation. The other assumes that the flame can transiently move into the burner, and it must be ensured that the flame must extinguish in the burner, once the transience is over. In this case the critical phenomenon is extinction. In the present essay, both perspectives are addressed.

Chapter 2

Concepts Related to Combustion and Flow in Premix Burners

Abstract

This chapter provides an overview of some fundamental concepts relating to the combustion and aerodynamics in premix gas turbine burners. First, the laminar flame speed is defined and the one-dimensional flame structure following the thermal theory is outlined. The role of species diffusion is discussed and the notion of “preferential diffusion” is introduced. The effect of flame stretch and flame curvature on laminar flame speed is outlined. Then, the structure of turbulent premixed flames is briefly discussed, identifying combustion regimes with different characteristics. Finally, the aerodynamics of swirl premix burners, where vortex breakdown is utilized for flame stabilization, is described.

Keywords

Flame stretch and curvature

laminar flame speed

preferential diffusion

swirl burner

turbulent flame speed

turbulent premixed combustion

vortex breakdown

2.1. Laminar premixed flames

2.1.1. The Laminar Flame Speed

The laminar flame speed is the speed at which a flame will propagate through a quiescent, homogeneous mixture of unburned reactants, under adiabatic conditions (Turns, 2012). The laminar flame speed for a planar, unstretched flame ($S_{\text{L}}^0$

) shall be dealt with first. Consider a one-dimensional, planar flame front within the laminar flow of a homogenous mixture, where the unburned reactants approach the flame front with the constant velocity u. Steady-state conditions, i.e., a spatially stationary flame front is obtained, when $u=S_{\text{L}}^0$

. In the following, as is common practice, a number of assumptions are made. It is assumed that the Mach number is low, and mechanical energies, the viscous dissipation, as well as the pressure difference across the flame front are negligible. Furthermore, it is assumed that the specific heat capacity, the thermal conductivity, and the diffusivity take constant values across the flame front (values corresponding to the unburned mixture are taken), the flame is thin (high activation energy), the Lewis number is unity (Le = 1), and the chemical kinetics is governed...

Table of contents

• Cover
• Title page
• Table of Contents
• Copyright
• Author bios
• Preface
• Chapter 1: Introduction
• Chapter 2: Concepts Related to Combustion and Flow in Premix Burners
• Chapter 3: Properties of Hydrogen-Containing Fuels
• Chapter 4: An Overview of Flashback Mechanisms
• Chapter 5: Flashback by Autoignition
• Chapter 6: Flashback Due to Combustion Instabilities
• Chapter 7: Flashback Due to Turbulent Flame Propagation in the Core Flow
• Chapter 8: Flashback Due to Flame Propagation in Boundary Layers
• Chapter 9: Combustion-Induced Vortex Breakdown–Driven Flashback
• Chapter 10: Flameholding by Fuel Injection Jets
• References
• Nomenclature