Syngas Spark Ignition Behavior at Simulated Gas Turbine Startup

Combustion Science and Technology

ISSN: 0010-2202 (Print) 1563-521X (Online) Journal homepage: http://www.tandfonline.com/loi/gcst20

Syngas Spark Ignition Behavior at Simulated Gas Turbine Startup Conditions Suhui Li, Xiaoyu Zhang, Di Zhong, Fanglong Weng & Min Zhu To cite this article: Suhui Li, Xiaoyu Zhang, Di Zhong, Fanglong Weng & Min Zhu (2014) Syngas Spark Ignition Behavior at Simulated Gas Turbine Startup Conditions, Combustion Science and Technology, 186:8, 1005-1024, DOI: 10.1080/00102202.2014.900058 To link to this article: http://dx.doi.org/10.1080/00102202.2014.900058

Accepted online: 28 Mar 2014.

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Date: 19 October 2015, At: 02:03

Combust. Sci. Technol., 186: 1005–1024, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 0010-2202 print / 1563-521X online DOI: 10.1080/00102202.2014.900058

SYNGAS SPARK IGNITION BEHAVIOR AT SIMULATED GAS TURBINE STARTUP CONDITIONS Suhui Li, Xiaoyu Zhang, Di Zhong, Fanglong Weng, and Min Zhu

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Key Laboratory for Thermal Science and Power Engineering, Department of Thermal Engineering, Tsinghua University, Beijing, China Syngas spark ignition experiments were conducted using a single nozzle model combustor under typical gas turbine startup conditions. Ignition performance was characterized by measuring lean ignition limits. Effects of fuel composition (H2 , CO2 , and N2 ) and inlet air flow velocity on ignition performance were investigated. Results show that H2 content and air flow velocity both have a strong impact on ignition performance. A small amount of H2 can significantly improve the ignition performance of syngas. Data analysis indicates that chemical time, rather than ignition delay time, is more suited for characterizing the reactivity of syngas influencing spark ignition performance. Effects of fuel composition and air flow velocity can be captured by a Damköhler number–flame temperature correlation. A 3step ignition theory is proposed to explain the correlation. Compared to natural gas, syngas has a better ignition performance due to the presence of H2 . Considering that gas turbines operating on syngas still have to start with natural gas or diesel, this study demonstrates that syngas is a promising ignition fuel for gas turbine startup. Keywords: Gas turbine startup; Hydrogen; Lean ignition limit; Spark ignition; Syngas

INTRODUCTION The interest in fuel flexibility of gas turbines is increasing due to concerns over global warming, environmental degradation, reduced availability of conventional fuels, and national energy independence. Syngas, or synthesis gas, is an attractive alternative to natural gas as gas turbines fuel because of its abundant availability and clean combustion characteristics (Lieuwen et al., 2009). Indeed, several IGCC (integrated gasification combined cycle) power plants have been successfully demonstrated operating on syngas. Syngas can be produced from a wide range of feedstock (including coal, biomass, petroleum coke, and landfill waste) and processes (such as gasification, coking, and fermentation). The large variation in feedstock and production processes inherently induces large variation in syngas composition (Higman and van der Burgt, 2008). Except for H2 and CO, small hydrocarbons (CH4 and C2 H6 ) and inert diluents (N2 and CO2 ) are common Received 25 October 2013; revised 11 January 2014; accepted 27 February 2014. Address correspondence to Suhui Li, Key Laboratory for Thermal Science and Power Engineering, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China. E-mail: leesuhui@ msn.com Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ gcst.

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impurities present in syngas. For example, volumetric H2 /CO ratio varies from 0.33–40, the percentage of diluent gases (N2 , CO2 ) from 1–51%, and the percentage of water from 0–40%, depending on the feedstock and production processes (Moliere, 2002). The inherent variability in composition and heating value of syngas, however, is a significant barrier towards its usage (Zhang et al., 2007). Consequently, compositional effects on syngas combustion have been studied intensively in terms of lean blowoff characteristics, combustion dynamics, laminar and turbulent flame speeds, autoignition delay time, and so on. For example, Zhang et al. (2007, 2010) investigated the lean blowoff, flashback, and dynamics behavior of H2 /CO/CH4 mixtures. They found that the lean blowoff limits can be captured by the classical Damköhler number approach and are dominated by H2 content. Counter-intuitively, H2 has much less impact on flashback behavior (Zhang et al., 2010). Mathieu et al. (2013b) and Brower et al. (2013) studied the effect of H2 and CH4 blending on autoignition delay time and laminar flame speed at gas turbine conditions using a numerical approach. Their studies show that H2 dominates the chemical kinetics and subsequently dictates the autoignition delay and laminar flame speed when H2 content exceeds a significant level (around 50 vol%), whereas CH4 has an adverse effect compared to H2 . Petersen et al. (2007) and Dryer and Chaos (2008) showed that there is incomplete understanding on the syngas autoignition behavior at high pressure and low temperature conditions. In light of this, Mansfield and Wooldridge (2012, 2013) systematically investigated syngas autoignition behavior at these conditions (above 10 atm and below 1000 K) and significantly advanced the understanding of the kinetics mechanism at these conditions. Their data validated the Li et al. (2007) mechanism on the accurate prediction of autoignition delay time applied in a zero-dimensional homogeneous reactor and revealed the correlation between the ignition transition (strong/weak) and H2 /O2 explosion limits. Baumgartner and Sattelmayer (2013a, 2013b) found that flashback of premixed H2 /air swirling flames has distinctive mechanisms at different equivalence ratios. Flashback is initiated by boundary layer propagation for lean mixtures, whereas it is initiated by combustion-induced vortex breakdown at equivalence ratios above 0.75. Venkateswaran et al. (2011, 2013) measured the turbulent flame speeds of various H2 /CO mixtures and concluded that the turbulent flame speeds can be correlated by the maximum laminar flame speed and a chemical time scaling ratio scale. Lin et al. (2013) investigated the flashback of high-H2 syngas and developed a methodology to correlate flashback propensity with turbulent flame speed. These studies significantly advance the understanding of syngas combustion, focusing on effects of fuel contents (H2 /CO/CH4 ), i.e., ideal syngas. Compared to the characteristics mentioned above, however, less work has been done on the spark ignition of syngas. Although the spark ignition of conventional fuels (natural gas, diesel, and jet fuel) has been widely studied (Akindele et al., 1982; Ballal and Lefebvre, 1975, 1977; Bradley and Lung, 1987; Calcote et al., 1952; Kailasanath et al., 1982; Lewis and von Elbe, 1961; Swett, 1957), the spark ignition behavior of syngas is less well understood, particularly for gas turbine startup applications that feature turbulent inlet flow. Previous studies on syngas ignition were mostly focused on pure syngas (containing only H2 /CO/CH4 ) in quiescent environments, i.e., stationary fuel/air mixture in a chamber. For example, Walton et al. (2007) studied ignition process of H2 /CO mixture with diluents using a rapid compression facility and found that the ignition time is affected by content of dilute gas, rather than the ratios of H2 /CO. Ono et al. (2007) and Ono and Oda (2008) found that minimum ignition energy (MIE) of quiescent H2 /air mixtures is strongly affected by H2 content. In gas turbine startup, however, fuel/air mixture is injected through a nozzle (or a burner) and the flow aerodynamics certainly influences the ignition performance.

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Thus, their study cannot capture the effect of turbulent flow aerodynamics on the ignition performance. In addition, there is no general agreement on which parameter is suited for quantitatively characterizing the effects of syngas composition (mainly chemical reactivity) on spark ignition behavior. Although ignition delay time is a good measure of the effect of syngas composition (H2 content, hydrocarbon addition) on the autoignition behavior, it is unclear if it is suited for spark ignition. In particular, previous studies (Mathieu et al., 2013a, 2013b; Brower et al., 2013) show that H2 dominates autoignition delay time when H2 content exceeds a significant level (about 50%), it may not need such a large amount of H2 to dominate the spark ignition behavior. Furthermore, effect of inert diluents (N2 /CO2 ) on ignition behavior of practical syngas has not been well understood. Presence of N2 and CO2 can impact the reactivity and fuel heating value, which have subsequent influence on the ignition, flame propagation, and lean blowoff behavior. In particular, N2 and CO2 can decrease the reactivity of syngas and flame temperature, thus impairing the propagation of flame in a spark ignition process. As far as the authors know, gas turbines in current commercial IGCC power plants still have to start with either natural gas or diesel fuels. Starting on natural gas or diesel fuels requires a dedicated fuel circuit and pump, plus additional fuel cost. Moreover, diesel fuel produces significantly more NOx emissions than syngas during gas turbine startup, when the exhaust temperature is too low for after-treatment devices (e.g., selective catalytic reduction) to work effectively. Therefore, it is desirable to start the syngas turbine with syngas fuel, eliminating the separate natural gas or diesel startup system. This work is motivated by the lack of understanding on the spark ignition behavior of practical syngas under gas turbine startup conditions. The article presents an experimental study on the effects of inlet air flow velocity and fuel composition on spark ignition of syngas fuels using a swirl fuel nozzle and a model combustor. MATERIALS AND METHODS Syngas Fuels Syngas fuels used in this study are obtained by mixing a H2 -rich fuel with an inertrich fuel. Coke oven gas (COG), which is produced in the coke oven, typically contains 50–60% H2 and is used as the H2 -rich fuel. Blast furnace gas (BFG), which is produced in a blast furnace where iron ore is reduced to iron by coke, is used as the inerts-rich fuel because it contains about 50–60% N2 and 10–20% CO2 . The syngas mixtures contain 10%, 20%, 30%, and 40% COG molar fraction and are designated as SYN1, SYN2, SYN3, and SYN4, respectively. Properties of the syngas mixtures and natural gas fuels used in this study are listed in Table 1. The syngas mixtures have a H2 content ranging from 7.14% to 24.36%, and a N2 content ranging from 51.40% to 36.70%. Experimental Apparatus and Methods Ignition experiments were conducted using a swirl nozzle in a model combustor under atmospheric pressure. The section view of the head end of the combustor with the swirl nozzle is schematically shown in Figure 1. The nozzle exit inner diameter (ID) is 3.2 cm. The combustor has an inner diameter of 12 cm and a length of 50 cm. All of the hardware is made of stainless steel. The fuel nozzle consists of a center body fuel injector, a radial air swirler, and an air sleeve. Syngas fuel is injected through fuel orifices on the

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Table 1 Properties of syngas mixtures and natural gas used in this study Component

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H2 (vol%) CO (vol%) CH4 (vol%) C2 H6 (vol%) N2 (vol%) CO2 (vol%) Sum LHV (kJ/Nm3 )

SYN1 (10% COG) SYN2 (20% COG) SYN3 (30% COG) SYN4 (40% COG) Natural gas 7.14 22.61 2.38

12.88 20.92 4.76

18.62 19.23 7.14

24.36 17.54 9.52

51.40 16.47 100 4254

46.50 14.94 100 5449

41.60 13.41 100 6644

36.70 11.88 100 7840

93.1 5.0 1.9 100 34,701

Figure 1 Schematic of the model combustor with fuel nozzle.

center body. Air is injected through the radial swirler, which generates swirling air flow and promotes fuel-air mixing. A small fraction of the air is injected through the air sleeve and forms a cool air shield to protect the inner combustor liner. Fuel and air flow rates are controlled by Bronkhorst mass flow controllers. Four type-K thermocouples are embedded onto the nozzle tip with equal spacing to detect flashback. A spark plug igniter is installed on top of the combustor for ignition. The spark plug tip is located 5 cm downstream of the nozzle and 1.6 cm from the nozzle centerline (which flushes the inner surface of the nozzle exit) in the radial direction. The spark plug has a sparking energy of 120 mJ and a maximum sparking frequency of 120 Hz. Ballal and Lefebvre (1975) found that the minimum ignition energy is about 3 mJ for turbulent methane and propane gases. Thus, the spark energy of the igniter is sufficient for this experiment considering the fact that H2 requires much lower ignition energy than methane and propane (Ono and Oda, 2008; Ono et al., 2007). Flame detection is achieved by three methods: flame observation, heat release, and pressure oscillation. Two optical windows are installed on front and back sides of the

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Table 2 Experimental conditions for conducting spark ignition experiments Parameter

Value

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Fuel and air inlet temperature (C) Inlet air flow velocity (m/s) Combustor pressure (atm) Igniter spark energy (mJ) Igniter sparking frequency (Hz)

20 10—22 1 120 20

combustor for flame observation and chemiluminescence measurement. A high speed camera (Phantom V210) working at 1000 fps and 640 × 480 resolution is used to record the video of the ignition process. A photomultiplier tube (PMT) with a narrow band filter centered at 309 nm is used to detect the chemiluminescence of OH radicals (a measure of heat release) emitted in the ignition process. Dynamic pressure inside the combustor is recorded with 2 PCB pressure transducers. Ignition is indicated by a sustained flame detected by analyzing the data recorded by the three aforementioned methods. Experiments were performed under atmospheric pressure and ambient fuel and air inlet temperature conditions. Air flow velocity at the nozzle exit is varied in the range of 10–22 m/s, typical of heavy-duty gas turbine startup conditions. The inlet air flow velocity is defined as the mass-averaged velocity of the air flow at the nozzle exit plane, and is calculated by: Uexit =

m ˙ air 1/4πd02

(1)

in which Uexit is the inlet air flow velocity, m ˙ air is the mass flow rate of air, and d0 is the inner diameter of the nozzle exit. The experimental conditions are summarized in Table 2. In light of the fact that sparking ignition strongly depends on the statistical coincidence of spark kernel formation and suitable inlet fuel/air mixture composition, the maximum spark duration is set to 5 s with a 20-Hz sparking frequency. These 100 spark pulses are expected to give a statistically averaged ignition limit.

Experimental Procedure Ignition characteristics of a particular fuel are characterized by the lean and rich ignition limits at a constant air flow velocity. The lean ignition limit (LIL) is the lowest equivalence ratio (φLIL ) at which successful ignition can be achieved at fuel lean condition, whereas the rich ignition limit is the highest equivalence ratio at which successful ignition can be established. By increasing inlet air flow velocity, a saddle point can be reached at around stoichiometric ratio, beyond which no ignition can be achieved. The loop of ignition limits is the ignition boundary, as conceptually illustrated in Figure 2. Here, only the lean ignition limits were measured because gas turbine usually starts at fuel lean conditions on concern over safety. The lean ignition boundary is used to assess the ignition performance of a gas turbine combustor at a constant combustor inlet pressure, temperature, and air flow velocity (Lefebvre and Ballal, 2010). In fact, the lower the ignition boundary, the wider the operation window is for gas turbine to light off. In this study, the lean ignition limits

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Figure 2 Hypothetical ignition boundary of syngas fuels with respect to air flow velocity.

of syngas fuels were measured following the principle established by Lefebvre and Ballal (2010). The experimental procedure is briefly summarized as follows:

1. Purge the combustor for five times of its volume to remove any existing combustible gas. 2. At a constant air flow rate and COG content, set the fuel flow for an equivalence ratio of 1, and establish a successful ignition. Allow 5 s maximum ignition time for each attempt. Successful ignition is indicated by continued flame after switching off the igniter. Switch off the fuel. 3. Attempt ignition on successive test points by decreasing the equivalence ratio until the lean ignition limit is determined. 4. At increased air flow velocity, repeat steps 1–3 until the maximum air flow velocity is reached. Then the lean ignition boundary is obtained for the specific syngas mixture. 5. At increased COG content, repeat steps 1–4 until the maximum H2 content is reached. Then an ignition map can be obtained for the syngas fuel mixtures at various blending ratios.

RESULTS AND DISCUSSION Effect of Air Flow Velocity The lean ignition limits of syngas fuels, φ LIL , are plotted in Figure 3 in terms of inlet air flow velocity. The lean ignition limits are dependent on both air flow velocity and syngas composition. In general, the lean ignition limit increases with air flow velocity and decreases with COG content. In particular, ignition becomes impossible at the highest flow velocity for SYN1, which contains only 10% COG. The ignition performance decreases with increasing air flow. This phenomenon agrees well with previous studies on ignition characteristics of hydrocarbon and syngas fuels (Ballal and Lefebvre, 1977; Walton et al., 2007).

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1 10% COG

Ø_LIL

0.8

20% COG

0.6

30% COG

0.4

40% COG

0.2 0

0

5

10 15 20 Air Flow Velocity (m/s)

25

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Figure 3 Lean ignition limits of syngas mixtures with respect to air flow velocity.

Traditional theory considers spark ignition in flowing gases as a two-step process (Walton et al., 2007; Lefebvre and Ballal, 2010). In the first step, the igniter supplies sufficient energy to the surrounding fuel-air mixture to create a flame kernel, which is called localized ignition. In the second step, the flame kernel propagates to surrounding or incoming fuel-air mixtures, which is called self-propagation. The flame kernel formed in the first step needs to survive the straining of the inlet air flow to be able to progress into the second step. The key to successful self-propagation is to overcome the heat loss (usually with the aid of continued sparking, spark-assisted ignition) because the fuel-air mixture needs to be heated to the adiabatic temperature to be ignited. The inlet air flow has dual effects on the ignition process. As air flow velocity increases, Reynolds number and thus the turbulence intensity of the flow increase. Turbulence promotes fuel-air mixing, turbulent burning velocity of the syngas fuel (Venkateswaran et al., 2011, 2013), and spreading of spark and flame kernel into surrounding fuel-air mixtures, which facilitates self-propagation of the ignition process (Walton et al., 2007). Turbulence, however, can cause convective heat loss of spark energy to downstream of the sparking location, reducing the energy available to heat the fuel-air mixture, thus impairing the first step of the ignition process. Ballal and Lefebvre (1975) observed that the minimum ignition energy required to ignite a premixed fuel/air gas flow increases with flow velocity, due to increased heat loss from the spark kernel. Turbulence also increases strain rate, which will weaken the initially formed flame kernel in the first step. In addition, increasing flow velocity reduces residence time of the spark kernel to stay in the recirculation zone of the combustor to ignite the incoming fuel-air mixture. Ignition is indeed a competition between heat injection and heat loss for turbulent flows. Data in Figure 3 suggests that the overall effect of increasing air flow velocity is negative on spark ignition at turbulent flow conditions. This explains why ignition cannot be achieved when air flow velocity exceeds a critical value, even for a stoichiometric mixture. Effect of Fuel Composition As expected, the effect of syngas composition on LIL is significant. At the same air flow velocity, φ LIL decreases with COG content in syngas mixture. This trend is particularly dramatic when COG content increases from 10% to 20%. However, the LIL then decreases just slightly when COG content increases from 20% to 40%. The general trend that LIL decreases with COG content can be attributed to the decrease of inert diluents

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Fuel or Inert Content (vol%)

1012

100

Total Fuel

80

Inert Diluents

60 40 20 0 0

10

20 30 40 COG Content (vol%)

50

in the syngas mixture. The syngas composition can be classified as two categories: fuels (H2 , CH4 , and CO) and inert diluents (N2 and CO2 ). Inert diluents thermally influence the ignition performance by reducing the temperature of the fuel/air mixture that the spark can heat to and the temperature of the initial flame kernel. The inert diluents content decreases with COG content, as shown in Figure 4. As such, the decrease of inert diluents lowers lean ignition limit by facilitating the formation of a flame kernel in the ignition process. The inert diluents can also affect the lean ignition limit by changing the chemical reactivity of the syngas mixture, which will be discussed later. Regarding the phenomenon that the lean ignition limit drops just slightly when COG content is increased from 20% to 40%, it indicates the dominating effect of H2 ignition. To further illustrate this phenomenon, the lean ignition limits are plotted against H2 content in Figure 5. At a constant air inlet velocity, the lean ignition limit drops significantly when H2 content initially increases from 7% to 13%, whereas the lean ignition limit then decreases slowly with respect to H2 content. For syngas, ignition is indeed a process that involves ignition of multiple fuel components, H2 , CO, and CH4 . It has been well demonstrated that H2 has the highest reactivity among the three fuel components, which results in the lowest minimum ignition energy (Ono and Oda, 2008; Ono et al., 2007) and shortest ignition delay time (Cheng and Oppenheim, 1984; Samuelsen et al., 2006). In fact, the H2 /O2 reaction is the starting point

0.8 0.7 0.6 Ø_LIL

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Figure 4 Variation of fuel and inert diluents composition with respect to COG content in syngas mixtures.

0.5 0.4

10 m/s

0.3

14 m/s

0.2

18 m/s

0.1 0

22 m/s 0

5

10 15 20 H2 Content (Vol%)

25

30

Figure 5 Lean ignition limits with respect to H2 content of syngas mixtures.

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in the hierarchy of the oxidation of hydrocarbon fuels (Hong et al., 2011). The effect of H2 reactivity is represented by providing H radicals for the main branching reaction: H + O2 = OH + O

(R1)

Reaction (R1) controls the ignition delay time and chemical time, which are generally regarded as indicators of the chemical reactivity of the fuel. Thus, adding H2 into the fuel is instrumental to ignition. Reaction (R1) is critical in generating OH radicals for subsequently attacking CH4 by the chain-propagating reaction: OH + CH4 = CH3 + H2 O

(R2)

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and the main heat release reactions: OH + CO = CO2 + H

(R3)

OH + H2 = H2 O + H

(R4)

Reactions (R3) and (R4) are possible exothermic reactions that heat up the surrounding fuel/air mixture to sustain the flame. Reaction (R3), however, is well known as the slowest oxidation reaction (Lewis and von Elbe, 1961). Moreover, it was found that the main heat release reaction shifts from reaction (R3) to (R4) when H2 mole fraction increases in the syngas fuel, due to the H2 preferential diffusion effect (Zhang et al., 2011). Therefore, CO has no contribution to ignition compared to H2 . It competes on OH radicals via reaction (R3). Indeed, Walton et al. (2007) and Gersen et al. (2012) reported weak correlations between ignition delay time and CO concentration in syngas. CH4 also competes with reaction (R1) on H radicals via the chain-propagating reaction: H + CH4 = CH3 + H2

(R5)

Consequently, it is the H2 in the syngas that initiates the overall reaction system, and the presence of CH4 and CO has an adverse effect on ignition. As shown in Figure 6, the H2 content in the syngas increases with syngas blending and facilitates ignition. Meanwhile, the CH4 content also increases with syngas blending, which impairs ignition by consuming H and OH radicals. As shown in Figure 5, the lean ignition limit decreases significantly when H2 increases to 12.7% (correspondingly CH4 increases to 5%), indicating that H2 has a stronger effect than CH4 on the ignition of syngas when H2 content exceeds 10%. The H2 preferential diffusion also makes a significant contribution to the dominant effect of H2 on the ignition behavior of syngas. Physically, the H2 preferential diffusion increases H2 concentration in the flame front, making the flame more like H2 /air flame and burn more intensely. It was demonstrated that the H2 preferential diffusion can extend the lean extinction limit (Zhang et al., 2013) or lean blowoff limit of syngas flame (Zhang et al., 2007). In addition to the preferential effect, the stretch effect also strongly influences the syngas flame because the syngas has a non-unity Lewis number. It is well known that flame temperature and extinction behavior are dependent on the Lewis number (Law and Sung, 2000). The lean syngas/air mixture has a Lewis number less than 1, and enrichment

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Fuel Content (vol%)

30 H2

25

CH4

20

CO

15 10 5 0

0

10

20

30

40

50

60

COG Content (vol%)

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Figure 6 Variation of H2 , CO, and CH4 concentration with respect to COG content in the syngas mixture.

of H2 in the flame front essentially reduces the Lewis number. For a laminar flame with a Lewis number less than 1, positive stretch increases the maximum flame temperature and subsequently the burning intensity. The turbulent flow condition in this study further enhances the burning intensity by generating more curvatures (Venkateswaran et al., 2011). Therefore, the H2 preferential diffusion coupled with the Lewis number effect and flame stretching further strengthens the dominant effect of H2 in the ignition behavior of syngas. The kinetic effect of inert diluents on ignition lies in consuming H radicals via the chain-terminating reaction: H + O2 + (M) = HO2 + (M)

(R6)

Zhang et al. (2011) demonstrated that extinction of CH4 and syngas flames is determined by the competition between reactions (R1) and (R6). CO2 and N2 can participate in reaction (R6) as the intermediate, thus competing on H radicals with reaction (R1) and impairing ignition. Reaction (R6) is well known to be active at low temperatures (Brower et al., 2013; Lewis and von Elbe, 1961). The adiabatic flame temperature of syngas fuels is around 1100–1300 K at lean ignition limits. Therefore, the inert diluents may have a negative impact on ignition by suppressing the subsequent flame propagation following the initial flame kernel. As shown in Figure 4, the inert diluents increase with decreasing COG content, thus impairing ignition. Consequently, the significant increase in lean ignition limit can be attributed to the large amount of inert diluents in the SYN1 mixture. That is, inert diluents dictate the ignition behavior of syngas at low H2 content, e.g., below 10% H2 . Previous studies (Brower et al., 2013; Mathieu et al., 2013a, 2013b) on syngas autoignition suggested that the autoignition delay time, an important indicator of chemical reactivity, is dominated by H2 when H2 content exceeds a critical value, about 50%. For spark ignition of syngas, it can be speculated similarly that H2 effect will dominate at a certain value. To reveal the combined kinetic effects of fuel components and diluents on ignition, chemical time is chosen as an indicator of the chemical reactivity of syngas mixtures. The reason to choose chemical time over ignition delay time is that spark ignition in a gas turbine combustor involves sustaining of the initially formed flame kernel, and the flame extinction behavior is better represented by chemical time (Kobayashi and Kitano, 1989; Wang et al., 2004). The chemical time, τ chem , is a characteristic time scale for evaluating the kinetic reaction rate of a premixed flame and the resistance of the flame to sustain straining. A short

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chemical time indicates that the reaction needs a short time to complete and, thus, the flame can survive a high strain rate. The chemical time can be calculated based on laminar flame properties: δL 2α = 2 SL SL

(2)

in which δL is the flame (reaction zone) thickness, SL is the laminar flame speed, and α is the mixture-averaged thermal diffusivity of the reactants (Plee and Mellor, 1979; Radhakrishnan et al., 1981). Calculation of τchem using Eq. (2) is hindered by the difficulty in calculating the laminar flame speed, which has converging problems at fuel lean conditions, such as the lean ignition limits of syngas mixtures. An alternative approach to numerically calculate the chemical time is to use the blowoff time (τblowoff ) of a flame in a perfectly stirred reactor (PSR). The blowoff time is the minimum residence time for a flame to stay burning in the reactor, and can be used to correlate chemical time. Zhang et al. (2007) demonstrated that the chemical time and blowoff time can be correlated linearly for syngas flames. Moreover, it has been a long practice to model the flame zone of a swirled gas turbine combustor as a PSR (Longwell et al., 1953; Oates, 1997; Spaulding, 1955). In this work, therefore, the chemical time is estimated using blowoff time, recognizing the fact that the blowoff time is practically easier to calculate at fuel lean conditions. Blowoff time is calculated with the Chemkin program using the C5 mechanism and a perfectly stirred reactor model. The C5 mechanism, developed by Metcalfe et al. (2012) at the National University of Ireland, Galway (NUIG), has been demonstrated to have excellent prediction on ignition delay time and laminar flame speed of syngas fuels containing H2 , CO, and CH4 (Mathieu et al., 2013; Brower et al., 2013). Results are presented in Figure 7 as a function of H2 content at different equivalence ratios. The blowoff time drops sharply when H2 content increases from 7% to 12%, and then drops slightly when H2 content increases from 12% to 24%. The sharp drop of blowoff time when H2 content increases from 7% to 12% is particularly significant for lean mixtures, i.e., conditions close to lean ignition limits. This trend agrees well with the trend in Figure 5 that the lean ignition limit decreases significantly when H2 content increases from 7% to 12%. Therefore, it can be concluded that ignition behavior at a constant air flow velocity is dictated by chemical reactivity of the syngas, which is strongly influenced by the H2 content. In particular, the ignition behavior of syngas is dominated by H2 when H2 concentration exceeds 10%. 5 τ_blowoff (ms)

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τchem =

Ø=1

4

Ø = 0.5 3 2 1 0

0

5

10 15 20 H2 Content (vol%)

25

30

Figure 7 Blowoff time of syngas mixtures as a function of H2 content at different equivalence ratios.

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0.40

T = 1200 K, P = 1 atm

τ_AI (ms)

0.35 0.30 0.25 0.20

0

5

10 15 20 H2 Content (vol%)

25

30

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Figure 8 Autoignition delay time of syngas mixtures as a function of H2 content.

In comparison, the autoignition delay time (τAI ) of the syngas fuels is calculated with the Chemkin program using the NUIG mechanism (Metcalfe et al., 2012) and a 0-D reactor model. Calculation is performed for stoichiometric reactants at 1200 K and 1 atm, and the results are presented in Figure 8. It is interesting to note that the ignition delay time increases with H2 content, probably due to the simultaneous increase of CH4 , which consumes H and OH radicals through reactions (R2) and (R5). Indeed, Mathieu et al. (2013a) found that, in shock tube experiments, CH4 addition can increase the ignition delay time through the reaction (R2). As a reference, the autoignition delay time of natural gas is about 27 ms at the same conditions. Data in Figure 5, however, suggests that the chemical reactivity of syngas should increase with H2 content. This analysis indicates that autoignition delay time is not well suited for measuring the chemical reactivity associated with spark ignition. The major difference between autoignition and spark ignition is the heat injection by sparking. Production of H and OH radicals by continued sparking compensates or exceeds consumption of H and OH radicals by CH4 through (R2) and (R5). Autoignition, however, has no external source to help generate H and OH radicals and the consumption of these radicals by CH4 is truly rate-limiting at low H2 concentration. Therefore, ignition delay time, which is used to measure fuel reactivity concerning the autoignition behavior, is not suited for spark ignition. Chemical time, or blowoff time, is more appropriate for evaluating the chemical reactivity that influences spark ignition of syngas because CH4 is no longer the limiting factor with the sparking-assisted production of H and OH radicals. Correlation of Ignition Limits The above analysis on LIL suggests that both turbulent flow properties and fuel composition impact the spark ignition of syngas fuels. The overall effect of increasing flow velocity is adverse based on the trend of lean ignition limit with respect to air flow velocity. Fuel composition effect is reflected by the chemical reactivity of the syngas, which is dominated by the H2 content. A small amount of H2 significantly lowers the lean ignition limit, improving the ignition performance. When H2 content is below 10%, ignition is dominated by inert diluents. When H2 content exceeds 10%, the H2 chemistry dominates the ignition behavior of syngas. To better understand the physics behind the effects of turbulent flow and fuel composition on ignition, a model is proposed to correlate the ignition limits with flow properties and fuel composition characteristics.

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The adverse effects of turbulent flow on ignition mainly include straining the flame kernel and causing convective heat loss from spark plug tip to downstream. At increased air flow velocity, the strain rate increases and the spark energy is also carried away from the igniter tip to downstream. The radiative heat loss, however, is negligible because it scales with the 4th order of flame temperature, which is relatively low (1100–1300 K) for the syngas mixtures at lean ignition conditions due to the low heating values of the fuels. The strain rate and convective heat loss are dependent on the Reynolds number, which is defined as: ρUd μ

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Re =

(3)

where ρ is flow density, U is flow velocity, μ is flow viscosity, and d is characteristic length. Here the flow refers to inlet air, U is the mass-average flow velocity evaluated at the nozzle exit, and d is the diameter of the nozzle. In addition, increasing flow velocity reduces residence time of spark kernel to stay in the recirculation zone of the combustor to ignite the incoming fuel-air mixture. Therefore, the effects of inlet air flow can be linked to velocity. The residence time of flame kernel in the recirculation zone of the combustor can be calculated by: τres =

dzone Uref

(4)

where τres is the residence time, dzone is the characteristic length of the recirculation zone (e.g., recirculation zone length), and Uref is the reference flow velocity. Here, we use the nozzle tip diameter d0 (defined in Eq. (1)) as the characteristic length because the recirculation zone length scales with nozzle diameter for swirling flames (Weber and Breussin, 1998). The air inlet velocity at nozzle exit (Uexit , defined in Eq. (1)) is chosen as the reference flow velocity. Indeed, the ratio d0 /Uexit represents the inertial time scale of the flow and is often called the convective time scale. At high Reynolds numbers, all time properties measured in the flow, except for those associated with molecular dissipation process, are proportional to d0 /Uexit (Weber and Breussin, 1998). By examining Eqs. (3) and (4), we can find that they share a common component, the flow velocity. As discussed above, when Reynolds number increases (only when the air flow velocity increases because the density, viscosity, and characteristic length are constants for the air and nozzle used in this study), the air flow has a negative effect on ignition because of increased flame straining and heat loss. Similarly, when residence time decreases (air flow velocity increases), the air flow has a negative effect on ignition because less time is available for the flame kernel to stay in the recirculation zone to ignite the incoming fuelair mixture. Therefore, Re and τ res have a similar effect on ignition when air flow velocity increases, and just one of them needs to be included in the model. Now, let us examine which one shall be included in the model. The fuel composition effect can be related to the fuel reactivity, which is characterized by chemical time, or blowoff time, as calculated in the previous analysis. Indeed, the ratio of residence time to blowoff time is the Damköhler number: Da =

τres τblowoff

(5)

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Figure 9 Hypothetical fate of spark kernel in ignition process.

Da has been successfully used to correlate the lean blowoff limits of premixed flames in gas turbine combustors assuming a PSR model (Longwell et al., 1953; Oates, 1997; Spaulding, 1955; Zhang et al., 2007, 2010). It is essentially a measure to determine if the reactants have sufficient time to complete combustion reactions in the reactor. For spark ignition of syngas in a swirling flow combustor featuring a recirculation zone, success of ignition relies on the initially formed flame kernel to propagate to the incoming fuel-air mixture through the recirculation zone. In particular, a spark kernel that enters the recirculation zone and propagates back to the combustor inlet has a chance to ignite the incoming fuelair mixture. Meanwhile, the incoming fuel-air mixture is easier to be ignited if it has a high chemical reactivity, or needs less ignition energy. As such, a longer residence time of the flame kernel compared to the chemical time of the fuel-air mixture will promote ignition. To elaborate the concept, the spark ignition process is hypothetically illustrated in Figure 9. In addition to the flame kernel formation and flame propagation of the traditional two-step ignition theory, a final step is added in this concept—stabilization of the propagating flame to establish a sustained flame. For a flame kernel generated by spark, it will have a better chance to ignite the incoming fuel-air mixture if it has more time to stay in the recirculation zone, whereas it has a lower chance to make a contribution to ignition if it is blown to the downstream of the combustor. Basically, a large Da indicates long spark residence time, low strain rate, less heat loss, and high fuel reactivity, and ultimately, a strong flame kernel. Meanwhile, Da is also a representative measure of the stabilizing flame in the combustion zone (Zhang et al., 2007). Therefore, Da is chosen over Re for characterizing the propensity of ignition (particularly the first step of the ignition process) because it contains properties of both flow aerodynamics and fuel reactivity. Regarding the second step of the ignition process, flame propagation, an ideal parameter to consider is the turbulent flame speed. Turbulent flame speed (or turbulent burning velocity, turbulent consumption speed) is a key parameter characterizing the flame propagation at turbulent conditions. Turbulent flame speed can be correlated to laminar flame speed (Venkateswaran et al., 2011, 2013). Calculation of laminar flame speed, however, is hindered by the convergence difficulty encountered at highly fuel lean conditions. It is known that the laminar flame speed is positively related to the burned gas temperature as volumetric expansion of burned gas accelerates flow speed (Natarajan et al., 2007; Turns, 2012)

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Figure 10 Conceptual model to correlate syngas ignition behavior using Da and Tflame .

Therefore, premixed flame temperature is chosen to characterize the flame propagation tendency. Based on the two parameters, Damköhler number and premixed flame temperature, a model is proposed to correlate the lean ignition limits of syngas at flowing conditions and is conceptually illustrated in Figure 10. Damköhler number, Da, is the ratio of spark flame kernel residence time to the blowoff time of the flame and represents the strength of the initially formed flame kernel by spark. Premixed flame temperature, T flame , represents the tendency of flame propagation in the fuel-air mixture. According to the model, successful ignition is favored at high Damköhler number and high flame temperature. It is interesting to note the two extreme conditions corresponding to T max and T min . T max is the maximum flame temperature at stoichiometric condition (equivalence ratio equal to 1). The stoichiometric fuel-air mixture has the highest flame speed to propagate and strongest resistance to flame straining, thus, it can be ignited at a small Damköhler number (high flow velocity or low fuel reactivity). Previous studies (Ballal and Lefebvre, 1975, 1977; Ono and Oda, 2008; Ono et al., 2007) demonstrated that the minimum ignition energy drops sharply when equivalence ratio approaches 1. In contrast, T min is the lowest flame temperature that can be reached when Damköhler number approaches infinity. According to Eqs. (4) and (5), the flow velocity approaches 0 when Damköhler number approaches infinity. As a result, the lean ignition limit corresponding to minimum flame temperature is essentially the lean flammability limit (or lean extinction limit) of quiescent fuel-air mixture. Caution needs to be exercised, however, when interpreting T min of non-premixed flame. The reason is that for non-premixed flame, the overall equivalence ratio can be lower than the lean flammability limit of the premixed fuel-air mixture. Correspondingly, the minimum flame temperature calculated assuming premixed combustion will be below the flame temperature at the lean flammability limit. To validate the model, Damköhler number of syngas mixtures is plotted against the adiabatic premixed flame temperature at lean ignition limits and the results are presented in Figure 11. Damköhler number correlates reasonably well with flame temperature, substantiating the correct choice of Damköhler number and flame temperature as the parameters to represent the flow and fuel properties that are associated with ignition behavior. The effects of air flow velocity and fuel reactivity are well captured by the Da − T flame correlation. Data scattering mainly occurs in the low temperature-high Damköhler number region,

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40 7% H2

Da

30

13% H2 19% H2

20

25% H2 10 0 500

1000

1500

2000

T_flame (K)

0.5

1250

0.4

1200

0.3

1150

0.2 0.1 0

1100

Lean Flammability Limit Premixed Flame Temperature 0

5

10 15 20 25 H2 Content (vol%)

Tflame (K)

Ø_LFL

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Figure 11 Correlation of lean ignition data using Da and Tflame .

1050 30

1000

Figure 12 Lean flammability limit and corresponding premixed flame temperature of syngas mixtures as a function of H2 content.

which may be due to the measurement error of mass flow controller, or more importantly, the reduced fuel-air mixing at these conditions. At the low temperature-high Damköhler number region, both the air flow rate and fuel flow rate are relatively low, falling into the low-range side of the mass flow meters, which can have relatively large errors in measuring flow rate. The fuel-air mixing is relatively poorer because the Reynolds number decreases at low air flow velocity and the flame shifts towards non-premixed combustion. Recall the minimum flame temperature discussed in Figure 10, the flame temperature at lean ignition limit may fall below the flame temperature at the lean flammability limit. To verify this analysis, the lean flammability limit (φ LFL ) of the syngas mixtures is calculated using Chemkin following the counter-flow flame method proposed by Law and Egolfopoulos (1990) and the result is presented in Figure 12 together with the corresponding premixed flame temperatures. The lowest lean flammability limit occurs for the syngas with 25% H2 because it has the highest fuel reactivity. The lowest flame temperature, 1134 K, is slightly higher than the minimum flame temperature in Figure 11, which is about 1100 K. Therefore, the fuel-air mixture is ignited at a non-premixed condition with an overall equivalence ratio below the lean flammability limit. Curve fitting of the data in Figure 11 gives the exponential correlation between Damköhler number and premixed flame temperature with an R2 of 0.9:   Da = 5993.7exp −0.005Tflame

(6)

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0.8 0.7

Ø_LIL

0.6 0.5 0.4 0.3

10% COG

0.2

20% COG

0.1

Natural Gas

0 0

10 20 Air Inlet Velocity (m/s)

30

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Figure 13 Comparison of lean ignition limits of natural gas and syngas.

Equation (6) indicates that the lean ignition limit is embedded in the correlation because premixed flame temperature is directly related to equivalence ratio. Comparison with Natural Gas Data in Figure 3 shows that practical syngas, even with a significant amount of inert diluents, can be ignited at gas turbine startup conditions. To replace natural gas as startup fuel for syngas turbines, it is necessary to compare the ignition performance of syngas with natural gas. Consequently, the lean ignition limits of natural gas are measured at the same air flow conditions and the results are presented in Figure 13. The ignition performance of natural gas, in terms of lean ignition limit, lies in between the syngas containing 7% H2 and the syngas containing 13% H2 . This comparison suggests that syngas, even with a significant amount of inert diluents, can have better ignition performance than natural gas, as long as there is more than 10% H2 . CONCLUSION Syngas spark ignition experiments were performed using a swirl nozzle in a model combustor under typical gas turbine startup conditions. The effects of fuel composition and air flow on ignition behavior were studied by varying the fuel composition and air flow velocity. Results show that the lean ignition limit, a measure of ignition performance, is strongly dependent on both the fuel composition and air flow velocity. The fuel composition influences the ignition limit in terms of chemical reactivity, which can be characterized by chemical time, or blowoff time. In particular, H2 dominates the spark ignition behavior when there is a small amount (around 10%) H2 in the syngas mixture, compared to the previous observation that a significant amount of H2 (about 50%) is needed to dominate autoignition behavior. Other fuel components, such as CH4 and CO, have little influence on ignition. The air flow plays a significant role in ignition by affecting heat transfer from spark to surrounding reactants and flame kernel to incoming fuel-air mixture. Increasing air flow velocity results in more heat loss from spark and flame kernel to downstream, as well as higher flame straining, thus impairing ignition. The impact of air flow and fuel composition can be captured by a Damköhler-flame temperature correlation. The correlation can be used to predict the lean ignition limits based on the flow velocity and fuel composition. A threestep ignition theory is proposed to describe the ignition process. It includes stabilization of

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the propagating flame as the final step, in addition to the flame kernel formation and flame propagation in the traditional two-step ignition theory. Compared to natural gas, syngas has better ignition performance because of the presence of H2 . ACKNOWLEDGMENT The authors gratefully acknowledge Dr. Qingguo Zhang for the support in Chemkin calculations.

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