Premixed Combustion of Hydrogen and Syngas …formatex.info/energymaterialsbook/book/946-957.pdfPremixed Combustion of Hydrogen and Syngas Fuels in Gas Turbine Combustors Onur Tuncer

Premixed Combustion of Hydrogen and Syngas Fuels in Gas Turbine Combustors

Onur Tuncer Istanbul Technical University, Dept. of Aeronautical Engineering, Maslak, 34469, Istanbul, Turkey

Power generation industry often relies on land based gas turbine engines for energy conversion from liquid and/or gaseous fuels. When it comes to gaseous fuels, from a historical perspective, combustors were often designed for methane fuel, since it is the major constituent of natural gas, an abundant fuel resource. Nevertheless, with the recent proliferation of the IGCC (Integrated Gasified Combined Cycle) technology, the use of hydrogen rich fuels is dictated for many power generation installations. Fuel flexibility is a major issue for these gas turbine engines. For the last two decades lean premixed combustion has become the industry standard for many applications as it enables single digit NOx emissions. Hydrogen enrichment significantly alters flame behaviour of hydrocarbon fuels due to its much faster propagation speeds. This presents challenges in terms of flame holding, flashback and thermo-acoustic instability.

There are several methods which the power generation industry utilizes in order to alleviate this situation. Each has advantages and disadvantages in terms of key performance metrics such as volumetric heat release, pattern factors and emissions. This paper aims to present the state-of-the-art on hydrogen rich fuel combustion in gas turbine environment with examples from the open literature. Firstly, conventional swirl stabilized hydrogen enriched flames are presented with a thorough discussion on associated challenges; secondly a novel technique that relies on aerodynamic flame stabilization is discussed. This second technique is also termed as low-swirl combustion. Benefits and possible drawbacks for each premixed flame configuration are discussed.

Keywords: hydrogen; syngas; combustion; gas turbine engine; flame holding

1. Introduction

Synthesis gas (syngas), an environmentally clean source of energy, is a variable mixture of primarily hydrogen and carbon monoxide and some other species such as hydrogen sulphur, hydrogen cyanide, hydrogen chloride and ammonia in smaller proportions. Syngas is either obtained from gasification of coal or biomass. Depending on the gasification process, and which solid is gasified substantial changes in the resulting syngas composition can occur [1]. Variability of the syngas composition (Table 1) can significantly alter the flame behaviour [2]. For example, an increase in hydrogen proportion will assure better flame stability but, at the very same time, it will make the combustor more susceptible to flashback and thermo-acoustic instability. In addition it will also increase nitric oxide emissions due to higher temperatures. On the other hand an increase in carbon monoxide content will deteriorate flame stability. Consequently, it is necessary that a syngas combustor be able to tackle the variability in fuel composition without requiring any design changes [3]. Low BTU fuel as syngas has less chemical bonding energy per unit weight. Therefore, in order to achieve a desired power output from syngas high mass flow rates needs to be used [4]. High mass flow rates normally translate into higher injection speeds, which pose a problem in flame holding. Furthermore, the main reaction zone residence time determines the pollutant levels [5-6]. For the combustion of syngas high turbulence intensity (between 10-20%) at the burner exit is needed since flame speed is related to the turbulence level in the main reaction zone. Therefore the design of the syngas injection nozzle and combustor should incorporate the challenges posed by flame holding, low BTU per unit mass and emissions. Table 1 Syngas composition variation across different sites in USA [2].

PSI Tampa Sierra-Pacific H2 24.8 37.2 14.5 CO 39.5 46.6 23.6 CH4 1.5 0.1 1.3 CO2 9.3 13.3 5.6 N2+Ar 2.3 2.5 49.3 H2O 22.7 0.3 5.7 H2/CO Ratio 0.63 0.80 0.79 Diluent Steam N2 Steam LHV (kJ/m3) 8224 9962 5024

Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.)____________________________________________________________________________________________________

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In two separate works Borghi and Peters [7-8] have identified a regime diagram for premixed flames which is shownin Figure 1. Note that practical combustors mostly operate within the flamelet regime. Here reaction takes place in athin front that separates the products and the reactants.

Syngas utilization in a multi fuel combustion environment is another challenge that needs to be addressed. In manyof the existing installations, other fuels like natural gas and/or fuel oil are readily available on site. Therefore it isdesirable to understand how mixing natural gas and syngas in different proportions effects flame holding, flashback,and combustion dynamics.

For premixed syngas injection, one of the biggest challenges is flashback, since hydrogen flame speeds are quite high[9-10]. Flashback into the pre-mixer section leads to thermal overload and destruction of the hardware therefore it mustbe avoided at all load conditions [11]. Flashback can be prevented by using specially designed flame holders or byinjecting syngas in a separate non-premixed arrangement. However, in transitioning from natural gas as the fuel ofchoice to syngas, it is desirable to keep hardware changes to a minimum, given the extensive body of knowledge withcurrent natural gas related hardware.

Combustors are usually operated at near lean blowout limits. Lean premixed combustion has a high potential of NOxabatement. A stable combustion of lean mixtures with low flame speeds is therefore necessary in order to obtainemission levels below 10 ppm [11]. Yet, as the adiabatic flame temperature is lowered, carbon monoxide emissionstend to increase. An optimum operating point needs to be sought so as to guarantee both low nitric oxide and carbonmonoxide emissions.

Finally, the most important issues with SGH (syngas and hydrogen) fuel combustion are identified as; ability of thecombustor to burn variable mixtures of syngas without necessitating a design change, flame flashback, auto-ignitionphenomena, combustion dynamics such as thermo-acoustic instability, near-lean-limit flame instability related to issues,feed system coupling, flow interaction and other unsteady fluid mechanics phenomena.

In order to investigate fuel flexibility issues and associated challenges experimentally a laboratory scale test rig isbuilt. The effect of hydrogen enrichment of methane fuel is investigated. Conventional and aerodynamic flamestabilization techniques are also discussed.

2. Experimental setup

Experimental setup includes the combustor, gas bottles, metering systems, and the instrumentation. The combustorsystem consists of the combustor shell, the inlet fuel and air-delivery system, and the premixing section defines thedump plane. Quartz windows enable optical access into the main recirculation zone above the dump plane. At the exitthere is a cylindrical stainless steel shell followed with as conical constriction section where the exhaust flow diameterdrops down to 12 mm. Combustor is shown in Figure 2.

On the setup there are ports to enable pressure transducer mounting and there is also a port to enable gas sampling.The basic design of the fuel-air premixing section represents a generic configuration with characteristic features similarto industrial gas turbine systems where the fuel is injected into the swirling cross flow and mixes within a downstreamdistance before reaching the dump plane. Combustor is operated up to a power rating of 20 kW.

Fig. 1 Borghi-Peters regime diagram for premixedflames (note that conventional gas turbinecombustors operate within the thin reaction zonesregime).


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Hydrogen and methane gases are individually supplied from compressed tanks and mixed within a manifold prior to combustor inlet. Their flow rates are controlled by separate mass flow meters. Mass flow rates are adjusted separately in order to achieve the desired fuel composition. Air necessary for combustion is supplied from a compressor. Volumetric airflow rate is measured by a rotameter, and a pressure gauge. Further details on the experimental setup can be found in [12].

Fig. 2 Overall view of the combustor used for experimental study. Combustion air is fed through a swirl vane (Figure 3). The blades on the swirl vane incorporate a swirling motion to the fluid flow. Swirl provides stabilization at the dump plane and facilitates the entrainment of fuel jets within the cross flow at the pre-mixer. The design is modular such that it can incorporate both a conventional swirl vane and a low swirl vane.

a) b)

Fig. 3 Conventional (a) and low swirl (b) vanes.

This flow then interacts with the by-pass stream at the center. By-pass jets are intentionally made with small diameter such that the flame does not flashback. When the flow reaches the dump plane conservation of angular momentum causes the flow to expand and separate. The periphery of the central separated flow is covered with a low intensity swirling flow that gets weaker in the streamwise direction. A linear drop in the mean flow velocity occurs due to the mean flow gradient. These set the conditions for the propagation of a premixed turbulent flame. Flame is stabilized at the location where the local flow velocity is of the same magnitude but in opposite direction to the local flame propagation speed [13-14]. Furthermore since the turbulence intensity is directly proportional to the flow velocity a feedback loop is formed. Due to this mechanism acceleration and deceleration of the flame under different load conditions is assured [15]. The calculation of the swirl number can be performed with the following formula (Eq. 1). and are the corresponding radii of the center channel and the burner. The parameter denotes their ratio (Eq. 2). Similarly, and

are the mass fluxes through the center channel and the swirl annulus. Therefore the parameter is termed as the by-pass ratio (Eq. 3). Note that when there is no-bypass flow through the center (i.e. ), then this situation corresponds to a traditional high-swirl burner [13]. The introduction of the by-pass stream provides an additional


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degree-of-freedom for the designer. Hence not only the desired swirl number can be obtained by altering the flow split, but also the desired divergent flow profile can be achieved [13].

(1)

(2)

(3)

3. Effects of hydrogen enrichment in conventional swirl stabilized configuration

Swirl stabilization offers unique characteristics due to the flame stability, high combustion performance and efficiency both for premixed and non-premixed flames. Present gas turbine combustors and industrial systems alike both utilize strong swirl [16]. In these systems the swirl component is strong enough to establish a recirculation zone within the combustion chamber. Such a flow structure is shown in Figure 4. This re-circulation zone traps hot combustion products within itself. Therefore, in traditional premixed combustion systems the re-circulation zone provides the heat source and the free radical pool necessary for continuous ignition.

3.1 Adiabatic Flame Temperature and Flame Speeds

Following the assumptions of Yu et al. [17] an equivalence ratio is defined as follows (Eq. 4). This equation implies that the hydrogen in the blend is completely oxidized and the remaining oxygen is used to burn the methane content. This is a reasonable assumption since the hydrogen oxidation proceeds much faster than methane oxidation.

( )( )

F A H H A st

F A st

C C C C C

C Cφ

− =

(4)

Figure 5 shows the effect of hydrogen enrichment on adiabatic flame temperatures as a function of equivalence ratio and hydrogen fraction. Adiabatic flame temperatures increase with increasing hydrogen percentage. Note that this change is quite significant especially for lean mixtures. Figure 6 on the other hand illustrates the effect of hydrogen enrichment on flame propagation speeds. Addition of hydrogen into the mixture significantly enhances the laminar flame speeds. This in turn can have profound effects in terms of flame stabilization.

Fig. 4 Flow structure in a conventional swirl stabilized flame (ReD=19400, Sw=0.74, ) [12].


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3.2 Lean Blowout Measurements

Contemporary land based gas turbine engines are operated near their lean blow-off limits due to emissions considerations. Here blow-off limits for the laboratory scale combustor are identified with respect to the particular choice of fuel or fuel composition for that matter. Effects of two possible blowout mechanisms were discussed; one approach based on front propagation, the other based on a well stirred reactor approximation. As fuel flexibility is an issue and different fuel blends are used, it is possible that burning occur in different regimes.

Hydrogen enrichment considerably extends the lean blowout limits of the methane fuel as evidence to this argument is shown in Figure 7. These effects regarding the extension of lean blowout limits through hydrogen addition are consistent with the observations reported by [18-20]. Blowout data scales with an empirical parameter , where α is the hydrogen volume fraction. The higher the volumetric flow rate (hence velocity) the more the amount of hydrogen added to methane in order to keep the reaction going. It is observed that for hydrogen enriched methane lean blowout equivalence ratio is not fixed it both depends on the extent of enrichment and also upon the flow rate. In flamelet combustion regime blowout occurs when the local flame speed is less then the oncoming fluid velocity everywhere in the flame. Thus the stabilization mechanism for flamelet-like combustion is flame front propagation [20]. For flamelet combustion a loading parameter is defined as follows (Eq. 5).

Φ

T ad(K

)

0.5 0.75 1 1.25

1600

1800

2000

2200

2400

% 0 H2%20 H2%40 H2%60 H2%80 H2

Equivalence Ratio

Lam

inar

Flam

eSp

eed

(m/s

)

0.6 0.8 1 1.2 1.40.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3% 0 H2% 20 H2% 40 H2% 60 H2% 80 H2

Fig. 5 Adiabatic flame temperatures of methane/hydrogen mixtures as a function of equivalence ratio and hydrogen fraction (hydrogen percentage by volume).

Fig. 6 Laminar flame speeds of methane/hydrogen mixtures (hydrogen percentage by volume).


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TSL U= (5)

Turbulent flame speed is often expressed as in terms of laminar flame speed multiplied by a function which depends both on turbulence intensity and geometry. For all conditions tested in the laboratory combustor the geometry is fixed so those can be factored out when correlating the blow out behavior. Assuming turbulence intensities are similar as a first order approximation the loading parameter can be expressed in terms of the laminar flame speed of the fuel mixture [20]. Re-expressing the loading parameter in terms of laminar speed as in Eq. 6.

LSL U= (6)

a.

b.

Fig. 7 Relationship between blowout equivalence ratio and flamelet based loading parameter. Figure 7a shows the relationship between lean blowout equivalence ratio and flamelet based loading parameter. Data for blowout is recorded in a wide range of operating conditions , as well as a wide range of fuel compositions (from pure methane to 50% methane/50 % hydrogen). As it can be seen from this figure data points are scattered along a single line with a correlation coefficient of R2=0.69. Chemical time scales for hydrogen and methane oxidation are quite different. Therefore considering the turbulence in practical combustors combustion process can indeed cover a wide spectrum of Damköhler numbers. Another reactor loading parameter L2 based on a well stirred reactor approach can be defined as well (Eq. 7). Following the work of Hoffman et al. [21] azimuthal velocity component U and combustor diameter D are used as the appropriate scaling parameters considering the fluid dynamic effects. If the well-stirred reactor based combustion is dominant then the appropriate loading parameter L2 is defined as per the following equation. Here α denotes the thermal diffusivity of the reactant mixture.

2 2L

UL

S D

α= (7)

Results for reactor based correlation are shown in Figure 7b. For this parameter L2 correlation is weaker with respect to the previous one. The correlation coefficient is only R2=0.41. Especially for low hydrogen content mixtures one observes that there is a very large scatter in the data. This in turn points out that well-stirred reactor based loading does not offer a good explanation for methane rich mixtures. On the other hand, one can see that for hydrogen rich mixtures data seems to correlate well with this loading parameter. Therefore, no single assumption could entirely explain the lean blowout behaviour of the combustor with methane hydrogen mixtures and tightly collapse all data points on a single line. Flamelet based loading parameter on the other hand offers a much better correlation for low hydrogen content mixtures. For hydrogen rich mixtures on the other hand there is a better correlation regarding the reactor based loading parameter.

Equivalance Ratio at Blowout

SL/U

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.70

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.60% H210% H220% H230% H240% H250% H2

Equivalance Ratio at Blowout

Rea

ctor

Load

ing

Para

met

erL2

0.3 0.4 0.5 0.6 0.70

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

% 0 H2% 30 H2% 40 H2% 50 H2


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3.3 Pressure, Heat Release and Flashback Measurements

A number of piezo-electric pressure transducers are mounted along the combustor wall measures dynamic pressure variations in the combustor. So as to examine the waveform of the combustion instability pressure inside the reactor is to be measured at several stations along the entire length of the combustor. The CH/OH radical light intensity is recorded using a photodiode looking at the flame equipped with an appropriate band-pass optical filter. Photodiode reading is taken as a measure of integral heat release fluctuations in the main reaction zone of the reactor. Figure 8 shows the Fourier spectra of CH light, flashback and pressure signals at ReD=7200 and Φ=0.7. All three exhibit oscillatory behaviour as governed by the acoustic modes of the combustor. These modes depend on the boundary conditions at the inlet and outlet. Interestingly a sudden shift in the dominant frequency occurs after certain hydrogen content within the fuel is exceeded. In Figure 8, one can identify two distinct regions one with high frequency and the other one with low frequency. With increasing hydrogen content a low frequency flashback mode is observed. This behaviour occurs within a wide range of load conditions as evidenced by Figure 9. Another way fuel composition affects the feedback route between heat release and pressure fluctuations within the thermo-acoustic loop is the change that occurs when the flame center of heat release is altered due to change in flame speed. As the flame speed increases with the addition of hydrogen the flame becomes shorter and the distance to the flame center of heat release decreases. This yields to a reduced convective time for the equivalence ratio perturbations which can cause a stable flame become unstable and vice versa [22]. Figure 10 shows the change in flame center of mass with varying fuel composition while keeping cold flow velocity constant to factor out the aerodynamic effects. These are time averaged two dimensional OH* chemiluminescence images captured with an intensified CCD camera equipped with an appropriate band pass filter. OH* radical is a good indicator for heat release. One can notice the change in the location between two flow conditions tested. On the right hand side with 50% methane most of the heat release takes place in spatial locations closer to the dump plane.

a. b.

Fig. 8 Pressure, heat release and flashback spectra, ReD=7200, Φ=0.7 (a. Pure Methane, b. %50 H2 by volume). Another important phenomenon in the study of pre-mixed flames is flame flashback. Flashback can be triggered by acoustic velocity fluctuations and is facilitated by higher flame speeds. Different fuels therefore have varying degrees of susceptibility to flashback. This is not only due to a change of flame speed other factors are also present. For example a change in the flame height alters the location of heat release or a change in the flame temperature affects the acoustics by shifting the dominant mode. Flame front movement and thermo-acoustic phenomena are in fact closely coupled to one another though the dynamic equations. Velocity fluctuations cause the flame to move. This flame movement causes the spatial location of heat release to move thus shifting the instantaneous phase between pressure and heat release. A sequence phase locked of images is recorded showing the flashback cycle. This cyclic behaviour can be seen in the CH chemiluminescence images of Figure 12. Initially flame cone makes a steeper angle with the dump plane in order to balance the increase in the oncoming fluid velocity. Shortly afterwards flame departs from its attachment point and continues to propagate upstream. At one point flame reaches its maximum propagation distance. As the total velocity becomes positive it pushes the flame tip back to its attachment point. This behaviour is repetitive due to the nature of velocity fluctuations occurring inside the combustion chamber. Figure 11 depicts the cycle of events leading to periodic flame flashback. The attached wedge shaped flame moves upstream due to a flow reversal shortly following pressure build-up. Vertical arrows pointing upwards and downwards indicate total flow velocity and flame speed respectively. Whenever the flame is detached from the injector tip the flame anchoring point (tip of the wedge shaped flame) moves with a velocity which is the sum of the flame speed and fluid

Frequency (Hz)

Pre

ssur

e(d

B)

Hea

tRel

ease

,Fla

shba

ck(d

B)

0 100 200 300 400 50040

50

60

70

80

90

20

30

40

50

60PressureHeat ReleaseFlashback

No flashback

86 Hz

164 Hz

Frequency (Hz)

Pre

ssur

e(d

B)

Hea

tRel

ease

,Fla

shba

ck(d

B)

0 100 200 300 400 50040

50

60

70

80

90

20

25

30

35

40

45

50

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60

PressureHeat ReleaseFlashback

156 Hz

40 HzFlashback mode


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velocity. As it is seen from the figure at the mid-cycle flame is entirely inside the pre-mixer. As the cycle progresses and the total fluid velocity attains a positive value that beats the flame speed which always points towards the reactants side flame front recovers its shape and re-attaches to the center body tip.

a. Pure Methane b. 50% H2 by volume

Fig. 10 OH* chemiluminescence images demonstrating effect of fuel composition on the distance to the flame center of mass at a fixed flow rate and equivalence ratio (ReD =7200 , Φ=0.7, Sw=0.74).

3.4 Emissions

Figure 13 demonstrates the effect of hydrogen enrichment on nitric oxide emission index. Emission index is defined as the grams of pollutant generated per kg of fuel burnt. This non-dimensionalization offers an unbiased comparison. It is observed that EINO values rises monotonically with increasing hydrogen fraction. This effect is due to increased flame temperatures. Figure 14 shows the correlation between adiabatic flame temperature and nitric oxide emissions index (EINO) in a wide parameter space including fuel variability effects. Note that for the flow condition tested residence time changes by almost a factor of three. Residence time is one of the determining parameters for the equilibrium concentration of species. However as the points scatter along a line depending on the adiabatic flame temperature one can argue that residence times are longer than formation timescales and the associated effects are relatively smaller. Points on the scatter plot are color coded according to the binary fuel mixture composition. Holding all the other parameters constant, and increasing hydrogen volume fraction within the fuel blend, yields an increase in the nitric oxide emissions index. A possible explanation for such an affect is the increased abundance of hydroxyl radicals within the radical pool. Higher hydroxyl concentrations can increase the forward reaction rates in the nitric oxide formation mechanism where

0 10 20 30 40 50

0.60.7

0.80.9

1

0

20

40

60

80

100

120

Freq

uenc

y (H

z)

% Hydrogen

Eq. Ratio

Fig. 9 Dominant frequency (Hz) with respect to fuel composition and equivalence ratio (ReD=7200).


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hydroxyl takes part in. Figure 14 suggests that the main scaling parameter is the adiabatic flame temperature for oxygen based EINO. It can be concluded that the extended Zeldovich mechanism (thermal NOx) is the dominating pathway to NO formation.

4. Aerodynamic flame stabilization

Low swirl aerodynamic flame stabilization, as introduced by Cheng [13], is a rather new concept. Its working principle is based on the propagation of the premixed flame front. Premixed flames create a continuous reaction due to their self-propagation ability. The flame speeds depends on the concentration of the reactants, thermodynamic conditions and turbulence intensity. In low swirl combustion in order to hold turbulent fast propagating flames a diverging flow field is utilized [14]. A diverging flow field is established under conditions such as vortex breakdown when the swirl strength is low. Therefore in a low swirl combustor the flame is stabilized aerodynamically. This is the fundamental difference between low swirl combustion and traditional techniques. The following discussion provides an insight to this problem and enables one to reach certain conclusions. The axial velocity at the flame anchoring location is provided by the following equation (Eq. 20) [13].

(8)

0 degrees

0

20

40

60

80

100

45 degrees 90 degrees 135 degrees

180 degrees 225 degrees 270 degrees 315 degrees

Fig. 12 Phase locked CH radical images demonstrating a flashback cycle (ReD= 6600, φ=0.7, 40 % hydrogen by volume).

Fig. 11 An illustration showing the cycle of events regarding flashback.


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In the literature it has been demonstrated that for low swirl flames the turbulent flame speed demonstrates a linear correlation for a wide range of fuels including methane, propane, ethylene and diluted hydrocarbons [23-24]. This relationship is provided below through Eq. 9.

Fig. 13 Effect of hydrogen enrichment on emissions index (ReD=4870, Sw=0.74).

(9)

Hence if one divides this expression (Eq. 8) by the axial velocity and substitutes the above correlation (Eq. 9) for the turbulent flame speed , then the following expression would eventually emerge (Eq. 10) [15]. Now this expression (Eq. 10) can be analyzed term by term. Note that at the left hand side is invariant due to flow

similarity, furthermore is asymptotic at large , which is often the case in gas turbine combustors. At the right hand side the term is rather small; the other term also remains nearly constant. As a consequence, flow field similarity and turbulent flame speed correlations do explain why the flame can be stabilized aerodynamically and remains stationary through a wide range of velocities and equivalence ratios [13-15].

(10)

% Hydrogen Volume Fraction

EIN

O(g

NO

/kgf

uel)

10 20 30 40 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7 φ=0.7

T/Tref

EIN

O(O

2B

ased

)

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.20

0.05

0.1

0.15

0.2

0.25%0 H2%10 H2%20 H2%30 H2%40 H2%50 H2

Fig. 14 Relationship between adiabatic flame temperature and emissions index in a wide range of load conditions (0.4<φ<1.4, 2900<ReD<8200, Sw=0.74).


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5. Discussion and conclusion

Hydrogen-enriched confined methane combustion is studied in a laboratory scale premixed combustor. Whenever the equivalance of hydrocarbon fuels is of concern, the gas turbine industry utilizes a parameter called Wobbe index defined as per Eq. 11. Here HHV indicates the higher heating value. For pure methane this index is 12.7 whereas it 11.5 for pure hydrogen.

(11)

Hydrogen enrichment both increases the adiabatic flame temperatures and the flame speeds. Since the flame propagation speed of hydrogen is much faster that of methane shifts in the combustion regime were observed. Hydrogen enrichment increases nitric oxide emissions due to rising adiabatic flame temperatures, however at the same time this also enables very lean mixtures to be burnt (thus reducing flame temperatures) and the overall effect becomes a reduction in the nitric oxide emissions. It is thought that the thermal path (Zeldovich mechanism) is dominant in the production of nitric oxides. Correlating parameters for lean blowout (LBO), pressure amplitudes and emissions are examined. Two loading parameters are examined to correlate the lean blowout results: one based on a flamelet approach the other based on a well-stirred reactor approach. Dominant acoustic mode can go sudden changes with increasing hydrogen content. Such behavior is observed for the present combustor beyond 40% H2 by volume and suddenly a low frequency mode becomes dominant. Furthermore with the conventional swirl vane flame flashback becomes a troubling issue with increasing hydrogen content within the fuel mixture. These conclusions altogether suggest that Wobbe index is not an appropriate scaling parameter when methane/hydrogen mixtures are of concern. Due to profound changes in flame behavior, a novel design methodology is needed in order to burn hydrogen rich fuels in premixed gas turbine combustors. The divergent flow structure offered by the low swirl vane suggests that it would be the appropriate solution for premixed methane/hydrogen fuel mixtures. The flame in this configuration would be stabilized purely by aerodynamics and since at each load condition the aerodynamics would be such that an acceleration or deceleration would be balanced by the oncoming flow structure the flame would burn without actually touching anywhere inside the combustor (i.e. a center body would not be necessary for flame holding). Thus the disparity between flame speeds of methane and hydrogen would not be of concern. Moreover, this approach would offer a simple retrofit for the existing gas turbines. Replacing the conventional swirler with a low swirl vane shall fix the problem and offer a wide range of fuel variability. This aerodynamically stabilized hydrogen enrichment approach then indeed can pave the road towards a full hydrogen economy in the future.

Acknowledgements The support by Turkish Scientific and Technical Research Council (TÜBİTAK) through grant number 109M426 is gratefully acknowledged.

Fig. 15 Diverging flow pattern past a low swirl vane, ReD=50700, Sw=0.62 (velocities non-dimensionalized with respect to maximum axial velocity).


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