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Stability limits and exhaust NO performances of ammonia-methane-air swirl flames
Journal Pre-proofs Stability limits and exhaust NO performances of ammonia-methane-air swirl flames Abdulrahman A. Khateeb, Thibault F. Guiberti, Xuren Zhu, Mourad Younes, Aqil Jamal, William L. Roberts PII: DOI: Reference:
S0894-1777(19)32025-4 https://doi.org/10.1016/j.expthermflusci.2020.110058 ETF 110058
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
28 November 2019 6 January 2020 22 January 2020
Please cite this article as: A.A. Khateeb, T.F. Guiberti, X. Zhu, M. Younes, A. Jamal, W.L. Roberts, Stability limits and exhaust NO performances of ammonia-methane-air swirl flames, Experimental Thermal and Fluid Science (2020), doi: https://doi.org/10.1016/j.expthermflusci.2020.110058
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Stability limits and exhaust NO performances of ammonia-methane-air swirl flames Abdulrahman A. Khateeb1, Thibault F. Guiberti1, Xuren Zhu1, Mourad Younes2, Aqil Jamal2 William L. Roberts1 1King
Abdullah University of Science and Technology (KAUST), CCRC, Thuwal 23955-6900, Saudi Arabia. 2Saudi
Aramco, Dhahran 31311, Saudi Arabia.
Corresponding author email: [email protected]
Highlights
Pure ammonia-air swirl flames can be stabilized for a wide range of equivalence ratios.
Ammonia addition to methane reduces the flames’ propensity to flashback.
Ammonia addition to methane widens the stable range of operation.
Good NO performances are only achieved for slightly-rich equivalence ratios.
Abstract Ammonia is a promising fuel for heat and power generation because it is carbon-free and it can be produced from abundant chemicals and renewable energy sources. However, ammonia-air mixtures feature a low reactivity and stabilizing turbulent ammonia-air flames is challenging. In this study, the stability limits of technically-premixed ammonia-methane-air mixtures are measured for a wide range of ammonia additions in a laboratory-scale swirl burner. Results are compared to that obtained for baseline methane-air mixtures. Data show that increasing the ammonia addition increases the equivalence ratio at the lean blowout limit but also reduces the flames’ propensity to flashback. If the volume fraction of ammonia in the fuel blend exceeds a critical value, experiments also show that increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blow-out occurs instead. This significantly widens the range of equivalence ratios yielding stable flames. The critical ammonia volume fraction is a function of the Reynolds number and is xNH3 = 0.42 for Re = 7000 and xNH3 = 0.70 for Re = 3000. Because the ability to stabilize ammonia-methane-air flames is not practically relevant if NOx emissions are 1
too large compared to that of conventional methane-air flames, exhaust NO concentrations are also measured. Consistent with previous studies, data show that, for non-marginal ammonia fuel fractions, good NO performances are not found for lean ammonia-methane-air mixtures and can only be achieved with slightly rich mixtures, which are within measured stability limits. Keywords: Ammonia; Methane; Swirl flame; Stability limits; NO Nomenclature xNH3 𝑚𝐴 ub 𝜌 t Sg dA Sl Re r0 𝑚𝜃 At 𝜇 VNH3 VCH4
Ammonia volume fraction in the fuel blend Axial momentum Bulk velocity Density Equivalence ratio Exposure time Geometric swirl number as given by Eq. 1 Injection throat inner diameter Laminar burning velocity Reynolds number as given by Eq. 3 Tangential inlet radius (inlet b in Fig. 1) Tangential momentum Total area of tangential inlets (inlet b in Fig. 1) Dynamic viscosity Volumetric flow rate of ammonia Volumetric flow rate of methane
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1. Introduction Global warming is one of the greatest challenges that humanity is facing. Mitigation of global warming must be achieved by a combination of strategies including the improvement of energy efficiency and the reduction of the emissions of greenhouse gases, such as carbon dioxide. There is a strong potential for reductions of greenhouse gases emissions via the use of eco-friendly technologies. Among others, technologies relying on renewable energy sources are particularly promising. Renewable energy sources, such as solar and wind, are intermittent and issues related to the storage of electricity currently limit their potential [1]. Therefore, combustion technologies relying on energy dense fuels can play an important role in supporting the growth of renewable energies. Carbon-free fuels such as hydrogen and ammonia can be produced from abundant chemicals using renewable energy [2] and can be stored, transported, and later burnt in gas turbines, furnaces, and boilers. Compared to typical hydrocarbons, hydrogen has a poor volumetric energy density and must be compressed or liquefied to achieve high energy density. This raises challenges related to cost and safety [3]. Alternatively, hydrogen can be stored chemically in a liquid hydrogen carrier such as ammonia, NH3. Due to its use in most fertilizers, infrastructures for the production and distribution of ammonia are widely available [4–6]. In addition, the volumetric energy density of ammonia is large, roughly half of that of ethanol. This makes ammonia a good candidate for the chemical storage of renewable energies. However, the use of ammonia as a fuel for gas turbines, furnaces, and boilers raises a number of challenges. Ammonia-air mixtures are much less reactive than more conventional methane-air mixtures [3,7] and this is an issue for flame ignition and stabilization [8–10]. In addition, ammonia-
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air flames tend to yield large NOx emissions [11,12]. The body of knowledge related to the use of ammonia in gas turbines, furnaces, and boilers is limited, with a relatively small number of published experimental studies. Early research studies related to ammonia combustion were published in the 1960s and showed that ammonia has a higher ignition energy than conventional hydrocarbon fuels such as methane [13–15]. Verkamp et al. (1967) [14] investigated the flammability limits of pure ammonia-air mixtures and concluded that the equivalence ratio range () yielding ammonia-air flames is relatively narrow, 0.73, compared to that of methane-air flames, 1.15. Pratt (1967) [15] attempted to burn ammonia in a gas turbine and concluded that this was a challenging task due to the weak reactivity of this fuel. Recently, Hayakawa et al. (2015) [16] measured the laminar burning velocity (Sl) of pure ammonia-air flames for different pressures and equivalence ratios. Authors reported a maximum laminar burning velocity of Sl 7 cm/s for ammonia ( 1.05, 298 K, and 1 bar), about one fifth that of methane, Sl 35 cm/s. Han et al. (2019) [17] measured the laminar burning velocities for ammonia blended with hydrogen, carbon monoxide, and methane. Authors concluded that the low burning velocity of ammonia can be boosted by these blends, and that blending with hydrogen is the most effective. Recent experiments in laboratory-scale swirl burners at atmospheric [7,9,11,18,19] and elevated pressure [20] and industrial micro gas turbines [12,21,22] demonstrated that it is possible to stabilize turbulent swirl flames of pure ammonia or ammonia blended with methane [9,19–21] or hydrogen [8,18]. However, these studies all targeted very specific ammonia volume fractions in the fuel blend and consistent measurements of stability limits for large ranges of ammonia volume fractions (from no ammonia to pure ammonia) were never reported for a single burner
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configuration. This is the first objective of the present study and it is done with ammonia-methane fuel blends in an atmospheric laboratory-scale swirl burner. Being able to stabilize swirl flames fueled by ammonia is not practically relevant if emissions, including NOx, are too large. The propensity of ammonia-air flames to yield large NO emissions is then another challenge associated with the use of ammonia as a fuel [23]. In the recent years, the NO chemistry in ammonia-air, ammonia-methane-air, and ammonia-hydrogen-air flames has received as much, if not more, attention than flame stabilization [21,24,25]. When ammonia is present, NO can be produced through the three classical chemical pathways; thermal-NO, promptNO, and fuel-NO [26]. Due to the presence of nitrogen in ammonia, the fuel-NO pathway can play a large role, which is not the case for methane and hydrogen, potentially yielding exhaust NO emissions far exceeding current regulations. Consistent across different fuel blends and burner configurations, experiments [21–24] and simulations [20,25] demonstrated that burning ammonia blends at lean equivalence ratios typical of gas turbines yields very poor NO performances [20– 25]. However, resorting to slightly rich equivalence ratios allows recovering acceptable NO performances in par with that of lean premixed methane-air flames [21]. If associated with a secondary combustion stage, rich burning of ammonia blends then becomes an acceptable strategy [27]. As for stability limits, measurements of exhaust NO emissions across large ranges of equivalence ratios and ammonia fuel fractions in the fuel blend for a single burner configuration are scarce. Filling this gap is the second objective of the present study.
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2. Experimental setup and methods In this study, a laboratory-scale generic swirl burner is used and its main features are shown in Figure 1. This swirl burner has initially been developed for swirl flames of highly oxygenated fuels and it has been modified to accommodate ammonia-air flames. Details about this burner are available in [28] and [29] and only the burner’s main features are described here.
Figure 1. Cartoon rendering of the swirl burner showing where reactants are injected in the plenum; (a) two axial air inlets, (b) four tangential fuel-air inlets (dimensions are in mm).
Reactants are injected into the plenum via two axial inlets installed at the bottom of the assembly (inlets a in Figure 1) and four tangential inlets (inlets b in Figure 1), regularly distributed azimuthally and located 140 mm upstream of the burner outlet. Only air flows through inlets a while a mixture of fuel and air flows through inlets b. A ceramic honeycomb of 20-mm thickness is located 80 mm downstream of inlets a to distribute the air uniformly before mixing with the fuel-air mixture.
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Swirl is imparted to the flow because of tangential injection [30]. The swirl number can be adjusted by varying tangential momentum 𝑚𝜃 and the axial momentum 𝑚𝐴. The swirl number is defined here by Equation (1) [31,32]: Sg =
𝜋𝑟0𝑑𝐴
(
𝑚𝜃 𝑚𝜃 + 𝑚𝐴
2𝐴𝑡
2
)
(1)
where r0 stands for the radius of each tangential inlet, dA is the diameter of the injector throat, and At denotes the total area of the four tangential inlets. The flame is confined within a cylindrical ceramic wall to mitigate heat losses and flat quartz windows are mounted on one side to provide optical access to the flame. The cylindrical ceramic wall is 330-mm in length and it has a diameter of 152 mm. A refractory chimney rests on top of the ceramic confinement. Effects of the Reynolds number on the flames’ topology and stability limits are of interest. The Reynolds number is defined here with bulk flow properties: Re =
𝜌𝑑𝐴𝑢𝑏 𝜇
(2)
where 𝜌 stands for the density of the fuel-air mixture, dA is the diameter of the injector throat (44.4 mm), ub stands for the bulk velocity at the injector throat, and 𝜇 is the dynamic viscosity of the fuel-air mixture. Different ammonia additions to a reference methane-air mixture are examined, spanning the full range of ammonia fuel fractions from 𝑥𝑁𝐻3 = 0.00 to 1.00. The ammonia fuel fraction is adjusted by varying the ratio of the volumetric flow rate of ammonia VNH3 to the volumetric flow rate of methane VCH4. The ammonia fuel fraction value is defined here by Equation (3): 𝑥𝑁𝐻3 =
𝑉𝑁𝐻3 𝑉𝑁𝐻3 + 𝑉𝐶𝐻4
(3) 7
The flow rates of ammonia, methane, and air are prescribed with digital flow controllers (MKS Instruments) with an accuracy better than 2 %. While a fraction of the total air is perfectly mixed with the fuel blend prior to injection in inlets b, mixing with the remaining air, injected through inlets a, occurs within the burner. Therefore, it is possible that the reactant mixture is not perfectly premixed at the injection plane. However, it was verified using non-reactive RANS simulations that the mixture inhomogeneities at the injection plane was smaller than 3.5 %. One may refer to these flames as technically-premixed and the equivalence ratio () mentioned in what follows stands for the global equivalence ratio. Furthermore, careful examination of the time-averaged broadband flame images (shown later) confirm that if inhomogeneity exists, it is not large. Such images are recorded with a digital single lens reflex camera (DSLR Nikon D700, AF-S NIKKOR 24-70 mm F2.8G ED), for exposure times ranging from t = 0.60 to 2.00 seconds, an aperture of f/7.1, and an ISO number of 250. Each fuel-air mixture is initially ignited at an equivalence ratio slightly higher than that of the lean blowout limit. To measure stability limits, the fuel and air flow rates are progressively adjusted with a rate of change of 0.01 per second until lean blowout, rich blowout, or flashback occurs. For each test, measurements are repeated at least three times. For exhaust NO concentration measurements, combustion products are sampled on the centerline of the burner at the chimney’s outlet, 450 mm downstream of the injection plane, using a stainless-steel probe. By probing samples at different radial locations of the chimney’s outlet, it was verified that the stream of exhaust products is uniform. The stainless-steel probe, featuring a 4-mm inner diameter and an 80-mm length, is connected to a thermally-insulated flexible tube that feeds a gas analyzer (Horiba, model VA-3000, chemiluminescent sensor). The flexible tube is heated to 150C to prevent water condensation.
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Mixtures of nitrogen (99.9% purity) with 500/1500/3000 ppm NO concentrations are used for calibration. For each test condition, measurements are repeated at least three times and reproducibility is within 5% of the mean. The sensitivity of the gas analyzer is around 100 ppm and its precision is ±1%. The measured NO concentrations are corrected for 6% of O2. Table 1 shows the relevant fundamental combustion characteristics of ammonia compared to methane. The densities of ammonia and methane are close, at 1 bar and in gas phase. However, the heating value of ammonia is 19 MJ/kg which is less than half of that of methane 50 MJ/kg. At atmospheric pressure, the maximum laminar burning velocity of ammonia is Sl 7.00 cm/s [4], about one fifth that of methane with Sl 35 cm/s. The flammable range of ammonia-air flames is quite narrow with 0.83 compared to 1.21 for methane-air flames. Table 1. Fundamental combustion characteristics of ammonia and methane. Data are taken from [4,7].
Autoignition temperature Adiabatic flame temperature (@ = 1) Density (𝝆) Flammability limit range () Heating value Laminar burning velocity (Sl)
Unit
NH3
CH4
K K kg/m3 MJ/kg cm/s
930 1850 0.73 0.63 – 1.46 19 7
859 2223 0.66 0.48 – 1.69 50 35
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3. Results and discussion Understanding effects of ammonia addition on the flame stability requires a well-characterized reference. Here, mixtures of methane and air, that do not include ammonia, are used as a baseline and flame topology and stability limits are measured for ranges of swirl number and Reynolds number. Measurements are first conducted for baseline methane-air mixtures to provide a suitable understanding of the burner’s behavior as well as choosing an appropriate swirl number and Reynolds number for further examination. Effects of ammonia addition on the flames’ topology and on stability limits are examined later in Secs. 3.2 and 3.3. Effects of ammonia addition on the exhaust NO concentrations are studied in Sec. 3.4. 3.1. Stability limits of baseline methane-air flames Figure 2 shows time-averaged broadband images of methane-air flames for five different swirl numbers ranging from Sg = 0.50 to 1.50 for a Reynolds number Re = 5000 and an equivalence ratio = 0.60. The exposure time is set to t = 2.00 seconds.
Figure 2. Time-averaged broadband images of methane-air flames for five different swirl numbers ranging from Sg = 0.50 to 1.50 for Re = 5000 and = 0.60.
While all the swirl flames feature a conical V shape, increasing the swirl number increases the flame’s base angle and makes the flame more compact. This is an expected feature. Since the fuel mass flow rate and equivalence ratio () are kept constant, the more intense chemiluminescence 10
(here mainly from the CH* radical) observed for the largest swirl number highlights how increasing the swirl number increases the volumetric heat-release rate. Figure 3 shows the measured stability limits (red and black symbols) of methane-air flames as a function of the swirl number for different Reynolds numbers (Re = 4000, 6000, and 8000). The flammability limits of methane-air mixtures are also provided (red and black lines) and the grey area is the flammable range. Consistent with expectations, the swirl flames’ stability limits are within the flammable range. However, the stable range (blue area) is a very small subset of the flammable range and is limited to lean equivalence ratios. Comparisons of square, circle, and triangle symbols shows that the stability limits are fairly independent of the Reynolds number. Data also shows that stability limits are not a strong function of the swirl number. The range of stable equivalence ratios is smallest for Sg = 0.75 and largest for Sg = 1.50 but, overall, lean blowout occurs for 0.5 and flashback occurs for 0.7. The swirl number Sg = 0.75 roughly corresponds to the largest swirl number for which part of the flame is still located inside of the injection tube and is also expected to be close to the swirl number yielding vortex breakdown [33]. The sudden change of flame structure, most-likely associated to vortex breakdown, may explain the reduced stable range found for Sg = 0.75. Velocity fields were not measured in this study and this hypothesis cannot be verified.
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Figure 3. Stability limits of methane-air swirl flames as a function of the swirl number for Re = 4000 (square), Re = 6000 (circle), and Re = 8000 (triangle). The grey area corresponds to the flammable range and is bounded by the lean (red line) and rich (black line) flammability limits. The blue area shows the stable range bounded by lean blowout (red symbols) and flashback (black symbols) limits.
With this burner and methane-air mixtures, the lean blowout occurs at an equivalence ratio close to the lean flammable limit and flashback also occurs for rather lean mixtures, yielding a narrow stable range. However, less reactive ammonia-methane-air mixtures are targeted in this study and these are expected to be less prone to flashback. Based on Figure 3, values Sg = 1.00 and Re = 5000 yield a suitable flame topology -the flame is compact and not protruding inside the injection tube- and are therefore selected for the remaining of this study. 3.2. Stability limits of ammonia-methane-air flames Effects of ammonia addition on the flames’ topology and stability limits are now examined. Figure 4 shows time-averaged broadband images of ammonia-methane-air flames for different ammonia additions xNH3. The thermal power, equivalence ratio, and camera’s exposure time are all varied in Figure 4 (see Table 2) so quantitative comparisons are not meaningful. The swirl number is fixed to Sg = 1.00 and the Reynolds number is fixed to Re = 5000. Note that it is not
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possible to stabilize flames with xNH3 = 0 and xNH3 = 1 for the same equivalence ratio due to different stability limits (shown later). Ammonia addition does not seem to have a large influence on the global flame structure. However, one can observe the orange-yellow color that increases in intensity with increased ammonia addition. This color is mainly attributed to the NH2 ammonia alpha band [34], and is sometimes referred to as nitrogen glow [35]. Table 2. Thermal power (Pth), equivalence ratio (), and camera’s exposure time (t) for the broadband images of ammonia-methane-air flames shown in Figure 4. xNH3
Pth (kW)
t (s)
0.00
5.00
0.58
2.00
0.25
5.50
0.64
1.30
0.50
6.00
0.71
1.00
0.75
6.50
0.86
0.77
1.00
7.00
0.92
0.63
Figure 4. Time-averaged broadband images of ammonia-methane-air swirl flames for five different ammonia additions ranging from xNH3 = 0.00 to 1.00 for Sg = 1.00 and Re = 5000. Figure 5 shows the flammability and stability limits of ammonia-methane-air flames for ammonia additions ranging from xNH3 = 0.00 to 1.00. The swirl number is fixed to Sg = 1.00 and the Reynolds number is fixed to Re = 5000. Note that the flammability limits are not measured here and are linearly interpolated between that reported in the literature for methane-air and ammonia-air mixtures (see Table 1). The flammable range (grey area) narrows with increased ammonia addition. Replacing the methane by ammonia in the fuel mixture reduces the equivalence ratio () of the rich flammability limit from = 1.69 to = 1.46, while the equivalence ratio () 13
of the lean flammability limit increases from = 0.48 for methane-air to = 0.63 for ammoniaair [7].
Figure 5. Stability limits of ammonia-methane-air swirl flames as a function of the ammonia fuel fraction for Sg = 1.00 and Re = 5000. The grey area shows the flammable range bounded by the lean (red line) and rich (black line) flammability limits. The blue area shows the flame stability range bounded by lean blowout (red triangles), flashback (black circles), or rich blowout (blue squares). Error bars give an idea of the variability of the data over at least three samples.
Figure 5 shows that the range of stable equivalence ratios widens with ammonia addition and that it becomes relatively large for ammonia additions larger than 50%, where some transition occurs. In contrast with methane-air mixtures (see Figure 3), stability is now bounded by three different limits; the lean blowout limit (red triangles), the flashback limit (black circles), and now also the rich blowout limit (blue squares). Consistent with expectations and with lean flammability limits (red solid line), the equivalence ratio at lean blowout increases regularly with ammonia addition. It is = 0.50 for pure methane (xNH3 = 0.00) and = 0.79 for pure ammonia (xNH3 = 1.00). Up until xNH3 = 0.50, the equivalence ratio at flashback also increases regularly with ammonia addition. The rate of increase of the equivalence ratio yielding flashback is larger for
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xNH3 > 0.40 than for xNH3 < 0.40. The equivalence ratio at flashback increases from = 0.83 for xNH3 = 0.40 to 0.96 for xNH3 = 0.50. In addition to a flashback limit, a rich blowout limit is also provided for xNH3 = 0.50. This is because, depending on the test number, flashback or rich blowout occurred during measurements. This behavior is attributed to the fact that the equivalence ratio at flashback is close to stoichiometric for xNH3 = 0.50. Han et al. (2019) [17] reported that regardless of the ammonia fuel fraction, the equivalence ratio yielding the largest laminar burning velocity is 1.05. It can then be argued that this equivalence ratio would also yield the largest turbulent burning velocity and, in turn, would feature the largest propensity to flashback, regardless of ammonia addition. Extrapolation of the flashback limit (black symbols) to ammonia additions larger than xNH3 = 0.50 suggests that flashback should occur for equivalence ratios larger than 1.05, which is not possible if flashback did not already occur for 1.05. Figure 5 shows that, for Re = 5000, conditions are not met for ammonia-methane-air flames to flashback if xNH3 > 0.50, and rich blowout occurs instead, at a much larger equivalence ratio. The large error bar found for the flashback limit for xNH3 = 0.50 is due to this transitional behavior. Consistent with rich flammability limits, the equivalence ratio at rich blowout decreases regularly with ammonia addition. It is = 1.40 for xNH3 = 0.50 and = 1.24 for xNH3 = 1.00. Transition from flashback to rich blowout as the ammonia addition is increased above 50% is due to the weaker reactivity of ammonia compared to methane and it explains why the range of stability becomes much wider than that of methane-air with this burner. Additional experiments were conducted for different Reynolds numbers to assess the universality of the xNH3 = 0.50 transitional ammonia addition.
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3.3. Effects of the Reynolds number on stability limits of ammonia-methane-air flames Figure 6 shows direct broadband images of ammonia-methane-air flames for different Reynolds numbers ranging from Re = 3000 to 7000, for Sg = 1.00, = 1.00, and xNH3 = 0.80. The bulk velocity increases from ub = 1.00 m/s at Re = 3000 to ub = 2.33 m/s at Re = 7000. The exposure time is fixed to t = 1.30 seconds. Figure 6 shows that the flame shape is fairly insensitive to the Reynolds number if Re > 3000. The flame length increases slowly with the Reynolds number, except for Re = 3000, where the flame is slightly longer than that for Re = 4000. This is probably due to flow laminarization. The chemiluminescence intensity is largest for the largest Reynolds number, which is associated to more intense turbulence and volumetric heat-release rate.
Figure 6. Time-averaged broadband images of ammonia-methane-air swirl flames for five different Reynolds number ranging from Re = 3000 to 7000 for the Sg = 1.00, = 1.00, and xNH3 = 0.80.
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Figure 7. Flashback and rich blowout limits near the transitional ammonia addition for different Reynolds numbers Re = 3000 (green), Re = 4000 (gold), and Re = 7000 (orange) for Sg = 1.00. The blue area shows the stable range for Re = 5000 and is repeated from Figure 5. Figure 7 shows additional measured stability limits for three different Reynolds numbers Re = 3000 (green), Re = 4000 (gold), and Re = 7000 (orange). The stable range measured for Re = 5000 (blue area) is repeated from Figure 5 for comparison purposes. It is evident that the transitional behavior (flashback to rich blowout limit) can be found for different Reynolds numbers. Regardless of the Reynolds number, the largest equivalence ratio yielding flashback is close to stoichiometric. However, the transitional ammonia addition increases when the Reynolds number decreases. It is xNH3 = 0.42 for Re = 7000 and xNH3 = 0.70 for Re = 3000. This is because decreasing Reynolds number decreases proportionally the jet velocity and yields conditions more suitable for flashback. As a consequence, flashback continues to occur for less reactive mixtures featuring a smaller turbulent burning velocity and obtained for larger ammonia additions. Note that increasing Reynolds number for a fixed fuel blend does not change the laminar burning velocity but may increase the turbulent burning velocity. This is because turbulent velocity fluctuations become larger. Figure 7 shows that increasing Reynolds number retards flashback
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and, as a consequence, the potential increase of turbulent burning velocity with increasing Reynolds number does not offset the increase of jet velocity. To restrict the test matrix to a manageable size, stability limits for methane-ammonia blends were not measured for different swirl numbers. Nevertheless, effects of the swirl number on the sharp transition from flashback to rich blowout can be predicted. Increasing swirl has been shown before to promote flashback in comparable combustors (see for example [36]). As a consequence, one would assume that increasing swirl will shift transition from flashback to rich blowout to larger ammonia additions. Figure 7 shows that the burner selected in this study allows stabilizing ammonia-methane-air flames for large ranges of ammonia additions and equivalence ratios. It is worth mentioning that the range of stable equivalence ratios increases with ammonia addition, which is contrary to what typically intuited for ammonia. However, ammonia is known to be prone to large NO emissions [23] and the possibility to stabilize ammonia-methane-air flames is not sufficient to constitute practically relevant operating conditions. It is also required that NO emissions are comparable or smaller than that of typical methane-air flames. Therefore, NO emissions were also measured for many of the stable conditions described in Figs. 4-7. These results are now described. 3.4. Exhaust NO concentration in ammonia-methane-air flames Figure 8 shows the measured exhaust NO concentration as a function of equivalence ratio for four different ammonia additions xNH3 = 0.50, 0.60, 0.80, and 1.00 for Sg = 1.00 and Re = 5000. Note that for each ammonia addition, the range of available equivalence ratios is limited by the flames’ stability limits (see Figure 5).
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Figure 8. Measured exhaust NO concentration in parts per million (ppm) as a function of equivalence ratio for xNH3 = 0.50 (brown square), xNH3 = 0.60 (orange circle), xNH3 = 0.80 (yellow triangle), and xNH3 = 1.00 (green triangle) for Sg = 1.00 and Re = 5000. In this study, the largest measured NO concentration is 6540 ppm and it is for xNH3 = 0.50 and = 0.85. A few conditions yield NO concentrations smaller than the analyzer’s sensitivity ~100 ppm. For xNH3 = 0.60, the NO concentration increases when the equivalence ratio increases from = 0.70 to = 0.85. Although some data points are missing due to blowout, this trend most likely applies as well to the other ammonia additions investigated here: xNH3 = 0.50, 0.80, and 1.00. For > 0.85, the NO concentration decreases when the equivalence ratio decreases. The NO concentration is smaller than 100 ppm for xNH3 = 0.50 and = 1.10. These trends are compatible with that recently measured in similar premixed swirl flames by Okafor et al. [21]. Overall, reasonably low NO concentrations (< 100 ppm) can only be found for stoichiometric or rich equivalence ratios, which is consistent with previous studies [11,21]. Figure 8 also shows that for a fixed equivalence ratio, the NO concentration decreases when the ammonia addition increases. For = 0.85, increasing the ammonia addition from xNH3 = 0.50 to xNH3 = 1.00 decreases the NO concentration by a factor of ~ 6 from 6540 ppm to 1025 ppm. 19
Effects of the ammonia addition are further analyzed with Figure 9 that plots the measured NO concentration as a function of the ammonia addition for different equivalence ratios = 0.70, 0.85, 1.00, and 1.05. For xNH3 > 0.50 and 0.70, the NO concentration consistently decreases with ammonia addition. For = 0.70, the NO concentration increases slightly when the ammonia addition increases from xNH3 = 0.20 to 0.50. These trends are in agreement with those reported earlier by others [19,24,37]. Figure 8 shows that pure ammonia-air flames have better NO performances than ammoniamethane-air flames across a large range of equivalence ratios. This is an expected feature [23,38,39] that is attributed to the role of OH radicals in the NO chemical pathways of ammoniamethane-air flames. Large OH radical concentrations promote the oxidation of NH2 and NH radicals leading to NO production [40,41]. The production of OH radicals is smaller in pure ammonia-air flames than in ammonia-methane-air flames, yielding lower NO concentrations. Similarly, large OH radical concentrations are found for slightly lean equivalence ratios ( ~ 0.85) compared to rich cases, promoting NO. These slightly lean equivalence ratios do not lead to large NO concentrations in methane-air flames because NH2 and NH radicals are not present. Table 1 shows that increasing ammonia addition at a fixed equivalence ratio reduces the adiabatic flame temperature. Therefore, it is reasonable to think that increasing ammonia addition at a fixed equivalence ratio reduces the temperature downstream of the swirl flame. As a consequence, it is possible that thermal NOx pathways also play some role in the reduction of the NO concentration observed in Fig. 9. However, revealing the relative contribution of thermal NO production to the total NO production requires a detailed analysis of all chemical pathways and is beyond the scope of this study.
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Figure 9. Measured exhaust NO concentrations in parts per million (ppm) as a function of ammonia addition for different equivalence ratios = 0.70 (gold diamond), 0.85 (blue pentagon), 1.00 (navy triangle), and 1.05 (brown triangle) for Sg = 1.00 and Re = 5000. Figure 9 shows that good NO performances are obtained for slightly rich mixtures and Figure 5 shows that these equivalence ratios yield stable flames for a large range of ammonia additions. Therefore, these operating conditions are practically relevant. This was also reported before [11] for a subset of the operating conditions examined here. However, a second combustion stage is required to burn the excess ammonia and avoid efficiency and unburnt ammonia emissions penalties [21,42]. Figure 9 also shows that good NO performances cannot be obtained for nonmarginal ammonia additions and lean equivalence ratios. Even though the NO concentration decreases when the equivalence ratio becomes small enough (most-likely due to a reduced OH radical concentration), blowout occurs before good NO performances can be recovered. The tradeoff between NO and unburnt ammonia emissions implies that the current combustor performs optimally for ~ 1.10 for ammonia additions in excess of 50%. Recent work by Valera-Medina et al. (2017) [8] suggests that blending ammonia with hydrogen instead of methane alleviates this issue because flames with very lean equivalence ratios may be stabilized due to the highest
21
reactivity of hydrogen. Measuring the flames’ stability limits and NO performances for hydrogenammonia-air mixtures with this swirl burner is the topic of ongoing research. 4. Conclusions The stability limits and exhaust NO emissions of ammonia-methane-air flames were measured in an atmospheric laboratory-scale swirl burner. The main findings are:
Pure ammonia-air flames can be stabilized for a wide range of equivalence ratios (0.79 1.24).
Gradually replacing methane with ammonia increases the equivalence ratio leading to lean blowout and to flashback.
The range of equivalence ratios yielding stable flames widens when ammonia is added to the fuel blend.
Flashback defines the upper stability limits for ammonia additions smaller than a critical value. Above this critical ammonia addition, rich blowout controls the upper stability limit. The critical ammonia addition is the smallest ammonia addition for which the most reactive mixture with 1.05 does not yield conditions suitable for flashback. It is a function of the Reynolds number with xNH3 = 0.42 for Re = 7000, xNH3 = 0.50 for Re = 5000, and xNH3 = 0.70 for Re = 3000.
At a fixed equivalence ratio and if xNH3 ≥ 0.50, the exhaust NO concentration decreases with increased ammonia addition. For non-marginal ammonia additions, good NO performances (relative to lean methane-air flames) are only found for slightly rich ammonia-methane-air mixtures.
22
For ammonia-methane-air mixtures, decreasing equivalence ratio much below 0.85 could improve NO performances significantly but these very lean equivalence ratios cannot be reached with this swirl burner due to lean blowout.
Acknowledgments The paper is based upon work supported by Saudi Aramco Research and Development Center under research agreement number RGC/3/3837-01-01 and by the clean combustion research center at King Abdullah University of Science and Technology (KAUST). The collaborative research aims at supporting orderly energy transitions and managing emissions from hydrocarbons.
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Author statement Abdulrahman A. Khateeb: Conceptualization, Methodology, Investigation, Writing-Original Draft Thibault F. Guiberti: Conceptualization, Methodology, Writing-Review & Editing Xuren Zhu: Investigation Mourad Younes: Funding acquisition, Aqil Jamal: Funding acquisition William L. Roberts: Conceptualization, Supervision, Project administration, Funding acquisition.
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30
S0894-1777(19)32025-4 https://doi.org/10.1016/j.expthermflusci.2020.110058 ETF 110058
To appear in:
Experimental Thermal and Fluid Science
Received Date: Revised Date: Accepted Date:
28 November 2019 6 January 2020 22 January 2020
Please cite this article as: A.A. Khateeb, T.F. Guiberti, X. Zhu, M. Younes, A. Jamal, W.L. Roberts, Stability limits and exhaust NO performances of ammonia-methane-air swirl flames, Experimental Thermal and Fluid Science (2020), doi: https://doi.org/10.1016/j.expthermflusci.2020.110058
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Stability limits and exhaust NO performances of ammonia-methane-air swirl flames Abdulrahman A. Khateeb1, Thibault F. Guiberti1, Xuren Zhu1, Mourad Younes2, Aqil Jamal2 William L. Roberts1 1King
Abdullah University of Science and Technology (KAUST), CCRC, Thuwal 23955-6900, Saudi Arabia. 2Saudi
Aramco, Dhahran 31311, Saudi Arabia.
Corresponding author email: [email protected]
Highlights
Pure ammonia-air swirl flames can be stabilized for a wide range of equivalence ratios.
Ammonia addition to methane reduces the flames’ propensity to flashback.
Ammonia addition to methane widens the stable range of operation.
Good NO performances are only achieved for slightly-rich equivalence ratios.
Abstract Ammonia is a promising fuel for heat and power generation because it is carbon-free and it can be produced from abundant chemicals and renewable energy sources. However, ammonia-air mixtures feature a low reactivity and stabilizing turbulent ammonia-air flames is challenging. In this study, the stability limits of technically-premixed ammonia-methane-air mixtures are measured for a wide range of ammonia additions in a laboratory-scale swirl burner. Results are compared to that obtained for baseline methane-air mixtures. Data show that increasing the ammonia addition increases the equivalence ratio at the lean blowout limit but also reduces the flames’ propensity to flashback. If the volume fraction of ammonia in the fuel blend exceeds a critical value, experiments also show that increasing the equivalence ratio at a fixed bulk velocity does not yield flashback and rich blow-out occurs instead. This significantly widens the range of equivalence ratios yielding stable flames. The critical ammonia volume fraction is a function of the Reynolds number and is xNH3 = 0.42 for Re = 7000 and xNH3 = 0.70 for Re = 3000. Because the ability to stabilize ammonia-methane-air flames is not practically relevant if NOx emissions are 1
too large compared to that of conventional methane-air flames, exhaust NO concentrations are also measured. Consistent with previous studies, data show that, for non-marginal ammonia fuel fractions, good NO performances are not found for lean ammonia-methane-air mixtures and can only be achieved with slightly rich mixtures, which are within measured stability limits. Keywords: Ammonia; Methane; Swirl flame; Stability limits; NO Nomenclature xNH3 𝑚𝐴 ub 𝜌 t Sg dA Sl Re r0 𝑚𝜃 At 𝜇 VNH3 VCH4
Ammonia volume fraction in the fuel blend Axial momentum Bulk velocity Density Equivalence ratio Exposure time Geometric swirl number as given by Eq. 1 Injection throat inner diameter Laminar burning velocity Reynolds number as given by Eq. 3 Tangential inlet radius (inlet b in Fig. 1) Tangential momentum Total area of tangential inlets (inlet b in Fig. 1) Dynamic viscosity Volumetric flow rate of ammonia Volumetric flow rate of methane
2
1. Introduction Global warming is one of the greatest challenges that humanity is facing. Mitigation of global warming must be achieved by a combination of strategies including the improvement of energy efficiency and the reduction of the emissions of greenhouse gases, such as carbon dioxide. There is a strong potential for reductions of greenhouse gases emissions via the use of eco-friendly technologies. Among others, technologies relying on renewable energy sources are particularly promising. Renewable energy sources, such as solar and wind, are intermittent and issues related to the storage of electricity currently limit their potential [1]. Therefore, combustion technologies relying on energy dense fuels can play an important role in supporting the growth of renewable energies. Carbon-free fuels such as hydrogen and ammonia can be produced from abundant chemicals using renewable energy [2] and can be stored, transported, and later burnt in gas turbines, furnaces, and boilers. Compared to typical hydrocarbons, hydrogen has a poor volumetric energy density and must be compressed or liquefied to achieve high energy density. This raises challenges related to cost and safety [3]. Alternatively, hydrogen can be stored chemically in a liquid hydrogen carrier such as ammonia, NH3. Due to its use in most fertilizers, infrastructures for the production and distribution of ammonia are widely available [4–6]. In addition, the volumetric energy density of ammonia is large, roughly half of that of ethanol. This makes ammonia a good candidate for the chemical storage of renewable energies. However, the use of ammonia as a fuel for gas turbines, furnaces, and boilers raises a number of challenges. Ammonia-air mixtures are much less reactive than more conventional methane-air mixtures [3,7] and this is an issue for flame ignition and stabilization [8–10]. In addition, ammonia-
3
air flames tend to yield large NOx emissions [11,12]. The body of knowledge related to the use of ammonia in gas turbines, furnaces, and boilers is limited, with a relatively small number of published experimental studies. Early research studies related to ammonia combustion were published in the 1960s and showed that ammonia has a higher ignition energy than conventional hydrocarbon fuels such as methane [13–15]. Verkamp et al. (1967) [14] investigated the flammability limits of pure ammonia-air mixtures and concluded that the equivalence ratio range () yielding ammonia-air flames is relatively narrow, 0.73, compared to that of methane-air flames, 1.15. Pratt (1967) [15] attempted to burn ammonia in a gas turbine and concluded that this was a challenging task due to the weak reactivity of this fuel. Recently, Hayakawa et al. (2015) [16] measured the laminar burning velocity (Sl) of pure ammonia-air flames for different pressures and equivalence ratios. Authors reported a maximum laminar burning velocity of Sl 7 cm/s for ammonia ( 1.05, 298 K, and 1 bar), about one fifth that of methane, Sl 35 cm/s. Han et al. (2019) [17] measured the laminar burning velocities for ammonia blended with hydrogen, carbon monoxide, and methane. Authors concluded that the low burning velocity of ammonia can be boosted by these blends, and that blending with hydrogen is the most effective. Recent experiments in laboratory-scale swirl burners at atmospheric [7,9,11,18,19] and elevated pressure [20] and industrial micro gas turbines [12,21,22] demonstrated that it is possible to stabilize turbulent swirl flames of pure ammonia or ammonia blended with methane [9,19–21] or hydrogen [8,18]. However, these studies all targeted very specific ammonia volume fractions in the fuel blend and consistent measurements of stability limits for large ranges of ammonia volume fractions (from no ammonia to pure ammonia) were never reported for a single burner
4
configuration. This is the first objective of the present study and it is done with ammonia-methane fuel blends in an atmospheric laboratory-scale swirl burner. Being able to stabilize swirl flames fueled by ammonia is not practically relevant if emissions, including NOx, are too large. The propensity of ammonia-air flames to yield large NO emissions is then another challenge associated with the use of ammonia as a fuel [23]. In the recent years, the NO chemistry in ammonia-air, ammonia-methane-air, and ammonia-hydrogen-air flames has received as much, if not more, attention than flame stabilization [21,24,25]. When ammonia is present, NO can be produced through the three classical chemical pathways; thermal-NO, promptNO, and fuel-NO [26]. Due to the presence of nitrogen in ammonia, the fuel-NO pathway can play a large role, which is not the case for methane and hydrogen, potentially yielding exhaust NO emissions far exceeding current regulations. Consistent across different fuel blends and burner configurations, experiments [21–24] and simulations [20,25] demonstrated that burning ammonia blends at lean equivalence ratios typical of gas turbines yields very poor NO performances [20– 25]. However, resorting to slightly rich equivalence ratios allows recovering acceptable NO performances in par with that of lean premixed methane-air flames [21]. If associated with a secondary combustion stage, rich burning of ammonia blends then becomes an acceptable strategy [27]. As for stability limits, measurements of exhaust NO emissions across large ranges of equivalence ratios and ammonia fuel fractions in the fuel blend for a single burner configuration are scarce. Filling this gap is the second objective of the present study.
5
2. Experimental setup and methods In this study, a laboratory-scale generic swirl burner is used and its main features are shown in Figure 1. This swirl burner has initially been developed for swirl flames of highly oxygenated fuels and it has been modified to accommodate ammonia-air flames. Details about this burner are available in [28] and [29] and only the burner’s main features are described here.
Figure 1. Cartoon rendering of the swirl burner showing where reactants are injected in the plenum; (a) two axial air inlets, (b) four tangential fuel-air inlets (dimensions are in mm).
Reactants are injected into the plenum via two axial inlets installed at the bottom of the assembly (inlets a in Figure 1) and four tangential inlets (inlets b in Figure 1), regularly distributed azimuthally and located 140 mm upstream of the burner outlet. Only air flows through inlets a while a mixture of fuel and air flows through inlets b. A ceramic honeycomb of 20-mm thickness is located 80 mm downstream of inlets a to distribute the air uniformly before mixing with the fuel-air mixture.
6
Swirl is imparted to the flow because of tangential injection [30]. The swirl number can be adjusted by varying tangential momentum 𝑚𝜃 and the axial momentum 𝑚𝐴. The swirl number is defined here by Equation (1) [31,32]: Sg =
𝜋𝑟0𝑑𝐴
(
𝑚𝜃 𝑚𝜃 + 𝑚𝐴
2𝐴𝑡
2
)
(1)
where r0 stands for the radius of each tangential inlet, dA is the diameter of the injector throat, and At denotes the total area of the four tangential inlets. The flame is confined within a cylindrical ceramic wall to mitigate heat losses and flat quartz windows are mounted on one side to provide optical access to the flame. The cylindrical ceramic wall is 330-mm in length and it has a diameter of 152 mm. A refractory chimney rests on top of the ceramic confinement. Effects of the Reynolds number on the flames’ topology and stability limits are of interest. The Reynolds number is defined here with bulk flow properties: Re =
𝜌𝑑𝐴𝑢𝑏 𝜇
(2)
where 𝜌 stands for the density of the fuel-air mixture, dA is the diameter of the injector throat (44.4 mm), ub stands for the bulk velocity at the injector throat, and 𝜇 is the dynamic viscosity of the fuel-air mixture. Different ammonia additions to a reference methane-air mixture are examined, spanning the full range of ammonia fuel fractions from 𝑥𝑁𝐻3 = 0.00 to 1.00. The ammonia fuel fraction is adjusted by varying the ratio of the volumetric flow rate of ammonia VNH3 to the volumetric flow rate of methane VCH4. The ammonia fuel fraction value is defined here by Equation (3): 𝑥𝑁𝐻3 =
𝑉𝑁𝐻3 𝑉𝑁𝐻3 + 𝑉𝐶𝐻4
(3) 7
The flow rates of ammonia, methane, and air are prescribed with digital flow controllers (MKS Instruments) with an accuracy better than 2 %. While a fraction of the total air is perfectly mixed with the fuel blend prior to injection in inlets b, mixing with the remaining air, injected through inlets a, occurs within the burner. Therefore, it is possible that the reactant mixture is not perfectly premixed at the injection plane. However, it was verified using non-reactive RANS simulations that the mixture inhomogeneities at the injection plane was smaller than 3.5 %. One may refer to these flames as technically-premixed and the equivalence ratio () mentioned in what follows stands for the global equivalence ratio. Furthermore, careful examination of the time-averaged broadband flame images (shown later) confirm that if inhomogeneity exists, it is not large. Such images are recorded with a digital single lens reflex camera (DSLR Nikon D700, AF-S NIKKOR 24-70 mm F2.8G ED), for exposure times ranging from t = 0.60 to 2.00 seconds, an aperture of f/7.1, and an ISO number of 250. Each fuel-air mixture is initially ignited at an equivalence ratio slightly higher than that of the lean blowout limit. To measure stability limits, the fuel and air flow rates are progressively adjusted with a rate of change of 0.01 per second until lean blowout, rich blowout, or flashback occurs. For each test, measurements are repeated at least three times. For exhaust NO concentration measurements, combustion products are sampled on the centerline of the burner at the chimney’s outlet, 450 mm downstream of the injection plane, using a stainless-steel probe. By probing samples at different radial locations of the chimney’s outlet, it was verified that the stream of exhaust products is uniform. The stainless-steel probe, featuring a 4-mm inner diameter and an 80-mm length, is connected to a thermally-insulated flexible tube that feeds a gas analyzer (Horiba, model VA-3000, chemiluminescent sensor). The flexible tube is heated to 150C to prevent water condensation.
8
Mixtures of nitrogen (99.9% purity) with 500/1500/3000 ppm NO concentrations are used for calibration. For each test condition, measurements are repeated at least three times and reproducibility is within 5% of the mean. The sensitivity of the gas analyzer is around 100 ppm and its precision is ±1%. The measured NO concentrations are corrected for 6% of O2. Table 1 shows the relevant fundamental combustion characteristics of ammonia compared to methane. The densities of ammonia and methane are close, at 1 bar and in gas phase. However, the heating value of ammonia is 19 MJ/kg which is less than half of that of methane 50 MJ/kg. At atmospheric pressure, the maximum laminar burning velocity of ammonia is Sl 7.00 cm/s [4], about one fifth that of methane with Sl 35 cm/s. The flammable range of ammonia-air flames is quite narrow with 0.83 compared to 1.21 for methane-air flames. Table 1. Fundamental combustion characteristics of ammonia and methane. Data are taken from [4,7].
Autoignition temperature Adiabatic flame temperature (@ = 1) Density (𝝆) Flammability limit range () Heating value Laminar burning velocity (Sl)
Unit
NH3
CH4
K K kg/m3 MJ/kg cm/s
930 1850 0.73 0.63 – 1.46 19 7
859 2223 0.66 0.48 – 1.69 50 35
9
3. Results and discussion Understanding effects of ammonia addition on the flame stability requires a well-characterized reference. Here, mixtures of methane and air, that do not include ammonia, are used as a baseline and flame topology and stability limits are measured for ranges of swirl number and Reynolds number. Measurements are first conducted for baseline methane-air mixtures to provide a suitable understanding of the burner’s behavior as well as choosing an appropriate swirl number and Reynolds number for further examination. Effects of ammonia addition on the flames’ topology and on stability limits are examined later in Secs. 3.2 and 3.3. Effects of ammonia addition on the exhaust NO concentrations are studied in Sec. 3.4. 3.1. Stability limits of baseline methane-air flames Figure 2 shows time-averaged broadband images of methane-air flames for five different swirl numbers ranging from Sg = 0.50 to 1.50 for a Reynolds number Re = 5000 and an equivalence ratio = 0.60. The exposure time is set to t = 2.00 seconds.
Figure 2. Time-averaged broadband images of methane-air flames for five different swirl numbers ranging from Sg = 0.50 to 1.50 for Re = 5000 and = 0.60.
While all the swirl flames feature a conical V shape, increasing the swirl number increases the flame’s base angle and makes the flame more compact. This is an expected feature. Since the fuel mass flow rate and equivalence ratio () are kept constant, the more intense chemiluminescence 10
(here mainly from the CH* radical) observed for the largest swirl number highlights how increasing the swirl number increases the volumetric heat-release rate. Figure 3 shows the measured stability limits (red and black symbols) of methane-air flames as a function of the swirl number for different Reynolds numbers (Re = 4000, 6000, and 8000). The flammability limits of methane-air mixtures are also provided (red and black lines) and the grey area is the flammable range. Consistent with expectations, the swirl flames’ stability limits are within the flammable range. However, the stable range (blue area) is a very small subset of the flammable range and is limited to lean equivalence ratios. Comparisons of square, circle, and triangle symbols shows that the stability limits are fairly independent of the Reynolds number. Data also shows that stability limits are not a strong function of the swirl number. The range of stable equivalence ratios is smallest for Sg = 0.75 and largest for Sg = 1.50 but, overall, lean blowout occurs for 0.5 and flashback occurs for 0.7. The swirl number Sg = 0.75 roughly corresponds to the largest swirl number for which part of the flame is still located inside of the injection tube and is also expected to be close to the swirl number yielding vortex breakdown [33]. The sudden change of flame structure, most-likely associated to vortex breakdown, may explain the reduced stable range found for Sg = 0.75. Velocity fields were not measured in this study and this hypothesis cannot be verified.
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Figure 3. Stability limits of methane-air swirl flames as a function of the swirl number for Re = 4000 (square), Re = 6000 (circle), and Re = 8000 (triangle). The grey area corresponds to the flammable range and is bounded by the lean (red line) and rich (black line) flammability limits. The blue area shows the stable range bounded by lean blowout (red symbols) and flashback (black symbols) limits.
With this burner and methane-air mixtures, the lean blowout occurs at an equivalence ratio close to the lean flammable limit and flashback also occurs for rather lean mixtures, yielding a narrow stable range. However, less reactive ammonia-methane-air mixtures are targeted in this study and these are expected to be less prone to flashback. Based on Figure 3, values Sg = 1.00 and Re = 5000 yield a suitable flame topology -the flame is compact and not protruding inside the injection tube- and are therefore selected for the remaining of this study. 3.2. Stability limits of ammonia-methane-air flames Effects of ammonia addition on the flames’ topology and stability limits are now examined. Figure 4 shows time-averaged broadband images of ammonia-methane-air flames for different ammonia additions xNH3. The thermal power, equivalence ratio, and camera’s exposure time are all varied in Figure 4 (see Table 2) so quantitative comparisons are not meaningful. The swirl number is fixed to Sg = 1.00 and the Reynolds number is fixed to Re = 5000. Note that it is not
12
possible to stabilize flames with xNH3 = 0 and xNH3 = 1 for the same equivalence ratio due to different stability limits (shown later). Ammonia addition does not seem to have a large influence on the global flame structure. However, one can observe the orange-yellow color that increases in intensity with increased ammonia addition. This color is mainly attributed to the NH2 ammonia alpha band [34], and is sometimes referred to as nitrogen glow [35]. Table 2. Thermal power (Pth), equivalence ratio (), and camera’s exposure time (t) for the broadband images of ammonia-methane-air flames shown in Figure 4. xNH3
Pth (kW)
t (s)
0.00
5.00
0.58
2.00
0.25
5.50
0.64
1.30
0.50
6.00
0.71
1.00
0.75
6.50
0.86
0.77
1.00
7.00
0.92
0.63
Figure 4. Time-averaged broadband images of ammonia-methane-air swirl flames for five different ammonia additions ranging from xNH3 = 0.00 to 1.00 for Sg = 1.00 and Re = 5000. Figure 5 shows the flammability and stability limits of ammonia-methane-air flames for ammonia additions ranging from xNH3 = 0.00 to 1.00. The swirl number is fixed to Sg = 1.00 and the Reynolds number is fixed to Re = 5000. Note that the flammability limits are not measured here and are linearly interpolated between that reported in the literature for methane-air and ammonia-air mixtures (see Table 1). The flammable range (grey area) narrows with increased ammonia addition. Replacing the methane by ammonia in the fuel mixture reduces the equivalence ratio () of the rich flammability limit from = 1.69 to = 1.46, while the equivalence ratio () 13
of the lean flammability limit increases from = 0.48 for methane-air to = 0.63 for ammoniaair [7].
Figure 5. Stability limits of ammonia-methane-air swirl flames as a function of the ammonia fuel fraction for Sg = 1.00 and Re = 5000. The grey area shows the flammable range bounded by the lean (red line) and rich (black line) flammability limits. The blue area shows the flame stability range bounded by lean blowout (red triangles), flashback (black circles), or rich blowout (blue squares). Error bars give an idea of the variability of the data over at least three samples.
Figure 5 shows that the range of stable equivalence ratios widens with ammonia addition and that it becomes relatively large for ammonia additions larger than 50%, where some transition occurs. In contrast with methane-air mixtures (see Figure 3), stability is now bounded by three different limits; the lean blowout limit (red triangles), the flashback limit (black circles), and now also the rich blowout limit (blue squares). Consistent with expectations and with lean flammability limits (red solid line), the equivalence ratio at lean blowout increases regularly with ammonia addition. It is = 0.50 for pure methane (xNH3 = 0.00) and = 0.79 for pure ammonia (xNH3 = 1.00). Up until xNH3 = 0.50, the equivalence ratio at flashback also increases regularly with ammonia addition. The rate of increase of the equivalence ratio yielding flashback is larger for
14
xNH3 > 0.40 than for xNH3 < 0.40. The equivalence ratio at flashback increases from = 0.83 for xNH3 = 0.40 to 0.96 for xNH3 = 0.50. In addition to a flashback limit, a rich blowout limit is also provided for xNH3 = 0.50. This is because, depending on the test number, flashback or rich blowout occurred during measurements. This behavior is attributed to the fact that the equivalence ratio at flashback is close to stoichiometric for xNH3 = 0.50. Han et al. (2019) [17] reported that regardless of the ammonia fuel fraction, the equivalence ratio yielding the largest laminar burning velocity is 1.05. It can then be argued that this equivalence ratio would also yield the largest turbulent burning velocity and, in turn, would feature the largest propensity to flashback, regardless of ammonia addition. Extrapolation of the flashback limit (black symbols) to ammonia additions larger than xNH3 = 0.50 suggests that flashback should occur for equivalence ratios larger than 1.05, which is not possible if flashback did not already occur for 1.05. Figure 5 shows that, for Re = 5000, conditions are not met for ammonia-methane-air flames to flashback if xNH3 > 0.50, and rich blowout occurs instead, at a much larger equivalence ratio. The large error bar found for the flashback limit for xNH3 = 0.50 is due to this transitional behavior. Consistent with rich flammability limits, the equivalence ratio at rich blowout decreases regularly with ammonia addition. It is = 1.40 for xNH3 = 0.50 and = 1.24 for xNH3 = 1.00. Transition from flashback to rich blowout as the ammonia addition is increased above 50% is due to the weaker reactivity of ammonia compared to methane and it explains why the range of stability becomes much wider than that of methane-air with this burner. Additional experiments were conducted for different Reynolds numbers to assess the universality of the xNH3 = 0.50 transitional ammonia addition.
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3.3. Effects of the Reynolds number on stability limits of ammonia-methane-air flames Figure 6 shows direct broadband images of ammonia-methane-air flames for different Reynolds numbers ranging from Re = 3000 to 7000, for Sg = 1.00, = 1.00, and xNH3 = 0.80. The bulk velocity increases from ub = 1.00 m/s at Re = 3000 to ub = 2.33 m/s at Re = 7000. The exposure time is fixed to t = 1.30 seconds. Figure 6 shows that the flame shape is fairly insensitive to the Reynolds number if Re > 3000. The flame length increases slowly with the Reynolds number, except for Re = 3000, where the flame is slightly longer than that for Re = 4000. This is probably due to flow laminarization. The chemiluminescence intensity is largest for the largest Reynolds number, which is associated to more intense turbulence and volumetric heat-release rate.
Figure 6. Time-averaged broadband images of ammonia-methane-air swirl flames for five different Reynolds number ranging from Re = 3000 to 7000 for the Sg = 1.00, = 1.00, and xNH3 = 0.80.
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Figure 7. Flashback and rich blowout limits near the transitional ammonia addition for different Reynolds numbers Re = 3000 (green), Re = 4000 (gold), and Re = 7000 (orange) for Sg = 1.00. The blue area shows the stable range for Re = 5000 and is repeated from Figure 5. Figure 7 shows additional measured stability limits for three different Reynolds numbers Re = 3000 (green), Re = 4000 (gold), and Re = 7000 (orange). The stable range measured for Re = 5000 (blue area) is repeated from Figure 5 for comparison purposes. It is evident that the transitional behavior (flashback to rich blowout limit) can be found for different Reynolds numbers. Regardless of the Reynolds number, the largest equivalence ratio yielding flashback is close to stoichiometric. However, the transitional ammonia addition increases when the Reynolds number decreases. It is xNH3 = 0.42 for Re = 7000 and xNH3 = 0.70 for Re = 3000. This is because decreasing Reynolds number decreases proportionally the jet velocity and yields conditions more suitable for flashback. As a consequence, flashback continues to occur for less reactive mixtures featuring a smaller turbulent burning velocity and obtained for larger ammonia additions. Note that increasing Reynolds number for a fixed fuel blend does not change the laminar burning velocity but may increase the turbulent burning velocity. This is because turbulent velocity fluctuations become larger. Figure 7 shows that increasing Reynolds number retards flashback
17
and, as a consequence, the potential increase of turbulent burning velocity with increasing Reynolds number does not offset the increase of jet velocity. To restrict the test matrix to a manageable size, stability limits for methane-ammonia blends were not measured for different swirl numbers. Nevertheless, effects of the swirl number on the sharp transition from flashback to rich blowout can be predicted. Increasing swirl has been shown before to promote flashback in comparable combustors (see for example [36]). As a consequence, one would assume that increasing swirl will shift transition from flashback to rich blowout to larger ammonia additions. Figure 7 shows that the burner selected in this study allows stabilizing ammonia-methane-air flames for large ranges of ammonia additions and equivalence ratios. It is worth mentioning that the range of stable equivalence ratios increases with ammonia addition, which is contrary to what typically intuited for ammonia. However, ammonia is known to be prone to large NO emissions [23] and the possibility to stabilize ammonia-methane-air flames is not sufficient to constitute practically relevant operating conditions. It is also required that NO emissions are comparable or smaller than that of typical methane-air flames. Therefore, NO emissions were also measured for many of the stable conditions described in Figs. 4-7. These results are now described. 3.4. Exhaust NO concentration in ammonia-methane-air flames Figure 8 shows the measured exhaust NO concentration as a function of equivalence ratio for four different ammonia additions xNH3 = 0.50, 0.60, 0.80, and 1.00 for Sg = 1.00 and Re = 5000. Note that for each ammonia addition, the range of available equivalence ratios is limited by the flames’ stability limits (see Figure 5).
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Figure 8. Measured exhaust NO concentration in parts per million (ppm) as a function of equivalence ratio for xNH3 = 0.50 (brown square), xNH3 = 0.60 (orange circle), xNH3 = 0.80 (yellow triangle), and xNH3 = 1.00 (green triangle) for Sg = 1.00 and Re = 5000. In this study, the largest measured NO concentration is 6540 ppm and it is for xNH3 = 0.50 and = 0.85. A few conditions yield NO concentrations smaller than the analyzer’s sensitivity ~100 ppm. For xNH3 = 0.60, the NO concentration increases when the equivalence ratio increases from = 0.70 to = 0.85. Although some data points are missing due to blowout, this trend most likely applies as well to the other ammonia additions investigated here: xNH3 = 0.50, 0.80, and 1.00. For > 0.85, the NO concentration decreases when the equivalence ratio decreases. The NO concentration is smaller than 100 ppm for xNH3 = 0.50 and = 1.10. These trends are compatible with that recently measured in similar premixed swirl flames by Okafor et al. [21]. Overall, reasonably low NO concentrations (< 100 ppm) can only be found for stoichiometric or rich equivalence ratios, which is consistent with previous studies [11,21]. Figure 8 also shows that for a fixed equivalence ratio, the NO concentration decreases when the ammonia addition increases. For = 0.85, increasing the ammonia addition from xNH3 = 0.50 to xNH3 = 1.00 decreases the NO concentration by a factor of ~ 6 from 6540 ppm to 1025 ppm. 19
Effects of the ammonia addition are further analyzed with Figure 9 that plots the measured NO concentration as a function of the ammonia addition for different equivalence ratios = 0.70, 0.85, 1.00, and 1.05. For xNH3 > 0.50 and 0.70, the NO concentration consistently decreases with ammonia addition. For = 0.70, the NO concentration increases slightly when the ammonia addition increases from xNH3 = 0.20 to 0.50. These trends are in agreement with those reported earlier by others [19,24,37]. Figure 8 shows that pure ammonia-air flames have better NO performances than ammoniamethane-air flames across a large range of equivalence ratios. This is an expected feature [23,38,39] that is attributed to the role of OH radicals in the NO chemical pathways of ammoniamethane-air flames. Large OH radical concentrations promote the oxidation of NH2 and NH radicals leading to NO production [40,41]. The production of OH radicals is smaller in pure ammonia-air flames than in ammonia-methane-air flames, yielding lower NO concentrations. Similarly, large OH radical concentrations are found for slightly lean equivalence ratios ( ~ 0.85) compared to rich cases, promoting NO. These slightly lean equivalence ratios do not lead to large NO concentrations in methane-air flames because NH2 and NH radicals are not present. Table 1 shows that increasing ammonia addition at a fixed equivalence ratio reduces the adiabatic flame temperature. Therefore, it is reasonable to think that increasing ammonia addition at a fixed equivalence ratio reduces the temperature downstream of the swirl flame. As a consequence, it is possible that thermal NOx pathways also play some role in the reduction of the NO concentration observed in Fig. 9. However, revealing the relative contribution of thermal NO production to the total NO production requires a detailed analysis of all chemical pathways and is beyond the scope of this study.
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Figure 9. Measured exhaust NO concentrations in parts per million (ppm) as a function of ammonia addition for different equivalence ratios = 0.70 (gold diamond), 0.85 (blue pentagon), 1.00 (navy triangle), and 1.05 (brown triangle) for Sg = 1.00 and Re = 5000. Figure 9 shows that good NO performances are obtained for slightly rich mixtures and Figure 5 shows that these equivalence ratios yield stable flames for a large range of ammonia additions. Therefore, these operating conditions are practically relevant. This was also reported before [11] for a subset of the operating conditions examined here. However, a second combustion stage is required to burn the excess ammonia and avoid efficiency and unburnt ammonia emissions penalties [21,42]. Figure 9 also shows that good NO performances cannot be obtained for nonmarginal ammonia additions and lean equivalence ratios. Even though the NO concentration decreases when the equivalence ratio becomes small enough (most-likely due to a reduced OH radical concentration), blowout occurs before good NO performances can be recovered. The tradeoff between NO and unburnt ammonia emissions implies that the current combustor performs optimally for ~ 1.10 for ammonia additions in excess of 50%. Recent work by Valera-Medina et al. (2017) [8] suggests that blending ammonia with hydrogen instead of methane alleviates this issue because flames with very lean equivalence ratios may be stabilized due to the highest
21
reactivity of hydrogen. Measuring the flames’ stability limits and NO performances for hydrogenammonia-air mixtures with this swirl burner is the topic of ongoing research. 4. Conclusions The stability limits and exhaust NO emissions of ammonia-methane-air flames were measured in an atmospheric laboratory-scale swirl burner. The main findings are:
Pure ammonia-air flames can be stabilized for a wide range of equivalence ratios (0.79 1.24).
Gradually replacing methane with ammonia increases the equivalence ratio leading to lean blowout and to flashback.
The range of equivalence ratios yielding stable flames widens when ammonia is added to the fuel blend.
Flashback defines the upper stability limits for ammonia additions smaller than a critical value. Above this critical ammonia addition, rich blowout controls the upper stability limit. The critical ammonia addition is the smallest ammonia addition for which the most reactive mixture with 1.05 does not yield conditions suitable for flashback. It is a function of the Reynolds number with xNH3 = 0.42 for Re = 7000, xNH3 = 0.50 for Re = 5000, and xNH3 = 0.70 for Re = 3000.
At a fixed equivalence ratio and if xNH3 ≥ 0.50, the exhaust NO concentration decreases with increased ammonia addition. For non-marginal ammonia additions, good NO performances (relative to lean methane-air flames) are only found for slightly rich ammonia-methane-air mixtures.
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For ammonia-methane-air mixtures, decreasing equivalence ratio much below 0.85 could improve NO performances significantly but these very lean equivalence ratios cannot be reached with this swirl burner due to lean blowout.
Acknowledgments The paper is based upon work supported by Saudi Aramco Research and Development Center under research agreement number RGC/3/3837-01-01 and by the clean combustion research center at King Abdullah University of Science and Technology (KAUST). The collaborative research aims at supporting orderly energy transitions and managing emissions from hydrocarbons.
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Author statement Abdulrahman A. Khateeb: Conceptualization, Methodology, Investigation, Writing-Original Draft Thibault F. Guiberti: Conceptualization, Methodology, Writing-Review & Editing Xuren Zhu: Investigation Mourad Younes: Funding acquisition, Aqil Jamal: Funding acquisition William L. Roberts: Conceptualization, Supervision, Project administration, Funding acquisition.
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