(MILD) Combustion from a Single Jet Burner in a Laboratory-Scale

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Premixed Moderate or Intense Low-Oxygen Dilution (MILD) Combustion from a Single Jet Burner in a Laboratory-Scale Furnace Pengfei Li,† Jianchun Mi,*,† Bassam B. Dally,‡ Richard A. Craig,‡ and Feifei Wang† †

State Key Laboratory of Turbulence and Complex Systems, Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China ‡ Center of Energy Technology and School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia ABSTRACT: Impacts of initial conditions on the characteristics of premixed moderate or intense low-oxygen dilution (MILD) combustion from a single jet burner in a laboratory-scale furnace are investigated through Reynolds-averaged Navier
Stokes (RANS) modeling and experiments. Different initial conditions examined include the area of the nozzle (A), equivalence ratio (Φ), thermal input (P), and initial dilution of reactants (f). Very low emissions of NOx, CO, and H2 are measured for the MILD conditions when the furnace is operated under the premixed mode. The numerical results have shown that premixed MILD combustion can occur at the present furnace and burner system only when Re exceeds a critical value (Rec). When Re g Rec, a stable MILD combustion can be established, irrespective of the variation of A, Φ, or f. The diagram of the stability limits for the premixed MILD combustion based on the furnace temperature and recirculation rate is also presented.

1. INTRODUCTION Moderate or intense low-oxygen dilution (MILD) combustion1 has been identified to have great potential in enhancing thermal efficiency while reducing pollutant emissions. Distinct from the conventional combustion with visible flames, this combustion is nearly invisible or flameless because of suppressed radiation from intermediate species at reduced temperatures brought about by the strong flue gas recirculation.2
5 The strong recirculation yields a distributed reaction zone and moderate temperatures, which result in a significant reduction in exhaust emissions of pollutants, in particular, NOx.6
8 The intense internal recirculation coupled with the distributed reaction ensures a more uniform temperature distribution, hence enhancing the product quality mainly in industrial applications involved in steel production and heating processes. Different names have been used to describe this combustion regime, including flameless oxidation (FLOX)9 or high-temperature air combustion (HiTAC)10 because the combustion air is usually highly preheated for industrial regenerative combustor systems.11 Considerable research has been carried out in this field, and it has been well-recognized that strong hot exhaust gas recirculation to dilute the reactant streams before the reaction takes place is the key to achieve and maintain stable MILD combustion.12
15 Also essential for MILD combustion is to maintain the furnace temperature above autoignition to ensure that the distributed fuel stream can be fully oxidized before exiting the furnace. The approach to achieve such a regime requires careful design of the input streams to ensure dilution of reactants with the products prior to the reaction taking place.16,17 Additionally, the fuel jet momentum was found to play a key role in the dilution of the fuel and the establishment of MILD combustion.16,17 Such approaches are mainly required for MILD combustion, where the fuel and air streams are introduced separately into the furnace. The industrial MILD combustion application system (HiTAC) r 2011 American Chemical Society

uses regenerators to transfer energy from hot exhaust gases to fresh air, and the flow paths of the system are switched alternately to heat and cool the thermal storage medium. In this way, the combustion air is highly preheated to well above the temperature of fuel self-ignition (e.g., 1600 K for liquefied petroleum gas as fuel).10 This (non-premixed) method significantly enhances the thermal efficiency of the HiTAC system and simultaneously reduces exhaust emissions of NOx to a very low level ( 0) and side downward flue gas flows mainly at four corners (vz < 0). The most dynamical and strongest recirculation clearly occurs in the reference case whose D is smallest. As D is increased, both the cross-sectional area of the upward flow and its velocity at z = 200 mm decrease (see the lower part of Figure 6), so that the jet entrainment of the flue gas (m_ e) is reduced. The total entrainment rate can be estimated by m_ e = m_ up
(m_ a þR m R_ f), and the upward flow mass flux m_ up is calculated from m_ up = Fvz(x, y)dxdy for vz > 0, where m_ a and m_ f are the initial air and fuel mass fluxes. It follows that the ratio m_ e/(m_ a þ m_ f) = m_ up/(m_ a þ m_ f)
1, which is similar to the recirculation rate by W€unning and W€unning,9 who defined it as Kv = m_ r/(m_ a þ m_ f), with m_ r being the recirculation flux, for the case of the forward-flowing furnace, where m_ e = m_ r. We thus choose the same symbol Kv to denote the ratio, i.e., Kv = m_ e/(m_ a þ m_ f), for the present backward-flowing furnace. The magnitudes of Kv are indicated on the upper plots for z = 200 and 400 mm. Clearly, as D is increased, Kv decreases rapidly. Hence, an increase in A reduces the amount of flue gases recirculated and then entrained for diluting the reactants. The present experiment confirmed that, for D = 7.2 and 26.6 mm, MILD combustion can be established and the flame is invisible (see Figure 3d). The modeled results of these cases also show the MILD combustion characteristics; e.g., as seen in Figure 7, their overall temperature is relatively low, no high temperature peak occurs, and consequently, their temperature distribution is very uniform. Accordingly, the numerical and experimental results are overall consistent. Note that, when D = 7.2 mm, the initial velocity is approximately 86 m/s. When D is increased from 7.2 to 34 mm, Kv is reduced from 8.01 to 0.60 (see Figure 6). Such a low value of Kv = 0.6 for D = 34 mm means a low entrainment of 2787

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Figure 7. Temperature contours (K) on the central xz plane (y = 0) obtained by CFD. P = 10 kW, and Φ ≈ 0.8.

Figure 8. Centerline normalized distributions of (a) YO2 and (b) temperature obtained by CFD. P = 10 kW, and Φ ≈ 0.8.

Figure 9. Temperature contours (K) in the central xz plane (y = 0) using an annular nozzle obtained by CFD. P = 10 kW, and Φ ≈ 0.8.

flue gases that may not lead to sufficient dilution of the reactants for establishing MILD mode. This is indeed the

case, as demonstrated in Figure 7; for D g 34 mm, the peak temperature occurring around the jet boundary reaches as high as 2788

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Figure 10. Centerline normalized distributions of (a) YO2 and (b) TCL obtained by CFD. D = 7.2 mm, and Φ = 0.8.

Figure 11. Temperature contours (K) on the central xz plane (y = 0) obtained by CFD. D = 7.2 mm, and Φ = 0.8.

2000 K. By comparison, for D e 30 mm, the maximum temperature is only approximately 1400 K and the axial and lateral temperature gradients are relatively much smaller. Therefore, for present cases, as D is increased from 7.2 to 34 mm and, thus, Kv is decreased drastically, the combustion is predicted to switch from the MILD to conventional mode. These results are further analyzed using the mass fraction of O2 (YO2) and the mean temperature (TCL) along the centerline of the furnace. The YO2 and TCL are further normalized, respectively, by the oxygen mass fraction at the inlet and the maximum temperature (Tmax) in the furnace and are presented in panels a and b of Figure 8. It is clear that, as the jet diameter D is decreased, the jet potential core, where little mixing occurs between reactants and flue gases, becomes smaller, so that, as seen in panels a and b of Figure 8, TCL and YO2 start to change at a smaller downstream distance z. Concurrently, decreasing D (thus, increasing the injection velocity and then momentum) results in an increase in the internal recirculation rate and, therefore, a faster dilution of the premixed reactants in the jet mixing layer. It is thus evident in Figure 8 that, in the region downstream from the potential core, the oxidation occurs more slowly, as reflected by both YO2 decreasing and TCL increasing at a smaller rate, for smaller D (e30 mm). By comparison, for D g 34 mm, the larger mixture jet cannot entrain enough hot flue

gases and, hence, conventional flame takes place at z = 360
450 mm. Consequently, both YO2 drops and the temperature grows rapidly. Figure 9 shows the effect of the exit area of an annular nozzle, instead of a central tube, on the temperature distributions obtained in our previous investigation on the premixed combustion.23 Although the recirculation zone behind the bluff body may change the trajectory of reactant stream and tends to stabilize the flame, the influence of the exit area follows a similar trend to that of the central circular tube. Therefore, no matter what nozzle configuration is used, if combustion is conventional for some size of the nozzle, reducing the nozzle size significantly will switch it from conventional to MILD mode. 5.4. Effect of the Thermal Input (Modeling). To investigate the effect of the thermal input (P), we performed simulations of the isothermal premixed combustion, with no heat flux through the furnace walls, for D = 7.2 mm and Φ = 0.8 at P = 0.5
10 kW. Figure 10 shows the normalized mass fraction of O2 (YO2 /YþO2) and temperature TCL/Tmax along the centerline of the furnace. When P e 1 kW, the oxygen is rapidly consumed at the vicinity of z = 80 mm and an abrupt increase in temperature occurs there. These trends of YO2/YþO2 and TCL/Tmax are similar to those for D = 34 and 40 mm (P = 10 kW) whose combustion is conventional (Figure 7), suggesting that the 2789

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Figure 12. Temperature contours (K) on the central xz plane (y = 0) of the cases of varying the initial reactant dilution obtained by CFD. D = 34 mm, P = 10 kW, and Φ = 0.8.

Figure 13. Dependence of (a) Kv and (b) maximum temperature (Tmax) on Re for P = 10 kW.

combustion for 0.5 and 1 kW (D = 7.2 mm) is also conventional. By comparison, when P g 2.5 kW, the consumption rate of oxygen is reduced and the temperature distribution becomes more uniform, thus pointing to the MILD mode. Indeed, the MILD combustion was experimentally observed for P = 7.5 and 10 kW. Figure 11 displays contours of the normalized temperature. Clearly, relatively high temperatures (>0.9Tmax) for P = 0.5 and 1 kW occur only over quite a small zone, which looks very much like the conventional lifted flame. When P is increased beyond 2.5 kW, the zone of T/Tmax > 0.9 largely expands and occupies nearly the entire furnace; namely, the temperature distribution becomes much more uniform. 5.5. Effect of the Reactant Dilution (Modeling). Our recent study24 investigated the dilution effect of the air
fuel mixture on MILD combustion. In this case, a large diameter (D = 34 mm) is used where MILD combustion under premixed conditions can be established. The present investigation is performed on the basis of the same diameter with P = 10 kW and Φ = 0.8. Figure 12 shows temperature distributions for YO2 = 0
29.45%, where YO2 denotes the initial fraction of CO2 over the total premixed mixture. For YO2 = 0, the combustion is conventional. As YO2 is increased, the reaction rate decreases and the temperature reduces and eventually switches to the MILD mode.

The dilution of the initial premixed reactant increases the initial injection velocity and, hence, the momentum but decreases the initial concentration of O2, both helping to establish the MILD combustion.

6. GENERAL DISCUSSION 6.1. Effects of Re on Combustion. The results presented above in Figures 4
12 indicate that all initial conditions of A, Φ, P, and f have significant effects on the premixed combustion. To generalize the influence of these parameters, it is essential to find a non-dimensional parameter, which may control the stability and performance of the premixed MILD combustion. The Reynolds number of the premixed reactant jet (Re) appears to be the appropriate candidate, which can reflect the influence from all of the above parameters. The influence of Re on Kv for the P = 10 kW case is shown in Figure 13a. Clearly, under constant P and Φ conditions, Re increases with a decreasing A or an increasing f and there is an approximately linear growth of Kv. By comparison, varying Re through changing Φ results in little change in Kv. Hence, the relationship between Kv and Re appears to be complex and different for different cases. Nevertheless, some likely explanations are proposed here. For the cases of varying Φ, it is expected 2790

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Figure 14. Ta
Kv plots of both the premixed and non-premixed combustions of natural gas (symbols) versus the stability limits of the non-premixed MILD combustion of methane (dashed curves) by W€unning and W€unning.9 The solid symbols correspond to the premixed MILD regime, with those indicated by arrows being verified by experiments.

that the in-furnace flow field and temperature distribution should be similar in structure. These similarities should keep Kv nearly constant because it is a non-dimensional quantity for the global flow. On the other hand, when varying A and, thus, the nozzle dimension, the ratio of D to the furnace depth is changed and then the above similarities are not kept. Likewise, varying the reactant dilution via the CO2 concentration in the mixture changes the density ratio of the jet to the surroundings, so that the similarities are not maintained. Accordingly, in all of these cases, Kv must vary considerably. Despite the discrepancy observed above, it appears that, for a fixed burner configuration and constant P, there should be a critical Reynolds number (Rec) for the premixed MILD combustion to occur and that this Rec may be nearly constant, no matter whether A, Φ, or f is changed. On the basis of the results of Figures 6, 9, and 13b, Rec is estimated to be roughly 1.0  104 and 1.6  104 for the circular tube and annular tube burners, respectively. This Rec difference for the two cases is not surprising because, for the latter burner, which has an axisymmetric bluff body at the exit, the traditional flame is easily held by the bluff body and blowing it off requires a higher injection velocity of the mixture and, thus, a higher Reynolds number. The dependence of Tmax on Re is clearly evident in Figure 13b and provides a strong support for the above claims. Differently from the Kv
Re relationship, the dependence of Tmax on Re seems to be consistent for different ways of varying the Reynolds number. That is, simply Tmax decreases as Re increases. Importantly, the data points for all of the cases appear to fall onto the same curve. More specifically, there is a rapid drop of Tmax from over 1800 to below 1500 K around Rec for both the central circular tube and annular tube burners. When Re g Rec, Tmax is below 1400 K, suggesting the occurrence of the MILD mode. Besides, for Re > Rec, an increase in Re yields an insignificant change in Tmax and, more generally, in the overall in-furnace temperature. Our previous study20 has also concluded that, when the MILD combustion has been achieved, no significant influence on the MILD combustion results from either variation of the initial momentum rate or that of the initial fuel
air unmixedness.

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It is worth noting that, at P = 0.5 and 1 kW, although Kv ≈ 7 (not presented), seemingly sufficient for MILD combustion, their reactions are still conventional (see Figures 10 and 11). This can be explained because their Re is too low, resulting in a low strain rate and making it hard for the flame front to be extinguished. It is thus suggested that, to establish the premixed MILD combustion, both Kv and Re should be high enough. A high value of Kv reflects a high rate of recirculation of flue gases relative to the reactant supply rate but not necessarily a high initial jet velocity. For the premixed combustion, when the initial jet velocity is low, even though sufficient exhaust gases are recirculated for diluting reactants far downstream, the flame front may still appear near the nozzle exit. This differs from the non-premixed combustion. For the latter, when the initial jet velocity is low, the flame front cannot exist near the nozzle exit because there is no mixing between fuel and air streams. 6.2. Stability Limits of Premixed MILD Combustion. W€unning and W€unning9 have provided a stability limit diagram for the non-premixed MILD combustion of methane (CH4) in a particular forward-flowing furnace system (Figure 14). When the furnace temperature was related to Kv, three combustion regimes were identified and termed as regions A, B, and C. Region A was characterized by stable conventional flames. Region B was characterized by an unstable transitional regime, where both conventional and MILD combustions may occur intermittently. Region C was characterized by the MILD regime. For comparison, the present results of Ta versus Kv obtained at z = 400 mm (premixed) and those of the non-premixed combustion obtained at the merging point of the fuel and air jets in the same furnace system of Figure 1 (both firing natural gas)36 are also presented in Figure 14. Here, Ta is the mean temperature averaged over the cross-section where Kv is obtained. Note that, on the plot, the solid symbols for the premixed cases and the letter symbols bound by a dashed-line rectangle for the non-premixed cases36 correspond to the MILD regimes, with those indicated by arrows being verified by experiments. Evidently, the present stability operating map for the premixed MILD combustion is very different from that of the nonpremixed MILD combustion. For the premixed cases of the circular tube, although several points fall into their corresponding region, the majority of data is located near the A
B limit boundary and mainly in region B. The data set for the nonpremixed combustion appears to shift right but still not falling into the right regions of the diagram obtained by W€unning and W€unning9 for firing CH4. The discrepancy between the nonpremixed and premixed cases appears to suggest that the MILD mode occurs over a wider range of conditions for the latter case. This is confirmed by the present experiment and our previous study (those cases indicated by arrows are indeed MILD combustion in experiments). Furthermore, it is found that the stability diagram of the premixed MILD combustion varies from furnace to furnace. For the present study, the establishment of the MILD combustion using the annular tube needs higher Kv, relative to that of the central tube case, because of the influence of the recirculation zone behind the annular tube. 6.3. Relationship between the Reynolds Number and Momentum Rate of a Jet. For the present study, it has been found that there is a critical value of the Reynolds number, Re, below which the stable premixed MILD combustion cannot occur and above which a stable MILD combustion can be established. The previous study20 on premixed MILD combustion also found a critical initial value of the momentum. The jet 2791

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momentum rate J can be expressed on the basis of the mean velocity, v, the fluid density, F, and the tube cross-sectional area, S, as J = Fv2S, while the Reynolds number is defined as Re = vDeq/ υ, where Deq is the equivalent diameter and υ is the kinetic viscosity. It follows that J ¼ ðFπ=4Þν2 Re2 ¼ CRe2 where C is a constant related to the kinematic viscosity of the jet mixture. Note that, different from the momentum J, Re is a dimensionless parameter and, therefore, should be more useful in comparing different systems. Nevertheless, it is expected that Rec will be dependent upon the size of furnaces or combustors; i.e., most likely it will increase as the dimensions are enlarged. The investigation of this issue is beyond the scope of the present study.

7. CONCLUSION The operational limits of a single jet MILD combustor have been determined through experiments and numerical simulations. The influences of the inlet area of the premixed reactant, equivalence ratio, thermal input, and initial dilution of reactants have been investigated. The present investigation leads to the following conclusions: (1) The premixed MILD combustion can be established without air preheating. For the reference case of the present study, the premixed MILD combustion was established and very low emissions of NOx, CO, and H2 were obtained. (2) The Reynolds number of the premixed reactant jet (Re) is a key non-dimensional parameter, which can effectively control the stability of the premixed MILD combustion. If the combustion is initially conventional, it will switch to the MILD mode as Re is increased sufficiently. Namely, there is a critical Reynolds number, Rec, beyond which the premixed MILD combustion occurs. At this critical Reynolds number, the corresponding recirculation rate, Kv, should be sufficient for the MILD combustion to occur. For fixed burner and furnace configurations and a constant input power rate, Rec appears to be approximately identical, regardless of whether Re is changed by varying the inlet area, air equivalence ratio, or inlet dilution of reactants. When the premixed MILD combustion has developed at Re g Rec, a further increase of Re does not appear to significantly change the performance of the combustion. (3) For the premixed MILD combustion, Kv is not sensitive to the equivalence ratio, Φ, and, importantly, for Φ = 0.5
0.98, the equivalence ratio does not significantly influence the overall characteristic of the reactions. Moreover, when Φ ≈ 1, although the NOx emission is still low, extremely high emissions of CO and H2 are obtained, indicating that the fuel is not completely oxidized in the furnace. (4) According to our computational and experimental results, the stability limits for the MILD combustion appear to occur over a wider range of Kv for the premixed case than the non-premixed case. Moreover, the stability limit diagram of the MILD combustions is dependent upon the burner conditions and also varies from furnace to furnace. Note that the present results for both the premixed and non-premixed combustions were obtained in the same furnace system but different burner arrangements. Finally, Szeg€o et al.16 have revealed that the flame behaviors of natural gas and liquefied petroleum gas are very similar in the furnace presently investigated fuels. Hence, the above conclusions may still hold for other gaseous fuels.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge the support from the Foundation of the State Key Laboratory of Coal Combustion of China and the Institute for Mineral and Energy Resources (IMER) of The University of Adelaide. We also thank all of the reviewers for providing insightful comments, the addressing of which has significantly strengthened our paper. ’ REFERENCES (1) Cavaliere, A.; de Joannon, M. Prog. Energy Combust. Sci. 2004, 30, 329–366. (2) de Joannon, M.; Langella, G.; Beretta, F.; Cavaliere, A.; Noviello, C. Combust. Sci. Technol. 2000, 153, 33–50. (3) Cavigiolo, A.; Galbiati, M. A.; Effuggi, A.; Gelosa, D.; Rota, R. Combust. Sci. Technol. 2003, 175, 1347–1367. (4) Parente, A.; Galletti, C.; Tognotti, L. Int. J. Hydrogen Energy 2008, 33, 7553–7564. (5) Galbiati, M. A.; Cavigiolo, A.; Effuggi, A.; Gelosa, D.; Rota, R. Combust. Sci. Technol. 2004, 176, 1035–1054. (6) Dally, B.; Karpetis, A. N.; Barlow, R. S. Proc. Combust. Inst. 2002, 29, 1147–1154. (7) Medwell, P. R.; Kalt, P. A. M.; Dally, B. B. Combust. Flame 2008, 152, 100–113. (8) Medwell, P. R.; Kalt, P. A. M.; Dally, B. B. Combust. Flame 2007, 148, 48–61. (9) W€unning, J. A.; W€unning, J. G. Prog. Energy Combust. Sci. 1997, 23, 81–94. (10) Tsuji, H.; Gupta, A. K.; Haskgawa, T.; Katsuki, M.; Kishimoto, K.; Morita, M. High Temperature Air Combustion—From Energy Conservation to Pollution Reduction; CRC Press: Boca Raton, FL, 2003. (11) Katsuki, M.; Hasegawa, T. Proceedings of the 27th International Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; pp 3135
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