Fundamental Studies of NOx Emission Characteristics in Dimethyl Ether

Jan 23, 2009 - The NOx emission characteristics of dimethyl ether (DME) in laminar co-axial jet and counterflow non-premixed flames were investigated ...
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Energy & Fuels 2009, 23, 754–761

Fundamental Studies of NOx Emission Characteristics in Dimethyl Ether (DME)/Air Non-premixed Flames Cheol-Hong Hwang,†,‡ Chang-Eon Lee,*,‡ and Kee-Man Lee§ School of Mechanical Engineering, Inha UniVersity, 253, Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea, and School of Mechanical and Aerospace Engineering, Sunchon National UniVersity, 315, Maegok-dong, Sunchon, Jeonnam 540-742, South Korea ReceiVed October 23, 2008. ReVised Manuscript ReceiVed December 19, 2008

The NOx emission characteristics of dimethyl ether (DME) in laminar co-axial jet and counterflow nonpremixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NOx emissions of DME were compared to those of C2H6, which has equivalent methyl structures but lacks an oxygen atom. Experimental results showed that, in the co-axial jet case, the combustion of DME had the characteristics of a partial premixed flame. Additionally, it had a shorter flame and lower NOx emissions compared to the C2H6 flame. It is thus concluded that the major cause of low NOx emissions from DME co-axial jet flames may be the short flame length because of the lower stoichiometric air/fuel ratio. The activation of reburning NO chemistry because of the characteristics of the partially premixed flame may also play a role. The computational results of the DME counterflow non-premixed flame revealed that the EINO decreased by approximately 50% relative to that of the C2H6 flame. Although the overall NOx reaction path of the DME flame is similar to that of the C2H6 flame, it is concluded that the DME non-premixed flame has a distinct NO reduction mechanism. This is associated with reburning NO chemistry in the fuel-rich region because of fast pyrolysis and oxidation reactions in comparison to that of the C2H6 flame.

1. Introduction Dimethyl ether (DME; CH3-O-CH3) is the simplest ether; it consists of two methyls and one oxygen atom. DME is considered to be an attractive alternative fuel for the replacement of petroleum in terms of availability, economics, acceptability, environmental factors, and emissions.1 In particular, DME appears to be an excellent and efficient alternative fuel for use in diesel engines.2 This is because of its high cetane number (55-60), which is sufficient for self-ignition, its almost instantaneous vaporization upon injection into the cylinder, and its smoke-free combustion characteristics.3,4 The smoke (soot) reduction is directly related to the chemical structure of DME, which has no carbon-carbon bonds to act as seeds for polymerization, leading to the formation of polyaromatic hydrocarbons. In addition to these advantages, the physical properties of DME, which are similar to those of liquefied propane gas (LPG), also allow for the use of DME in homogeneous charge-compression ignition (HCCI) engines as an alternative fuel to LPG.5 Furthermore, many studies have been conducted on the effects of blending DME with diesel * To whom correspondence should be addressed. Telephone: +82-32860-7323. Fax: +82-32-868-1716. E-mail: [email protected]. † Current address: National Institute of Standards and Technology, Gaithersburg, MD 20899-8663. ‡ Inha University. § Sunchon National University. (1) Semelsberger, T. A.; Borup, R. L.; Greene, H. L. J. Power Sources 2007, 156, 497–511. (2) Arcoumanis, C.; Bae, C.; Crookes, R.; Kinoshita, E. Fuel 2008, 87, 1014–1030. (3) Seko, T. Jpn. Automot. Res. 1998, 20, 13–20. (4) Kajitani, S.; Chen, Z. L.; Konno, M.; Rhee, K. T. SAE Tech. Pap. 972973, 1997. (5) Yamada, H.; Sakanashi, H.; Choi, N.; Tezaki, A. SAE Tech. Pap. 2003-01-1819, 2003.

fuel, LPG, and liquefied natural gas (LNG) to improve combustion and exhaust characteristics.6,7 For successful use of DME with respect to clean combustion, research has largely focused on NOx emissions in diesel engines because DME already intrinsically offers smoke-free combustion. Comparative NOx emissions from DME and diesel fuel vary depending upon the engine conditions and the fuel supply system, as described in the literature.2 Kajitani et al.4 reported that the NOx level was higher with DME than with diesel fuel in a compression ignition engine that was tested at the same injection timing recommended for diesel fueling. This observation was attributed to the longer duration of the peak combustion temperature in the initial combustion period because of the shorter ignition delay of DME. Cipolat8 noted that the concentrations of NOx with DME fueling were higher at low engine speeds and similar at high engine speeds in comparison to those of diesel fueling at the same conditions of injection timing and injector opening pressure. On the other hand, when the operating conditions of the engine were optimized for each fuel, the NOx from DME was lower than that from diesel.9 Longbao et al.10 achieved low NOx emissions from DME compared to those of diesel fuel by adjusting the injection time. Christensen et al.11 also identified a reduction in NOx emissions of DME with a decrease in the injector opening pressure using favorable spray atomization and vaporization characteristics. With regard to NOx (6) Ying, W.; Longbao, Z.; Hewu, W. Atmos. EnViron. 2006, 40, 2313– 2320. (7) Chen, Z.; Konno, M.; Goto, S. J. Soc. Automot. Eng. ReV. 2001, 22, 265–270. (8) Cipolat, D. Appl. Therm. Eng. 2007, 27, 2095–2103. (9) Kapus, P.; Ofner, H. SAE Tech. Pap. 950062, 1995. (10) Longbao, Z.; Hewu, W.; Deming, J.; Zuohua, H. SAE Tech. Pap. 1999-01-3669, 1999. (11) Christensen, R.; Sorenson, S. C.; Jensen, M. G.; Hansen, K. F. SAE Tech. Pap. 971665, 1997.

10.1021/ef800921q CCC: $40.75  2009 American Chemical Society Published on Web 01/23/2009

DME/Air Non-premixed Flames

emissions of DME in a HCCI engine, Yamada et al.12 noted that DME provides the capability of burning at a wide range of equivalence ratios without soot formation. This can lead to reduced NOx formation through the favorable operations of exhaust gas recirculation (EGR) and improved application of postcombustion de-NOx processes. The aforementioned studies have provided valuable information about the effects of operating conditions on NOx emissions from DME in practical engine systems. However, it is difficult to delineate the intrinsic NOx formation characteristics of DME compared to those of hydrocarbon fuels because the change of fuel type in a practical engine can significantly modify the combustion environments that impact NOx formation (including flame temperature, pressure, and residence time at higher temperatures). To date, most research on the NOx emissions of DME has concentrated on engine applications, while few studies have explored the fundamental combustion characteristics of DME, such as flame structure and NOx emissions. Thus, to better understand the intrinsic NOx formation characteristics of DME, fundamental studies employing simple flames are required prior to studies with complex engine systems. Motivated by such consideration, the current study presents experimental and computational results concerning the NOx emission characteristics of DME/air non-premixed flames. First, an experimental study was conducted to identify the quantitative characteristics of the flame shape and the NOx emissions of DME in co-axial jet non-premixed flames. Second, the counterflow configuration was adopted in a numerical study, so that more comprehensive chemistry and transport models could be derived for a detailed investigation of the structure and emission characteristics of DME/air non-premixed flames. In particular, the characteristics of the DME flames were compared to C2H6 flames to investigate the effect of the oxygen atom in the chemical structure of DME on NOx emissions. NOx emissions could be optimized in practical systems, where it is anticipated that the current fundamental results would enhance the understanding of the intrinsic NOx emission characteristics for DME non-premixed flames. 2. Experimental and Numerical Methods 2.1. Experimental Methods. A laminar co-axial jet flame was adopted to experimentally identify the flame shape and NOx emissions of DME compared to C2H6. The combustor consisted of a centered fuel nozzle and an ambient air nozzle. The inner diameter and thickness of the fuel nozzle were fixed at 8 and 1 mm, respectively. A contracted air nozzle (inner diameter of 50 mm) designed by Morel’s suggestion13 was used to sustain a uniform velocity distribution at the nozzle exit. The fuel mass flow rate was adjusted from 6.56 × 10-3 to 13.12 × 10-3 g/s. DME and C2H6 having the purity of more than 99.9% were used. The ambient airflow was fixed at 30 slpm (standing for standard liters per minute). The temperatures of fuel and air stream were measured at approximately 298 K. To intercept the external flow and measure the NOx concentration in the downstream region, a 1 m long Pyrex chimney was used. The NO concentration was measured using a NOx analyzer (GreenLine 9000); details of the combustor and measurements can be found in the literature.14 2.2. Numerical Methods. For a detailed investigation of the structure and NOx emission characteristics of the DME/air nonpremixed flames, an axisymmetric counterflow configuration was adopted, as shown in Figure 1. Using the boundary layer approximation in the axial direction, the conservation equations for (12) Yamada, H.; Suzaki, K.; Sakanashi, H.; Choi, N.; Tezaki, A. Combust. Flame 2005, 140, 24–33. (13) Morel, T. J. Fluids Eng. 1975, 97 (2), 225–233. (14) Lee, C. E.; Oh, C. B.; Kim, J. H. Fuel 2004, 83, 2323–2334.

Energy & Fuels, Vol. 23, 2009 755

Figure 1. Schematics of a counterflow non-premixed flame.

mass, radial and axial momentum, energy, and species along the centerline of symmetry are written as15,16

G(x) ) dF(x)/dx H-2 Fu Fu

2

(1)

d d G d FG 3G + + µ )0 dx F F dx dx F

( )

[ ( )]

dYk d + (FY V ) - ω ˙ kWk ) 0, dx dx k k

dT 1 d dT F λ + dx cp dx dx cp

( )

∑c

pkYkVk

k

(2)

k ) 1, ..., K

dT 1 + dx cp

(3) q˙r

∑ h ω˙ - c k

k

k

)0

p

(4) where G(x) ) -Fu/r, F(x) ) Fu/2, and the radial pressure gradient, H ) (1/r)(∂p/∂r) ) constant, is an eigenvalue of the problem. A detailed derivation of the governing equations was presented somewhere else.15 Given these formulations, the axial and radial velocity components at the nozzle exit can be independently specified and the pressure eigenvalue may be computed as part of the solution. The boundary conditions for the fuel and oxidizer streams at the nozzles are

x ) 0: F )

FFuF , G ) 0, T ) TF, FuYk + FYkVk ) (FuYk)F 2

x ) L: F )

FOuO , G ) 0, T ) TO, FuYk + FYkVk ) (FuYk)O 2

where the velocities at both nozzle exits were maintained symmetric such that uF ) uO. The main contributions to radiative heat loss were assumed to be attributed to CO2, H2O, and CO species, where the radiative heat loss based on the optically thin approximation17 is calculated as follows:

q˙r ) -4σKp(T4 - T∞4)

(5)

Kp ) PCO2KCO2 + PH2OKH2O + PCOKCO

(6)

where σ is the Stefan-Boltzmann constant and T and T∞ are the local and ambient temperatures, respectively. Kp is the Plank mean absorption coefficient. Pk and Kk are the partial pressure and the Plank mean absorption coefficient of a species, respectively. The Plank mean absorption coefficient is approximately obtained as follows: 5

Kk )

∑A

j kjT ,

k ) CO2, CO, H2O

(7)

j)0

(15) Kee, R. J.; Miller, J. A.; Evans, G. H.; Dixon-Lewis, G. Proc. Combust. Inst. 1988, 22, 1479–1494. (16) Lutz, A. E.; Kee, R. J.; Grcar, J. F.; Rupley, F. M. Sandia Report SAND96-8243, Sandia National Laboratories, Albuquerque, NM, 1997. (17) Tien, C. L. AdV. Heat Transfer 1968, 5, 253–324.

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Here, Akj is the polynomial coefficient18 of the k-th species expressed as a function of the temperature. Chemkin-II19 and the Transport package20 were used to solve the governing equations shown above. The nozzle separation distance was 2 cm in the present simulation. The fuel and air stream temperatures were fixed at 298 K. The global strain rate,21 defined by the following equation, was fixed at a moderate strain rate of ag ) 100 s-1.

ag )

[

2(-uO) uF 1+ L (-uO)

] FF FO

(8)

The NO emission index (EINO) proposed by Takeno et al.22 was introduced to quantitatively compare the NO formation characteristics for each flame (shown below in eq 9)

EINO )

(∫ ω˙ L

0

) ⁄ (-∫ ω˙

NOdx

L

0

)

fueldx

(9)

where ω ˙ k represents the production rate based on the mass for both NO and fuel. The rate of production analysis (ROPA) methodology23 was employed to identify dominant reactions that make major contributions to the NO production rate. On the basis of this analysis, the NOx emission characteristics were compared for each fuel. 2.3. Chemical Kinetic Model. Although the NOx formation is directly related to reactions of N-containing species (i.e., N2, NNH, NH, HCN, etc.), it is well-known that hydrocarbon radicals (CH, CH2, etc.), as well as O, H, OH, and H2O that are generated in the process of fuel oxidation play an important role in NOx production and destruction.24 Therefore, to reasonably estimate the NOx formation characteristics, a detailed comprehensive mechanism that can simultaneously predict the processes of fuel oxidation and NOx formation is required. However, there exists no reaction mechanism for the two fuels (C2H6 and DME) that can simultaneously predict fuel oxidation and NOx formation (recalling that reaction mechanisms for the oxidation processes of these fuels have been suggested25,26). To solve these problems, previous studies investigated the NOx formation characteristics of various fuels through a combination process that featured individually validated reaction mechanisms for fuel oxidation and NOx formation. For example, Xue et al.27 and Naha et al.28 studied the NOx emissions of n-heptane/air partially premixed flames by integrating the specific oxidation mechanism and the NOx reaction mechanism. They found from their comprehensive comparison that the combined mechanism yielded an excellent agreement between the measured and predicted NOx profiles. It is thus currently concluded that a combination of verified fuel oxidation and NOx reaction mechanisms can yield reasonable results for NOx prediction, provided that NOx-related reactions added to the fuel oxidation mechanism do not affect the major oxidation processes of the fuel, such as flame structure and the production of important radicals. In the present study, the oxidation chemistry of C2H6 was modeled using a mechanism reported by Sung et al.25 (referred to (18) Ju, Y.; Guo, H.; Maruta, K.; Liu, F. J. Fluid Mech. 1997, 342, 315–334. (19) Kee, R. J.; Rupley, F. M.; Miller, J. A. Sandia Report SAND898009B, Sandia National Laboratories, Albuquerque, NM, 1989. (20) Kee, R. J.; Dixon-Lewis, G.; Warnatz, J.; Coltrin, M. E.; Miller, J. A. Sandia Report SAND86-8246, Sandia National Laboratories, Albuquerque, NM, 1994. (21) Chellian, H. K.; Law, C. K.; Ueda, T.; Smooke, M. D.; Williams, F. A. Proc. Combust. Inst. 1990, 23, 503–511. (22) Takeno, T.; Nishioka, M. Combust. Flame 1993, 92, 465–468. (23) KINALC, version 1.9, http://www.chem.leeds.ac.uk/Combustion/ kinalc.html. (24) Drake, M. C.; Blint, R. J. Combust. Flame 1991, 83, 185–203. (25) Sung, C. J.; Li, B.; Wang, H.; Law, C. K. Proc. Combust. Inst. 1988, 27, 1523–1529. (26) Kaiser, E. W.; Wallington, T. J.; Hurley, M. D.; Platz, J.; Curran, H. J.; Pitz, W. J.; Westbrook, C. K. J. Phys. Chem. A 2000, 104, 8194– 8206. (27) Xue, H.; Aggarwal, S. K. Combust. Flame 2003, 132, 723–741. (28) Naha, S.; Aggarwal, S. K. Combust. Flame 2004, 139, 90–105.

Figure 2. Flame images of C2H6 and DME in co-axial jet non-premixed flames at various fuel flow rates (air flow rate is fixed at 30 slpm).

as the modified C3 mechanism) that involves 92 species and 619 elementary reactions. Many researchers have validated such a mechanism in a variety of configurations. Sung et al.25 validated this mechanism using experimental data for flame structures and sooting limits in counterflow CH4/air and C3H8/air non-premixed flames. Lee et al.29 validated this mechanism for burning velocities in laminar C3H8/air premixed flames. Similarly, the oxidation chemistry of DME was modeled using a mechanism developed by Kaiser et al.,26 which involved 79 species and 351 elementary reactions. The initial version of this mechanism was suggested by Curran et al.30 Kaiser et al.26 recently developed and validated this mechanism for a flame structure, including species profiles in DME/ air premixed flames. Several detailed mechanisms are used to describe NOx chemistry. These include the NOx mechanism used in GRI-3.0,31 the Li-Williams mechanism,32 and the original and modified Miller and Bowman mechanisms.33,34 Through validations of combinations of these NOx chemistries and fuel oxidation mechanisms, it was found that the modified Miller and Bowman NOx mechanism (hereafter called the MMB) reproduced the experimental data better than the other three NOx mechanisms in the case of counterflow non-premixed flames. Therefore, the present study employs a combination of the MMB NOx mechanism with the oxidation mechanisms developed by Sung et al. (for C2H6) and Kaiser et al. (for DME). Details of such a combined approach, including numerical results, are presented in the Results and Discussion.

3. Results and Discussion 3.1. Experimental Results for Co-axial Jet Flames. To show the flame shape of a DME non-premixed flame, Figure 2 illustrates DME flame images at a variety of fuel flow rates. The fuel flow rate varied from 6.59 × 10-3 to 13.12 × 10-3 g/s. A flame image of C2H6 for a fuel flow rate of 6.59 × 10-3 g/s is also shown for reference. For the same flow rate of 6.59 × 10-3 g/s, the flame length of the DME flame became shorter than the C2H6 flame by a factor of 3. In addition, half of the length of the DME flame showed a blue color for all fuel flow rates. The shapes of the DME flame were similar to that of a hydrocarbon flame that was burning under partial premixed conditions with an equivalence ratio of roughly 4-8.14 The difference between the flame images of DME and general hydrocarbon fuels may be attributed to the existence of an O atom in the DME (by the existence of O-C bonds instead of C-C bond), as well as the difference in the air/fuel ratio, as (29) Lee, C. E.; Oh, C. B.; Jung, I. S.; Park, J. Fuel 2002, 81, 1679– 1686. (30) Curran, H. J.; Pitz, W. J.; Westbrook, C. K.; Dagaut, P.; Boettner, J. C.; Cathonnet, M. Int. J. Chem. Kinet. 1998, 30, 229–241. (31) GRI Mech, version 3.0, http://www.me.berkeley.edu/gri_mech/. (32) Li, S. C.; Williams, F. A. Combust. Flame 1999, 118, 399–414. (33) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287–338. (34) Williams, B. A.; Pasternack, L. Combust. Flame 1997, 111, 87– 110.

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Table 1. Physical Properties of C2H6 and DME density (kg N-1 m-3) (A/F)stoichiometric (kg/kg) Tadiabatic(K) lower heating value (MJ/kg)

C2H6

DME

1.230 15.98 2265.3 47.8

1.884 8.94 2294.7 28.6

shown in Table 1. The stoichiometric air/fuel ratios of DME and C2H6 are 8.94 and 15.98, respectively, such that the same air flow rate of 30 slpm would be far sufficient for oxidizing the DME compared to the C2H6. The DME must be supplied by a mass 1.5 times greater than the C2H6 to produce similar heat and power in combustion appliances because the heating value per unit mass of DME (28.6 MJ/kg) is less than that of C2H6 (47.8 MJ/kg) by about 1.5 times. For similar thermal delivery conditions, the flame length of DME was half that of C2H6, as can be seen by comparing flame images (particularly, the DME flame image at 10.93 × 10-3 g/s and the C2H6 flame image at 6.56 × 10-3 g/s). The flame color confirms that the DME flame generated much less soot than those associated with the C2H6 flame, as reported in previous research.35 Figure 3 shows the NOx emissions obtained under the same conditions as the case illustrated in Figure 2. Emissions were measured far downstream of the flame and were corrected to 4% O2 in the product gas. For the same fuel mass flow rate conditions (6.56 × 10-3 g/s), the NOx emission of the DME flame was reduced by 33%, and for the same thermal delivery conditions (10.93 × 10-3 g/s), it was reduced by 8% relative to that of the C2H6 flame. There are several possible explanations for the comparatively low NOx production in the DME nonpremixed flame. First, considering that the adiabatic flame temperature between the two fuels showed a small difference of approximately 25 K, it follows that the major cause of low NOx emissions for DME was the short flame length, as indicated by the lower stoichiometric air/fuel ratio. In this sense, the short flame length is associated with a reduction in the residence time, which in turn decelerates the thermal NOx formation in the region of temperatures higher than 1800 K. Second, the DME non-premixed flame was similar to the partially premixed flame in terms of flame shape and color. This may be attributed to the fast chemical reaction rate of DME, likely owing to the existence of the oxygen atom in the molecular structure. The characteristics of the partially premixed flame for DME can lead to a reduction of NOx via the activation of reburning NO chemistry.33 However, in terms of chemical reactions, the role of the O atom in the pyrolysis and oxidation processes in the context of NOx emission has not been identified to date. A detailed analysis of the NOx emissions in the co-flow jet flame can be conducted through cross-sectional comparisons using numerical simulation. However, the simulation of multidimensional configurations using detailed DME chemistry requires prohibitively lengthy computational times. Consequently, to clarify the difference in NOx emission characteristics near the flame surface of the DME and hydrocarbon fuels, numerical simulations of the counterflow non-premixed flame were carried out using detailed reaction mechanisms. It is known that the NO formation characteristics based on the onedimensional flame might be different from the multi-dimensional flame in terms of NO formation mechanisms and emission indices. In fact, various chemical environments related to NO formation are distributed in the up- and downstream reaction (35) Westbrook, C. K.; Pitz, W. J.; Curran, H. J. J. Phys. Chem. A 2006, 110, 6912–6922.

Figure 3. NOx emissions of C2H6 and DME in co-axial jet non-premixed flames at various fuel flow rates.

zone in the co-axial jet flame.36 Therefore, the current numerical analysis using the counterflow configuration is applicable only within the limited range of the upstream portion of the co-flow jet flame, where the reaction between fresh fuel and air actually occurs. 3.2. Numerical Results for Counterflow Flames. Various NOx mechanism options were evaluated to determine which is to be combined with the DME and the modified C3 mechanism (e.g., GRI-3.0,31 Li-Williams mechanism,32 and MMB34). The results of assessing the Li-Williams mechanism are not presented here because a significant discrepancy existed between the predicted and measured data. Parts a and b of Figure 4 show the current comparisons of the predicted NO profiles with experimental data obtained by Ravikrishna et al.37 for C2H6 and CH4 non-premixed flames established at global strain rates of ag ) 96 and 68 s-1, respectively. The NO concentration is expressed as a function of the distance from the fuel nozzle. The experiments were carried out for low-temperature flames (below 1700 K), where very little thermal NO is generated because the fuel used contains a large quantity of nitrogen. For the C2H6 flame in Figure 4a, the NO concentration profiles based on the MMB and the GRI-3.0 mechanisms were in good agreement with the measurements. For the CH4 flame in Figure 4b, while both numerical results predicted the experimental result reasonably well, overall, the NOx mechanism of the MMB generated a more accurate prediction than the GRI-3.0. Figure 4c presents numerical results compared to experimental data obtained by Lim et al.38 The flame used in their experiment was a pure methane counterflow flame established at a global strain rate of ag ) 162 s-1. The NO concentration profiles based on the MMB NOx mechanism were in good agreement with the measurements, while those based on the NOx mechanism of GRI-3.0 were 2-fold overpredicted, relative to the measurements. This is consistent with results reported in previous investigations.32,39 From these comparisons, it is clear that the NOx mechanisms in GRI-3.0 overpredicted the thermal NO values when the flame temperature increased, because of the decrease in nitrogen dilution. However, the NOx mechanism in MMB predicts NOx formation characteristics well for hydro(36) Zhu, X. L.; Nishioka, M.; Takeno, T. Proc. Combust. Inst. 1988, 22, 1369–1376. (37) Ravikrishna, R. V.; Laurendeau, N. M. Combust. Flame 2000, 120, 372–382. (38) Lim, J.; Gore, J.; Viskanta, R. Combust. Flame 2000, 121, 262– 274. (39) Giles, D. E.; Som, S.; Aggarwal, S. K. Fuel 2006, 85, 1729–1742.

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Figure 5. Axial profiles of temperature, velocity, and major species for C2H6 and DME non-premixed flames with and without the NOx mechanism.

Figure 4. Comparison of simulated NO profiles to experimental data for C2H6 and CH4 non-premixed flames.

carbon flames. Even though validation of the NOx reaction mechanism for a DME flame was not carried out because of a lack of suitable experimental data for comparison, the NOx formation process in the DME flame can be assumed to be similar to that of a hydrocarbon flame. Thus, the NOx mechanism of MMB was combined with the oxidation mechanism of DME. As previously mentioned, combining the fuel oxidation mechanism and the NOx formation mechanism requires considerable caution because the processes of fuel oxidation may change because of those reactions related to the important radicals included in the NOx mechanism. To illustrate the change in flame structure with and without the NOx mechanism, Figure 5 presents comparisons with regard to the temperatures, major species, and velocity profiles both with and without the NOx mechanism for C2H6 and DME non-premixed flames established at a global strain rate of ag ) 100 s-1. The symbols represent the simulation results when only the oxidation mechanism of the fuel is taken into consideration. The lines represent data yielded by the combined mechanism for fuel oxidation and NOx formation. The two figures show that combining the MMB NOx mechanism with the C3 and Kaiser mechanisms does not affect the numerical simulation of the flow field, temperature, and major chemical species. In other words, the combination of the

NOx mechanism with the two fuel oxidation mechanisms has no effect on flame structure, except for the NOx formation processes. In addition, the two flames are very similar in terms of flame structure (including flame thickness) upon directly comparing the two flames under the same strain rate conditions. Differences, however, were found as follows: the CO concentration in the DME flame is much higher than the C2H6 flame; the maximum temperature of DME flame is 56 K higher than that of C2H6 flame; and the locations of the DME stagnation plane and flame surface move about 1 mm toward the air side because of an increase in DME momentum on the fuel side. However, the relative distance between the stagnation plane and the flame surface is almost the same for the two flames. It is also important to note that a difference of maximum temperature between DME and C2H6 flame may be increased because of less soot formation in the DME flame in comparison to that of C2H6 flame. Although the soot model was not included in the present study, studies on the detailed interaction between soot and NOx emission are in progress. Figure 6 shows the axial profiles of the minor species related to NOx formation for C2H6 and DME flame established under the same conditions as employed for the case illustrated in Figure 5. A combination of the NOx mechanism with the two fuel oxidation mechanisms does not cause any difference in radical concentrations (e.g., OH, O, and HCCO) except for CH distribution in the DME flame. When the NOx mechanism is considered in the DME flame, the CH radical concentration is decreased by a factor of 2. This result implies that the NOx chemistry in the DME flame has a much closer relationship with the CH reaction related to prompt NO or the reburning NO mechanism in comparison to the C2H6 flame. In addition, the concentration of formaldehyde (CH2O) in the DME flame is

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Energy & Fuels, Vol. 23, 2009 759

Figure 7. Axial profiles of the NO concentration for C2H6 and DME non-premixed flames. Table 2. Comparison of the Results Relating to NOx Formation in C2H6 and DME Non-premixed Flames Tmax (K) δT > 1800 (mm) ∫0Lω ˙ fueldx (g cm-2 s-1) ∫0Lω ˙ NOdx (g cm-2 s-1) EINO (g/kg)

Figure 6. Axial profiles of minor species as a function of the relative distance from the location of the maximum temperature for C2H6 and DME non-premixed flames with and without the NOx mechanism.

200 times higher, and the maximum concentration is closer to the fuel nozzle compared to the C2H6 flame. Considering that CH2O is an intermediate species generated by low-temperature reactions during the oxidation of many hydrocarbon fuels or an indicator of combustion onset,40 it is concluded that the DME flame has faster pyrolysis and oxidation reactions than the C2H6 flame. The NOx formation characteristics that arise because of these differences are discussed later. Figure 7 presents the axial profiles of NO at relative distances from the location of the maximum temperature for the C2H6 and DME flames under the same conditions as in Figure 5. The DME flame shows a similar NO profile, except that the NO concentration on the left side of the flame surface is lower than that of the C2H6 flame. This indicates that the formation and destruction characteristics of NO in the initial stage of DME oxidation are very different from those in C2H6 oxidation. In terms of the maximum NO concentration, the two flames have nearly the same values near the flame surface. However, this does not imply that the NO emission of DME is similar to that of C2H6, considering that the stoichiometric air/fuel ratios of each fuel are considerably different, as summarized in Table 1. It was noted that the NO concentrations in this figure are only relative volume fractions calculated from the total mole numbers of species in the reacting field for each fuel. In fact, the total mole number is very different according to the fuel type. In (40) Brackmann, C.; Pengloan, G.; Andersson, O. Combust. Sci. Technol. 2006, 178, 1165–1184.

C2H6/air

DME/air

1955.1 0.66 -1.407 × 10-2 7.404 × 10-6 0.526

2010.9 1.07 -2.656 × 10-2 6.541 × 10-6 0.246

addition, an identical global strain rate of 100 s-1 for both flames changes the mass flow rates of the fuels, although it also yields a similar flame structure, including flame thickness and relative location of the flame surface from the stagnation plane (SP). Therefore, examination using the NO emission index is necessary to quantitatively compare the NO emissions between the two fuels. The NO emission indices (EINO) calculated using eq 9 are listed in Table 2. While the integrated fuel production rate of DME is higher than C2H6 by a factor of 2, the integrated NO production rates exhibit similar values. Thus, the EINO of the DME flame with a magnitude of 0.246 g/kg is as much as 50% lower than that of the C2H6 flame (having a value of 0.526 g/kg). Figure 8 presents the fuel production rates as a function of the relative distance from the maximum temperature for each flame to examine the distribution of the fuel production rate related to the NO emission index. This figure shows that the DME is consumed in the upstream region and the corresponding

Figure 8. Axial profiles of the fuel production rate for C2H6 and DME non-premixed flames.

760 Energy & Fuels, Vol. 23, 2009

Hwang et al.

Figure 10. Reaction path diagram of NOx formation at x ) -1.0 mm from the location of Tmax in the DME non-premixed flame.

Figure 9. Reaction path diagrams of NOx formation for C2H6 and DME non-premixed flames.

rate is much higher than in C2H6. In particular, the DME oxidation process starts upstream of the stagnation plane. From the results of Figures 7 and 8, it is found that, even though the DME flame consumed twice as much fuel as the C2H6 flame, the NO concentration profiles were similar for two flames. This supports the result that the EINO of DME flame is as much as 50% lower than that of the C2H6 flame. Thus, it can be inferred that the NOx emissions are significantly affected by the initial stage of the DME chemistry. The fast and high consumption rate of DME can be explained by the existence of C-O bonds in the chemical structure of DME. Arcoumanis et al.2 reported that C-O bonds break more easily than C-H bonds because of the low C-O bond energy, as well as the distortion of C-O bonds. As such, the pyrolysis of DME may initiate a chain reaction at a lower temperature than that associated with the case of other hydrocarbon fuels. As a result, in the fuel-rich region located on the left side of the flame surface, methyl radicals and other radicals related to NO chemistry generated

through a chain reaction by the initial pyrolysis of DME can significantly affect NO production and destruction. The progress of NO reactions in such a region will be discussed using the reaction path diagram presented in Figure 10. Figure 9 shows the reaction path diagram for the formation of NOx in C2H6 and DME non-premixed flames. The thickness of the arrow, divided into six levels, gives a visual indication of the relative importance of particular reaction paths with respect to their reaction rates integrated over the entire computational domain. The NOx formation route for the C2H6 flame shown in Figure 9a can be explained by the major production and destruction routes. The major production path of N2 f HCN f NCO f NH f N f NO shows that NO is produced through complex interactions between thermal NO and the prompt NO mechanisms. That is, the most important thermal NO reaction is N + OH f NO + H, while the N radical of this reaction is produced from reactions of NCO and NH species related to prompt NO reactions. The destruction of NO mainly occurs through reburning chemistries of NO, with intermediates generated from pyrolysis and oxidation of fuel, such as C, CH, CH2, and HCCO. When comparing the reaction paths of NOx for C2H6 and DME, we found that the overall routes of NOx formation are very similar. However, in the DME non-premixed flame shown in Figure 9b, the contribution of a thermal NO mechanism (N + OH f NO + H) shows a greater increase compared to the case of the C2H6 flame. This phenomenon is also explained by comparing the maximum flame temperature and flame thickness in the region above 1800 K, as listed in Table 2. On the other hand, the NO destruction also increases consistently with various reburning NO chemistries. In particular, the roles of the NO + C f CN + O and NO + H (+M) f HNO + M reactions in NOx destruction of the DME flame become more prominent than in the case of the C2H6 flame.

DME/Air Non-premixed Flames

Figure 10 shows the reaction path diagram of NOx at a relative distance of x ) -1 mm from the location of the maximum temperature of the DME flame (as shown in Figure 7). This allows for the examination about NOx reduction in the fuelrich region of the DME flame. NOx is not generally produced in this region and is mainly diffused from the flame surface. As shown in Figure 10, the NO production by the thermal and prompt NO mechanisms is very small, while NO destruction actively occurs through reburning reaction paths, such as NO + HCCO f HCNO + CO and NO + CH3 (or +CH2) f HCN + H2O (or +OH). For the C2H6 flame, the reaction path diagram is not presented in the current study because the local NO reaction rate at the relative distance of x ) -1 mm is too small in comparison to the case of DME flame. Most NO is consumed by the reaction of NO + O + M f NO2 + M, and its reaction rate is 2.3 × 10-21 mol cm-3 s-1. That is, there is virtually no production or destruction of NO at the fuel-rich region for the C2H6 flame. It is concluded from these results that, although the overall NOx reaction path of the DME flame is similar to that exhibited by the C2H6 flame, the DME flame has a distinct NO reduction mechanism through reburning NO chemistry in the fuel-rich region because of fast pyrolysis and oxidation reactions.

Energy & Fuels, Vol. 23, 2009 761

structure of DME in NOx emissions was examined through a comparison to C2H6 flames. The experimental results suggested that the DME flame had the characteristics of a partial premixed flame in co-axial jet flames and that the flame length decreased by up to 1/3 of that of C2H6 under the same fuel mass flow rate conditions. The NOx emission of the DME flame was reduced by 33% at the same fuel mass flow rate and by 8% at the same thermal delivery conditions, relative to that of the C2H6 flame. As a result, it is inferred that the major cause of low NOx emissions for DME flames is the short flame length because of the lower stoichiometric air/fuel ratio, as well as the activation of reburning NO chemistry caused by the characteristics of partially premixed flames. In our calculations pertaining to the DME counterflow nonpremixed flame, the EINO decreased by approximately 50% relative to a C2H6 flame. Although the overall NOx reaction path of DME is similar to that of other hydrocarbon fuels, it is concluded that the DME non-premixed flame has a distinct NO reduction mechanism that operates by means of reburning NO chemistry in the fuel-rich region because of the fast pyrolysis and oxidation reactions in comparison to that of the C2H6 flame.

4. Conclusions Experimental and computational studies were conducted to investigate the NOx emission characteristics of a DME/air nonpremixed flame. The effect of oxygen atom on the chemical

Acknowledgment. This work was supported by an Inha University Research Grant. EF800921Q