Experimental and numerical study on the CO formation mechanism in methane MILD combustion without preheated air

Fuel 192 (2017) 140–148

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Experimental and numerical study on the CO formation mechanism in methane MILD combustion without preheated air Yang Liu a, Jia Cheng a, Chun Zou a,⇑, Lei Cai b,⇑, Yizhuo He a, Chuguang Zheng a a b

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China

h i g h l i g h t s
Local high O2 concentration was the reason for high CO concentration zone appeared.
R166, R167, R168 plays a significant role in CO formation during MILD combustion.
In the central zone, the main CO2 formation path was CH4 ? CH3 ? CH3O ? CH2O ? HCO ? CO ? CO2.
R99 dominated in the CO consumption in the recirculation zone.

a r t i c l e

i n f o

Article history: Received 10 July 2016 Received in revised form 5 October 2016 Accepted 6 December 2016

Keywords: MILD combustion CO formation mechanism Non-preheated air Computational fluid dynamics Reaction rate

a b s t r a c t Moderate or intense low-oxygen dilution (MILD) combustion has increasingly attracted the attention of scholars because of its high efficiency and low-pollutant emission, particularly its emission of CO and NOX. MILD combustion without preheated air was achieved in this study. Numerical results obtained by a computational fluid dynamics model with GRI-Mech 3.0 conform well to those from the experiments. Three zones, namely, central zone, high CO concentration zone, and recirculation zone, were found in the furnace. The CO formation mechanisms were analyzed in the three zones by examining the rate of production. Analysis results showed that the reason for the appearance of high CO concentration zone was local high O2 concentration. High O2 concentration enhanced the reaction O + CH3 , H + CH2O, which resulted in an amount of CH2O. High O2 concentration also enhanced the radical pool reaction H + O2 , OH + O, which produced an amount of O radicals. Thus, reaction O + CH4 , OH + CH3 was enhanced, and it had the largest production rate of OH and CH3. The OH radical led to an amount of HCO through the reaction OH + CH2O , HCO + H2O, and CH3 radical favored the production of CH2O. An amount of HCO was converted to CO by HCO + O2 , HO2 + CO, HCO + H2O , H + CO + H2O, and HCO + M , H + CO + M. The main CO formation path was CH4 ? CH3 ? CH3O ? CH2O ? HCO ? CO in the central zone. The majority of CO in the recirculation zone came from the central or high CO concentration zones by convection or diffusion, and it was consumed by OH + CO , H + CO2. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Given the serious air pollution and strict environmental policy, the efficiency and low emission of combustion technology have become the subjects of intense research in the combustion field. Moderate or intense low-oxygen dilution (MILD) combustion [1], also denoted as flameless combustion [2,3], flameless oxidation [4,5], and colorless distributed combustion [6,7], has increasingly attracted research attention because of its high efficiency and low pollutant emission, particularly its emission of CO and NOX. ⇑ Corresponding authors. E-mail addresses: [email protected] (C. Zou), [email protected] (L. Cai). http://dx.doi.org/10.1016/j.fuel.2016.12.010 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

In MILD combustion, the temperature of the reactant mixture is higher than the mixture auto-ignition temperature, and the oxygen concentration in the reactant mixture is very low, typically 3–5%, by high dilution with hot combustion products [8]. The high temperature and low oxygen concentration of the reactant mixture lead to a uniform temperature field, no visual flame in the furnace, and low CO and NOX emissions [9,10]. Cavaliere and Jonnan systematically explained MILD combustion from the physical, chemical, and thermodynamic aspects to fully understand the fundamental applications of MILD combustion [1]. In the past few years, many researchers investigated MILD combustion [11–13]. Experimental and numerical modeling of a jet in hot coflow (JHC) burner under different oxygen mole fractions

Y. Liu et al. / Fuel 192 (2017) 140–148

was conducted by Dally et al. [14,15]. They concluded that different oxygen mole fractions in the reactant mixtures affect the flame structure because the pathways and chemical mechanisms of CO, OH, and NO formations in MILD combustion mode are different from those in conventional combustion. Szegö et al. [16] reported that CO emission is related to the mixing patterns of the combustion and furnace temperature rather than to reaction quenching by the heat exchanger. Many studies on the MILD combustion of gas fuel [17,18], sawdust [19], and coal [20] have indicated that CO emissions in the MILD combustion mode are much lower than those in the conventional combustion mode. Mardani et al. investigated methane MILD combustion in a JHC burner through numerical simulation [21–23]. They reported that the ethane oxidation path (CH3 ? C2H6 ? C2H5 ? C2H4 ? C2H3 ? C2H2 ? CO) is weakened in the methane MILD combustion regime; this phenomenon is an important reason for the low CO emission. However, they did not provide the CO formation mechanisms. Recently, Tu et al. [24] found a high CO concentration zone in the furnace when MILD combustion was achieved. In our previous study [25], we also observed a high CO concentration zone in the furnace, although the CO emission was 17 ppm in MILD combustion without preheated air. This phenomenon showed that, although CO emission was very low in the MILD combustion mode, a local high CO zone could have appeared in the furnace. An important progress in MILD combustion is that MILD combustion without preheated air has been achieved [25]. Zou et al. studied the characteristics and mechanisms of CO formation in the MILD combustion without preheated air regime using opposed diffusion flame with simultaneous diluted and preheated oxidant and fuel [26], and found that the elementary reaction OH + CO , H + CO2 has a predominate function in the MILD combustion regime. Therefore, the present work aimed (1) to explain the phenomenon that, although CO emission was very low in the MILD combustion mode, a local high CO zone could have appeared in the furnace, and (2) to analyze the CO formation mechanism in MILD combustion without preheated air. MILD combustion without preheated air was performed using methane as fuel in a laboratory-scale MILD combustion furnace (MCF). A computational fluid dynamics model with GRI-Mech 3.0 was employed to analyze the CO formation mechanisms in this combustion mode. By analyzing the rate of production, the phenomenon that a local high CO concentration zone existed in the furnace although MILD combustion has ultralow CO emission was explained, and the paths of CO formation in the MILD combustion without preheated air were obtained.

2. Experimental The experiments were conducted in a laboratory-scale MCF. The experimental process and measurement had been described in detail in our previous work [25], and thus only a brief introduction was provided in the present work. Fig. 1 shows the schematic diagram of the MCF and burner system. The combustion chamber height was 550 mm, and it had a square cross-section of 250  250 mm2. The furnace was equipped with a 50 mm-thick high-temperature ceramic and 75 mm-thick extremetemperature refractor to insulate the combustion chamber. As shown in Fig. 1, four exhaust ports were symmetrically arranged in the furnace bottom. These exhaust ports were used to enhance the flue gas cycling inside the combustion chamber. Air and fuel were well mixed, and thus the stability of MILD combustion was established. The distance between the port and the central tube was 43 mm. Fuel was injected into the furnace through a bluff body, and air was injected through an annular channel around

141

Fig. 1. Schematic diagram of the MCF and burner system.

the bluff body. The diameters of the exhaust port, fuel tube, bluff body, and air annular channel were 26, 6, 22, and 26 mm, respectively. The viewing window was uniformly distributed on the two sides of the furnace, as shown in Fig.1. Each side had five openings and was insulated by the firebrick. Three rows of ports were spaced in the back of furnace, and each row had six ports. The distance between the centers of these six ports and the bottom of the furnace was 45, 135, 225, 315, 405, and 495 mm. The high temperature furnace flue gas is cooled by a water cooling sampling probe before it enters into the gas analyzer. The cooling probe was designed with double layer, inner layer enter flue gas and outer layer enter water. Furnace gas composition is measured by Kane 9106 portable gas analyzer. Estimating the analyzer measurements accuracies are as follows: O2 (±0.1%, 0–25%), CO (±20 ppm, 0–400 ppm; ±5%, 400–2000 ppm; ±10%, 2000– 10,000 ppm; ±0.1%, 1–10%), CO2 (±0.3%, 0–99.9%), SO2 (±5 ppm, 0–100 ppm; ±5%, 100–5000 ppm). A bare, fine-wire type R (Pt-Pt13% Rh) thermocouple was used to measure the time-average furnace temperature. The burner designed maximum thermal power is 20 kW. In this study, CH4 was used as the fuel, the thermal power is 9.5 kW, the inlet fuel velocity Ufuel = 9.32 m/s and the temperature is 306 K, the inlet air velocity Uair = 22.38 m/s and the temperature is 306 K, the global excess air rate is 1.25. 3. Modeling Steady Favre-averaged 3D Navier–Stokes equations were solved by finite volume code FLUENT using the ANSYS 15.0 package. The modified k-e model coupled with a standard wall function was adopted to simulate the flow filed. According to Dally and Christo [15], the coefficient Ce1 in the eddy dissipation equation changed

142

Y. Liu et al. / Fuel 192 (2017) 140–148

from 1.44 to 1.6 to improve the modeling prediction accuracy. In MILD combustion, the radiative heat transfer between the flame and the wall cannot be neglected, and the optical thickness is unclear; consequently, the discrete ordinate (DO) radiation model incorporating the weighted sum of gray gas model (WSGGM) was adopted [27]. The interaction between turbulence and chemistry was solved by the eddy dissipation concept (EDC) model [28]. The GRI-Mech 3.0 was applied to the simulations in this work according to Wang et al. [29] and de Joannon et al. [30]. The SIMPLE algorithm was used to handle the pressure–velocity coupling. The second-order scheme was used to pressure discretization, and the second-upwind scheme was adopted to solve the other transport equations to enhance the accuracy of the simulation. The furnace was a symmetrical rectangle, and thus the computational domain covered half of the furnace and was 550 mm long and 125 mm wide. A grid independence study was conducted by the number of cells ranging from 600,000 to 1,200,000, and the 900,000 refined structured cells were selected. The residuals were kept lower than 106 for energy and DO intensity, and 105 for all other variables as a convergence criterion.

through the origin O. Fig. 3 shows the measured values and numerical values of the temperature and concentrations of CO and O2 in the XOZ plane in the MILD combustion without preheated air mode. As shown in Fig. 3, the numerical results conform well to those from the experiments. In other words, the mode used in the present study was applicable to analyze MILD combustion without preheated air.

4. Results and discussions

M ¼ n=nst

4.1. MILD combustion realization The definition of MILD combustion suggested by Cao et al. [25] as follows: (1) no visible flame in the furnace, (2) according to Kumar’s definition [31], the normalized spatial temperature variaR tion, b, is defined as b ¼ T 0 =T mean , in which T mean ¼ TdV=dV, R 0 R 2 02 T ¼ ðT  T mean Þ dV= dV, the value of variation should be <15% for a nearly uniform temperature field and (3) the emissions of NOx and CO should be <100 ppm in the flue gas. Photographs of conventional combustion and the MILD combustion without preheated air mode in our experiments are shown in Fig. 2. In the MILD combustion without preheated air mode, no visible flame was found inside the furnace, and the entire furnace was hot and bright. The temperature values of 108 measuring points were uniform, and the normalized spatial temperature variation was 8.4%. The CO and NOX concentrations in the flue gas were 17 and 8 ppm, respectively. Therefore, according to the definition in the literature [25], MILD combustion without preheated air was achieved. 4.2. Validation of simulation As shown in Fig. 1, the center of the furnace bottom surface is defined as the coordinate origin O, the central axis is defined as the Z axis, the X axis is a horizontal axis that crosses the fuel jet center, and axis Y is perpendicular to the XOZ plane and passes

4.3. Flow and mixture In order to help understanding of dynamic characteristic of the furnace, the gas flow pattern for combustion without preheating air is shown in Fig. 4. Fig. 4 showed the velocity vector on the XOZ plane for without preheating. It can be found from Fig. 4 that the arrangement of fuel jet and out port in the same side facilitates the recirculation of combustion products, and the large quantity of hot combustion products are again entrained into air and fuel steam to heat and dilute the mixtures of fuel and air. To evaluate the mixture uniformity of the furnace, a mixing factor was introduced and defined as follows:

ð1Þ

where n is mixture fraction and nst is the mixture fraction at stoichiometric ratio. Mixture fraction is defined according to Bilger’s formula [32] and Christo and Dally [15]

n¼

2ðZ C  Z C;o Þ=W C þ ðZ H  Z H;o Þ=ð2W H Þ  ðZ O  Z O;o Þ=W O 2ðZ C;f  Z C;o Þ=W C þ ðZ H;f  Z H;o Þ=ð2W H Þ  ðZ O;f  Z O;o Þ=W O

ð2Þ

where Wi denotes the atomic mass of element I. The subscripts C, H and O correspond to the elements C, H and O, respectively, while the subscripts f and o refer to the fuel and the oxidizer mixture conditions, respectively. Zi is conserved scalar given by the total mass fraction of element i, which can be prescribed as:

Zi ¼

N X

lj;i Y j

ð3Þ

j¼1

where lj;i denotes the mass fraction of element i in species j, Y j is denotes the mass fraction of species j. In this study, Zi was calculated using all species in the GRI-Mech 3.0. As for the oxidant is air, the nst = 0.055. Fig. 5 shows the counter map of the mixing factor in the furnace during the MILD combustion without preheating air. As shown in Fig. 5, in the most of furnace zone, the values of the mixing factor are in the range of 0.6–0.8, which means the mixture uniformity inside the furnace is very well. Moreover, the normalized spatial temperature variation based the experimental values is 8.4%, which means the temperature filed was nearly uniform.

Fig. 2. Photographs of flames with (a) conventional flame and (b) stable MILD combustion.

143

Y. Liu et al. / Fuel 192 (2017) 140–148

O2 concentration(%)

(a)

18 12 6

Ƶ

modeling measurements

Z=495mm

18 12 6

Z=405mm

18 12 6

Z=315mm

18 12 6

Z=225mm

18 12 6

Z=135mm

0

20

40

60

80

100

120

140

Distence from furnace center (mm)

Temperature (K)

(b) 1800

1200 600

Z=495mm

1800 1200 600

Z=405mm

1800 1200 600

Z=315mm

1800 1200 600

Z=225mm

Fig. 4. Velocity vector on the XOZ plane.

1800 1200 600

Z=135mm

0

20

40

60

80

100

120

140

Distence from furnace center (mm)

CO concentration (ppm)

(c) 6000

Z=495mm

4000 2000 0 6000 4000 2000 0

Z=405mm

6000 4000 2000 0

Z=315mm

6000 4000 2000 0

Z=225mm

6000 4000 2000 0

Z=135mm

0

20

40

60

80

100

120

140

Distence from furnace center (mm) Fig. 3. Comparison between the predicted and the experimental results for Tair = 306 K and Tfuel = 306 K: (a) mole fraction of O2, (b) temperature, (c) mole fraction of CO.

Therefore, the mixture uniformity was performed inside the furnace.

4.4. CO formation characteristic analysis 4.4.1. CO distribute in the furnace Fig. 6 shows the contour map of the CO mole fraction in the XOZ plane. The arrow marked F represents fuel in, the arrow marked A

Fig. 5. Counter map of the predicted n/nst on the XOZ plant.

represents air in, and the arrow marked E represents exhaust. As shown in Fig. 6, the furnace has three regions. The first region (I) is the central zone in which the CO mole fraction is very low. The second region (II) is the high concentration zone in which the CO mole fraction is much higher than that in the central zone. The third region (III) is the recirculation zone in which the CO mole fraction is very low and comparable with that in the central zone. Among the three zones mentioned above, the recirculation zone accounts for the largest proportion of the furnace area, whereas the central and high-concentration zones are relatively small.

144

Y. Liu et al. / Fuel 192 (2017) 140–148

Fig. 6. Counter map of the predicted CO mole fraction.

4.4.2. CO formation characteristic analysis at line Y = 0 mm, Z = 210 mm To elucidate the CO production mechanism in MILD combustion, we chose the characteristic line (Y = 0 mm, Z = 210 mm) to investigate and verify the characteristics of CO formation. The profiles of CO mole fraction and net reaction rate of CO at the characteristic line are shown in Fig. 7. The central zone was from 0 mm to 7.9 mm, in which the reaction rate was relatively small and increased very slowly. The high CO concentration zone was from 7.9 mm to 28.1 mm. The mole fraction of CO reached a peak at 15.3 mm, wherein the net reaction rate of CO had the highest value. When the location exceeded 15.3 mm, the mole fraction of CO rapidly decreased and reached a valley point at 18.4 mm. The net reaction rate of CO rapidly decreased from positive to negative value. When the location was beyond 18.4 mm, the net reaction rate of CO increased again to a value that is comparable with that in the central zone, while the CO mole fraction decreased. Clearly,

0.8

A

Net Reaction Rate of co

Mole fraction of CO (ppm)

6000

Mole fraction of CO

II

I

5000

0.6

III 0.4

4000 0.2

3000 2000

D

C

0.0

1000

-0.2

Net Reaction Rate of co(kg/m3-s)

7000

the net reaction rate of CO is closely associated with the high CO concentration in the zone. The recirculation zone is located from 28.1 mm to 125 mm, where the mole fraction of CO is a stale value of 1000 ppm and the net production rate of CO is very low. To study the CO formation mechanism in MILD combustion, we selected four characteristic points in the net reaction rate profile: point A (x = 15.3 mm) and point B (x = 18.4 mm) were the maximum and minimum net reaction rate points, respectively, point C (x = 0 mm) was located in the central zone, and point D (x = 99.9 mm) was located in the recirculation zone. At point A, the top three positive production rates of CO were R168 (HCO + O2 , HO2 + CO), R166 (HCO + H2O , H + CO + H2O), and R167 (HCO + M , H + CO + M), which were much larger than the others. These reactions produced CO from HCO. As shown in Fig. 8 the mole fraction of CO increased and decreased with the increase and decrease in the mole fraction of HCO, respectively, and the location of the CO maximum mole fraction was the same as that of HCO. Consequently, HCO became the main radical for the CO formation. The profiles of the mole fractions of HCO and OH are shown in Fig. 9. The mole fraction of HCO drastically increased with the mole fraction of OH. The reaction R101 (CH2O + OH , HCO + H2O) had the largest production rate of HCO because of the high mole fraction of OH, and the location of its maximum reaction rate was the same as that of the HCO. Accordingly, reaction CH2O + OH , HCO + H2O was the important reaction for the CO formation. R99 (OH + CO , H + CO2) had the largest reaction of consumption rate, and it was several magnitudes more than the other reactions. As a result, R99 was the most important reaction of CO consumption in MILD combustion. Notably, R99 was also the reaction of OH consumption, and it competed with R101 for OH. Thus, as shown in Fig. 9, the reaction rates of R101 and R99 increased with the increase in OH, but R101 increased more sharply than R99 in the distance ranging from 10.2 mm to 15.3 mm. Accordingly, the net production rate and the concentration of CO increased to the maximum value. In the high CO concentration zone, R11 (O + CH4 , OH + CH3) had the highest production rate of OH and the highest consumption rate of O, and thus the location of the OH maximum concentration was the same as that of the O concentration, as shown in Fig. 10. High OH concentration due to high O concentration resulted in R101 outcompeting R99 for OH, which led to the drastic increase in the concentrations of HCO and CO.

B

0 0

20

40

60

80

100

120

-0.4 140

Distence from the furnace center (mm) Fig. 7. Profile of the CO mole fraction and net reaction rates of CO at line Y = 0 mm, Z = 210 mm.

145

Y. Liu et al. / Fuel 192 (2017) 140–148

7000

0.30

Mole fraction of CO

5000

0.20

4000 0.15 3000 0.10

2000

0.05

Mole fraction of CO(ppm)

Mole fraction of HCO (ppm)

6000

Mole fraction of HCO

0.25

1000

0.00

0 0

10

20

30

40

50

60

Distence from the furnace center (mm)

Mole fraction of HCO (ppm)

Mole fraction of HCO Mole fraction of OH

0.25

Arrhenius Rate of Reaction-99 Arrhenius Rate of Reaction-101

0.20

3200

0.32

2800

0.28

2400

0.24

2000

0.15

1600 1200

0.10

800 0.05

Mole fraction of OH(ppm)

0.30

400

0.00

0 0

10

20

30

40

50

60

0.20 0.16 0.12 0.08 0.04

Reaction Rate (kg/m3-s)

Fig. 8. Profile of the CO and HCO mole fractions at line Y = 0 mm, Z = 210 mm.

0.00 -0.04

Distence from the furnace center (mm) Fig. 9. Profile of the HCO and OH mole fractions and the Arrhenius rate of R99 and R101 at line Y = 0 mm, Z = 210 mm.

1600

Mole fraction of O Mole fraction of OH

1400

Mole fraction (ppm)

1200 1000 800 600 400 200 0 0

10

20

30

40

50

Distance from the furnace center (mm) Fig. 10. Profile of the O and OH mole fractions at line Y = 0 mm, Z = 210 mm.

60

146

Y. Liu et al. / Fuel 192 (2017) 140–148

Fig. 11. Paths of CO2 formation in the central zone (a), high concentration zone (b) and recirculation zone (c). Reaction rates are shown in parentheses in kgmol/cm3 s.

At point B, the top five reactions that had a positive production rate of CO were the same as those at point A, and these values were much smaller than those at point A. For CO consumption, the consumption rate of R99 was still several magnitudes more than the other CO consumption reactions. As shown in Fig. 9, when the location exceeded 15.3 mm, the reaction rate of R101 dropped

drastically, and thus the reaction rate of R99 also dropped. However, R101 decreased more sharply than R99. Consequently, the net production rate and concentration of CO decreased. At point C in the central zone, R168 dominated the CO production, and the CO consumption rate of R99 was higher in several magnitudes than the other consumption reactions. Given that the

Y. Liu et al. / Fuel 192 (2017) 140–148

CO production rate of R168 was slightly larger than the CO consumption rate of R99, the mole fraction of CO increased slowly in the zone. At point D in the recirculation zone, the top two reactions of CO production rate were R12 (O + CO (+M) , CO2 (+M)) and R120 (HO2 + CO , OH + CO2). Both R12 and R120 were the decomposition reactions of CO2, and their CO production rates were much larger than the other CO production reactions. Therefore, CO was mainly produced from the CO2 decomposition in the recirculation zone. R99 still dominated in the CO consumption in the recirculation zone. Notably, the majority of CO in the recirculation zone came from zone II and zone I by convection and diffusion, and was mostly consumed by R99. Accordingly, the CO concentration was very low in the recirculation zone. 4.4.3. Path of CO2 formation The CO formation path in the central zone is shown in Fig. 11a. In the central zone, the main CO2 formation path was CH4 ? CH3 ? CH3O ? CH2O ? HCO ? CO ? CO2. CH3O was the main intermediate species formatted from CH3 by R119 (HO2 + CH3 , OH + CH3O) as a result of the high concentration of HO2. Subsequently, CH3O was mainly converted to CH2O by R170 (CH3O + O2 , HO2 + CH2O) and produced HO2. The two reactions used HO2 radical as linkage, which made the path (CH3 ? CH3O ? CH2O) become the main path of CH3 converted to CH2O. Fig. 11b shows the path of CO2 formation in the high concentration zone. The main CO2 formation path was CH4 ? CH3 ? CH2O ? HCO ? CO ? CO2. High O2 concentration enhanced the reaction O + CH3 , H + CH2O, which resulted in an amount of CH2O. High O2 concentration also enhanced the radical pool reaction H + O2 , OH + O, which produced an amount of O radicals. Thus, reaction O + CH4 , OH + CH3 was enhanced, and it had the largest production rate of OH and CH3. The OH radical formed an amount of HCO through the reaction OH + CH2O , HCO + H2O, and CH3 radical favored the production of CH2O. An amount of HCO was converted to CO by HCO + O2 , HO2 + CO, HCO + H2O , H + CO + H2O, and HCO + M , H + CO + M. As a result, high O2 concentration (approximately 10%) was the main reason for the high CO concentration. The path of CO2 formation in the recirculation zone is shown in Fig. 11c. R99 dominated in the CO consumption in the recirculation zone. The production of CO was mainly from CO2, but it was much lower than the consumption amount of CO in the recirculation zone. The majority of CO in the zone came from the central or the high CO concentration zone by convection or diffusion, and was consumed by R99. This phenomenon resulted in a very low CO emission. 5. Conclusions Through experimentation and numerical simulations, the CO formation mechanism was investigated under methane MILD combustion without preheated air. The following conclusions were obtained. 1. Although MILD combustion without preheated air was conducted, a high CO concentration zone could have appeared in the furnace. Local high O2 concentration (approximately 10%) was the reason for this phenomenon. High O2 concentration enhanced the reaction O + CH3 , H + CH2O, which resulted in an amount of CH2O. High O2 concentration also enhanced the radical pool reaction H + O2 , OH + O, which produced an amount of O radicals. Thus, reaction O + CH4 , OH + CH3 was enhanced, and it had the largest production rate of OH and CH3. The OH radical led to an amount of HCO by reaction OH

147

+ CH2O , HCO + H2O, and CH3 radical favored the production of CH2O. An amount of HCO was converted to CO by HCO + O2 , HO2 + CO, HCO + H2O , H + CO + H2O, and HCO + M , H + CO + M. 2. In the central zone, the main CO2 formation path was CH4 ? CH3 ? CH3O ? CH2O ? HCO ? CO ? CO2. The reaction HO2 + CH3 , OH + CH3O and the reaction CH3O + O2 , HO2 + CH2O used HO2 radical as linkage, which made the path (CH3 ? CH3O ? CH2O) become the main path of CH3 converted to CH2O. 3. R99 dominated in the CO consumption in the recirculation zone. The majority of CO in the zone came from the central or the high CO concentration zones by convection or diffusion, it and was consumed by R99. This phenomenon resulted in a very low CO emission.

Acknowledgements This work was supported by the general program (51176055) of the National Natural Science Foundation of China. References [1] Cavaliere A, de Joannon M. Mild combustion. Prog Energy Combust Sci 2004;30:329–66. [2] Sánchez M, Cadavid F, Amell A. Experimental evaluation of a 20 kW oxygen enhanced self-regenerative burner operated in flameless combustion mode. Appl Energy 2013;111:240–6. [3] Wünning J. Flameless combustion and its applications. Nat Gas Technol Orlando (USA) 2006:30. [4] Wünning J, Wünning J. Burners for flameless oxidation with low NOx formation even at maximum air preheat. Gaswärme Int 1992;41:438–44. [5] Milani A, Wünning J. What are the stability limits of flameless combustion. Combust File 2002:1–6. [6] Khalil AE, Arghode V, Gupta AK. Colorless distributed combustion (CDC) with swirl for gas turbine application. Proceedings of ASME power conference, POWER2010, Chicago, IL; 2010. p. 77–88. [7] Arghode VK, Gupta AK. Effect of flow field for colorless distributed combustion (CDC) for gas turbine combustion. Appl Energy 2010;87:1631–40. [8] De Joannon M, Saponaro A, Cavaliere A. Zero-dimensional analysis of diluted oxidation of methane in rich conditions. Proc Combust Inst 2000;28:1639–46. [9] Kumar S, Paul P, Mukunda H. Investigations of the scaling criteria for a mild combustion burner. Proc Combust Inst 2005;30:2613–21. [10] Tsuji H, Gupta AK, Hasegawa T, Katsuki M, Kishimoto K, Morita M. High temperature air combustion: from energy conservation to pollution reduction. CRC Press; 2002. [11] Galbiati MA, Cavigiolo A, Effuggi A, Gelosa D, Rota R. Mild combustion for fuelNOx reduction. Combust Sci Technol 2004;176:1035–54. [12] Katsuki M, Hasegawa T. The science and technology of combustion in highly preheated air. In: Symposium (International) on combustion. Elsevier; 1998. p. 3135–46. [13] Wünning J, Wünning J. Flameless oxidation to reduce thermal NO-formation. Prog Energy Combust Sci 1997;23:81–94. [14] Dally BB, Karpetis A, Barlow R. Structure of turbulent non-premixed jet flames in a diluted hot coflow. Proc Combust Inst 2002;29:1147–54. [15] Christo F, Dally BB. Modeling turbulent reacting jets issuing into a hot and diluted coflow. Combust Flame 2005;142:117–29. [16] Szegö G, Dally B, Nathan G. Operational characteristics of a parallel jet MILD combustion burner system. Combust Flame 2009;156:429–38. [17] Cavigiolo A, Galbiati MA, Effuggi A, Gelosa D, Rota R. Mild combustion in a laboratory-scale apparatus. Combust Sci Technol 2003;175:1347–67. [18] Veríssimo A, Rocha A, Costa M. Operational, combustion, and emission characteristics of a small-scale combustor. Energy Fuels 2011;25:2469–80. [19] Shim S, Dally B, Ashman P, Szegö G, Craig R. Sawdust burning under MILD combustion conditions. Proceedings of the Australian combustion symposium; 2007. p. 9–11. [20] Li P, Wang F, Tu Y, Mei Z, Zhang J, Zheng Y, et al. Moderate or intense lowoxygen dilution oxy-combustion characteristics of light oil and pulverized coal in a pilot-scale furnace. Energy Fuels 2014;28:1524–35. [21] Mardani A, Tabejamaat S, Hassanpour S. Numerical study of CO and CO2 formation in CH4/H2 blended flame under MILD condition. Combust Flame 2013;160:1636–49. [22] Mardani A, Tabejamaat S, Mohammadi MB. Numerical study of the effect of turbulence on rate of reactions in the MILD combustion regime. Combust Theory Model 2011;15:753–72. [23] Mardani A, Tabejamaat S, Ghamari M. Numerical study of influence of molecular diffusion in the MILD combustion regime. Combust Theory Model 2010;14:747–74.

148

Y. Liu et al. / Fuel 192 (2017) 140–148

[24] Tu Y, Liu H, Su K, Chen S, Liu Z, Zheng C, et al. Numerical study of H2O addition effects on pulverized coal oxy-MILD combustion. Fuel Proc Technol 2015;138:252–62. [25] Cao S, Zou C, Han Q, Liu Y, Wu D, Zheng C. Numerical and experimental studies of NO formation mechanisms under methane moderate or intense low-oxygen dilution (MILD) combustion without heated air. Energy Fuels 2015;29:1987–96. [26] Zou C, Cao S, Song Y, He Y, Guo F, Zheng C. Characteristics and mechanistic analysis of CO formation in MILD regime with simultaneously diluted and preheated oxidant and fuel. Fuel 2014;130:10–8. [27] Chui E, Raithby G. Computation of radiant heat transfer on a nonorthogonal mesh using the finite-volume method. Numer Heat Transf 1993;23:269–88.

[28] Gran IR, Magnussen BF. A numerical study of a bluff-body stabilized diffusion flame. Part 2. Influence of combustion modeling and finite-rate chemistry. Combust Sci Technol 1996;119:191–217. [29] Wang F, Mi J, Li P, Zheng C. Diffusion flame of a CH4/H2 jet in hot low-oxygen coflow. Int J Hydrogen Energy 2011;36:9267–77. [30] de Joannon M, Sorrentino G, Cavaliere A. MILD combustion in diffusioncontrolled regimes of Hot Diluted Fuel. Combust Flame 2012;159:1832–9. [31] Kumar S, Paul P, Mukunda H. Studies on a new high-intensity low-emission burner. Proc Combust Inst 2002;29:1131–7. [32] Bilger RW, Stårner SH, Kee RJ. On reduced mechanisms for methane air combustion in nonpremixed flames. Combust Flame 1990;80:135–49.