Experimental study of combustion in a double-layer burner packed with alumina pellets of different diameters

Applied Energy 100 (2012) 295–302

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Experimental study of combustion in a double-layer burner packed with alumina pellets of different diameters Huai-Bin Gao, Zhi-Guo Qu ⇑, Ya-ling He, Wen-Quan Tao Key Laboratory of Thermo-Fluid Science and Engineering of MOE, Energy and Power Engineering School, Xi’an Jiaotong University, Xi’an 710049, China

h i g h l i g h t s " Stable operating region was enlarged with increased alumina pellet diameter. " Flame temperature decreased with increased alumina pellet diameter. " Optimal sphere diameter corresponding to lowest NOx emissions was found. " HC emissions decreased with increased pellet diameter.

a r t i c l e

i n f o

Article history: Received 17 February 2012 Received in revised form 15 May 2012 Accepted 20 May 2012 Available online 21 July 2012 Keywords: Double-layer burner Pellet diameter Flame stability limits Flame temperature Emissions

a b s t r a c t The combustion performance in a double-layer burner packed with alumina pellets of different diameters was experimentally studied. The effects of the diameter of the alumina pellet on the flame stability limits, flame temperature, pressure drop, and pollutant emissions were examined. The 3 mm diameter alumina pellets were located in the upstream, while the 6, 8, 10, and 13 mm diameter alumina pellets were located in the downstream. A single-layer burner packed with 3 mm diameter pellets was also investigated as a reference. The flame can be more effectively stabilized near the interface between the two sections of double-layer burner. The flame stability limits could apparently be extended in the double-layer burner compared with the single-layer burner. The flame and surface temperatures increased more evidently with increased flame speed for the single-layer burner than for the double-layer burner. The flame temperature decreased with increased alumina pellet diameter, whereas the surface temperature was insensitive to the pellet diameter. An optimal pellet diameter corresponding to the lowest NOx emissions was found. The CO emissions of the double-layer burner were lower than those of the single-layer burner at a low velocity range (S < 35 cm/s) and was almost identical for pellets of different diameters at a high velocity range (S > 35 cm/s). The unburned hydrocarbon emissions decreased with increased alumina pellet diameter within the entire experimental velocity range. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Combustion in porous media has been studied as an advanced combustion technology, which can offer superior advantages such as extended lean flammability limits, low pollutant emissions, high radiant outputs, higher power densities and flame speeds compared with free-flame combustion systems. Comprehensive reviews [1–4] of the theoretical background and experimental/ numerical studies have been dedicated to further understanding of combustion in porous media. These reviews focused on the effects of operating/design parameters including equivalence ratio, thermal load, inlet temperature and porous matrix material on combustion performance such as flame stability, emissions and radiant output. ⇑ Corresponding author. E-mail address: [email protected] (Z.-G. Qu). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.05.019

Porous burners can be classified into single layer and multilayer types from the standpoint of geometric structures. Initially, most studies [5–9] focused on methane combustion in single-layer porous burners. Echigo et al. [5] theoretically and experimentally shown that porous medium combustion can extend the combustion flame limit. Sathe et al. [6] numerically analyzed combustion in inert porous media to understand the flame stabilization in porous radiant burners. Hsu et al. [7] numerically investigated premixed combustion within a highly porous inert medium and indicated that the lean limit can be extended to an equivalence ratio of about 0.36 for a methane/air flame. Akbari et al. [8] numerically investigated laminar premixed flame propagation of methane/air mixture in a porous medium and indicated that the stable performance range of the burner is extended when the equivalence ratio increases; however, the blow-out region expands with an increase in the firing rate. Later study of Akbari and Riahi [9] indicated that flame stability and thermal characteristics of the

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Nomenclature A BTM E EB ES ET ETh E/ k(T) M R S Smax

cross-section area of the burner (m2) pre-exponential factor activation energy uncertainty of the terminal block (°C) uncertainty of the flame speed (cm/s) uncertainty of the temperature measurement system (°C) uncertainty of the thermocouples (°C) uncertainty of the equivalence ratio rate constant temperature exponent standard molar constant flame speed (cm/s) maximum flame speed (cm/s)

burner are strongly dependent on the inlet mixture specifications and the solid matrix structural properties. Two-section porous burners have received a great deal of attention for its good flame stabilization in the last two decades. Most of two-section burners were constructed of reticulated porous ceramics such as partially stabilized zirconia (PSZ) foam by Hsu et al. [10], Khanna [11] and Barra et al. [12], alumina oxide (Al2O3) foam by Zhou and Pereira [13], partially stabilized zirconia (PSZ), yttria-stabilized zirconia/alumina composite (YZA) and FeCrAlY foam by Ellzey and coworkers [14–16]. The above two-section burner designs have been shown to stabilize the flame easily at the interface between two different porosity ceramic blocks over a wide range of conditions. Malico and coworkers [17–19] presented a 3D numerical model of a two-layer porous burner composed of a 10 PPI SiC porous foam preceded by a perforated alumina (Al2O3) plate and the results showed that using smaller pore diameters can avoid flashback. Similar to the previous work by Hsu et al. [10], Djordjevic et al. [20,21] studied two-section burner that stacked small pore sponge (zirconia partially stabilized with magnesia; 45 PPI)in the upstream and either 10 or 20 PPI Al2O3 and SiSiC foams in the downstream section. Francisco et al. [22] investigated combustion of hydrogen-rich gaseous fuels with a low calorific value in a foam burner, which consists of two distinct layers: a preheating region (40 PPI) and a stable burning region (10PPI). The laminar flame speed increased as the H2 content in the mixture increased to lead to extended flame stability limits. By using the same burner, Catapan et al. [23] studied the flame stabilization operating in controllable wall temperature. They found an increase in the burner flame temperature occurred in heated environment as compared to adiabatic condition. Reticulated foams are the most commonly used materials in porous burners due to the advantages such as improved gas mixing, high porosities and low pressure drop. However, the foams are easy to damage after exposure to high combustion environment and thermal shock. Packed beds of discrete particles are a commonly used alternative to foams. The main stated advantage of discrete particles is the significantly improved durability and heat capacity because the particles are small robust shapes compared to a rigid and continuous structure. Zhdanok et al. [24] studied low-velocity filtration combustion of methane/air mixture in a packed bed of 5.6-mmdiameter alumina spheres. A temperature can be reached much in excess of the adiabatic reaction temperature and thus self-propagating reactions are possible even in a weakly exothermic medium. Subsequently, Foutko et al. [25] and Henneke and Ellzey [26] simulated the experiments of Zhdanok et al. [24]. Foutko et al.

T TC V_ V_ max V_ air V_ airmax V_ CH4 V_ CH max 4

e / /max

temperature, K centerline temperature (°C) total volumetric flow rate of fuel and air (m3/s) maximum total volumetric flow rate of fuel and air (m3/s) volumetric flow rate of air (m3/s) maximum volumetric flow rate of air (m3/s) volumetric flow rate of CH4 (m3/s) maximum volumetric flow rate of CH4 (m3/s) porous matrix surface emissivity equivalence ratio maximum equivalence ratio

[25] suggested that the temperature increase for the gas element may be treated as a thermal explosion and Henneke and Ellzey [26] stated that gas-phase transport is important for wave propagation only at high equivalence ratios. Bubnovich et al. [27,28] studied double layer burners with alumina spheres of 4.8–13 mm diameter and 2.5–5.6 mm diameter to determine the stabilization ranges. Gao et al. [29] experimentally studied biogas combustion in a two-layer porous burner packed with alumina spheres of 3.0 mm (upstream) and 8.0 mm (downstream) diameter. The results showed that CO2 concentration in the biogas had influences on the flame temperature, stable flame region, pressure drops and pollutant emissions. However, the effect of the diameter of stacked beads on flame stabilization has not been reported experimentally. From the above review, the previous studies mainly focused on combustion in ceramic foams with fixed porosities or packed beds with fixed diameters. The variation in packed bed diameter can result in various porosity, which have great impacts on flame stability, flame temperature and pollutant emissions. The main objectives of the current work are the determination of the gas velocity and equivalence ratio stability ranges in a porous medium burner comprising two layers of alumina pellets with diameters of 3 mm at the upstream section, as well as 6, 8, 10, and 13 mm at the downstream section. The effects of pellet diameter on the flame stability limits, flame temperature profile, pressure drop, and pollutant emissions were examined. 2. Experimental section 2.1. Experimental setup A schematic diagram of the experimental apparatus is shown in Fig. 1. The test rig and procedure was similar to the previous study of the present authors [29] which consists of four parts: a mixing chamber, the supply system for fuel/air, a porous burner, measuring and data processing equipment. The mixing chamber is comprised a 5 cm diameter stainless steel cylinder with two opposite entrances for methane and air, as well as one exit for the gas mixture. Methane and air flows were regulated using two mass flow controllers (SierraC10Smart-Trak) to satisfy the required conditions before the gases enter the mixing chamber. Methane (99.5%) was stored in high-pressure bottles. The air was supplied by a compressor connected to an air storage tank for pressure stabilization and was then filtered prior to entering the experimental apparatus. The burner was a 50 mm ID corundum tube, 5 mm in thickness and 200 mm in length, which has a two-layer porous

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Gas analyzers O2

CO

NOx

HC

Gas

Packed balls

Data acquisition system

Perforated plate

P MFC

Filter

Mixer

CH 4 Air

Filter

MFC

Compressor Fig. 1. Schematic of the experimental setup.

25 mm Kaowool Thermocouples X/mm

100 mm 12.5 mm

TC1 TC2 TC3 TC4 TC5

P

TC6 TC7 TC8 TC9

0

Perforated plate

3mm balls 100 mm

Fuel/air Fig. 2. Schematic of the porous burner.

ence by a Kaowool high-temperature insulation material layer with a thickness of 45 mm. The burner had nine B-type thermocouples located at 1.25 cm intervals along the flow direction. The first thermocouple was located at the entrance of the burner, and the fifth thermocouple was located at the interface of the small and large packed pellets. The pressure drop across the porous media was measured using a Rosemount 3051 pressure transducer connected from a pressure tap just upstream of the packed pellets to the exit of the burner. All signals for the thermocouples and pressure transducer were recorded using an Agilent data acquisition system. A stainless steel probe is used to sample the exhaust gases at top of the burner and the concentrations of NOx, CO, HC in the exhaust from the burner were measured using a TESTO 350 Pro gas analyzer. The experimental procedure and uncertain analysis can be consulted with Gao et al. [29] of the present authors. The details of the experiment uncertainties are available in Table 2. The equivalence ratio ranged from 0.60 to 0.70. For comparison, the test conditions, porous burner configuration, and porous medium materials of previous studies [11,12,14,16] using double-layer burners for methane are summarized in Table 3. 2.2. Parameter definitions

Table 1 Porosity of the packed beds. Diameter (mm)

Porosity

3 6 8 10 13

0.40 0.43 0.45 0.47 0.52

structure as shown in Fig. 2. The first layer was a flame holder that comprised a 50 mm long packed bed of 3 mm Al2O3 beads. The second layer was where the flame occurred with a 50 mm long packed bed of Al2O3 beads with different diameters (3, 6, 8, 10, and 13 mm). The random packing of uniformly sized pellets with 3 mm to 13 mm diameters yielded porosities of 0.40 to 0.52, as shown in Table 1. The burner was insulated around the circumfer-

The chemical reaction was shown in Eq. (1), the equivalence ratio /, the flame speed S, which equals to the mean flow velocity, were shown in Eqs. (2) and (3).

CH4 þ

1þ/ ðO2 þ 3:76N2 Þ ! CO2 þ 2H2 O þ 7:52N2 / 1/ ðO2 þ 3:76N2 Þ þ /

ð1Þ

/¼

V_ CH4 ðmfu =mair Þactu ¼ 9:52  ðmfu =mair Þstoic V_ air

ð2Þ

S¼

V_ V_ CH4 þ V_ air ¼ A A

ð3Þ

_ A are the total volumetric flow rate of the fuel and air, the where V, inner cross sectional area of the burner, respectively. In Eqs. (2) and

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Table 2 Uncertainties for measured variables. Variable

Formula qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ET ¼ E2B þ E2Th rhffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiiffi  2 @S dV _ þð @S dAÞ2 @A @ V_ Es ¼ S

Temperature Flame speed

Parameters

Uncertainty

EB = ± 1.26 °C ETh = ± 8.5 °C

8.59 °C

@S @ V_

1.2%

¼ A1 ¼ 0:0509 cm2

¼ V_ max 12 ¼ 4:07 cm=s A qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi _ 2 _ 2 þ ðdVÞ dV_ ¼ ðdVÞ

@S @A

air

CH4

¼ 1:005 L= min ¼ 16:75 cm3 =s V_ max ¼ 94:23 L= min ¼ 1570:5 cm3 =s dV_ air ¼ 1:0 L= min; V_ airmax ¼ 87:78 L= min dV_ CH ¼ 0:1 L= min; V_ CH max ¼ 6:45 L= min 4

4

dA ¼ 0:04 cm2 ; Smax ¼ 80 cm=s s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2  2

Equivalence ratio E/ ¼

@/ @ V_ CH 4

dV_ CH4

þ

@/ @ V_ CH4

@/ dV_ air @ V_ air

¼ 9:52 V_

1 airmax

2.5%

¼ 0:155 min=L

/ @/ @ V_ air

¼ 9:52

V_ CH4 max V_ 2

¼ 8:0  103 min=L

airmax

/max = 0.70 Pressure drop NOx emission CO emission HC emission

1.5 Pa 2 ppm 2 ppm 10 ppm

Table 3 Summary chart of previously studied burners compared with the burners in the present study. Fuel

Methane Methane Methane Methane

Equivalence ratio

Material and its porosity

0.60–0.70 0.60–0.80 0.55–0.80 0.55–0.70

Al2O3 e = 0.4–0.52 PSZ e = 0.835,0.87 PSZ e = 0.835,0.87 PSZ/YZA e = 0.835,0.85

Upstream

Downstream

PPC (foams) or diameter (packed beads, mm)

Thickness (mm)

PPC (foams) or diameter (packed beads, mm)

Thickness (mm)

3.0 mm 25.6 PPC 25.6 PPC 23.6 PPC

6.0 3.5 3.5 5.08

3.0, 6.0,8.0, 10.0, and 13.0 mm 3.9 PPC 3.9 PPC 3.9 PPC

6.0 2.55 2.55 5.08

(3), the mfu and mair are the mass flow rates of the fuel and air, respectively, V_ CH4 and V_ air were the respective volumetric flow rates of CH4 and air at standard conditions, which were obtained from the mass flow controllers.

3. Results and discussion Porosity is an important factor that controls burner stability by influencing the location of flame formation and stabilization. The flame stability limits for equivalence ratio ranging from 0.60 to 0.70 for alumina pellets with different diameters of the downstream were obtained. These ranges were then compared with the computational results of Barra et al. [12] and the experiment results of Khanna [11] for pure CH4 combustion in a two-section foam burner of partially stabilized zirconia, as shown in Fig. 3. The flame stability limits is defined as the maximum and minimum flame speed that the flame can sustained in the porous region at given equivalence ratio. The lower stability limit was designated as the flow velocity at which the flame front reached the position of thermocouple TC8 (seen in Fig. 2); below this velocity, a flashback happened. The upper flame stability limits was defined as the velocity at which the flame floated on the burner surface. The present and previous results showed similar trends. The stable operating range increased and shifted to larger values with increased equivalence ratio. Both the upper and lower flammability limits of the present study were significantly lower than the computational results of Barra et al. [12] because the radial heat losses

References and data type

Present study, Exp. Barra et al. [12], Num. Khanna [11], Exp. Ellzey et al. [14,16], Exp.

were not considered in the calculations. However, the flame limits were slightly higher than the experimental results of Khanna [11] because of the higher heat capacity and lower porosity of the packed beds. The stable operating range significantly increased with increased packed pellets diameter in the downstream. This result can be attributed to a higher void volume, resulting in more radiative preheating. The temperature profiles along the centerline for / = 0.65 for the different mean flow velocities are shown in Fig. 4. The flame location can be identified as the position where the temperature reaches the highest values and the flame temperature is identified as the peak temperature among the 8 thermocouples at the given flame speed and equivalence ratio. The flame for the single-layer burner was located at the upstream at a low flame speed (S < 35 cm/s) and located at the downstream at a high flame speed (S > 35 cm/s). In contrast, the flame for a double-layer burner can be more effectively stabilized near the interface between the upstream and downstream sections as indicated in the studies of Hsu et al. [10], Zhou and Pereira [13], Mathis and Ellzey [14], as well as Vogel and Ellzey [15]. The flame temperature increased and the flame location tended to move from the interface to the downstream with increased flame speed. Wheareas, the overall shapes of the temperature profiles for the alumina pellets with different diameters were similar. The premixed flame of the larger pellets in the downstream was considerably thicker than that of the smaller pellets primarily because the larger pellets contributed to higher void volumes, and radiative preheating resulted in higher heat recirculation.

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Velocity (cm/s)

Single layer

120

Vmin 3mm

100

Double layer Vmin 3-6mm Vmin 3-8mm Vmin 3-10mm Vmin 3-13mm Vmin Barra et al.[12] Vmin Khanna et al.[11]

80

Vmax 3mm Vmax 3-6mm Vmax 3-8mm Vmax 3-10mm Vmax 3-13mm Vmax Barra et al.[12] Vmax Khanna et al.[11]

60

40

20

0 0.58

0.60

0.62

0.64

0.66

0.68

0.70

0.72

φ Fig. 3. Flame stability limits.

1600 1200

upstream 20cm/s

downstream

1400

1000

1200

800

1000

upstream downstream 40cm/s

800

600 Single layer 3mm Double layer 3-6mm 3-8mm 3-10mm 3-13mm

400 200 0 0

2

4

6

8

Single layer 3mm Double layer 3-6mm 3-8mm 3-10mm 3-13mm

600 400 200 0 0

10

2

Axial position (cm)

4 6 Axial position (cm)

upstream

10

(c)

(a) 1400

8

downstream

1600

30cm/s

1200

1400

1000

1200

upstream

downstream

45cm/s

1000

800 600

Single layer 3mm Double layer 3-6mm 3-8mm 3-10mm 3-13mm

400 200 0

800 Single layer 3mm Double layer 3-6mm 3-8mm 3-10mm 3-13mm

600 400 200 0

0

2

4

6

Axial position (cm)

(b)

8

10 0

2

4

6

8

10

Axial position (cm)

(d)

Fig. 4. Effect of the diameter of the alumina balls on the temperature profiles for different velocities (/ = 0.65): (a) 20, (b) 30, (c) 40, and (d) 45 cm/s.

H.-B. Gao et al. / Applied Energy 100 (2012) 295–302

1500 1400

Flame temperature ( )

Fig. 5 shows the flame temperature as a function of the flow velocity for the alumina pellets with different diameters at / = 0.65. The flame temperature increased with increased flame speed owing to the higher heat input. However, the maximum flame temperature did not correspond to the maximum flame speed, particularly near the exit where the heat loss was more significant. The flame temperature increased more evidently with increased flame speed for a single-layer burner compared with a two-section burner. This result can be attributed to the flame being more easily stabilized in a two-section burner than in a singlelayer burner with fluctuations in flame speed. For a two-section burner, the flame temperature decreased with increasing pellet diameter at the downstream at fixed equivalence ratios and flame speeds. That is, at a fixed heat input condition, more heat was taken from the combustion zone to the preheat zone and downstream by radiative heat transfer for the larger alumina pellets which resulted in a larger porosity. The effect of the diameter of alumina pellets on the maximum flame temperature at three equivalence ratios is shown in Fig. 6. The maximum flame temperature was defined as the maximum value of flame temperatures at a fixed equivalence ratio with different flame speeds. The maximum flame temperature increased significantly with an increased equivalence ratio owing to the high power input per unit volume. At a fixed equivalence ratio, the maximum flame temperature decreased with increased packed pellet diameter because more heat was taken from the combustion zone to the preheat zone and downstream by radiation. Thus, the diameter of the alumina pellets of the downstream had a considerable effect on the maximum flame temperature. The surface temperature as a function of flow velocity for the alumina pellets with different diameters at / = 0.65 is shown in Fig. 7. The surface temperature is identified as the temperature at the exit where the thermocouple TC1 was located. The surface temperatures of the single-layer burner increased more dramatically with increased flame speed than those of the double-layer burner. The surface temperature of the single-layer burner was found to be lower at low flame speeds (S < 35 cm/s) and higher at high flame speeds (S > 35 cm/s) compared with that of the double-layer burner. This finding can be explained by the flame being easily stabilized near the interface of the two sections and more slowly moved to the downstream with increased flame speed, as indicated in Fig. 4. Increased pellet diameter can result in increased void volume with decreased surface area. Hence, the radiative heat transfer from the reaction zone to the downstream increased because of the increased void volume, but the corresponding total convective heat transfer rate decreased because of the decreased surface area. These conditions resulted in a slight surface temperature discrepancy for the double-layer burner with pellets of different diameters. The pressure drop in the reaction flow as a function of flow velocity for the alumina pellets with different diameters at an equivalent ratio of 0.65 is shown in Fig. 8. The pressure drop of the cold flow is plotted as a reference. The pressure drop for the reaction flow was higher than that of the corresponding cold flow. The pressure drop for both the reaction and cold flows increased with increased flow velocity or decreased pellet diameter. The pressure drop in the single-layer burner was significantly higher than that of the double-layer burner, particularly for the reaction flow, owing to the lower porosity of the packed beds. The discrepancy among the pressure drops of the particles with different diameters was higher in the reaction flows than in the cold flows because of the significant gas density discrepancy in the reaction flow due to various flame temperature profiles, as shown in Fig. 4. The NOx, CO, and HC emissions with an equivalence ratio of 0.65 measured by the TESTO 350 Pro gas analyzer are shown in Figs. 9– 11. The NOx emissions as a function of flow velocity for the alumina

1300 1200 1100

Single layer 3mm Double layer 3-6mm 3-8mm 3-10mm 3-13mm

1000 900 800

10

20

30

40

50

Velocity (cm/s) Fig. 5. Flame temperature as a function of flow velocity for alumina balls with different diameters (/ = 0.65).

1600 φ=0.70 φ=0.65 φ=0.60

1500

1400

1300

1200

1100 4

6

8

10

12

14

Diameter (mm) Fig. 6. Effect of the diameter of the alumina balls on the maximum flame temperature.

1400 1300

Surface Temperature ( )

300

1200 1100 1000

Single layer 3mm Double layer 3-6mm 3-8mm 3-10mm 3-13mm

900 800 700 600 500

10

20

30

40

50

Velocity (cm/s) Fig. 7. Surface temperature as a function of flow velocity for alumina balls with different diameters (/ = 0.65).

pellets with different diameters at / = 0.65 are shown in Fig. 9. NOx formation was governed by the combination of gas residence time and flame temperature. An increased flame speed can result in decreased residence time and increased flame temperature. The for-

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Pressure drop (Pa)

700 600 500

Single layer 3mm 3mm Double layer 3-6mm 3-6mm 3-8mm 3-8mm 3-10mm 3-10mm 3-13mm 3-13mm Reaction flow Cold flow

Single layer 3mm Double layer 3-6mm 3-8mm 3-10mm 3-13mm Mathis et al.[11] Knanna et al.[14] Smucker et al.[16]

10 8

NOx (ppm)

800

400 300 200

6 4 2

100 0

0 0

10

20

30

40

50

60

0

Velocity (cm/s)

30

40

50

60

ð4Þ

The reaction rate constant k(T) for the Eq. (5) can be obtained from GRI-Mech 2.11 and was expressed as

ð5Þ

Fig. 9. NOx emissions as a function of flow velocity for alumina balls with different diameters (/ = 0.65).

Single layer 3mm Double layer 3-6mm 3-8mm 3-10mm 3-13mm Mathis et al.[11] Knanna et al.[14] Smucker et al.[16]

1000

CO (ppm)

800 600 400 200 0 0

10

20

30

40

50

60

Velocity (cm/s) Fig. 10. CO emissions as a function of flow velocity for alumina balls with different diameters (/ = 0.65).

Single layer 3mm Double layer 3-6mm 3-8mm 3-10mm 3-13mm Mathis et al.[11] Smucker et al.[16]

10000 8000

HC (ppm)

mer plays a negative role in NOx formation, and the latter favors thermal NOx production. Hence, the NOx emissions generally increased initially and then decreased with increased flow velocity. This variation in NOx emissions differed from foam burners studied by Khanna [11], Mathis and Ellzey [14] and Smucker and Ellzey [16]. In the foam burner, the NOx emissions increased with increased flow velocity. In the current work, the NOx emissions varied more evidently with increased flame speed for a single-layer burner than for a double-layer burner owing to the similar flame temperature variation trend with the flame speed, as indicated in Fig. 5. For the two-section burner, the NOx emission decreased in the order of 3–10 mm, 3–6 mm, 3–13 mm, and 3–8 mm. This behavior exhibited an optimal pellet diameter (3–8 mm) to achieve a lower NOx emission. With decreased packed pellet diameter, the flame temperature increased to accelerate thermal NOx production. However, the residence time of the reaction flow decreased to surpress low NOx production. The two abovementioned dominant factors competed with each other, thereby facilitating the identification of the above optimal diameter of alumina pellets. The CO emissions as a function of flow velocity for the alumina pellets with different diameters at / = 0.65 are shown in Fig. 10. The CO emissions for methane in the foam burner studied by Khanna [11], Mathis and Ellzey [14] and Smucker and Ellzey [16] were almost independent of the flame speed because the tortuous path network in the foam structure exhibited good interconnectivity. However, the CO emission in the present double-layer burner was strongly dependent on the flame speed. The CO emissions were primarily determined by the flame temperature, as well as the homogeneous mixing of fuel and air. At low flame speeds (S < 35 cm/s), the CO emissions decreased more significantly because the ceramic beads in the present study only had a near point contact with neighboring beads. The CO emissions of the double-layer burner were lower than those of the single-layer burner because the flame temperature of the former is generally higher than that of the latter, as indicated in Fig. 5. At high flame speeds (S > 35 cm/s), the CO emissions for both the double- and single-layer burners were almost identical and maintained a low constant value. The flame temperature discrepancy among the alumina pellets with different diameters became less obvious, as also indicated in Fig. 5. The major source for CO source is from the reversible reaction:

kðTÞ ¼ BTM eE=RT

20

Velocity (cm/s)

Fig. 8. Pressure drop as a function of flow velocity for alumina balls with different diameters (/ = 0.65).

CO þ OH CO2 þ H

10

6000 4000 2000 0 0

10

20

30

40

50

60

Velocity (cm/s) Fig. 11. HC emissions as a function of flow velocity for alumina balls with different diameters (/ = 0.65).

where BTM, M, E, R are the pre-exponential factor, temperature exponent, activation energy, standard molar constant respectively and their values are shown in the following:

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B ¼ 4:76  107 mol=cm3 ; M ¼ 1:23 J=mol; E ¼ 293 J=mol;

R ¼ 8:314 J=ðmol KÞ

References

ð6Þ

The rate constant k(T) increased as temperature increased at higher flame speed. Hence, the CO pollutant concentration was reduced with increased flame speed. On the other hand, the fuel mixing homogeneity was improved to a great extent by the enhanced gas convection at a relatively higher velocity. The effect of mixing homogeneity became more prominent than that of flame temperature. The HC emissions as a function of flow velocity for the alumina pellets with different diameters at / = 0.65 are shown in Fig. 11. The HC emissions in the foam burner studied by Mathis and Ellzey [14] and Smucker and Ellzey [16] were significantly lower than those in the present burner because of the homogeneous mixing generated by the tortuous path network. The HC emissions decreased with increased flame speed for both single- and doublelayer burners. However, the HC emissions for the single-layer burner decreased more dramatically with increased flame speed. The HC emissions were lower than those of the double-layer burner at low flame speeds (S < 25 cm/s). For the double-layer burner, the HC emissions decreased with the increased diameter of the packed pellets of the downstream. This phenomenon can be attributed to the fact that the residence time of the unreacted mixture passing through the reaction zone decreased owing to the increased porosity, thus resulting in the incomplete burnout of the HC fuel.

4. Conclusions The flame location in a double-layer burner was less insensitive to the flame speed compared with that in a single-layer burner. The stable operating region can be enlarged with the increased diameter of packed pellets. The flame and surface temperatures increased more evidently with increased flame speed for the single-layer burner than for the double-layer one. The flame temperature for the double-layer burner decreased with the increased pellet diameter of the downstream at a fixed equivalence ratio and flame speed. The discrepancy among the pressure drops of the particles with different diameters was higher in the reaction flows than in the cold flows. NOx formation was governed by the combination of gas residence time and flame temperature. The NOx emissions generally increased initially and then decreased with increased flame speed. An optimal diameter of the packed pellets was identified and found to corresponding to the lowest NOx emissions. The CO and HC emissions decreased with increased flame speed. The effect of the pellet diameter on CO emissions strongly depended on the flow velocity. The CO emissions were almost independent of the pellet diameter at high flame speeds (S > 35 cm/s). The HC emissions decreased with increased pellet diameter for the entire range of experimental flow velocity.

Acknowledgments The present work was supported by the National Key Projects of Fundamental R/D of China (973 Project: No. 2011CB610306) and the National Natural Science Foundation of China (No. 51176149).

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