Homogeneous Combustion of Fuel Ultra-Lean Methane–Air Mixtures

ARTICLE pubs.acs.org/EF

Homogeneous Combustion of Fuel Ultra-Lean Methane
Air Mixtures: Experimental Study and Simplified Reaction Mechanism Yikun Wang, Yinhe Liu, Qi Cao, Chang’an Wang, and Defu Che* State Key Laboratory of Multiphase Flow in Power Engineering, China School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China ABSTRACT: The use of ventilation air methane (VAM) has attracted more and more attention in recent years. The experimental investigation on homogeneous combustion of fuel ultra-lean methane
air mixtures is carried out in reactors packed with and without a monolith. The experimental results show that the methane oxidation follows a consecutive reaction scheme. In the reaction process, although a little CH4 may be directly oxidized to CO2 at low temperatures, most CH4 is oxidized to CO as the intermediate product and then further oxidized to CO2 at high temperatures. The present experimental results also show that the presence of a monolith has an obvious effect on the combustion of methane. The methane combustion can be enhanced by both expanding the specific surface area and decreasing the diameter of the monolith channel. However, an extremely small diameter may lead to the quenching of combustion. Moreover, it is found that the effect of homogeneous combustion cannot be neglected for the catalytic flow reversal reactor (CFRR) at high temperatures. On the basis of the analysis of the present experimental data, a simplified two-step consecutive mechanism is developed and the kinetic parameters are determined to describe the methane combustion in an empty reactor and a reactor packed with a monolith. The present research is helpful for understanding the reaction mechanism for VAM use.

1. INTRODUCTION Methane is the second most important greenhouse gas after carbon dioxide. Its greenhouse effect is tens of times higher than that of carbon dioxide at the same concentration. According to statistics from China Coalbed Methane Clearinghouse, the pure methane emission from ventilation air methane (VAM) is 161  109 m3 during coal mining in China in 2008, equating to 15.1  106 tons oil equivalent (toe) or 21.6  106 tons coal equivalent (tce).1 Therefore, a large amount of methane emitted into the atmosphere with ventilation air from coal mines may be a potential energy source. However, the methane concentration of VAM is very low (usually less than 1.0 vol %) and unsteady; therefore, it cannot be burned in traditional combustion mode. Hence, efficient use of this low-rank energy has become a challenge around the world. Several systems have been developed to use VAM. MEGTEC Systems offers the VOCSIDIZER, a thermal flow reversal reactor (TFRR) that usually operates at above 1000 °C, exceeding the ignition temperature of methane. TFRR is evolved from the reactor for thermal oxidation of volatile organic compounds (VOCs). Because the maintenance of the self-sustained operation of TFRR mainly depends upon the homogeneous combustion in porous medium, it is of great significance to study the homogeneous combustion mechanism of methane. The homogeneous combustion of methane is usually accompanied by the production of a large number of radicals and may consist of dozens or even hundreds of consecutive elementary reactions. Many works have been performed on this complex reaction, and various reaction mechanisms were proposed. After the data in methane pyrolysis and combustion were evaluated, Tsang et al.2 proposed a detailed mechanism including 27 species and 371 reactions. In 1989, Miller et al.3 put forward r 2011 American Chemical Society

another detailed mechanism for methane combustion, comprising 49 species and 227 elementary reactions. Smith et al.’s mechanism of natural gas combustion (i.e., GRI-Mech) also obtained recognition and is widely used in others’ studies.4 Besides, the detailed mechanism for the methane oxidation in a perfectly stirred and tubular flow reactor was investigated by Barbe et al.5 The aforementioned mechanisms can provide a good simulation of the homogeneous combustion of methane. However, the high computational cost of the detailed mechanism in simulation hinders its application in industrial design. To reduce the cost of industrial simulation, a simplified single-step mechanism has been used frequently in most of the previous studies.6
8 Nonetheless, the single-step mechanism can neither predict all of the flame characteristics nor accurately predict the composition of combustion products because the formation processes of CO, H2, etc. are not seen as part of this single-step mechanism.9 Therefore, for the purpose of reducing the cost of industrial design and accurately predicting the products of methane combustion, several simplified mechanisms of methane combustion are developed under various conditions. Glarborg et al.10 developed a four-step simplified reaction mechanism for the methane oxidation. This simplified mechanism takes CH4, O2, H, H2, CO, CO2, and H2O as independent reactants, and its calculation results are in good agreement with the detailed mechanism11 in the prediction of the temperature and main species concentration. A simple three-step six-component (CH4, O2, H2, CO, CO2, and H2O) reaction mechanism for Received: March 30, 2011 Revised: July 20, 2011 Published: July 20, 2011 3437

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Energy & Fuels partial oxidation of methane in inert porous medium was developed by Dobrego et al.12 Their simulation results show that the error of species concentration computed by the simplified mechanisms is within (4%. Recently, it has been confirmed that the stable products that could be detected are mainly CO, CO2, and H2O for homogeneous combustion of fuel ultra-lean methane
air mixtures.13,14 H2 could hardly be detected under fuel-lean conditions.7,13
15 Therefore, the mechanisms for combustion of fuel ultra-lean methane
air mixtures could be further simplified. Combustion is susceptible to fluid flow, heat, and mass transfer. Consequently, different reaction conditions will result in diverse simplified mechanisms of homogeneous combustion. By comparing the oxidation of methane before and after packing small ceramic balls into ceramic tubes under fuel-lean conditions, Slepterev et al.13 indicated that the methane oxidation follows a consecutive reaction scheme, in which CH4 is first oxidized to CO and subsequently to CO2. However, according to Gosiewski et al.,7,14 the methane reaction mechanisms for the empty reactor and a monolith with wide channels could be regarded as a consecutive scheme, while the methane combustion in a monolith with narrow channels could be better described as a consecutive
parallel scheme. To broaden the range of working conditions of TFRR, a catalyst is used to reduce the ignition temperature of methane, which results in the catalytic flow reversal reactor (CFRR). Canadian Mineral and Energy Technologies (CANMET) developed CFRR for VAM combustion.16 Different from TFRR, the self-sustained operation of CFRR mainly depends upon the heterogeneous combustion on the surface of the catalyst. Subsequently, numerous experimental and simulation studies on CFRR have been carried out in the following years.17
20 However, the homogeneous combustion may also occur in CFRR because of the increased operation temperature in the system if the methane concentration is increased. Hence, the contribution of homogeneous combustion in CFRR to methane conversion should be taken into consideration. Because the processes of homogeneous combustion depend upon reaction conditions to a large extent, it is not appropriate to directly compare the ignition temperatures of methane in an empty reactor and a reactor with a monolith under various gas velocities, such as in the study by Gosiewski et al.14 The effect of the porous medium presence in fuel ultra-lean methane
air mixture combustion is contradictory in the conclusions by Slepterev et al.13 and those by Gosiewski et al.14 Furthermore, the turning point of the CO2 concentration reported by Slepterev et al.13 is not identified in the study by Gosiewski et al.14 Therefore, a further study should be carried out. To reduce the computational cost of simulating homogeneous combustion in an industrial-scale unit, a reliable simplified model needs to be built based on the simple combustion products of fuel ultra-lean methane
air mixtures. Such a model must be able to simplify industrial design and provide accurate results simultaneously. Besides, the phenomena of different CO2 generation profiles reported by Slepterev et al.13 and Gosiewski et al.14 also need to be clarified. In this paper, an experimental reaction system is specially designed to investigate the influence of the monolith on the homogeneous combustion of fuel ultra-lean methane
air mixtures and to build a reliable simplified model for industrial-scale unit simulation. The impacts of gas velocity, methane concentration of feeding gas, and reactor temperature are studied.

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Figure 1. Schematic diagram of the experimental system: (1) air compressor, (2) air filter, (3) methane cylinder, (4) pressure-reducing valve, (5 and 6) mass flow controllers, (7) gas chromatography, (8) FTIR gas analyzer, (9) mixing tank, (10) pressure gauge, (11) thermocouples, (12) temperature-controlled oven, and (13) reactor.

Moreover, a simplified reaction mechanism is obtained by comparing the calculation results from different reaction mechanisms, and the kinetic parameters are also determined. The present research is also helpful to CFRR design and operation.

2. EXPERIMENTAL SECTION An experimental system is constructed to test the performance of homogeneous combustion of fuel ultra-lean methane
air mixtures, as shown in Figure 1. First, the fused-silica reactor is preheated by a thermostat oven to a certain temperature above the ignition temperature of fuel ultra-lean methane
air mixtures. Then, high-purity methane gas (over 99.99 vol %) and compressed air are mixed in the mixing tank before going into the reactor to become burned. The temperature in the reactor is measured by six K-type thermocouples (1 mm outer diameter, with a precision of (1 °C) located along the axis of the reactor. The height of the oven is 30 cm, and its maximum heating power is 3 kW. The composition of gas is measured both online and offline by a GASMET DX4000 Fourier transform infrared (FTIR) gas analyzer and a gas chromatograph (GC), respectively. Hydrogen is detected by a 13 molecule sieve-packed column (3 mm inner diameter and length of 2 m) and a thermal conductivity detector (TCD). The optimized working conditions of the GC are as follows: carrier gas (argon) flow rate, 30 mL min
1; column temperature, 50 °C; injector temperature, 50 °C; and detector temperature, 100 °C. Carbon monoxide, carbon dioxide, and methane are detected by a TDX-01 carbon molecule sieve-packed column (3 mm inner diameter and length of 1 m) and a flame ionization detector (FID). The analysis conditions are as follows: carrier gas (helium) flow rate, 30 mL min
1; column temperature, 50 °C; injector temperature, 50 °C; and converter temperature, 380 °C. No observable hydrogen is formed in this study, which is consistent with the results by Slepterev et al.13 and Gosiewski et al.14 NOx is detected when the temperature exceeds 900 °C, and no other species is detected in the present experiment. Therefore, it can be concluded that the final products are mainly CO, CO2, and H2O. Measured results of CO, CO2, and unreacted CH4 by FTIR are in good agreement with those by GC. The difference between the FTIR analysis results and the GC analysis results is within (5%. Carbon balance is checked for each sample, and a discrepancy within (6% is shown from the results. The reported concentration in this paper is obtained by a GC. 3438

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Figure 2. Simplified reaction mechanism for homogeneous combustion of fuel ultra-lean methane
air mixtures: (a) consecutive mechanism and (b) consecutive
parallel mechanism.

3. SIMPLIFIED MECHANISM According to the previous experimental results,13,14 the mechanism for homogeneous combustion of methane under fuel ultra-lean conditions is shown in Figure 2. In the consecutive mechanism, as shown in Figure 2a, CH4 is first oxidized to CO, which is further oxidized to CO2 at high temperatures. In the consecutive
parallel mechanism, as shown in Figure 2b, CH4 is directly oxidized to both CO and CO2, among which CO is further oxidized to CO2. In the consecutive
parallel mechanism, as the temperature increases, more and more CO2 is produced by the oxidation of the primary product CO. The reactions for the above two simplified mechanisms are as follows: 2CH4 þ 3O2 f 2CO þ 4H2 O

ð1Þ

2CO þ O2 f 2CO2

ð2Þ

CH4 þ 2O2 f CO2 þ 2H2 O

ð3Þ

The influence of the oxygen concentration can be ignored because of the large excess of oxygen in VAM. Therefore, the rates of these reactions can be expressed as follows: (1) For the consecutive mechanism
con  dCCH4
E1 con ¼ k1, 0 exp
rCH4 ¼ ð4Þ CaCH4 dt RT
rCO ¼

dCCO ¼ kcon 2, 0 dt


con 
E2 exp CbCO RT

ð5Þ

(2) For the consecutive-parallel mechanism con
par


rCH4

dCCH4
E1 con
par ¼ k1, 0 ¼ exp dt RT


rCO

dCCO
E2 con
par ¼ k2, 0 ¼ exp dt RT


rCH4

dCCH4
E3 con
par ¼ k3, 0 ¼ exp dt RT

con
par

! CaCH4

ð6Þ

equal to zero. To determine the ignition temperature, the temperature interval is preset to 2 °C. Once CO or CO2 is detected from the reacted gas, it is considered that the combustion has started and the temperature at that moment is defined as the ignition temperature. Because the length of the oven is greater than that of the combustion zone (where the combustion occurs), the combustion zone is defined as the zone where the temperature is higher than the ignition temperature. It is assumed that the temperatures measured by thermocouples are temperatures of the gas mixture. Furthermore, the average temperatures of the combustion zone measured by thermocouples are chosen as the reaction temperatures during the calculation of the kinetic parameters. The temperature measurement error caused by radiation may be significant in the experiment. The temperature correction can be performed on the basis of the following assumptions: (1) the thermocouple junction is spherical; (2) the thermocouple has uniform temperature distribution along the cross-section; (3) the radiation of clean reaction gas and the thermal conduction of thermocouples are ignorable; (4) (Tgas
Tth)/Tgas , 1, the corrected formula for temperature proposed by Luo,21 can be expressed as Tgas
Tth ¼

con
par

Nu ¼ hdth =λgas ¼ 2 þ ð0:4Re1=2 þ 0:06Re2=3 ÞPr 0:4 ðμ∞ =μw Þ1=4 ð10Þ which is valid for 3.5 < Re < 8  104 and 0.7 < Pr < 380, where dth is the diameter of the thermocouple junction, λgas is the thermal conductivity of the gas, Pr is the Prandtl number, μ∞ is the viscosity of gas at the environment temperature, and μw is the viscosity of gas at the wall temperature. The properties of gas are calculated by the following equations:23

ð7Þ

! CcCH4

ð8Þ

λgas ¼ 2:127  10
11 Tgas 3
5:93883  10
8 Tgas 2 þ 1:06196  10
4 Tgas
6:48978  10
4

ð11Þ

Fgas ¼ 373:74211Tgas
1:01165

ð12Þ

cp, gas ¼ 1:7464  10
10 Tgas 4
7:47131  10
7 Tgas 3 þ 1:1  10
3 Tgas 2
0:4416Tgas þ 1059:75

ð13Þ

μgas ¼
4:48913  10
18 Tgas 4 þ 2:62003

4. RESULTS AND DISCUSSION 4.1. Calculation of Kinetic Parameters. The initial step of the experiment is to determine the ignition temperature of fuel ultralean methane
air mixtures. The ignition temperature is defined as the highest temperature at which the methane conversion is

ð9Þ

where Tgas is the temperature of the gas, Tth is the temperature of the thermocouple, σ is the Stefan
Boltzmann constant, εth is the emissivity of the thermocouple junction, and h is convective heattransfer coefficient. The Nusselt number (Nu) can be expressed as22

! CbCO

σεth Tgas 4 h þ 4σεth Tgas 3

10
14 Tgas 3
5:47545  10
11 Tgas 2 þ 7:5235  10
8 Tgas þ 1:4769  10
7

ð14Þ

where cp,gas is the specific thermal capacity of the gas and μgas is the viscosity of the gas. 3439

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3440

a monolith

reactor with

empty reactor

parallel

consecutive


consecutive

parallel

consecutive


consecutive

reaction

mechanism

reaction

condition

CH4 þ 2O2 f CO2 þ 2H2 O

con
par k3, 0

2CO þ O2 f 2CO2

k2, 0

con
par

2CH4 þ 3O2 f 2CO þ 4H2 O

k1, 0

con
par

kcon 2, 0

2CO þ O2 f 2CO2

2CH4 þ 3O2 f 2CO þ 4H2 O

kcon 1, 0

CH4 þ 2O2 f CO2 þ 2H2 O

k3, 0

con
par

2CO þ O2 f 2CO2

con
par k2, 0

2CH4 þ 3O2 f 2CO þ 4H2 O

k1, 0

con
par

kcon 2, 0

2CO þ O2 f 2CO2

2CH4 þ 3O2 f 2CO þ 4H2 O

kcon 1, 0

reaction

3

216.21  10

304.14  10 3

3

208.06  103

98.99  103

150.16  103

256.14  10

294.27  10

8.79  10

16

1.59  10

17

1.02  1012

4.91  106

6.07  108

1.94  10 17

3.57  10 16

4.76  1012

204.31  103 3

1.27  107

123.74  103

0.8

1.7

0.6

1.1

1.0

2.10

1.80

1.10

0.90

1.01

5.44  107

134.07  103

a, b, and c

ki,0 (s
1)

E (J mol
1)

Table 1. Kinetic Parameters for Different Simplified Reaction Mechanisms

24.87

13.20

16.83

16.75

(%)

ΔcCO

42.78

19.14

53.39

27.69

(%)

ΔcCO2

16.18

5.81

16.62

4.98

(%)

ΔcCH4

0.960

0.996

0.910

0.986

F2

200.30

2006.73

63.92

432.32

F

F0.01

4.68

5.01

(p, N
p)

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Figure 3. Calculated versus measured methane conversion: (a) empty reactor and (b) monolith (xexp, experimental results of methane conversion; xcalc, calculation results of methane conversion).

Figure 5. Methane conversion versus the preset temperature for the combustion in an empty reactor (tr, 2.91 s). Figure 4. Concentration profiles of outlet gas versus the preset temperature for the combustion in an empty reactor (tr, 8.86 s; CCH4,in, 0.50%).

results can be expressed by the following equations: Δc ¼

The emissivity of the thermocouple junction can be calculated by the fitting formula24

1 N jCexp, i
Ccalc, i j  100% N i¼1 Cexp, i

∑

ð17Þ

N

εth ¼ 0:475788 þ 0:000127059Tth

ð15Þ

which is valid for 232.4 °C < Tth < 940.7 °C. Substituting eqs 10
15 into eq 9, we can obtain a quintic equation for Tgas, which can be solved by iteration. The kinetic parameters for the consecutive and consecutive
parallel mechanism are calculated with computer programs. The commonly used fourth-order Runge
Kutta scheme is adopted to solve eqs 4
8. The direct search method is used for parameter optimization, with the objective function as follows: Gobj ¼

N

∑ ðxexp, i
xcalc, i Þ2 i¼1

ð16Þ

A simplified mechanism of homogeneous combustion of VAM depends upon the ultimately calculated kinetic parameters and average relative error. Analysis of variance (ANOVA) is used to validate the kinetic model. The reliability of the regression

F2 ¼ 1


∑ ðxexp, i
xcalc, i Þ2 i¼1 N

∑ ðxexp, i Þ

ð18Þ

2

i¼1

½ F ¼

N

N

∑ ðxexp, i Þ2
i∑¼ 1ðxexp, i
xcalc, i Þ2 =p i¼1 ½

N

∑

i¼1

ð19Þ

ðxexp, i
xcalc, i Þ2 =ðN
pÞ

where Δc is the average relative error, F2 is the correlation coefficient, and F is the test statistic of ANOVA. Generally, the regression result is acceptable when F2 > 0.9 and F > 10F0.01 (p, N
p).25 The calculation results are shown in Table 1. The comparison of xexp and xcalc is shown in Figure 3. 4.2. Combustion in an Empty Reactor. The homogeneous combustion of fuel ultra-lean methane
air mixtures is carried out in a cylindrical fused-silica reactor (29 mm inner diameter and length of 800 mm), with the preset temperature range of 500
960 °C, the flow rate of 0.86
4.63 L min
1 (i.e., the gas 3441

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Figure 6. Concentration profiles of outlet gas versus the preset temperature for the combustion in an empty reactor (tr, 1.68 s; CCH4,in, 0.75 vol %).

Figure 8. Concentration profiles of outlet gas versus the preset temperature for an empty reactor and a reactor packed with a monolith (tr, 1.72 s; CCH4,in, 0.75 vol %).

Figure 7. Monolith with refractory fiber.

velocity is 0.022
0.116 m s
1), and the methane concentration of 0.5
1.0 vol %. The composition of the reactor outlet gas is shown in Figure 4. It can be seen that the maximum concentration of CO occurs at about 775 °C when the corresponding conversion of methane to CO is 57.96%. Besides, CO is oxidized to CO2 rapidly when the methane concentration is less than 500 ppmv. This phenomenon indicates that methane can inhibit CO from being oxidized, which is consistent with previous research results.13,26 It can also be seen from Figure 4 that the CO2 formation profile is rather complicated: a small peak (around 660 °C, about 56 ppmv), followed by a trough, and finally, a sharp rise are observed. Such phenomenon occurs in all present experiments and is consistent with the study by Slepterev et al.13 Because no other species is experimentally detected, this phenomenon can probably be explained by the following reaction: CH4 þ 3CO2 a 4CO þ 2H2 O

ð20Þ

Limited by chemical equilibrium, the reaction shifts to the right side with the increase of the temperature (the equilibrium constant of eq 20 becomes larger than 1 at temperatures above 700 °C).13 At high temperatures (which is more important for the investigation), the formation of CO2 is mainly due to the oxidation of CO (generated by the primary oxidation of methane).

Figure 9. Methane conversion versus the preset temperature for the combustion in amonolith (tr, 1.72 s).

The methane conversions of the present reactor at different preset temperatures are shown in Figure 5. The ignition temperature is about 719 °C. It can be seen that the methane conversions are independent of the inlet methane concentration under fuel-lean conditions. This conclusion is in agreement with that by Gosiewski et al.7,14 According to the calculation results in Table 1, the small average relative error and large correlation coefficient obtained by the consecutive mechanism indicate that the mechanism of fuel ultra-lean methane
air mixture combustion in an empty reactor can be simplified to a consecutive one. The calculated reacted gas concentrations for the consecutive mechanism and measured data are shown in Figure 6. 4.3. Combustion in a Monolith. The homogeneous combustion of fuel ultra-lean methane
air mixtures in a monolith is carried out in a cylindrical fused-silica reactor (29 mm inner diameter and length of 800 mm) packed with three monolith segments (19.4  19.4 mm and length of 90 mm). The pore size of the monolith is 1.5  1.5 mm, and the wall thickness of the monolith is 0.4 mm. The pore number of the monolith is 100. The specific surface area of the monolith is 1303 m2 m
3, and its porosity is 63%. The monolith could be operated at the temperature up to 1400 °C. To prevent gas leakage from 3442

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Figure 10. Temperature profiles along the reactor for the constant preset temperature (Vgas, 2.63 L min
1).

Figure 11. Concentration profiles of outlet gas versus the preset temperature for the combustion in a monolith (t r , 1.79 s; C CH 4,in , 0.5 vol %).

upstream to downstream through the gap between the monolith and the reactor wall, the monolith is wrapped with refractory fiber, as shown in Figure 7. The experiment is carried out in the preset temperature range of 500
960 °C, with the flow rate of 1.51
2.63 L min
1 (i.e., the gas velocity is 0.067
0.116 m s
1) and the methane concentration of 0.3
1.0 vol %. With a fixed gas velocity, the outlet gas concentration of an empty reactor and that of a monolith are displayed in Figure 8, which is corresponding to the results by Gosiewski et al.7 However, different from the previous research,7 CO is detected in all experiments. Therefore, the reaction cannot be simplified to a simple single-step mechanism. In the presence of a monolith, the ignition temperature of fuel ultra-lean methane
air mixtures decreases. The ignition temperatures of gas in the present experiment are about 744 and 632 °C for an empty reactor and a monolith, respectively, both of which are higher than that reported by Gosiewski et al.7 (about 530 °C). It can also be seen from Figure 8 that the methane conversion increases with the presence of a monolith. Meanwhile, the preset temperature for the maximum CO formation decreases from 920 to 880 °C. The methane conversions of homogeneous combustion in a monolith with different inlet methane concentrations are shown in Figure 9. It can be seen that the methane conversion is found to be independent of the inlet methane concentration, which was also observed in the previous study.14 When Figures 5 and 9 are compared, it can be concluded that, although the gas velocity doubles, the methane conversion for the homogeneous combustion in a monolith is close to that in an empty reactor at the same preset temperature. This further proves that the presence of a monolith improves the methane combustion.14,27 The methane conversion exceeds 3% at the preset temperature above 800 °C, as shown in Figure 9. Therefore, the influence of homogeneous combustion in a monolith for CFRR should be taken into account at high temperatures. Figure 10 shows the temperature profiles along the reactor for an empty reactor and a monolith at the preset temperature of 900 °C. From Figure 10, it can be seen that the temperature profiles are practically independent of the methane concentration, which was consistent with the result by Gosiewski et al.14 Because the reaction heat is negligible compared to the heat flux

of the oven, it is difficult to distinguish the temperature profiles of different methane concentrations. According to the calculation results in Table 1, the small average relative error and large correlation coefficient obtained by the consecutive mechanism show that the mechanism of homogeneous combustion of fuel ultra-lean methane
air mixtures in a monolith can also be simplified to a consecutive mechanism. The calculated outlet gas concentrations for the consecutive mechanism and measured ones are shown in Figure 11, with the mean relative error of the gas concentration below 8%. On the basis of the calculation results, the homogeneous combustion mechanism of methane in an empty reactor and a reactor packed with a monolith can both be simplified to the consecutive mechanism. This conclusion is consistent with the previous studies;9,11,28 i.e., methane is oxidized mainly to CO at low temperatures, and this part of CO is oxidized to CO2 at high temperatures. The presence of a monolith leads to a positive feedback to the reaction through transferring the combustion heat back to the upstream gas. Because the process of heat transfer is strongly dependent upon the channel diameter of the monolith as well as the area of the monolith contacting the gas phase, the kinetic parameters in this paper are not universal. The enlargement of the contact area with the gas phase by either decreasing the channel diameter or increasing the specific surface area can lead to a lower ignition temperature. As shown in the experiments, when the gas velocity is 0.067 m s
1 (equal to the gas velocity for monolith B14 with a channel width of 3 mm), the ignition temperature is 590 °C, which is lower than that for monolith B (about 675 °C). Ignition of the gas-phase reaction requires the buildup of a critical concentration of highly reactive radicals. However, an extremely small channel diameter may lead to an obvious radical scavenging effect on the monolith wall, because the mean free path of the molecules is in the same order of magnitude as the channel diameter. Consequently, radical recombination reactions balance radical initiation reactions, and an extremely small channel diameter may result in a complete quenching of homogeneous combustion. The quenching is not simply a result of the heat loss of the reactor but rather a result of a kinetic effect.29,30 Therefore, an optimal diameter, which can decrease the ignition temperature and maintain the 3443

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Energy & Fuels stability of combustion, should be determined for a specific gas velocity.

5. CONCLUSION In present work, the homogeneous combustion of fuel ultralean methane
air mixtures in an empty reactor and a monolith is studied. The kinetic parameters of homogeneous combustion methane are determined on the basis of the two simplified reaction mechanisms. The main conclusions of the current studies are as follows: (1) The CO2 concentration increases at first and reaches a local maximum value; then, it decreases and, finally, increases with the increase of the temperature. It is possible that a small amount of CO2 is formed by the direct oxidation of methane at low temperatures. This part of CO2 then reacts with CH4 and is reduced to CO at high temperatures. At higher temperatures, CO2 is mainly produced by the CO oxidation. (2) The presence of a monolith, which has an obvious effect on the combustion of methane, will decrease the ignition temperature and enhance the combustion of methane. (3) The contact area of a monolith with the gas phase affects the combustion of methane significantly because a larger contact area leads to a lower ignition temperature. Thus, either increasing the specific surface area of a monolith or decreasing the channel diameter of a monolith can enhance the combustion. However, an extremely small diameter may lead to a complete quenching of combustion. Hence, an optimal diameter should be determined at a given gas velocity to ensure the stability of combustion and decrease the ignition temperature efficiently. (4) The simplified two-step consecutive mechanism is accurate enough to describe the combustion in both an empty reactor and a monolith according to the calculation results. (5) If CFRR works at high temperatures over 800 °C, the effect of homogeneous combustion should be taken into consideration. ’ AUTHOR INFORMATION Corresponding Author

*Telephone: +86-29-82665185. Fax: +86-29-82668703. E-mail: [email protected].

’ ACKNOWLEDGMENT This work has been financially supported by the National High Technology Research and Development Program (2006AA05Z244). ’ NOMENCLATURE a, b, and c = exponents at methane concentration in the kinetic equation cp,gas = specific thermal capacity of the gas (kJ kg
1 k
1) C = concentration (%) dth = diameter of the thermocouple junction (m) E = activation energy (J mol
1) F = test statistic of ANOVA Gobj = objective function of parameter optimization h = convective heat-transfer coefficient (W m
2 K
1) ki = reaction rate constant in the kinetic equation (s
1) ki,0 = pre-exponential factor in the kinetic equation (s
1) Nu = Nusselt number N = number of calculation points used in the estimation of kinetics

ARTICLE

p = number of kinetic parameters used in the estimation of kinetics Pr = Prandtl number R = gas constant (J mol
1 K
1) Re = Reynolds number T = temperature (K or °C) Tign,f = ignition temperature of methane combustion in an empty reactor (K or °C) Tign,m = ignition temperature of methane combustion in a monolith (K or °C) r = reaction rate of methane combustion (s
1) t = time (s) tr = residence time (s) V = flow rate of the fuel ultra-lean methane
air mixture (L min
1) v = gas velocity (m s
1) x = methane conversion (%) Greek Letters

Δc = average relative error (%) σ = Stefan
Boltzmann constant (5.67  10
8 W m
2 K
4) εth = emissivity of the thermocouple junction μ = viscosity (kg m
1 s
1) F = density (kg m
3) F2 = correlation coefficient (%) λ = thermal conductivity (W m
1 K
1) Subscripts and Superscripts

con = consecutive mechanism con
par = consecutive
parallel mechanism calc = calculation results CO = carbon monoxide CO2 = carbon dioxide CH4 = methane exp = experimental results gas = gas mixture i = ith calculation point j = jth calculation point in = inlet th = thermocouples w = wall ∞ = environment

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