A Modelling Study of the Impact of Blending N2, CO2 and H2O on

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A Modelling Study of the Impact of Blending N2, CO2 and H2O on Characteristics of CH4 Laminar Premixed Combustion Fei Ren, Longkai Xiang, Huaqiang Chu, Hantao Jiang, and Yuchen Ya Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b02108 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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A Modelling Study of the Impact of Blending N2, CO2 and H2O on Characteristics of CH4 Laminar Premixed Combustion Fei Rena,b, Longkai Xianga, Huaqiang Chua,c*, Hantao Jianga, Yuchen Yaa aSchool

of Energy and Environment, Anhui University of Technology, Ma’anshan 243002, Anhui Province, China

bKey

Lab. for Power Machinery and Engineering of M. O. E., Shanghai Jiao Tong Universit, Shanghai, China cEngineering

Technology Research Center of Energy saving & pollutant control in

metallurgical process, Ma’anshan 243002, Anhui Province, China

A paper submitted to

Energy & Fuels

*

Corresponding author:

Email: [email protected]

1 ACS Paragon Plus Environment

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Abstract N2, CO2 and H2O are impurity components in the biogas fuel production process, which are added at an appropriate amount to the combustion of hydrocarbon fuel that can effectively reduce the emissions of NOx and soot precursors (Polycyclic Aromatic Hydrocarbons, PAHs). In this paper, using N2, CO2 and H2O as dilution gas, Chemkin II/Premix Code with detailed chemical reaction mechanism GRI 3.0 was chosen to calculate to the premixed combustion characteristics and NOx emissions of CH4. At different equivalence ratios (Ф=0.8, 1.0 and 1.2) and blending ratio (0-40%), the physical and chemical effects of different dilution gases were systematically studied through the introduction of hypothetical substances FN2, FCO2 and FH2O. The results show that the laminar burning velocity and adiabatic flame temperature of CH4 were decreased by adding N2, CO2 and H2O, and the influence of the three diluents increases with the increase of the blending ratios following the order of CO2 > H2O > N2. Moreover, the physical and chemical effects are greatest at stoichiometric, and physical effects are much greater than chemical effects. In particular, at Ф = 1.2, the chemical effect of H2O leads to the adiabatic flame temperatures of CH4 gradually increase as the Dr (Dilution ratio) increases. The sensitivity analysis of the main elementary reactions, which play a leading role in the influence of NO generation, show that the adiabatic flame temperature of CH4 is reduced after adding N2, CO2 and H2O, depressing the generation of NO. The generation path of NO is mainly prompt NOx, and the responsible reactions are R38 H + O2 O + OH, R240 CH + N2 = HCN+N and R52 H + CH3(+M) CH4(+M).

Keywords: Chemical and physical effect, Laminar premixed combustion, Net heat release rate, NO formation


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1. Introduction Today, the organic waste is produced everywhere, creating greenhouse gases, which is generated at every stage and aspect of life and pollutes our environment. The use of biogas technologies can help people clean up the planet and create business value. Biogas, a convenient and clean gaseous fuel whose short carbon cycle in combustion process will refrain from increasing greenhouse gas emissions, can burn directly and also can be used for small power plants as well as for internal combustion engines. For its environmental characteristics, in many countries, biogas has been widely used for heating and vehicle fuels.1-6 Biogas is generally composed of 40-80% CH4, 20-60% CO2, 0-5% N2, less than 1% H2, 0.4% O2 and 0.1-3% H2S, etc. Its composition depends on its origin, production methods and materials.2,4,6,7 It is remarkable that N2, CO2 and H2O (steam) are impurity components in the biogas fuel production process, which are added at an appropriate amount to the combustion of hydrocarbon fuel that can effectively reduce the emissions of NOx and soot precursors (Polycyclic Aromatic Hydrocarbons, PAHs).8 Laminar combustion flame has the advantages of simple and stable structure and easy control of operating parameters. It can be used to reflect the combustion characteristics of fuel, such as laminar combustion velocity (LBV), ignition energy, maximum flame temperature, ignition temperature and concentration, ignition delay period and so on. In order to identify the impurity gas(N2, CO2 and H2O) in methane play a role in its laminar premixed combustion process, Miao et al.9,10 experimentally studied the effects of dilution of CO2 and N2 on laminar combustion characteristics of natural gas-hydrogen blended fuel in a constant volume combustion bomb and found that dilution of CO2 and N2 dilution reduce the LBVs. Besides, The methane laminar premixed flame with CO2 and N2has been studied experimentally and numerically by Zhang et al..11 The results show that the existence of CO2 and N2 inhibits the formation of NO and the specific dilution effect of reducing NO emission is related to the equivalence ratio. Halter et al.12 studied the LBV of methane/air in a spherical combustion bomb under elevated pressure and hydrogen doping ratios by using the classical shadowing method and investigated the effects of hydrogen doping on laminar and turbulent premixed methane flame by spherical propagation flame and pilot flame at high pressure13. Moreover, the influence of N2 and CO2 addition on LBV of methane and isooctane was analyzed by introducing the concept of FCO2 and three methods for obtaining LBV and Markstein length from combustion bomb flames also have been summarized and compared with experimental data by Halter et al.14,15 In addition, Galmiche et al. 3 ACS Paragon Plus Environment

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experimentally and numerically studied the effects of dilution of N2, CO2, H2O on the LBV of methane. The physical and chemical effects of CO2 and H2O on the LBVs were analyzed by defining FCO2 and FH2O which did not participate in chemical reactions, and the correlation between LBV and heat capacity of mixtures were proposed and validated.16 Mazas et al.17,18 measured the LBVs of oxygen-rich methane premixed flames diluted by CO2 and water vapor under different caliber Bunsen flames. The effects of CO2 and H2O in different oxidant atmosphere were compared and analyzed, and the thermal and chemical effects of H2O were emphatically analyzed. Through Bunsen burner and numerical simulation, Hu et al. investigated the flame structure, free radical concentration and chemical reaction path in the atmosphere of O2/CO2 and O2/N219 and the chemical effects of CO2 at different equivalence ratios and oxygen concentration during methane laminar premixed combustion process which found that the three-body collision effect of CO2 decreased the LBV at low oxygen concentration and promoted the LBV at high oxygen concentration20. In addition, they also found that increasing the initial temperature accelerated the combustion reaction and increased the LBV. The chemical effect CO2 on the LBV was more obvious than that of transport and radiation effect.21 Zou et al.22 and Song et al.23 numerically explored the chemical reaction mechanism of the effect of water vapor and CO2 on oxygen-enriched methane counter-flow flame. The counter-flow diffusion flame of biogas BG75 (75%CH4 and 25%CO2)/hydrogen mixture was numerically studied by Amar et al.24 It was found that the maximum combustion temperature, the species of CO, C2H2 and CH2O and NO production are reduced with dilution by H2O. Shareh et al.25 analyzed the effects of adding N2, CO2 and H2O on the oxidation characteristics of methane/oxygen mixture through simulation calculation. It was concluded that water vapor could promote methane oxidation and shorten ignition delay time, while carbon dioxide was the opposite. Chica Cano et al.26 performed an experiment to study the effect of CH4 mixing with water vapor at high temperature, high pressure and different oxygen concentration on its LBV in a constant volume combustion bomb combined with shadow imaging technology and found that the differences between calculated LBVs with and without FH2O increase with water mole fraction and decrease with pressure. From the above researches it can be concluded that the effects of adding N2, CO2 and H2O on the fuel, oxidant or fuel/oxidant mixtures have been carried out to study the laminar flames characteristics of methane. However, there are still several limitations in the previous studies to analyze the physical and chemical effects of N2, CO2 and H2O adding to fuel on the 4 ACS Paragon Plus Environment

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laminar premixed combustion characteristics, especially the net heat release rate (NHRR) and NO formation. The aim of our work is to explore the effects of these diluents on laminar premixed combustion which can help carry out further kinetic studies on biogas combustion and provide theoretical basis for optimizing the combustion of biogas and methane. 2. Numerical model In this paper, the effects of adding N2, CO2 and H2O on the laminar premixed combustion characteristics of methane were simulated using the premixed free-propagating flame model based on ChemkinII27/Premix Code28. In the calculation process, the multicomponent transport model is used in all operating conditions and the Soret effect is taken into account. The maximum number of meshes is set to 500. The parameters of gradient and curvature equal to 0.04. The left boundary of computational domain was at 0.2 cm and the right boundary location was set to between 6 and 10 cm to achieve zero gradients for all variables, which satisfied the calculation requirements. The formula for calculating the amount of N2, CO2 and H2O doping is as follows

Dr 

Vdiluent Vdiluent  VCH 4

(1)

where Vdiluent and VCH4 are volume fractions of the diluents (N2, CO2 and H2O) and methane, respectively. Dr represents the proportion of diluents in the fuel/diluent mixture and is generally used to study the effects of different fuel mixtures. It can also be used to investigate the influence of impurity gases, such as CO2 and N2 in fuels (such as biogas). Based on previous research results14-24 and our previous work29, the widely recognized kinetic mechanisms: GRI-Mech 3.030 (containing 325 elementary reactions and 53 species) that can be applied to oxidation combustion of methane was selected to analyze the laminar premixed combustion characteristics of the mixtures of methane with dilutions. Moreover, to determine the physical and chemical effects of N2, CO2 and H2O, three fictious substances, FN2, FCO2 and FH2O, were added to the GRI 3.0 mechanism. The number of elementary reactions in the mechanism does not increase, nor does it affect the calculation results. Correspondingly, the three hypothetical substances have the same thermodynamic and transport properties of N2, CO2 and H2O respectively, and they do not directly participate in any chemical reaction and do not crack molecules, but only affect the combustion chemical reaction process through three-body collision, similar to inert gases. 3. Results and discussion 5 ACS Paragon Plus Environment

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3.1. The effects of N2/CO2/H2O addition on LBV Figure 1 shows the calculation results of LBVs of CH4 blended with N2/CO2/H2O under lean (Φ=0.8), stoichiometric (Φ=1.0) and rich (Φ=1.2) combustion conditions, at initial temperature 398 K and initial pressure 1 atm, unless otherwise explicitly indicated. From Figure 1(a) - (c), it can be seen that the LBVs of CH4 decreased with N2, CO2 and H2O addition at different equivalence ratios, and the drop is decreased with the increase of dilution ratios. In Figure 1(a), taking the laminar premixed combustion of CH4 with 30% CO2 addition as an example, the LBV decreases with the dilution of CO2 and FCO2, and the influence of CO2 was higher than that of FCO2 at the same Dr. This reflects that both the physical and chemical effects of CO2 reduce the LBV, and the physical effects are greater than the chemical. Similarly, the physical and chemical effects of N2 and H2O also decrease the LBV of CH4, and the physical effects are dominant. For the low chemical reactivity of N2, which is often regarded as inert gas, the effect of FN2 on LBV is very slightly different from that of N2. Therefore, there is no special explanation that the Dr marked at 40% is the LBV of methane diluted with N2. According to the literature22,23 (giving the characteristic parameters of N2, CO2 and H2O at 100 kPa and 1000 K), it is known that the order of specific heat capacity of the three diluents is CO2 > H2O > N2. Therefore, comparing Figure 1(a) - (c), it is seen that the decreasing range of the physical effects on LBV follows the order of CO2 > H2O > N2 under different equivalence ratios, and the tendency of the chemical effects is the same as that of physical. This is mainly due to the addition of CO2, which participates in the reaction of OH + CO H + CO2 competing for H atoms with H + O2 OH + OH and H + O2 O + OH, which increases the ratio of OH/H, affecting the propagation of the reaction chain and reducing the overall combustion reaction rate, thus inhibiting the oxidation of CH4 and decreasing the LBV. The addition of H2O increases the concentration of OH by participating in the reaction H2O + H OH + H2 and H2O + O OH + OH consuming H and O while increasing the concentration of OH, and it affects the process of H + O2 O + OH and CH4 + O CH3 + OH, so that the combustion reaction rate decreases and the LBV decreases. At different equivalence ratios, the effect of three diluents on the decrease of LBV is the greatest at rich combustion. The chemical effect is the greatest at stoichiometric condition, and the physical effect is the greatest at rich combustion.


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(a)

39 36 33 0

N2

FN2

H2O

FH2O

CO2

FCO2

10

38.77

Chemical effect

 = 0.8

Physics effect

42

Laminar burning velocity (cm/s)

45.32 Combined effect

45

37.03 35.95 34.40 31.78

20

30

(b)

60

60.01

57 54 51

50.98

 = 1.0

48 45 42 39

40

0

48.15 46.94 44.96

N2

FN2

H2O

FH2O

CO2

FCO2

10

39.29

20

Dr/(%)

30

40

Dr/(%)

57 Laminar burning velocity (cm/s)

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Laminar burning velocity (cm/s)

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(c)

54

54.77

51 48 45

44.02

 = 1.2

42

40.37 39.90

39 36 33 30

0

N2

FN2

H2O

FH2O

CO2

FCO2

10

37.01 32.36

20

30

40

Dr/(%)

Figure 1. LBVs of CH4 blended with N2/CO2/H2O at lean, stoichiometric and rich conditions. 3.2. The effects of N2/CO2/H2O addition on AFTs The adiabatic flame temperatures (AFTs) of CH4 blended with N2/CO2/H2O under lean, stoichiometric and rich combustion conditions are given in Figure 2. From Figure 2 (a) - (c), it can be seen that the maximum temperature of CH4 adiabatic flame decreases gradually with the mixing of N2, CO2 and H2O. The effects of three diluents increase with the increasing Dr, and their physical and chemical effects are the greatest at stoichiometric ratio. The physical effects are far greater than chemical effects. In particular, it can be found from Figure 2 (c) that the chemical effect of H2O increases the AFT of CH4, and the effect increases with the increasing Dr. However, the chemical effect of CO2 still reduces the AFT. According to references8,24, the chemical effect is higher than that of physical effect at high temperature through adding appropriate amount of H2O to CH4, which makes the AFT rise. The reason for that is the H2O existence in the fuel could promote the consumption of O radicals through OH + OH O + H2O simultaneously increasing the OH mole fraction which can modify the flame structure to produce higher flame temperature in a specific condition (a certain dilution ratio of H2O in the methane) due to the chemical effect of H2O. 7 ACS Paragon Plus Environment

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(a)

2020

 = 0.8

Adiabatic flame temperature (K)

2040

2066

Physics effect

Combined effect

2060

2004

2000 1980 1960 0

N2

FN2

H2O

FH2O

CO2

FCO2

10

Chemical effect


1986 1984 1967 1963

20

30

(b)

2280

2277

2260 2240 2220

 = 1.0

2214

2200 2180 2160 0

40

N2

FN2

2193 2188

H2O

FH2O

CO2

FCO2

2172

10

2159

20

30

40

Dr/(%)

Dr/(%)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)

2200

2204

2160 2120

 = 1.2

2080

N2

FN2

H2O

FH2O

CO2

FCO2

2040 0

2110

10

2085 2082 2053 2048

20

30

40

Dr/(%)

Figure 2. AFTs of CH4 blended with N2/CO2/H2O at lean, stoichiometric and rich conditions 3.3. The effects of N2/CO2/H2O addition on NHRR Figure 3 shows the relationship between the NHRR of CH4 diluted by N2/CO2/H2O varied with flame temperature at dilution ratios of 0%, 20% and 40%. The results show that the NHRR of CH4 decreases with the increase the Dr of N2, CO2 and H2O. At the same equivalence ratio, the flame temperature corresponding to the peak of NHRR of methane decreases gradually with the increasing diluents volume. As can be seen from the above section, this is because the increasing amount of three diluents leads to the decrease of flame temperature and the decrease of laminar premixed combustion rate. At the same Dr, the vary temperature value of the peak pair of net heat release rate follows the order of N2 > H2O > CO2, and the order of the effect of three diluents on the decreasing NHRR is CO2 > H2O > N2. In addition, as shown in Figure 3, when the same volume diluent is added, the temperature range corresponding to heat release decreases gradually with the increasing Dr. With different diluents and the same blending ratio, the order of corresponding temperature range of heat release under the dilution of three diluents is N2 > H2O > CO2. By comparing FN2, FCO2 and FH2O, the physical and chemical effects of N2, CO2 and H2O reduced the 8 ACS Paragon Plus Environment

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NHRR. The temperature value corresponding to the peak NHRR is higher with adding fictious substance. In addition, the chemical effects of CO2 and H2O have a significant impact on the decrease of NHRR, while the NHRR curves of N2 and FN2 basically coincide, and there is no significant difference even though at different equivalence ratios.

2.5 2.0 1.5 1.0 0.5 0.0

0 20% CO2 20% FCO2 40% FCO2 20% N2 40% N2 20% FN2 40% FN2 20% H2O 40% H2O 20% FH2O 40% FH2O

600

900

5

-1

40% CO2

b  = 1.0

1200

1500

1800

0 20% CO2 40% CO2

-3

-3

-1

3.0

6

a  = 0.8

Net heat release rate (kJcm s )

3.5


4 3

20% FCO2 40% FCO2 20% N2 40% N2 20% FN2

2 1

40% FN2 20% H2O 40% H2O 20% FH2O 40% FH2O

0 600

2100

5

-1 -3

4

900

1200

1500

1800

2100

Tempreature (K)

Tempreature (K)


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c  = 1.2 0 20% CO2 40% CO2 20% FCO2

3

40% FCO2 20% N2 40% N2

2

20% FN2 40% FN2

1

20% H2O 40% H2O 20% FH2O

0

40% FH2O

600

900

1200

1500

1800

2100

Tempreature (K)

Figure 3. NHRRs of CH4 blended with N2/CO2/H2O at lean, stoichiometric and rich conditions. 3.4. Temperature sensitivity analysis 3.4.1. The effect of N2 on the temperature sensitivity In order to further analyze the effects of N2, CO2 and H2O on the temperature and NHRR of methane laminar premixed flame, the specific effects of three diluents on the important dominated reactions of temperature were determined. Firstly, Figure 4 shows the sensitivity analysis of the important elementary reactions on the temperature change at the maximum temperature gradient at different equivalence ratios with N2 and FN2 blending. From Figure 4 (a) - (c), it can be concluded that with the increase of equivalence ratio, the most important reaction contributing to the increase of temperature is R38 H + O2 O + 9 ACS Paragon Plus Environment

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OH. R52 H + CH3(+M) CH4(+M) plays a leading role in hindering the temperature rise. N2 doping makes the sensitivity coefficients of both of them increase. At different equivalence ratios, the effect of N2 addition on temperature is enhanced. Compared with FN2, the physical effect of N2 increases the temperature sensitivity of the reactions. The chemical effects of N2 on R38 and R52 with Dr of 20% at lean combustion and 20% and 40% at rich combustion promote their effects on temperature. In other cases, the chemical effect of N2 reduces its flame temperature sensitivity. (a)   

R52 H+CH3(+M)CH4(+M) R35 H+O2+H2OHO2+H2O

40% FN2

R168 HCO+O2HO2+CO

20% FN2 40% N2

R167 HCO+MH+CO+M

20% N2

R284 O+CH3=>H+H2+CO

0%

R166 HCO+H2OH+CO+H2O R119 HO2+CH3OH+CH3O R97 OH+CH3CH2(S)+H2O R99 OH+COH+CO2 R38 H+O2O+OH

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Sensitivity coefficient

(b)   

R52 H+CH3(+M)CH4(+M)

40% FN2

R35 H+O2+H2OHO2+H2O

20% FN2

R55 H+HCOH2+CO

40% N2

R167 HCO+MH+CO+M

20% N2

R284 O+CH3=>H+H2+CO

0%


-0.4

-0.2

0.0

0.2

0.4



0.6

0.8

1.0

1.2

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(c)   

R52 H+CH3(+M)CH4(+M) R55 H+HCOH2+CO

40% FN2

R158 2CH3(+M)C2H6(+M)

20% FN2 40% N2

R74 H+C2H4(+M)C2H5(+M)

20% N2

R167 HCO+MH+CO+M

0%

R284 O+CH3=>H+H2+CO R97 OH+CH3CH2(S)+H2O R166 HCO+H2OH+CO+H2O R119 HO2+CH3OH+CH3O R38 H+O2O+OH

-0.5

0.0

0.5

1.0

1.5


Figure 4. The effect of N2 on the temperature sensitivity analysis with N2 addition at different equivalence ratios. 3.4.2. The effect of CO2 on the temperature sensitivity The sensitivity analysis of the important elementary reactions on temperature change at the maximum temperature gradient with CO2 blending at different equivalence ratios is shown in Figure 5. It is found from Figure 5(a)-(c) that at different equivalence ratios, R55 H + HCO H2 + CO, R99 OH + CO H + CO2, R166 HCO + H2O H + CO + H2O and R284 O + CH3 H + H2+ CO decrease their sensitivity to CH4 temperature increase with CO2 addition, but promote the increase of sensitivity coefficients of other reactions. This is mainly due to the CO2 concentration increased in the combustion process through R99 OH + CO H + CO2 with CO2 addition. Comparing with blended FCO2, it can be found that the physical effects of CO2 enhance the sensitivity of reactions for temperature increase except for R284 O + CH3 H + H2+ CO at lean combustion. The chemical effect of CO2 increases the temperature sensitivity of R38 at lean and stoichiometric conditions as well as R35, R166 and R119 at different equivalence ratios decreases the sensitivity of other reactions with increasing Dr. In addition, it is to be noted that the sensitivity coefficients of R52 and R38 on temperature increase with the increase of equivalence ratio, while for other reactions it decreases with increasing equivalence ratios.


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(a)   


40% FCO2

R168 HCO+O2HO2+CO

20% FCO2 40% CO2

R167 HCO+MH+CO+M

20% CO2

R284 O+CH3=>H+H2+CO

0%


-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Sensitivity coefficient (b)   


40% FCO2

R35 H+O2+H2OHO2+H2O

20% FCO2

R55 H+HCOH2+CO

40% CO2

R167 HCO+MH+CO+M

20% CO2

R284 O+CH3=>H+H2+CO

0%


-0.4 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2


(c)   


40% FCO2

R158 2CH3(+M)C2H6(+M)

20% FCO2 40% CO2

R74 H+C2H4(+M)C2H5(+M)

20% CO2

R167 HCO+MH+CO+M

0%


-0.5

0.0

0.5

1.0

1.5


Figure 5. The effect of CO2 on the temperature sensitivity analysis with N2 addition at different equivalence ratios. 3.4.3. The effect of H2O on the temperature sensitivity Figure 6 illustrates the sensitivity analysis of the important elementary reactions contributing to temperature rise with H2O addition. In lean combustion, R99 OH + CO H 12 ACS Paragon Plus Environment

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+ CO2, R97 OH + CH3 CH2(S) + H2O and R284 O + CH3 H + H2+ CO on the increasing CH4 temperature are gradually weakened with the increase of H2O blending ratio. The sensitivity coefficients of other reactions increased accordingly. The sensitivity coefficients R97, R284 and R167 at stoichiometric ratio and R284 at rich combustion on temperature rise decrease with the increase. Besides, the sensitivity of R55 at rich combustion also decreases. Comparing to FH2O, it can be also found that except for R167 at any equivalent ratio, R 55 and R 166 at stoichiometric ratio and R 158 at rich combustion, the physical effect of H2O enhances the sensitivity of other reactions to temperature with the increase of Dr. The chemical effect of H2O increases the temperature sensitivity of R38 and R35 and R166 at lean combustion with different equivalence ratios, while the sensitivity coefficient of other reactions decreases gradually with increasing Dr. (a)   


40% FH2O

R168 HCO+O2HO2+CO

20% FH2O

R167 HCO+MH+CO+M

40% H2O 20% H2O

R284 O+CH3=>H+H2+CO

0%


-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Sensitivity coefficient (b)   


40% FH2O

R35 H+O2+H2OHO2+H2O

20% FH2O

R55 H+HCOH2+CO

40% H2O

R167 HCO+MH+CO+M

20% H2O

R284 O+CH3=>H+H2+CO

0%


-0.4

-0.2

0.0

0.2

0.4



0.6

0.8

1.0

1.2

Energy & Fuels

(c)   


40% FH2O

R158 2CH3(+M)C2H6(+M)

20% FH2O 40% H2O

R74 H+C2H4(+M)C2H5(+M)

20% H2O

R167 HCO+MH+CO+M

0%


-0.5

0.0

0.5

1.0

1.5


Figure 6. The effect of H2O on the temperature sensitivity analysis with N2 addition at different equivalence ratios. 3.5. NO formation The mole fraction profiles of NO under lean, stoichiometric and rich conditions with 40% N2/CO2/H2O addition to CH4 are given in Figure 7. It is seen that increasing concentration of N2, CO2 and H2O can lead to the reduction of NO emission in methane/air mixtures at all equivalence ratios. And the influence of CO2 on decreasing NO formation is larger than that of H2O and N2. Compared to the fictious substances, from Figure 7, it is seen that the chemical effects of CO2 and H2O decrease the formation of NO. The chemical effect of N2 promotes NO production in combustion. However, for all three diluents, their physical effects significantly promote the reduction of NO formation. Besides, it is worth pointing out that in lean flames the NO concentration is much lower than stoichiometric and rich conditions. Obviously, blending N2, CO2 and H2O to methane can effectively reduce the NO fomation during combustion. But for the post-treatment of NO, the way of using catalyst for adsorption, storage and reuse can be adopted. 31,32 -5

-4

7.00x10

3.5x10

(a)    0 40% N2

-5

5.60x10

40% FH2O

-5

40% CO2 40% FCO2

-5

2.80x10

-5

1.40x10

0.00

0 40% N2

-4

40% H2O

4.20x10

(b)   

2.8x10

40% FN2

Mole fraction of NO

Mole fraction of NO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

40% FN2 40% H2O

-4

2.1x10

40% FH2O 40% CO2 40% FCO2

-4

1.4x10

-5

7.0x10

0

1

2

3

4

5

0.0

0

1

2

3

Distance from jet (cm)



4

5

Page 15 of 23

-5

9.00x10

(c)   

-5

7.50x10

Mole fraction of NO

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

-5

6.00x10

-5

4.50x10

0 40% N2

-5

3.00x10

40% FN2 40% H2O 40% FH2O

-5

1.50x10

40% CO2 40% FCO2

0

1

2

3

4

5


Figure 7. The effect of N2/CO2/H2O addition on NO formation at different equivalence ratios. To further explore the specific effects of physical, chemical and comprehensive effects of three diluents(N2/CO2/H2O) on the important reaction of NO formation, the sensitivity analysis on NO formation was carried out. Figures 8-10 show the sensitivity coefficients of NO with N2/CO2/H2O dilution at Φ= 0.8, 1.0 and 1.2. Through sensitivity analysis, in Figures 8-10 the mechanism of NO production in methane flame is controlled mainly through the prompt route in stoichiometric condition. The sensitivity coefficients of the reactions directly responsible for NO formation are decreased with N2/CO2/H2O dilutions. The sensitivity analysis of the main elementary reactions, which play a leading role in the influence of NO generation, show that the AFT is reduced after adding N2, CO2 and H2O, depressing the generation of NO. With N2/CO2/H2O addition, the generation path of NO is mainly prompt NOx, and the responsible reactions are R38 H + O2 O + OH, R240 CH + N2 = HCN+N, R52 H + CH3(+M) CH4(+M) and R125 CH + O2 O + HCO. In addition, with the increase of equivalence ratio, the sensitivity coefficients of important elementary reactions which play an important role in NO formation increase. Moreover, the physical and chemical effects of N2, CO2 and H2O on the main reaction are significantly depending on the equivalence ratios.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

(a) R35 H+O2+H2O HO2+H2O R52 H+CH3(+M) CH4(+M)

 = 0.8 40% FN2 20% FN2 40% N2 20% N2 0

R186 HO2+NO NO2+OH R178 N+NO N2+O R119 HO2+CH3 OH+CH3O R126 CH+H2 H+CH2 R97 OH+CH3 CH2(S)+H2O R240 CH+N2 HCN+N R99 OH+CO H+CO2 R38 H+O2 O+OH

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Sensitivity coefficient

(b) R125 CH+O2 O+HCO R52 H+CH3(+M) CH4(+M)

 = 1.0 40% FN2 20% FN2 40% N2 20% N2 0

R284 O+CH3 H+H2+CO R186 HO2+NO NO2+OH R178 N+NO N2+O R119 HO2+CH3 OH+CH3O R97 OH+CH3 CH2(S)+H2O R99 OH+CO H+CO2 R240 CH+N2 HCN+N R38 H+O2 O+OH

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Sensitivity coefficient (c) R52 H+CH3(+M) CH4(+M) R125 CH+O2 O+HCO R98 OH+CH4 CH3+H2O R186 HO2+NO NO2+OH R284 O+CH3 H+H2+CO R119 HO2+CH3 OH+CH3O R166 HCO+H2O H+CO+H2O R126 CH+H2 H+CH2 R97 OH+CH3 CH2(S)+H2O R240 CH+N2 HCN+N R38 H+O2 O+OH

 = 1.2 40% FN2 20% FN2 40% N2 20% N2 0

-0.5

0.0

0.5

1.0

1.5

2.0


Figure 8. Sensitivity coefficients of NO with N2 addition at different equivalence ratios.


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(a) R35 H+O2+H2O HO2+H2O R52 H+CH3(+M) CH4(+M) R45 H+HO2 O2+H2 R186 HO2+NO NO2+OH R178 N+NO N2+O R119 HO2+CH3 OH+CH3O R126 CH+H2 H+CH2 R97 OH+CH3 CH2(S)+H2O R240 CH+N2 HCN+N R99 OH+CO H+CO2 R38 H+O2 O+OH

 = 0.8 40% FCO2 20% FCO2 40% CO2 20% CO2 0

-0.5

0.0

0.5

1.0

1.5

Sensitivity coefficient (b) R125 CH+O2 O+HCO R52 H+CH3(+M) CH4(+M) R98 OH+CH4 CH3+H2O R284 O+CH3 H+H2+CO R186 HO2+NO NO2+OH R178 N+NO N2+O R119 HO2+CH3 OH+CH3O R99 OH+CO H+CO2 R97 OH+CH3 CH2(S)+H2O R240 CH+N2 HCN+N R38 H+O2 O+OH

 = 1.0 40% FCO2 20% FCO2 40% CO2 20% CO2 0

-0.5

0.0

0.5

1.0

1.5


(c)

 = 1.2

R52 H+CH3(+M) CH4(+M) R125 CH+O2 O+HCO R53 H+CH4 CH3+H2 R98 OH+CH4 CH3+H2O R55 H+HCO H2+CO R45 H+HO2 O2+H2 R186 HO2+NO NO2+OH R284 O+CH3 H+H2+CO R74 H+C2H4(+M) C2H5(+M) R166 HCO+H2O H+CO+H2O R119 HO2+CH3 OH+CH3O R97 OH+CH3 CH2(S)+H2O R240 CH+N2 HCN+N R38 H+O2 O+OH

-1.0

40% FCO2 20% FCO2 40% CO2 20% CO2 0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5


Figure 9. Sensitivity coefficients of NO with CO2 addition at different equivalence ratios.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

(a) R35 H+O2+H2O HO2+H2O

 = 0.8 40% FH2O 20% FH2O 40% H2O 20% H2O 0

R52 H+CH3(+M) CH4(+M) R186 HO2+NO NO2+OH R178 N+NO N2+O R119 HO2+CH3 OH+CH3O R126 CH+H2 H+CH2 R97 OH+CH3 CH2(S)+H2O R240 CH+N2 HCN+N R99 OH+CO H+CO2 R38 H+O2 O+OH -0.5

0.0

0.5

1.0

1.5

Sensitivity coefficient (b) R125 CH+O2 O+HCO R52 H+CH3(+M) CH4(+M)

 = 1.0 40% FH2O 20% FH2O 40% H2O 20% H2O 0

R98 OH+CH4 CH3+H2O R284 O+CH3 H+H2+CO R186 HO2+NO NO2+OH R178 N+NO N2+O R119 HO2+CH3 OH+CH3O R97 OH+CH3 CH2(S)+H2O R99 OH+CO H+CO2 R240 CH+N2 HCN+N R38 H+O2 O+OH -0.5

0.0

0.5

1.0

1.5

Sensitivity coefficient (c)

 = 1.2

R52 H+CH3(+M) CH4(+M) R125 CH+O2 O+HCO R53 H+CH4 CH3+H2

40% FH2O 20% FH2O 40% H2O 20% H2O 0

R98 OH+CH4 CH3+H2O R186 HO2+NO NO2+OH R284 O+CH3 H+H2+CO R74 H+C2H4(+M) C2H5(+M) R119 HO2+CH3 OH+CH3O R166 HCO+H2O H+CO+H2O R97 OH+CH3 CH2(S)+H2O R240 CH+N2 HCN+N R38 H+O2 O+OH

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0


Figure 10. Sensitivity coefficients of NO with H2O addition at different equivalence ratios. 4. Conclusions Using the premixed free-propagating flame model based on ChemkinII/Premix Code, a numerical study on the effects of adding N2, CO2 and H2O on the laminar premixed 18 ACS Paragon Plus Environment

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combustion characteristics of methane was carried out. The physical and chemical effects of N2, CO2 and H2O on LBVs, AFTs, NHRRs and the main elementary reactions responsible for temperature rise and NO formation of methane laminar premixed flames were discussed under lean (Ф=0.8), stoichiometric (Ф=1.0) and rich (Ф=1.2) conditions at different dilution ratios(0-40%). The main conclusions are summarized as follows. (1) Both the physical and chemical effects of N2, CO2 and H2O reduce the LBV, and the physical effects are greater than the chemical. The AFT of CH4 decreases gradually with the mixing of N2, CO2 and H2O. The effects of three diluents increase with the increasing Dr, and their physical and chemical effects are the greatest at stoichiometric ratio. In particular, under rich condition, the chemical effect of H2O increases the AFT of CH4, and the effect increases with the increasing Dr. (2) For the same Dr, the order of temperature value of the peak pair of net heat release rate is N2 > H2O > CO2, and the order of the effect of three diluents on the decreasing NHRR is CO2 > H2O > N2. With different diluents and the same blending ratio, the order of corresponding temperature range of heat release under the dilution of three diluents is N2 > H2O > CO2. And both the physical and chemical effects of N2, CO2 and H2O reduced the NHRR. (3) The chemical effects of CO2 and H2O decrease the formation of NO while the chemical effect of N2 promotes NO production. Besides, for all three diluents, their physical effects significantly promote the reduction of NO formation. The generation path of NO is mainly a prompt way, and the responsible reactions are R38 H + O2 O + OH, R240 CH + N2 = HCN+N, R52 H + CH3(+M) CH4(+M) and R125 CH + O2 O + HCO. Acknowledgements The authors would like to thank the National Natural Science Foundation of China (Grant Nos. 51827808, 51676002) and the Project of support program for outstanding young people in Colleges and Universities (Grant No. gxyqZD201830) for their ﬁnancial support of this study.


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A Modelling Study of the Impact of Blending N2, CO2 and H2O on

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