行政院國家科學委員會專題研究計畫成果報告

(1)

行政院國家科學委員會專題研究計畫成果報告

過氧化氫輔助次極限貧油燃燒之實驗與數值研究研究成果報告(精簡版)

計畫類別：個別型

計畫編號： NSC 99-2221-E-216-007-

執行期間： 99 年 08 月 01 日至 100 年 07 月 31 日執行單位：中華大學機械工程學系

計畫主持人：鄭藏勝共同主持人：陳冠邦

計畫參與人員：碩士班研究生-兼任助理人員：林士勛博士班研究生-兼任助理人員：許耀中

處理方式：本計畫可公開查詢

中華民國 100 年 10 月 21 日

(2)

行政院國家科學委員會補助專題研究計畫 ■ 成果報告

□期中進度報告

過氧化氫輔助次極限貧油燃燒之實驗與數值研究

Experimental and Numerical Investigations of Hydrogen Peroxide Assisted Sub-lean Limit Combustion

計畫類別：▓ 個別型計畫 □ 整合型計畫計畫編號：NSC 992221E216007

執行期間： 99 年 8 月 1 日至 100 年 7 月 31 日

計畫主持人：鄭藏勝教授

共同主持人：陳冠邦副研究員

計畫參與人員：趙怡欽教授、李約亨(博士後研究員)、許紘瑋(博士後研究員)、許耀中(博士生)、林士勛(碩士生)

成果報告類型(依經費核定清單規定繳交)：▓精簡報告 □完整報告

本成果報告包括以下應繳交之附件：

□赴國外出差或研習心得報告一份

□赴大陸地區出差或研習心得報告一份

□出席國際學術會議心得報告及發表之論文各一份

□國際合作研究計畫國外研究報告書一份

處理方式：除產學合作研究計畫、提升產業技術及人才培育研究計畫、列管計畫及下列情形者外，得立即公開查詢

□涉及專利或其他智慧財產權，□一年□二年後可公開查詢

執行單位：中華大學機械工程學系、國立成功大學航空太空工程學系

中華民國 100 年 10 月 21 日

(3)

1 行政院國家科學委員會99年度專題研究計畫成果報告

過氧化氫輔助次極限貧油燃燒之實驗與數值研究

Experimental and Numerical Investigations of Hydrogen Peroxide Assisted Sub-lean Limit Combustion

計畫編號: NSC 99  ²²²¹  ^E  ²¹⁶  ⁰⁰⁷

執行期限: 99 年 8 月 1 日至 100 年 7 月31 日主持人: 鄭藏勝中華大學機械工程學系

共同主持人：陳冠邦國立成功大學航空太空工程學系 E-mail: tscheng@chu.edu.tw; gbchen26@gmail.com

1. 摘要

本計畫主要是以發展應用過氧化氫作為反應的促進劑或取代空氣當作氧化劑，輔助次極限貧油燃燒，進而達到節能與降低污染物生成的目標。由於目前正在使用之工業燃燒爐、渦輪引擎燃燒室、內燃機引擎等燃燒器之設計尚無法完全採用次極限貧油燃燒，因此，利用高濃度過氧化氫的特性來輔助燃燒，使其達到次極限貧油燃燒的操作條件是一值得研究的主題。因此，本計畫將以實驗與數值方法分年研究過氧化氫作為燃料添加劑或氧化劑對貧油燃燒特性之影響。本年度的工作將利用 CHEMKIN Collection 軟體結合 GRI-Mech 3.0 完整化學反應機構，探討添加過氧化氫對化學反應路徑、層流燃燒速度、火焰結構及污染生成之影響。本計畫有關實驗部分原安排於第 2 及第 3 年，惟因計畫僅核准一年，故本報告將僅呈現第 1 年之數值研究成果。

關鍵字：過氧化氫、次極限貧油燃燒、觸媒分解、

數值模擬

2. Abstract

The main objective of this research project is to employ the purified hydrogen peroxide (H2O2) as a reaction stimulator or oxidizer for assisting sub-lean limit combustion and to achieve the goal of energy savings and pollutant reductions.

However, the applications of hydrogen peroxide assisted sub-lean limit combustion to practical combustion systems strongly rely on fundamental understanding of the characteristics of hydrogen peroxide combustion. The main focus of the present proposal is to thoroughly investigate the detailed chemical kinetics, flame stability limits, flame structures, and pollutant formation mechanisms of hydrogen peroxide assisted lean combustion through experimental investigations and numerical

simulations. The work of this year is to numerically investigate the effects of hydrogen peroxide addition on the reaction path, laminar burning velocity, flame structure, and pollutant formation using the CHEMKIN Collection in conjunction with GRI-Mech 3.0 chemical kinetic mechanisms.

Experimental investigations will be performed in the second and third years.

Keywords: Hydrogen peroxide, Sub-lean limit

combustion, Catalytic decomposition, Numerical simulation

3. Introduction

Lean combustion is generally considered as one of the timely solutions for the more stringent environmental regulations and global warming concerns in the new century. However, lean combustion suffers from combustion instability, such as flame pulsation, flame flickering and blowout, due to low heat release rate and high local extinction. In addition, it is generally accompanied with incomplete combustion with high CO and UHC (unburned hydrocarbon) emissions in most lean combustion applications. Therefore, the key problem for lean combustion applications is to enhance combustion for flame stabilization and for complete combustion to avoid high CO and UHC emissions [1-2]. Some strategies for stabilizing lean premixed flames and extending lean flammability limit have been proposed and extensively studied for decades [1-3]. Particularly, feasible and pragmatic approaches to extend lean flammability limit can be roughly summarized into three categories: (i) flow structure adjustment [3-5], (ii) increase in temperature and pressure of flow fields [6-8], and (iii) alteration in flame chemical properties [9-14]. Utilizing swirler and bluff body to generate recirculation zone for enhancing flow residence time and fuel-air mixing is a promising

(4)

manner by means of flow structures modification.

Heat exchanger, flue gas recirculation mechanism as well as turbocharger are generally applied in engines and furnaces to enhance temperature and pressure in the flow field, and to extend flammability limits. As to chemical characteristics alteration in flames, employing catalyst to accelerate chemical reaction [7, 9] and inducing active chemical radicals (H, OH) by imposing microwave [10] or plasma [11] in the flame have received increasing attention recently. Nevertheless, these approaches generally have to further assemble extra mechanical auxiliaries and modify combustor configuration, so that thorough mechanical retrofit design and accurate fabrication are necessitated.

Employing active fuels (such as hydrogen, [12, 13]) or adding strong oxidants [14] to fuels is an attractive alternative to enhance lean flame combustion without involving any moving part or configuration modification. Hydrogen peroxide (H2O2) is an environmentally friendly oxidant with strong oxidability. The oxidizing power of hydrogen peroxide is just inferior to that of fluorine [15], which has the strongest electronegativity in the periodic table. Hydrogen peroxide under normal temperature is in liquid state, so that it is easy to store and handle. After chemical dissociation, hydrogen peroxide produces only oxygen and steam, and plus exothermicity (approximately 2884.6 kJ/kg) without toxic products. The decomposition reaction may be facilitated by heating or using catalyst and it is defined as:

H2O2 H2O + 1/2 O2 + △H (1) When temperature achieves 450℃, hydrogen peroxide will decompose with fierce exothermic heat release. Due to its inherently high decomposition temperature and high energy release, hydrogen peroxide with high concentration is often considered as a monopropellant [16]. It can also combine with hydrocarbon fuels to be bipropellants [17]. As to combustion applications, hydrogen peroxide can be used to replace air for reducing NOx emission and increasing combustion temperature.

A few recent studies have indicated potential promises in utilizing hydrogen peroxide for improving practical combustion process.

Golovitchev et al. [18] examined the possibility of promoting methane auto-ignition in air using 5-10%

hydrogen peroxide. Ting and Reader [19] used PREMIX code to investigate the effects of hydrogen peroxide on the premixed methane-air flame under atmospheric conditions. The maximum hydrogen peroxide concentration used to replace air was 6.25%. Hydrogen peroxide was found to be

effective in enhancing the burning velocity, and this was particularly true for the richer mixtures considered. Kim et al. [20] discovered that hydrogen peroxide assisted the conversion of harmful nitric oxide to nitrogen dioxide in diesel exhaust gas. In addition, Born and Peters [21] found that proper injection of hydrogen peroxide into a diesel engine reduced soot and NOx drastically.

Martinez et al. [22] reported that the concentrations of UHC, CO and NOx from their industrial pilot plant combustion chamber fueled with natural gas were lowered significantly by injection of a few hundred ppm of hydrogen peroxide. Based on the above studies, promotion to stable lean combustion seems effective with the addition of hydrogen peroxide than that with hydrogen. Hydrogen is usually considered as an easily ignitable fuel and helpful for enhancing combustion. Nevertheless, hydrogen peroxide was also shown to act as a gas catalyst, to shift chemical pathways, and to further enhance chemical radicals [18].

Although the effects of hydrogen peroxide on combustion enhancement have been reported, systematic studies of the roles of hydrogen peroxide in combustion enhancement have not yet been conducted. Therefore, in the present study, the effects of hydrogen peroxide on premixed methane/air reaction pathway, laminar burning velocity, adiabatic flame temperature, and species formation are investigated numerically using the PREMIX code of Chemkin collection 3.5 with the GRI-Mech 3.0 chemical kinetic mechanisms [23]

and detailed transport properties.

4. Numerical model

In this work, the PREMIX code of CHEMKIN Collection is used to calculate the adiabatic, unstrained, free propagation velocities of the laminar premixed CH4/air/H2O2 flames. It solves the equations governing steady, isobaric, quasi-one-dimensional flame propagation. To obtain the accurate flame speed, the boundaries should be sufficiently far from the flame to avoid temperature and species gradients at the boundaries. Firstly, an initial run is performed with a computational domain just wide enough to encompass the flame.

Then, the domain is gradually expanded until the solution is domain-independent. In addition, the adiabatic flame temperature is calculated by using the EQUIL code of the Chemkin Collection. An initial reactant mixture is specified and equilibrium of constant enthalpy and constant pressure is constrained. To obtain accurate adiabatic flame temperature, besides reactants and products, all radical species that might occur in the flame are also included.

The GRI-Mech 3.0 chemical kinetic

(5)

3

mechanism composing of 53 chemical species and 325 reaction steps and detailed transport properties are used without any modifications. The reaction mechanism also includes the formation of NOx. This mechanism has been used satisfactorily to simulate non-catalytic H2O2 decomposition [24]. At the cold boundary, the unburned reactants are supplied at 423 K and 1 atm. This temperature presets at the boiling temperature of H2O2 and it is expected that all reactants are in gas phase. To further study into the role of hydrogen peroxide in the combustion enhancement of premixed methane flames, numerical simulations are performed for two different characteristic types of hydrogen peroxide addition: (1) as the oxidizer substituent by partial replacement of air (2) as the oxidizer supplier by using different concentrations of H2O2. Decomposition of one mole H2O2 can produce half mole O2 and one mole H2O. Therefore, the stoichiometric CH4/H2O2 ratio is 0.25 and the global reaction is defined as:

4 2 2 2 2

CH +4H O  CO +6H O

(2) In the case of partially replacing air by H2O2, the total O2 amount is maintained to keep the equivalence ratio constant. Therefore, the reduced amount of O2 from air is supplied from the decomposition of H2O2. For a stoichiometric condition, the reaction is defined as:

4 2 2 2 2 2 2

CH 4 H O



 (2 2 )(O



3.76N )CO (4



2)H O

3.76(2 2 )N  

₂ (3) where α is the replacement percentage of air by H2O2. The reduction of air leads to reduction of N2

in the oxidizer stream. For cases of using H2O2 as the oxidizer, the volumetric concentration of H2O2

is considered ranging from 30% to 100%.

5. Results and Discussion

5.1 Effects of H

₂

O

₂

on combustion characteristics

In order to examine the effects of H2O2 on enhancement of premixed methane flames and to study further into the modification of combustion characteristics, the results of premixed stoichiometric CH4/air and CH4/80% air + 20%

H2O2 are compared for illustration. The spatial coordinate ranges from cold boundary to 0.4 cm within the flame. The resultant temperature and species concentration profiles are shown in Fig. 1.

Results indicate that the premixed flame with 20%

air replaced by hydrogen peroxide has a higher adiabatic flame temperature due to the reduction of nitrogen dilution and heat release from thermal decomposition of hydrogen peroxide. The temperature increases approximately by 140 K as compared to the pure air case. For pure air case, the

reactant CH4 is completely consumed within 1.0 mm of the spatial coordinate but 0.85 mm for the 20% H2O2 replacement case. Hydrogen peroxide dedicates to enhance the methane consumption. In addition, the increase of H2O2 content in air obviously results in an increase of H2O production due to the product of H2O2 decomposition. It appears to slightly decrease CO2 formation and increase CO formation. Besides, some intermediate radicals, such as OH, H and O, show increasing trends with hydrogen peroxide addition. Especially, the increasing trends of HO2, HCO, CH2O and CH3O are more significant. These facts suggest that the dominant reactions of methane combustion are altered by H2O2 addition. Hydrogen peroxide decomposition increases the active radicals, enhances the reaction rate, and then accelerates the laminar burning velocity. The computed laminar burning velocity of stoichiometric CH4/air with the inlet temperature 423 K is about 0.71m/s, while the flame speed is increased to about 1.25 m/s when 20% air is replaced by H2O2.

5.2 Laminar burning velocity

Figure 2 shows the effect of partial replacement of air by H2O2 on the laminar burning velocity and adiabatic flame temperature for three different equivalence ratios (ER). The maximum percentage of air replaced by H2O2 is 100%. It can be seen that the laminar burning velocity is increased with increasing the equivalence ratio and the percentage of hydrogen peroxide replacement.

However, the effect of equivalence ratio on flame speed becomes mild for high hydrogen peroxide replacement percentage cases. When air is completely replaced by H2O2, the laminar burning velocity approaches 4.7 m/s for all equivalence ratio conditions. This is because that the oxidizer is completely provided from decomposition of hydrogen peroxide and hydrogen peroxide dominates the reaction rate of methane oxidation.

Figure 2 also shows that the adiabatic flame temperature increases with increasing the percentage of H2O2 replacement. The temperature increase for fuel lean conditions is larger than that for stoichiometric condition. The maximum temperature increase is up to 900 K for ER = 0.6, but 680 K and 520 K for ER = 0.8 and 1.0, respectively. The effect of temperature increase is primarily induced from the heat release of hydrogen peroxide decomposition since it is much higher than that released from methane reactions. Table 1 shows the maximum heat release rates (HRR) for different reactant conditions. The maximum heat release rates are about 6.09×10⁹ J/m³-s and 1.04×10⁹ J/m³-s for ER = 1.0 and 0.6 CH4/air flames, respectively.

For the case of CH4/50% H2O2 + 50% air the

(6)

maximum heat release rates are about 5.42 × 10¹⁰ J/m³-s and 2.77 × 10¹⁰ J/m³-s for ER = 1.0 and 0.6, respectively. Similarly, when air is completely replaced by H2O2, the maximum heat release rate approaches 4.49×10¹¹ J/m³-s and 4.37×10¹¹ J/m³-s, respectively. From Table 1 it is also noted that equivalence ratio contributes less significantly to the maximum heat release rate when air is replaced or partially replaced by hydrogen peroxide, but their HRR values are two orders of magnitude higher than that for CH4/air case. This “orders of magnitude” difference comes from hydrogen peroxide decomposition. It proves that the decomposition of H2O2 dominates the heat release and then determines the adiabatic flame temperature.

To investigate the characteristics of CH4/H2O2

flames, the volumetric concentration of H2O2 is varied from 30 to 100% while ER is kept at 0.6, 0.8, and 1.0. Figure 3 shows the laminar burning velocity and adiabatic flame temperature of the CH4/H2O2 flames with ER = 0.6, 0.8, and 1.0.

Results show that both laminar burning velocity and adiabatic flame temperature increase with increasing H2O2 concentration. With 30 vol.% of H2O2 the laminar burning velocity is higher than that of CH4/air flame for three different equivalence ratios, but the adiabatic flame temperature is lower than that of CH4/air flame. For the case of ER = 1.0 the temperature difference between two cases is about 250 K. However, for the case of ER = 0.6, the adiabatic flame temperature is almost the same as that of CH4/air flame and the temperature difference is only 10 K. In addition, the laminar burning velocity is increased to 0.63 m/s, which is much higher than that of CH4/air flame (0.27 m/s). When the H2O2 concentration is increased to 40 vol.%, the adiabatic flame temperature is higher than that of pure CH4/air flame for three different equivalence ratios. Comparisons of Fig. 2 and Fig. 3 suggest that using H2O2 with various concentrations as an oxidizer the role of fuel equivalence ratio on the laminar burning velocity and adiabatic flame temperature becomes less important.

In order to understand the effect of chemical reaction on the flame speed of CH4/air/H2O2 flames, the first-order sensitivity analysis of laminar burning velocity is shown in Fig. 4 for different reactant compositions at stoichiometric condition.

In the case of pure air, the dominant reactions for laminar burning velocity are,

OH O H

O

₂

  

(R38)

M CH M

CH

H 

₃

 

₄



(R52)

CO

2

H CO

OH   

(R99)

For the hydrogen peroxide replacement cases, the dominant reactions shift to the following chemical steps:

OH O H

O

₂

  

(R38)

M O H M

2OH  

₂ ₂



(R85)

O H HO O

H

OH 

₂ ₂



₂



₂

(R89)

O H CH CH

OH 

₄



₃



₂

(R98)

Among these reactions, (R85) and (R89) are the most important chemical reactions, so that hydrogen peroxide modifies the reaction pathway, and significantly enhances the reaction rate leading to flame speed enhancement.

6. Conclusions

In the present study, the effects of hydrogen peroxide on methane/air premixed flames are systemically and numerically investigated under the atmosphere condition with GRI-Mech 3.0 mechanism. Hydrogen peroxide is used as the oxidizer for two different conditions: (1) replacing partial air by H2O2 and (2) using H2O2 as an oxidizer but with different concentrations.

Especially, the characteristics of laminar burning velocity, adiabatic flame temperature and species concentration are studied. The following findings are obtained from this study.

1.

The laminar burning velocities and the adiabatic temperature are obviously increased with the addition of H2O2. When air is completely replaced by H2O2, the laminar burning velocity is almost not affected by the equivalence ratio. The decomposition of H2O2

dominates the net heat release rate and then affects the adiabatic flame temperature.

2.

When the concentration of H2O2 increases, the dominant reactions for laminar burning velocity are shifted. Hydrogen peroxide affects the reaction pathway, enhances the reaction rate, and then increases the flame speed.

3.

Hydrogen peroxide affects the species concentration and production/consumption rate. CH4 are completely consumed more upstream as H2O2 is added. H2O2 addition increases H2O concentration. However, CO emission is increased and CO2 concentration is decreased. Using H2O2 with a lower concentration will help to control CO emission.

4.

When air is partial replaced by H2O2, N2

reactant concentration is decreased. However, H2O2 enrichment enhances NO production reaction and NO emission concentration is increased due to the high flame temperature.

(7)

5 7. Self Evaluation

In this project, numerical investigations of the effects of hydrogen peroxide addition on the reaction path, laminar burning velocity, flame structure, and pollutant formation have been made as they were proposed in the proposal. Numerical simulations used the CHEMKIN Collection in conjunction with GRI-Mech 3.0 chemical kinetic mechanisms. Results of this year’s research have been published in International Journal of Hydrogen Energy (see Appendix). Experimental investigations will be performed in the second and third years.

8. References

[1] Mcmanus KR, Poinsot T, Candel SM. (1993) A review of active control of combustion instabilities. Prog Energy Combust Sci 2003;

19: 1-29.

[2] Proscia B., Peraccio Z. NOx As a Function of Fuel for C1-toC16 Hydrocarbons and Methanol Burned in a High Intensity, Lean-Premixed, Combustion Reactor. ASME 1998; 98-GT-269.

[3] Feikema D, Chen RH, Driscoll JF.

Enhancement of flame blowout limits by the use of swirl. AIAA 1989; 89-0158.

[4] Tangirala V, Chen RH, Driscoll JF. Effect of heat release and swirl on the recirculation with swirl-stabilized flame. Combust Sci Tech.

1987; 51:77-95.

[5] Schadow KC, Gutmark E, Wilson KJ. Active combustion control in a coaxial dump combustor. Combust Sci Tech 1992;

81:285-300.

[6] Jones AR, Lloyd SA, Weinberg FJ.

Combustion in heat exchangers. Proc R Soc Lonf A 1978; 360:97-115.

[7] Ahn J, Eastwood C, Sitzki L, Ronney PD.

Gas-phase and catalytic combustion in heat-recirculating burners. Proc Combust Inst 2005; 30:2463-2472.

[8] Van den Schoor F, Verplaetsen F. The upper flammability limit of methane/hydrogen/air mixture at elevated pressure and temperatures.

Int J Hydrogen Energy 2007; 32:2548-2552.

[9] Pfefferle WC, Pfefferle LD. Catalytically stabilized combustion. Prog. Energy Combust.

Sci.1986; 12: 25-41.

[10] Stockman ES, Zaidi SH, Miles RB, Carter CD, Ryan MD. Measurements of combustion properties in a microwave enhanced flame.

Combust Flame 2009; 156:1453-1461.

[11] Vincent-Randonnier A, Larigaldie S, Magre P,

Sabel’nikov V. Plasma assisted combustion:

effect of a coaxial DBD on a methane diffusion flame. Plasma Sources Sci Technol 2007; 16:149-160.

[12] Schefer RW. Hydrpgen enrichment for improved lean flame stability. Int J Hydrogen Energy 2003; 28:1131-1141.

[13] Juste GL. Hydrogen injection as additional fuel in gas turbine combustor: Evaluation of effects. Int J Hydrogen Energy 2006;

31:2112-2121.

[14] Lin HC, Chen BC, Ho CC, Chao YC. A comprehensive study of two interactive parallel premixed methane flames on lean combustion. Proc Combust Inst 2009;

32:995-1002.

[15] Zabetakis MG. Flammability characteristics of combustible gases and vapors. U.S.

Department of Mines Bulletin 627.

[16] Kuan CK, Chen GB, Chao YC. Development of a 100 mN HTP Monopropellant Microthruster. AIAA J. of Propulsion and Power 2007; 23(6):1313-1320.

[17] Humble RW. Bipropellant Engine Development Using Hydrogen Peroxide and a Hypergolic Fuel. AIAA-2000-3554, 36th AIAA/ ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, July 17-19, 2000, Huntsville, Alabama.

[18] Golovitchev VI, Pilia ML, Bruno C.

Autoignition of methane mixtures: the effect of hydrogen peroxide. Journal of Propulsion and Power 1996; 12(4):699-707.

[19] Ting DK, Reader GT, (2005) “Hydtogen peroxide for improving permixed methane-air combustion. Energy, 2005; 30:313-322.

[20] Kim I, Park J, Goto S. Conversion of nitric oxide to nitrogen dioxide using hydrogen peroxide. SAE Paper 2000-01-1931, 2000.

[21] Born C, Peters N. Reduction of soot emission at a DI diesel engine by additional injection of hydrogen peroxide during combustion. SAE Paper 982676, 1998.

[22] Martinez AI, Corredor LF, Tamara W.

Reduction of combustion emissions using hydrogen peroxide in a pilot scale combustion chamber. Proceedings of the 1997 Air and Waste Management Association’s 90th Annual Meeting and Exhibition, Paper 97-TP30B.01 1997.

[23] Smith G, Golden D, Frenklach M, Moriaty N, Eiteneer B, Goldenber M, Bowman C, Hanson R, Song S, Gardiner W, Lissianski V, Qin Z,

GRI-Mech 3.0, 1999, http://euler.me.berkeley.edu/gri_mech.

[24] Debasis Sengupta, Sandip Mazumder, J.

Vernon Cole, and Samuel Lowry , Controlling

(8)

Non-Catalytic Decomposition of High Concentration Hydrogen Peroxide, CFD Research Corporation, technical report, FEB 2004.

[25] R.J. Kee, F.M. Rupley, E. Meeks, J.A. Miller, Sandia National Laboratories Report SAND96-8216, 1996.

[26] Jinhua Wang, Zuohua Huang, Chenglong Tang, Haiyan Miao, and Xibin Wang, Numerical study of the effect of hydrogen addition on methane-air mixtures combustion, International Journal of Hydrogen Energy, 34 (2009) 1084-1096.

Table 1 Maximum Heat release rate for different reactant conditions

Reactant HRR(J/m³-s) (ER = 1)

HRR(J/m³-s) (ER = 0.6)

CH4/air 6.09×10⁹ 1.04×10⁹ CH4/50% H2O2 + 50%

air

5.42×10¹⁰ 2.77×10¹⁰

CH4/50% H2O2 + 50%

H2O

7.38×10¹⁰ 5.27×10¹⁰

CH4/H2O2 4.49×10¹¹ 4.37×10¹¹

Fig. 1. Profiles of temperature and species concentration of stoichiometric CH4/air/H2O2

flames. (a) CH4/100% air.

(b) CH4/80% air/20% H2O2.

Fig. 2. Computed laminar burning velocity and adiabatic flame temperature of methane/air flames with different percentages of air replaced by H2O2.

(9)

7

Fig. 3. Computed laminar burning velocity and adiabatic flame temperature of methane/H2O2

flames with various H2O2 concentrations.

Fig. 4. Sensitivity analysis of laminar burning velocity for CH4/air/H2O2 flames (ER = 1.0).

(10)

Appendix

(11)

Effects of hydrogen peroxide on combustion enhancement of premixed methane/air flames

Guan-Bang Chen

^a,

*, Yueh-Heng Li

^a,b

, Tsarng-Sheng Cheng

^c

, Hung-Wei Hsu

^d

, Yei-Chin Chao

^b,

**

aResearch Center for Energy Technology and Strategy, National Cheng Kung University, No.1, University Road, Tainan 701, Taiwan, ROC

bDepartment of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan, ROC

cDepartment of Mechanical Engineering, Chung Hua University, Hsinchu 300, Taiwan, ROC

dAerospace Science and Technology Research Center, National Cheng Kung University, Tainan 701, Taiwan, ROC

a r t i c l e i n f o Article history:

Received 21 May 2011 Received in revised form 7 July 2011

Accepted 16 July 2011

Available online 7 September 2011

Keywords:

Numerical simulation Hydrogen peroxide Premixed methane flame Laminar burning velocity

a b s t r a c t

Hydrogen peroxide is generally considered to be an effective combustion promoter for different fuels. The effects of hydrogen peroxide on the combustion enhancement of premixed methane/air flames are investigated numerically using the PREMIX code of Chemkin collection 3.5 with the GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. To study into the enhancement behavior, hydrogen peroxide is used for two different conditions: (1) as the oxidizer substituent by partial replacement of air and (2) as the oxidizer supplier by using different concentrations of H2O2. Results show that the laminar burning velocity and adiabatic flame temperature of methane flame are significantly enhanced with H2O2addition. Besides, the addition of H2O2increases the CH4

consumption rate and CO production rate, but reduces CO2productions. Nevertheless, using a lower volumetric concentration of H2O2as an oxidizer is prone to reduce CO formation. The OH concentration is increased with increasing H2O2 addition due to apparent shifting of major reaction pathways. The increase of OH concentration significantly enhances the reaction rate leading to enhanced laminar burning velocity and combustion. As to NO emission, using H2O2as an oxidizer will never produce NO, but NO emission will increase due to enhanced flame temperature when air is partially replaced by H2O2.

1. Introduction

Lean combustion is generally considered as one of the timely solutions for the more stringent environmental regulations and global warming concerns in the new century. However, lean combustion suffers from combustion instability, such as flame pulsation, flame flickering and blowout, due to low heat

release rate and high local extinction. In addition, it is generally accompanied with incomplete combustion with high CO and UHC (unburned hydrocarbon) emissions in most lean combustion applications. Therefore, the key problem for lean combustion applications is to enhance combustion for flame stabilization and for complete combustion to avoid high CO and UHC emissions[1,2]. Some strategies for stabilizing

* Corresponding author. Tel.:þ886 6 2757575x31450; fax: þ886 6 2095913.

** Corresponding author. Tel.:þ886 6 2757575x63690; fax: þ886 6 2389940.

E-mail addresses:gbchen26@gmail.com(G.-B. Chen),ycchao@mail.ncku.edu.tw(Y.-C. Chao).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h e

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 5 4 1 4e1 5 4 2 6

doi:10.1016/j.ijhydene.2011.07.074

(12)

lean premixed flames and extending lean flammability limit have been proposed and extensively studied for decades[1e3]. Particularly, feasible and pragmatic approaches to extend lean flammability limit can be roughly summarized into three categories: (i) flow structure adjustment[3e5], (ii) increase in temperature and pressure of flow fields[6e8], and (iii) alteration in flame chemical properties[9e14]. Utilizing swirler and bluff body to generate recirculation zone for enhancing flow residence time and fuel-air mixing is a promising manner by means of flow structures modification. Heat exchanger, flue gas recirculation mechanism as well as turbocharger are generally applied in engines and furnaces to enhance temperature and pressure in the flow field, and to extend flammability limits. As to chemical characteristics alteration in flames, employing catalyst to accelerate chemical reaction [7,9e11] and inducing active chemical radicals (H, OH) by imposing microwave[12] or plasma[13]in the flame have received increasing attention recently. Nevertheless, these approaches generally have to further assemble extra mechanical auxiliaries and modify combustor configuration, so that thorough mechanical retrofit design and accurate fabrication are necessitated.

Employing active fuels (such as hydrogen [14e16],) or adding strong oxidants[17]to fuels is an attractive alternative to enhance lean flame combustion without involving any moving part or configuration modification. Hydrogen peroxide (H2O2) is an environmentally friendly oxidant with strong oxidability. The oxidizing power of hydrogen peroxide is just inferior to that of fluorine[18], which has the strongest electronegativity in the periodic table. Hydrogen peroxide under

normal temperature is in liquid state, so that it is easy to store and handle. After chemical dissociation, hydrogen peroxide produces only oxygen and steam, and plus exothermicity (approximately 2884.6 kJ/kg) without toxic products. The decomposition reaction may be facilitated by heating or using catalyst and it is defined as:

H2O2/H2Oþ 1=2 O2þ DH (1)

When temperature achieves 450C, hydrogen peroxide will decompose with fierce exothermic heat release. Due to its inherently high decomposition temperature and high energy release, hydrogen peroxide with high concentration is often considered as a monopropellant[19]. It can also combine with hydrocarbon fuels to be bipropellants[20]. As to combustion applications, hydrogen peroxide can be used to replace air for reducing NOx emission and increasing combustion temperature.

A few recent studies have indicated potential promises in utilizing hydrogen peroxide for improving practical combustion process. Golovitchev et al. [21] examined the possibility of promoting methane auto-ignition in air using 5e10% hydrogen peroxide. Ting and Reader [22] used PREMIX code to investigate the effects of hydrogen peroxide on the premixed methane-air flame under atmospheric conditions. The maximum hydrogen peroxide concentration used to replace air was 6.25%. Hydrogen peroxide was found to be effective in enhancing the burning velocity, and this was particularly true for the richer mixtures considered.

Kim et al. [23] discovered that hydrogen peroxide assisted the conversion of harmful nitric oxide to nitrogen dioxide in

Fig. 1e Profiles of temperature and species concentration of stoichiometric CH/air/H O flames. (a) CH/100% air, (b) CH/ i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 5 4 1 4e1 5 4 2 6

15415

(13)

diesel exhaust gas. In addition, Born and Peters[24]found that proper injection of hydrogen peroxide into a diesel engine reduced soot and NOxdrastically. Martinez et al.[25]

reported that the concentrations of UHC, CO and NOxfrom their industrial pilot plant combustion chamber fueled with natural gas were lowered significantly by injection of a few hundred ppm of hydrogen peroxide. Based on the above studies, promotion to stable lean combustion seems effective with the addition of hydrogen peroxide than that with hydrogen. Hydrogen is usually considered as an easily ignitable fuel and helpful for enhancing combustion.

Nevertheless, hydrogen peroxide was also shown to act as a gas catalyst, to shift chemical pathways, and to further enhance chemical radicals[21].

Although the effects of hydrogen peroxide on combustion enhancement have been reported, systematic studies of the roles of hydrogen peroxide in combustion enhancement have not yet been conducted. Therefore, in the present study, the effects of hydrogen peroxide on premixed methane/air reaction pathway, laminar burning velocity, adiabatic flame temperature, and species formation are investigated

numerically using the PREMIX code of Chemkin collection 3.5 with the GRI-Mech 3.0 chemical kinetic mechanisms[26]and detailed transport properties.

2. Numerical model and chemical mechanism

In this work, the PREMIX code of CHEMKIN Collection is used to calculate the adiabatic, unstrained, free propagation velocities of the laminar premixed CH4/air/H2O2 flames. It solves the equations governing steady, isobaric, quasi-one- dimensional flame propagation. The equations are written as follows:

Continuity : _M¼ ruA (2)

Energy : _MdT dx1

Cp

d dx

lAdT

dx

þA

Cp

XK

k¼1_wkhkWk¼ 0 (3) Fig. 2e Computed laminar burning velocity and adiabatic flame temperature of methane/air flames with different

percentages of air replaced by H2O2.

15416

(14)

Species : _MdYk

dx þ d

dxðrAYkVkÞ þ A _wkWk¼ 0

k¼ 1; .; Kg (4)

Equation of state : r ¼PW

RT (5)

In these equations, x denotes the spatial coordinate, _M is the mass flow rate, r is the fluid density, u is the fluid velocity, A is the cross-section of the stream tube encom- passing the flame normalized by the burner area, T is the temperature,l is the thermal conductivity of the mixture, _wk

is the molar production rate of the kth species, hk is the specific enthalpy of the kth species, Wk is the molecular weight of the kth species, Ykis the mass fraction of the kth

species, Vkis the diffusion velocity of the kthspecies, and P is the pressure.

For a freely propagation flame, _M is an eigenvalue and must be determined as part of the solution. The initial guess is set to be 0.04 g/cm³/s. An additional constraint is required and a flame-fixed coordinate system is established by fixing the temperature at 500 K. To obtain the accurate flame speed, the boundaries should be sufficiently far from the flame to avoid temperature and species gradients at the boundaries. Firstly, an initial run is performed with a computational domain just wide enough to encompass the flame. Then, the domain is gradually expanded until the solution is domain-independent.

In addition, the adiabatic flame temperature is calculated by using the EQUIL code of the Chemkin Collection. An initial reactant mixture is specified and equilibrium of constant enthalpy and constant pressure is constrained. To obtain accurate adiabatic flame temperature, besides reactants and products, all radical species that might occur in the flame are also included.

The GRI-Mech 3.0 chemical kinetic mechanism composing of 53 chemical species and 325 reaction steps and detailed transport properties are used without any modifications. The reaction mechanism also includes the formation of NOx. This mechanism has been used satisfactorily to simulate non- catalytic H₂O₂decomposition[27]. The reaction rate constant is represented by the modified Arrhenius expression,

k¼ AT^bexp

Ea

RT

(6)

where A is the pre-exponential factor, b is the temperature exponent, and Ea is the activation energy. The chemical kinetics with CHEMKIN format is used in the code. Details of the chemical reaction rate formulation and CHEMKIN format can be found in the user’s manual[28].

At the cold boundary, the unburned reactants are supplied at 423 K and 1 atm. This temperature presets at the boiling temperature of H2O2and it is expected that all reactants are in gas phase. To further study into the role of hydrogen peroxide in the combustion enhancement of premixed methane flames, numerical simulations are performed for two different characteristic types of hydrogen peroxide addition: (1) as the oxidizer substituent by partial replacement of air (2) as the oxidizer supplier by using different concentrations of H₂O₂. Decomposition of 1 mol H2O2can produce half mole O2and 1 mol H2O. Therefore, the stoichiometric CH4/H2O2ratio is 0.25 and the global reaction is defined as:

CH4þ 4H2O2/CO2þ 6H2O (7)

In the case of partially replacing air by H2O2, the total O2

amount is maintained to keep the equivalence ratio constant.

Therefore, the reduced amount of O2from air is supplied from the decomposition of H2O2. For a stoichiometric condition, the reaction is defined as:

CH4þ 4aH2O2þ ð2 2aÞðO2þ 3:76N2Þ/CO2þ ð4a þ 2ÞH2O

þ3:76ð2 2aÞN2 (8)

wherea is the replacement percentage of air by H2O2. The reduction of air leads to reduction of N₂ in the oxidizer stream. For cases of using H2O2 as the oxidizer, the volumetric concentration of HO is considered ranging from 30%

Table 1e Maximum Heat release rate for different reactant conditions.

Reactant HRR (J/m³-s)

(ER¼ 1) HRR (J/m³-s) (ER¼ 0.6)

CH4/air 6.09 10⁹ 1.04 10⁹

CH4/50% H2O2þ 50% air 5.42 10¹⁰ 2.77 10¹⁰ CH4/50% H2O2þ 50% H2O 7.38 10¹⁰ 5.27 10¹⁰

CH4/H2O2 4.49 10¹¹ 4.37 10¹¹

30 40 50 60 70 80 90 100

Hydrogen peroxide concentration (%) 1600

1800 2000 2200 2400 2600 2800 3000

Adiabatic flame temperature (K)

0 1 2 3 4 5

Laminar burning velocity (m/s)

Equivalence ratio=1.0 Equivalence ratio=0.8 Equivalence ratio=0.6 Equivalence ratio=1.0 Equivalence ratio=0.8 Equivalence ratio=0.6

a

b

Fig. 3e Computed laminar burning velocity and adiabatic flame temperature of methane/H O flames with various

15417

(15)

3. Results and discussion

3.1. Effects of H2O2on combustion characteristics

In order to examine the effects of H2O2on enhancement of premixed methane flames and to study further into the modification of combustion characteristics, the results of premixed stoichiometric CH₄/air and CH₄/80% air þ 20%

H2O2are compared for illustration. The spatial coordinate ranges from cold boundary to 0.4 cm within the flame. The resultant temperature and species concentration profiles

are shown inFig. 1. Results indicate that the premixed flame with 20% air replaced by hydrogen peroxide has a higher adiabatic flame temperature due to the reduction of nitrogen dilution and heat release from thermal decomposition of hydrogen peroxide. The temperature increases approximately by 140 K as compared to the pure air case.

For pure air case, the reactant CH4is completely consumed within 1.0 mm of the spatial coordinate but 0.85 mm for the 20% H2O2replacement case. Hydrogen peroxide dedicates to enhance the methane consumption. In addition, the increase of H2O2 content in air obviously results in an increase of H2O production due to the product of H2O2

Fig. 4e Sensitivity analysis of laminar burning velocity for CH4/air/H2O2flames (ER[ 1.0).

Fig. 5e Effect of H2O2on normalized CH4mass fraction for two cases. (a) replacing partial air by H2O2and (b) H2O2as an oxidizer with different concentrations.

15418

(16)

decomposition. It appears to slightly decrease CO₂ formation and increase CO formation. Besides, some intermediate radicals, such as OH, H and O, show increasing trends with hydrogen peroxide addition. Especially, the increasing trends of HO2, HCO, CH2O and CH3O are more significant.

These facts suggest that the dominant reactions of methane combustion are altered by H2O2addition. Hydrogen peroxide decomposition increases the active radicals, enhances the reaction rate, and then accelerates the laminar burning velocity. The computed laminar burning velocity of stoichiometric CH4/air with the inlet temperature 423 K is about 0.71 m/s, while the flame speed is increased to about 1.25 m/

s when 20% air is replaced by H2O2.

3.2. Laminar burning velocity

Fig. 2shows the effect of partial replacement of air by H2O2

on the laminar burning velocity and adiabatic flame temperature for three different equivalence ratios (ER). The maximum percentage of air replaced by H2O2is 100%. It can be seen that the laminar burning velocity is increased with increasing the equivalence ratio and the percentage of hydrogen peroxide replacement. However, the effect of equivalence ratio on flame speed becomes mild for high hydrogen peroxide replacement percentage cases. When air is completely replaced by H2O2, the laminar burning velocity approaches 4.7 m/s for all equivalence ratio conditions. This is because that the oxidizer is completely provided from decomposition of hydrogen peroxide and hydrogen peroxide dominates the reaction rate of methane oxidation.Fig. 2also shows that the adiabatic flame temperature increases with increasing the percentage of H2O2 replacement. The

temperature increase for fuel lean conditions is larger than that for stoichiometric condition. The maximum temperature increase is up to 900 K for ER¼ 0.6, but 680 K and 520 K for ER¼ 0.8 and 1.0, respectively. The effect of temperature increase is primarily induced from the heat release of hydrogen peroxide decomposition since it is much higher than that released from methane reactions.Table 1shows the maximum heat release rates (HRR) for different reactant conditions. The maximum heat release rates are about 6.09 10⁹J/m³-s and 1.04 10⁹J/m³-s for ER¼ 1.0 and 0.6 CH4/ air flames, respectively. For the case of CH4/50% H2O2þ 50%

air the maximum heat release rates are about 5.42 10¹⁰J/

m³-s and 2.77 10¹⁰J/m³-s for ER¼ 1.0 and 0.6, respectively.

Similarly, when air is completely replaced by H2O2, the maximum heat release rate approaches 4.49 10¹¹ J/m³-s and 4.37 10¹¹J/m³-s, respectively. FromTable 1it is also noted that equivalence ratio contributes less significantly to the maximum heat release rate when air is replaced or partially replaced by hydrogen peroxide, but their HRR values are two orders of magnitude higher than that for CH4/air case. This “orders of magnitude” difference comes from hydrogen peroxide decomposition. It proves that the decomposition of H₂O₂dominates the heat release and then determines the adiabatic flame temperature.

To investigate the characteristics of CH4/H2O2flames, the volumetric concentration of H2O2is varied from 30 to 100%

while ER is kept at 0.6, 0.8, and 1.0.Fig. 3shows the laminar burning velocity and adiabatic flame temperature of the CH4/H2O2 flames with ER¼ 0.6, 0.8, and 1.0. Results show that both laminar burning velocity and adiabatic flame temperature increase with increasing H2O2 concentration.

With 30 vol.% of H2O2the laminar burning velocity is higher

Fig. 6e Effect of HO on normalized CH consumption rate for different reactant conditions. (a) pure air, (b) 100% HO , (c) i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 5 4 1 4e1 5 4 2 6

15419

(17)

than that of CH₄/air flame for three different equivalence ratios, but the adiabatic flame temperature is lower than that of CH4/air flame. For the case of ER¼ 1.0 the temperature difference between two cases is about 250 K. However, for the case of ER¼ 0.6, the adiabatic flame temperature is almost the same as that of CH4/air flame and the temperature difference is only 10 K. In addition, the laminar burning velocity is increased to 0.63 m/s, which is much higher than that of CH4/air flame (0.27 m/s). When the H2O2concentra- tion is increased to 40 vol.%, the adiabatic flame temperature is higher than that of pure CH4/air flame for three different equivalence ratios. Comparisons ofFigs. 2 and 3suggest that using H2O2with various concentrations as an oxidizer the role of fuel equivalence ratio on the laminar burning velocity and adiabatic flame temperature becomes less important.

In order to understand the effect of chemical reaction on the flame speed of CH4/air/H2O2flames, the first-order sensitivity analysis of laminar burning velocity is shown inFig. 4for different reactant compositions at stoichiometric condition.

In the case of pure air, the dominant reactions for laminar burning velocity are,

O2þ H4O þ OH (R38)

Hþ CH3þ M4CH4þ M (R52)

OHþ CO4H þ CO2 (R99)

For the hydrogen peroxide replacement cases, the dominant reactions shift to the following chemical steps:

2OHþ M4H2O2þ M (R85)

OHþ H2O24HO2þ H2O (R89)

OHþ CH44CH3þ H2O (R98)

Among these reactions, (R85) and (R89) are the most important chemical reactions. Hydrogen peroxide promotes the product of OH radical, so that hydrogen peroxide modifies the reaction pathway, and significantly enhances the reaction rate leading to flame speed enhancement. The effect of hydrogen peroxide on OH radical is further discussed in the next section.

3.3. Major species and OH radical

The effects of H2O2on the major species and OH radical of the CH4/air/H2O2premixed flames are investigated. In the study, the CH4 reactant concentration is changed with different percentages of hydrogen peroxide additions, and it affects the concentration of carbon-related species in the products[29].

Normalization of concentration of the carbon-related species is performed to eliminate this problem by the following formula,

Fig. 7e Effect of H2O2on H2O mass fraction for two cases. (a) replacing partial air by H2O2and (b) H2O2as oxidizer with different concentrations.

15420

(18)

y_k;n¼ yk;nyCH4ðairÞ

yCH4ðnÞ (9)

where yk,nis the mass fraction or the rate of production of the carbon-related species k in flame n; yCH4ðairÞ is the mass

fraction or the rate of methane in pure air case and yCH4ðairÞ is in flame n.

Fig. 5shows the normalized CH4mass fraction for ER¼ 1.0 and 0.6. The normalized CH4mass fraction at inlet is the same for different H2O2concentrations and the CH4consumption Fig. 8e Effect of H2O2on normalized CO2mass fraction for two cases. (a) replacing partial air by H2O2and (b) H2O2as oxidizer with different concentrations.

Fig. 9e Effect of H O on normalized CO mass fraction for two cases. (a) replacing partial air by HO and (b) HO as oxidizer i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 5 4 1 4e1 5 4 2 6

15421

(19)

trend can be clearly revealed. In stoichiometric (ER¼ 1.0) and fuel lean (ER ¼ 0.6) conditions earlier accomplishment of complete methane consumptions is noted with increasing hydrogen peroxide replacement percentage. Similarly, increasing the hydrogen peroxide concentration also enhances rapid methane consumption in both stoichiometric and fuel lean conditions, except for the case of 30 vol.% H2O2

in stoichiometric condition.Fig. 6shows the normalized CH4

consumption rate for different reactant conditions at stoichiometric. With the increase of hydrogen peroxide addition, the total consumption rate of CH4is obviously increased. The primary CH₄consumption reactions are,

Oþ CH44OH þ CH3 (R11)

Hþ CH44H2þ CH3 (R53)

OHþ CH44CH3þ H2O (R98)

The main reactions for CH4consumption are the abstrac- tion reactions initiated by radicals, such as H, O, and OH and yield CH3. Reaction (R98) is the principal reaction for CH4

consumption rate. Since hydrogen peroxide increases the production of O, H, and OH radicals, the reactions(R11), (R53), and (R99)are then enhanced to promote CH₄consumption.

Fig. 7shows the effect of H2O2on H2O mass fraction for ER¼ 1.0 ad 0.6. Since H2O2decomposes to O2and H2O, the H2O in product gas is primarily from H2O2 decomposition and secondarily from the CH4 combustion. It appears that the mass fraction of H2O increases with increasing H2O2in the reactant. However, in the cases of using H2O2as an oxidizer with various concentrations, the lower H2O2 concentration means more water vapor will be produced in the product

stream. It turns out that H₂O mass fraction increases with decreasing H2O2volumetric concentration.

The normalized CO2mass fraction profiles with different H2O2concentrations for ER¼ 1.0 and 0.6 are shown inFig. 8.

The rise of CO2production curve shifts upstream, especially for ER¼ 0.6. In the case of hydrogen peroxide replacement, the normalized mass fraction of CO2is obviously decreased with increasing H2O2for ER¼ 1.0. Nonetheless, for ER ¼ 0.6, the decrease of CO2 mass fraction is not obvious when the hydrogen peroxide replacement is below 30% in total air. In the case of using different concentrations of H2O2 as the oxidizer, CO₂mass fraction is also decreased with increasing H2O2concentration. However, CO2mass fraction with 30%vol.

concentration of H2O2is less than that of pure air case for ER¼ 1.0. For ER ¼ 0.6, the decrease of CO2mass fraction is also not obvious when the hydrogen peroxide concentration is below 30% in volume.

CO and CO2are the main carbon-related products in CH4

flames. The decrease of CO2production is often accompanied with the increase of CO production. Both of them have a trade off tendency. Therefore, the normalized CO mass fraction obviously increases with H2O2addition.Fig. 9shows the effect of H₂O₂on normalized CO mass fraction or ER¼ 1.0 and 0.6.

Similar to CO2effect, with H2O2addition, CO production shifts upstream, especially for ER ¼ 0.6. The CO concentration increases drastically in the preheat zone and then decreases in the reaction zone. The replacement of partial air with H2O2

increases the peak of CO concentration. It also increases the CO formation in the post reaction zone except in the lower H2O2concentration (30%).Fig. 10shows the normalized CO production rate for different reactant conditions. The dominant reactions for CO production are,

Fig. 10e Effect of H2O2on normalized CO production rate for different reactant conditions. (a) pure air, (b) 100% H2O2, (c) 50%

airD 50% H2O2, and (d) 50% H2O2D 50% H2O.

15422

(20)

HCOþ H2O4H þ CO þ H2O (R166)

O2þ HCO4HO2þ CO (R168)

Oþ CH34H þ H2þ CO (R284)

And the main CO consumption reaction is

OHþ CO4H þ CO2 (R99)

Reaction (R99) is the most important reaction in CH4/air combustion and most of the heat release is from this exothermic reaction. According toFig. 10, these reactions are enhanced with H2O2addition and reaction(R166)is promoted to become the most important reaction in CO formation. This is due to HCO decomposition ameliorated by massive H2O.

With the increase of hydrogen peroxide, the total production rate of CO is obviously increased.

The effects of H2O2on OH mass fraction for ER¼ 1.0 and 0.6 are shown in Fig. 11. The increase of OH concentration in flames manifests the severe chemical reactions, especially for the case of using H2O2as the oxidizer. OH radicals are significantly yielded upstream and its corresponding concentration obviously increases when H2O2replacement or vol. concentration is increased to 50%. It elucidates that the effect of hydrogen peroxide consists of thermal and chemical effects.

When H2O2replacement or vol. concentration is increased to above 80%, the OH concentration is almost unaffected by the equivalence ratio. This is because that under these conditions, OH radicals are primarily yielded from H2O2reaction pathway, instead of from the original CH4combustion reactions.Fig. 12

shows the OH production rate for various reactant conditions.

In order to clearly display the main reactions, the values of some reactions are reduced several folds. In the case of pure air, the dominant reactions for OH production and consumption are,

OHþ H24H þ H2O (R84)

The chain branching reaction(R38)is the main reaction for forming OH and(R84)is the main reaction for consuming OH.

When hydrogen peroxide enrichment gradually increases, the effect of these above reactions is weakened and the dominant reactions shift to the following mechanisms:

2OHþ M4H2O2þ M (R85)

OHþ H2O24HO2þ H2O (R89)

Hydrogen peroxide modifies the reaction pathway. With the increase of hydrogen peroxide, the total production rate of OH is increased.

3.4. NO formation

The effect of H2O2on NO mass fraction for ER¼ 1.0 and 0.6 are sown inFig. 13. Since the concentration of N2is changed with partial replacement of air by hydrogen peroxide, it affects the concentration of NO in the products. Similar to the carbon- related species, the normalized concentration of NO is used to eliminate this effect by the following formula,

Fig. 11e Effect of HO on OH mass fraction for two cases. (a) replacing partial air by H O and (b) HO as oxidizer with i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 1 5 4 1 4e1 5 4 2 6

15423

(21)

y_NO;n¼ yNO;nyNOðairÞ

y_NOðnÞ (10)

where yNO,nis the mass fraction or the rate of production of NO in flame n, yNO (air) is the mass fraction or the rate of production of NO in pure air case, and yCH4ðnÞ is in flame n.

Since there is no nitrogen in the oxidizer, using different volumetric concentrations of H2O2as the oxidizer is unable to produce NO emission. As to the cases of hydrogen peroxide replacement, NO production increases with H2O2enrichment.

The higher adiabatic flame temperature leads to higher yield of thermal NOx, so that NO emission formation overwhelms the benefit on reduction of N2 reactant reduction. The normalized NO mass fraction always gradually increases with increasing H2O2addition and it represents the existence of H2O2 in hydrocarbon flame may accelerate thermal NO formation due to inherently high flame temperature.

Fig. 14 shows the normalized NO production rate for various reactant conditions. The total production rate of NO is increased with the increase of H2O2addition. The dominant reactions for NO production are,

Nþ O24NO þ H (R179)

Nþ OH4NO þ H (R180)

HO2þ NO4OH þ NO2 (R186)

NO2þ H4NO þ OH (R189)

Oþ NH4H þ NO (R190)

Hþ HNO4H2þ NO (R214)

CH2þ NO4H þ HNCO (R249)

It can be seen that reactions(R179), (R180), (R190), (R214), and (R249)dominate at high temperature zone and reactions (R186) and (R189)dominate at lower temperature zone. With the addition of H2O2, the flame temperature increases and Fig. 12e Effect of H2O2on OH production rate for different reactant conditions.

Fig. 13e Effect of H2O2on NO mass fraction with replacing partial air by H2O2.

15424

(22)

these reactions are enhanced and(R186)becomes the dominant reaction for NO consumption.

4. Conclusions

In the present study, the effects of hydrogen peroxide on methane/air premixed flames are systemically and numerically investigated under the atmosphere condition with GRI-Mech 3.0 mechanism. Hydrogen peroxide is used as the oxidizer for two different conditions: (1) replacing partial air by H2O2and (2) using H2O2as an oxidizer but with different concentrations.

Especially, the characteristics of laminar burning velocity, adiabatic flame temperature and species concentration are studied. The following findings are obtained from this study.

1. The laminar burning velocities and the adiabatic temperature are obviously increased with the addition of H₂O₂. When air is completely replaced by H2O2, the laminar burning velocity is almost not affected by the equivalence ratio. The decomposition of H2O2dominates the net heat release rate and then affects the adiabatic flame temperature.

2. When the concentration of H2O2increases, the dominant reactions for laminar burning velocity are shifted.

Hydrogen peroxide affects the reaction pathway, enhances the reaction rate, and then increases the flame speed.

3. Hydrogen peroxide affects the species concentration and production/consumption rate. CH4 are completely consumed more upstream as H2O2is added. H2O2addition increases H2O concentration. However, CO emission is increased and CO2concentration is decreased. Using H2O2

with a lower concentration will help to control CO emission.

4. When air is partial replaced by HO, N reactant concen-

NO production reaction and NO emission concentration is increased due to the high flame temperature.

Acknowledgment

This research was supported by the National Science Council of the Republic of China under the grant number NSC99- 2221- E-216-007.

r e f e r e n c e s

[1] Mcmanus KR, Poinsot T, Candel SM. A review of active control of combustion instabilities. Prog Energy Combust Sci 1993;2003(19):1e29.

[2] Proscia B, Peraccio Z. NOxAs a Function of Fuel for C1-toC16 Hydrocarbons and Methanol Burned in a High Intensity, Lean-Premixed, Combustion Reactor. ASME; 1998. 98-GT- 269.

[3] Feikema D, Chen RH, Driscoll JF. Enhancement of flame blowout limits by the use of swirl. AIAA; 1989. 89-0158.

[4] Tangirala V, Chen RH, Driscoll JF. Effect of heat release and swirl on the recirculation with swirl-stabilized flame.

Combust Sci Tech 1987;51:77e95.

[5] Schadow KC, Gutmark E, Wilson KJ. Active combustion control in a coaxial dump combustor. Combust Sci Tech 1992;81:285e300.

[6] Jones AR, Lloyd SA, Weinberg FJ. Combustion in heat exchangers. Proc R Soc Lond A 1978;360:97e115.

[7] Ahn J, Eastwood C, Sitzki L, Ronney PD. Gas-phase and catalytic combustion in heat-recirculating burners. Proc Combust Inst 2005;30:2463e72.

[8] Van den Schoor F, Verplaetsen F. The upper flammability Fig. 14e Effect of H2O2on NO production rate for different reactant conditions. (a) pure air, (b) 70% airD 30% H2O2, (c) 50%

airD 50% H2O2, and (d) 20% airD 80% H2O2.

行政院國家科學委員會專題研究計畫 成果報告