- Email: [email protected]
Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures
international journal of hydrogen energy 35 (2010) 7246–7252
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures Jinhua Wang, Zuohua Huang*, Chenglong Tang, Jianjun Zheng State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China
article info
abstract
Article history:
Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures
Received 16 November 2009
was investigated experimentally and numerically. The flame propagating photos of pre-
Received in revised form
mixed combustion and direct-injection combustion was obtained by using a constant
28 December 2009
volume vessel and schlieren photographic technique. The pressure derived initial
Accepted 3 January 2010
combustion durations were also obtained at different hydrogen fractions (from 0% to 40%
Available online 20 January 2010
in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II
Keywords:
program with GRI 3.0 mechanism. The results showed that the initial combustion process
Hydrogen
of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in
Natural gas
the case of both premixed combustion and direct-injection combustion. This phenomenon
Lean combustion
is more obvious at leaner mixture condition near the lean limit of natural gas. The mole
Spark ignition
fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the flames. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
With ever increasing of energy demand and concern of environmental protection, the research on high efficiency clean combustion has attracted increased attention. Lean mixture combustion has a great potential to achieve higher thermal efficiency and lower emissions [1]. While the relatively high lean flammability limit of hydrocarbon fuel makes it difficult to achieve stable combustion near the lean burning regime [2], and this is even more severe for natural gas. As the main composition of natural gas, methane has unique tetrahedral molecular structure with large C–H bond energies, thus demonstrates some unique combustion
characteristics such as high ignition temperature and low flame propagation speed [3]. Hydrogen is an excellent additive to improve the combustion of hydrocarbon fuel due to its low ignition energy, high reactivity, high diffusivity and fast burning velocity [4,5]. Many researches have conducted on engine fueled with hydrogenenriched natural gas. Ma et al. [6–9] conducted engine study fueled with hydrogen enriched natural gas and found that lean limit of natural gas engine was extended with hydrogen enrichment. The thermal efficiency of natural gas engine can be improved with hydrogen addition combined with optimum retarded ignition timing. The authors conducted the engine study fueled with hydrogen-enriched natural gas in a direct-
* Corresponding author at: School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China. E-mail address: [email protected] (Z. Huang). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.01.004
international journal of hydrogen energy 35 (2010) 7246–7252
injection spark ignition engine [10–12] and port mixing spark ignition engine [13]. It showed that the combustion of natural gas engine was enhanced with hydrogen addition. The combustion duration was decreased and the optimum ignition timing can be retarded as hydrogen is added to natural gas. The HC and CO emissions decreased while the NOx emissions increased significantly with hydrogen addition. This indicated that hydrogen addition should be used combined with lean combustion and/or large EGR ratio to obtain lower NOx emissions. The stable lean limit of natural gas engine can be extended with hydrogen addition. Researches have also conducted on the auto-ignition characteristics of methane– hydrogen blends based on the background of homogeneous charge compression ignition (HCCI) engine [14–16]. The experimental studies were conducted on shock tube test facility and/or rapid compression machine and results showed that hydrogen addition has a modest effect on the measured ignition delay time at low mole fractions of hydrogen (less than 20%). At 50% hydrogen fraction in the fuel blends, substantial reduction of measured ignition delay time was observed. However, fewer studies reported on the effect of hydrogen addition to natural gas and/or methane on spark ignition and early flame propagating process. Positive ignition of the fuel– air mixtures in cylinder of spark ignition engine is essential for good performance and efficiency [17]. Short initial combustion duration corresponding to fast early flame growth will be beneficial to high thermal efficiency and low cyclic variations in engines [18]. This is even more important for the lean combustion and/or using large EGR ratios which have the lower flame speed. Traditionally, high ignition energy system was used to ensure stable spark ignition with short initial combustion duration [9,10]. Hydrogen addition presents a potential to enhance the combustion characteristics and extend the stable lean limit of hydrocarbon fuels [19,20]. It is very difficult to understand the ignition and flame propagating process in a real engine because it is very complicated. The objective of the present study is to clarify the effect of hydrogen addition on ignition and early flame growth of lean burn natural gas–air mixtures in a constant volume vessel. The flame propagating photos and pressure derived initial combustion duration was obtained by using a constant volume vessel with schlieren photographic technique. The laminar premixed flames at the same condition as that of the experimental flames were calculated with PREMIX code of CHEMKIN II program. The monotonic correlation between initial combustion duration and the reciprocal maximum OH mole fraction in the flames was observed.
2. Experimental procedures and computational method The experiments of lean burn natural gas–hydrogen–air mixtures are conducted in a constant volume vessel. The experimental methods are similar to that of the earlier work [21] and will be described briefly here. The constant volume vessel used in this experiment is a cylindrical type vessel with inside diameter of 130 mm, inside length of 130 mm, and the volume of 1.725 L. Two sides of this vessel are transparent to make it optically accessible. The gas in the vessel is drawn out
7247
by a vacuum pump, and the fresh air is introduced into the vessel via the inlet valve at the initial pressure of 0.1 MPa and initial temperature of 300 K. Five minutes is awaited to ensure the air in the vessel motionless. Then the fuel blends with a specific hydrogen fraction and the overall equivalence ratio was injected into the vessel. The injection pressure is maintained constantly at 8.0 MPa in this experiment. The injected fuel jet generates the turbulence in the vessel and forms a turbulent inhomogeneous fuel–air mixture in the vessel, similar to that in a gas direct-injection engine. The fuel–air mixture is ignited by the centrally located electrodes at a given timing after the ending timing of injection (Tig). The change of Tig will be corresponding to different turbulence and stratification intensity condition in the vessel at the timing of spark ignition. The stoichiometric fuel–air ratio in volumetric of hydrogen is about one-fourth to that of natural gas, thus the fuel injection duration is increased in the case of hydrogen addition for the same equivalence ratio as illustrated in ref. [21]. The total fuel jet momentum at different hydrogen fractions gives the closely comparable value regardless of hydrogen fraction. This indicated that the turbulence intensity generated by the fuel jet is closely comparable with hydrogen addition. A homogeneous premixed mixture is also prepared in this vessel and provides the information on laminar homogeneous mixture combustion. A standard capacitive ignition system with ignition energy of 45 mJ is used to produce the spark. The pressure of the vessel during the combustion process is recorded by a piezoelectric Kistler absolute pressure transducer. The natural gas and hydrogen used in this experiment is also identical to that of ref. [21]. The methane–hydrogen–air freely propagated laminar premixed flames at normal temperature and atmospheric pressure at equivalence ratio of 0.6 and 0.8 were calculated by using PREMIX code of CHEMKIN II program with GRI-Mech 3.0 mechanism [22]. The mole fraction profiles of OH and O radicals in the flames are derived from the calculation results.
3.
Results and discussions
Flame propagating photos are an effective tool on combustion diagnoses. The effect of hydrogen addition on the flame propagating process of laminar premixed mixture combustion is illustrated in Fig. 1. It showed that the flame kernel occupied large volume at the same time after ignition with hydrogen addition. This indicated that the flame propagating speed was increased as hydrogen is added to natural gas. The flame is elliptical for pure natural gas–air mixture combustion due to the buoyancy effect near the lean burn limit with slow flame propagating speed. This effect is eliminated with a small amount of hydrogen addition. It can be seen that cracks can be observed in the flame surface when the hydrogen fraction is reached 32%. The cracks in the flame surface will increase the flame area thus accelerate the flame propagating process. The Markstein number which is an indicator of flame instability decreased with the increase of hydrogen fraction in natural gas–hydrogen fuel blends [23]. This indicated a self acceleration tendency with the increase of hydrogen fraction due to
7248
international journal of hydrogen energy 35 (2010) 7246–7252
Fig. 1 – Photos show the effect of hydrogen addition on flame propagating process of natural gas–air mixtures laminar premixed combustion (4 [ 0.6).
the increase of flame instability which will lead to the enhancement of combustion. The effect of hydrogen addition on early flame growth of natural gas–air mixtures combustion with Tig ¼ 45 ms at equivalence ratio of 0.6 is given in Fig. 2. It can be seen that early flame growth of natural gas–air turbulent combustion was enhanced as hydrogen is added. The flame kernel size at initial combustion process increases with the increase of hydrogen fraction. It is very obvious even at 4.0 ms after spark ignition timing. The flame kernel is very small and the shape is much irregular for natural gas which has the tendency to extinguish under the influence of turbulent flow. The flame kernel is more concentrated to the spark position and the flame shape is much more regular as hydrogen is added to
natural gas. This indicates that the spark ignition of lean burn natural gas–air mixtures is more stable with hydrogen addition. The effect of turbulent flow on spark ignition and flame propagation process can be weakened as hydrogen is added to natural gas. This would be due to the low ignition energy and high reactivity of hydrogen which accelerate the combustion rate of natural gas combustion. The high diffusivity of hydrogen weakened the effect of turbulent flow on flame propagating process. Fig. 3 shows the pressure history during the combustion process for lean premixed combustion and direct-injection combustion of natural gas–hydrogen blends. It shows that the initial pressure rise is increased remarkably as hydrogen is added to natural gas. This phenomenon becomes more
international journal of hydrogen energy 35 (2010) 7246–7252
7249
Fig. 2 – Photos show the effect of hydrogen fraction on early flame growth of natural gas–air mixtures turbulent combustion with Tig [ 45 ms (4 [ 0.6).
obvious for the leaner mixture combustion (4 ¼ 0.6) in the case of both premixed combustion and direct-injection combustion. This study reveals the enhancement of the initial combustion process by hydrogen addition in both laminar combustion and turbulent combustion. For turbulent combustion in this experimental, the ignition timing was set to be constant at 45 ms after the end of fuel injection, thus, the turbulent condition in the vessel is comparable for all the fuels at ignition timing. Hydrogen addition is the only one reason to contribute to the enhancement of initial combustion process. The low required ignition energy and high reactivity of hydrogen are responsible for the improvement of ignition and
early flame propagation. However, it should be noted that the effect of hydrogen addition on laminar premixed combustion and direct-injection turbulent combustion of natural gas is much different. In the case of laminar premixed combustion, the hydrogen addition promotes the fuel–air diffusion and combustion reaction. The promotion of combustion is almost linear with the increase of hydrogen fraction at equivalence ratio of both 0.6 and 0.8. However, in the case of direct-injection combustion, hydrogen addition will promote the spark ignition and initial flame development for the stratified mixture combustion and maintains stable flame propagation from the spark position to the surrounding relatively leaner
7250
international journal of hydrogen energy 35 (2010) 7246–7252
a
a
b
b
Fig. 3 – Pressure history during combustion process in the constant volume vessel (a: 4 [ 0.6, b: 4 [ 0.8).
mixture region. This is more obvious for condition at equivalence ratio of 0.6 which is near the lean limit of natural gas. The hydrogen addition will suppress the flame local
Fig. 4 – OH and O mole fraction profiles of methane– hydrogen–air flames (a: 4 [ 0.6, b: 4 [ 0.8).
distinguish at propagating process from the spark position to the surrounding relatively leaner mixture region. The effect of hydrogen addition on natural gas direct-injection combustion
international journal of hydrogen energy 35 (2010) 7246–7252
a
7251
The correlation between the initial combustion duration of the fuel blends for premixed combustion and direct-injection combustion and the corresponding maximum OH mole fraction in the laminar flames are given in Fig. 5. The initial combustion duration used in this paper is defined as the interval from the ignition timing to the timing of pressure rise up to 0.15 MPa [18]. The initial combustion duration decreases as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This indicates the enhancement of initial combustion process of lean burn natural gas combustion with hydrogen addition as described in previous. Meanwhile, a monotonic correlation between initial combustion duration and the reciprocal maximum OH mole fraction in the laminar premixed flame is observed, and this indicates that the decrease of initial combustion duration with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the fuel blends.
b 4.
Conclusions
Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The main results are summarized as follows:
Fig. 5 – Correlation between initial combustion duration and the reciprocal of maximum OH mole fraction (a: 4 [ 0.6, b: 4 [ 0.8).
is not linear. It is interesting to note that the difference between 32% hydrogen fraction and 40% hydrogen fraction is quite small, especially at initial combustion process. The effect of hydrogen addition is much weaker at equivalence ratio of 0.8 for direct-injection turbulent combustion. OH and O radicals play an important role in combustion chemical reaction of hydrocarbon fuels [24]. Yamamoto et al. found that the maximum OH concentration had a linear correlation to the laminar burning velocity for both 1-D and 2D flames [25]. The OH and O mole fraction profiles in the laminar premixed methane–hydrogen–air flames are calculated and illustrated in Fig. 4. Both the mole fractions of OH and O are increased with the increase of hydrogen fraction and the maximum OH and O mole fraction will move closing to the unburned mixture side. The main reaction pathways attributed to OH and O formation identified from the rate of production analysis are OH þ H2<¼>H þ H2O (R84) and H þ O2<¼>O þ OH (R38). The increase of H2 mole fraction will increase the reaction rate of R84 and thus forms more H radical and promotes the reaction rate of R38.
(1) The initial combustion process is significantly enhanced with hydrogen addition for both laminar premixed and direct-injection turbulent combustion at lean mixture condition. (2) The OH and O mole fraction increase with hydrogen addition and the position of maximum OH and O mole fraction moves closing to the unburned mixture side. (3) The enhancement of spark ignition of lean natural gas–air mixtures with hydrogen addition can be ascribed to the increase of OH and O mole fraction in the flames.
Acknowledgements This study was supported by National Basic Research Project (2007CB210006), National Natural Science Foundation of China (50821604) and State Key Laboratory of Engines (SKLE 200907).
references
[1] Bade Shrestha SO, Karim GA. Hydrogen as an additive to methane for spark ignition engine applications. International Journal of Hydrogen Energy 1999;24(6):577–86. [2] Naha S, Briones AM, Aggarwal SK. Effect of fuel blends on pollutant emissions in flames. Combustion Science and Technology 2005;177(1):183–220. [3] Turns Stephen R. An introduction to combustion: concepts and applications. 2nd ed., Boston: McGraw-Hill; 2000. pp. 158. [4] Bell SR, Gupta M. Extension of the lean operating limit for natural gas fueling of a spark ignited engine using hydrogen blending. Combustion Science and Technology 1997;123(1–6): 23–48.
7252
international journal of hydrogen energy 35 (2010) 7246–7252
[5] Karim Ghazi A. Hydrogen as a spark ignition engine fuel. International Journal of Hydrogen Energy 2003;28(5):569–77. [6] Ma Fanhua, Wang Yu, Liu Haiquan, Li Yong, Wang Junjun, Zhao Shuli. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. International Journal of Hydrogen Energy 2007;32(18):5067–75. [7] Ma Fanhua, Liu Haiquan, Wang Yu, Li Yong, Wang Junjun, Zhao Shuli. Combustion and emission characteristics of a port-injection HCNG engine under various ignition timings. International Journal of Hydrogen Energy 2008;33(2):816–22. [8] Ma Fanhua, Wang Yu, Liu Haiquan, Li Yong, Wang Junjun, Ding Shangfen. Effects of hydrogen addition on cycle-by-cycle variations in a lean burn natural gas spark-ignition engine. International Journal of Hydrogen Energy 2008;33(2):823–31. [9] Ma Fanhua, Wang Yefu, Ding Shangfen, Jiang Long. Twenty percent hydrogen-enriched natural gas transient performance research. International Journal of Hydrogen Energy 2009;34(15):6523–31. [10] Huang CQ, Wei LX, Yang B, Wang J, Li YY, Sheng LS, et al. Lean premixed gasoline/oxygen flame studied with tunable synchrotron vacuum UV photoionization. Energy and Fuels 2006;20(4):1505–13. [11] Huang Zuohua, Wang Jinhua, Liu Bing, Zeng Ke, Yu Jinrong, Jiang Deming. Combustion characteristics of a direct-injection engine fueled with natural gas–hydrogen blends under various injection timings. Energy and Fuels 2006;20(4):1498–504. [12] Wang Jinhua, Huang Zuohua, Fang Yu, Liu Bing, Zeng Ke, Miao Haiyan, et al. Combustion behaviors of a directinjection engine operating on various fractions of natural gas–hydrogen blends. International Journal of Hydrogen Energy 2007;32(15):3555–64. [13] Wang Jinhua, Chen Hao, Liu Bing, Huang Zuohua. Study of cycle-by-cycle variations of a spark ignition engine fueled with natural gas–hydrogen blends. International Journal of Hydrogen Energy 2008;33(18):4876–83. [14] Cheng RK, Oppenheim AK. Autoignition in methane– hydrogen mixtures. Combustion and Flame 1984;58(2):125–39.
[15] Lifshitz Assa, Scheller Karl, Burcat Alexander, Skinner Gordon B. Shock-tube investigation of ignition in methane–oxygen–argon mixtures. Combustion and Flame 1971;16(3):311–21. [16] Gersen S, Anikin NB, Mokhov AV, Levinsky HB. Ignition properties of methane/hydrogen mixtures in a rapid compression machine. International Journal of Hydrogen Energy 2008;33(7):1957–64. [17] Dale JD, Checkel MD, Smy PR. Application of high energy ignition systems to engines. Progress in Energy and Combustion Science 1997;23(5–6):379–98. [18] Heywood JB. Internal combustion engine fundamentals. , New York: McGraw-Hill; 1988. p. 413–23. [19] Orhan Akansu S, Dulger Zafer, Kahraman Nafiz, Nejat Veziroglu T. Internal combustion engines fueled by natural gas–hydrogen mixtures. International Journal of Hydrogen Energy 2004;29(14):1527–39. [20] Bauer CG, Forest TW. Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: effect on S.I. engine performance. International Journal of Hydrogen Energy 2001;26(1):55–70. [21] Wang Jinhua, Huang Zuohua, Miao Haiyan, Wang Xibin, Jiang Deming. Characteristics of direct injection combustion fuelled by natural gas–hydrogen mixtures using a constant volume vessel. International Journal of Hydrogen Energy 2008;33(7):1947–56. [22] Smith Gregory P, Golden David M, Frenklach Michael, Moriarty Nigel W, Eiteneer Boris, Goldenberg Mikhail, et al, http://www.me.berkeley.edu/gri_mech/. [23] Huang Zuohua, Zhang Yong, Zeng Ke, Liu Bing, Wang Qian, Jiang Deming. Measurements of laminar burning velocities for natural gas–hydrogen–air mixtures. Combustion and Flame 2006;146(1–2):302–11. [24] Law Chung K. Combustion physics. , New York: Cambridge University Press; 2006. [25] Yamamoto K, Ozeki M, Hayashi N, Yamashita H. Burning velocity and OH concentration in premixed combustion. Proceedings of the Combustion Institute 2009;32(1):1227–35.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures Jinhua Wang, Zuohua Huang*, Chenglong Tang, Jianjun Zheng State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China
article info
abstract
Article history:
Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures
Received 16 November 2009
was investigated experimentally and numerically. The flame propagating photos of pre-
Received in revised form
mixed combustion and direct-injection combustion was obtained by using a constant
28 December 2009
volume vessel and schlieren photographic technique. The pressure derived initial
Accepted 3 January 2010
combustion durations were also obtained at different hydrogen fractions (from 0% to 40%
Available online 20 January 2010
in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II
Keywords:
program with GRI 3.0 mechanism. The results showed that the initial combustion process
Hydrogen
of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in
Natural gas
the case of both premixed combustion and direct-injection combustion. This phenomenon
Lean combustion
is more obvious at leaner mixture condition near the lean limit of natural gas. The mole
Spark ignition
fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the flames. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
With ever increasing of energy demand and concern of environmental protection, the research on high efficiency clean combustion has attracted increased attention. Lean mixture combustion has a great potential to achieve higher thermal efficiency and lower emissions [1]. While the relatively high lean flammability limit of hydrocarbon fuel makes it difficult to achieve stable combustion near the lean burning regime [2], and this is even more severe for natural gas. As the main composition of natural gas, methane has unique tetrahedral molecular structure with large C–H bond energies, thus demonstrates some unique combustion
characteristics such as high ignition temperature and low flame propagation speed [3]. Hydrogen is an excellent additive to improve the combustion of hydrocarbon fuel due to its low ignition energy, high reactivity, high diffusivity and fast burning velocity [4,5]. Many researches have conducted on engine fueled with hydrogenenriched natural gas. Ma et al. [6–9] conducted engine study fueled with hydrogen enriched natural gas and found that lean limit of natural gas engine was extended with hydrogen enrichment. The thermal efficiency of natural gas engine can be improved with hydrogen addition combined with optimum retarded ignition timing. The authors conducted the engine study fueled with hydrogen-enriched natural gas in a direct-
* Corresponding author at: School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China. E-mail address: [email protected] (Z. Huang). 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.01.004
international journal of hydrogen energy 35 (2010) 7246–7252
injection spark ignition engine [10–12] and port mixing spark ignition engine [13]. It showed that the combustion of natural gas engine was enhanced with hydrogen addition. The combustion duration was decreased and the optimum ignition timing can be retarded as hydrogen is added to natural gas. The HC and CO emissions decreased while the NOx emissions increased significantly with hydrogen addition. This indicated that hydrogen addition should be used combined with lean combustion and/or large EGR ratio to obtain lower NOx emissions. The stable lean limit of natural gas engine can be extended with hydrogen addition. Researches have also conducted on the auto-ignition characteristics of methane– hydrogen blends based on the background of homogeneous charge compression ignition (HCCI) engine [14–16]. The experimental studies were conducted on shock tube test facility and/or rapid compression machine and results showed that hydrogen addition has a modest effect on the measured ignition delay time at low mole fractions of hydrogen (less than 20%). At 50% hydrogen fraction in the fuel blends, substantial reduction of measured ignition delay time was observed. However, fewer studies reported on the effect of hydrogen addition to natural gas and/or methane on spark ignition and early flame propagating process. Positive ignition of the fuel– air mixtures in cylinder of spark ignition engine is essential for good performance and efficiency [17]. Short initial combustion duration corresponding to fast early flame growth will be beneficial to high thermal efficiency and low cyclic variations in engines [18]. This is even more important for the lean combustion and/or using large EGR ratios which have the lower flame speed. Traditionally, high ignition energy system was used to ensure stable spark ignition with short initial combustion duration [9,10]. Hydrogen addition presents a potential to enhance the combustion characteristics and extend the stable lean limit of hydrocarbon fuels [19,20]. It is very difficult to understand the ignition and flame propagating process in a real engine because it is very complicated. The objective of the present study is to clarify the effect of hydrogen addition on ignition and early flame growth of lean burn natural gas–air mixtures in a constant volume vessel. The flame propagating photos and pressure derived initial combustion duration was obtained by using a constant volume vessel with schlieren photographic technique. The laminar premixed flames at the same condition as that of the experimental flames were calculated with PREMIX code of CHEMKIN II program. The monotonic correlation between initial combustion duration and the reciprocal maximum OH mole fraction in the flames was observed.
2. Experimental procedures and computational method The experiments of lean burn natural gas–hydrogen–air mixtures are conducted in a constant volume vessel. The experimental methods are similar to that of the earlier work [21] and will be described briefly here. The constant volume vessel used in this experiment is a cylindrical type vessel with inside diameter of 130 mm, inside length of 130 mm, and the volume of 1.725 L. Two sides of this vessel are transparent to make it optically accessible. The gas in the vessel is drawn out
7247
by a vacuum pump, and the fresh air is introduced into the vessel via the inlet valve at the initial pressure of 0.1 MPa and initial temperature of 300 K. Five minutes is awaited to ensure the air in the vessel motionless. Then the fuel blends with a specific hydrogen fraction and the overall equivalence ratio was injected into the vessel. The injection pressure is maintained constantly at 8.0 MPa in this experiment. The injected fuel jet generates the turbulence in the vessel and forms a turbulent inhomogeneous fuel–air mixture in the vessel, similar to that in a gas direct-injection engine. The fuel–air mixture is ignited by the centrally located electrodes at a given timing after the ending timing of injection (Tig). The change of Tig will be corresponding to different turbulence and stratification intensity condition in the vessel at the timing of spark ignition. The stoichiometric fuel–air ratio in volumetric of hydrogen is about one-fourth to that of natural gas, thus the fuel injection duration is increased in the case of hydrogen addition for the same equivalence ratio as illustrated in ref. [21]. The total fuel jet momentum at different hydrogen fractions gives the closely comparable value regardless of hydrogen fraction. This indicated that the turbulence intensity generated by the fuel jet is closely comparable with hydrogen addition. A homogeneous premixed mixture is also prepared in this vessel and provides the information on laminar homogeneous mixture combustion. A standard capacitive ignition system with ignition energy of 45 mJ is used to produce the spark. The pressure of the vessel during the combustion process is recorded by a piezoelectric Kistler absolute pressure transducer. The natural gas and hydrogen used in this experiment is also identical to that of ref. [21]. The methane–hydrogen–air freely propagated laminar premixed flames at normal temperature and atmospheric pressure at equivalence ratio of 0.6 and 0.8 were calculated by using PREMIX code of CHEMKIN II program with GRI-Mech 3.0 mechanism [22]. The mole fraction profiles of OH and O radicals in the flames are derived from the calculation results.
3.
Results and discussions
Flame propagating photos are an effective tool on combustion diagnoses. The effect of hydrogen addition on the flame propagating process of laminar premixed mixture combustion is illustrated in Fig. 1. It showed that the flame kernel occupied large volume at the same time after ignition with hydrogen addition. This indicated that the flame propagating speed was increased as hydrogen is added to natural gas. The flame is elliptical for pure natural gas–air mixture combustion due to the buoyancy effect near the lean burn limit with slow flame propagating speed. This effect is eliminated with a small amount of hydrogen addition. It can be seen that cracks can be observed in the flame surface when the hydrogen fraction is reached 32%. The cracks in the flame surface will increase the flame area thus accelerate the flame propagating process. The Markstein number which is an indicator of flame instability decreased with the increase of hydrogen fraction in natural gas–hydrogen fuel blends [23]. This indicated a self acceleration tendency with the increase of hydrogen fraction due to
7248
international journal of hydrogen energy 35 (2010) 7246–7252
Fig. 1 – Photos show the effect of hydrogen addition on flame propagating process of natural gas–air mixtures laminar premixed combustion (4 [ 0.6).
the increase of flame instability which will lead to the enhancement of combustion. The effect of hydrogen addition on early flame growth of natural gas–air mixtures combustion with Tig ¼ 45 ms at equivalence ratio of 0.6 is given in Fig. 2. It can be seen that early flame growth of natural gas–air turbulent combustion was enhanced as hydrogen is added. The flame kernel size at initial combustion process increases with the increase of hydrogen fraction. It is very obvious even at 4.0 ms after spark ignition timing. The flame kernel is very small and the shape is much irregular for natural gas which has the tendency to extinguish under the influence of turbulent flow. The flame kernel is more concentrated to the spark position and the flame shape is much more regular as hydrogen is added to
natural gas. This indicates that the spark ignition of lean burn natural gas–air mixtures is more stable with hydrogen addition. The effect of turbulent flow on spark ignition and flame propagation process can be weakened as hydrogen is added to natural gas. This would be due to the low ignition energy and high reactivity of hydrogen which accelerate the combustion rate of natural gas combustion. The high diffusivity of hydrogen weakened the effect of turbulent flow on flame propagating process. Fig. 3 shows the pressure history during the combustion process for lean premixed combustion and direct-injection combustion of natural gas–hydrogen blends. It shows that the initial pressure rise is increased remarkably as hydrogen is added to natural gas. This phenomenon becomes more
international journal of hydrogen energy 35 (2010) 7246–7252
7249
Fig. 2 – Photos show the effect of hydrogen fraction on early flame growth of natural gas–air mixtures turbulent combustion with Tig [ 45 ms (4 [ 0.6).
obvious for the leaner mixture combustion (4 ¼ 0.6) in the case of both premixed combustion and direct-injection combustion. This study reveals the enhancement of the initial combustion process by hydrogen addition in both laminar combustion and turbulent combustion. For turbulent combustion in this experimental, the ignition timing was set to be constant at 45 ms after the end of fuel injection, thus, the turbulent condition in the vessel is comparable for all the fuels at ignition timing. Hydrogen addition is the only one reason to contribute to the enhancement of initial combustion process. The low required ignition energy and high reactivity of hydrogen are responsible for the improvement of ignition and
early flame propagation. However, it should be noted that the effect of hydrogen addition on laminar premixed combustion and direct-injection turbulent combustion of natural gas is much different. In the case of laminar premixed combustion, the hydrogen addition promotes the fuel–air diffusion and combustion reaction. The promotion of combustion is almost linear with the increase of hydrogen fraction at equivalence ratio of both 0.6 and 0.8. However, in the case of direct-injection combustion, hydrogen addition will promote the spark ignition and initial flame development for the stratified mixture combustion and maintains stable flame propagation from the spark position to the surrounding relatively leaner
7250
international journal of hydrogen energy 35 (2010) 7246–7252
a
a
b
b
Fig. 3 – Pressure history during combustion process in the constant volume vessel (a: 4 [ 0.6, b: 4 [ 0.8).
mixture region. This is more obvious for condition at equivalence ratio of 0.6 which is near the lean limit of natural gas. The hydrogen addition will suppress the flame local
Fig. 4 – OH and O mole fraction profiles of methane– hydrogen–air flames (a: 4 [ 0.6, b: 4 [ 0.8).
distinguish at propagating process from the spark position to the surrounding relatively leaner mixture region. The effect of hydrogen addition on natural gas direct-injection combustion
international journal of hydrogen energy 35 (2010) 7246–7252
a
7251
The correlation between the initial combustion duration of the fuel blends for premixed combustion and direct-injection combustion and the corresponding maximum OH mole fraction in the laminar flames are given in Fig. 5. The initial combustion duration used in this paper is defined as the interval from the ignition timing to the timing of pressure rise up to 0.15 MPa [18]. The initial combustion duration decreases as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This indicates the enhancement of initial combustion process of lean burn natural gas combustion with hydrogen addition as described in previous. Meanwhile, a monotonic correlation between initial combustion duration and the reciprocal maximum OH mole fraction in the laminar premixed flame is observed, and this indicates that the decrease of initial combustion duration with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the fuel blends.
b 4.
Conclusions
Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The main results are summarized as follows:
Fig. 5 – Correlation between initial combustion duration and the reciprocal of maximum OH mole fraction (a: 4 [ 0.6, b: 4 [ 0.8).
is not linear. It is interesting to note that the difference between 32% hydrogen fraction and 40% hydrogen fraction is quite small, especially at initial combustion process. The effect of hydrogen addition is much weaker at equivalence ratio of 0.8 for direct-injection turbulent combustion. OH and O radicals play an important role in combustion chemical reaction of hydrocarbon fuels [24]. Yamamoto et al. found that the maximum OH concentration had a linear correlation to the laminar burning velocity for both 1-D and 2D flames [25]. The OH and O mole fraction profiles in the laminar premixed methane–hydrogen–air flames are calculated and illustrated in Fig. 4. Both the mole fractions of OH and O are increased with the increase of hydrogen fraction and the maximum OH and O mole fraction will move closing to the unburned mixture side. The main reaction pathways attributed to OH and O formation identified from the rate of production analysis are OH þ H2<¼>H þ H2O (R84) and H þ O2<¼>O þ OH (R38). The increase of H2 mole fraction will increase the reaction rate of R84 and thus forms more H radical and promotes the reaction rate of R38.
(1) The initial combustion process is significantly enhanced with hydrogen addition for both laminar premixed and direct-injection turbulent combustion at lean mixture condition. (2) The OH and O mole fraction increase with hydrogen addition and the position of maximum OH and O mole fraction moves closing to the unburned mixture side. (3) The enhancement of spark ignition of lean natural gas–air mixtures with hydrogen addition can be ascribed to the increase of OH and O mole fraction in the flames.
Acknowledgements This study was supported by National Basic Research Project (2007CB210006), National Natural Science Foundation of China (50821604) and State Key Laboratory of Engines (SKLE 200907).
references
[1] Bade Shrestha SO, Karim GA. Hydrogen as an additive to methane for spark ignition engine applications. International Journal of Hydrogen Energy 1999;24(6):577–86. [2] Naha S, Briones AM, Aggarwal SK. Effect of fuel blends on pollutant emissions in flames. Combustion Science and Technology 2005;177(1):183–220. [3] Turns Stephen R. An introduction to combustion: concepts and applications. 2nd ed., Boston: McGraw-Hill; 2000. pp. 158. [4] Bell SR, Gupta M. Extension of the lean operating limit for natural gas fueling of a spark ignited engine using hydrogen blending. Combustion Science and Technology 1997;123(1–6): 23–48.
7252
international journal of hydrogen energy 35 (2010) 7246–7252
[5] Karim Ghazi A. Hydrogen as a spark ignition engine fuel. International Journal of Hydrogen Energy 2003;28(5):569–77. [6] Ma Fanhua, Wang Yu, Liu Haiquan, Li Yong, Wang Junjun, Zhao Shuli. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. International Journal of Hydrogen Energy 2007;32(18):5067–75. [7] Ma Fanhua, Liu Haiquan, Wang Yu, Li Yong, Wang Junjun, Zhao Shuli. Combustion and emission characteristics of a port-injection HCNG engine under various ignition timings. International Journal of Hydrogen Energy 2008;33(2):816–22. [8] Ma Fanhua, Wang Yu, Liu Haiquan, Li Yong, Wang Junjun, Ding Shangfen. Effects of hydrogen addition on cycle-by-cycle variations in a lean burn natural gas spark-ignition engine. International Journal of Hydrogen Energy 2008;33(2):823–31. [9] Ma Fanhua, Wang Yefu, Ding Shangfen, Jiang Long. Twenty percent hydrogen-enriched natural gas transient performance research. International Journal of Hydrogen Energy 2009;34(15):6523–31. [10] Huang CQ, Wei LX, Yang B, Wang J, Li YY, Sheng LS, et al. Lean premixed gasoline/oxygen flame studied with tunable synchrotron vacuum UV photoionization. Energy and Fuels 2006;20(4):1505–13. [11] Huang Zuohua, Wang Jinhua, Liu Bing, Zeng Ke, Yu Jinrong, Jiang Deming. Combustion characteristics of a direct-injection engine fueled with natural gas–hydrogen blends under various injection timings. Energy and Fuels 2006;20(4):1498–504. [12] Wang Jinhua, Huang Zuohua, Fang Yu, Liu Bing, Zeng Ke, Miao Haiyan, et al. Combustion behaviors of a directinjection engine operating on various fractions of natural gas–hydrogen blends. International Journal of Hydrogen Energy 2007;32(15):3555–64. [13] Wang Jinhua, Chen Hao, Liu Bing, Huang Zuohua. Study of cycle-by-cycle variations of a spark ignition engine fueled with natural gas–hydrogen blends. International Journal of Hydrogen Energy 2008;33(18):4876–83. [14] Cheng RK, Oppenheim AK. Autoignition in methane– hydrogen mixtures. Combustion and Flame 1984;58(2):125–39.
[15] Lifshitz Assa, Scheller Karl, Burcat Alexander, Skinner Gordon B. Shock-tube investigation of ignition in methane–oxygen–argon mixtures. Combustion and Flame 1971;16(3):311–21. [16] Gersen S, Anikin NB, Mokhov AV, Levinsky HB. Ignition properties of methane/hydrogen mixtures in a rapid compression machine. International Journal of Hydrogen Energy 2008;33(7):1957–64. [17] Dale JD, Checkel MD, Smy PR. Application of high energy ignition systems to engines. Progress in Energy and Combustion Science 1997;23(5–6):379–98. [18] Heywood JB. Internal combustion engine fundamentals. , New York: McGraw-Hill; 1988. p. 413–23. [19] Orhan Akansu S, Dulger Zafer, Kahraman Nafiz, Nejat Veziroglu T. Internal combustion engines fueled by natural gas–hydrogen mixtures. International Journal of Hydrogen Energy 2004;29(14):1527–39. [20] Bauer CG, Forest TW. Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: effect on S.I. engine performance. International Journal of Hydrogen Energy 2001;26(1):55–70. [21] Wang Jinhua, Huang Zuohua, Miao Haiyan, Wang Xibin, Jiang Deming. Characteristics of direct injection combustion fuelled by natural gas–hydrogen mixtures using a constant volume vessel. International Journal of Hydrogen Energy 2008;33(7):1947–56. [22] Smith Gregory P, Golden David M, Frenklach Michael, Moriarty Nigel W, Eiteneer Boris, Goldenberg Mikhail, et al, http://www.me.berkeley.edu/gri_mech/. [23] Huang Zuohua, Zhang Yong, Zeng Ke, Liu Bing, Wang Qian, Jiang Deming. Measurements of laminar burning velocities for natural gas–hydrogen–air mixtures. Combustion and Flame 2006;146(1–2):302–11. [24] Law Chung K. Combustion physics. , New York: Cambridge University Press; 2006. [25] Yamamoto K, Ozeki M, Hayashi N, Yamashita H. Burning velocity and OH concentration in premixed combustion. Proceedings of the Combustion Institute 2009;32(1):1227–35.