Energy & Fuels 2009, 23, 1431–1436
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Premixed Combustion of Diluted Hydrogen-Air Mixtures in a Constant Volume Bomb Haiyan Miao,* Qian Huang, Erjiang Hu, Zuohua Huang, and Deming Jiang State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong UniVersity, Xi’an 710049, China ReceiVed September 17, 2008. ReVised Manuscript ReceiVed NoVember 9, 2008
Combustion characteristics of diluted hydrogen-air premixed combustion were studied in a constant volume bomb over wide ranges of diluent ratios and equivalence ratios using two kinds of diluent gases. A two-zone model was applied to calculate the heat release rate and combustion duration based on the pressure data. The study showed that, by adding diluent gas into the hydrogen-air mixture, both the combustion development duration and the combustion duration are lengthened. Increasing the diluent ratio reduces the value of the peak pressure, and the reduction is larger on the rich mixture than on the lean one. CO2 diluent gas has greater effects on the normalized mass burning rate, the combustion development duration, and the combustion duration than nitrogen. Within the experimental range of this study, the minimum value of both the combustion development duration and the combustion duration lies within the range of φ ) 1.4-1.6.
Introduction With the increasing concern on fossil fuel shortage and environmental issues, the development of alternative fuel engines has attracted more and more attention. Hydrogen is one of the most attractive fuels because of its high reactivity. Recently, the interest of using pure hydrogen as an energy source is increased because of the concern of global warming as well as its ability to be derived from various sources, many of which are renewable. However, there exist considerable challenges to use pure hydrogen associated with storage, transport, and the performance of internal combustion engines1 because of its high reactive and diffusive nature. The other ways to use hydrogen are as an additive to hydrocarbon fuels for improved engine performance (e.g., ref 2-3) or as a main fuel with propane substitution to moderate cell formation and suppress both diffusional thermal and hydrodynamic cellular instabilities.4 Experiments showed that adding hydrogen improves the lean-burn capability of the engine and decreases the unburned hydrocarbon emissions with a cost of increased NOx emission. The more the hydrogen added, the higher the NOx emission.2,3 Exhaust gas recirculation (EGR) is an effective approach to reduce NOx emission.5 Thus far, most * To whom correspondence should be addressed: School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China. Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail: hymiao@ mail.xjtu.edu.cn. (1) White, C. M.; Steeper, R. R.; Lutz, A. E. The hydrogen-fueled internal combustion engine: A technical review. Int. J. Hydrogen Energy 2006, 31 (10), 1292–1305. (2) Akansu, S. O.; Dulger, Z.; Kahraman, N.; Veziroglu, T. N. Internal combustion engines fueled by natural gas-hydrogen mixtures. Int. J. Hydrogen Energy 2004, 29 (14), 1527–1539. (3) Swain, M.; Yusuf, M.; Dulger, Z. The effects of hydrogen addition on natural gas engine operation. Society of Automotive Engineers (SAE), Warrendale, PA, 1993; 932775. (4) Law, C. K.; Kwon, O. C. Effects of hydrocarbon substitution on atmospheric hydrogen-air flame propagation. Int. J. Hydrogen Energy 2004, 29 (8), 867–879. (5) Abd-Alla, G. H. Using exhaust gas recirculation in internal combustion engines: A review. Energy ConVers. Manage. 2002, 43 (8), 1027– 1042.
research is concentrated on the engine.6-8 Because there are many factors influencing engine operation, it is therefore difficult to separate the influence of these factors. This motivated us to perform a fundamental investigation on the combustion characteristics of the hydrogen-air mixtures diluted by nitrogen and CO2, which are the main constituents of exhaust gas. The laminar burning velocities of hydrogen-air mixtures were measured intensively in previous studies.9-14 Hermanns15 used the heat flux method with nitrogen as the diluent and measured the laminar burning velocity of the hydrogen-airdiluent mixture at equivalence ratios ranging from 0.7 to 3.1, (6) Heffel, J. W. NOx emission reduction in a hydrogen fueled internal combustion engine at 3000 rpm using exhaust gas recirculation. Int. J. Hydrogen Energy 2003, 28 (11), 1285–1292. (7) Subramanian, V.; Mallikarjuna, J. M.; Ramesh, A. Intake charge dilution effects on control of nitric oxide emission in a hydrogen fueled SI engine. Int. J. Hydrogen Energy 2007, 32, 2043–2056. (8) Prasad, P.; Mahalingam, S. Exhaust gas recirculation effects on hydrogen-air combustion. Combust. Sci. Technol. 2007, 179 (6), 1131– 1157. (9) Milton, B. E.; Keck, J. C. Laminar burning velocities in stoichiometric hydrogen and hydrogen-hydrocarbon gas mixtures. Combust. Flame 1984, 58 (1), 13–22. (10) Ilbas, M.; Crayford, A.; Yilmaz, I.; Bowen, P.; Syred, N. Laminarbuming velocities of hydrogen-air and hydrogen-methane-air mixtures: An experimental study. Int. J. Hydrogen Energy 2006, 31 (12), 1768–1779. (11) Bradley, D.; Lawes, M.; Liu, K.; Verhelst, S.; Woolley, R. Laminar burning velocities of lean hydrogen-air mixtures at pressures up to 1.0 MPa. Combust. Flame 2007, 149 (1-2), 162–172. (12) Aung, K.; Hassan, M.; Faeth, G. Flame stretch interactions of laminar premixed hydrogen/air flames at normal temperature and pressure. Combust. Flame 1997, 109 (1-2), 1–24. (13) Sun, C.; Sung, C.; He, L.; Law, C. Dynamics of weakly stretched flames: Quantitative description and extraction of global flame parameters. Combust. Flame 1999, 118 (1-2), 108–128. (14) Dahoe, A. E. Laminar burning velocities of hydrogen-air mixtures from closed vessel gas explosions. J. Loss PreV. Process Ind. 2005, 18 (3), 152–166. (15) Hermanns, R.; Konnov, A.; Bastiaans, R.; de Goey, L. Laminar burning velocities of diluted hydrogen-oxygen-nitrogen mixtures. Energy Fuels 2007, 21 (4), 1977–1981. (16) Kwon, O.; Faeth, G. Flame/stretch interactions of premixed hydrogen-fueled flames: Measurements and predictions. Combust. Flame 2001, 124 (4), 590–610.
10.1021/ef8007823 CCC: $40.75 2009 American Chemical Society Published on Web 01/20/2009
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while refs 16-19 measured the laminar burning velocities of nitrogen-diluted hydrogen-air mixtures16-18 or H2-airHe-CO2 mixtures19 at different equivalence ratios (for example, from 0.6 to 4.5 in ref 16 and from 0.45 to 4 in ref 17) using outwardly expanding spherical flames with high-speed photography. Recently, Qiao, Kim, and Faeth20 extended the range of diluent ratios up to 40% by volume and studied the effect of various diluent gases, including nitrogen, carbon dioxide, helium, and argon. However, because the equivalence ratios were fixed at 1.0 and 1.8 in ref 20, the results did not provide information on how the diluent gas affects the combustion processes of hydrogen-air mixtures at lean conditions. The information is very important because to reduce the NOx emission from a combustion engine fueled with either hydrogen or hydrogen-enriched hydrocarbon fuels it was recommended that the engine should be operated at rather lean conditions1,2 with the EGR system.5 Therefore, further studies are needed to obtain a better understanding on the effect of diluent gases, especially at lean conditions. Motivated by the above needs, we conducted investigations on the laminar combustion of diluted hydrogen-air mixtures at the equivalence ratios from 0.6 to 1.8 with the diluent ratio up to 40% by volume for nitrogen diluent gas and 30% by volume for carbon dioxide diluent gas. Both laminar burning velocities and Markstein lengths were obtained from schlieren photography and summarized in ref 21. On the other hand, we also analyzed the pressure traces obtained during the experiments. In this paper, the combustion characteristics of the diluted hydrogen-air mixtures were studied on the basis of the analysis of the heat release of the combustible mixtures and its derived combustion durations using a two-zone model, which was developed to calculate the heat release rates based on the pressure data. The effects of N2 or CO2, the main constitutes of EGR, and their diluent ratios on the combustion characteristics were studied in this paper.
Miao et al.
Figure 1. Structure of the constant volume combustion bomb.
Figure 2. Schematic diagram of the two-zone model.
Experimental Section The experiment was conducted in a constant volume combustion bomb with schlieren photography. The combustion bomb used is a cylindrical type, with the diameter of 130 mm and length of 130 mm, as shown in Figure 1. Two sides of the bomb are transparent to provide optical access, allowing for observation of the combustion inside. The combustible mixture was prepared by adding hydrogen, air, and diluent gas (N2 or CO2) at the calculated partial pressures according to its corresponding equivalence ratio and dilution ratio φr, which is defined as the volume fraction of the diluent gas in the whole mixture. Hydrogen, N2, and CO2 with purities of 99.995, 99.99, and 99.99%, respectively, were used. Enough time was required to let the mixture go to quiescence. The fuel-air mixture was then ignited by the centrally located electrodes, and a standard capacitive discharge ignition system was used for producing the spark. In this study, the ignition energy is 45 mJ. The pressure inside the bomb was recorded by a piezoelectric Kistler pressure transducer, with a resolution of 0.01 kPa. (17) Aung, K.; Hassan, M.; Faeth, G. Effects of pressure and nitrogen dilution on flame/stretch interactions of laminar premixed H2/O2/N2 flames. Combust. Flame 1998, 112 (1-2), 1–15. (18) Tse, S.; Zhu, D.; Law, C. Morphology and burning rates of expanding spherical flames in H2/O2/inert mixtures up to 60 atm. Proc. Combust. Inst. 2000, 28, 1793–1800. (19) Lamoureux, N.; Djeba¨y´li-Chaumeix, N.; Paillard, C. Laminar flame velocity determination for H2-air-He-CO2 mixtures using the spherical bomb method. Exp. Therm. Fluid Sci. 2003, 27 (4), 385–393. (20) Qiao, L.; Kim, C.; Faeth, G. Suppression effects of diluents on laminar premixed hydrogen/oxygen/nitrogen flames. Combust. Flame 2005, 143 (1-2), 79–96. (21) Miao, H.; Huang, Q.; Huang, Z. Measurement of laminar burning velocities and Markstein lengths of premixed hydrogen-air-diluent mixtures. Int. J. Hydrogen Energy 2009, manuscript submitted.
Figure 3. Combustion pressure and normalized mass burning rate of hydrogen-air mixtures at various equivalence ratios.
The initial condition was strictly controlled in the experiments to realize the same initial pressure and temperature. In the experiments, the initial pressure was set at 0.1 MPa and the initial temperature was set at 300 K. To avoid the influence of the wall temperature on the mixture temperature, enough interval between two experiments was required, providing enough time for the wall to cool and, therefore, keeping the same initial temperature. Calculation Model. A two-zone model was used for the combustion analysis. The model was originally proposed by J. B.
Diluted Hydrogen-Air Premixed Combustion
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Figure 4. Effect of the diluent ratio on the combustion pressure of nitrogen/CO2-diluted hydrogen-air mixtures at φ ) 1.0.
Figure 5. Effect of the diluent ratio on the normalized mass burning rate of nitrogen/CO2-diluted hydrogen-air mixtures at φ ) 1.0.
Figure 6. Effect of the diluent ratio on the flame development duration at various equivalence ratios.
Heywood for the thermodynamic analysis of spark-ignition engine combustion.22 As shown in Figure 2, the spherical flame front divides the combustion chamber of the constant volume bomb into two zones, namely, the burned and unburned zones. The symbols p, T, V, and m represent the pressure, temperature, volume, and mass of the gases inside the combustion chamber, respectively. Qr is the amount of heat release from fuel. The subscripts (u and b) represent the unburned and burned states, respectively. (22) Heywood, J. B. Internal Combustion Engine Fundamentals; McGraw-Hill Book Company: New York, 1988; pp 90-92 and 376381.
Several assumptions were made as follows: (1) The gases inside the chamber are treated as ideal gas. (2) When the unburned gas enters the burned zone, combustion takes place very rapidly and thoroughly. (3) The pressure in the chamber reaches its equilibrium value instantaneously, and there exists no pressure difference between the burned and unburned zones. (4) There is no gas leakage, and both zones have their own uniformly distributed temperature. (5) The unburned gases in the unburned zone are regarded as the mixture of hydrogen, air, and diluents. (6) The gas properties of the unburned and burned gases are calculated by the corresponding fractions of their constituted gases.
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Miao et al.
Figure 7. Effect of the diluent ratio on the combustion duration at various equivalence ratios.
Because no pressure difference is considered between the burned and unburned zones, eq 4 can be established
P)
mbRbTb muRuTu ) Vb Vu
(4)
From the above four equations, the following equations can be derived:
˙u Q Tu dP + dTu P dt muRu ) dt 1 ∂uu +1 Ru ∂Tu
(
˙b+Q ˙ r + muRu Q
(
)(
)
dTu 1 ∂ub - PdV +1 dt Rb ∂Tb
( (
dP 1 ∂ub mb ∂ub Vu + + dt Rb ∂Tb V ∂Tb V ∂ub Ru (ub - uu) + T - Tb Rb u ∂Tb V
dmb ) dt
(5)
)
)
(6)
)
dVb 1 dmu 1 dTu 1 dP dV ) Vu + + dt mu dt Tu dt p dt dt
(7)
The heat-transfer model takes into account both convection heat transfer and radiation. The coefficient of convection heat transfer is derived from the plate-plate convective heat-transfer correlation as
λ R ) C Re Lc
(8)
where λ is the gas conductive coefficient, Lc is characteristic length, and Re is Reynolds number. The radiant heat flux q˙ is calculated by Figure 8. Effect of the diluent ratio on the maximum pressure at various equivalence ratios.
The mass conservation equation is
dmb dmu )dt dt
(9)
where σ is Boltzmann constant ()5.67 × kW ˙ u and the transient Thus, the transient heat transfer to the wall Q ˙ b are determined heat transfer from burned gas to unburned gas Q by eqs 10 and 11, respectively. 1011
(1)
From energy conservation, the following two equations can be established:
dVu dmu d(muuu) dQu ) -P + hu + dt dt dt dt dVu dmu dQb dQr d(muuu) ) -P + h + + dt dt dt u dt dt
q˙ ) Kσ(T4 - Ti4)
(2) (3)
m-2
K-4).
˙ u ) A[R(Tu - Tw) + Kσ(Tu4 - Tw4)] Q
(10)
˙ b ) Af[R(Tb - Tu) + Kσ(Tb4 - Tu4)] Q
(11)
They are typical Annand’s heat-transfer formula, where the constant K uses the value of 1.5 in this study, A is the wall surface area, and Af is the spherical flame front area. Tu and Tb are the gas
Diluted Hydrogen-Air Premixed Combustion temperatures of the unburned and burned zones, respectively, while Tw is the wall temperature. In the model, the gas temperatures are assumed to be uniform in the unburned and burned zones. That is, no temperature gradient was considered in the model. With respect to model calculation, dP/dt is obtained from the pressure data and dV/dt is zero for the constant volume bomb. The internal energy and gas constant of mixtures (ub, uu, Rb, Ru, ∂ub/∂Tb, and ∂uu/∂Tu) are calculated using the formula given in the literature22 according to the fraction of each species. Thus, the unknown variables in these thermodynamic equations are mb, Tb, and Tu. The initial burned gas temperature Tb uses the adiabatic flame temperature Tad, and by using a fourthorder Runge-Kutta scheme, the mb, Tb, and Tu, together with the burning rate dmb/dt, can be obtained. During the combustion process, gas compositions and properties are calculated through chemical equilibrium with 11 species and 7 equations.22
Results and Discussion As we know, the pressure inside the combustion chamber is device-dependent because (1) the amount of fuel inside the combustion chamber before ignition at a given initial pressure and temperature is determined by the volume of the chamber and (2) the heat-transfer-related parameters, such as the effective area of heat transfer, etc. (which control the amount of the heat transferred through the chamber wall and, therefore, the pressure drop), are varied with the design of the combustion chamber. To eliminate these effects, we introduced the concept of the normalized mass burning rate, which is defined as (1/m)(dmb/ dt), where m is the total mass of the combustible gases and dmb/dt is the burning rate obtained from the two-zone model described in the previous section. The normalized mass burning rate (especially before the flame reaches the chamber wall) could be reproducible in a bomb with different size. Figure 3 gives the pressure inside the combustion chamber after ignition and its corresponding normalized mass burning rate of the hydrogen-air mixtures at different equivalence ratios. The effect of the equivalence ratio on the pressure rise inside the chamber is clearly shown in Figure 3a. Generally speaking, the lean mixtures have a much slower pressure increase rate and relatively lower value of peak pressure compared to the stoichiometric and rich mixtures up to φ ) 1.8. Although the stoichiometric mixture has the highest value of peak pressure, it takes more time to reach its peak value than the rich mixture with an equivalence ratio between 1.2 and 1.8. In the case of lean mixtures, the value of the peak pressure decreases and the time to reach the peak value is delayed with the decrease of the equivalence ratio. In the case of rich mixtures, the value of the peak pressure decreases with the increase of the equivalence ratio. In the range of our experiments, it is when φ ) 1.6 that uses the shortest time to reach its peak pressure. From Figure 3b, it can be seen that the mass burning rates have a similar pattern at different equivalence ratios. They all increase with an increasing rate and decrease quite sharply after reaching the highest value. When the equivalence ratio increases, the highest value of the mass burning rate increases until φ reaches 1.4 and then starts to decrease. It is also when φ ) 1.6 that uses the shortest time to reach the highest value of the mass burning rate. Because fast burning reduces both the time to reach the peak pressure and the highest value of the mass burning rate, Figure 3 suggests that it is when φ ) 1.6 that the hydrogen-air mixture has the fastest flame speed, which is well agreed with the measured data.9 Note: in fact, it takes the stoichiometric hydrogen-air mixtures 6.3 ms to reach its peak pressure (see Figure 3a), while
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the stoichiometric natural gas-air mixture needs 63 ms to do so.23 This observation clearly suggests that, to apply pure hydrogen directly into the existing transport engines, it is rather difficult to control the combustion process and the heat release rate of the engine when using rich or stoichiometric mixtures. The effect of nitrogen and CO2 diluent gases on hydrogen combustion is illustrated in Figures 4 and 5, providing both the measured combustion pressure and the calculated normalized mass burning rate for stoichiometric mixtures with various diluent ratios of nitrogen and CO2 diluent gases. These figures show that the value of the peak pressure for both nitrogen- and CO2-diluted cases is reduced dramatically with the increase of the diluent ratio, which is mainly due to the reduction of the amount of hydrogen that is introduced into the combustion chamber (the dilute effect), the additional heat absorption of the diluent gases that reduces the flame temperature of the mixture (the thermal effect), and the enhanced heat transfer with the lengthened combustion duration. On the other hand, the time needed to reach peak pressure is increased, indicating that the flame propagation speed is reduced with the increase of the diluent ratio. When parts a and b of Figure 5 are compared, it can be seen that CO2 diluent gas has greater effects on the normalized mass burning rate than nitrogen. When 10% by volume CO2 is introduced into the total mixture, the highest value of the normalized mass burning rate is reduced to half that of the mixture without dilution, while less than 1/3 is reduced by introducing 10% by volume nitrogen. As mentioned before, it takes the stoichiometric hydrogen-air mixture 6 ms to complete its combustion. When φr ) 0.2, using nitrogen diluent gas extends this duration to 10 ms, while using CO2 extends this duration to almost 20 ms. To obtain a better understanding of the diluent ratio on the combustion process, experiments were conducted at various equivalence ratios ranging from 0.6 to 1.8. Both pressure curves and their corresponding normalized mass burning rates were obtained. Furthermore, we introduce the definitions widely used in analyzing the in-cylinder pressure in a spark-ignition engine into analyzing the pressure inside the combustion chamber. In this paper, the flame development duration is defined as the time after ignition in which 10% total mass of the fuel is combusted or consumed and the combustion duration is the time in which 90% total mass of the fuel is combusted. It is the flame development duration that is more likely to be reproduced in a combustion chamber with different size. Figures 6 and 7 summarized the effect of the diluent ratio on the flame development duration (which indicates the flame development at the early stage of the combustion after ignition) and combustion duration (which shows how long the combustion lasts after being ignited). The results show that, with the increase of the equivalence ratio, both the combustion development duration and the combustion duration of diluted or nondiluted hydrogen-air mixtures are decreased at first until reaching their corresponding minimum values and then they starts to increase with the increase of the equivalence ratio. Within the experimental range of this study, the minimum value of the combustion development duration and the combustion duration lies in the range of φ ) 1.4-1.6. By adding diluent gas into the hydrogen-air mixture, both the combustion development duration and the combustion duration are lengthened as a result of the decrease of the combustion speed and temperature. Nitrogen (23) Huang, Z.; Zhang, Y.; Ke, Z.; Liu, B.; Wang, Q.; Jiang, D. Natural gas-hydrogen-air premixed mixture combustion with a constant volume bomb. Energy Fuels 2007, 21, 692–698.
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and CO2 have quite similar effects on the combustion development duration and the combustion duration. However, the effect of CO2 is stronger than that of nitrogen. This is mainly due to the thermal effect difference of these two diluent gases. Because carbon dioxide has a larger specific heat than nitrogen, using carbon dioxide can increase the specific heat of the nonfuel portion in the combustible mixture and, therefore, cause more temperature reduction than nitrogen. Figure 8 summarizes the effect of the diluent ratio on the peak pressure. It shows that, in the range of the experiments, increasing the diluent ratio reduces the value of the peak pressure and, moreover, the reduction is to a greater extend on the rich mixture side than on the lean mixture side. This indicates that using diluted hydrogen or introducing EGR into the engines fueled with hydrogen-enriched hydrocarbon fuels could possibly bring about negative effects on the engine performance. Conclusions Combustion characteristics of diluted hydrogen-air premixed combustion were studied in a constant volume bomb over wide ranges of diluent ratios, equivalence ratios, and two kinds of diluent gases. A two-zone model was applied to calculate the heat release rate and combustion durations based on the pressure data. The main results are summarized as follows: (1) By adding diluent gas into the hydrogen-air mixture, both the combustion development duration and the combustion duration are lengthened and the time needed to reach the peak pressure is increased, which indicates that the flame propagation speed is reduced with the increase of the diluent ratio. (2) CO2 diluent gas has greater effects on the normalized mass burning rate, the combustion development duration, and the combustion duration than nitrogen. (3) Increasing the diluent ratio reduces the value of the peak pressure, and the reduction is to a greater extend on the rich mixture side than on the lean mixture side. (4) Within the experimental range of this study, the minimum value of both the combustion development duration and the combustion duration lies in the range of φ ) 1.4-1.6.
Miao et al. Acknowledgment. This study was supported by the National Natural Science Foundation of China (50606029) and the National Basic Research Program of China (Grant 2007CB210006).
Nomenclature A ) wall area (m2) Af ) flame front area (m2) h ) enthalpy (J) L ) molar number of air Lc ) characteristics length (m) m ) mass of gases (g) P ) gas pressure (Pa) Po ) initial mixture pressure (MPa) q˙ ) radiant heat-transfer flux (W/m2) ˙ b ) heat-transfer rate from burned gas to unburned gas (J/s) Q Qr ) amount of heat release by fuel combustion (J) ˙ u ) heat-transfer rate from the unburned zone to the wall (J/s) Q R ) gas constant Re ) Reynolds number T ) gas temperature (K) Tw ) wall temperature (K) u ) internal energy (J) ν ) velocity (m/s) V ) volume (m3) t ) time (s) tfd ) flame development duration (s) tcom ) combustion duration (s) φ ) equivalence ratio φr ) diluent ratio R ) heat-transfer coefficient (W m-1 K-1) λ ) gas conductive coefficient (W m-1 K-1) F ) gas density (kg/m3) σ ) Boltzmann constant µ ) viscosity (kg m-1 s-1) Subscripts b ) burned gas u ) unburned gas EF8007823
