Combustion and emission characteristics of HCNG in a constant ...

Journal of Mechanical Science and Technology 25 (2) (2011) 489~494 www.springerlink.com/content/1738-494x

DOI 10.1007/s12206-010-1231-5

Combustion and emission characteristics of HCNG in a constant volume chamber† Seang-Wock Lee1,*, Han-Seung Lee2, Young-Joon Park2 and Yong-Seok Cho1 1

School of Automotive Engineering, Kookmin University, Seoul, 136-702, Korea Graduate School of Automotive Engineering, Kookmin University, Seoul, 136-702, Korea

2

(Manuscript Received May 19, 2010; Revised November 1, 2010; Accepted December 20, 2010) ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract Finding an alternative fuel and reducing environmental pollution are the main goals for future internal combustion engines. Hydrogenmethane (HCNG) is now considered an alternative fuel due to its low emission and high burning rate. An experimental study was carried out to obtain fundamental data for the combustion and emission characteristics of pre-mixed hydrogen and methane in a constant volume chamber (CVC) with various fractions of hydrogen-methane blends. A pre-mixed chamber was used to obtain a good mixture of these gases. Exhaust emissions were measured using a Horiba exhaust gas analyzer for various fractions of hydrogen-methane blends. The results showed that the rapid combustion duration was shortened, and the rate of heat release elevated as the hydrogen fraction in the fuel blend was increased. Moreover, the maximum mean gas temperature and the maximum rate of pressure rise also increased. These phenomena were attributed to the burning velocity, which increased exponentially with the increased hydrogen fraction in the fuel blend. Exhaust HC and CO2 concentrations decreased, while NOX emission increased with an increase in the hydrogen fraction in the fuel blend. Our results could facilitate the application of hydrogen and methane as a fuel in the current fossil hydrocarbon-based economy and the strict emission regulations in internal combustion engines. Keywords: Hydrogen-methane (HCNG); Constant volume chamber (CVC); Combustion; Emission ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction The increased use of gasoline and diesel fuels in internal combustion engines causes air pollution and contributes to the greenhouse effect. Many studies have attempted to find solutions for these problems. It has been reported that the share of methane as an alternative fuel would increase gradually due to the growing interest in low emission vehicles and development of alternative fuels for public transportation in Korea. Methane fuel is relatively cheaper than diesel and gasoline, and it can be applied to the existing conventional engines without modification. Methane’s octane number, which is higher than gasoline’s, is 120, thereby increasing the compression ratio and anti-knocking. In addition, methane operates with higher thermal efficiency and power output than diesel and gasoline engines. Methanepowered vehicles emit less NMHC and NOX compared to dieselpowered vehicles in general [1, 2]. The addition of hydrogen increases the H/C ratio of hydrogen†

This paper was recommended for publication in revised form by Associate Editor Ohchae Kwon Corresponding author. Tel.: +82 2 910 4819, Fax.: +82 2 910 4178 E-mail address: [email protected] © KSME & Springer 2011 *

methane (HCNG) fuel. A higher H/C ratio results in less CO2 per unit of energy produced, thereby reducing greenhouse gas (GHG) emissions. Improvements in thermal efficiency could also help reduce GHG emissions [3]. However, vehicles using methane have a disadvantage due to slow flame speed. The laminar burning speed for methane is approximately 0.4 m/s, whereas hydrogen has a flame speed about six times higher [4]. Therefore, hydrogen mitigates the limitations of methane. Also, HCNG vehicles emit almost no emission gases because hydrogen is a clean fuel and does not contain carbon atoms. We obtained fundamental data on the combustion and emission characteristics of pre-mixed hydrogen and methane in a constant volume chamber (CVC) for various fractions of hydrogen-methane blends.

2. Experimental apparatus and method 2.1 Experimental apparatus A diagram of our experimental apparatus is shown in Fig. 1. A CVC with a bore and width of 86 mm and 39 mm, respectively, was used to determine the combustion characteristics of a spark ignition with pre-mixed hydrogen and methane. The CVC, which included a swirl motor, configuration of combustion

490

S.-W. Lee et al. / Journal of Mechanical Science and Technology 25 (2) (2011) 489~494

Table 2. Properties of hydrogen and methane [5].

Table 1. Experimental conditions. Bore × Width

86 mm × 39 mm

Property

Hydrogen

Methane

Displacement

228 cm3

Density (gaseous) [kg/m3]

0.09

0.716

Ambient Pressure

0.8 MPa

Molecular weight [kg/kmol]

2.016

16.043

Ambient Temperature

298K

0.029

0.058

Swirl Motor

500 rpm

Stoichiometric F/A [kg fuel /kg air] Energy density (gaseous) [MJ/dm3]

3.0

12.6

Ignition position

The center of CVC

Lower heating value [MJ/kg]

120

50

H2 Concentration

0, 15, 30 vol.%

Auto ignition Temp. in air [K]

858

810

Equivalence ratio

0.6, 0.7, 0.8, 1.0, 1.2

Minimum spark ignition energy [mJ]

0.02

0.29

Octane number

-

RON 120

Laminar flame speed [cm/s]

≈ 230

≈ 42

Fig. 1. Diagram of experimental apparatus.

chamber and location of spark plugs not applicable to an actual engine, was designed for several experimental conditions. A premixing chamber was used to obtain a uniform gas mixture. A high-speed camera was used to investigate the combustion phenomena, and exhaust emissions were measured with a Horiba exhaust gas analyzer (MEXA-554JKNOX), for various fractions of hydrogen-methane blends. Intake and exhaust valves, a pressure sensor, spark plugs, and an optical window (for photography) were installed in the CVC chamber. The swirl motor is controlled at 500 rpm to make a uniform gas mixture in the CVC. Residual gas was removed using a vacuum pump and decompression tank. The experimental conditions are given in Table 1. The combustion pressure data were obtained through a data acquisition card (DAQ Card 6024E based on Labview technology). All signals from the HCNG ignition and the photographic timing were controlled using Labview. 2.2 Experimental method The temperature in the CVC was maintained at 298 K, and the equivalence ratio was adjusted using partial pressures of HCNG and air with respect to the ambient pressure in the CVC. Hydrogen and methane were mixed in the pre-mixing chamber. The gases flowed into the CVC and were ignited by spark plugs (tungsten, 1 mm) so that we could visualize the developing flame. A 4000 fps high-speed digital camera was set up. The rates of the heat released from combustion were obtained and analyzed using

a piezo-type pressure sensor. Tests were carried out with hydrogen-methane with 0, 15, and 30 vol. % hydrogen at equivalence ratios of 0.6, 0.7, 0.8, 1.0, and 1.2. The equivalence ratio is adjusted by fixing the amount of air and changing the fuel amount. The fuels were used in volume unit for this experiment. Therefore, it was necessary to convert the fuels to their mass unit. According to the mass fraction of the two fuels, the F/A ratio was decided in proportion to F/A ratio of each fuel on the mass unit base. Table 2 shows properties of hydrogen and methane. A lower heating value is higher, and the laminar flame speed is faster, in hydrogen gas than in methane gas.

3. Results and discussion 3.1 Combustion characteristics The parameters we investigated are the equivalence ratios and different values for the enrichment of the blends. Fig. 2 shows combustion visualization for equivalence ratios and various fractions of hydrogen-methane blends. Figs. 3-7 show combustion pressures and rates of the heat released for various fractions of hydrogen-methane blends at various equivalence ratios. It is clear that the speed of combustion increased and the maximum combustion pressures and rates of the heat released also increased due to the increase in the hydrogen portion of the HCNG fuel. This is because of the fast combustion speed of hydrogen and a high lower-heating value. Hydrogen on the improvement of combustion pressure was significantly effective under lower equivalence ratio [6]. It misfired under all conditions at equivalence ratios of 0.6 and 0.7, except when hydrogen 30% was added at an equivalence ratio of 0.7. When the equivalence ratio was 0.8, the maximum combustion pressure and rate of the heat released rapidly increased with the increased amount of added hydrogen. The maximum combustion pressure and heat release rate were highest when the equivalence ratio was 1.0. Fig. 8 shows the ignition delay from the beginning of the spark to just before the pressure increase in terms of equivalence ratios and blending rates. This shows the process of passing through the stages of cool flame, blue flame, and to the state where the

491


CH4 100%

CH4 85%+ H2 15%

CH4 70%+ H2 30%

Fig. 2. Combustion visualization for equivalence ratios and various fractions of hydrogen-methane blends.

Fig. 3. Combustion pressure and rate of heat release for various fractions of hydrogen-methane blends at an equivalence ratio of 0.6.


492





Fig. 8. Ignition delay for equivalence ratios and various fractions of hydrogen-methane blends.

Fig. 9. Time required to reach maximum pressure for equivalence ratios and various fractions of hydrogen-methane blends.


493

Fig. 10. Combustion duration for equivalence ratios and various fractions of hydrogen-methane blends.

Fig. 12. HC emission for equivalence ratios and various fractions of hydrogen-methane blends.

Fig. 11. CO emission for equivalence ratios and various fractions of hydrogen-methane blends..

Fig. 13. CO2 emission for equivalence ratios and various fractions of hydrogen-methane blends.

hot flame occurred, just up to the point where actual ignition combustion began [7]. The ignition delay was the shortest when the equivalence ratio was around 1.0 and when the amount of added hydrogen increased. On the other hand, in the rich areas, the ignition delay seemed to be longer. Fig. 9 shows the time to reach maximum combustion pressure, and Fig. 10 represents main combustion durations that demonstrate the pressure at the beginning through maximum pressure. The time to reach the maximum combustion pressure was the fastest and the ignition delay was the shortest at an equivalence ratio of 1.0 with 30% hydrogen added fuel. 3.2 Characteristics of emission Figs. 11-14 show the exhaust emission characteristics of HCNG under different conditions. In Fig. 11, as the equivalence ratio increased, the CO rapidly increased as well (especially in the rich area) due to incomplete combustion. As shown in Fig. 12, HC decreased as more hydrogen was added. On the other hand, HC increased as the equivalence ratios were increased; Fig. 13 shows the CO2 emission amount. CO2 is known as the major source of air pollution, and much less was emitted by adding hydrogen in a lower equivalence ratio or lean area. Fig. 14 shows the amount of NOX emission. NOX rapidly increased at an equivalence ratio of about 1.0, and was hardly emitted in a rich area due to the lack of oxygen. In addition, for 30%

Fig. 14. NOX emission for equivalence ratios and various fractions of hydrogen-methane blends.

H2 addition, NOX increased more than in the other cases. We suggest that the cause of this phenomenon is the increase in the amount of thermal NOX due to the maximum combustion temperature caused by rapid hydrogen combustion.

4. Conclusion To understand the combustion and the exhaust emission characteristics of HCNG, we changed the amount of hydrogen added and the equivalence ratio in the CVC. The following conclusions were made based on our pressure and exhaust emission results. (1) From the results of the pre-mixed hydrogen-methane com-

494


bustion experiment, as the amount of added hydrogen increased, the maximum combustion pressure, rate of the heat released, and speed of flame spread also increased. (2) The maximum combustion pressure and rate of the heat released were the highest when the equivalence ratio was around 1.0 with 30% hydrogen fuel added. (3) The ignition delay was shorter and the burning velocity was faster when the amount of added hydrogen increased. (4) As the amount of hydrogen increased in the blended fuel, HC and CO2 decreased but NOX increased. This might be caused by the increased amount of thermal NOX due to the maximum combustion temperature occurring in rapid hydrogen combustion. (5) As the amount of added hydrogen increased, combustion is more possible in the lean area with reduced emissions.

Acknowledgment This work was supported by the Eco-STAR project of the Ministry of Environment and by a Korea Research Foundation Grant funded by the Korean Government, which we appreciate very much.

References [1] F. Linch and Hythane, A bridge to an ultraclean renewable hydrogen energy system, Atti del Workshop IEA, Denver (1991). [2] S. K. Fulcher, B. F. Gajdeczko, P. G. Felton and F. V. Bracco, The effects of fuel atomization, vaporization, and mixing on the cold-start UHC emissions of a contemporary S.I. engine with intake-manifold injection, SAE Paper No. 952482 (1995).

[3] S. R. Munshi, C. Nedelcu, J. Harris, T. Edwards, J. Williams, F. Lynch, M. Frailey, G. Dixon, S. Wayne and R. Nine, Hydrogen Blended Natural Gas Operation of a Heavy Duty Turbocharged Lean Burn Spark Ignition Engine, SAE Paper No. 2004-01-2956 (2004). [4] T. Iijima and T. Takeno, Effects of Temperature and Pressure on Burning Velocity, Combustion and Flame 65, Dept. of Mechanical Engineering, Faculty of Engineering, Tokai, Japan (1986) 35-43. [5] P. Grabner, A. Wimmer, F. Gerbig and A. Krohmer, Hydrogen as a Fuel for Internal Combustion Engines – Properties, Problems and Chances, 5th Intern. Colloquium FUELS (2005) 3-13. [6] T. Wallner, H. K. Ng and R. W. Peters, The Effects of Blending Hydrogen with Methane on Engine Operation, Efficiency, and Emissions, SAE Paper No. 2007-01-0474 (2007). [7] J. B. Heywood, Internal combustion engine fundamentals, McGraw-Hill, Inc., Massachusetts Institute of Technology, Massachusetts, USA (1988) 371-375.

Seang-Wock Lee received his BS and MS in Mechanical Engineering in 1996 and 1998 from Kookmin University, and then the PhD in 2003 from Waseda University. He is a Professor at Kookmin University in Seoul, Korea. His research interests are in thermal dynamics, internal combustion engines, and alternative fuel engines.