Energy Fuels 2010, 24, 365–374 Published on Web 11/23/2009
: DOI:10.1021/ef9008532
Ignition and Combustion Characteristics of Gas-to-Liquid Fuels for Different Ambient Pressures Dung Ngoc Nguyen,* Hiroaki Ishida, and Masahiro Shioji Graduate School of Energy Science, Kyoto University Honmachi, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan Received August 5, 2009. Revised Manuscript Received November 5, 2009
Gas-to-liquid (GTL) fuel exhibits potential as a clean alternative diesel fuel, suitable for addressing problems of energy security and environmental pollution. The main objective of this research was to provide fundamental data for the ignition and combustion of GTL fuels. Experiments were conducted in a constantvolume combustion vessel to investigate the effects of ambient temperature on ignition delay and combustion characteristics for various ambient pressures. Three kinds of GTL fuels with different distillation properties and their blends in gas-oil (conventional diesel fuel) were tested. The experimental results showed that all tested fuels exhibited similar ignition-delay trends: ignition delay increased as ambient temperature and ambient pressure decreased. The variation of ignition-delay values was small at temperatures higher than 700 K but large at temperatures less than 700 K. In addition, the results showed that the ignition-delay trends of GTL fuels depended significantly on distillation properties. GTL fuels with high cetane number corresponded to shorter ignition delay and smoother heat-release rate than those for gas-oil at the same temperature and pressure. Particularly, their good ignitabilities at low temperature might make it feasible to premixed-charge compression-ignition (PCCI) operations. In addition, the shadowgraph images showed that GTL fuels evaporated and mixed with the hot air quicker than gas-oil. Moreover, the blend GTL fuel helped improve ignition and combustion compared to gas-oil. The obtained results contribute to find the optimal condition of design and operation in diesel engines fuelled by GTL fuels. chemical reaction in which a mixture of carbon monoxide and hydrogen is converted into liquid hydrocarbons of various forms.2 Synthetic fuels are classified based on the feedstock that is used to create them. At the moment, the three most prominent processes are gas-to-liquids (GTL),3,4 coal-toliquids (CTL),5,6 and biomass-to-liquids (BTL).7,8 Gas-to-liquid fuel is a synthetic ultraclean diesel fuel, synthesized from natural gas (NG) via the Fischer-Tropsch process. Gas-to-liquid products are virtually pure paraffinic hydrocarbons that have excellent combustion properties and burn with a smooth, controlled flame.2 Produced from NG, GTL fuel has many advantages in comparison with other alternative diesel fuels. First, it is made from NG, a clean and versatile fuel with abundant reserves around the world, and for this reason, it can be guaranteed to supply the market at a large scale. Second, GTL fuel has virtually zero sulfur and low aromatics. These advantages offer a significant improvement in air quality and ensure that the use of GTL fuel in diesel engines can satisfy future stringent regulations.9,10
1. Introduction Fossil fuels have played a crucial role in the world energy market. According to the estimation of the Energy Information Administration in 2005, burning fossil fuels produced 86% of the world’s energy. In the last three decades, the world has been confronted with energy crises owing to the decrease of fossil resources, along with the increase in environment constraints and prices of oil. Lowering of world carbon dioxide emissions to reduce the risk of climate change requires a major restructuring of the energy systems. This situation consequently brings the search of alternatives and renewable fuels, which have to be not only sustainable, but also friendly with respect to the environment as well as techno-economically competitive. Gaseous fuels of hydrogen, natural gas, and biogas and liquid fuels such as ethanol, vegetable oil, biomass, synthetic fuels, and biodiesel fuel are starting to be of high interest to the developed countries.1 Nevertheless, the application of alternative fuels is facing many difficulties such as storage, delivery, and amount of fuel supplied for a large-scale approach. Among these, gas-to-liquid (GTL) synthetic fuels are considered to be some of the most promising fuels, since they can replace conventional petroleum and reduce tailpipe emissions. Synthetics fuels are liquid fuels produced from coal, natural gas, or biomass through the Fischer-Tropsch process. This process has been developed for almost a century, since the 1920s by the German researchers Franz Fischer and Hans Tropsch. Basically, the Fischer-Tropsch process is a catalyzed
(2) Alleman T. L.; McCormick, R. SAE Tech. Pap. 2003-01-0763, 2003. (3) Hall, K. R. Catal. Today 2005, 106, 243–246. (4) Vosloo, A. C. Fuel Process. Technol. 2001, 71, 149–155. (5) Mantripragada, H. C.; Rubina, E. S. Energy Procedia 2009, 1, 4331–4338. (6) Steynberg, A. P.; Nel, H. G. Fuel 2004, 83, 765–770. (7) Tijmensen, M. J. A.; Faaij, A. P. C.; Hamelinck, C. N.; van Hardeveld, M. R. M. Biomass Bioenergy 2002, 23, 129–152. (8) Lapidus, A.; Krylova, A.; Paushkin, Y.; Rathousky, J.; Zukal, A.; Starek, J. Fuel 1994, 73, 583–590. (9) Jonson, J. W.; Berlowitz, P. J.; Ryan, D. F.; Wittenbrink, R. J.; Genetti, W. B.; Ansell, L. L.; Kwon, Y.; Rickeard, D. SAE Tech. Pap. 2001-01-3518, 2001. (10) Kitano, K.; Ichiro, S.; Richard, C. SAE Tech. Pap. 2005-013763, 2005.
*To whom correspondence should be addressed. Telephone/Fax: þ81-75-753-5249. E-mail:
[email protected]. (1) Coronado, C. R.; Carvalho, J. A. D., Jr.; Yoshioka, J. T.; Silveira, J. L. Appl. Therm. Eng. 2009, 29, 1887–1892. r 2009 American Chemical Society
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Furthermore, GTL fuel can be used directly on existing diesel engines without engine modifications. The advantages of GTL fuel have attracted many researchers to apply GTL fuel for engines and vehicles.11-15 In most papers, the results showed that GTL fuel reduced exhaust gas emissions of carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter compared to gas-oil (conventional diesel fuel). Normally, the research started from a short-term test, where a diesel engine was switched to run with GTL fuel for collecting the performance and emission data. From the obtained results, the engine was modified to achieve higher performance and to reduce exhaust gas emissions.16 However, those papers did not provide sufficient data for analyzing the effects of physical and chemical properties of GTL fuels on ignition and combustion before adapting it for diesel engines. Cetane number is normally used to measure ignition quality of liquid fuels in diesel engines.17 A higher cetane number fuel normally exhibits more complete combustion. The ignition delay is a very important parameter in determining diesel engine characteristics such as engine performance, fuel conversion efficiency, exhaust emissions, and misfire limits. Ignition delay is strongly dependent on cetane number, ambient temperature, ambient pressure, oxygen mole-fraction, and properties of the fuel.18 Gas-to-liquid fuel has a higher cetane number and a lower autoignition temperature than gas-oil. These characteristics have important effects on ignition delay and combustion characteristics in diesel engines. From this point of view, a thorough understanding of the effect of GTL properties on ignition delay and combustion characteristics for diesel engines was very important. Therefore, the main objective of the present research was to obtain fundamental data regarding the ignition delay and combustion characteristics of GTL fuels under direct-injection compression-ignition conditions. Experiments were conducted in a constant-volume vessel under a variety of conditions, including ambient temperatures (Ti) and ambient pressures (pi). Three special GTL fuels with different distillation properties were used to investigate theirs physical and chemical effects on the ignition and combustion. In particular, based on the obtained results, the mechanisms of ignition and combustion characteristics in GTL fuels were discussed with the emphases on mixture formation and heat release rate.
Figure 1. Cross-section of constant-volume combustion vessel.
experimental facility was similar to the previous study for gaseous fuel.19 Figure 1 shows a schematic of the cross section of the combustion chamber. The combustion chamber is a circular cylinder, 80 mm in diameter, 30 mm in depth, and approximately 150 cm3 in volume. The two quartz windows fitted on both sides of the chamber allow full optical assessment. The chamber had six important parts, necessary to simulate diesel combustion conditions. The first one was a manual intake valve used to charge and adjust the initial combustible premixed gas in the evacuated chamber. The second one was an exhaust valve for removing burned-gas. The port from the exhaust valve was connected to a vacuum pump to evacuate the combustion chamber. The third one was a conventional spark ignition system for igniting premixed gas in the combustion chamber. The fourth one was a stirrer (or mixing fan) used to maintain a uniform temperature in the chamber before fuel injection; stirrer speed was then kept constant at 8500 rpm in this study. The fifth one was a pressure transducer, piezoelectric absolute-pressure transducer (Kistler 6052A), used to record combustion pressure and corresponds to calculate ignition delay and heat-release rate. The last part was a single-shot injector, mounted at the top of the chamber, used to injected liquid fuel into the combustion chamber. A pressure-storage type injection system, a modified Denso’s common-rail injection system, was used to drive and control the injected fuel. This system could inject the fuel at pressures of 40-120 MPa. Reacting liquid spray penetration and flame development were visually studied using a high-speed shadowgraph using a CMOS digital camera (FASTCAM SA1.1). To reduce vapor condensation on the combustion windows and to obtain high-quality shadowgraph photos, these windows were heated to 230 °C in an oven before setting at every experiment. In this research, photographs were captured at a frame rate of 30 000 fps, a shutter speed of 14 μs, and a resolution of 400 400 pixels. 2.2. GTL Fuel Properties. It is generally known that removing high boiling point components from fuel can decrease the unburnt emissions from the engines.20 With the purpose of using GTL fuel to improve combustion efficiency and to further emissions reduction, tested fuels were produced using the distillation temperature as a parameter, and the effects on the ignition and combustion were investigated. In this research, three GTL fuels were produced and supplied by Showa Shell Sekiyu K.K. to clarify the effect of fuel properties. The research was also conducted with a blend fuel of GTL and gas-oil or diesel fuel (JIS No.2). The blended fuel (BGTL) was produced by mixing by volume 20% GTL1 and 80% gas-oil. In addition, standard gas-oil was tested as a reference fuel.
2. Experimental Setup and Procedure 2.1. Constant-Volume Vessel. In our present study, experiments on ignition and combustion for various GTL fuels were conducted using a constant-volume combustion vessel. This (11) Akio, Y.; Yukihiro, T.; Toshiaki, T. SAE Tech. Pap. 2004-012960, 2004. (12) Xinling, L.; Zhen, H. Sci. Total Environ. 2009, 407, 2234–2244. (13) Alleman, T. L.; Eudy, L.; Miyasato, M.; Oshinuga, A.; Allison, S.; Corcoran, T.; Chatterjee, S.; Jacobs, T.; Cherrillo, R. A.; Clark, R.; Virrels, I.; Nine, R.; Wayne, S.; Lansing, R. SAE Tech. Pap. 2004-012959, 2004. (14) Nakajima, T.; Nishiumi, R.; Kitano, K.; Sakata, I.; Clark, R. JSAE Tech. Pap. 2008-54-01, 2008. (15) Alleman, T. L.; Tennant, C. J.; Hayes, R. R.; Miyasato, M.; Oshinuga, A.; Barton, G.; Rumminger, M.; Duggal, V.; Nelson, C.; May, M.; Cherrillo, R. A. SAE Tech. Pap. 2005-01-3766, 2005. (16) Alleman, T. L.; Barnitt, R. A.; Eudy, L.; Miyasato, M.; Oshinuga, A.; Corcoran, T. P.; Chatterjee, S.; Jacobs, T.; Cherrillo, R. A.; Clark, N. N.; Wayne, S. SAE Tech. Pap. 2005-01-3769, 2005. (17) Siebers, D. L. SAE Tech. Pap. 852102, 1985. (18) Heywood, J. B. Internal Combsution Engine Fundamentals; McGraw-Hill: New York, 1988. (19) Ishiyama, T.; Shioji, M.; Ihara, T.; Inoue, N. SAE Tech. Pap. 2003-01-1922, 2003.
(20) Kitano, K.; Mori, M.; Sakata, I.; Clark, R. SAE Tech. Pap. 2007-01-2004, 2007.
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Energy Fuels 2010, 24, 365–374
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Nguyen et al. Table 1. Fuel Properties
fuel analysis density at 15 °C (g/cm ) viscosity at 30 °C (mm2/s) flash point (°C) flow point (°C) cloudy point (°C) CFPP (°C) 10% distillation (°C) 50% distillation (°C) 90% distillation (°C) sulfur content (ppm) cetane number C (wt%) H (wt%) lower calorific value (MJ/kg) 3
method
GTL1
GTL2
GTL3
Gas-oil
JIS-K-2249 JIS-K-2283 JIS-K-2265 JIS-K-2269 JIS-K-2269 JIS-K-2288 JIS-K-2254 JIS-K-2254 JIS-K-2254 JIS-K-2541 JIS-K-2280 ASTM-D-5249 ASTM-D-5249 JIS-K-2279
0.7773 3.126 86.5 -35.0 -18 -18 226.5 264.5 307.0 700 K. However, at Ti < 700 K, the τ values of GTL1
Figure 7. Effects of Ti on τ for neat GTL fuels, blend GTL fuel and gas-oil at pi = 4 MPa. Graphs are separated for clarity.
tend to be shorter than that of GTL2 and GTL3. The difference in τ among GTL fuels could be caused by the change in fuel properties. The different distillation processes have modified the chemical structure and properties of these synthetic fuels. As shown in Table 1, the T10 temperature of GTL2 and GTL3 is around 190 (°C), and it is at 226.5 (°C) GTL1. In addition, the flash points of GTL2 and GTL3 are around 64 (°C), whereas it is 86.5 (°C) for GTL1. The trend of shorter ignition delays for GTL1 at Ti lower than 700 K may possibly be affected by slightly higher cetane number. In low temperature combustion at the same pressure, the cetane number has a strong effect on the variation of the ignition delay value as temperature changes. Moreover, in Figure 7, the τ values and trends of the blend BGTL fuel correspond to slightly shorter than that of gas-oil. The obtained τ of BGTL show a reasonable result of τ values owing to small amount of GTL fuel promoting the ignition. This obtained result implies that the use of blended GTL fuel in diesel engines 370
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: DOI:10.1021/ef9008532
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value can be distinguished by comparing dq/dt value in premixed combustion. At Ti = 650 K, however, the injected fuels are well mixed with air during the longer ignition delay. The blend BGTL and gas-oil have a little heat-release rate, whereas every one of the neat GTL fuels exhibits the typical course of dq/dt like the premixed combustion. This result shows the advantage of GTL fuels, with their high ignitability at low temperature combustion. The smoother combustion for GTL fuels can be observed by lower dq/dt in premixed combustion with the lower pressure and temperature increase due to combustion of GTL fuels. 3.3. Ignition Delay and Combustion Characteristics of GTL Fuels at Ambient Pressure of 2 MPa. Besides ambient temperature Ti, ambient pressure pi is another important parameter that strongly influences the ignition and the combustion. The changes of Ti and pi in a constant-volume vessel were related to the variation on compression ratio and operating conditions such as injection timing in real engine applications. To understand the effects of pi on ignition and combustion for different synthetic fuels, ambient pressure was reduced to 2 MPa for the experiment. Furthermore, the low ambient pressure of 2 MPa is to simulate for PCCI (premixed charge compression ignition) engine operation, which is part of future engine trends for low NOx emissions. The results of ignition delay for pi = 2 MPa are shown in Figure 9a and Figure 9b. Figure 9a shows the result of τ at pi = 2 MPa, which is calculated using single-stage ignition definition (first-stage ignition delay). In comparison with the result at pi = 4 MPa in Figure 7, the ignition delay trends for the different fuels are similar, in which GTL fuels exhibit much shorter τ than gasoil and the blend BGTL fuel. However, the ignition-delay value exhibits a large increase compared to the result at pi = 4 MPa in the entire temperatures. The increase of τ can be explained by a reducing chemical reaction rate in combustion vessel as pi decreases. The observation identifies that the Arrhenius ignition-delay trends at this condition are much different compared to the results at pi = 4 MPa in Figure 7. The “bend” in ignition delay curves in Figure 9a reflects the effect of cool flame and negative temperature coefficient regime on ignition and combustion for the tested fuels. More specifically, the difference of the bend (position and shape) and the difference of slope increase of Arrhenius curves suggest the different activation energy of chemical reaction. In comparison among neat GTL fuels, the ignition delay trends and values of GTL2 and GTL3 are shorter than those of GTL1 for Ti < 700 K. The lower initial boiling point, flash point, and viscosity (Table 1) of GTL2 and GTL3 make these fuels easier to evaporate at low Ti and pi conditions. On the other hand, for the comparison between BGTL and gas-oil, the ignition delay trend of BGTL exhibits a much shorter τ value than that of gas-oil at Ti < 800 K. The difference of τ at pi = 2 MPa is larger than at pi = 4 MPa. The GTL fuel with high ignitability blended to gas-oil helps promote the ignition, especially at low Ti and pi combustion conditions. The bend in the ignition delay curves is caused by changing ignition characteristics owing to cool flame effects. The definition of ignition delay for single-stage ignition or the pressure recovery definition contains limits, as it could not present an exact ignition delay value at low pi and Ti combustion conditions owing to the effects of cool flame combustion. In Figure 10, the results showed that the peak dq/dt in first-stage combustion is slightly higher at 0 MJ/s. The second-stage combustion or the hot-flame ignition is
Figure 8. Effects of Ti on dq/dt for neat GTL fuels, blend GTL fuel, and gas-oil at pi = 4 MPa.
may improve engine performance and increase combustion efficiency. Further observation shows that a reasonable τ around 1-2 ms for real engine application of the tested fuels can be obtained at Ti around 700-800 K. This temperature range seems to be a little lower than that of the TDC temperatures range in direct-injection compression-ignition engines with compression ratio 16:1, which is typically around 800 K.18 At Ti around 700 and around 800 K, the τ values of GTL1 fuel are 32 and 16% shorter than those of gas-oil. In addition, the observation result also recognizes that the misfire limit of GTL fuels with higher cetane number shift to a lower Ti around 605 K, against around 630 K for gas-oil. The lower misfire limit of GTL fuels indicates that the engine will be easy to start up, especially at cold-start conditions with low temperature. The variation of ignition-delay values affects the combustion processes of all tested fuel. Figure 8 shows the dq/dt of neat GTL fuels, blend GTL, and gas-oil at different temperatures. The observation results show that all fuels exhibit similar combustion processes. At Ti > 700 K, the typical progress of dq/dt is observed for all fuels. The premixed combustion occurs after the ignition delay. After a certain period of premixed combustion, the diffusive combustion follows. The fuel with a longer τ value indicates a higher peak dq/dt. In addition, the slight difference of ignition delay 371
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Figure 9. Effects of Ti on one- and two-stage ignition for neat GTL fuels, blend GTL fuel and gas-oil at pi = 2 MPa. Graphs are separated for clarity.
observed in the second rapid increase in dq/dt. Employing the threshold level of 0.03 MJ/s, the start of a rapid increase in dq/dt in second-stage combustion or the hot-flame ignition delay (τh) can be detected. The obtained results of τh are shown in Figure 9b. In comparison with the result of firststage ignition in Figure 9a, the difference of ignition delay exhibits only on the right side of the bend with a large increase of τh. The Arrhenius ignition delay curves are observed to be similar to the Arrhenius curves at pi = 4 MPa. In other words, the left side of the bend shows only single-stage ignition, and two-stage ignition happens within and right side of the bend. Figure 10 shows the dq/dt curves at pi = 2 MPa with different temperatures around the bend. At Ti = 900 K, the typical progress of dq/dt corresponds to all tested fuels. At Ti = 800 K, the dq/dt curves indicate the two-stage combustion for BGTL
and gas-oil while the dq/dt curves of neat GTL fuels are similar to those at Ti = 900 K. At Ti = 750 K, two-stage combustion is observed for all tested fuel. Owing to long ignition delays, the dq/dt curves of BGTL and gas-oil show only premixed combustion. At Ti = 700 K, the long ignition-delay causes a rapid dq/dt increase for neat GTL fuels, while it is a small increase in dq/dt BGTL and gas-oil owing to slow chemical reaction rate. In comparison with dq/dt at pi = 4 MPa, the slow chemical reaction rate at pi = 2 MPa can be observed by a somewhat smaller slope increase of dq/dt in premixed combustion and longer diffusive combustion duration. Combustion processes of GTL1, GTL3, and gas-oil are explained more illustratively by the shadowgraph images in Figure 11. In this figure, the first and second column photos display combustion progress in first-stage and in premixed combustion, while the next columns present flame 372
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: DOI:10.1021/ef9008532
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development in diffusive combustion, and the last column shows combustion process at the end of combustion.
In comparison between GTL1 and GTL3, the combustion processes are observed to be very similar in premixed and diffusive combustion. However, in the first column, GTL3 fuel, with a lower T90 temperature and lower viscosity, exhibits a shorter liquid phase length compared to GTL1 and gas-oil. The GTL3 fuel is also observed to be totally evaporated and to be mixed with air at a position around 60 mm from the nozzle tip. The shorter liquid phase length promotes more fuel evaporation and better fuel-air mixing. The result of thorough fuel-air mixing exhibits a somewhat brighter luminous flame in the second column photo and a little higher dq/dt in premixed combustion in comparison between GTL3 and GTL1 fuel. Combustion processes of gas-oil are very different from those of GTL1 and GTL3 fuels at this condition. The long ignition-delay values and the accumulated fuel cause a rapid increase in dq/dt. The gas-oil almost completely combusts, and its combustion starts to be extinguished at t = 7.5 ms, while the observation for GTL1 and GTL3 recognizes that it remains a burning fuel at t = 9 ms. 4. Conclusions Ignition delay and combustion characteristics of GTL fuels have been fundamentally studied in a constant-volume combustion vessel by changing temperature and pressure. The obtained results have provided useful information in rating the ignition delay of synthetic fuels. The results of neat GTL fuels have been compared to those of blend GTL and gas-oil. The results are summarized as follows: (1) Ignition delay of all tested fuels was very sensitive with the variation of temperature and pressure. A short ignition delay could be obtained by increasing Ti and pi. (2) The high cetane number and low autoignition temperature endowed GTL fuels with short ignition delay, smoother combustion, and a lower misfire limit in comparison with gas-oil. (3) The lower distillation temperature of GTL fuels showed that they easily evaporated and mixed with hot air in the combustion chamber. This
Figure 10. Effects of Ti on dq/dt for neat GTL fuels, blend GTL fuel, and gas-oil at pi = 2 MPa.
Figure 11. Shadowgraph images combustion progress for GTL1, GTL3, and gas-oil. Experiment was conducted at dN = 0.22 mm, pi = 80 MPa, rO2 = 21%, Ti = 750 K, and pi = 2 MPa.
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characteristic helped improve ignitability for GTL fuels at low Ti and pi. Furthermore, this obtained result promoted that GTL fuel could be appropriated for PCCI operation in future diesel engine application. (4) The blend of GTL fuel in gas-oil helps reduce the ignition delay. This result promotes the application of blend GTL for improving combustion
efficiency and reducing exhaust gas emission in conventional diesel engines. Acknowledgment. The authors would like to thank to Showa Shell Sekiyu K.K. for supplying GTL fuels for this research. Thank to Mr. Akihito Fujita for sharing his valuable time in experiment.
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