Energy & Fuels 2005, 19, 1561-1565
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Liquid-Phase Fuel Re-forming at Room Temperature Using Nonthermal Plasma Yoshihiko Matsui,* Souichirou Kawakami, Kazunori Takashima, Shinji Katsura, and Akira Mizuno Department of Ecological Engineering, Toyohashi University of Technology, 1-1 Tempaku-cho, Hibarigaoka, Toyohashi, Aichi, 441-8580, Japan Received September 1, 2004. Revised Manuscript Received March 29, 2005
A plasma reactor was investigated for fuel re-forming at room temperature. Aluminum or copper chips were placed between the plate electrodes in a vessel filled with oil. An intense plasma due to random sparking was generated between the chips and the plate electrodes to re-form the liquid-phase hydrocarbon fuels to gas and solid phases. 2,2,4-Trimethylpentane, hexadecane, tridecane, and diesel fuel were used as test fuels. When a pulsed square high voltage was applied, transparent hydrocarbon fuels rapidly changed to black due to formation of black powders. During the re-forming, gaseous products were also produced. Hydrogen gas concentration in the produced gas was about 60%-70% when the electrodes and chips consisted of aluminum. The generation rate of the hydrogen gas was 52 mL/min for an input energy of 32 W, with energy efficiency of 7.9 g/kWh. The production rate was proportional to the input energy. The selectivity of CH4 was higher than that of C2H4 when 2,2,4-trimethylpentane was used. On the other hand, the selectivity of C2H4 and C2H2 were higher than that of CH4 when other fuels were used. When the electrodes and chips were made of copper, the production rate was only half of that obtained by aluminum electrodes. The surfaces of the electrodes were analyzed by X-ray diffraction. This analysis demonstrated that a carbon compound (Al4C3) and sulfur compound (Cu5S8) were produced during the re-forming process.
1. Introduction Hydrocarbon fuel re-forming is a very important technology for various applications. For example, in the cracking process, it is necessary to obtain useful hydrocarbon gases and oils. Hydrogen generation from hydrocarbon fuels is also important for fuel cells. It is necessary to carry out fuel re-forming in which hydrogen is efficiently formed. Many catalytic methods were investigated for fuel re-forming to gaseous hydrocarbons and hydrogen gas. However, most catalysts include noble metals and expensive materials. Moreover, catalytic methods usually require high temperatures, around 500 to 800 °C to activate catalysts for re-forming. Decrease in operating temperature, therefore, saves energy and permits on-site re-forming. Recently, plasma and catalytic processes were studied by many researchers to re-form water,1 methane,2-5 or lower hydrocarbons. The main goal is to re-form hydrocarbon fuels, from liquid to gas phase using the plasma chemical reactions at low temperatures. Moreover, plasma methods have a possibility of fuel cracking control, producing various carbon compounds. The reforming method using induced plasma in liquid phase could be widely applicable in various fields. (1) Kabashima, H.; Einaga, H.; Futamura, S. IEEE, Trans. Ind. Appl. 2003, 39 (2 (Mar/Apr)), 340-345. (2) Okumoto, M.; Kim, H. H.; Takashima, K.; Katsura, S.; Mizuno, A. IEEE, Trans. Ind. Appl. 2001, 37 (6 (Nov/Dec)), 1618-1624. (3) Sobacchi, M. G.; Saveliev, A. V.; Fridman, A. A.; Kennedy, L. A.; Ahmed, S.; Krause, T. Hydrogen Energy 2002, 27, 635-642. (4) Nozaki, T.; Okazaki, K. Catal. Today 2004, 89 (1-2), 47-55. (5) Hammer, T.; Kappes, T.; Schiene, W. Plasma Catalytic Hybrid Reforming of Methane. Fuel Chem., Div. Prepr. 2002, 47 (1), 278.
Figure 1. Liquid-phase plasma reactor.
In this study, four kinds of hydrocarbon fuels, 2,2,4trimethylpentane (Wako), n-tridecane (Wako), n-hexadecane (Wako), and diesel fuel (JIS2) were investigated as test fuels. Plasma in liquid fuel can produce “high temperature, high pressure, and high energy” in a limited area and can activate catalytic reactions at low temperatures. Positive square-pulsed high voltage generates strong spark plasma in the fuels. The reactor temperatures can be kept at less than 60 °C. To reduce the onset of sparking in insulating liquids, metal chips were inserted in the electrode spacing. Using metal chips, the onset voltage of the sparking can be reduced significantly. We investigated two shapes of aluminum chips and copper chips. The products of the re-forming were different with the material of the metal chip.
10.1021/ef0497816 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/20/2005
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Figure 2. Photographs of three types of metal chips.
2. Experimental Procedure 2.1. Liquid Type Plasma Reactor. We investigated the liquid type plasma reactor for fuel re-forming as shown in Figure 1. It consisted of a plastic column vessel, two metal plates, and metal chips in the electrode spacing. The height and diameter of the column vessel were 113 mm and 70 mm, respectively. Two aluminum plates (or copper plates) were placed at both sides of the vessel: one was a high-voltage electrode and the
Figure 3. Experiment setup.
Matsui et al. other was a grounded electrode, and the electrodes gap was 62 mm. The reactor was filled with 100 mL of sample fuels. A magnetic stirrer and a stirring bar were placed in the reactor to stir the oil and the metal chips. Figure 2 shows three types of metal chips. The role type aluminum chips, rumpled aluminum foil, or copper chips were put into the liquid-phase plasma reactor, and their effect on the fuel re-forming was investigated. 2.2. Experimental Setup and Analysis Apparatus. Figure 3 shows the experimental setup in this study. The plasma was produced by positive pulsed square high voltage generated by a rotary spark gap (RSG) switch. Gas and black powder were generated from the fuel during the plasma application. Gas production rate was measured by a bubble soap method using a 1-mL pipet, and the gas was sampled with a gasbag. H2, N2, O2, CO, CO2, and other low molecular hydrocarbons were analyzed by GC-FID (Shimazu 14B) equipped with a metanizer (GL Science MT-21), GC-TCD (Shimazu 8A), and FT-IR (Bio-rad FTS3000). Black powder was observed using E-SEM (E-SEM-2700) and analyzed by X-ray microanalyzer (HORIBA EDX) and X-ray diffraction (XRD; RINT 2500) after evaporation of liquid fuels. For the E-SEM and XRD analysis, samples were prepared as follows: carbon powder deposited on the bottom of the reactor was filtered with a quartz fiber filter (Whatman QM-A) and dried for 6 h in an oven at 200 °C. Figure 4 shows the current and applied voltage with and without the discharge. Black lines indicate the waveform with the discharge, and gray lines, without the discharge. The rise time of the applied pulsed voltage was about 40 ns, and the repetition frequency of the pulse was set to 240 Hz. The voltage duration was about 8.3 ms. Using the liquid-phase plasma reactor with metal chip, it is possible that the onset voltage of discharge drastically lowers. Continuously intense plasma can be made by an applied voltage of 8 kV between a 60 mm electrode gap. The voltage rapidly fell down and a transient increasing current appeared when the spark discharge occurred. During the rising period, the spark discharges were produced, causing a few amperes of current.
3. Results and Discussion 3.1. Re-forming of 2,2,4-Trimethylpentane Fuel. A 100 mL aliquot of 2,2,4-trimethylpentane was investigated as the test fuel for the re-forming. During the plasma application, gas and black sediment were generated. The discharge time was 60 min. The produced gas was sampled in the period of 5-20, 20-40, and 40-60
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Figure 6. Gas generation rate using plasma reactor with aluminum chips.
Figure 7. E-SEM photograph of solid product after the fuel re-forming.
Figure 4. Current and voltage waveforms using liquid-phase plasma reactor with 2,2,4-trimethylpentane.
Figure 5. Time elapse change of gaseous products from liquid phase 2,2,4-trimethylpentane using the liquid-phase plasma reactor.
min. No sampling was taken within the first 5 min after starting the discharge in order to purge the gas in the reactor. The generated gas compounds from 5 to 20 min are shown in Figure 5. Generation of gaseous compounds was represented with the following order:
H2 > CH4 . C2H4 . CO, C2H6, C3, C4 More than 60% of the gas product was H2. As for H2 and CH4, 86% of selectivity (H2 + CH4) was obtained. The H2, CH4, and CO concentrations gradually decreased with time. On the other hand, the concentrations of C2H6, C3, and C4 hydrocarbons gradually increased with time. The black powder was rapidly produced in the liquid phase after starting high-voltage application, and the amount increased with time. After the experiment, the temperature of the liquid 2,2,4trimethylpentane was increased to 55-65 °C.
The gas generation rate using the plasma reactor with aluminum chips is shown in Figure 6. The total gas flow rates including H2, CH4, and other hydrocarbons were 17 and 80 mL/min for an applied power of 10 and 32 W, respectively. In this case, H2 gas was generated at 11 and 52 mL/min, respectively. The energy efficiency was 7.9 g/kWh in the case of 52 mL/min. The gas production also proportionally increased with the input energy in this case. After the plasma re-forming experiments, the surface of both electrodes and the aluminum chips had many black spots. These aluminum chips did not have such spots before the experiment. Moreover almost all the chips decreased their size and lost their edges. Plasma re-forming also produced the black solid products. The analysis of these powders is reported in this section. Figure 7 shows the E-SEM photograph of the solid product after the fuel re-forming on the quartz fiber filter. Spherical particles of sub-micrometer to several 10 micrometers in diameter were observed. These particles were determined to be aluminum powders from the X-ray microanalyzer. A carbon peak was also detected. A large aluminum peak was observed in XRD measurements. These results indicated that 2,2,4trimethylpentane was re-formed to gas and solids by the plasma. At the same time, aluminum electrodes and chips were melted by the heat locally generated by the spark discharges. It should be noted that these reactions were induced under moderate temperature around 60 °C in the whole reactor vessel. 3.2. Chip Shape Effect for Gaseous Products. The role type aluminum chips and rumpled aluminum foil were examined in this section. The rumpled aluminum foil has a more rough surface compared with the role shape chips. These rumpled chips were used in the liquid-phase plasma reactor to replace the role type aluminum chips, aiming at more intense discharge and higher re-forming performance.
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Figure 8. Comparison of gaseous products using rumpled aluminum foil chips and role-type aluminum chips.
Figure 9. Concentrations of the product gas of hydrogen, methane, and ethylene when using various hydrocarbon fuels.
Figure 8 shows the comparison of the gaseous products between the rumpled aluminum foil chips and the role type aluminum chips. The experimental conditions were as follows: input power was kept constant at 22 W; discharge time and sampling time were set at 5-20 min after starting the high-voltage application. As shown in this figure, hydrogen, methane, and ethylene concentrations were almost the same. The energy efficiency of the hydrogen generation has also the same value, around 8.0 g/kWh. These results indicated that the shape of the chip has little influence on both the rate and concentration of generated gas. 3.3. Re-forming of Various Hydrocarbon Fuels. To examine the re-forming characteristics of actual diesel oil, two types of diesel fuel were used. One was diesel fuel containing low sulfur concentration less than 50 ppm (fuel A). The other one contained sulfur up to 500 ppm (fuel B). We expected that some of the products can be used as additives for cleaning of exhaust gas, because hydrogen and some gaseous hydrocarbons were useful additives for the catalytic NOx treatment process. Figure 9 shows ratios of hydrogen gas, methane, and ethylene in the produced gas from various hydrocarbon fuels. Comparing 2,2,4-trimethylpentane with the other hydrocarbon fuels, the production rates of CH4 and C2H4 were significantly different. CH4 generation rate may depend on the number of methyl groups. Tridecane, hexadecane, and diesel fuel (fuel A) have high selectivity for ethylene and hydrogen. This is an advantage because ethylene and hydrogen are useful additives for exhaust gas treatment in the catalytic de-NOx process.6,7 Almost the same gaseous products were obtained using fuel B. 3.4. Re-forming of the Diesel Fuel for Sulfur Removal Using Aluminum Chips and Copper Chips. The aluminum and copper chips were compared for the diesel oil re-forming. Figure 10 shows the XRD pattern of the black powders produced by re-forming diesel fuel using three (6) Matsui, Y.; Sato, S.; Takashima, K.; Katsura, S.; Mizuno, A. SAE Technical Paper 2003-01-11185. (7) Macleod, N.; Lambert, R. M. Catal. Commun. 2002, 3, 61-65.
Figure 10. XRD patterns of black carbon powder using aluminum, copper, and aluminum-copper mixed metallic fragments.
types of metal chips and electrodes. Figure 10a shows the result using aluminum electrodes and chips, Figure 10b using copper chips, and Figure 10c,d using a mixture of aluminum (2 g) and copper (9 g) chips. The copper chips are shown in Figure 2. Figure 10d shows the result of re-forming of fuel B using the mixture of aluminum and copper with the same ratio as that of c. When the aluminum chips were used, aluminum peaks and Al4C3 peaks were observed. The corresponding peaks also appeared. Moreover the sulfur compound appeared, using aluminum and copper chips mixed (Figure 10d) with diesel fuel B. These results suggested that plasma could transform sulfur compounds into solid compounds when diesel fuel contained higher sulfur, indicating a possibility to remove sulfur compounds using the plasma treatment at low temperature. The peak of sulfur compounds have never been seen by using diesel fuel B with both metal chips of aluminum and copper, respectively. When chips were made of copper, only half of the hydrogen generation was obtained compared with the aluminum chip. Regarding the oil color, 3 h later from the plasma reforming experiment, the black powders in the fuel sample were precipitated at the bottom of the vessel, and oil color was cleared up when aluminum chips were used. On the other hand, oil color was still unchanged, being black, when the copper chips were used. When the mix of aluminum and copper chips were used, the medium property was observed between Al only and Cu only.
Liquid-Phase Fuel Re-forming at Room Temperature
4. Conclusions (1) The liquid-phase plasma reactor with metal chips can be possible to induce the continuously intense plasma in insulation oils by low onset voltage of about 8 kV. (2) Using the liquid-phase plasma reactor 65%-77% hydrogen was generated when 2,2,4-trimethylpentane, n-tridecane, n-hexadecane, or diesel oil were used as the sample fuel. When 2,2,4-trimethylpentane was used as a test fuel, the generation rate of the hydrogen gas was 52 mL/min for an input energy of 32 W, with an energy efficiency of 7.9 g/kWh. (3) CH4 concentration was higher than that of C2H4 when 2,2,4-trimethylpentane was used. On the other hand, the C2H4 and C2H2 selectivity was higher than that of CH4 when the other oils, n-tridecane, n-hexadecane, or diesel oil, were used.
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(4) Black powders were also generated during the fuel re-forming. Aluminum, copper, and the chemical compounds of metal chips and carbon (Al4C3) were found from XRD analysis of black powder. The sulfur compound (Cu5S8) was also obtained when the diesel oil contained a high sulfur concentration. Acknowledgment. The authors thank Prof. Graciela Prieto from the National University of Tucuman, Argentina, for the discussion given to the work. This work was partially supported by Japan Society for the Promotion of Science Research Fellowship for Young Scientists (Grant No. 8345) from the Ministry of Education, Culture, Sports, Science and Technology. EF0497816
