Energy & Fuels 2008, 22, 123–127
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Hydrogen-Rich Gas Production from Biogas Reforming Using Plasmatron† Young N. Chun,* Hyoung W. Song, Seong C. Kim, and Mun S. Lim BK 21 Team for Hydrogen Production · Department of EnVironmental Engineering, Chosun UniVersity, Gwangju 375, Republic of Korea ReceiVed May 28, 2007. ReVised Manuscript ReceiVed August 24, 2007
The purpose of this paper was to investigate the reforming characteristics and optimum operating conditions of the plasmatron-assisted CH4-reforming reaction for the hydrogen-rich gas production. In addition, to increase the hydrogen production and methane conversion rate, parametric screening studies were conducted, in which there were the variations of the CH4 flow ratio, CO2 flow ratio, steam flow ratio, and catalyst addition in the reactor. A high-temperature plasma flame was generated by air and arc discharge. The air flow rate and input electric power were fixed at 5.1 L/min and 6.4 kW, respectively. When the CH4 flow ratio is 38.5%, the production of hydrogen was maximized and the optimal methane conversion rate was 99.2%. Under these optimal conditions, the following syngas concentrations were determined: H2, 45.4%; CO, 6.9%; CO2, 1.5%; and C2H2, 1.1%. The H2/CO ratio was 6.6; the hydrogen yield was 78.8%; and reformer thermal efficiency was 63.6%.
1. Introduction Environmental pollution is becoming increasingly more serious as a result of the rising use of fossil fuels. With the consequent strengthening focus on developing alternative and clean energy sources, research attention is increasing on the clean energy, such as hydrogen fuel. Subsequently, according to the increase of the demand for hydrogen, various research on hydrogen production technology of cheaper hydrogen than fossil fuel is now undergoing. Fuel-reforming methods include steam reforming,1 CO2 reforming,2 and catalyst reforming3,4 from a reaction point of view. However, all of these methods have experienced technological limitations and economical constraining factors. The existing reforming processes have various technical limits, such as slow ignition characteristics, requirement of high outside thermal sources because of a strong thermal absorption reaction, and catalyst poison by a small amount of sulfur compound. Therefore, as a method to overcome such limits, the plasma-reforming5 method that uses electric discharge is getting attention. Plasma reforming is considered to be a method with the potential to overcome such limitations. Because plasma is high in energy and can transmit energy to other materials without difficulty, it has suitable characteristics to react easily with low-reactant mixtures.6,7 High-temperature †
Presented at the International Conference on Bioenergy Outlook 2007, Singapore, April 26–27, 2007. * To whom correspondence should be addressed. Tel: 82-62-230-7156. Fax: 82-62-230-7156. E-mail:
[email protected]. (1) Backhands, P.; Heinzel, A.; Mathiak, J.; Roes, J. J. Power Sources 2004, 127, 294. (2) Wang, S. G.; Li, Y. W.; Lu, J. X.; He, M. Y.; Jiao, H. J. Mol. Struct. 2004, 673, 181. (3) Tsang, S. C.; Claridge, J. B.; Green, M. L. H. Catal. Today 1995, 23, 3. (4) Tiejun, W.; Jie, C.; Pengmei, L. Energy Fuels 2005, 19, 637. (5) Deminsky, M.; Jivotov, V.; Potapkin, B.; Rusanov, V. Pure Appl. Chem. 2002, 74, 413. (6) Chun, Y. N.; Song, H. W. EnViron. Eng. Sci. 2006, 23, 1017. (7) Chun, Y. N.; Kim, S. W.; Song, H. W.; Chae, J. W. J. Air Waste Manage. Assoc. 2005, 55, 430.
plasma among plasma electric discharge can produce hydrogenrich gas because it has a very high reaction by maintaining a high-density ion status through the generation of thermal plasma that is ionized gas by direct current (DC) arc electric discharge in thermal dynamic equilibrium. In addition, using plasma’s own heat at the time of reforming and internal reaction heat according to partial oxidation, it is possible to apply the wide range of flow amounts and various gas forms because of fast starting within a few seconds and the response time. In this study, a plasma torch (so-called plasmatron) was designed for the advanced reformer. The experiments were carried out to investigate the optimal operating conditions for producing hydrogen-rich gas. 2. Theory and Test Method 2.1. Reforming Reaction. Because the reactions are very diverse when chemicals react in the plasma reformer, the expected reactions are selected as eqs 1–11.8–11 In case of the catalytic reactions of eqs 10 and 11, CH4 molecules are decomposed to surface carbon species and H2 by metallic Ni sites and then surface carbon species react with adsorbed oxygen atoms [O(ad)] present as NiOx species to produce CO selectively. Partial-oxidation-reforming reaction 1 CH4 + O2 f CO + 2H2 2 Carbon-dioxide-reforming reaction CH4 + CO2 f 2CO + 2H2
(1)
(2)
Steam-reforming reaction CH4 + H2O f CO + 3H2
(3)
(8) Demirbas, A. Energy ConVers. Manage. 2002, 43 (7), 897. (9) Nishimoto, H.; Nakagawa, K.; Ikenaga, N.; Nishitani, G. M.; Ando, T.; Suzuki, T. Appl. Catal., A 2004, 264 (1), 65. (10) Sreethawong, T.; Thakonpatthanakun, P.; Chavadej, S. Int. J. Hydrogen Energy 2007, 32, 1067. (11) Lu, Y.; Xue, J.; Yu, C.; Liu, Y.; Shen, S. Appl. Catal., A 1998, 174, 121.
10.1021/ef700302z CCC: $40.75 2008 American Chemical Society Published on Web 10/09/2007
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Figure 1. Schematic of the experimental apparatus.
C + H2O f CO + H2
(4)
Plasma (cracking) reforming reaction CH4 f C + 2H2
(5)
2CH4 f C2H2 + 3H2
(6)
2CO f C + CO2
(7)
Water–gas-shift reaction CO + H2O T CO2 + H2
(8)
CO oxidation 1 CO + O2 f CO2 2 Catalytic surface reactions
(9)
CH4 + NiO f C(s) + H2
(10)
C(s) + O(ad) f CO
(11)
2.2. Reactant Gas Conversion Rate. The methane conversion rate was calculated in eq 12 by the inflow and outflow concentrations measured at the exit. η)
[
VCH4 - VCH4 in
VCH4
out
in
]
× 100
(12)
where η is the conversion rate of the CH4, VCH4 is the influx in concentration of CH4 (vol %), and VCH4 is the efflux concentration out of CH4 (vol %). 2.3. Hydrogen Yield and Reformer Thermal Efficiency. The hydrogen yield and reformer thermal efficiency are calculated by eqs 13 and 14, respectively. hydrogen yield (%) )
H2 concentration × 100 2 × initial CH4 concentration (13)
reformer thermal efficiency (%) )
m ˙ syngas LHVsyngas × 100 m ˙ CH4 LHVCH4 (14)
where m˘ syngas and m˘ CH4 are the mass flow rate of syngas and CH4 and LHVsyngas and LHVCH4 are the low heating value of syngas and CH4, respectively.
2.4. Experimental Setup. The experimental apparatus setup that was used to construct an experiment for plasma reforming is shown in Figure 1. This consists of the plasmatron reformer, power-supply device, gas/steam feeding line, and measurement/analysis line. The plasmatron reformer consists of the plasma torch and reactor. To relieve the erosion phenomenon of the electrode at high temperatures, cold water was supplied to the two electrodes of the plasma torch and used castible for internal adiabatic and insulation. The power-supply device consists of a power generator, igniter, and trigger system, and the thermal plasma of a torch type was generated by input power at 6.4 kW (160 V and 40 A) after the igniter started with an input voltage of 30 kV. Gas/steam lines supply methane and air by precise flow calculation at each mass flow controller (MFC; Bronkhorst F201AC-FAC-22-V, The Netherlands). In addition, steam is supplied to the steam generator by adjusting the water supplied from the water tank with a flow meter (B-175-X052, U.S.A.) and metering valve. Measurement/analysis lines are divided into electricity characteristic measurements, temperature measurements, and gas analysis lines. The electricity characteristic measurement is measured by a digital oscilloscope (Tektronix TDS 3052, U.S.A.). The temperature measurement consists of a K-type thermal thermocouple with a diameter of 0.3 mm and a data analysis device (Fluke 2625A, U.S.A.). The thermal plasma temperature was continually monitored at a 10 cm distance from the end of the electrode in the plasma downstream direction. Gas analysis consists of a sampling line and gas chromatograph (Varian CP-4900, The Netherlands). 2.5. Experimental Method. The gas temperature within the reactor is stabilized at around 650 °C when the plasma was stabilized by feeding air. At this time, a methane partial oxidation reaction increased the gas temperature within the reactor up to around 1100 °C, and the experimental began at that point. To simulate biogas, each CH4 and CO2 was fed to the mixer and the fuel gases were precisely adjusted for flow quantity at each MFC. After water was adjusted by the metering valve, which is possible to adjust at a steam-supply quantity converted in the steam generator, it inflows together with input methane or carbon dioxide. The temperature in the steam generator was 335 °C. A power generator supplied high-voltage electric power to make plasma in the plasmatron. The input voltage and electric current of electrical characteristics were adjusted using the knob of the power generator, and each voltage and electric current were measured by
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Table 1. Experimental Conditions
conditions
CH4 content in feed (CH4/CH4 plus air)
CO2 content in feed (CO2/CH4 plus CO2 plus air)
H2O content in feed (H2O/CH4 plus H2O plus air)
range (%)
23.7–47.1
4.8–14.2
6.1–47.5
Table 2. Experimental Results from the Reference Conditions syngas concentration (dry vol %)
H2 CO CO2 C2H2 CH4 N2 45.4 6.9 1.5 a
1.1
0.9
45.1
reformer hydrogen thermal CH4 yield conversion H2/ efficiency (%)b rate (%)a CO (%)c 99.2
6.6
78.8
63.6
Calculated by eq 12. b Calculated by eq 13. c Calculated by eq 14.
Figure 3. Effects of variation in a CO2 flow ratio.
according to the methane flow rate, carbon dioxide flow rate, steam flow rate, and catalytic effect. The experimental conditions are in Table 1.
3. Results and Discussion
Figure 2. Effects of variation in a CH4 flow ratio.
a digital oscilloscope. Produced gases were sampled at sampling ports and analyzed by a gas chromatograph. The temperatures of the gas and reactor were measured at the measuring ports at the wall of the reactor. To investigate the reforming characteristics, the experiment set a reference condition for a maximum hydrogen production. The condition was that the methane and air flow rates were 1.8 and 5.1 L/min and the input power was 6.4 kW. In addition, parametric studies were achieved to know the optimum operating condition,
This research has produced syngas, including hydrogen-rich gas, by conducting a methane-reforming reaction after fixing the air input quantity and input power at 5.1 L/min and 6.4 kW, respectively, for the stable formation of the high-temperature plasma flame. Table 2 shows the result of reforming under the reference conditions. Under the reference condition, syngas generated by results of the methane-reforming reaction is H2 at 45.4%, CO at 6.9%, CO2 at 1.5%, and C2H2 at 1.1%. The H2/ CO ratio was 6.6, and it was a higher number than the ideal H2/CO ratio of 2.0 at the time that partial oxidation reforming progressed. This is because CO has broken into carbon and CO2 by plasma reforming in eq 7, and some CO have converted into CO2 by the CO oxidation reaction in eq 9; therefore, the CO concentration has become lowered. The methane conversion rate was 99.2%, and it was discovered that most input CH4 has been converted into syngas through the reforming reaction. At this time, the hydrogen yield generated by input methane was 78.8% and the reformer thermal efficiency was 63.6%. 3.1. Effect of the CH4 Content. Figure 2 is the result of experiments by changing the methane flow ratio (CH4/CH4 plus air) into 23.7–47.1%. Figure 2a shows representative selection
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Figure 4. Effects of variations in a steam flow ratio.
gas and the H2/CO ratio in the syngas. The H2 concentration was maximized as 45.4% at the CH4 flow ratio of nearly 38.5%, and this is because the partial-oxidation-reforming reaction in eq 1 and the plasma-reforming reaction in eqs 5 and 6 are progressed in superior. The CO concentration shows 8.1% against the changes of the methane flow ratio. This is because CO generated by the partial-oxidation-reforming reaction in eq 1 is continuously converted into CO2 by the reaction in eqs 7 and 9. Therefore, the 2% of CO2 that is not input as the reactant gas is generated as syngas. The H2/CO ratio was in the range of 3.4–6.5. The reason for a higher H2/CO ratio than 2.0 is because CO conversed into CO2 by the CO-shifting reaction in eq 9. Figure 2b shows the methane conversion rate and reformer thermal efficiency. The methane conversion rate and reformer thermal efficiency were at their highest values at 99.2 and 63.6%, respectively, with a methane flow ratio of 38.5%, where the H2 generation was at its maximum. At this time, the methane conversion rate was 96%. 3.2. Effect of the CO2 Content. Figure 3 is the result of the test by changing the carbon dioxide flow ratio (CO2/CH4 plus CO2 plus air) into 4.8–14.2%. The H2 concentration was 33.5%, which was lower value than without feeding CO2 (reference condition). This is because the reverse reaction in eq 8 is progressed stronger in the status of CO2 with sufficient H2, which is the major component of syngas. Figure 3a indicates a representative selection gas and H2/CO ratio among the syngas. The H2/CO ratio gradually decreases from 6.5 to 2.3 according
Chun et al.
Figure 5. Effects of the catalyst according to variations in a CH4 flow ratio.
to the increase of the CO2 flow ratio. Figure 3b indicates a reactant gas conversion rate and reformer thermal efficiency. The methane and carbon dioxide conversion rates showed a high conversion rate at 93.3 and 95.7%, respectively, against the changes of the carbon dioxide flow ratio. However, the reformer thermal efficiency decreased from 63.6 to 37.3% because of CO2 dilution. As a result, the changes of the carbon dioxide flow ratio are high in the conversion rate of the reactant gas, but there was a limit for the generation of hydrogen-rich gas. 3.3. Effect of the Steam Content. Figure 4 is the result of the experiment by changing the steam flow ratio (H2O/CH4 plus H2O plus air) into 6.1–47.5% by adding steam after fixing the methane flow ratio at 30.2%; in that, the hydrogen generation is at its maximum in the changes of the methane flow ratio. Figure 4a shows representative selection gas and the H2/CO ratio among the syngas. The H2 concentration showed a maximum value as 50.4% with a range of 6.1–47.5% of the steam flow ratio. This is because the regular reaction progressed strongly in the steam-reforming reaction in eqs 3 and 4, as well as the steam conversion reaction in eq 8. The CO concentration reduced more than 50% by the CO-shift reaction with steam addition. In addition, the CO2 concentration increased gradually. The H2/CO ratio gradually increased up to 10 in the range of 6.7–47.5% according to the increase of the steam flow ratio. Figure 4b shows the methane conversion rate and hydrogen yield. The maximum CH4 conversion rate was 99.7% with the
Reforming of Biogas Using Plasmatron
increase of the steam flow ratio of nearly 30.2%. At this time, the hydrogen yield shows 94.8%. However, at the steam flow ratio of 30.2%, the hydrogen yield is reduced drastically. As a result, it is effective to supply steam at less than 30.2% of the steam flow ratio to produce hydrogen-rich gas. 3.4. Catalyst Effect. Figure 5 shows the reforming characteristics by charging 6 wt % nickel catalyst within the reactor at the rear end of the plasmatron under the same conditions of the changes in the methane flow ratio. Figure 5a shows representative selection gas and the H2/CO ratio among the syngas and H2 concentration. The maximum H2 concentration for catalyst reforming was at a higher value (49.2%) than the value (45.4%) without catalyst reforming. When the CH4 flow ratio increases, the difference of the H2 production was narrow. That is why the catalyst activation was lower as carbons absorbed into catalytic. The CO concentration is at a similar distribution as the case with no catalyst. The H2/ CO ratio was 4.7, even though it decreases a little bit as the methane flow ratio increases. Figure 5b shows the methane conversion rate and reformer thermal efficiency. In the case with a catalyst, the methane conversion rate and reformer thermal efficiency showed high numbers when the methane flow ratio was below 36.6%. This is because the reforming reaction is actively progressed as the catalyst activation becomes larger because of the sufficient time
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for the reforming reaction with a catalyst as the retention time of the reactant gas within the reactor increases. 4. Conclusions In this research, the optimum reforming characteristics were investigated for the generation of syngas, including hydrogenrich gas, by the reforming reaction of methane. Under the reference conditions, the synthesis gas produced by the methane reforming with a catalyst, consisted of H2 at 45.4%, CO at 6.9%, CO2 at 1.5%, and C2H2 at 1.1%. The H2/ CO ratio, CH4 conversion rate, and reformer thermal efficiency were 6.6, 99.2, and 3.6%, respectively. The results on the parametric study are as follows: the H2 concentration was maximized as 45.4% at the CH4 flow ratio of nearly 38.5%. As carbon dioxide was added, H2 was at a lower value as 33.5% than without feeding CO2, which is the reference condition. At the steam flow ratio of 30.2%, the H2 concentration showed a maximum value as 50.4%. The H2 production yield of eq 13 with a catalyst was higher as 8.2% than the noncatalyst reforming. Acknowledgment. This research was conducted with a Grant from the Korea Science Foundation (R-01-2006-000-10355-0) with funds provided by the Ministry of Science and Technology in 2006. EF700302Z
