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Hydrogen production by Water-Gas Shift Reaction using an atmospheric steam plasma torch system with Reverse Vortex Reactor Sukhwal Ma, Daehyun Choi, Semin Chun, Sang Sik Yang, and Yongcheol Hong Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef501921k • Publication Date (Web): 03 Dec 2014 Downloaded from http://pubs.acs.org on December 5, 2014
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Hydrogen production by Water-Gas Shift Reaction using an atmospheric steam plasma torch system with Reverse Vortex Reactor †‡
†
†
‡
Suk H. Ma , , Dae H. Choi , Se M. Chun , Sang S. Yang , and Yong C. Hong∗,
†
†
Plasma Technology Research Center, National Fusion Research Institute, Gunsan 250-806, Korea ‡
Department of Electrical and Computer Engineering, Ajou University, Suwon 443-749, Korea
The water-gas shift reaction (WGSR) by using a pure steam plasma with CO as reactant was accomplished in this study. The steam plasma generated by microwaves at an atmospheric pressure provides highly active species and high temperature plasma flame about 6000K which enhancing the chemical reaction rate and eliminating the need for catalysts. We stably generated the steam plasma with the conditions of the power at 3 kW and the steam flow rate at 27-L/min with swirl flow. The energy efficiency and CO conversion were estimated. Using a cylinder type of reactor, the hydrogen 40 vol.% was produced at a given steam to carbon (S/C) ratio. For improving stability and residence time in the reaction, we designed a reverse vortex type of reactor (RVR). By using reverse vortex type of reactor (RVR), average 36 vol.% of hydrogen was produced and the stability of steam plasma was highly increased.
Keywords: WGS reaction, Microwave, Steam plasma, Hydrogen, Reverse vortex reactor
∗
Corresponding author. Tel.: +82 63 440 4110, fax: +82 63 466 7001. E-mail address:
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1. Introduction The search for environmentally benign energy sources is becoming increasingly urgent. Besides, recent advances in fuel cell technology have greatly improved the prospects of its use in electric power generation which uses hydrogen as a fuel and emits only H2O. Thus, efficient hydrogen production is very important as a clean energy source for use in fuel cells. However, hydrogen is very impractical in terms of storage and handling. The established technologies of storage hydrogen including liquid hydrogen, compressed hydrogen and advanced metal hydrides are faced with the problems to be solved. The problems cover poor energy density by volume versus hydrocarbon and insulation by design for liquid hydrogen tank to cool down the temperature.1 In spite of efforts to produce hydrogen by processes involving solar energy, wind energy, nuclear energy and biofuels, fossil fuels remain the most feasible feedstock in the near term. For a commercial scale of pure hydrogen production, steam reforming remains the most economic and efficient technology for a wide range of hydrocarbon feedstock.2 However, reforming hydrocarbon fuels to produce the needed hydrogen yields reformate streams containing CO2 as well as CO, which is toxic to the fuel cell at concentrations above 100 ppm. As the amount of CO permitted to reach the fuel cell increases, the performance of the fuel cell decreases until it ultimately stops functioning.3 Water-gas shift reaction (WGSR) is one of the well-known processes among steam reforming technologies. It is used in many industrial processes for hydrogen production to enhance the hydrogen generation and to reduce the carbon monoxide content. WGSR is a moderately exothermic reversible reaction and it is basically performed with catalysts.4,5 CO + H2O CO2 + H2, ∆H = −41.1kJ/mol
(1)
Although the equilibrium favors formation of products at lower temperatures due to its exothermic reaction, the reaction kinetics are faster at elevated temperatures. For this reason, 2 ACS Paragon Plus Environment
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the catalytic WGSR is initially carried out in a high temperature shift (HTS) reactor at 320~420oC. Conversion in the HTS reactor is limited by the equilibrium composition at the high temperature. To achieve higher conversions of CO to H2, the gas leaving the HTS reactor is cooled to 200~250oC and passed through a low temperature shift (LTS) reactor.6 For that reason, the existing WGSR reactors were difficult to be compact and minimized. In addition, the catalysts are considerably incurred expenses and environmental hazards. For example, current commercial catalysts, such as Fe-Cr, are toxic to humans and the environment, exhibit low activity, etc. As a result, there is a continued need for improved WGS catalysts that exhibit better activity in the WGS or found an alternative process.7 A pure steam torch generated by microwave radiation is one of the alternative methods to replace existing WGS reaction and to overcome its disadvantages. We observed the characteristics of steam plasma with CO gas generated by microwave through detecting emission spectrum and temperature. For the perfect combustion and high production rate of hydrogen, it should be needed to improve the gas residence time and stability of steam plasma. Thus, we designed the reverse vortex reactor (RVR) which focused on increasing gas residence time while decreasing heat losses from the reactor. We expect that using RVR leads to improve the stability of steam plasma at low steam to carbon (S/C) ratio and production rate of hydrogen.11 Here, we report the experimental results of the WGSR without catalysts by using a microwave steam plasma and compare the WGSR data between the cylinder-type reactor and reverse vortex reactor (RVR).
2. Experimental details The structure and operation of microwave plasma system has been summarized briefly. Further details are described in the references.12-14 Fig.1 (a) displays the structure of an atmospheric microwave plasma system consisting of the 2.45 GHz microwave generator, 3 ACS Paragon Plus Environment
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WR-340 waveguide components, including an isolator, a directional coupler, and a 3-stub tuner, and a microwave plasma torch as a field applicator. As shown in Fig. 1(a), the quartz holder for injecting discharge gas and reactive gas is attached to the tapered waveguide, holding the quartz tube. Fig. 1(b) shows that the microwave radiation generated from magnetron is guided through the tapered waveguide and it enters the discharge tube which consists of fused quartz. A commercially available steam generator produces 22 g of steam in a minute at an atmospheric pressure in experiments. The steam from a generator served as discharge gas and produced a stable steam plasma torch with swirl flow. The necessary steam temperature for ignition of the plasma in the discharge tube must be 150oC or higher. In order to measure the plasma emissions, we employed a monochromator (MonoRa750i, Dongwoo optron co., Ltd., Korea) with the grating (1200 and 2400 grooves/mm), an entrance slit aperture (10 µm), and a resolution (0.025 nm). The two types of reactor were used in this experiment. First reactor is cylinder type consisting of fused quartz that the length and inner diameter are 500 mm and 30 mm, respectively as shown in Fig. 1(b). The cylindrical quartz reactor is optimized for analyzing the characteristics of plasma flame including emission spectrum and temperature measurement because it is transparent and suitable for high temperature. CO as reactive gas was directly injected into the microwave-driven region as described in Fig. 1(b). One of the advantages of using cylindrical quartz reactor is that the reactive gas pass through the center of plasma where the high temperature region over 6000K with highly active chemical species. Thus, we expect that the performance of WGSR is more efficient. However, the straight flow of CO gas more than a given rate may influence on the stability of steam plasma with swirling flow, showing the disappearance of plasma. In order to overcome the problems related to the plasma stability, in this regard, we designed a reverse vortex reactor (RVR) for improving stability of steam plasma as well as gas residence time as shown in Fig. 1(c). It has 4 ACS Paragon Plus Environment
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the structure of double cylindrical tube type that the CO reactive gas enters the outer compartment through the tangential inlet which decreasing heat losses from the reactor. Fig. 1(c) indicates that the reaction gas is with vortex flow from the inlet to the torch and it mixed with the steam plasma stabilized with the vortex flow, which is of the same swirling flow direction, with no plasma disappearance. Thus, it may not affect to the stability of steam plasma which expected to improve hydrogen production. The RVR has a length of 470 mm, an outer diameter of 48.6 mm, and an inner diameter of 27.2 mm.15,16 For the accurate analysis, we cooled down the syngas through water circulating cooler and chiller which maintained the temperature under -40oC. Steam was supplied by commercial steam generator using tap water and the flow rate of steam was 27-L/min as fixed condition. Microwave power was fixed at 3 kW. We controlled the ratios of steam to carbon and injected CO gas through a mass flow controller.
3.
Results and discussion Hydrogen production via the WGS reaction by using steam plasma requires the highly
stable steam plasma. We confirmed stable flame in the cylindrical quartz tube by the condition of S/C ratio at 9 and 5.4, as shown in Fig. 2(a). However, the plasma flame became unstable with the decrease of S/C ratio (increase of CO gas flow). Fig. 2(b) displays an optical emission spectrum that identifies various excited plasma species produced from the steam plasma by the distance from the torch. The emission lines were mainly dominated by species that were produced from water dissociation; they contained the Balmer α at 656nm and γ at 434 nm of hydrogen, the hydroxyl radical (A2Σ+-X2Π) at 309 nm, and highly reactive atomic oxygen at 777 nm. These radicals mainly contribute to produce hydrogen and carbon dioxide in steam reforming reaction.17 For WGSR, we injected carbon monoxide into steam plasma. Fig. 3(c) shows its emission spectrum of the primary combustion zone that the 5 ACS Paragon Plus Environment
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radicals of CH, OH, O and H2 were detected between 300~600nm. We evaluated that the closer peak from the torch were having higher intensity of H2, OH and CH which means that steam reforming reaction occurs more at the high temperature plasma region. Water-gas shift reaction (WGSR) by pure steam plasma performs only with a pure steam and CO. Thus, other peaks of by-products such as methane or other hydrocarbon are not detected. We observed the comparison of characteristics between pure steam plasma and injecting carbon monoxide into steam plasma. The power of plasma was minimized for concerning power consumption and increasing the efficiency. The optimized power was 3 kW with stable flame of steam plasma. Generally, the energy efficiency η of the quasi-equilibrium plasma systems performed in thermal discharges is usually relatively low (less than 10-20%) considering the actual energy consumption Wplasma and reaction enthalpy ∆H, because it consumes electric power and the produced syngas should be cooled down which considerably limits energy efficiency. 25 η=
∆
(2)
We estimated the energy efficiency η of microwave steam plasma reacted with CO as reactant gas following under equation that showing 11% of energy efficiency. η =
!
"#$%&'!
= ()*+,* -'% . /0 (0℃, 1atm)
(3)
However, microwave steam plasma has advantages to countervail the energy efficiency. Among catalysts based WGSR, the response time of Kalamaras et al.8,9 used Pt/γ-Al2O3 was 5s and Pt/TiO2 catalyst was 5~10s. However, the response time of microwave steam plasma is only few micro seconds. Also, a steam torch contains highly active species enhancing the chemical reaction rate and eliminating the need for commercial catalysts. The advantages of WGSR without catalysts are not only reducing costs, also the reaction chamber can be compact due to simplified process. Moreover, it would be expected that a microwave steam 6 ACS Paragon Plus Environment
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plasma torch could provide near perfect combustion with an enlarged high temperature and large volume plasma flame by injecting CO as reactive gas.10 The produced syngas with high temperature needs to be cooled down by heat exchanger. During the cooling process, waste heat from heat exchanger is recovered and recycled for operating energy of steam generator. Thus, energy efficiency of the plasma generating system may increase while syngas is cooled down sufficiently. In addition, WGSR is commonly used as second process after SMR for reducing CO which is critical to the fuel cell and increasing efficiency of producing hydrogen. Thus, WGSR usually indicates their efficiency with CO conversion rate. Luengnaruemitchai et al.5 using Pt=CeO2 catalyst was performed at 200 - 360℃ with 18 – 85 % CO conversion rate. Maroño et al.26 using Fe-Cr based WGS catalysts was shown maximum 70 – 80 % of CO conversion at 380 - 460℃. In this regard, the CO conversion rate with the microwave steam plasma was 65 – 85 vol.% depending on steam to carbon (S/C) ratio. CO conversion rate (%) =
2! /030 /0 2! /0
x 100
(4)
The hydrogen selectivity was 32% with the condition of S/C ratio at 1.5 estimated by under equation. H2 selectivity (%) = 0.5 x
"4 +#!&*+ (,)') /0 7!8'%+ ! (,)')
x 100
(5)
In general, the optical emission of OH radicals is related to the rotational structure of diatomic gases providing information regarding the rotational temperature. Molecules in the rotational states and the neutral gas molecules are in the thermal equilibrium due to the low energies needed for rotational excitation and the short transition times. Therefore, the gas temperature of steam plasma can be obtained from the rotational temperature of OH radicals.18,19 Fig. 3(a) is the optical emission spectra at 305 - 312 nm showing the temperature of OH radicals measured by an optical emission spectroscopy. The experimental data of the 7 ACS Paragon Plus Environment
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optical signals were obtained through an optical fiber placed near a specified portion of z in the plasma torch as shown in Fig. 1(b). In Fig. 3(a), the bold line is measured and the dotted line is simulated using a commercial program, Spec Air. Here z, represents the distance from the center of torch, where the base of the torch is located inside the waveguide, designating z = 0 as the center of the flame. The temperature of steam plasma was about 6000K that it is enough to perform the WGS reaction without using catalysts or additional heat source as shown in Fig. 2(b). In addition, the water molecules at high temperatures dissociate into OH radicals and hydrogen atoms according to H2O OH + H with a bimolecular reaction αN = 2.66 x 10-7 exp(-57491/T) cm3/molecules, which is several orders higher than other dissociative reaction constants of water at high temperature.18,20 H2O OH + H
(6)
The produced OH radicals by high temperature steam plasma are important in establishing the water-gas shift reaction. OH radicals are highly reactive and enhancing the chemical reactions.21-24 OH + CO H + CO2
(7)
H + H2O H2 + OH
(8)
Fig. 4(a) shows that hydrogen was produced about 40 vol.% with the condition of S/C ratio at 9. On the other hand, a small amount of hydrogen ratio was decreased by decreasing S/C ratio. We designed reverse vortex reactor for the purpose of improving stability and residence time which expected to increase productivity of hydrogen. Fig. 4(b) displays that it has relatively lower concentration of hydrogen about 3 vol.% gap compared with the cylinder type. It is because of residence time of reactive gas to expose the center area of plasma where the high temperature zone (over 6000K). In case of cylindrical type, injecting CO as reactive gas into backside of holder passed through center of plasma, but CO into RVR swirled along 8 ACS Paragon Plus Environment
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the inner surface with steam. By contrast, the stability of steam plasma was greatly improved using RVR. Therefore, we could measure the gas composition data with the condition of S/C ratio at 1.5, as shown in Fig. 4(b). According to the WGS reaction, CO and H2O converted to CO2 and H2. However, carbon dioxide was decomposed to carbon monoxide again due to high temperature of steam plasma. In order to reduce carbon monoxide among the syngas and to increase efficiency of hydrogen production, it is necessary to find appropriate position of injecting CO in a relatively low temperature region. These experimental results indicate that WGS reaction by using steam plasma injecting CO with reverse vortex flows can be effective for producing hydrogen and reducing carbon monoxide without using catalysts.
4. Conclusions We overcome the disadvantages of existing WGS reaction including using commercial catalysts and complex structure of the reactor. Moreover, we improved the stability problem of steam plasma through designing reverse vortex type. The steam plasma was successfully generated with the conditions of the power at 3 kW and the steam flow rate at 27-L/min. We confirmed that hydrogen was produced average 36 vol.% by WGS reaction.
However, we
measured the concentration of carbon monoxide which was unexpectedly high. It is because of that the high temperature of plasma highly affects to the productivity of hydrogen and CO concentration. Accordingly, further research is necessary to develop the improved reactor and to find appropriate position of injecting CO by temperature. Overall, the results of this study suggest that WGS reaction by using steam plasma can be useful as an effective method in steam reforming process.
Acknowledgment 9 ACS Paragon Plus Environment
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This work was supported by 2013 R&D Convergence Program funded by Korea Research Council of Fundamental Science & Technology (R&D Convergence-13-5-NFRI). This work was also supported in part by the Degree & Research Center Program of the Korea Research Council of Fundamental Science and Technology.
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Figure Captions
Fig. 1. (a) Configuration of WGSR system with the steam microwave plasma torch. (b) Schematic diagram of WGSR system with cylindrical quartz reactor indicating emission detecting points (c) the drawing of the reverse vortex reactor (RVR). Fig. 2. (a) Photographs showing the stable steam plasma flame by different steam to carbon (S/C) ratio. Emission spectra showing the plasma emission lines of (b) pure steam microwave plasma and (c) pure steam plasma with CO gas. Fig. 3. (a) Optical emission spectra of OH molecules at 305 – 312 nm. The bold line was from the experimental measurements and the dotted line was obtained from the commercial simulation program. (b) The temperature plot of steam plasma along with the axial distance z from the center for the plasma flame at 3 kW. Fig. 4. Gas composition of WGSR using (a) the cylindrical quartz reactor using (b) the reverse vortex reactor (RVR).
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Figures
(a)
(b)
(c) Fig. 1
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(a)
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measurement simulation Temp= 6691K
1.0
Emission Intensity
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0.8
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Wavelength (nm) (a)
(b) Fig. 3
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(a)
(b) Fig. 4
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