Study on Hydrogen Formation Method through Carbon Monoxide by DME Reforming without Catalyst

Transcription

1 Study on Hydrogen Formation Method through Carbon Monoxide by DME Reforming without Catalyst Shunta Sagawa, Yuji Nakamura, Hiroyuki Ito Division of Mechanical and Space Engineering, Graduate School of Engineering Hokkaido University, Sapporo, Japan Abstract: Pyrolysis and partial oxidation characteristics of DME (dimethyl ether) have been investigated experimentally by using heated reactor. Final goal of the present study is to develop a new fuel reforming scheme of DME through CO (carbon monoxide) conversion without catalyst assistance. Targeted working temperature is below 300 degrees C, which is below the conventional working temperature of DME reforming by way of catalytic reactions. Experiments were performed with a 1500 mm-long, heated reaction tube and product gas species were analyzed by gas chromatograph. Mass flow rate of mixture, tested gas composition and preheated temperature were considered as experimental parameters. Two major findings were obtained: 1) pure DME starts to pyrolyze at 175 degrees C (without oxidizer), which is much lower than that of other conventional hydrocarbon fuels, 2) large amount of CO production in partial oxidation of DME is noted even below 300 degrees C of preheated temperature. Latter result suggests that a considerable amount of hydrogen could be produced by DME reforming without catalyst because CO can easily be converted to hydrogen through the so called water gas shift reaction: CO + H2O CO2 + H2 even below 250 degrees C. Although further study is needed to show the achievable performance, the possibility to form hydrogen from DME without catalyst is successfully revealed. Keywords: DME (dimethyl ether), reforming, CO (carbon monoxide), hydrogen formation 1. INTRODUCTION 1.1. Feature of DME Development of hydrogen fuel cell is expected as one of candidates to change the fossil-fuel-dependent world and to build up the sustainable society. Nonetheless, there is huge problem to consider the pure hydrogen as fuel; that is, its handling cost and safety. Hydrogen is quite difficult to liquefy, thus the system cost for storing and handling could be rather expensive. Additionally, since wide flammable range of the mixture is attained [1], the risk of explosion is vital. LNG (liquefied natural gas) and methanol are often considered for the fuel cell system as substitutions. In this study, we focus on DME (dimethyl ether) as hydrogen carrier in fuel cell system and the capability of reforming without catalyst would be discussed as main issue in this paper. DME has a preferable feature as the conventional fuel over LNG and methanol. The cost for storing and handling are comparatively inexpensive because DME can be liquefied easily (at minus 25 degrees C or 0.6 MPa). Additionally, DME is low toxity and safe for human body. Furthermore, it has been reported that DME Table 1 Physical properties of DME [1,2] Chemical formula CH3OCH3 Melting point C Boiling point C Ignition temperature 350 C Pyrolysis temperature 100~200 C can easily decompose (or pyrolyze) at low temperature [2]. In other words, DME has high reactivity at low temperature, which is suitable for the reforming with small amount of energy input. This reactivity is one of the key issues in our proposal by the way. Physical properties of DME are listed in Table Current DME-utilizing techniques Catalytic reaction is often utilized for gas reforming process in order to reduce the reaction-activated temperature. To this date, a variety of studies on DME steam reforming with catalyst have been reported [3-6]. One of recent major technique is utilizing methanol as source fuel to produce hydrogen. According to the report by Tanaka et al. [4], DME reforming process consists of two stages: at first stage DME is converted to methanol via hydrolysis reaction with γ-al2o3 catalyst (CH3OCH3 + H2O 2CH3OH), then at the following second stage, methanol is reformed by hot steam with Cu-based catalyst (CH3OH + H2O 3H2 + CO2). By this technique, 90 % of DME can be converted to form hydrogen at 350 degrees C. However, catalyst has indispensable disadvantage, namely, high cost, a difficulty in its maintenance and the stability [7,8]. If we could have new reforming technique without catalyst, it might be quite helpful for hydrogen society. In addition, it could be nice if we could reduce its working temperature below 300 degrees C to save energy in some senses Proposed scheme and objectives Our proposed path from DME to hydrogen is different from what is considered in DME steam reforming as mentioned above. Since DME reforming through metha- Corresponding author: Y. Nakamura, yuji-mg@eng.hokudai.ac.jp 277

2 Fig.1 Hydrogen formation path from DME nol needs high reaction temperature, we propose the path through CO which enable to reduce the working temperature. Our proposed path is shown in Fig.1. First, DME pyrolyze and partially oxidize to form methyl radical and formaldehyde. They are converted to CO by partial oxidation. It is well known that CO can easily be converted to hydrogen through the following water gas shift reaction: CO + H2O CO2 + H2O under 250 degrees C. If a considerable amount of CO formation is attained below 300 degrees C, this system has great advantage in the working condition: only need is below 300 degrees C, which is equivalent to waste heat in most of industries [9]. In addition, we do not need catalyst during the process so that the system is quite robust and no maintenance is required. In this paper, we set the target to form a large amount of CO by DME reforming without catalyst assistance below 300 degrees C and explore the feasibility of our proposed system. 2. EXPERIMENT Figs.2 and 3 show the photograph and the schematic of experimental apparatus used in this study, respectively. Reaction tube made by Pyrex is 12 mm of inner diameter and 1500 mm in length, and has three access branches for gas-sampling or temperature measurement in 500 mm, 1000 mm and 1500 mm, respectively. Thermocouple (HAKKO HTK0219) is used for temperature measurement. Gas analysis is performed with gas chromatography (GC) system (SHIMADZU GC-8A) with porapak T 50/80 and molecular sieve 5A. DME (CH3OCH3), CO, CO2, HCHO, CH4, CH3OH, H2O, H2, O2 and N2 can be detected with the current system. Flow rate of DME is controlled by the mass flow controller (KOFLOC /4VCR-DME-15SCCM). The mixture flows to the reaction tube heated by ribbon heater. Sampling gas is collected by the syringe, then injected to GC for each test. The typical chromatograms obtained in this study are shown in Figs.4 (molecular sieve 5A) and 5 (porapak T50/80), respectively. As seen in the figure, clear signals are detected, suggesting that high signal-to-noise ratio and enough reproducibility are expected. The area is controlled automatically by GC integrator (SHIMADZU C-R7A plus). Fig.4 Typical chromatogram given by molecular sieve 5A (DME partial oxidation at 250 degrees C and 1.0 of O2-to-DME molar ratio) Fig.2 Direct photograph of experimental apparatus Fig.3 Schematic of experimental apparatus Fig.5 Typical chromatogram given by porapak T50/80 (DME partial oxidation at 250 degrees C and 1.0 of O2-to-DME molar ratio) 278

3 3. RESULTS AND DISCUSSION 3.1. Pyrolysis temperature Produced amount of formaldehyde by way of DME pyrolysis under various reaction temperatures is shown in Fig.6. Formaldehyde is the first detected element in DME pyrolysis processes. From the figure, a formation of formaldehyde starts at 175 degrees C, indicating that DME starts to pyrolyze at that temperature. Since this temperature is much lower than that of other hydrocarbon fuels [10-12], it has been confirmed that DME has a high reactivity at low temperature. In other words, DME may not actually need the catalyst to be chemically-active state Partial oxidation Mixture effects The product gas composition with partial oxidation treatment at 275 degrees C of various O2-to-DME molar ratios (air flow rate is fixed at 40 ml/min, DME flow rate is 8.4, 4.2, 2.8 ml/min: O2-to-DME molar ratio is 1.0, 2.0, 3.0, respectively) is shown in Fig.7. In this figure, only carbon-related species are shown for convenient purpose to see how much the carbon in DME is converted to which species. It is found that more DME is converted to the other chemical compounds as the O2-to-DME molar ratio increases. Additionally, it is notified that CO is the major product under the condition studied. CO formation ratio, defined as the ratio of produced CO (mole) to consumed DME (mole), is 1.40, 1.38, 1.35 for case (a)-(c) in Fig.7, respectively. The result indicates that oxidizer-riched condition promote DME consumption, whereas depress CO formation ratio. Imposed condition of mixture should be done depending on what is the main interest by user Preheating effects The product gas composition with partial oxidation treatment at stoichiometric mixture ratio (O2-to-DME molar ratio is 3.0) under the various reaction temperatures (225, 250,275 degrees C) is shown in Fig.8. As noted, higher temperature could bring higher DME consumption. CO formation ratio shows the same trend, but this tendency is changed depending on the mixture ratio as explained in next DME reforming ratio and CO formation Fig.9 shows contour map of DME conversion ratio in temperature-mixture plane. Here, DME conversion ratio is defined by following formula: Fig.6 The area in chromatogram of formaldehyde formed by DME pyrolysis under various reaction temperatures DME consumption [mol] DME conversion ratio = DME input [mol] As indicated, DME conversion ratio becomes higher in Fig.7 The product gas composition with partial oxidation treatment at 275 degrees C of various O2-to-DME molar ratios of (a)1.0, (b)2.0, (c)3.0 Fig.8 The product gas composition at stoichiometric mixture ratio under the various reaction temperatures: (a)225, (b) 250, (c)275 degrees C 279

4 upper-right in the map; that is, higher O2 concentration and higher reaction temperature. Fig.10 shows contour map of CO formation ratio in temperature-mixture plane. As the same manner, CO formation ratio is defined by following formula: CO formation ratio = CO formation [mol] DME consumption [mol] At this time, CO formation ratio becomes higher in upper-left in the map; that is, lower O2 concentration and higher reaction temperature. Note that above trend is somewhat inconsistent in lower preheated temperature condition; for example, CO formation ratio actually decreases with 1.0 of O2-to-DME ratio and 225 degrees C of preheated temperature. At that condition, we could observe methanol as the product, which is much smaller in the other conditions, and it might be some effects to induce such inconsistency. We will dig it deeper in near future. Our target is high CO formation ratio and high DME conversion ratio, but two ratio tendencies are just opposite, as stated. When it is considered that CO formation ratio is more important, best condition is at 1.0 of O2-to-DME ratio and 250 degrees C of preheated temperature; where 1.45 of CO formation ratio is detected. 4. POSSIBLE CHEMICAL PATH OF DME PYROLYSIS AND OXIDATION DME could decompose in relatively low temperature, then it is suspected to be converted to lower-class of hydrocarbons (HCs). If we assume that it mainly goes to C1, production processes of CO as seen in Figs.7 and 8 would be somewhat similar to what observed in the partial oxidation process of methane [13]. First, methane goes to CH3 via dehydrogenation reaction. Produced CH3 is oxidized to form CH3O or HCHO, then CH3O are finally converted to HCHO. HCHO easily breaks to form CHO and CO through partial oxidation. Finally, CO2 is formed from CO oxidation. In this sense, the production paths from DME to CH3 and HCHO are quite important to understand the CO formation processes in the current system. In the followings, two possible paths with the DME consumption reactions are listed as its candidate. First one is DME consumption via pyrolysis: CH3OCH3 CH3 + CH3O This path is considered by Batt et al. [2]. As described above, CH3O can easily be converted to HCHO. Although it is understood that C-O bond is easy to break, our experimental results indicate that things are not so simple. DME conversion ratio by pyrolysis experiment at 250 degrees C is less than 1.0 % but it increases to 47.6% under oxidized environment (O2-to-DME molar ratio is 3.0). This fact implies that pyrolysis process is not the only responsible to form methyl radical and CH3O. Therefore oxidation reaction would play a role on somehow. Second is DME consumption via oxidation: CH3OCH3 + O2 CH3OCH2 + HO2 CH3OCH2 CH3 + HCHO C-O bond is easy to break as mentioned, but breakage of C-H bond might be also easy to occur [14,15]. Some other paths through CH3OCH2 are also proposed [16,17]. According to the report by Hidaka et al. [16], H radical is formed by DME partial oxidation and could affect the formation of CH3OCH2: CH3OCH3 + H CH3OCH2 + H2 In future, we will examine the reaction paths of CO formation by precise simulations. 5. CONCLUSION A new concept of hydrogen formation by DME reforming through CO without catalyst (instead, with mild preheat and partial oxidation) has been proposed. Pyrolysis and partial oxidation characteristics of DME with mild preheated treatment have been investigated experimentally. DME starts to pyrolyze at 175 degrees C or less and this temperature is much lower as compared with that of other hydrocarbon fuels as well as the working temperature for catalyst-assisted reforming. DME conversion ratio becomes high at high O2 concentration and high preheating temperature. CO formation ratio, on the other hand, becomes high at low O2 concentration and high temperature. So far 1.45 of CO formation ratio is achieved at 1.0 of O2-to-DME ratio and 250 degrees C of preheated temperature. Further study is required to look into detail chemical paths during the conversion process. Fig.9 DME conversion ratio in O2-to-DME molar ratio-preheated temperature plane Fig.10 CO formation ratio in O2-to-DME molar ratio-preheated temperature plane 280

5 ACKNOWLEDGEMENTS This work is partially supported by 2006 Hokkaido Gas Research Foundation, The Iwatani Naoji Foundation's Research Grant, General Sekiyu Research & Development Encouragement & Assistance Foundation, Suzuki Foundation. Authors would like to express sincere thanks for their supports. REFERENCES 1. Japan DME Forum, DME Handbook, ohmsha (2006), pp (in Japanese). 2. L. Batt, G. A. Salinas, I. A. B. Reid, The Combustion Institute, 1982, pp V. V. Galvita, G. L. Semin, V. D. Belyaev, T. M. Yurieva, V. A. Sobyanin, Applied Catalysis A: General, 216 (2001), pp Y. Tanaka, R. Kikuchi, T. Takeguchi, K. Eguchi, Applied Catalysis B: Environmental, 57 (2005), pp K. Takeishi, H. Suzuki, Applied Catalysis A: General, 260 (2004), pp M. Nilsson, P. Jozsa, L. J. Pettersson, Applied Catalysis B: Environmental, 76 (2007), pp S. Petal, K. K. Pant, Journal of Power Sources, 159 (2006), pp V. Agarwal, S. Patel, K. K. Pant, Applied Catalysis A: General, 279 (2005), pp T. Akiyama, J. Jpn Inst. Energy, 86 (2007), pp (in Japanese). 10. C. Gueret, M. Daroux, F. Billaud, Chemical Engineering Science, 52 (1997), pp A. Holmen, O. Olsvik, O. A. Rokstad, Fuel Processing Technology, 42 (1995), pp Z. Renjun, L. QIANGKUN, L. Zhiyong, J. Anal. Appl. Pyrolysis, 13 (1988), pp M. Fleys, Y. Simon, P. M. Marquaire, J. Anal. Appl. Pyrolysis, 79 (2007), pp A. M. E. Nahas, T. Uchimaru, M. Sugie, K. Tokuhashi, A. Sekiya, Journal of Molecular Structure: THEOCHEM, 722 (2005), pp Y. Pan, C. Liu, Fuel Processing Technology (to appear) 16. Y. Hidaka, K. Sato, M. Yamane, Combustion and Flame, 23 (2000), pp K. Suzaki, N. Kanno, K. Tonokura, M. Koshi, K. Tsuchiya, A. Tezaki, Chemical Pyrolysis Letters, 425 (2006), pp