THERMODYNAMIC EQUILIBRIUM PREDICTION FOR NATURAL GAS DRY REFORMING IN THERMAL PLASMA REFORMER

Transcription

1 Journal of the Chinese Institute of Engineers, Vol 31, No, pp (8) 891 THERMODYNAMIC EQUILIBRIUM PREDICTION FOR NATURAL GAS DRY REFORMING IN THERMAL PLASMA REFORMER Huan-Liang Tsai* and Chi-Sheng Wang ABSTRACT This paper presents the thermodynamic equilibrium predictions of natural gas dry reforming in a thermal plasma reformer using the HSC Chemistry 1 software package from a purely theoretical point of view The optimal operating condition has been set at a temperature of 8 C and a CO 2 /CH 4 mole flow rate ratio of 1 The predicted results show that molar fractions of hydrogen yield reach 436% on a wet basis for proton exchange membrane fuel cell applications For solid oxide fuel cell applications the molar fractions of total fuels in the reformate stream are over 94% on a wet basis With lower ratio of H 2 /CO and high concentration of fuel gases in the reformed synthesized gas, the reformate stream is much preferable to a solid oxide fuel cell system Key Words: thermodynamic equilibrium, dry reforming, thermal plasma reformer I INTRODUCTION Among all primary fossil fuels, natural gas is the cleanest and the most environment-friendly fuel resource in terms of its products of combustion Although it is a non-renewable energy resource, natural gas is naturally preferred as the first candidate among available fuels because of its wide availability (Dicks, 1996), high-efficiency hydrogen reforming (Ahmed and Krumpelt, 1; Brown, 1), environmental friendliness, and sufficient infrastructure for refueling, distribution, and storage Thus, natural gas will play an ever-increasing role in electric power systems in the future The chemical composition of natural gas varies according to the source and its principal component is usually methane (CH 4 ) For the past two decades catalytic reforming of CH 4 with carbon dioxide (CO 2 ), the so-called methane dry reforming (MDR), has been of great and growing interest for both industrial applications and environmental friendliness For industrial applications, the lower ratio of H 2 /CO in the reformate stream is suitable for the synthesis of valuable oxygenated derivatives, such *Corresponding author (Tel: ext 24; Fax: ; michael@maildyuedutw) H L Tsai is with the Department of Electrical Engineering, Da-Yeh University, Chang-Hua, Taiwan 1, ROC C S Wang is with the Clean Energy R&D Center, Da-Yeh University, Chang-Hua, Taiwan 1, ROC as methanol and Fischer-Tropsh syntheses From a standpoint of environmental friendliness, both CH 4 and CO 2 are known as greenhouse gases in abundance in the world The CH 4 reforming with CO 2 not only enhances the environment-friendly utilization of natural gas but contributes to the reduction of CO 2 Being more endothermic than methane stream reforming (MSR), the MDR process can be used in storing and transporting solar energy (Wörner and Tamme, 1998; Kodama et al, 1; Kodama et al, 2) in the form of chemical fuels to remote areas A major problem of the MDR reaction is continuous deactivation of catalyst with time, which is mainly due to coke deposition Many studies have focused on both material (Hou and Yashima, 4; Rynkowski, 4; Juan- Juan, 4; Roh et al, 4) and structure (Wang et al, 4; Guo et al, 4) of catalyst to improve coke resistance Some other researchers (Effendi et al, 3; Chen et al, 4; Jun et al, 4) have undertaken the problem of coke formation by optimizing the conditions of the catalyst bed In fact, natural gas is a sulfur-containing fuel Its reformate stream is primarily a mixture of H 2, CO, CO 2, CH 4, and H 2 O, and a trace of H 2 S as well All of the published studies are focused on catalytic reforming of CH 4 which poses the problems of both sulfur poisoning of and carbon deposition on the catalytic bed Sulfur compounds must be removed before the gas goes into the fuel reformer with a hydrodesulfurizer (HDS) by injecting hydrogen This

2 892 Journal of the Chinese Institute of Engineers, Vol 31, No (8) makes both design and implementation of a desulfurizing system somewhat complex We have progressively developed a stream reforming by a thermal plasma process (Wang, 3; Wang, 6a; Wang, et al, 6b; Tsai et al, 6a,b) which is a noncatalytic reforming and fuel-flexible method This reforming process combines flameless pyrolysis without combustion, non-equilibrium thermal plasma, and gas-phase superheated reforming technologies Without a catalyst being in the reformer, there are no problems of sulfur poisoning or coke deposition on the catalytic bed After reformation, the sulfur compound H 2 S in the reformate stream can easily be removed by a commercially available chemical agent like ZnO These features make the reformer much more compact, flexible enough to operate efficiently on a wide range of hydrocarbon and/or oxygenated hydrocarbon fuels, and give it better economic competitiveness Since a solid oxide fuel cell (SOFC) system can directly use hydrocarbon fuels, the main compounds of the MDR reformate stream such as H 2, CO, and CH 4 are useful fuels for SOFC-based power production systems For proton exchange membrane fuel cell (PEMFC) applications, both water gas shift (WGS) and preferential oxidation (PROX) reactors with additional water and air should be considered for the CO purification In our opinion, it is important to do thermodynamic analysis of CH 4 dry reforming in a thermal plasma reformer A limited number of studies can be found about the thermodynamic analysis of MDR reactions Wang et al (4) investigate a thermal reforming mechanism analysis using computational methods at temperatures in the range of K and 1atm It was concluded that to enhance the ability of CH 4 desorption and to enrich the activation of CO 2 are efficient methods to avoid carbon deposition for both catalytic and non-catalytic reforming processes In this paper, we adopt the HSC Chemistry 1 software package to carry out the thermodynamic equilibrium prediction of CH 4 dry reforming With these predicted working conditions, the experiments of CH 4 reforming with CO 2 will be conducted with a 1kWe thermal plasma reformer on hand in our laboratory The reformate stream is then analyzed in our hydrogen analysis laboratory immediately In order to estimate the compositions of the reformed stream, we have to first perform a thermodynamic equilibrium prediction for CH 4 dry reforming under various operating conditions Then we define the experimental plan that will be conducted utilizing a thermal plasma reformer on hand in our laboratory Finally, we will examine the experimental and predicted data for further thermodynamic analysis CH 4 dry reforming being thermodynamically favored for coke formation, we just focus on the prevention of coke formation at an optimal operating temperature with different of CH 4 fuel to CO 2 The main contribution of this paper is to perform a thermodynamic equilibrium prediction of CH 4 dry reforming in a thermal plasma reformer from a purely theoretical point of view The remainder of this paper is organized as follows For easy presentation, the thermal equilibrium prediction and some possible chemical reactions of CH 4 reforming with CO 2 are summarized in Section II In Section III, there is discussion of results obtained under various operation conditions Finally, brief conclusions are drawn in Section IV, and future directions for research are pointed out as well II THERMODYNAMIC ANALYSIS The thermodynamic equilibrium analysis of CH 4 reforming with CO 2 using thermal plasma is first conducted In our opinion, the thermodynamic equilibrium prediction should be done to forecast an optimal operating condition and some other working conditions for a thermal plasma reformer The experimental procedure for the test plan is then predefined with reference to the predicted results Without exact knowledge about possible reactions in CH 4 dry reforming, using thermal plasma reforming, we followed the minimization of Gibb s free energy to calculate the thermal equilibrium of CH 4 dry reforming reactions We adopted a computer code to calculate the molar fractions of equilibrium compounds under given operating conditions The main operating parameters are the temperature and pressure in the thermal plasma reformer, inlet temperatures of natural gas and CO 2, and their mass flow rates The inlet temperature of CH 4 and CO 2 put into the reformer is C 1 Thermodynamic Equilibrium Prediction We performed thermodynamic equilibrium predictions for CH 4 /CO 2 flow of 1/1, 1/1, 1/1, 1/17, and 1/2 The temperatures were set in the range of to 11 in C steps The controlling considerations taken into account during the thermodynamic equilibrium predictions for dry reforming of CH 4 using thermal plasma were: (1) minimizing carbon formation, (2) high concentration of hydrogen production, and (3) highly useful fuels for SOFC applications from a practical point of view The main compounds of the final reformate stream are C (solid residue), H 2, H 2 O, CO, CO 2, and CH 4 The corresponding mole fractions on a wet basis are depicted in Figs 1-6, where M and M CO2 are mole flow rates of CH 4 and CO 2, respectively Fig 1 shows that some carbon formations easily take place at lower temperatures Therefore, in order to prevent carbon deposition in the chamber of the thermal plasma reformer the temperature and M CO2 / M should be set and maintained at over 7 C and over, respectively Fig 2 reveals

3 H L Tsai and C S Wang: Natural Gas Dry Reforming in Thermal Plasma Reformer 893 C Mole Fraction (%) H 2 O Mole Fraction (%) 1 7 Fig 1 C(s) mole fractions on a wet basis at different M CO2 / M Fig 3 H 2 O mole fractions on a wet basis at different M CO2 / M H 2 Mole Fraction (%) Fig H 2 mole fractions on a wet basis at different M CO2 / Ṁ CH 4 CO Mole Fraction (%) Fig CO mole fractions on a wet basis at different M CO2 / M H 2 mole fractions at different CH 4 /CO 2 mole flow At M CO2 / M of 1, H 2 mole fractions increase with temperature and reach above 43% on a wet basis at temperatures in the range of 7-11 C For PEMFC applications, the more hydrogen selectivity and the less CO concentration the reformate stream has, the better the conversion efficiency of the reformer is The molar fraction of hydrogen yield reaches 436% on a wet basis at M CO2 / M of 1 and at temperature of 8 C On the other hand, the total mole fractions of SOFC fuels on a wet basis are shown in Fig 7 The total mole fractions on a wet basis achieve over 94% at temperatures in the range of 7- C Any operating temperature over 8 C can not effectively better the total mole fraction of SOFC fuels The higher the operating temperature is, the higher the power consumption and the less energy efficient the conversion is Therefore, an optimal operation condition of thermal plasma can be chosen at M /Ṁ CO2 CH 4 of 1 and at temperature of 8 C for SOFC application Figs 2-3 illustrate that higher of M /Ṁ CO2 CH 4 cause an increase of water and a reduction of hydrogen yield with the increase of temperature In order to unify the mass flow rate of CH 4 fuel and CO 2 as well as the temperature control design, we choose an optimal operation condition of thermal plasma at M CO2 / M of 1 and at temperature of 8 C for both PEMFC and SOFC applications Table 1 presents the mole fractions of H 2, H 2 O, CO, CO 2, and CH 4 on a wet basis at 8 C with M CO2 / M of 1

4 894 Journal of the Chinese Institute of Engineers, Vol 31, No (8) CO 2 Mole Fraction (%) Fig CH 4 Mole Fraction (%) Fig Possible Chemical Reactions 7 CO 2 mole fractions on a wet basis at different M CO2 / M CH 4 mole fractions on a wet basis at different M CO2 / M Table 1 Mole fractions of H 2, H 2 O, CO, CO 2, and CH 4 on a wet basis Mole fraction H 2 H 2 O CO CO 2 CH (%) 4 SOFC Fuels Mole Fraction (%) Fig 7 Being more environment-friendly than other fossil fuels, natural gas has become a highly desired fuel for commercial and residential energy generation applications CH 4 reforming with CO 2 for synthesized gas production involves a series of complex chemical reactions The reactant ratio, operational pressure and temperature are important factors to generate a high yield of synthesized gas without considerable side reactions and undesired by-products Since thermal plasma reforming is a process of flameless pyrolysis, several possible chemical reactions should be considered in dry reforming of CH 4 fuel from a thermodynamic point of view The thermal plasma reformer is assumed to operate at a temperature in the range of -11 C Having no knowledge of possible chemical reactions available from the limited number of literature we assume that dry reforming of CH 4 using thermal plasma should be modeled as simultaneous pyrolysis and dry reforming of CH 4 To describe the pyrolysis chemistry of natural gas, a pyrolysis dry reforming mechanism of CH 4 at high temperature is described as follows and Wet basis Total fuels mole fractions on a wet basis at different M CO2 / M CH 4 C + 2H 2 (3) 2CH 4 H 2 + 2CH 3 (4) 2CH 3 + 2CO 2 4CO + 3H 2 () In fact, the thermal direct decomposition of CH 4 begins to take place at 733K and is the main source of coke deposition (Wörner and Tamme, 1998) The main reaction of the MDR process is briefly described as CH 4 + CO 2 2CO + 2H 2 (6) The synthesized gas in the reformate stream mainly consists of some gases such as H 2, CO, CO 2, CH 4, and high-temperature steam The possible chemical reactions taking place in the dry reforming of gases are described as follows

5 H L Tsai and C S Wang: Natural Gas Dry Reforming in Thermal Plasma Reformer 89 and CO 2 + H 2 CO + H 2 O (7) CH 4 + H 2 O CO + 3H 2 (8) CO + 3H 2 CH 4 + H 2 O (9) 2CO C + CO 2 () 4CH 4 + CO H H 2 O CO + 42CO CH 4 (11) In addition, the fractions of MDR, RWGS, MSR, and methanation reactions taking place in the thermal plasma reformer theoretically approximate 869%, 19%, 2%, and 99%, respectively Eqs (7) and (8) are the main reactions of CH 4 dry reforming (Kodama et al, 2; Rynkowski et al, 4), which are called reverse water gas shift (RWGS) and methane steam reforming (MSR) reactions, respectively These two main reactions positively increase the concentration of hydrogen However, the reverse WGS reaction is favorable at higher temperatures At higher temperatures the generation of CO and H 2 O is favorable Eq (9) is called a methanation reaction that produces 1 mole of methane at the expense of 1 mole of CO and 3 moles of H 2 consumed This reaction significantly decreases the concentration of hydrogen and simultaneously increases the concentration of H 2 O Fortunately, the methanation reaction decreases with the increase of temperature The coke formation in Eq (3) is assumed as to be pure carbon deposition The analysis of carbon formation is also important, especially necessary for catalytic reforming applications It is well known that carbon deposition over the catalytic bed reduces the performance of the catalyst and leads to its rapid breakdown Eq () is called a Boudart reaction that can reduce carbon reformation by shifting the equilibrium concentration With a small increase of CO 2 concentration, coke formation can be reduced, as shown in Fig 1 Although there are four possible reactions of carbon formation from a mixture of H 2, CO, CO 2, and CH 4, we have found carbon formation increases with the increase of CO 2 and the decrease of CO This carbon formation prediction complies with the Boudard reaction of carbon formation In addition, Fig 3 unveils the reverse WGS reaction is thermodynamically favored at higher temperatures Our thermal plasma reformer is assumed to operate at temperatures in the range of -11 C It is well known that the standard free-energy change G of methanation becomes positive at temperature of C (Tsai et al, 6a,b) The methanation reaction is significantly suppressed when the temperature reaches 8 C The possible reactions including CO 2 reforming and direct thermal decomposition of CH 4 can be postulated The CH 4 reforming with CO 2 is assumed to be stoichiometrically completed With the predicted molar fractions of thermodynamic equilibrium at M CO2 / M of 1 and at temperature of 8 C, the overall reaction of CH 4 dry reforming in the thermal plasma reformer can be rewritten as III DISCUSSION For a sulfur-containing natural gas fuel, the sulfur becomes H 2 S in a reformate stream after reformation, which can easily be removed by a chemical agent like ZnO For a PEMFC application, the more hydrogen concentration and the less CO concentration the reformate has, the better the reforming efficiency is It is necessary to make CO concentration as low as possible for the PEMFC system High-temperature and low-temperature WGS reactors as well as a PROX reactor are used to purify CO out of the reformate stream With the molar fraction of H 2 O being much less than that of CO, it is necessary to inlet additional water and air into WAS and PROX reactors, respectively On the other hand, all of the H 2, CO, and CH 4 of the reformate stream following desulfurization are useful fuels for an SOFC stack The total molar fractions on a wet basis in Fig 7 approach 94% at temperatures in the range of 7-11 C Any higher operating temperature, over 8 C, can not effectively better the total molar fractions of SOFC fuels Higher operating temperature in the reformer leads to higher power consumption and less energy conversion efficiency as well Therefore, from a control point of view, an optimal operation condition for a thermal plasma reformer can be chosen at a mole flow ratio of CH 4 to CO 2 of 1: 1, ie, mass flow ratio of 16:, and at a temperature of 8 C for both PEMFC and SOFC applications IV CONCLUSIONS The thermodynamic analysis of CH 4 reforming with CO 2 is a first step for developing a CH 4 -fueled fuel cell system (FCS) From the foregoing analysis of thermodynamic equilibrium for CH 4 reforming with CO 2, we have found that an optimal working condition of thermal plasma reformer can be chosen at a temperature of 8 C and a mole flow ratio of CH 4 to CO 2 of 1: 1 (ie, mass flow ratio of 16:) With lower ratio of H 2 /CO and high conversion efficiency, the reformed synthesized gas is suitable for SOFC-based power generation applications For PEMFC applications, the CO removal processes involving WGS and PROX reactions is adopted to reduce CO to zero with the introduction of water and air, respectively This makes the overall system more complex than an SOFC-based system

6 896 Journal of the Chinese Institute of Engineers, Vol 31, No (8) ACKNOWLEDGMENT This research was sponsored by the project NSC C E from National Science Council of the Republic of China REFERENCES Ahmed, S, and Krumpelt, M, 1, Hydrogen from Hydrocarbon Fuels for Fuel Cells, International Journal Hydrogen Energy, Vol 26, No 4, pp Brown, L F, 1, A Comparative Study of Fuels for On-Board Hydrogen Production for Fuel-Cell- Powered Automobiles, International Journal Hydrogen Energy, Vol 26, No 4, pp Chen, X, Honda, K, and Zhang, Z G, 4, CO 2 - CH 4 Reforming over NiO/γ-Al 2 O 3 in Fixed-Bed/ Fluidized-Bed Switching Mode, Catalysis Today, Vol 93-9, pp Dicks, A L, 1996, Hydrogen Generation from Natural Gas for the Fuel Cell Systems of Tomorrow, Journal of Power Sources, Vol 61, No 1-2, pp Effendi, A, Hellgardt, K, Zhang, Z -G, and Yoshida, T, 3, Characterisation of Carbon Deposits on Ni/SiO 2 in the Reforming of CH 4 -CO 2 using Fixedand Fluidised- Bed Reactors, Catalysis Communications, Vol 4, No 4, pp 3-7 Guo, J, Lou, H, Zhao, H, Chai, D, and Zheng, X, 4, Dry Reforming of Methane over Nickel Catalysts Supported on Magnesium Aluminate Spinels, Applied Catalysis A: General, Vol 273, No 1-2, pp 7-82 Hou, Z, and Yashima, T, 4, Meso-Porous Ni/ Mg/Al Catalysts for Methane Reforming with CO 2, Applied Catalysis A: General, Vol 261, No 2, pp -9 Juan-Juan, J, Román-Martinez, M C, and Illán- Gómez, M J, 4, Catalytic Activity and Charcterization of Ni/Al 2 O 3 and NiK/Al 2 O 3 Catalysts for CO 2 Methane Reforming, Applied Catalysis A: General, Vol 264, No 2, pp Jun, J, Kim, J C, Shin, J H, Lee, K W, and Baek, Y S, 4, Effect of Electron Beam Irradiation on CO 2 Reforming of Methane over Ni/Al 2 O 3 Catalysts, Radiation Physics and Chemistry, Vol 71, No 6, pp 9-11 Kodama, T, Koyanagi, T, Shimizu, T, and Kitayama, Y, 1, CO 2 Reforming of Methane in a Molten Carbonate Salt Bath for Use in Solar Thermochemical Process, Energy & Fuels, Vol 1, No 1, pp 6-6 Kodama, T, Ohtake, H, Shimizu, K I, and Kitayama, Y, 2, Nickel Catalyst Driven by Direct Light Irradiation Solar CO 2 Reforming of Methane, Energy & Fuels, Vol 16, No, pp 6-6 Roh, H S, Potdar, H S, and Jun, K W, 4, Carbon Dioxide Reforming of Methane over Co-Precipated Ni-CeO 2, Ni-ZrO 2 and Ni-Ce-ZrO 2 Catalysts, Catalysis Today, Vol 93-9, pp Rynkowski, J, Samulkiewicz, P, Ladavos, A K, and Pomonis, P J, 4, Catalytic Performance of Reduced La 2-x Sr x NiO 4 Perovskite-Like Oxides for CO 2 Reforming of CH 4, Applied Catalysis A: General, Vol 263, No 1, pp 1-9 Tsai, H L, and Wang, C S, 6a, Analytical Studies of Ethanol Steam Reforming in the Thermal Plasma Reformer, Proceedings of The 1 ST National Conference on Hydrogen Energy and Fuel Cell, Sun- Moon Lake, Nantou, Taiwan, ROC, pp - Tsai, H L, Wang, C S, Chang, Y C, Chang, G E, and Kuo, J H, 6b, Thermodynamic Analysis of Ethanol Steam Reforming for 1kWe Thermal Plasma Hydrogen Reformer, Proceedings of 6 AASRC/CCAS Joint Conference, Chung- Li, Taiwan, Paper No 7_, pp 1-7 Wang, C S, 6a, Plasma Reformer for Hydrogen Production from Water and Fuel, US patent No: 7,7,634 B1, July 4 Wang, C S, and Huang, H, 3, Fuel -Flexible H 2 Reformer using Advanced Thermoelectric Technology, 3 Fuel Cell Seminar Abstracts, Miami Beach, FL USA, pp Wang, C S, Chang, Y C, Hong, S S, Lee, H B, Kuo, N H, Tsai, H L, Chang, K I, and Kuo, C H, 6b, Optimal Design of a 1kWe Thermal Plasma Reformer, 6 Fuel Cell Seminar, Honolullu, Hawaii, Poster Section 3 Wang, J B, Wu, Y S, and Huang, T J, 4, Effects of Carbon Deposition and De-Coking Treatments on the Activation of CH 4 and CO 2 in CO 2 Reforming of CH 4 over Ni/Yttria-Doped Ceria Catalysts, Applied Catalysis A: General, Vol 272, No 1-2, pp Wang, S G, Li, Y W, Lu, J X, He, M Y, and Jiao, H, 4, A Detailed Mechanism of Thermal CO 2 Reforming of CH 4, Journal of Molecular Structure (Theochem), Vol 673, No 1-3, pp Wörner A, and Tamme, R, 1998, CO 2 Reforming of Methane in a Solar Driven Volumetric Receiver-Reactor, Catalysis Today, Vol 46, No 2-3, pp Manuscript Received: Mar, 7 Revision Received: Nov 21, 7 and Accepted: Dec 31, 7