Study on steam reforming of CH4 and C2 hydrocarbons and carbon deposition on Ni-YSZ cermets
Introduction
Power generation by solid oxide fuel cells (SOFCs) is one of the most attracting energy conversion systems because of high efficiency, low pollution and multi-fuel compatibility. The high operating temperature gives rise to excellent fuel flexibility, which allows to reform hydrocarbon fuels on the anode internally in a SOFC module. Such simplified internal reforming operation of SOFC system results in low costs owing to the elimination of pre-reformer. Natural gas is regarded as a relatively cheep and popularly available fuel, which is suitable for SOFC. The main component of natural gas is methane, and the Ni-Y2O3-stabilized zirconia (Ni-YSZ) anode material catalyzes the following reforming of methane:Natural gas usually contains ca. 10% of higher hydrocarbons, which also react with steam:In steam reforming of methane, a large amount of steam in excess of the stoichiometric requirement of reaction (1) was usually supplied, and this addition promotes the shift reaction (2) to forward direction. The formed CO2 suppressed disproportionation (4):However, internal reforming of hydrocarbon often accompanies carbon deposition. The active sites of the anode are covered with deposited carbon, resulting in the deactivation, loss of cell performance and lower SOFC reliability [1], [2].
The majority of studies on Ni-YSZ-based cermet anode have focused on kinetics of internal reforming of hydrocarbons [3], [4], [5], the effect of CeO2 addition to the Ni-YSZ cermet [6], the influence of microstructure [7], [8], [9] and electrochemical performance as an electrode [10], [11]. Few reports dealt with carbon deposition on the anode and catalysis for reforming of fuel.
The Ni-YSZ cermet has been popularly used as the anode in most of the research and development of SOFCs. The reaction gas composed of high steam/carbon (S/C) ratios, typically over 2, was used to suppress carbon deposition; however, it is unattractive for fuel cells since steam dilution of the fuel lowers the conversion efficiency.
Steam reforming of methane on supported nickel catalysts has been studied by many workers [12], [13], [14], [15], [16], [17]. Some additives have been reported as effective in suppressing carbon deposition. For example, alkaline earth oxides were added to reduce carbon deposition [18], [19], [20]. The forms of supported nickel catalysts are different from Ni-YSZ cermet, and these catalysts have own pore structures and nickel particle sizes. The sizes of Ni grains in the cermet were much larger than those of the supported catalysts.
In the present investigation, the catalytic properties of Ni-YSZ cermets for steam reforming of methane and carbon deposition were investigated. Alkaline earth oxides (MgO, CaO, SrO) and CeO2 were added to Ni-YSZ cermets to avoid carbon deposition.
Section snippets
Partial pressure of oxygen and carbon deposition region by equilibrium calculation
The theoretical voltage of SOFC for an electrolyte with its ionic transference number of unity can be obtained by the Nernst equation (5):Partial pressure of O2 at the anode depends on the equilibrium composition of reaction gas, while partial pressure of O2 in cathode is usually constant. In this calculation, equilibrium compositions were determined by the following reactions ((6)–(8), or reactions (6)–(9) when carbon is deposited):
Shift reaction and steam reforming of methane on the conventional Ni-YSZ cermet
Shift reaction (2) and steam reforming of methane (1) were carried out at atmospheric pressure using a conventional flow system, where catalyst made by TOTO Ltd. was loaded. A commercial YSZ tube painted with the Ni-YSZ cermet was used for the reaction. The area of the active catalytic surface for the Ni-YSZ cermet was 25.8 cm2. Steam was generated in a temperature-controlled humidifier by bubbling the gas mixture. Reaction gas mixture composed of 22.2% H2O, 11.1% CO, 33.3% H2 and 33.4% N2 was
Partial pressure of oxygen and carbon deposition region by equilibrium calculation
The calculated equilibrium partial pressure of oxygen was expressed as a contour map in the CHO phase diagram at temperatures ranging from 400 to 1000 °C, as shown in Fig. 1. The open circuit voltage is directly connected to theoretical voltage of SOFC calculated by Eq. (5). As the fraction of oxygen in the CHO phase diagram increased, the cell voltage significantly decreased. The partial pressure of oxygen in the reducing atmosphere was generally lowered as the temperature was reduced. The
Conclusion
It is of primary importance for the investigation of internal reforming of SOFC to evaluate the thermodynamic equilibrium concerning on carbon deposition and oxygen potential in multi-component gas in reformate. The second approach should be elucidation of kinetic effects, such as reforming activity, the rate of carbon deposition, and electrochemical overpotential. The additive to the cermet significantly affected the kinetic parameters of steam reforming of CH4 on Ni-YSZ cermet. CaO-modified
Acknowledgements
This work was partly supported by International Joint Research Program grant (no. 2001EF004) awarded by the New Energy and Industrial Technology Development Organization (NEDO).
References (23)
- et al.
Catal. Today
(1998) J. Power Sources
(1996)- et al.
J. Power Sources
(2000) - et al.
Catal. Today
(2000) - et al.
J. Power Sources
(2001) - et al.
J. Power Sources
(2000) - et al.
Appl. Catal. A: Gen.
(2002) - et al.
Appl. Catal. A: Gen.
(1995) - et al.
J. Catal.
(2000) - et al.
J. Catal.
(2001)