Fuel processing in direct propane solid oxide fuel cell and carbon dioxide reforming of propane over Ni–YSZ

doi:10.1016/j.fuproc.2011.04.007

Fuel Processing Technology

Volume 92, Issue 8, August 2011, Pages 1611-1616

https://doi.org/10.1016/j.fuproc.2011.04.007 Get rights and content

Abstract

This work is aimed at understanding the reaction mechanism of propane internal reforming in the solid oxide fuel cell (SOFC). This mechanism is important for the design and operation of SOFC internal processing of hydrocarbons. An anode-supported SOFC unit with Ni–YSZ anode operating at 800 °C was tested with direct feeding of 5% propane. CO₂ reforming of propane was carried out in a reactor with Ni–YSZ catalyst to simulate internal propane processing in SOFC. The performance of this direct propane SOFC is stable. The C specie formed over the anode functional layer of SOFC can be completely removed. The major gas products of SOFC are H₂, CO, CH₄, C₂H₄ and CO₂. Pseudo-steady-state internal processing of propane in the anode catalytic layer of SOFC is associated with a CO₂/C₃H₈ molar ratio of about 1.26 and basically CO₂ reforming of propane. CO₂ dissociation to produce the O species to oxidize the C species from dehydrogenation and dissociation of propane and its fragments should be the major reaction during CO₂ reforming of propane.

Research highlights

► The performance of direct propane solid oxide fuel cell (SOFC) is stable. ► The C specie formed over SOFC anode functional layer can be completely removed. ► Internal processing of propane in SOFC anode catalytic layer is CO₂ reforming. ► CO₂ dissociation to produce O species to oxidize the C species is major reaction.

Introduction

Solid oxide fuel cell (SOFC) can utilize the liquefied petroleum gas (LPG), whose major component is propane, as the anode fuel. Propane can result in carbon deposition and thus the anode can be deactivated seriously. Therefore, external fuel processing of hydrocarbons such as propane is usually carried out to convert hydrocarbons to a gas mixture containing hydrogen and carbon monoxide. If internal fuel processing can have the same or similar efficiency in converting the fuel as the external one, the fuel can be fed directly to the anode and thus the system can be simplified. Anode-supported SOFC can use the anode side for fuel processing and thus decrease the extent of carbon deposition or even avoid the problem of coking. Notably, anode fuel of pure propane has been studied over electrolyte-supported SOFC with Ni–La_0.6Sr_0.4Fe_0.8Co_0.2O₃–gadolinia-doped ceria anode operating at 800 °C; stable output of current density has been observed during 120 h of operation; the occurrence of internal reforming of propane has been proposed [1].

During fuel processing of propane over the anode side, reforming reactions can occur by utilizing the anode product gas that contains CO₂ and/or H₂O:

CO₂ reforming (dry reforming) of propane [2], [3]:C₃H₈ + 3 CO₂ → 6 CO + 4 H₂Steam reforming of propane [4], [5], [6]:C₃H₈ + 3 H₂O → 3 CO + 7 H₂These reactions of propane reforming result in very different CO/H₂ ratio. Since H₂ is usually more efficient than CO as the SOFC fuel, it is desirable to understand the actual reaction type during internal fuel processing of propane. It is noted that the reactivity of syngas, i.e., the mixture of CO and H₂, over the SOFC anodes varies with the CO/H₂ ratio [7], [8].

Commercial anode-supported SOFCs use Ni–yttria-stabilized zirconia (YSZ) as the anode material. The thickness of the Ni–YSZ anode is usually around 800 μm. However, the anode functional layer has been considered to have a thickness less than 100 μm; Kong et al. [9] have shown that, with Ni–YSZ anode for anode-supported SOFCs, the thickness of the effective electrochemical reaction zone of the anode is about 40–50 μm. Thus, most portion of the Ni–YSZ anode in the anode-supported SOFCs can work only catalytically. It is noted that the anode functional layer is the portion of the anode where the oxygen ions can reach so to result in electrochemical oxidation; note also that the oxygen ions are migrated from the cathode to the anode via the electrolyte. Therefore, the catalytic portion of the Ni–YSZ anode can be simulated by a separate Ni–YSZ catalyst layer. However, the actual reaction in the anode catalytic portion has not yet been clarified, possibly due to the fact that the gas composition in this portion is hard to measure. Thus, it should be interesting to simulate the gas composition in the anode catalytic portion so to clarify the reaction type, especially whether it is CO₂ or steam reforming. This should help the design of the anode catalytic portion for propane processing.

In this work, propane was fed to an anode-supported SOFC unit cell with Ni–YSZ anode. A catalytic reactor with a layer of Ni–YSZ catalyst was used to simulate the reactions occurring in the Ni–YSZ catalytic portion of SOFC. CO₂ reforming of propane was carried out over Ni–YSZ catalyst and it has been shown that the reaction type during internal fuel processing of propane in the Ni–YSZ catalytic portion of SOFC should be CO₂ reforming of propane.

Section snippets

Preparation of NiO–YSZ powder

The YSZ powder was prepared by the method of solid state reaction. Yttria powder was mixed with zirconia powder in a ratio of Y/Zr = 8 mol%. The mixture was ball milled to carry out solid state reaction. Then, the mixture was heated to 1600 °C and held for 4 h. After cooling down to room temperature, the YSZ powder was obtained. The composition of YSZ is Zr_0.92Y_0.08O₂.

The NiO–YSZ powder was also prepared by the method of solid state reaction. The nickel oxide powder was mixed with the YSZ powder in

Fuel processing over Ni–YSZ anode in direct propane SOFC

Fig. 1 shows that the current density is constant. Note that this current density reaches the steady state immediately at the very beginning. This indicates that there is no deactivation of the SOFC performance with direct feeding of propane. Since the reactions are quite complex as shown by the formation of various products, the constant current density right from the very beginning may also indicate that its generation is controlled by the migration rate of the oxygen ions (O²⁻) from the

Conclusions

The SOFC performance has been shown to be stable, in terms of the generated current density, with direct propane feeding. The C specie formed over the anode functional layer of SOFC can be completely removed. The major gas products of fuel processing in direct propane SOFC are H₂, CO, CH₄, C₂H₄ and CO₂. Pseudo-steady-state internal processing of propane in the anode catalytic layer of SOFC is with CO₂/C₃H₈ molar ratio of about 1.26 and basically CO₂ reforming of propane. CO₂ dissociation to

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Fuel processing in direct propane solid oxide fuel cell and carbon dioxide reforming of propane over Ni–YSZ

Abstract

Research highlights

Introduction

Section snippets

Preparation of NiO–YSZ powder

Fuel processing over Ni–YSZ anode in direct propane SOFC

Conclusions

Electrochimica Acta

J. Catal.

Catal. Today

Chem. Eng. J.

Fuel

J. Power Sources

Electrochem. Commun.

J. Power Sources