Use of High Temperature Electrochemical Cells for Co-Generation of Chemicals and Electricity

Barnett, Scott

Northwestern University (Evanston, Ill.)

关键词: Electrochemical Cells;    Propane;    Current Density;    Efficiency;    03 Natural Gas;   

DOI  :  10.2172/924973
RP-ID  :  None
RP-ID  :  FC26-05NT42625
RP-ID  :  924973

美国|英语

来源: UNT Digital Library

【 摘 要 】

In this project, two key issues were addressed to show the feasibility of electrochemical partial oxidation (EPOx) in a SOFC. First, it was demonstrated that SOFCs can reliably operate directly with natural gas. These results are relevant to both direct-natural-gas SOFCs, where the aim is solely electrical power generation, and to EPOx. Second, it must be shown that SOFCs can work effectively as partial oxidation reactors, i.e, that they can provide high conversion efficiency of natural gas to syngas. The results of this study in both these areas look extremely promising. The main results are summarized briefly: (1) Stability and coke-free direct-methane SOFC operation is promoted by the addition of a thin porous inert barrier layer to the anode and the addition of small amounts of CO{sub 2} or air to the fuel stream; (2) Modeling results readily explained these improvements by a change in the gas composition at the Ni-YSZ anode to a non-coking condition; (3) The operation range for coke-free operation is greatly increased by using a cell geometry with a thin Ni-YSZ anode active layer on an inert porous ceramic support, i.e., (Sr,La)TiO{sub 3} or partially-stabilized zirconia (in segmented-in-series arrays); (4) Ethane and propane components in natural gas greatly increase coking both on the SOFC anode and on gas-feed tubes, but this can be mitigated by preferentially oxidizing these components prior to introduction into the fuel cell, the addition of a small amount of air to the fuel, and/or the use of ceramic-supported SOFC; (5) While a minimum SOFC current density was generally required to prevent coking, current interruptions of up to 8 minutes yielded only slight anode coking that caused no permanent damage and was completely reversible when the cell current was resumed; (6) Stable direct-methane SOFC operation was demonstrated under EPOx conditions in a 350 h test; (7) EPOx operation was demonstrated at 750 C that yielded 0.9 W/cm{sup 2} and a syngas production rate of 30 sccm/cm{sup 2}, and the reaction product composition was close to the equilibrium prediction during the early stages of cell testing; (8) The methane conversion to syngas continuously decreased during the first 100 h of cell testing, even though the cell electrical characteristics did not change, due to a steady decrease in the reforming activity of Ni-YSZ anodes; (9) The stability of methane conversion was substantially improved via the addition of a more stable reforming catalyst to the SOFC anode; (10) Modeling results indicated that a SOFC with anode barrier provides similar non-coking performance as an internal reforming SOFC, and provides a simpler approach with no need for a high-temperature exhaust-gas recycle pump; (11) Since there is little or no heat produced in the EPOx reaction, overall efficiency of the SOFC operated in this mode can, in theory, approach 100%; and (12) The combined value of the electricity and syngas produced allows the EPOx generator to be economically viable at a >2x higher cost/kW than a conventional SOFC.