Oxide Scale Morphology and Chromium Evaporation Characteristics of Alloys for Balance of Plant Applications in Solid Oxi
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INTRODUCTION
IN comparison with the conventional combustionbased technologies for electrical power generation, solid oxide fuel cell (SOFC) power systems offer well-documented advantages in terms of improved power plant efficiency (chemical to electrical), fuel flexibility (from coal to hydrocarbon to hydrogen), and reduction in the carbon foot print and emission of criteria pollutants (PMs, SOx, NOx, and VOC).[1–3] SOFCs also offer flexibility of systems design that is ideally suited for distributed and centralized power generation as well as carbon capture, combined heat and power generation, and water-independent operation.[4–6] A basic SOFC system flow sheet is shown in Figure 1. The SOFC operation, electrochemical processes, cell and stack component materials, technical challenges, and research trends have been documented in recent review articles.[7–9] A barrier to commercialization of the SOFC power generation systems has also been reviewed and attributed to the systems’ cost (cell stack and balance of plant) LE GE, Graduate Student, ATUL VERMA, Research Professor, and PRABHAKAR SINGH, Professor, are with the Department of Chemical, Materials & Biomolecular Engineering, Center for Clean Energy Engineering, University of Connecticut, Storrs, CT 06269. Contact e-mail: [email protected] RICHARD GOETTLER and DAVID LOVETT, Researchers, are with Rolls-Royce Fuel Cell Systems (US) Inc., North Canton, OH 44720. R.K. SINGH RAMAN, Professor, is with the Departments of Mechanical & Aerospace Engineering, and Chemical Engineering, Monash University, Melbourne, VIC 3800, Australia. Manuscript submitted December 21, 2011. Article published online November 6, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
and long-term performance degradation under nominal and transient operating conditions.[10,11] Several cost reduction schemes ranging from improvement in the power density to implementation of the lower cost materials (cell, stacK and balance of plant (BOP), and large scale manufacturing to innovative functional integration at the cell, stack, and subsystem levels have been developed and implemented by SOFC system manufacturers.[12,13] A detailed mechanistic understanding of the long-term performance of degradation processes associated with the electrochemical deactivation, interfacial reactions, and mechanical failure remains largely unknown. One of the major sources of electrical performance degradation in cells and stacks has been associated with the poisoning and deactivation of the air electrode due to interactions with chromium vapor species present in the air stream.[14–16] Chromium vapor species form at the exposed oxide surfaces of the metallic interconnects, stack manifold, air delivery tubes, and high temperature heat exchangers and enter the stack with the incoming air, and subsequently react with the bulk air electrode material either to form a stable compound or deposit as stable chromium oxide at the electrochemical triple phase boundary. Chromium evaporation from the bulk interconnect materials namely fer
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