Session: 30-03 Seals and Bearings

Paper Number: 153424

Environmental Degradation of Hard and Soft Magnetic Materials in Gaseous and Supercritical CO2 Environments 

Hermetic machinery for supercritical CO₂ (sCO₂) cycles eliminate CO₂ leakage that would be lost through shaft end seals in non-hermetic machinery, which is a benefit to emissions as well as operating costs for makeup of lost process fluid. Proposed hermetic machinery concepts replace traditional oil-lubricated bearings with process-lubricated bearings or actively-controlled magnetic bearings that operate in the process fluid environment; they also incorporate high-speed motors/generators within the hermetic casing rather than traditional low-speed versions with gearboxes. It is desirable that sCO₂ machinery components are capable of operating at temperatures close to the process conditions (e.g., 500°C to over 700°C for turbines), as it would minimize the need for active cooling.

Permanent magnets are used in the motors/generators of these machines as well as, potentially, permanent magnet-biased active magnetic bearings. An ongoing project is investigating the effect of CO₂ exposure on several hard (permanent magnets) and soft (easily magnetized and demagnetized) magnetic materials. Six hard magnetic material configurations were chosen: Alnico 9C, Alnico 5-7C, and 18-T550 grade SmCo, all with or without Ni plating. The soft magnetic material was Hiperco 50 with and without a C5 coating.The chosen soft magnetic material may be used for rotor and stator laminations of active magnetic bearings in high-temperature environments, which have superior maximum relative permeability, saturation flux density, and Curie temperature than typical materials for lower-temperature service. The C5 coating is also a common electrically insulating material used for stacked laminations.

All material samples are exposed to CO₂ at two conditions: (1) flowing gaseous conditions of 550°C and atmospheric pressure in a furnace and (2) supercritical conditions of 450°C and 103 bar in an autoclave. This paper presents results for total exposures of 1,000 hours and 2,000 hours. The preliminary mass change measured after exposures indicate that the 18-T550 grade SmCo is significantly less resistant to exposure to CO₂ than the Alnico and Hiperco 50 materials. Based on mass change only, all coatings appeared to be beneficial. However, some of the coated SmCo 18-T550 samples had cracked or became very brittle after exposures. The coatings on the other magnets improved protection. The absolute mass changes were higher on the specimens that had been in the furnace than in the autoclave. This indicates that temperature has a stronger effect on the degradation of the materials than the phase of CO₂. Indeed, at the same temperature, it could be expected that sCO₂ would be more corrosive than gaseous CO₂. At the time of this abstract submission, SEM imaging is planned; these results are expected for the full paper and will complement the mass change results and associated technical discussions.

Acknowledgement and Disclaimer:

This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Solar Energy Technologies Office, Award Number DE-EE0009823.

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Presenting Author: Aaron Rimpel Southwest Research Institute

Presenting Author Biography: Mr. Rimpel is a Group Leader in the Rotating Machinery Dynamics Section of the Machinery Department at SwRI. He graduated with a bachelor in mechanical engineering from Western Michigan University in 2005 and obtained his master in mechanical engineering from Texas A&M University in 2008. He joined Southwest Research Institute in 2009. His Group helps analyzing and solving problems associated with various types of machinery, designing and developing rotating machinery components and systems, developing component test stands for machinery applications, performing lateral/torsional rotordynamics analyses, and supporting root cause failure investigations for machinery systems at SwRI.
He has participated in several DOE-funded supercritical CO2 power cycle projects, including a large size seal test rig (project lead), turbine and compressor designs, and an integrally-geared compressor-expander design. He also led the development and testing of several high-speed bearing, dry gas seal, and high-speed, dry ice particle impingement erosion test rig. His expertise led him to have key roles in the development of other machines for aerospace, oil and gas, and energy applications.

Authors:

Florent Bocher Southwest Research Institute
Aaron Rimpel Southwest Research Institute
Wanming Zhang GE Vernova
Robert Lipham Southwest Research Institute
Dongil Shin GE Vernova
Amin Changizi GE Vernova