Final Report, Materials for Industrial Heat Recovery Systems, Task 1 Improved Materials and Operation of Recuperators for Aluminum Melting Furnaces
Production of aluminum is a very energy intensive process which is increasingly more important in the USA. This project concentrated on the materials issues associated with recovery of energy from the flue gas stream in the secondary industry where scrap and recycled metal are melted in large furnaces using gas fired burners. Recuperators are one method used to transfer heat from the flue gas to the air intended for use in the gas burners. By preheating this combustion air, less fuel has to be used to raise the gas temperature to the desired level. Recuperators have been successfully used to preheat the air, however, in many cases the metallic recuperator tubes have a relatively limited lifetime – 6 to 9 months. The intent of this project was to determine the cause of the rapid tube degradation and then to recommend alternative materials or operating conditions to prolong life of the recuperator tubes. The first step to understanding degradation of the tubes was to examine exposed tubes to identify the corrosion products. Analyses of the surface scales showed primarily iron oxides rather than chromium oxide suggesting the tubes were probably cycled to relatively high temperatures to the extent that cycling and subsequent oxide spalling reduced the surface concentration of chromium below a critical level. To characterize the temperatures reached by the tubes, thermocouples were mounted on selected tubes and the temperatures measured. During the several hour furnace cycle, tube temperatures well above 1000°C were regularly recorded and, on some occasions, temperatures of more than 1100°C were measured. Further temperature characterization was done with an infrared camera, and this camera clearly showed the variations in temperature across the first row of tubes in the four recuperator modules. Computational fluid dynamics was used to model the flow of combustion air in the tubes and the flue gas around the outside of the tubes. This modeling showed the distribution of air in the tubes was not at all uniform, and it was strongly affected by velocity and the turbulence created by items in the lower plenum such as the fork lift slots, structural members, and broken spiral fins originally intended to promote non-linear flow in the tubes. Finite element modeling of the stresses developed between tubes in the first and second rows showed how stresses could become large enough to account for the bending and bowing of tubes in the first row. To prevent rapid degradation and subsequently short lifetimes of recuperator tubes two approaches were offered. Alloys that form an aluminum oxide surface layer would be more likely to survive the excursions to very high temperatures. However, availability of these alloys in the tube sizes required might be an issue as might the ability of welding these alloys to the other recuperator components. The other approach would entail improving the design of the recuperator module and to change operating procedures to avoid conditions where very hot flue gases are flowing across the recuperator tubes at times when reduced combustion air is flowing through the recuperator tubes.
- Research Organization:
- Weyerhaeuser Company, Federal Way, WA, USA
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE); USDOE Office of Industrial Technologies (OIT) - (EE-20); Golden Field Office
- DOE Contract Number:
- FC36-04GO14035
- OSTI ID:
- 919037
- Report Number(s):
- DOE/GO14035/2; TRN: US200825%%178
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
42 ENGINEERING
99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE
AIR
ALLOYS
CHROMIUM OXIDES
COMBUSTION
COMPUTERIZED SIMULATION
CORROSION PRODUCTS
FLUE GAS
FLUID MECHANICS
FURNACES
GAS BURNERS
HEAT RECOVERY
HEAT TREATMENTS
IRON OXIDES
MELTING
Aluminum Melting Furnaces
Recuperator
Energy Recovery
Finite Element Modeling
Computational Fluid Dynamics