Final Report, Materials for Industrial Heat Recovery Systems, Tasks 3 and 4 Materials for Heat Recovery in Recovery Boilers
The DOE-funded project on materials for industrial heat recovery systems included four research tasks: materials for aluminum melting furnace recuperator tubes, materials and operational changes to prevent cracking and corrosion of the co-extruded tubes that form primary air ports in black liquor recovery boilers, the cause of and means to prevent corrosion of carbon steel tubes in the mid-furnace area of recovery boilers, and materials and operational changes to prevent corrosion and cracking of recovery boiler superheater tubes. Results from studies on the latter two topics are given in this report while separate reports on results for the first two tasks have already been published. Accelerated, localized corrosion has been observed in the mid-furnace area of kraft recovery boilers. This corrosion of the carbon steel waterwall tubes is typically observed in the vicinity of the upper level of air ports where the stainless clad co-extruded wall tubes used in the lower portion of the boiler are welded to the carbon steel tubes that extend from this transition point or “cut line” to the top of the boiler. Corrosion patterns generally vary from one boiler to another depending on boiler design and operating parameters, but the corrosion is almost always found within a few meters of the cut line and often much closer than that. This localized corrosion results in tube wall thinning that can reach the level where the integrity of the tube is at risk. Collection and analysis of gas samples from various areas near the waterwall surface showed reducing and sulfidizing gases were present in the areas where corrosion was accelerated. However, collection of samples from the same areas at intervals over a two year period showed the gaseous environment in the mid-furnace section can cycle between oxidizing and reducing conditions. These fluctuations are thought to be due to gas flow instabilities and they result in an unstable or a less protective scale on the carbon steel tubes. Also, these fluctuating air flow patterns can result in deposition of black liquor on the wall tubes, and during periods when deposition is high, there is a noticeable increase in the concentrations of sulfur-bearing gases like hydrogen sulfide and methyl mercaptan. Laboratory studies have shown that chromized and aluminized surface treatments on carbon steel improve the resistance to sulfidation attack. Studies of superheater corrosion and cracking have included laboratory analyses of cracked tubes, laboratory corrosion studies designed to simulate the superheater environment and field tests to study the movement of superheater tubes and to expose a corrosion probe to assess the corrosion behavior of alternate superheater alloys, particularly alloys that would be used for superheaters operating at higher temperatures and higher pressures than most current boilers. In the laboratory corrosion studies, samples of six alternate materials were immersed in an aggressive, low melting point salt mixture and exposed for times up to 336 h, at temperatures of 510, 530 or 560°C in an inert or reactive cover gas. Using weight change and results of metallographic examination, the samples were graded on their resistance to the various environments. For the superheater corrosion probe studies, samples of the same six materials were exposed on an air-cooled corrosion probe exposed in the superheater section of a recovery boiler for 1000 h. Post exposure examination showed cracking and/or subsurface attack in the samples exposed at the higher temperatures with the attack being more severe for samples 13 exposed above the first melting temperature of the deposits that collected on the superheater tubes. From these superheater studies, a ranking was developed for the six materials tested. The task addressing cracking and corrosion of primary air port tubes that was part of this project produced results that have been extensively implemented in recovery boilers in North America, the Nordic countries and many other parts of the world. By utilizing these results, boilers are being operated under more demanding conditions with fewer outages and reduced safety concerns about boiler explosions resulting from failed tubes in the lower boiler. The results of studies on the mid-furnace tube corrosion task and the superheater corrosion and cracking task have become available much more recently, and, consequently, the extent of implementation is far more limited. However, boiler operators are utilizing some of the operational information to minimize corrosion in the mid-furnace area. With construction of new boilers that are designed to operate at higher temperatures and pressures, the guidance on selection of superheater tube materials will almost certainly influence the selection of materials for the more critical areas of the superheaters.
- 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:
- 921898
- Report Number(s):
- DOE/GO14035/3; TRN: US200806%%175
- 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 FLOW
BOILERS
ALUMINIUM
CARBON STEELS
CORROSION
COVER GAS
FIELD TESTS
GAS FLOW
HEAT RECOVERY
HYDROGEN SULFIDES
MELTING
MELTING POINTS
SPENT LIQUORS
SUPERHEATERS
SURFACE TREATMENTS
Recovery Boiler
Superheater
Corrosion
Computational Fluid Dynamics
waterwall tubes
co-extruded tubes