Refractory material lines a gasifier and gives it ability to withstand extremely high operating temperatures. In the most severe slagging gasifiers, refractories can require replacement every three months, in which the gasifier system needs to be shut down for one to two weeks. These shut-downs can cost a plant millions of dollars. Research and development to improve the refractory materials will lead directly to increased gasifier availability.
Some typical causes of refractory failure include:
- Chemical corrosion from molten slag and hot gas/molten salt
- Spalling and other forms of physical wear
- Problems with the quality of the refractory material itself (its manufacture for example) or gasifier design
|Conventional (left) and phosphate-modified (right) chrome oxide refractory materials after rotary slag testing. New materials research and development, like this improved refractory material aims to improve gasifier availability. (Source)
Slag is fluidized ash and occurs in gasifiers where the operating temperature is above the ash fusion point. Slagging gasifiers are associated with very extreme conditions; refractory materials must withstand:
- Temperatures from 1,325 to 1,575°C
- Frequent thermal cycling
- Reducing and oxidizing environments
- Corrosive slag of varying composition
- Corrosive gases
- Pressures of 400 psi and higher
The dissolution of the refractory material begins another process of refractory failure: spalling. Spalling is the flaking away of the refractory material. The slag penetrates the refractory material, weakening it and causing significant material loss. Spalling shortens refractory life even more than chemical corrosion.
Chromium Oxide (Cr2O3) Refractories
Chromium oxide (Cr2O3) is a commonly used refractory material because of its ability to withstand extremely high temperatures and relatively slow rate of inevitable dissolution (chemical corrosion) from the molten slag. However, improvement is still important towards gasifier optimization.
Spalling in Cr2O3 refractories is a major issue. Fuel flexibility can sometimes necessitate a different refractory material because Cr2O3 may not be suitable with ash and slag that is high in alkalis and alkaline earths. Other industries have methods for repairing refractories to extend life and increase availability that do not work with Cr2O3 refractories. In addition, suppliers of high Cr2O3 refractories are dwindling, making an already expensive and difficult to produce product even more expensive. Therefore, research on novel materials for use in refractories is being done to improve plant availability.
Some of these novel materials are described below. More information can be found in the NETL presentation, Refractory Materials for Slagging Gasifiers [PDF].
Novel Materials (Cr2O3 Based)
- Phosphate Modified Cr2O3
- Decreased slag penetration
- Eliminates spalling
- Keeps resistance to chemical corrosion
- Aurex 95P (NETL-patented refractory material, Press Release)
- Field tests confirm elimination of spalling
- Keeps resistance to chemical corrosion
Failure of non-Cr2O3 refractories is expected to be similar to Cr2O3 refractories: dissolution and reaction with slag. Thermodynamics indicates that few materials will be as chemically stable as Cr2O3, but depending on ash chemistries, refractories of ZrO2 or Al2O3/MgO have potential. These are currently undergoing laboratory testing and scale-up for further testing.
Latest NETL-sponsored research aimed at refractory improvement is focused on refractory development as discussed above, and the impact of additives on carbon feedstock ash behavior and refractory wear. Refractory service life improvement through material development or modeling and control of slag chemistry will be evaluated via laboratory testing at NETL and through the cooperation of industry gasifier operators. The goal is to develop a slag model that allows operators to control slag viscosity, maximize refractory service life, and minimize downstream material issues like syngas fouling. It is expected this information will become part of an NETL developed database, allowing predictive modeling of coal and/or petcoke slags in gasifiers.
More information on NETL-supported R&D projects on refractory improvement and refractory durability testing is available.