Increasing Availability

Gasifiers currently operate under conditions that can be considered demanding. These conditions present several challenges, especially in regard to materials, where not only is the gasifier itself—more specifically the refractories—under severe stress, but so are any devices inserted to monitor and control the gasification process.

Availability is very important to the economics of a gasification plant. A shut-down gasifier haults synthesis gas (syngas) production and, therefore, final product output (electricity, liquid fuels, etc.). With current, state-of-the-art technology, many integrated gasification combined cycle (IGCC) designs incorporate a spare gasifier in order to achieve acceptable overall plant availability, even though this entails a higher capital cost. With continued operating experience and research, it is believed that an online availability higher than 90% can be achieved by a gasification plant, but currently, most plants cannot achieve that without redundancy or fuel backup:

• Tampa Electric Integrated Gasification Combined-Cycle Project – operating commercially since 1996, 82% gasifier availability and 74% plant availability.
• Wabash River Coal Gasification Repowering Project – operating commercially since 1995, reaching 79.1% plant availability, and 76% gasifier availability.
• Great Plains Gasification Plant [PDF-3.1MB] – operating since 1984, 98.7% days with production (i.e., not-zero production), although this isn’t traditionally how availability is measured.
• Elcogas SA IGCC, Puertollano, Spain – operating since 1998, provided 78.6% gasifier availability, and 48% overall plant availability as of 2007.

Research and development is being conducted to increase the availability of the gasifier and decrease the cost of operation and maintenance. Examples include advanced materials development for refractory and the development of a reliable, practical and cost-effective means of monitoring real-time temperature in the gasifier.

Refractory Failure
The refractory material lines the gasifiers and gives it its strength under extremely high operating temperatures. In the most severe slagging gasifiers, refractories can require replacement every three months, where the gasifier system then 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 have some very extreme conditions; refractory materials must withstand:

• Temperatures from 1,325 to 1,575ºC
• 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 a goal.

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-845KB].

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

Non-Cr₂O₃ Refractories
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.

Instrumentation
To maintain control of a gasifier under changing conditions—which allows for optimized performance—knowledge of key system parameters, pressure or temperature for example, must be available. Thermocouples are one of the most common methods of providing near real-time temperature data, but they don’t hold up well and can fail early in start-up. Replacement requires the gasifier to be shut down. A functioning thermocouple allows for greater control of the gasifier, which increases reliability and performance. A failed thermocouple decreases availability. These factors prompt research and development into extending the life of gasifier thermocouples.

Causes of thermocouple failure include:

• Issues with the placement and installation in the gasifier, slag binding and shear forces, and operational issues like temperature and throughput
• Design and fabrication defects
• Chemical corrosion from slag, vapor, and metallic iron

Several of these problems (e.g. slag) are possibly related to refractory issues.

	Thermocouple protection system for gasifiers. (Source)

NETL recommends a thermocouple fabrication procedure to reduce fabrication defects and has developed a filler material to reduce slag penetration. Improvements in well blocks in the refractory material could also better protect the thermocouples. In addition, thermocouples will be tracked in regard to cause and frequency of failure for better understanding. Please see the NETL presentation, Refractory Materials for Slagging Gasifiers [PDF-845KB] for more information.

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