Wear-Resistant Refractory Ceramic Materials

Wear is a progressive loss of material due to mechanical action such as continuous impingement of particles. For information on wear of boiler parts due to ash, please refer to 3.5.1.2.1. Corrosion is a chemical or electrochemical action, leading to loss of material, whereas wear is a mechanical phenomenon. Wear is prevented by protecting the parts by sacrificial material of high hardness made of either steel or refractory to suit the applica­tion. Steel protection is used for the inside parts of the boiler subject to very high tempera­ture and flue gas velocities. The outside parts, where there is no heat transfer, are protected by special ceramic linings and coatings.

Typically mill internals such as parts of mill housings and classifiers, coal pipes (more particularly the bends), and cyclone inlets of CFBC boilers are parts that experience varying levels of erosion, depending on the erosiveness of coal and ash and the air/gas velocities. Appropriate ceramic lining is used to protect the affected parts based on the temperature and the severity of erosion. A harder lining prolongs life.

Hardness has no single accepted definition. It is the resistance to local penetration, scratching, machining, wear, or abrasion, depending on the material and reference. Scratch hardness is measured by Mohs scale of minerals, which is an arbitrary measure ranging from 1 to 10 based on the relative hardness. Talc is the softest mineral and is placed at the bottom of the scale at 1, whereas diamond is the hardest at 10 (see Figure 5.9).

Some of the popular materials from which the abrasion resisting linings are made are shown in Table 5.21.

TABLE 5.21

Special Ceramic Linings for Wear Resistance

Type of Ceramic

Fused Cast Basalt

Clinker Tile

Cast Corundum

Dense Al2O3

Supplied form

Cast to size

Tile cut to

Tiles and special

Tiles, special shapes,

Size

Shapes

2 mm thin tiles

Raw materials

Dense basalt

Clay, kaolin, feldspar, and sand

Al and zircon

Al2O3

Main use

Friction-induced

Milder

Severe abrasion

Extreme abrasion

Wear

Abrasion

And high temperatures

And high temperatures

Density (te/m3)

2.8-2.9

2.4

3.4

About 3.5

Hardness on Mohs

8

6

9

9

Scale

Thermal conductivity

1-1.16

1.3-1.6

4.2

14

(W/m K)

Coefficient of

6-8

5

6.5

3.4

Expansion (i/K)

Temperature limit (°C)

350

350

1000

1000

Chemical resistance

To most acids and

To many

To many acids

To many acids and

Alkalies

Acids and alkalies

And alkalies

Alkalies

Areas of application

Coal pipes and

Separators,

Cyclones, pipe

Cyclones, fan blades,

Bends, cyclones,

Drag link

Bends

Conveyors, mixers

Hoppers, and silos

Feeders

Further Readings

Babcock and Wilcox Refractories, a company publication.

Bendick, W. et al., Replacement of P22 power station components by P91 for existing and new units, Mannesman Company Publication.

Boiler plate specifications—Section 2 of ASME B&PV Code, BS 1501, DIN 17155.

Boiler tube and pipe specifications—Section 2 of ASME B&PV, BS 3059 and 3062 & 3064, DIN 17175. Efficient use of fuel, 1957, Her Majesty’s Stationery Office, London.

Fontana, M. G., 1987, Corrosion Engineering, McGraw Hill Book Company, New York.

High pressure castings—Section 2 of ASME B&PV, BS 1504, DIN 17245.

High pressure forgings—Section 2 of ASME B&PV, BS 1503, DIN 17243.

Mack, W. C., 1972, How to select steel tubing for elevated temperature service, Babcock and Wilcox Tubular Products Division, Power, October.

Makansi, J., 1983, Corrosion, a special report of Power magazine, April.

Refractories, Harbison-Walker Refractory Publication.

Richmond, C. and Chaille, C. E., High performance castables for arduous applications, Morgan Refractories Limited.

Vishwanathan, R. and Bakker, W. T., Materials for boilers in USC plants, Proceedings of 2000 Inter­national Joint Power Generation Conference, EPRI, July 2000.

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