All the nonferrous parts of a boiler are known as BRIL, which is an acronym for bricks, refractory, insulation, and lagging. The boiler setting or enclosure, which is the assembly sur­rounding all the hot parts, is substantially made of BRIL so that

1. The heat is contained within the boiler.

2. Heat loss is restricted to a minimum.

3. Personnel safety is ensured.

As the boiler sizes have increased along with greater water wall cooling and more sophis­ticated steels, the significance of BRIL has considerably altered. With the improved aware­ness for esthetics, environment, and human comfort, there is more insulation and lagging in boilers and the increased use of membrane panels and the progressive discontinuance of many mass burning practices have made the refractory bricks practically obsolete and increased the use of castable refractories. I and L have grown at the expense of B and R.

Refractories

Refractories will be discussed only as applicable to boilers. The following are the impor­tant points to remember:

• Refractories are made of a combination of minerals.

• Their properties are dependent on the specific combination.

• They are also highly influenced by conditions where the minerals are sourced.

• The properties can be drastically affected even by small changes in the ingredients.

Functions. Refractories are nonmetallic materials suitable for construction or lining of high-temperature furnaces. Accordingly, the refractories are expected to withstand the following in hot conditions:

• Pressure from self-weight or the weight of furnace parts or contents

• Thermal shock resulting from rapid heating and cooling

• Chemical attack by heated solids, liquids, gases, and fumes

• Mechanical wear

Classification of Refractories

On the basis of properties or composition, refractories are classified into five main types:

1. Fireclay

2. High alumina

3. Silica

4. Basic

5. Insulating

They are also classified into many special types such as

• Silicon carbide

• Graphite

• Zircon

• Zirconia

Most are made from silica and alumina in various combinations.

Refractory materials are supplied in various ways:

1. Preformed shapes

2. Special-purpose clays

3. High-temperature cements

4. Bonding mortars

5. Plastic

6. Ramming mixes

7. Hydraulic setting castables

8. Gunning mixes

9. Granular materials

10. Ceramic fibers

Refractory Selection

Refractory selection is based on the following considerations:

• Besides high gas temperatures, refractories have to withstand specific types of chemical attacks from the ashes of flue gas.

• Selection of the type of refractory is also guided by the gas velocity.

— Ceramic fibers can be used up to a gas velocity of 7.5 m/s.

— Monolithics can be used up to a gas velocity of 60 m/s.

— Bricks can be used for gas velocities more than 60 m/s.

• Monolithics have no joints and present no risk of erosion of the mortar joints in brick construction provided the application can accept lower density and wear resistance.

Refractoriness

Refractoriness is temperature-withstanding ability. Refractoriness is measured by pyro — metric cone equivalent (PCE), which is the temperature at which a cone of specified size (usually 12 mm sides and 38 mm tall with one edge perpendicular to the base) begins to soften and turn roundish at the top when heated slowly at the rate of 5°C/min. Refractori­ness under load (2 kg/cm[4]) is a more valuable parameter as it represents the practical situ­ation better. PCEs and equivalent temperatures are given in Table 5.18.

Various Types of Refractories

• Fireclay refractories consist essentially of hydrated aluminum silicates (generally Al2O3 ■ 2SiO2 ■ 2H2O) along with smaller quantities of other minerals. Hard clays are the flint and semiflint clays, which form the principal component of the super — and high-duty fire bricks having PCE of 33-35. Plastic and semiplastic refractory clays, also called soft clays, vary considerably in refractoriness, bonding strength, and plasticity having PCE values ranging from 29 to 33.

• High-alumina refractories. Several minerals can be used in the making of alumina refractories. Most high-alumina refractories are made from bauxite (Al2O3 ■ H2O + Al2O3 ■ 3H2O) or diaspore (Al2O3 ■ H2O) or a mixture of the two blended with flint and plastic clay. Alumina types are more refractory than fireclay, approximately in proportion to their content of alumina. They are highly resistant to chemical attack by various slags and fumes and, in general, have greater resistance to pressure at high temperature than fireclay refractory.

• Silica refractories are made from high-purity crystalline mineral quartz. Their thermal expansion is high at low temperatures and negligible beyond 550°C. They possess high refractoriness, strength, and rigidity at temperatures close to their melting point. They are susceptible to thermal spalling (cracking) at 650°C, but at higher tempera­tures, they are free from spalling.

• Basic refractories. The raw materials used include magnesite, dolomite, and chrome ores.

• Insulating refractories are lighter and porous, as they trap a lot of air and hence possess much lower conductivity and heat storage capacity. They are normally used as the backing to the dense refractory facing. If the furnace conditions are clean, they can also be used as facing materials.

Bricks are preformed shapes obtained by pressing the green mass to the required shape and size and firing at the specified temperature until the refractory bond is formed by chemical action under heat. The word brick is used in the acronym BRIL to cover both bricks and the tiles. Standard refractory bricks are made in two thicknesses, namely, 227 mm X 116 mm X 76 mm and 63 mm (9 in. X 4 1/2 in. X 3 in./2_ in.). Tiles are flattened bricks usually made in the range of 50-76 mm thickness, with overlapping edges and an arrangement to hold the tile. Tiles can also be made in different shapes and in more advanced composition than bricks. Today, refractory brick construction is confined mainly to the following areas:

• Around the fire in pile burning, in horse shoe or ward furnaces in firing bagasse or similar fuels. This type of burning is nearly extinct.

• Shaped refractory arches in chain grate stokers for radiating the heat onto the bed.

• Boiler enclosure in brick set boilers.

• Brick lining for underground brick flues.

• Stack lining.

• Brick lining in cyclones and external HXs in CFBC boilers.

The use of refractory bricks in modern boilers is negligible in comparison to the former times. Since the bricks are pressed in hydraulic presses, they are strong, dense, and heavy, which makes them ideally suited to face the fire and dust-laden gases.

Several types of refractory bricks are manufactured to suit various types of furnaces:

Refractory Tiles

Refractory tiles are used in the following areas in modern boilers:

• On top of the floor tubes to protect the tubes from overheating in package boilers

• In burner quarls to give the right shape to the flame

• Between tubes to form gas baffles

• In the lining of cyclones

• In the lining of hot and dust-laden gas ducts and hoppers

Refractory Castables

These refractory materials are not preformed but are cast in situ to any desired shape. Because of this flexibility and the advances made in the materials and binders, the cast — able refractories have become greatly popular, displacing the shaped and formed types in many applications. They are available in special mixes or blends of dry granular or stiffly plastic refractory materials with which (a) practically joint-free linings (monolithic) can be made or (b) repair of masonry can be carried out. These are packed in a way that makes transportation and handling easy. The application is also made very easy with little or no preparation. There are four types of castables or monolithics:

The monolithics develop their strength by either air or hydraulic setting. The entire thickness becomes hard and strong at room temperatures. At higher temperatures, it becomes even stronger due to the development of the ceramic bond. Heat setting monolithic refractories have very low strength at low temperatures and develop their full strength only on attain­ing the full temperature. Linings on furnace wall tubes require that the water walls be fully drained before the application of a refractory layer, lest the water-cooled walls hinder the lining from attaining the necessary temperature. Usually castable linings need some anchor material to hold.

• Plastic refractories are mixtures of refractory materials prepared in stiff plastic con­ditions of proper consistency, for ramming into place with pneumatic hammers or mallets. Plastics are similar to castables in formulation, as both use calcined aggregates and a binder. However, the plastics that are premixed at the factory use phosphates or other heat-setting agents to develop a bond when fired. Castables use hydraulic cements that form the permanent bond when mixed with water.

• Plastic chrome ore (PCO) linings are proven lining materials for black liquor (BL) recovery boilers. The air-setting plastic compound is rammed into position on the studded walls to develop a dense monolithic layer, which has high resistance to spalling, erosion, hot gases, and smelt. High-alumina phosphate-bonded plastics are used in hot cyclones of circulating fluidized bed (CFB) boilers to withstand high erosion.

• Ramming mixes are ground refractory materials with minor amounts of other materials added to make the mixes workable. Most ramming mixes are sup­plied dry. Ramming mixes are required to be mixed with water and rammed into place, followed by drying and heating when they form a dense and strong monolithic refractory structure by self-bonding.

• Gun mixes are granular refractory materials prepared for spraying at high velocity and pressure by guns. The resulting lining is homogeneous and dense and free from lamination cracks. The spray can be either by dry mix or by wet mix. The gun has a water nozzle to moisten the mixture. The gun mixes can be either air or heat setting. Refractory lining of steel stacks is often done by gunning.

• Castables are granular refractory materials combined with suitable hydraulic set­ting bonding agent. They are supplied in dry form to be mixed, at site, with water and poured or cast in place to develop a strong hydraulic set. They are rammed or troweled or tamped into position and occasionally applied with air guns. These castables have negligible shrinkage in service and low coefficients of thermal expansion. They are resistant to spalling. Some are capable of withstanding severe erosion. Some are good insulators and others are good conductors.

— High-alumina dense castables with varying alumina contents in excess of 90% are used up to an operating temperature of 1800°C to withstand high erosion. Burner quarls and furnace linings are some examples of the usage of this castable.

— Low-cement, ultra low-cement, and no-cement castables are relatively recent develop­ments where refractory cement used as binder is progressively brought down to withstand high temperatures without weakening the lining. Less cement means reduced lime content and reduced water requirement for setting. The porosity is reduced by more than 50%, resulting in a very dense structure capable of stand­ing up to very high erosion as seen in cyclones of CFB boilers.

— Insulating castables serve as hot face lining in clean applications and as backup lining for dusty applications. They are light, strong, and low in conductivity, making the whole liner thin and cheap. They are made in a range of densities from —400 to 1600 kg/m3.

— Silicon carbide lining of FBC furnaces is done to withstand extreme levels of ero­sion and have good heat transfer to the water walls.

Insulation

Some of the important aspects of the insulating materials are as follows:

• Unlike the refractory materials, which are substantially mineral-based and there­fore capable of withstanding the highest temperatures and erosion, the insulation materials are nonmineral (except for ceramic fibers), can stand up to temperature no higher than —1650°C, and do not have any abrasion resistance.

• The insulating materials possess much lower heat conductivity, several times lower than those of the refractory materials.

• The insulating materials are fluffy and hold lot of air, which contributes to the reduced heat flow.

• Thermal conductivity for insulating materials rises steeply with temperature. Insulation materials in boiler practice are available in mainly four forms:

1. Reformed shapes and slabs

2. Mattresses

3. Plastic cement

4. Loose fill

The insulating materials used in boilers are as follows:

1. Calcium silicate in block forms

2. Mineral/slag wool

3. Ceramic fiber

4. High-temperature plastic

A. Calcium silicate (CaSiO3) is a popular block insulation up to 650°C, available in preformed shapes or blocks. It is particularly suitable for piping and valves. The blocks are ideal for insulating the hoppers and ducts from inside for metal protection where there is no abrasive ash.

B. Mineral or slag or rock wool is made from blowing the slag produced in steel mills into fine fiber by steam or air jets. Mineral wool has practically displaced the glass wool, which contains glass fibers because of human health considerations. It is also slightly cheaper.

C. In blanket form, the wool is held in place by wire mesh on one or both sides, depending on the application, and is used up to 650°C. For vertical walls of a furnace, resin bonding of the mattress is usually preferred, as the wool is held

In place without settling at the bottom due to the slight vibrations in the walls. Typically, densities of 100-140 kg/m3 are employed for normal applications.

D. When clay is used as a binder and the wool is molded at high temperature and pressure, mineral wool blocks are formed and can be used up to 1050°C. These blocks are used for insulating the boiler and membrane walls.

E. Ceramic fiber linings are produced by melting the alumina-silica china clay and blowing air to form fibers of 50-100 mm long and 3 jam in diameter. They are offered in block or mattress form capable of withstanding very high tempera­tures. The blocks substitute for insulating bricks, and the mattresses can be used for high temperatures. They have good acid resistance but lower alkali resistance. Ceramic fiber is useful when temperatures as high as 1650°C are encountered. This material is reserved only for special applications due to its high cost. Lining of the inlet duct downstream of burners in HRSG is a typical application.

F. High-temperature plastic is insulating plastic cement made from mineral wool and clay, and is suitable up to 1050°C for use on irregular shapes and for filling gaps in block insulation.

Table 5.20 lists the properties of various common insulating materials used in boilers.

Lagging

Lagging is the outermost covering to protect the insulation from damage, usually in the form of metal sheet of galvanized iron (GI) or aluminum and weather-resistant. GI is stron­ger but lacks the luster of Al. Usually plain GI sheeting for furnaces is used in thickness of 1 mm (19 SWG) and Al in 1.2 mm (18 SWG) plain or 0.9 mm (20 SWG) corrugated. For pipes,

0. 71 or 0.56 mm (22 or 24 SWG) of these metals is used for ease of bending. Both GI and Al are about the same in price. Al is gaining greater acceptance due to its better appearance.