News

Refractories for Byproduct Coke Oven Batteries

• satyendra
• August 21, 2014
• 0 Comments

Refractories for Byproduct Coke Oven Batteries

 A coke oven battery (COB) is basically a structure made of different varieties of refractory bricks and shapes held together by a steel framework. The battery is designed to operate at maximum temperatures up to 1450 deg C though it is constructed at ambient temperature. During operation, the maximum temperature can be expected to cycle downward by as much as 110 deg C as part of normal battery operation. Hence the battery must be constructed of refractory materials that can withstand the maximum as well as the cyclic temperatures, and that have known and predictable properties related to thermal expansion, strength and creep.

Coke oven batteries are normally operated for long periods like 25 to 30 years continuously. Therefore refractory bricks for lining a COB should have accurate shapes and precise dimensions, an excellent mechanical strength at high temperatures, hot modulus of rupture and excellent volumetric stability to work at temperatures of up to 1450 deg C.  Types of refractories used in different region of COB are shown in Tab 1.

Silica bricks have sufficiently high refractoriness under load (RUL)and reliable volumetric stability at high temperatures so a large volume of silica bricks of standard shapes are used to construct the coking and combustion chambers of COB. The roof, regenerator checkers and chimney flue of COB are constructed with high quality fireclay bricks. In some countries like Japan a campaign life of more than 40 years is achieved by employing high quality refractories and through control of standard operating parameters for coke oven batteries. Typical cross section from pusher side of 7 m high COB showing brickwork is at Fig 1

Fig 1 Pusher side typical cross section showing brickwork

 Silica refractories

The most abundant refractory used in the construction of a COB is silica brick. Silica is the refractory of choice primarily because, at normal COB operating temperatures, silica refractories are subject to minimal creep. Also, since nearly all of the expansion of silica brick takes place below 650 deg C, during normal operation of a COB, the moderate temperature fluctuations of the walls have no effect on the volume stability of the refractory comprising the wall.

The silica refractories are manufactured as multiple asymmetric shapes, which are normally keyed or interlocked with each other by means of tongues and grooves. A COB design can have well over 400 different shapes used in its construction. These shapes are installed with a silica mortar.

Silica mortar is primarily composed of the silica mineral quartz. In general, there are two types of mortar used, an air setting, which contains a small amount of sodium silicate, and a heat setting, which is basically the same mortar, but without the sodium silicate. Both the types of silica mortar typically do not bond to the silica brick at normal battery operating temperatures. Therefore, it does not impart any strength to the wall. Also, since the primary mineral constituent in the mortar is quartz, the mortar is not volume stable. The quartz in silica mortar installed in an operating battery slowly converts to the high temperature forms of silica—tridymite and cristobalite—during normal battery operation. This conversion is accompanied by a significant increase in volume. This conversion happens first on the hotter flue side of the wall. This means that the mortar in the horizontal joints assumes a wedge shape; thicker on the flue side and thinner on the oven side of the wall.

Silica bricks are manufactured mainly from the mineral quartz in finely crystalline form. These bricks are having the proper characteristics for conversion to cristobalite and tridymite, the high temperature crystalline forms of silica. The quartz is obtained primarily from crushed quartzite rock, which is washed to remove natural impurities. The crushed and washed quartzite is further ground and sized into specific size fractions which are then re-blended in specific proportions, along with 2 % to 3.5 % of lime (CaO), water and organic binders to achieve the proper chemistry of silica brick.

Brick shapes are produced by power and impact pressing. Those shapes that cannot be mechanically formed or only needed in small numbers are hand moulded. Immediately after forming, the green bricks, held together by the organic binders, are dried. Silica bricks are fired either in batch kilns or in continuous car tunnel kilns. The bricks are fired to convert the quartz to cristobalite and tridymite, the stable high temperature forms of silica, which in turn renders the brick volume stable for service. It is necessary to maintain a carefully planned time temperature cycle during firing because there are critical temperature ranges through which the silica brick must pass so that a strong, well bonded bricks are obtained. The normal permanent expansion of silica bricks during firing is 12 % to 15 %. Transformation of silica bricks during heating is shown in Fig 2.

Fig 2 Transformation of silica bricks during heating

 Silica bricks have a relatively high melting temperature of 1695 deg C to 1710 deg C. They have the ability to withstand a 172 to 345 kPa load to within 28 deg C to 56 deg C of the ultimate melting point. They have excellent creep properties. The purity of the brick is important. For example, if the sum of alumina, titania, and alkalis content in a brick is 1.0 %, the load to failure will be 28 deg C to 50 deg C lower than in another brick in which this sum is only 0.5 %.  At temperature of around 593 deg C, silica bricks are nearly volume stable and virtually free from thermal spalling, while at temperatures below 593 deg C, silica bricks are highly susceptible to thermal spalling.

Since the later part of 1950s there had been a general trend to use high bulk density (BD) silica bricks (BD greater than 1850 kg/cum) in COB construction, since increasing BD is accompanied by corresponding increases in cold strength and thermal conductivity. Therefore, it was presumed that higher heat transfer rates from flue to oven would be realized because of the higher thermal conductivity. However, the measurement of flue to oven temperature differentials in the operating batteries showed that higher BD silica brick did not result in improved heat transfer over regular BD (1740–1800 kg/cum) silica bricks. In addition, a series of tests on silica bricks of varying bulk density showed that the hot strength in the range of 650 deg C–1315 deg C did not appreciably change as a function of BD, and that the lower BD bricks were less brittle and less susceptible to failure from thermal cycling.

Due to the unusual dilation curve for silica bricks, the heating walls in a COB must be very slowly heated to prevent damage to the refractories. Once heated, the silica refractories can be expected to undergo 1.2 % to 1.3 % linear expansion. Almost all of this expansion will take place at temperatures that are below 650 deg C. Therefore, the length and height of a wall can increase during heat up of the COB.

Fireclay bricks

Fireclay bricks are used throughout the cooler parts of the COB that includes the regenerator chambers, checker, battery roof, in the pinion walls, and in the coke wharf. Normally there are five general classes of fireclay bricks. These are super duty, high duty, semi silica, medium duty, and low duty. Fireclay bricks used in COB construction are generally of the high duty class.

Blends of five or more ground and sized clays are used to make fireclay bricks. Some bricks, especially those in the low duty class, may be made from single clay. Mixes for high duty bricks commonly contain raw flint and bond clays, possibly with calcined clays. In the high duty class, a large proportion of the mix is pre-calcined to control firing shrinkage, as well as to stabilize the volume and control mineral composition of the final brick.

Fireclay shapes are moulded by several methods namely power pressing, extrusion and repressing, air ramming and hand ramming. The method of moulding is selected on the basis of shape complexity as well as the properties of the fireclay that are needed. However small lots of a given shape may be hand moulded. Fireclay brick shapes after the moulding operations are dried on hot floors or in tunnel or humidity driers. After drying, the shapes are usually fired in continuous car type tunnel kilns. Batch type, downdraft kilns are rarely used. The firing temperature is dependent on the nature of the clays used and on the intended service of the brick. Free and combined water are lost during firing, and iron and sulfur compounds as well as organic matter are oxidized. The particles of clay are ceramically bonded together to form a strong refractory. High duty fireclay shapes are more resistant to spalling than medium and low duty products, and they are burned hard enough so that they are highly resistant to carbon monoxide disintegration.

High duty fireclay brick for COB construction generally is refractory to about 1700 deg C. However it does not have the creep resistance of silica brick. High duty fireclay can be expected to deform 0.5 % to as much as 4.0 % under a 172 kPa load at 1350 deg C.

Fireclay bricks have nearly a linear thermal expansion from ambient temperature to battery operating temperatures, as compared to silica brick, which has nearly all of its expansion taking place below 593 deg C.  As a result high duty fireclay bricks can be repeatedly cycled through low temperature ranges without spalling failure. Medium and low duty fireclay refractories, being more dense, are more susceptible to thermal shock.

Mortars

There are two primary types of mortars used in COB construction, silica mortar and fireclay mortar. The silica mortar is further divided into a heat set and an air set type. Normally the silica brick in a COB is laid up with the heat set type.

Silica mortars are made from sized silica sand for aggregate along with some small amount of bond clay. The air set silica mortar is the same composition, but is has a small amount of dry sodium silicate added.

fireclay mortar is made from sized calcined kaolin with a small amount of bond clay. No mortar used in battery construction contains any type of cementitious bond. Silica mortars are refractory up to 1680 deg C and clay mortars up to 1600 deg C.

Shrinkage on drying from a workable consistency is an important parameter, and is specified as less than 2.5 % and 4.0 % for silica and clay mortars, respectively. Mortars used in COB construction do not develop a coherent bond with the refractory to which they are applied at normal battery operating temperatures. Mortars therefore do not contribute to the structural integrity of a battery. The mortar used in a battery serves two main functions. It is used to compensate for inconsistencies in brick size, and it acts as a sealant between brick to prevent gas leakage among the combustion air, fuel gas, and foul gas systems within the battery.

Castables

Castables are refractory concretes that are made with calcium aluminate (CA) cements and various refractory aggregates. They are used in different areas of a COB. The service capability of a castable is dependent on the purity level of the CA cement as well as of the aggregate.  Usually there are three purity levels of CA cement namely low purity, medium purity and high purity. The aggregates can range from calcined clays to high purity tabular alumina. Size grading of castable aggregates can also vary considerably.

Castables are used in COB include low and medium purity castables. In addition to regular cement castables, low cement and ultra low cement castables are used. Low purity castables are used to line the waste heat tunnels and flues. They can be formed up and cast in place or it can be gunited. The gunniting application is faster and less costly.

Low purity castables are also applicable to areas in the battery top, for example it is used as a filler material in the parapet area. Low purity castables used in COB construction or repairs is generally specified to satisfy strength requirements and carbon monoxide resistance. Medium purity castables are used for the higher temperature areas in a battery to which castables are applicable, for example door plug refractories and standpipe lining.

Expansion joint fillers

Expansion joints fillers are usually vermiculite, an expanded mica, or with ceramic fiber bats of adequate density and refractoriness.

Packing materials

Packing materials are used to seal between metal components and refractory brick, for example between the jamb and jamb brick, in stand-pipe split rings and slip joints and around air and waste heat boxes. Asbestos rope is the preferred and most commonly used packing material. Asbestos rope can be replaced by synthetic fiber rope.

Insulating materials

Various types of insulating materials are used in COB construction. These vary from insulating fire brick to low refractoriness insulating concrete made of an expanded clay or shale aggregate and calcium aluminate cement. Insulating fireclay bricks which are available from an 870 deg C class to a 1760 deg C  class are used to insulate the battery top, areas behind the buck stays, regenerator walls and pinion walls. Block insulation can be used in non load bearing areas. Insulating concretes can be employed as fillers for irregular areas throughout lower temperature areas of the COB.

Acid resistant refractory

The COB chimney lining is of acid resistant construction, which consists of high density acid resistant fireclay brick laid up with acid resistant mortars. Acid resistant brick are usually made by the stiff mud process and are high fired so as to have low porosity (6 % to –11 %) and permeability. Acid resistant mortars are generally potassium and sodium silicate bonded fireclay aggregate. Ambient temperature levels are critical for good acid resistant construction. Mortars generally will not cure at temperatures below 13 deg C.

Fire fused silica

Silica (quartz) that is electrically fused, cooled, reground, then formed and re-fired is referred to as a fire fused silica shape. Fused silica is essentially amorphous (non crystalline), containing low levels of quartz and cristobalite. Fired fused silica has a nearly flat thermal expansion curve, expansion being of the order of 0.72 K??. It has excellent resistance to thermal shock. A fairly common use of fired fused silica is for door plug refractories. However, refractory cost is two to three times that of a castable or brick door plug. Fired fused silica has also been used as jamb brick with varying degrees of success. Fused silica is useful where resistance to carbon buildup and is desired.a

Fused silica grain along with calcium aluminate cement is used to make fused silica castables. The castables, however, do not have the thermal expansion properties of the fired bodies. In general, the castables made with fused silica aggregates are subject to similar temperature volume relationships as normal silica alumina castables in that aggregate sizing and cement content can have marked effects on volume stability.

Common red bricks

Common red bricks are used in a number of non critical areas in COB construction, primarily in areas of low temperature and minimal exposure to thermal cycling. Common brick is a dense fired clay body. It is used as filler in the battery roof, backup material in the pinion walls and often as an underlying course on the pad.