THE USE OF HEAT-RESISTANT CONCRETE AT HOT REPAIRS OF COKE OVEN BATTERIES
© Levchenko Anatoliy Aleksandrovich, chief Specialist
Artickle is published in journal "Coke and chemistry" 2013 № 4 p. 60-62
Various types of heat-resistant concrete are manufactured nowadays. Portland cement, clinker-bearing slag cement, calcium aluminate cement or periclase cement, phosphate binders, soluble water glass etc. are used as cementing components for its manufacture. In many cases, cementing components carry in fine ground additives. Crushed refractory or refractory rock formations, hard-burned refractory products and other materials are used as fillers. Heat-resistant concretes are divided into high-refractory (refractoriness above 1770 ° C), fireproof (1580-1770 ° C), heat-resistant (less than 1580° C).
The use of heat-resistant concrete for coke oven batteries is less common and largely limited to using it for lining of doors, charging hole lids and gas take off standpipes. There are developments of American (United States Patent 5,423,152) and German (Fosbel's MONOWALL ™) specialists in the use of heat-resistant concrete in hot repair of coke oven batteries.
Suggestions of American experts on the use of concrete blocks have a number of disadvantages, such as:
- consistent use of lifting equipment, which is difficult at operating battery;
- mandatory installation of transverse and longitudinal thrust jacks on unrepaired brickwork that should be considered a great obstacle for laying of concrete blocks;
- the need for pre-fabrication and burning of units (as well as for the preparation of silica brick) does not meet the conditions of the urgency of repair;
- the inability to establish serial manufacture of blocks due to different length of heating walls on repaired batteries.
This technology is more suitable for construction of new batteries.
More interesting is the use of heat-resistant concrete technology offered by "Fosbel" (MONOWALL™), which is much more suitable for hot repairs. This technology envisages the use of fused silica for hot repairs with 0.06% expansion coefficient at 1000° C, with 99% SiO2 content. This technology was developed to address the following issues:
- long-term manufacture and supply of silica and fireclay bricks;
- lack of manpower (skilled refractory-men);
- attrition of stocks of materials;
- intensification of installation works.
As of April 2010 "Fosbel" repaired 207 heating channels and five heating walls throughout the entire length by this method. Small number of heating walls and channels is explained by the reason that this technology has been recently applied.
The use of heat-resistant concrete in place with the use of cast formwork fully meets the requirements of urgent hot repair when the first necessity is to extend the service life of heating wall up to the basic repair. Due to the fact that concrete casting requires the use of cast formwork, repair area is restricted by this condition. For example, you can not fill failure of the heating channel with concrete or restore oven’s roofing without creating conditions for cast formwork etc. However, in practice, there are many examples in by product coke oven industry when there is no time to order and supply the necessary refractories as there is an urgent need to rebrick the heating wall to the depth of two or more heating channels. In such a case the question of durability of concrete and other brickwork requirements is not considered but there is a necessity to extend the service life of heating wall up to the basic repair.
Cast concrete is well connected with the old brickwork and if the temperature of the old brickwork at the junction with concrete will be maintained within 800-900 ° C, which is quite realistic, no displacement between brickwork and concrete will occur. To avoid appearance of cracks in the brick at contact with the cold liquid concrete mass it is recommended to paste a layer of concrete with low water content on the brickwork(using liquid glass or orthophosphoric acid).
The advantages of heat-resistant concrete - low coefficient of thermal expansion, high refractory properties, ease of preparation and application significantly increase the proportion of its participation in hot repair of coke oven batteries. Not to debate the issues of durability of concrete heating walls, material costs and other factors, we consider the feasibility of its application where there is no other possibility to carry out the necessary work immediately.
At relaying the areas of fireclay ovens roofing a great number of fireclay brick shapes is required that can be successfully substituted by the heat-resistant concrete. For example, often the cause of failure of the cross anchor- tie rods is burning of gas under the flash plates through the fireclay bricking-in. In this case, relining of the fireclay ovens facade under the flash plates is required, which can be replaced with heat resistant concrete with a sheet steel form.
In cases where it is necessary to extend normal operation of the oven’s chamber without replacement of the flash plates which deviate from brickwork and flaps of flash plates burn, heat-resistant concrete can be used as follows:
- П-shaped metal formwork is fabricated to fit the top of the camera (the height of the formwork must close the gap of the flash plate deviation from the brickwork, the width of the formwork covering the distance from the brickwork to the door frame);
- the diaphragm or the screen is set to the depth of one heating channel;
- brick facade over the flash plate is dismantled;
- formwork is inserted and thrusted with jacks;
- the cord is attached to the door frame in the project places and concrete is poured to the top of the frame;
- metal formwork is installed on the façade and filled up to the top with concrete.
Operating capability of six coke ovens with chamber volume 41.6 m3 was restored by this method, the repaired ovens working successfully for a year at the battery pending for basic repair. In addition, 12 heating walls of this battery needed urgent rebricking of 2-4 heating channels to provide the work of half of the battery during the basic repair of the other half by cold rebricking from the seventh range. There were no complete set of silica refractories. There were fireclay repairing wall brick shapes ("guitars", " spoons", "binders") and chambers roofing brick shapes. The author of this article suggested to rebrick the walls with fireclay brickshapes and use heat-resistant concrete, withstanding 1450° C temperature manufactured by the local concrete products plant, instead of jamb bricks, heating channels’ and ovens’ roofing brickshapes.
Rebricking was carried out according to the rules of hot repair maintaining the temperature in the adjacent heating walls and unrepaired part within the range of 800°C. For removal of "spoons" tailings at making the tuss at the junction of new and old brickwork solid walls were installed so that the proximal septum edge was strictly on the axis of "binder". Partitions were set tightly with choking of remaining slots with mortar, thrust jacks being no longer needed. Connection of new brickwork with the old one is performed by the tuss on each row without heating of the new brickwork. Rebricking was performed by accelerated rates by 10 rows per shift at four and 20 rows per shift at two heating flues without a fear that new brickwork undermines the old one when heated.
End (pillar) heating flue is made in the same way as the others, only the "stretchers" were cut for passing of the heating wall behind the edge of the flash plate. The space between the flash plate and brickwork was filled with concrete with a layer of mineral wool (blanket) with a thickness of 25 mm from flash plate. The heating wall to oven roof was made out of wall brick and heating flues roof - from long "binders" forming the basis of shafts. The partitions above the heating flues ("binders") were not laid, but (tin) pipes dia. 120 mm were installed instead of shafts at the height of "pot " (brick shape under the inspection hole) and filled with concrete to the level of oven roof. After laying of the oven’s roofing (with the arrangement of the gas hole from normal and installation of cast formwork on the front ) the whole volume was filled with concrete up to the “facing brickwork”.
The rebricking of four heating flues of one heating wall required not more than seven days (one day for dismantling of the old brickwork and installation of jacks, 2-3 days for laying of walls up to the chamber’s roofing, a day for chambers and oven’s roofing, two days for heating up).
Partitions were removed and the door was installed after the ovens roofing. Gas for heating was supplied in a day after the installation of door and a day later the oven was charged. Temperatures in the rebricked heating flues should not exceed 1250 ° C, which is quite enough at the oven turnover of 17 hours.
Thus, repaired by fireclay and concrete, heating walls successfully worked for 1.5 year till the start up of the second half of the battery after overhaul. When disassembling these heating walls a hoisting crane had to be used for removal of concrete monolith. Continuous cracks or gas leakages in concrete were not found.
The available experience of the use of blocks made of heat -resistant concrete at the construction of new batteries suggests that in the future heat resistant concrete can completely replace single-piece silica and fireclay refractories. However it is necessary to improve the quality concrete in terms of thermal conductivity and abradability. Minimal linear expansion, no need for thermal treatment, lack of significant cracks in the lower zone of the battery at heating and operation, significant reduction of the battery construction periods and multiply accelerated process of its heating, reduction of the required number of highly skilled bricklayers- refractory men and minimal use of manual labor make it a promising direction.
Blocks of heat-resistant concrete can also be used in hot repairs of brickwork under middle charging holes in the zone of heating flues roofing. Hot repairs in this area are particularly difficult because of the high temperatures and the need to perform a large scope of work for temperature protection. Dismantling of the old brickwork can be conducted with the use of crowbars and long tongs, and erection of blocks - with cantilever and winch on the charging car.
Brick shapes of ovens roofing can be replaced by entire panels. This method significantly reduces the time of repair and allows to do things that are impossible at repair by bricks.
Arrangement of front walls over the flash plates after heating up of new battery can be performed by heat-resistant concrete, which, according to the author of the article, will be more stable than bricks.
Conclusions
The use of heat-resistant concrete for hot repairs in coke industry at this stage is not considerable not because of the negative effect, but due to the lack of attention to this issue.
Heat-resistant concrete successfully replaces fireclay part of brickwork of coke oven batteries and is characterized by the following advantages:
- the possibility of performing urgent repairs without expectation of manufacturing, supply and the need to keep a large stock of refractory bricks;
- improved gas proofness of cast block of fireclay brickwork;
- significantly greater strength of monolithic performance over brickwork;
- reduction of the repair period;
- reduction of labor costs;
- a sharp reduction in number of high qualification refractory men.
At improvement of the quality of heat-resistant concrete in thermal conductivity and abradability large perspectives of coke oven batteries brickwork replacement by concrete blocks are revealed.