STOVE WITH A SELF-SUPPORTING DOME AND AN INTERNAL COMBUSTION CHAMBER

Jan Pisak and Stanislav Gencur  

Conventional stoves with a blast temperature of about 1000°C were previously used to heat the blast for the blast furnaces at the metallurgical plant in Kosice, Slovakia. An increase in the useful volume of these furnaces made it necessary to heat a larger volume of blast and increase its temperature. A new stove with a self-supporting mushroom-shaped dome was built. The stove was provided with a checker work made of high-quality refractories and a lining with keyed joints was installed in the combustion chamber. The combustion products from the combustion chamber are discharged in the vertical direction. The lining of the dome is not connected to the outside lining of the stove.

The first pig iron made in Slovakia was produced in the town of Kosice on three 1719-m³ blast furnaces. The furnaces came on line in 1965, 1967, and 1969, and the blast air was heated in conventional stoves to roughly 1000°C

As the useful volume of the furnaces increased, it became necessary to heat a larger volume of blast and increase the overall temperature of the latter. New stoves with an external combustion chamber and a heating surface of 100 m²/m³ volume were installed for the second blast furnace, which now has a volume of 2413 m³. The introduction of the new stoves made it possible to increase blast temperature to 1200°C. A new stove with a self-supporting mushroom-shaped dome was designed for furnaces No. 1 (volume 1880 m³;) and No.3 (volume 2150 m³ ). The reconstruction project also involved the installation of new checker work composed of quality refractories. The combustion chamber was lined using keyed joints. The factory also installed a ceramic burner in which the combustion products travel in the vertical direction. The lining of the dome is not connected to the outer lining of the stove. The dome lining, resting on the dome's supporting ring, was constructed of high-quality refractories. The idea behind the self-supporting lining is the installation of quality refractories on its internal part that compensate for the lining's expansion and the use of an expanding material for the lining proper. The mushroom shape of the dome makes the shell and the lining more stable.

A traditional blast-furnace stove has a built-in limitation on the maximum temperature of the dome (up to 1250°C) and is also characterized by inadequate heating surface (33 435 m²).

The new type of stove makes maximum use of the properties of the refractory material: dome temperature is increased to 1500°C, heating surface is increased to 42122 m², and installation of the stove on the site of the old stave does not require a new foundation or new pipes. The main difference between the traditional and new stoves is the design of the self-supporting dome.

The main parts of the stove are the steel shell with its unions, the checker work, and the refractory lining. The specifications of the furnace are shown below:

The unions are welded to the steel. The cylindrical lower part - the part where the unions are attached - is reinforced. The lower part also includes a load-bearing ring.

The checkerwork (Fig. 1) consists of posts and gratings that create a space for entry of the cold air and escape of the flue gases. Supports for installation of the heat-storing checker work are also provided in the stove.

In terms of its functions, the refractory liningis divided amongst the outer facing, the heat –storing checker work, the ceramic burner, and the dome. In terms of height, the lining is divided into zones I, II, III, and IV (Fig. 2.)

Fig.1. Equipment under the checkerwork.

Fig. 2. Structure of the lining of the stove.

The outer facing consists of four layers extending over the entire height of the stove. The innermost ring of this facing corresponds to the quality of the materials in the individual zones of the checker work. The lining (Fig. 3.) of the combustion chamber consists of two layers (over the entire height) that are formed by refractory beams and are connected by dowels.

The heat-storing checker work (Fig. 4) is the main part of the refractory lining. The function of the checker work is to store the heat of the flue gases and transfer it to the region occupied by cold air. Two types of specially shaped blocks (N105 and N106) are used, with each block having 11 cells (average cell diameter 38 mm). The blocks are laid in parquet fashion, so that the cells are continuous over the entire height of the check work.

Fig.3. Lining of the combustion chamber.

Fig.4. Heat-storing checkereork.

The ceramic burner (Fig. 5) is located in the lower part of the combustion chamber. The feed of gas and air is along the vertical axis of the chamber. The lining of the burner consists of special blocks connected by dowels. The composition and form of the blocks are such as to allow air to be fed into the upper part of the burner at a 45° angle to the gas flow. This leads to optimum combustion of the gas.

The lining of the dome (Fig. 6) consists of five layers connected to the shell of the self-supporting dome without an intervening gap. The bottom layer rests directly on the supporting ring of the dome. The arrangement of the special lined blocks used far the dome corresponds to the dome's mushroom shape. All of the pipes for this system are provided with special valves and gates. Platforms, catwalks, gangways, and ladders are provided to access the individual components. A crane is used to perform repairs or to replace the reinforcing lining.

Fig.5. Ceramic burner.

Fig.6. Dome of the stove.

All the parameters of the incoming and outgoing flows are measured in the stove. The most important characteristics are the temperature in the dome of the combustion chamber and the temperature of the flue gases. All the measured quantities and the signals indicating the condition of the equipment are analyzed in a data-processing system and in the systems that control the heating and overall operation of the stove. There is an interlock to ensure safe operation of the various components.

In designing a stove with a self-supporting dome, it was necessary to closely examine the utilization of the flue gases' heat. With the goal of keeping capital costs as low as possible, we chose a system that has a built-in metal heat exchanger of the rotor type (Fig. 7).

Fig.7. Main parts of the heat exchanger

The specifications of the heat exchanger are shown below:

The system that utilizes the heat of the flue gases (Fig. 8) operates by the following principles:

• the flue gases travel through the flue-gas sector;
• the gases give up some of their heat to the checker work;
• the gases are sucked out of the system and into a smokestack;
• the checker work is heated throughout by rotating the rotor in this sector;
• the air used for combustion enters through the air sector;
• the preheated air travels from the heat exchanger to the stove.
   

Fig.8. System for feeding combustion air to different stoves

Conclusion

1. It is known that the best results in blast-furnace smelting are obtained when the furnace has a stove in which the total weight of the checker work satisfies the following condition: 1 ton checkerwork/1 ton pig iron per 24 h. Thus, three stoves of the type described here are sufficient for one blast furnace.
2. Four stoves are needed for operation in the parallel regime. In this regime, the temperature of the blast is regulated without the addition of cold air. This makes it possible to increase the temperature of the blast and lower coke consumption. Thus, given the current prices for coke, it is more advantageous to build four stoves.
3. The lining of the checker work of stoves with a self-supporting dome consists of several parts extending over the height of this component. The top part is made of dinas, while high-quality refractoties are used in the lower parts.
4. The ceramic burners, with vertically directed combustion products, are designed so as to ensure mixing of the gas and air in the required proportion and prevent additional combustion of the gas under the dome and in the upper layers of the checker work.
5. The use of keyed joints in the lining of the combustion chamber and ceramic burner gives these two elements strength in the vertical direction.