Since the first set of 3-section top fired stoves were put into operation for more than 10 years, the combustion chamber has issues of hot blast outlet getting red hot and the conical dome failure, and the damage rate has been up to 90%. This seriously affects hot blast temperature and benefits of steel plants. Despite a number of optimization measures taken, the hidden trouble of the stress concentration is like a time bomb, collapse risks may occur at any time. Therefore, based on the structural defects of 3-section top fired stoves, a 4-section top fired stove is developed.

In 2003, China's ironmaking enterprises gradually accepted and adopted 3-section top fired stoves as a new type of energy-saving and high temperature stove type which has been used for more than ten years. 3-section top fired stoves are favored by the majority of clients because of its reasonable cost performance, low stove body, low investment and high hot blast temperature, and have been rapidly developed. So far, the usage of this type kind accounts for half of stove market. It has made a contribution to the development of China's iron and steel industry.

1 Structure and feature of 3-section top fired stove

1.1 Stove shell getting red hot and failure of HBO and dome

More than a decade of operation practices has shown that the combustion chamber of 3-section top fired stoves has more problems when delivering high hot blast, and issues of hot blast outlet getting red hot and dome failures have been normalized, and brings huge troubles to stove operators. Meanwhile, iron production and energy saving are negatively affected. Through a long-term market research and maintenance of this kind, we found that the conical dome of 3-section top fired stove directly perches on hot blast outlet wall leading to the stress concentration upon this structural defect. This the main factor that results in collapse of hot blast outlet and dome (During the technical exchange with HBIS ironmaking specialists regarding its relocation projects, technical experts pointed out that the structural problems of 3-section top fired stoves are more than 90%). This discovery provides scientific basis for optimizing existing stove structure, eliminating hidden troubles, avoiding extensive failures and designing an optimised stove type.

1.2 Problem analysis

Structural characteristics of 3-section top fired stove (see three-dimensional view): The first section is precombustion chamber (gas and air premixed chamber), the second section is combustion chamber (conical dome and hot blast outlet wall), and the third section is checker chamber (checker bricks installed and cold blast chamber). It can be seen from the section arrangement that the conical dome sits on hot blast outlet wall directly. The stress concentration of gravity of refractories, expansion force, overlap expansion force, sliding force, shear force, impact force and bounce of steel shell renders this area to be subject to the most issues. As a result, high hot blast temperature cannot be used normally, resources and energy are wasted and cost stays at a high level.

How to optimize the solution to structural defects, eliminate structural hidden dangers? Since 2003, hot blast outlet and the conical dome of more than 100 nos of 3-section top fired stoves for 450-2500m3 BFs have been repaired and rebuilt. However, the shortcomings of this kind still exist.

2 Structure and feature of 4-section top fired stove

2.1 Structure research and exploration of 4-section top fired stove

Firstly, the conical dome is independent from hot blast outlet wall (see right figure), and supported b stove shell. As a result, the concentrated stress is dispersed and the stability of the conical dome and hot blast outlet is ensured, high temperature impact, stove shell getting red hot and the failure of the conical dome are avoided. After the modification, 4-section top fired stove is able to sustain high hot blast temperature. In simple words, upper stove shell diameter of hot blast outlet is bigger than that of stove shell at hot blast outlet area. This is a distinguishing feature of 4-section top fired stove.

2.2 Stability proof for 4-section top fired stove

The combustion chamber of 4-section top fired stove is a proven and stable structure, which derives from the optimization of the combustion chamber of Nippon Steel ECC stove. In fact, the profile/outline and overall construction of refractory linings of 3-section top fired stove is the same as Nippon Steel ECC stove, which is a copy version. The difference lies in that the conical dome base of 3-section top fired stove is provided with a section of wall where hot blast outlet is located. However, hot blast outlet of Nippon Steel ECC stove is arranged at the small dome. The arrangement of hot blast outlet locate at wall of 3-section top fired stove leads to the stress concentration. As the refractory lining of the conical dome of Nippon Steel ECC stove is supported on stove shell, the stress concentration does not exist. Therefore, the service life of Nippon Steel ECC stoves for large capacity blast furnaces operating in Baosteel, TISCO, Masteel, Japan, etc is more than 30 years, and there is no record of conical dome failures. Hence one can see that the conical dome shall be independent from hot blast outlet wall, and directly sits on stove shell so as to the stress is relieved then longevity and long term high hot blast temperature of stoves can be guaranteed.

3 Modification practice of 3-section top fired stove with 4-section top fired stove as replacement

9 3-section top fired stoves for 2500m³ BFs at Hebei Zhongtie Plant have issues of hot blast outlet getting red hot and the conical dome failures. Grouting and nitrogen cooling are frequently adopted by the client as emergency measures. Hot blast temperature is less than 1200 oC. From September 9, 2017 to July 8, 2018, Yuxing Limited performed modification of the conventional 3-section top fired stoves with 4-section design as replacement. Such modifications cost 100 million China Yuan approximately which is a huge loss to the client.

It can be seen from above photos that a new section of stove shell is placed between the original combustion chamber and the conical dome. The diameter of the new section of stove shell is bigger than the existing one. The conical dome linings are directly supported on stove shell, and are also independent from hot blast outlet wall. After these optimizations, original 3-section top fired stoves are converted into 4-section top fired stoves. After the conversion, hot blast temperature is 20-40 oC higher that previous hot blast and stoves are running stably.

The main purpose of 3-section top fired stove conversion is to eliminate the complicated stress concentration and ensure a stable structure and high hot blast temperature.

4 Application of 4-section top fired stove

At present, a new round of capacity replacement and relocation of iron and steel plants is coming, which will make the majority of ironmaking clients, workers and scholars spend more energy in understanding, researching stove and choosing stove type. Recently, technical experts of Yuxing Limited were invited by HBIS to attend technology exchange and discussion meetings and learned about 34 years’ service life of the conical dome of Nippon Steel ECC stoves operating at BaoSteel. After a number of multi-level communications and discussion, it is concluded that the conical dome directly supported on stove shell and being independent from hot blast outlet wall is a crucial technology which is able to eliminate the complex stress and hidden troubles completely. Finally, Yuxing 4-section top fired stoves are approved by HBIS and applied in its 3x3580m3 BFs.

The combustion chamber of 4-section top fired stove has no structural defects and hidden dangers, which is the technical innovation and development direction of high temperature top fired stoves in China and even in the world. The stove profile, hot blast temperature, life span and structural characteristics of 4-section top fired stove can be comparable to that of Nippon Steel ECC stove. Moreover, footprint of 4-section top fired stove is smaller and total cost will be cheaper.