Up to now, top combustion hot blast stove has become the best energy saving, environmental protection and high blast temperature technology in iron making industry. Among them, Yuxing catenary top combustion hot blast stove has been more and more recognized by experts and favored by users because of its characteristics such as high hot blast temperature, high efficiency, stability and ultra-low NOx emission and longevity.

1 Development situation of Yuxing catenary top fired stove

On December 28, 2007, Yuxing catenary top-fired hot blast stove for 450m³ blast furnace of Tongcai Company of Shanxi Jianbang Group was put into operation. It is the first 3 Yuxing catenary top-fired hot blast stoves put into operation in the world, which marks its successful industrial application. With air preheating temperature of 200℃, design blast temperature is 1150℃. Checkerwork is made of 7-flue high alumina checker with flue dia. 40mm. Up to now, these three hot blast stoves have been running safely and stably for 12 years. During shutdown, specialists enter the stove for inspection. Inspection of cooled stoves revealed that burner, dome, stove proper and hot blast outlet are not damaged, and stoves have not been repaired in a medium or minor way. Since the operating hot blast temperature is 100℃ higher than the designed hot blast temperature, high alumina checker bricks adapting to 1150℃ are adopted in accordance with design hot blast temperature and the requirement of the client. Due to the increase in operating hot blast temperature, slagification and deformation of high alumina checker bricks happed. Checkers are thus choked. So, high alumina checkers are replaced by silica checkers. So far, these three 3 stoves are still running stably under high hot blast temperature.

High hot blast temperature and stable structure of the first 3 Yuxing catenary top-fired hot blast stove have attracted the attention and praise of many ironmaking operators, which promoted the rapid development of Yuxing catenary top-fired hot blast stove technology.

In July 2012, Yuxing catenary top fired stove with design hot blast of 1300 oC for 1860m³ BF at Shanxi Tongcai Steel, which affects the iron and steel industry, was put into operation. The blast furnace is designed to be equipped with four top-fired hot stoves and two preheating stoves. In October and November of the same year, the average monthly hot blast temperature was up to 1314.7℃, which created the highest blast temperature record in China's history and has not been surpassed yet so far. In April 2018, BF capacity was expanded. Inspection of cooled stoves shown that three stoves and all elements are in good conditions except one stove having 1.5m2 spalling area with thickness of 10cm. There is no convex and concave at top of checker bricks showing even distribution of combustion products. Practice has proved that Yuxing top combustion hot stove has the advantages of stability, longevity, energy saving and high hot blast temperature.

Since the first set of 3 Yuxing catenary top-fired hot stoves putting into production in 2007, there have been more than 400 catenary top-fired hot blast stoves (excluding more than 160 top-fired hot blast stoves with a small dome design) for blast furnaces with volume from 450-3580m3 operating at home and abroad. Yuxing catenary top fired hot blast stoves have been erected in China, India, Vietnam and Indonesia, and are working with European metallurgical design institutes to promote Yuxing catenary top fired stove in European and American countries.

2 Features of Yuxing catenary top fired stove

A number of application cases have proved that Yuxing catenary top combustion hot blast stove has obvious advantages over other hot blast stoves, mainly manifesting from the following aspects.

2.1 Safety, stability and longevity

Reasonable structure is the basis for safety, stability and longevity of catenary top combustion hot stove. Catenary dome, ceramic burner and straight section hot blast outlet of catenary top-fired hot stove are independently supported on the stove shell, with stress decentralized and reasonable structure. The residual CO (carbon monoxide) in gas ring and gas short pipe connected with the burner is safely handled without popping, pulsation and backfire, and thus damages to the burner is avoided. Annular staggered arrangements of burner air, gas nozzles located at the upper end of the precombustion chamber adopt vertical jet short flame combustion technology. The central high temperature combustion area of combustion chamber is separated from the combustion chamber lining via the annular gas curtain. Waste gas flows result in swirl flows and vortex, even mixing and complete combustion.

2.2 High hot blast temperature and high thermal efficiency

Yuxing catenary top-fired hot blast stove has a high thermal efficiency of 83.5%-88.41%. When the excess coefficient of air is 1.02-1.06, BF gas is completely burned out. Hot blast temperature is increased and gas consumption amount is saved. The monthly average hot blast temperature of 1314.7℃ is attained, which is the Chinese highest historical record.

2.3 Minor temperature difference between catenary dome and hot blast

The difference between dome temperature and hot blast temperature is 30-50℃. Since the difference between dome temperature and hot blast is small, the increase in hot blast temperature is of great potential. While increasing the hot blast temperature, dome temperature is reduced, and the generation of NOx is greatly reduced. This conforms to the environmental protection policy and the occurrence of SCC of stove shell is avoided as well.

2.4 Environmental protection

Yuxing catenary top combustion hot stove can achieve ultra-low NOx emission. In June and July 2019, Yuxing Limited entrusted two authoritative environmental monitoring organisations to perform waste gas composition analysis of 14 Yuxing catenary dome stove for 1000-3000m3 BF operating at Hebei Jingye Steel, Hebei Aosen Steel, Zhongtie Plant, Qinhuangdao Anfeng Steel, Tangshan Ruifeng Steel and Zongheng Fengnan Steel. Test results show that NOx emission ranges from 20 to 40 mg/Nm3 in waste gas.

2.5 Self-regulating waste gas and cold blast flows

Pressure and flow equalizing checkers can adjust the distribution of waste gas and cold blast flow field by itself, which can greatly improve the utilization rate of checkers, increase hot blast temperature, save energy and reduce consumption.

Yuxing catenary top combustion hot blast stove technology is not a copy, but is an optimised technology via the production practice, continuous investigation, research and analysis, flexible innovations. Long-term practice and continuous improvements of Yuxing catenary dome combustion hot stove render its thermal efficiency 3.5%-8.5% higher than other stove technology, hot blast temperature of 1314.7 ℃ to be achieved, NOx emission 81.25% lower less than the national standards, CO emission in waste gas less than 100 PPM, dome temperature and hot blast temperature difference less than 50 ℃. In particular, pressure and flow equalizing checkers with flue mutually connected developed by Yuxing can effectively improve the utilization rate of checker brick and enhance hot blast temperature. After nearly 20 years of in-depth research, investigation and practice, continuous improvement and optimization, Yuxing top combustion hot blast stove has become the mainstream and leading international technology.

3 Typical damage case analysis and correct operation method

At present, Yuxing hot blast stoves have been operating all over China and even in the world. However, there are some problems in the process of improvement and development of Yuxing hot blast stove technology. Statistical data show that due to improper operation, inadequate management, changes in safety operation procedures and other factors, a few Yuxing top-fired hot blast stoves were damaged, and the damage rate is 2.18%. For this reason, Yuxing Limited has investigated some typical damage cases of 550 Yuxing top combustion hot blast stoves under high hot temperature operation and formulated corresponding technical measures.

Case 1

Yuxing catenary top fired stove for 1200m3 BF at Hanzhong Steel, commissioned in 2010, hot blast temperature ranging from 1200-1280 ℃. The dome is made of high alumina bricks. After 5 years of high hot blast temperature operation, dome plug bricks and adjacent surrounding refractory bricks started falling off in 2015. Besides, there is 1 meter hole happening at the middle part of a stove dome. However, hot blast outlet, nozzles of ceramic burner and checker bricks are still in good conditions. Yuxing Limited has conducted a serious investigation and analysis, and believes that the main reasons are as follows:

1) Design conflict

The designed hot blast temperature of hot blast stove is required to be greater than 1200℃ (actual operation hot blast over 1250℃). For the high temperature area of the combustion chamber dome, Yuxing plans to use high-quality silica bricks, but the user insists on using low creep and high alumina bricks leading to local damages of the dome. Since then, Yuxing Limited insists on using silica brick for high temperature hot blast stoves. The issue never happens again. Only by adhering to the design concept can similar problems be completely avoided and the hidden danger of subsequent damage to the dome be eliminated.

2) Installation factor

As catenary dome has a certain natural radian, installation has to be done by erection team with rich experience. Otherwise, it will cause serious damages or shorten the service life of stove dome. Figure 1 is erection photo of dome plug.

3) Improper maintenance

Hot blast stove is like a car. When it runs for a certain period of time, a certain gap is formed between the stove shell and refractory materials due to the thermal shrinkage performance of high alumina bricks. This requires grouting maintenance. However, due to the user's lack of maintenance awareness, the long-term high temperature between the stove shell and refractories causes refractories falling off and continuous damages. Therefore, high alumina bricks are not allowed to apply in high temperature area in the design stage. Experienced installation and construction organisations are required for the installation of refractories. Maintenance should be focused during the operation and stability and longevity can be ensured.

Figure 1 Erection photo of dome plug.

Case 2

Yuxing catenary top fired stove for 1200m3 BF at Hanzhong Steel, commissioned in 2012, hot blast temperature ranging from 1230-1280 ℃. At the beginning of 2015, it was clear observed that gas and air nozzles were seriously damaged through manhole and peephole. At the same time, it was gratifying to see that dome, hot blast outlet and checkers are in good conditions. According to the damage status of nozzles, Yuxing experts hold the view that there was a problem with nitrogen purging, and then went to control room to know the situation. They confirmed that the real reason for the damage of the burner nozzles lying in that nitrogen purge was suspended or nitrogen purge was insufficient (see Figure 2). A similar case also appeared in another steel plant. Therefore, nitrogen purging of hot blast stoves is an important condition to ensure the structure stability, longevity, energy saving, high efficiency and high hot blast temperature of stoves. Yuxing also hopes clients respect the design concept and do not arbitrarily change the operating procedures in order to avoid similar accidents.

Case 3

Ringwall and checkers of stove are seriously damaged and checker support system was damaged in a steel plant. Due to the high hot blast temperature of current hot blast stoves, high temperature areas of stove use high quality silica bricks with poor thermal shock resistance but good high temperature resistance property. Under high temperature conditions, fragmentation will occur when silica bricks meet with a large amount of water. The high-temperature water-cooled valve of this steel plant was seriously damaged. Leaking water entered the inside of hot blast stove resulting in failures of ringwall and sinking of checkers. (see Figure 3, water flow trace and damages at the lower end of hot blast outlet). As a result, overhaul of stove is required and the steel plant suffered heavy economic losses. Be sure to keep in mind that water is not allowed to enter hot blast stove. Once it enters stove, in addition to damaging stove structure, steam instantly produced may cause explosion. Therefore, the comprehensive responsibility management for the stability and longevity of hot blast stoves is very important. Enterprises should strengthen the management and implement the responsibilities in place.

After years of development, Yuxing catenary hot stoves have been widely used in blast furnaces. In practice, the failure rate of Yuxing hot blast stove is very low. Damage rate of Yuxing hot blast stove caused only by improper operation and change of operation procedure is only about 2%, which reflects the simple structure and reasonable force of stove design. From the investigation of typical damage cases, it can be seen that emerging issues are not design problems. Nevertheless, we still need to deeply study the various factors affecting the stability, longevity and high hot blast temperature of hot blast stoves so as to make greater contributions to the technological progress of China's iron and steel industry.

Figure 3 Water flow trace and damages at the lower end of hot blast outlet