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Blast furnace hot stoves

Hot blast stoves (or hot stoves, hereinafter “HS”) are regenerative heat exchangers. Each of them represents a cylindrical structure filled with multiple course grid (checker-work) which is made of refractory bricks. Checker-work is the basic constructive element of hot stove, which defines the process of heat transfer from combustion products to cold blast.
 
 Full working period of regenerative hot stoves consists of two operating cycles:
  • Cycle of checker-work heating (gas period) when products of combustion of fuel gas (blast furnace gas or mixed gas) enter from the top and, passing through the checker-work, they heat it up;
  • Blast cycle (blast period) when air (cold blast) enters the previously heated checker-work from the bottom and, passing through it upwards, is heated up. The checker-work is thus cooled down.
In this regard, the number of hot stoves cannot be less than two. Normally, a system of three or four (extremely rare – five) stoves are used in the blast furnace operation.
 
Hot blast stoves define in many respects the techno-economics of blast furnace operation. The rise in the hot blast temperature leads to the reduction in the specific coke rate and enhancement of the specific productivity of blast furnaces, which essentially reduces the operating cost. Economic efficiency of blast heating depends on capital expenses for construction and repair of hot blast stoves. The level of maintained hot blast temperature, capital expenses for construction of hot blast stoves as well as the service life between relines of hot stoves strictly depend on their design.

Conventional designs of hot stoves

Until recently, only two types of hot blast stoves were used: the one with internal combustion chamber (when the combustion chamber is located within the stove proper along with the checker-work chamber) and the second one with external combustion chamber (when the combustion chamber is separated from the checker-work and is located in an external shell). These stoves have a number of disadvantages, basically stemming from the presence of the combustion chamber.

1. Hot stoves with internal combustion chamber

At present, hot blast stove with internal combustion chamber (see Fig. 1), where both the checker chamber and the combustion chamber are located within the same steel shell, represent the most commonly used type of hot blast stoves in iron making around the world. This design was introduced by a British engineer and inventor E.A. Cowper more than 150 years ago. Operation of this type of stove has revealed a number of essential weaknesses as follows:

  • “Short circuit”, or direct leakage of fuel gases through the separation wall between the combustion chamber and the checker chamber through cracks and joints between bricks. During the gas period this leads to a substantial increase of CO content in the waste gases reaching 5000 mg/m³, while the European norms are 100 mg/m³ only resulting in a significant increase in CO emissions into atmosphere and to deterioration of the ecosystem. Due to the same short circuiting reason during blast cycle, part of cold blast leaks from checker-work chamber into combustion chamber and gets mixed with hot blast leading to reduction of hot blast temperature (sometimes by 100ºС)
  • Leaning of the combustion chamber towards the checker chamber (“banana” effect) that leads to damage of both combustion chamber and checker brickwork as well as to displacement of checkers and partial loss of open channels and to a significant increase in hydraulic resistance of hot blast stove;
  • High-temperature creep of refractory bricks under the influence of high temperatures and brickwork load in the bottom part of the combustion chamber that causes deformation and collapse of brickwork, especially of hot blast branch;
  • Pulsating combustion, which originates from the acoustic excitation of tall combustion chamber similar to that of organ pipe and leads to strong vibration of structures and to a collapse of its brickwork, which in turn deteriorates the performance of hot blast stoves;
  • Uneven distribution of combustion products along the checker-work surface which may go up to ±15% and this reduces the efficiency of the checker-work performance and results in temperature fluctuation across the checker-work and in development of cracks in its bricks and in the checker supporting grid;
  • Cracking of refractories in the bottom part of combustion chamber (in the area where the burner is installed) due to thermal shock caused to refractory bricks during the change-over from gas period to blast period and visa versa.

These design deficiencies lead to frequent failures of the combustion chamber and, therefore, the combustion chamber is considered as the weakest element of the hot blast stove with internal combustion chamber. Long-term hot blast temperature operation of these stoves is limited to 1200ºС. The higher the operating hot blast temperature, the more frequent are shutdowns and relines of stoves which affect the blast furnace techno-economics.

Fig. 1 – Hot stove with the internal combustion chamber

2. Hot stoves with external combustion chamber

The most essential modification in hot blast stoves with internal combustion chamber was introduction of external combustion chamber where the latter is arranged in a separate shell (see Fig. 2). However, only two disadvantages are thus eliminated, namely- “short circuiting” and “banana” effects.
These hot stoves have a much more complicated dome design and a complex system of temperature expansion compensation of the checker chamber shell and of the combustion chamber shell and therefore, their cost is 30-35% higher. Moreover, much more space is required for such stoves due to a larger footprint, which makes it impossible to introduce in operating shops. Besides, the shell of this type of stoves has a high tendency to Stress Corrosion Cracks (SCC). Nevertheless, these stoves have a more reliable structure and they are mainly installed in large capacity blast furnaces. Long-term maximum operating hot blast temperature of these stoves is 1250ºC.

Fig. 2 – Hot stoves with the external combustion chamber

Shaftless hot stoves

The main disadvantages of conventional hot blast stoves with internal and external combustion chamber described above can be resolved if the combustion chamber (known also as the “shaft”) is eliminated as such. Hence, hot blast stoves having no combustion chamber in the conventional sense have been named “shaftless” also known as “dome combustion stoves”. Therefore, inter-reline service life of shaftless hot stoves is defined by the service life of its dome brickwork, life of the burner device located on top of it and that of the checker-work but not by the combustion chamber life.

1. Hot stove with annular pre-chamber in the dome base

The first shaftless hot stove in the USSR was developed by the All-Union Scientific Research Institute for Metallurgical Thermal Engineering (VNIIMT) under the guidance of Iakov Kalugin, has been introduced in 1982 in blast furnace No.4 of 1513 m3 volume at Nizhny Tagil Iron and Steel Works (NTMK; Russia, Sverdlovsk region). A short annular pre-chamber about 1 meter high (see Fig. 3), having 50 ceramic burners of small diameter, is arranged at the base of the enlarged dome made of silica. Ceramic Burners and other elements of this stove have been designed and tested by means of special testing facilities. Almost complete combustion of gas was achieved at the outlet of the pre-chamber and there was no pulsating combustion during any mode. Unevenness of distribution of combustion products across the checker-work is only ±5% (in a conventional hot stove it is ±15%). 

Fig. 3 – Shaftless hot blast stove with annular pre-chamber

This hot blast stove was initially tested for operation with design dome temperature of 1450ºC and hot blast temperature of 1350ºC. It has been operating reliably in the system along with two conventional hot stoves without any reline for the last 27 years delivering hot blast temperature of 1200ºC. Inspections of cooled stove (during BF shutdowns) after 9, 16 and 27 years of operation revealed good condition of all its elements. The inter-reline service life of shaftless hot stove is therefore defined by the life of the dome which is several times longer than the service life of combustion chamber, and for silica dome it reaches 30 years.
The above stove design was developed as an experimental analogue of hot blast stoves for magnetic-hydrodynamic power plant (known as MGDES-500) with blast heating temperature of 1700ºC, blast flow rate of 10,000 Nm³/min and blast pressure of 1.0 MPa. Because of an enlarged diameter of dome, such a stove cannot be fit in the existing dimensions of blast furnace stove unit having internal combustion chamber stoves in order to carry out one-by-one reconstruction (the unit at the BF No.4 of volume 1513 m³ at Nizhny Tagil Iron and Steel Works was incepted as a stand-alone stove). Therefore, the above design of hot stoves was not applied anywhere else.

2. Kalugin shaftless hot stoves (KSS)

Kalugin shaftless hot stove with a small diameter pre-chamber at the top of the dome (Fig. 4) has become the further development of the dome combustion concept. Gas is combusted in the ceramic burner device of “pre-chamber” type where the jet-vortex flow of gas and air is arranged in the vertical axis of the stove. Annular headers of gas and air are arranged inside the brickwork of the KSS pre-chamber and the latter is independently supported on the dome shell. Such a design has been incorporated for the first time in BF No.1 of Satka Iron-Melting Works in 1992, and the above hot stove has been operating failure-free up to the present moment.

 Fig. 4 – Kalugin hot stove

Numerous trials were carried out at special combustion and aerodynamic test facilities for several versions of pre-chamber burner devices. According to the acquired data, the most reliable and suitable design of the pre-chamber burner was selected from the viewpoint of technical parameters, and simplicity and reliability of performance of the refractory lining as well. Furthermore, a method of 3D numerical computer calculation has been developed based on those tests, a detailed comparison of calculated and experimental results has been made and an excellent correlation was established. At present, calculations of all pre-chamber burner devices of Kalugin hot blast stoves is made based on this 3D numerical computer calculation method.

Jet vortex of gas and air in the pre-chamber (Fig. 5) ensures intensive and uniform mixing & combustion of gas and the process of combustion is over in the middle part of the dome before entering the checker-work. The optimum degree of jet vortex is obtained experimentally and can also be verified through theoretical calculations. Thus, unevenness of distribution of combustion products across the checker-work surface does not exceed 3-5%.

Fig. 5 – Jet vortex of gases in the pre-chamber of Kalugin hot stove

Pre-chamber burner design for each KSS is of tailor-made engineering, as well as that of the optimum number of nozzles, their location and angle, gas and air velocities and the degree of jet vortex are also calculated individually. Based on the data obtained, the patterns of gas combustion, temperatures and flow distribution are prepared and subsequently engineering of the refractory lining and steel shell is carried out.

An example of pre-chamber burner 3D numerical calculation results is shown in Fig. 6. It is obvious that gas combustion starts in the pre-chamber and completes in the top part of the dome so that before entering the checker-work, the gas is almost burnt out having the content of carbon monoxide max 50 ppm. When gas is moving down the checker-work, after-burning of remaining carbon monoxide occurs and at the outlet of KSS the content of CO is less that 20 mg/m³. Hence, the ceramic burner system of the pre-chamber ensures a very intensive mixing and instant combustion of gas owing to the jet vortex flow of gas and air

Jet vortex flow distribution may be different for Kalugin hot stoves of different thermal capacities. Research works carried out in the operating KSS with oxygen (O2) content in exhaust waste gas in the range from 0.3 to 5.1% during full thermal capacity operation have shown that the jet-vortex burner ensures the content of carbon monoxide (CO) not more than 0.0016%, or 20 mg/m3, which is 5 times less than the European standards (Fig. 7). The Kalugin ceramic burner performs better than the most commonly used slot-hole ceramic burner of DME design (Germany).

 

Fig. 7 – Relation of the carbon monoxide (CO) concentration in exhaust waste gas on the oxygen concentration

Fig. 6 – Location of the gas combustion zone in KSS. The content of carbon monoxide (CO) is determined based on 3D numerical computer calculation for full thermal capacity operation

As obvious, there is no “short circuiting” in KSS and the hot blast stove will remain ecologically friendly throughout the entire service life.
Since there is no pulsating of combustion, it is possible to enhance operating modes of KSS without great pressure fluctuations and vibrations, and therefore, damage to brickwork and structures is avoided.
Hydraulic resistance of the hot blast stove is insignificant and a normally available pressure of gas (4.0-5.0 kPa) at the burner inlet is sufficient for full capacity operation.

Jet vortex of gas and air in the pre-chamber of Kalugin hot blast stove ensures an even combustion of gas and subsequent distribution of flows of combustion products across the checker-work surface (see Fig. 8) and requires no adjustment of the ceramic burner before commissioning. Refractory lining of most of the parts of KSS is quite simple except that of the pre-chamber.

Fig. 8 – Distribution of velocities of combustion products at the checker inlet:
 
1 – Hot stove with the internal combustion chamber;
2 – Hot stove with the external combustion chamber (such as Didier-Werke);
3 – Kalugin hot stove

Since there is no the direct impact of flame on the brickwork in KSS, local overheating is avoided and this also provides a symmetric distribution of temperatures along the dome, checker, refractory lining and steel shell. Hence, the thermal stress on all design elements is reduced significantly and the service life of the stove is improved.

As measurements have shown, the average level of temperatures in the pre-chamber brickwork is low (about 900ºС on the average), and fluctuations in temperatures of the brickwork between the gas and blast periods are close to fluctuations in temperatures in the brickwork of the ceramic burners of the first shaftless hot stove with the annular pre-chamber (stove No.4 (23) of BF4, NTMK) which has been operated since 1982 without capital repairs and is still in a good condition. This allows us to determine long term service life of the pre-chamber brickwork, and thus the Kalugin stove lifetime is specified by the service life of the silica dome, which reaches 30 years without reline.

 Fig. 9 – Scheme of the KSS operation (Animation)

Design of the dome proper also promotes a longer service life at high temperature operation because the wider part of sphere above the checker-work passes to the conical part and further to the narrow neck accomplished in the ceramic burner dome, which has a much smaller radius and operates at relatively low temperatures. The diameter of the KSS dome having an independent support of its brickwork on the shell, as compared with the typical hot stove, is only slightly wider, and therefore Kalugin hot stoves can be fit well into the dimensions of existing stove units in case of their modification into a shaftless stove unit.

Cut-off valves of gas and combustion air were installed in the dome area in the first Kalugin dome combustion stove. Presently, new arrangement and technological solutions are developed, in which all technological equipment of KSS system (hot blast valves, cut-off and control valves of gas and air) is located either on the existing maintenance platforms at the bottom, or on new platforms in the upper part of the shell. As a rule, all equipment is located on one side of the hot stoves system (see Fig. 10) and this reduces the space for its arrangement. Technological equipment of KSS is serviced with lifting equipment installed in the Stove House. In general, the operation process of KSS practically does not differ from the operation of conventional hot stove system.

Fig. 10 – Plan and section of the system of Kalugin hot stoves

Благодаря точному теплотехническому расчёту и рациональному размещению огнеупорных материалов в благоприятных для них температурных зонах, достигается их высокая эксплуатационная стойкость. При этом устраняется забивание насадки пылью и шлаком (см. рис. 11), включая насадку с диаметром канала 20 мм. Это обеспечивает срок службы воздухонагревателя без капитального ремонта до 30 лет.

Степень использования тепла продуктов сгорания, то есть количество тепла, переданное нагреваемому воздуху дутья, и температура его зависят от величины теплообменной поверхности насадки, от коэффициента теплообмена, от аккумулирующей массы насадки и её теплофизических свойств.

В первых бесшахтных воздухонагревателях Калугина насадка выполнялась, как и в обычных ВН, из типовых насадочных шестигранных блоков с диаметром каналов 40 мм (см. рис. 12,а). ЗАО «КАЛУГИН» специально для вновь сооружаемых воздухонагревателей были разработаны новые виды насадки:

  • с диаметром канала 30 мм (рис. 12,б) в двух вариантах: цилиндрический канал (поверхность нагрева – 48,0 м²/м³) и конический канал (поверхность нагрева – 48,7 м²/м³);
  • с диаметром канала 20 мм (рис. 12,в) и поверхностью нагрева 64,0 м²/м³.

Рис. 11 – Состояние поверхностей динасовых насадочных блоков верхнего ряда насадки бесшахтного ВН ДП №4 НТМК через 9 (а) и 27 (б) лет эксплуатации

За счёт развитой поверхности нагрева и высокого коэффициента теплоотдачи этих насадок, а также устранения камеры горения, высота насадки значительно снижается (на 40-50%). При сохранении тепловой мощности воздухонагреватель становится малогабаритным, достигается существенная экономия огнеупорных материалов (до 50%) по сравнению с обычными воздухонагревателями, то есть вместо одного обычного ВН может быть построено два ВНК. На рис. 13 представлено сравнение габаритов старых воздухонагревателей для доменных печей V=3000 м³ ОАО «ЗСМК» (а) и V=5500 м³ ОАО «Северсталь» (б) и новых ВНК, построенных и строящихся взамен на их месте.

Рис. 12 – Насадочные блоки

Рис. 13 – Сравнение габаритов воздухонагревателей:
а) ОАО «ЗСМК», V=3000 м³; б) ОАО «Северсталь», V=5500 м³

ВНК позволяют работать с подогревом газа и воздуха горения до 600ºС. Это даёт возможность достичь температуры горячего дутья до 1400ºС при сжигании одного доменного газа с теплотой сгорания до 3000 кДж/м³ (около 720 ккал/м³ в пересчёте на сухой газ). На практике ВНК уже работают с подогревом газа до 200ºС и воздуха горения до 570ºС.
Применение утилизации дымовых газов и подогрева газа и воздуха позволяет получить технологические преимущества за счёт снижения расхода кокса и увеличения производительности доменных печей, а также существенные выгоды по экологии благодаря уменьшению тепловых выбросов (снижение температуры дыма, выбрасываемого в атмосферу) и уменьшению выбросов CO2 (замена добавок газа с высокой теплотой сгорания на физическое тепло).

За счёт улучшения условий службы для огнеупоров в ВНК при использовании широко применяемых огнеупорных материалов (динас, муллитокорунд, шамот) можно получить температуру горячего дутья до 1400ºС, что невозможно при других конструкциях ВН. Это даёт возможность перейти в доменном производстве на новый уровень нагрева дутья – до температур 1300-1400ºС.

Значительное снижение капитальных затрат и большая экономия на ремонтах благодаря увеличению межремонтного срока службы, возможность увеличения температуры нагрева дутья на 100-200ºС и возможность размещения бесшахтных воздухонагревателей на месте существующих с установкой типового основного оборудования на существующей рабочей площадке, малое гидравлическое сопротивление и работа без пульсаций с весьма низким содержанием вредных выбросов в дыме дают воздухонагревателям Калугина значительные преимущества по сравнению с существующими аппаратами и определяют их как наиболее перспективную конструкцию высокотемпературных воздухонагревателей.

Конструкция ВНК запатентована в России, Украине и Китае, патентуется в Японии, Индии и других странах.

На стадии, предшествующей детальному проектированию воздухонагревателей Калугина, специалистами нашей компании последовательно выполняются:

  1. общий теплотехнический расчёт блока воздухонагревателей;
  2. расчёт насадки;
  3. расчёт форкамеры с определением количества сопел, их размеров и расположения, скорости газа и воздуха и степени закрутки потоков;
  4. расчёт футеровки с определением количества, толщин и температур слоёв различных огнеупорных материалов;
  5. прочностной расчёт кожуха;
  6. прочностной расчёт поднасадочного устройства;
  7. компоновка блока ВНК с трубопроводами и оборудованием;
  8. гидравлический расчёт воздухонагревателя и системы трубопроводов с определением их диаметров и требуемых давлений сред;
  9. определение характеристик оборудования и составление его перечня (предварительной спецификации).