DANIELI CORUS HOT BLAST STOVESONCE IN A LIFETIME

In the primary metals industry, market economics have created the need for increased productivity, extended campaign life and optimized process stability, all at the lowest cost per ton of hot metal.

In a blast furnace, high hot blast temperatures are essential as they reduce the furnaces coke requirement substantially and facilitate the injection of auxiliary fuels such as pulverized coal as a replacement for expensive metallurgical coke. This can have a significant effect in terms of reducing the cost of hot metal.

Danieli Corus high temperature hot blast stoves have now been developed to such a degree that a thirty year lifetime can be achieved at full capacity and without major repairs or maintenance, even when going through necessary cooldowns and heatups required for blast furnace repairs.

The design concept can also be implemented as a performance upgrade in an existing shell. In almost all cases, a higher capacity and higher duty design can be installed in the existing shell.

In general, only the nozzles are replaced in case of reuse of the shell.

Modern blast furnace operation has become increasingly demanding for the hot blast system. Continuous operation at high online hot blast temperatures requires exceptional stability of the steel shell and refractory design.

To this end, the Danieli Corus design incorporates a number of sophisticated design features, such as the ceramic burner, the acclaimed mushroom dome and the partition wall.

At the request of the client, the stove design can be based on alumina refractories. For most stoves, however, silica refractories are the material of choice for improved stability owing to the elimination of expansion movements in the upper structure during operation.

Silica refractories have an additional advantage over alumina since they are resistant to dust accumulation. For this reason, seven layers of silica checkers should be installed at the top of the checker shaft, in aluminabased stoves.

Silica stoves can nowadays be heated up and cooled down within a period as short as fourteen days. This can be achieved multiple times without detrimental effects to the stove.

Danieli Corus Hot Blast Stoves

1 General Arrangements: Retrofit in existing shell vs. new

2 Typical layout of a three stove system3 Failure of hemispherical dome4 Failure of traditional partition wall5 Danieli Corus design dome after 22 years of

operation6 Danieli Corus design partition wall after 32

years of operation7 Mushroom dome design and expansion

allowance for ring walls

1

Waste Gas

Blast Furnace Gas

Combustion Air

Cold Blast

Hot Blast

2

Traditional hemispherical domes, although simple in shape, have a natural instability with a tendency for the upper part of the dome to collapse first. An inverted catenary shape dome has a statically balanced shape and can be built with a minimum of special shape bricks. Since the mushroom dome refractory will also expand and contract, a hinged support construction will allow for these movements, without exerting any force on the structure.

Another problem of traditional designs is that the dome is supported directly by the ring wall. Since there is a difference in temperature between the combustion shaft and checker chamber, the ring wall will have different thermal expansion behaviour in these areas. This will cause cracks, emerging from the partition wall connection. This problem can be prevented by having a lintel support the dome, allowing for free expansion of the ring wall. Cracking due to radial expansion at the base of the dome can be eliminated by introducing a support based on a cantilever refractory construction with hinged elements.

In traditional stove designs, stability of the internal partition wall was always a critical issue. The lower part of this wall is exposed to the intermittant radiation heat from the burner while having the cold checker mass to the opposite side, causing steep temperature gradients. Different thermal expansion behaviour at both faces of the wall will cause cracking, jeopardize gas tightness and eventually destroy the wall.

Short circuiting during the blast cycle will reduce the temperature of the hot blast. Eventually, leaks might lead to overheating of checkers or even the grid and support columns.

In the Danieli Corus design, an intermediate insulation layer reduces much of the thermal stresses on the wall. Expansion allowance will relieve the wall of mechanical stresses due to thermal expansion behavior. In addition, heat resistant metal sheets are applied to eliminate gas leaking in the lower and middle area of the partition wall.

43

6

5

10-11 m typical

Free Expansionof vertical walls

Dome supporteddirecly by steel shell

7

Repairs, Retrofits and Upgrades

Danieli Corus promotes a design philosophy for hot blast stoves that is oriented towards achieving at least 20 to 30 year campaign lives. This holds for new systems as well as repairs, retrofits and upgrades. Such campaign lives have been achieved with hot blast stoves after repair by Danieli Corus.

If the objective is to not just repair the stove, but to retrofit a mushroom type dome or improved partition wall, the condition of the existing stove shell is assess with respect to e.g. intercrystalline stress corrosion and the ability to carry the heavier dome. Also, a reduction in stove height through the application of higher efficiency checkers can be considered.

Increased production targets for existing blast furnaces might induce the need to also increase the hot blast systems capacity. The existing hot blast stoves can be upgraded or, space permitting, a fourth stove can be built next in row.

1 New, fourth hot blast stove built next in row to an existing three stove system

2 Dome damage after 13 years of operation (retrofit in existing shell)

3 Dome damage after 29 years of operation (retrofit in existing shell)

4 Typical repair in partition wallring wall connection area after 24 years of operation (retrofit in existing shell)

31

4

2

Since the expansion behaviour of silica is such that above 600C no volume changes occur, it is the preferred material for hot blast stoves. Between 200C and 600C however, there is relatively large expansion.

In order for the material to remain stable during heatup to operating temperatures, welldefined heatup schedules are required.

Danieli Corus has a wide experience with heating up both internal combustion chamber type hot blast stoves of Danieli Corus design as well as external combustion chamber type stoves of other design. Cooling down or heating up stoves can confidently be performed within only fourteen days, and many projects were actually executed within even shorter periods. This results in minimum downtime and minimum production loss.

A simple heatup burner is used for the procedure. The burner is made out of carbon steel and can be made in house relatively cheaply. Designs for natural gas and coke oven gas are available. If used properly, the burners can be used for more than one heatup.

As each stove is unique in size, Danieli Corus determines the schedule and process parameters individually in each case. Based on our experience in engineering over 180 hot blast stoves, our schedules incorporate safe margins while minimizing the interruption to production.

Heatup and Cooldown Services

0

2

4

6

8

0 240 480 720 960Time (h)

MP

a

0 120 240 360 480Time (h)

0 12 24 36 48 60 Time (h)

Stress calculation results for (from left to right) 1C per hour, 2C per hour and 15C per hour heatup rates. The red line shows the tensile strength of the material, the other lines horizontal, vertical and maximum stress. The importance of controlled heatup is clearly demonstrated as in the right diagram: the tensile strength of the material is exceeded by stresses after around twelve hours.

Waste Gas Heat Recovery

In an integrated steelworks, the hot blast stoves account for 10 to 15% of the total energy requirement. Therefore, improvement of the efficiency of the hot blast stoves will result in substantial energy savings and operational expenditure savings on enrichment gas consumption.

To minimize the costs of energy, recovery of waste heat can be applied. The design waste gas temperature of modern stoves is approximately 400C. The remaining heat in the waste gas can be recovered and used for preheating of the combustion gas and/or combustion air for the stove.

A typical waste heat recovery unit will be capable of reducing the final waste gas temperature to just over 130C, minimizing enrichment gas consumption.

Benefits are largely based upon plantspecific parameters such as local energy prices and whether the BF gas is used for power generation or other integrated purposes.

The Ceramic Burner

For hot blast stove burners, reliability is not the only issue. They have also turned out to be essential in accomplishing long campaigns for some of the other parts of the stove. Clean combustion and equal heat distribution contribute to stability of refractories, and the checkerwork in particular.

Danieli Corus provides two ceramic burner designs based on the Maxlife and Maxe technologies. The Maxlife ceramic burner, developed at Hoogovens IJmuiden (now Corus) has a lower pressure drop over the entire burner and large air and gas slots to prevent pollution/plugging. The Maxe ceramic burner has a wider operating range and lower emissions, but may require more space depending on the diameter of the hot blast stoves. The Maxe bu