DANIELI CORUS HOT BLAST STOVES ONCE IN A LIFETIME

DANIELI CORUS HOT BLAST STOVES
ONCE IN A LIFETIME
Danieli Corus Hot Blast Stoves
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.
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.
In a blast furnace, high hot blast
temperatures are essential as
they reduce the furnace’s 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.
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.
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 cool–downs
and heat–ups 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 re–use of the
shell.
1 General Arrangements: Retrofit in existing
shell vs. new
2 Typical layout of a three stove system
3 Failure of hemispherical dome
4 Failure of traditional partition wall
5 Danieli Corus design dome after 22 years of
operation
6 Danieli Corus design partition wall after 32
years of operation
7 Mushroom dome design and expansion
allowance for ring walls
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 alumina–
based 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.
1
Cold Blast
Combustion Air
Blast Furnace Gas
Hot Blast
Waste Gas
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.
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.
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.
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.
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5
Free Expansion
of vertical walls
Dome supported
direcly by steel shell
10-11 m typical
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6
4
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 system’s capacity.
The existing hot blast stoves can
be upgraded or, space permitting,
a fourth stove can be built next
in row.
2
1
3
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 wall–ring wall
connection area after 24 years of operation
(retrofit in existing shell)
4
Heat–up and Cool–down Services
Since the expansion behaviour of
silica is such that above 600°C
no volume changes occur, it is
the preferred material for hot
blast stoves. Between 200°C and
600°C however, there is relatively
large expansion.
In order for the material to remain
stable during heat–up to operating
temperatures, well–defined
heat–up 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 heat–up 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 heat–up.
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.
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.
To minimize the costs of energy, recovery of waste heat can be
applied. The design waste gas temperature of modern stoves is
approximately 400°C. 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 130°C, minimizing
enrichment gas consumption.
Benefits are largely based upon plant–specific parameters such
as local energy prices and whether the BF gas is used for power
generation or other integrated purposes.
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6
MPa
Stress calculation results for (from left to right) 1°C
per hour, 2°C per hour and 15°C per hour heat–up
rates. The red line shows the tensile strength of
the material, the other lines horizontal, vertical
and maximum stress. The importance of controlled
heat–up is clearly demonstrated as in the right
diagram: the tensile strength of the material is
exceeded by stresses after around twelve hours.
4
2
0
0
240
480
Time (h)
720
960
0
120
240
Time (h)
360
480
0
12
24
36
Time (h)
48
60
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 ‘Max–life’ and ‘Max–e’
technologies. The ‘Max–life’
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 ‘Max–e’ 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 ‘Max–e’
burner has been installed in many
external combustion chamber
type hot blast stoves and was
recently modified to fit in internal
cumbustion chambers. The
‘Max–life’ burner has proven
performance in many hot blast
stoves since the 1970’s.
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2
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4
Mixing Chamber
The hot blast system includes a
mixer for controlling the hot blast
temperature by mixing cold air
with the hot blast. In case of the
preferred central mixing method,
it is located in the hot blast main.
The latest design mixer has four
inlets for cold air, as opposed to
eight in the traditional design.
Computational Fluid Dynamics
calculation were excuted to
assess the mixing performance of
this mixer design and compare it
with the results of older designs.
momentum of the cold airflow
per pipe of the redesigned
chamber is twice as high as the
momentum of the cold air per
pipe of the traditional chamber.
The cold airflow will therefore
penetrate better into the hot blast
flow and this will result in better
mixing, and hence a more evenly
distributed hot blast temperature
to each tuyere.
4
The results of the analyses show
that the newer mixer performs
better than the traditional design.
The reason for this is that the
1 ‘Max–life’ ceramic burner after 12 years in
operation
2 Even heat distribution across checkers using
‘Max–life’ ceramic burner
3 Top of checkerwork left unaffected by
good combustion (top) and damaged by
ill–performing burner (bottom)
4 Artist’s impression of ‘Max–e’ burner
5 Mixing chamber, typical design
6 Mixing performance of traditional design:
longitudinal section over 5 meters and
cross–sections after 5 and 30 meters
7 Mixing performance of new design:
longitudinal section over 5 meters and
cross–sections after 5 and 30 meters
Computational Fluid Dynamics: blue
indicates low temperatures, red indicates
high temperatures. More even color gradients
in cross–sections demonstrate better mixing.
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Danieli, the Reliable Innovative Team in the Metals Industry
Danieli Headquarters
Danieli Corus
Via Nazionale, 41
33042 Buttrio (UD)
Italy
T +39 0432 1958111
F +39 0432 1958289
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1970 CA IJmuiden
The Netherlands
T +31 (0)251 500500
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E info@danieli–corus.com
W www.danieli–corus.com
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