Blast furnace-Inconsistent blowing and its effects

Frequent stoppages of blast furnace operation and consequences
Unstable operation
or Low production
When the metal below the tap hole level solidifies it has a large
mass and good thermal conductivity to the cooled bottom.
If other conditions are promoting inactivity a maintenance stop
can lead to a long-standing inactivity of the deadman.
The sensitivity to maintenance stops depends on the furnace
size, furnace cooling and hearth erosion.
Large furnaces have a smaller cooling surface calculated per ton
hot metal in the hearth, so they have a larger heat capacity.
Too effective hearth cooling increases the risk of inactivation
consequences
or Low production
rate increases the
risk of an inactive
deadman
the production rate being low the
residence time of coke in contact
with CO2 and alkalies becomes
longer and the coke properties
weaken.
Heat losses through the hearth
lining are relatively constant at
the beginning of the slow
production period. Heat
transport to the deadman slows
down and a risk of solidification
grows.
A planned or unplanned stop
during slow production may
inactivate the deadman
Frequent start and stop of blast
furnaces and cohesive
akmistrymistry@gmail,com,8895500177,9968605977

Coke lumps are weakened in the
shaft by solution loss reaction:C +
CO2 → 2 COc
CSR and the length of raceway cavity
In the high temperature zone coke loses its alkalies
and some SiO2. It is also graphitized to some
extent. These phenomena together with an abrasion
in the raceway lead to a considerable formation
of fines. High wind velocity makes the raceway act
like a jet mill producing increasing amounts of
coke dust.
With high injection rates the residence
time of coke in the lower part of the blast
furnace increases and the gasifying
degree increases resulting in lower
strength.
Coke is weakened also by an alkali
attack - both by circulating alkali
compounds and those present in
the mineral phases of coke
Coke properties

Steps in coke degradation during inconsistent blow

At the stockline, near the centre there is a
THE inverted V comes from two main
factors, distribution of raw materials at the
stockline and high blast velocity at the
tuyeres. All coke in the centre promotes
hot gas flow up the centre of the furnace (
hence need for high blast velocity of 180
m/sec min or more).
Wind reduction leads to change in cohesive
This continues until the high amount of
ore at the walls is in contact with less hot
gas meaning it reaches the CZ temp last.
The control of distribution is designed to
ensure this happens at the bosh parallel
level. Normally 40 to 45% coke at the
walls gives this result.
At the stockline, near the centre there is a
small amount of ore that comes into
contact with the hot gas and the Gas/ore
ratio is high so the ore reaches the CZ
temp early, at the top of the furnace. The
hot gas enters the lumpy zone via the coke
slits so now the situation of more ore in
contact with less gas resulting the ore
needing more time to reach the CZ temp..
Wind reduction leads to change in cohesive
from inverted –inverted-V to w-shape cohesive
,hence when we revert back to normal
operation ,fce resist to accept wind

Cohesive profile during Full blast and reduced blast effects, slip during
increasing wind from reduced blast condition
Cold material
which comes
to fce bottom
earliest
during
increasing
Cohesive profile
during full blast
increasing
blow
Duel
cohesive
during
reduced
blast
Short Blast Penetration

Reduced wind blowing will cause
the tuyere velocity to be at a low
level. As a result blast penetration
can be significantly reduced.
This can cause increased heat load
on the bosh area of the furnace.
Increased heat load in the bosh
caused the refractory bricks ( SiC
bricks ) to be lost.
Given this, It is recommended that
,pay very close attention to the
heat load in the bosh and if have a
lintel expansion joint at the
bottom of the bosh parallel , again
monitor this very carefully. Watch
for any cracking/increased gas
losses at this area.

Between the deadman and
the cohesive zone coke is
flowing down and towards
the raceways. Coke lumps
are partly fluidized and
flow freely down. If the
clearance between the
deadman and the cohesive
zone is too narrow coke
can not flow and the
burden slip.
Narrow section occurs
frequently with the
hanging cohesive zone
Because of the high
amount of fines, you are
facing two problems.
Erratic burden descent
and probably hanging and
slipping
wind acceptance and
therefore low tuyere
velocity. Frequent hang
and slip of blast furnaces
Wide Active layer
fludizing effect is more
granular volume is less
due to high cohesive
Narrow Active layer
fludizing effect is less
granular volume is high
due to low cohesive

Evolution of
clogged hearth
problem
Change in lower part of blast furnace
Monitoring DCI

The expression for carbon
saturation in hot metal is
shown below and there is a
strong temperature
dependence. It is suspected
that they have a very high slag
alumina content and high slag
volume - like all Indian
practices - and that they could
well benefit by raising the
metal temp.
%Csat = 1.3 +
2.57×10-3 x T -
0.31 x %Si-
0.33 x %P - 0.4
x %S+ 0.028 x %Mn
The probable reason for
the low carbon content is
the low metal temperature.

Reflection on tapping
Declining slag ratio is a clear consequence of an inactive deadman.
Molten hot metal, slag and coke fill the space between the hearth walls and the
inactive deadman.
Because this space is narrow the slag/metal interface area becomes smaller and
the slag flow is delayed (like in a chemist’s separating funnel).
the slag flow is delayed (like in a chemist’s separating funnel).
The slag tapping time is quite short and the simultaneously tapped amount of
hot metal relatively small.
The amount of remaining slag in the hearth grows to a higher level due to rapid
slag tapping.

Hot metal temperature
During a normal
operation the molten
metal below the tap
hole level is cooled by
conduction Heat
flows to the bottom
and wall cooling
systems.
When the deadman
becomes inactive a
great part of the
bottom metal is solid
and cannot mix with
the fresh hot metal.
Mainly hearth walls
cool this hot metal
and the contact area
is smaller in this case
compared with the
cooling from the
hearth walls and
bottom, resulting in a
smaller temperature
drop in hot metal.
During tapping this
cold metal is mixed
with hot newly
melted metal
resulting in a certain
hot metal
temperature.
As described above,
freshly melted hot
metal is collected in
the space between
the deadman and the
hearth walls.
In this case the silicon
content of hot metal
and fuel rate remain
practically unchanged
but the hot metal
temperature becomes
higher.

Soot formation
pulverised coal injection have become a new risk of a clogged hearth
These coke fines descend to the hearth and may clog the deadman
If the amount of injectant is too high ascending gas to the shaft brings increasing amounts of soot or char. Usually the coke fines in the
burden are consumed by solution loss reaction . In this case soot and char are more reactive and they are consumed instead of coke fines
The raceway tends to shorten and to bend up towards the bosh wall
Too high injection rate retards the consumption of coke breeze in the raceway making the bottom and the back wall of the raceway tight
(bird's nest)
inside the tuyeres produces more gas (CO and H2) and thus increases the gas velocity in front of the tuyeres. Increasing velocity will increase
the whirling speed of the tuyere coke and promote fines generation

Problem solver
• The first measure is to get coarse and good coke
to the hearth. The oldest measure to control the
blocked hearth has been coke size coarsening.
Charging of nut coke (10-28 mm) has been
stopped and the best quality coke has been
Inactivation of
deadman and
measures taken
to maintain hot
metal quality as
Charging of nut coke (10-28 mm) has been
stopped and the best quality coke has been
used
• The second measure is to push heat into the
inactive deadman to break its structure.
Maintaining the highest possible wind rate, high
flame temperature and high oxygen enrichment
does this to get more heat to the deadman
to maintain hot
metal quality as
good as
possible;by
increasing slag
rate and coke
rate

Identification of inactive deadman
Slag tapping ratio decreases
Hot metal carbon decreases
Oxygen potential (FeO and MnO in slag) increases resulting in decreased desulphurization
of hot metal
Hearth wall temperatures increase
Temperatures in the hearth bottom decline
Shortening of the tap hole.
Back walls of the raceways are hard (rod test)
Shortening of the raceway can be observed
Hot metal temperature is higher than usual with the same energy consumption and hot
metal composition
Slag tapping ratio decreases

Photos of tuyere probe sampling and position of fine particle layer in
front of raceway

Pressure loss during the tap cycle. The upper panel depicts the evolution when the
dead man sits at the bottom and the lower panel the evolution for a hearth with a
floating dead.

Evolution of the pressure loss at low (upper panel) and at high (lower panel) mean tapping rates
of iron. The tapping rates in the lower panel are approximately 50% higher than the tapping
rates in the upper panel and burden descend rate during the tap cycle

Possible explanation for the observed change in the tap internal evolution of the silicon content: For a permeable hearth (left figure), the iron is
drained to a lower level, and the variation in Si induced by the tap cycle is shifted “vertically”, causing mixing of the “layers” above and below the
top-hole during tapping

Strategy in plant and in BF Process - for stable operation
To have a stable operation,
we would like to run the BF
with a constant wind rate.
Constant wind rate should
One good example is If BOF
shop, is not consuming hot
metal as fast as we would
like to. There are less and
less empty torpedo ladle
cars for casting the BF. Some
actions need to be taken. In
some companies, this would
trigger the action of
dumping hot metal. (we call
In other case, if there is too
much fines into the furnace,
the delta P of the Bf is
higher than acceptable
level. It is absolutely
justifiable to reduce wind
Constant wind rate should
be something BF operators
strive for. However,
depends on the priority,
sometimes we do have to
adjust the wind rates to
minimize a bad
consequence because of the
situation.
dumping hot metal. (we call
it beaching the hot metal) so
that they can keep the wind
constant. For some others,
like TATA Steel in
Nehterland (Ijmuiden), they
will try to meter the
furnace. It means that at
certain number of empties,
it would trigger the action of
reducing production. This
would involve wind rate
reduction, reduce % oxygen
enrichemnt and top
pressure etc.
justifiable to reduce wind
rate. The way to do it is to
do small incremental wind
reductions. To have a good
furnace oepration, The wind
rate must match furnace
permeability. If you have
high delta P, you have to
blow less wind and not try
to overblow the furnace by
keeping constant wind rate.
Another situation which you
should not keep constant
wind rate is when you have
a very low stocklines.

Cause:Frequent stoppages force you to arrive at
situation where furnacec is not accepting wind due to
high blast pressure,due to following
•materials are bridging,
When there is "stiffness of furnace", It is presume
that the furnace is descending very slowly and not
smoothly. As we may well aware, burden descent is a
result of the balance of internal forces. When burden
has difficulty to descend, it implies that the upward
forces from the ascending gas or from the liquid is too
high.
•Wind reduction will give an opportunity to break
Soft blow when fce pressure increasing-wind acceptance is bad
•materials are bridging,
•slag is hanging up,
•irregular gas distribution due to fines,
•high liquid level,
•and bosh gas volume too high.
•Wind reduction will give an opportunity to break
the material bridging, redistribution the gas
distribution, let the slag drops down etc...

It’s hardly to monitor the work
state of hearth by direct way, so
The hearth of the blast furnace
is the starting and ending point
of the smelting process,
the work state of hearth could
influence the campaign life,
stable state and hot metal
quality of blast furnace.
state of hearth by direct way, so
the auxiliary monitoring method
such as thermocouple
temperature, hot metal and slag
tapping, hot metal temperature
were used to estimate the work
state of hearth

With the effective volume of the
blast furnace is upsizing in recent
years, the effective volume and the
hearth diameter of largest blast
furnace are reached to 5 800 m3
and 15 m in China, respectively.
Consequently, the flame
temperature of raceway zones are
different. According to the
calculation model of raceway
zone,the gas pressures in different
tuyere are also various, causing the
size of raceway zones are different,
affecting the first gas distributions
in the blast furnace hearth.
The numbers of raceway zones are
increased from one dozen to
several dozens, the amount of hot
blast and coal injection rate are
different in the various
tuyere,causing the combustion
conditions of coke and coal are
different for various raceway zones.

Blast furnace hearth are very crucial and disturb by frequent stoppages
of fce
If the uniformity and the activity
of the blast furnace hearth are
worsening, the permeability
index of blast furnace will be
while the tapping time will be
decreased, the amount of
tapping hot metal and slag in
different tapping turn will be
also unequal.
The uniformity and
activity of blast furnace
hearth are very crucial
to produce the high
quality hot metal and
prolong the campaign
life of blast furnace, it
also could
influence the burden
descend rate in the
peripheral direction.
Indirectly influence the
utilization rate of CO, production
cost, operation indexes and
energy consumption of a blast
furnace.
index of blast furnace will be
decreased, the blast will be
increased, then the BF cooling
water temperature will be
increased, and the fuel rate per
one ton hot metal will also be
increased,

It is necessary to establish the practical and effective system to obtain the evaluation system for
the blast furnace hearth. The flame temperature of raceway zone could directly influence the
melting slag and iron as well as heat transfer and mass transfer of furnace, which has a direct
relationship with the work state of the blast furnace hearth.Few extract of authors from
lierature.
Sasaki investigated the quenched No. 2 blast
furnace at Kokura works, and found that the
depths of various raceway zones were different.
And the shapes and positions of cohesive zones
for different tuyere were also different
Baosteel has proposed an active index to reflect
the work state of blast furnace hearth,12) but it
could just suitable to use in the Baosteel blast
furnaces.
Chen developed a new index to evaluate activity
index of blast furnace hearth based on the
flowing resistance coefficient of slag and hot
metal, which is hardly being used to estimate the
work state of blast furnace hearth because the
calculated parameters are not easy to gain.
Dai has modified the activity index of blast
furnacehearth based on the Chen’s
model,however, it also has the problem in
obtaining the calculated parameters. Xu has
established the activity index of blast furnace
hearth based on the pressure difference and
temperature of deadman, but it’s also hard to
get those two parameters
Besides China, in recent years, other
countries also suffered frequent
safety accidents of blast furnace
hearth.
Hearth is an important part of blast
furnace body. As a high temperature
and high pressure reactor, hearth
bears direct reduction reaction, high
temperature melting, frequent cast
and other tasks.
At the same time, hearth suffers most
severe high temperature reaction,
abrasion, erosion and other damage.
However, hearth lining can not be
replaced under medium repair, so
hearth safe has become a key link
limiting a whole campaign life of blast
furnace.
Long hearth service life design is a
systematic engineering, which is close
related to hearth structure design,
refractory selection, cooling system
arrangement and monitoring system
establishment

• Please send your comments regarding this
topic.
• Thanks for your valuable time for this
presentation.
presentation.

Blast furnace-Inconsistent blowing and its effects

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Blast furnace-Inconsistent blowing and its effects

Similar to Blast furnace-Inconsistent blowing and its effects (20)

Recently uploaded

Recently uploaded (20)

Blast furnace-Inconsistent blowing and its effects