blast furnace -dry wet coke in use and effect

1. Dry coke to wet-Actions

2. Changes during dry to wet coke
screen jam due to high moisture
Nut coke outer surface covered with fines
Burden head end and tail end lead with nut coke
Coke Bunker levels difficult to maintain
Fines increases result in reduction in wind acceptance
Underutilisation of nutcoke to take care of acceptance impact on low level of coke result in fines due to high fall.
Centre coke surface coke proportion change
Process upsets /control on process

3. Process Control
Temperature control
•Thermal Control scheme
Gas control
•Burden distribution
Chemical control
•Burdening

4. Thermal Control
Purpose of;
Ensure temperature of
hot metal is as required
by steel plant, while
maintaining a fluid slag
Controlled by;
Coke rate
Measured
with;
Hot metal temperature
maintaining a fluid slag
Tuyere injectants Top temperature
Heat losses

5. Chemical Control
Purpose of; chemical
control
• Ensuring the correct HM
composition for steel plant,
and slag granulate for
customer
Controlled by;
• Burden chemistry
Measured with;
• Hot metal analysis
• Slag analysis
• Top gas analysis

6. Steam and PCI: T Flame and Hydrogen
• The furnace content sees only:
– Gas, consisting of CO ( CO2), H2 ( H2O), N2
– Heat
– The furnace doesn’t know where hydrogen comes from. Moisture? Coal?
• Hydrogen
– Normal operation for all coke
• HBT 1010, moisture 40 gr/NM3, no oxygen enrichment, O2/tHM 245 NM3,
– blast volume = 1200 NM3/t, 48 kg steam per ton: 5,5 kg H2/tHM of which ~2
kg comes from ambient moisture
kg comes from ambient moisture
– Coal contains 4-5% hydrogen, 3,5 kg hydrogen comes from ~80-90 kg
PCI/tHM
– At PCI of 80-90 kg/tHM, steam injection can be 0
• Step by step, reduce first to moisture in HB to total 30 gr/NM3
• Learn to use PCI for “safety”
– Availability PCI
– Works only if T Flame is > 2050 C
• Heat: control T Flame 2050-2250(?) C
– Too high: burden descent can become erratic
– Too low: PCI “doesn’t work”

7. Out of Control Sheet
Be aware of
furnace ‘out of
control’ when;
Burden descent is
poor
Keep furnace
under control by;
Reducing blast
rate
Furnace cooling
down
Water ingress
into furnace
Long stops
Casting
Using ‘safe’
burden (low
basicity, high coke
rate)

8. Defining ‘Out of Control’
• Furnace not reacting to standard measures in
the usual time span
• Frequent slips
• Cooling trend that cannot be reversed with
Operating outside • Cooling trend that cannot be reversed with
corrective actions
• High blast pressure
• Cause can not be found
• Have investigated and eliminated possible
causes
• Unexplained casting irregularities
Operating outside
normal procedures,
that is:

9. Protecting the Furnace
Add extra fuel by way of a coke blank, increase coke rate by 3% and increase fuel injection
Reduce the blast rate, but keep fuel injection on the furnace
Cast the furnace
Expert systems are not designed to operate during chills, should not be relied upon at such times.
These actions are done to Protect the furnace for the time that is required to find the problem.
Investigate the situation, now that the furnace process has been protected
Call Management to inform them of the situation
Reduce basicity by 10%

10. Raise the cohesive zone with a coke blank
• Charge a 15 t coke blank
• Short interruption of production
• Situation fully back to normal after six hours
• No burden calculation required – original is still applicable
• Automatically reduces basicity

11. Gas Flow Control
• Giving enough contact between
the ore and the gas for sufficient
indirect reduction to occur
Purpose
of;
Controlled • Burden distribution
Controlled
by;
• Above burden probes
• stack temperatures
• pressure taps
• Top temperature
• Top gas analyses
Measured
with;

12. Effect of a tuyere without fuel injection
Asymmetric cohesive zone
Cohesive zone with one tuyere off

13. Pressure Drop and Coke Rate

14. Peep-sight Observations (2)
4. Oval tuyere
surround
• Tipped tuyere
• If lance
position can
not be seen,
impingement is
possible
5. Partially
blocked tuyere
• May be
blocked with
slag, char or
unreduced
material
6. Fully blocked
tuyere
• Injection is no
longer possible

15. Peep-sight Observations (1)
1. Normal tuyere
flame
• Lance in
correct
position
• Bright flame
2. Dull appearance
of tuyere flame
• Possibly water
leakage in the
furnace
3. Lance position
incorrect
• This can lead
to wearing of
the tuyere,
causing tuyere
leakage

16. Peep-sight Observations (3)
7. Slag entering the
tuyere
• Shows a high
liquid level, usually
when blast
pressure is being
reduced
8. Solid material in
front of the tuyere
• This can be scabs,
or a concentration
of fines coming
down in front of
the tuyere

17. Summary of Gas Flow Control
Monitor stability of
burden descent
Stockrods
Stack Pressure Taps
Monitor stability of
central coke chimney and
central gas flow
CO/CO2
Above burden probe
central gas flow Above burden probe
In burden probe
Monitor wall gas flow Skinflow TC’s
Heat flux (esp upper
belly, lower stack)
Wall temperatures
Above burden probe
In burden probe

18. Gas Flow Control
Blast Furnace Parameters
for Gas Flow Control
Burden
• Size
• Weights
• Coke moisture gauges
Above Burden Probe
Profile Meter
Top Gas
• Analysis
• Temperature (uptakes or
dust catcher)
• Pressure
Infra-red camera
Stock Rod
Skin flow thermocouple
Profile Meter
In–burden Probe
(fixed or retractable)
Hot Blast
• Flow Rate
• Temperature
• Tuyere input (H20, O2, Fuel…)
Skin flow thermocouple
• Intermediate Pressure Taps
• In–wall Thermocouple
• Heat Loss Measurements
• Stave body temperatures

19. Effect of thick ore layer in Furnaces
• Ore flows more towards the
centre
• It overflows the coke
• When this ore starts to melt, it
will have poor permeability

20. Permeability of coke and ore layers
High resistance to gas flow
• Ore burden particles are smaller and have less voidage.
• Gas flows 4 times easier through coke than through ore.
Low resistance to gas flow

21. What to do in case of poor burden descent?
• Too high blast pressure: decrease
blast
• Poor hearth drainage: cast, open
2nd taphole, open with bigger
drill diameter
• Too hot furnace (means high
position of cohesive zone): cool
Burden weight
Upward force by
position of cohesive zone): cool
down with additional moisture
injection
• Too many fines in burden:
eliminate sources of fines e.g.
poor quality burden materials
Upward force by
blast
Upward force by
liquid

22. Summary Sheet
• Poor burden descent is indicator of disturbed blast furnace process
• In sequence of severity:
– Slow burden movement and slight slips
– Hanging and slipping
– Kicks
• Will lead to fuel shortage in furnace
• Corrective actions:
– Decrease blast volume
– Compensate with extra fuel

23. Feed tank failure, what to do?
• Low injection levels (< 25 t/hr)
– Switch off oxygen when not injecting PCI
– Cycle time of filling feed tank is 15-20 minutes
– Feed tank can deliver 22 ton per cycle (1 hour or more)
– For limited time, go to PCI on/off cycle. Take oxygen also on/off.
feed tank cycle available for injection
hour 0 1 2 3 4 5
feed tank 1
feed tank 2
feed tank 1
feed tank 2 not available

24. Causes of Poor Burden Descent
• High blast pressure caused by
• Retained liquids in hearth
• Hot furnace: cohesive zone high in furnace
• Too many fines

25. Effects of Slips on Furnace Process
• Cold material in hearth
• Energy escapes from furnace not
being used in the process: high
top temperature, high CO/CO2,
high wall temperatures
• Cooling trend
• What to do:
• What to do:
– Make up energy loss with
additional fuel
– Recover burden level
– Recover normal layer
structure, if needed, by
operating for a complete
furnace fill on lower blast
volume (4-6 hours)

26. T Flame effects
• 5 ton coal/hr reduces T Flame ~ 65 C
• 1% oxygen increases Tflame ~ 54 C
• - 10 g/NM3 moisture = -2,5 ton steam/hr
increases T Flame with ~53 C

27. Burden Slips
• Voidage collapses
• Burden level low
• Severe disruption of layer
structure
• Gas flow impeded, gas jets along
furnace wall

28. Racing/Fast Filling / High Charging Rate
High charging rate is
consequence of
High oxygen input into
furnace
Wind
Too low fuel rate
Coke charged
Effects:
Low top
temperature
Reaction: slow down
production rate by
increasing fuel
injection
Oxygen
enrichment
Water!
(Leakage)
Fuel injection
level
Too much
fuel used in
process
Low CO/CO2
(High
solution loss)
Cooling trend

29. How to manage Low Bin Levels
Potential Consequences
for the furnace?
•Material falls further
•More fines are produced
How can Control Room
Operator find out if bin What are steps to take?
•More fines are produced
•Either these will be screened
out, giving a yield loss
•Or, they will be charged to the
furnace
•Blast pressure can increase
with more fines
•Wind rate has to be reduced
•So production is reduced
Operator find out if bin
levels are too low?
•Reported by
personnel/automatic system
•Known interruptions in supply
•Cameras and/or level indicators
What are steps to take?
•Stagger the restart of the bins
to avoid high-fines material
being charged from two bins
simultaneously
•Try to keep all bins at >80% full

30. Bin Contamination
•Fuel shortage/excess
•Basicity changes
•Contaminated central coke chimney
•High wall flows
Potential consequences for the furnace
•Reported to him when it occurs, or is noticed
How can Control Room Operator find out about contamination?
•Reported to him when it occurs, or is noticed
•Furnace top cameras
•Increase/change in return fines
•Chemistry changes in hot metal or slag
•Filling times
•Moisture levels
•isolate the bunker
•Assess the impact on the furnace of the material already charged
•Charge as a different specified material
•Or Charge a much smaller quantity to empty the bunker
•Or empty out
Steps to Take

31. Practical Examples
Bin
Contamination
What are the potential
consequences for the
furnace?
Bin Levels too
low
What are the
potential
consequences for
furnace?
How can the Control
Room Operator find
out of bins are
contaminated?
What are the steps to
take when bins are
known to be
contaminated? Why?
consequences for
the furnace?
How can the Control
Room Operator find
out if the bin levels
are too low?
What are the steps
to take when bin
levels are too low?
Why?

32. Coke break-up, nut coke covered with more fines
As weak coke breaks up voids fill with smaller pieces and the gas flow and
liquid metal and slag flow becomes more difficult.

33. Fines Descending through Furnace
Descend
1
2
Fines charged
near wall
Descend
straight down,
do not melt or
reduce
(impermeable
to gases)
Solids seen
through
peephole
Blast pressure
may increase,
furnace can
hang
2
3

34. Blast Pressure & the Process
Below the
equipment-set
The maximum blast
pressure for the Above the
Reducing the blast
pressure before the
When Top pressure
is kept constant;
equipment-set
blast pressure limit,
the blast pressure
fluctuates up to a
defined limit for
the process.
pressure for the
process is the point
at which the
furnace can
operate without
the risk of hanging
and slipping.
Above the
maximum blast
pressure for the
process the furnace
may start to hang
or slip.
pressure before the
maximum is
reached can
stabilise the
process before
hanging or slipping
occurs.

35. Required Properties of Material
• Moisture content controlled – may be measured and corrected for by
gauges
• Strength at ambient temperature – to prevent breaking up before charging
• Strength at high temperature – to prevent breaking up in the furnace
• Chemistry – to make sure there is enough of what is required, and not too
much of anything else
much of anything else
• Size distribution – limit the amount of fines
• Average size – should be within required range

36. Thermal Control
• CO Utilisation
• Heat Load to the Walls
• Top Gas Temperature
• Hot Metal Temperature and Silicon Content
• Charging Rate-Racing furnace
• Loss of Hot Blast Temperature
• Loss of Stockline

37. Racing furnace-example
• Number of charges (0-2) - Average number of charges per
hour over the last 2 hours
• Number of charges(0-8) - Average number of charges per
hour over the last 8 hours
• If Number of charges/hour C(0-2) = Number of
For example
• If Number of charges/hour C(0-2) = Number of
charges/hour(0-8) + 2
• We say furnace racing, making more melt, thermal sink
• Consequences- reduce the blast volume by 1800 Nm3/hr
For example

38. Racing Furnace, Direct Reduction (DR) and Wu
Racing Furnace, Direct Reduction (DR) and Wu
FeO + CO = Fe + CO2
Fe₂O₃ + CO = 2FeO + CO₂
110⁰C
1300⁰C
4/7/2021 38
FeO + CO = Fe + CO2
C + CO₂ = 2CO (>900oC: Solution loss)
Why fce cool during racing condition?
Racing furnace happens due to more DR which
consumes heat and consumes Carbon which does not
burn at the tuyeres
So Furnace cools. Due to poor reduction.
Wu is the residual heat in the lower zone = heat of
combustion – heat of solution loss + heat of blast
temp – heat to split steam and PCI
1500⁰C
2150⁰C
DR

39. Guidelines for operators-How to keep thermal stability in racing furnace
• For racing furnace, consider the 4 hour charging rate, as well as the 8 hour, because there are
times when this will give earlier warning, especially when the charging rate rapidly
accelerates.
• Racing furnace actions if the 4 hour charging rate is at least 9% above expected or the 8 hour
is at least 5% above expected.
• If the charging rate is increasing, (4 hour or 8 hour), or we are building up blast volume from
a reduced blast period, do not set coal variation to negative.
a reduced blast period, do not set coal variation to negative.
• Be very wary of setting the coal variation below -5 for more than a few hours. Increase
steam first. i.e. only if charging rate slow and furnace hot.
• Do not go below -5 if charging rate slow but thermally OK.
• Do not go below -5 if furnace hot but charging rate OK.
• When the top gas analysis is under maintenance, Wu ends up well out of range and
misleading. If Wu does not look right, cross check the top gas analysis with the power station
on the expert system trend.
• Note that the time for a PCI rate change to affect metal quality is 7 to 11 hours, whereas
steam is 3 to 5 hours.

40. Delays and effect times
PCI: 3 kg/thm = 10 deg C = 0.07% Si. 7 to 11 hours
Steam: -4g/Nm3 = 8 deg C. 3 to 5 hours
PCI to Wu: 3 kg/thm = 50 MJ/thm. 4 to 6 hours.
0
2
4
6
8
10
12
0 1 2 3 4 5 6 7 8 9 10 11 12
Hours
Effect time PCI change to hotmetal temp
PCI change (kg/thm)
Change in metal temp (deg C)
50
60
70
80
90
100
5
6
7
8
9
10
Change
in
Wu
MJ/thm
Change
in
PCI
ratekg/thm
Effect time PCI change to Wu
PCI change (kg/thm)
Change in Wu (MJ/thm)
Wu to HMT: 50 MJ/thm = 10 deg C. 3 hours
0
10
20
30
40
0
1
2
3
4
0 1 2 3 4 5 6 7 8 9 10 11 12
Change
in
Wu
MJ/thm
Change
in
PCI
ratekg/thm
Hours
-5
-3
-1
1
3
5
7
0 1 2 3 4 5 6 7 8 9 10 11 12
Hours
Effect time steam change to hotmetal temp
Steam change (kg/thm)
Change in metal temp (deg C)
Note that the furnace is not an incinerator.
Increasing PCI rate does not immediately generate more heat
as the furnace will just burn coal instead of coke.
The furnace will eventually warm up because the burden
in the stack will descend more slowly so will be exposed
to more reducing gas.
It will then be better reduced and need less direct reduction.

41. What is oxy/thm factor?
• Volume of oxygen assumed to make 1 thm
• 6300 BV at 3% enrichment: oxygen in blast is
6300 * (21+3)/100 = 1512 Nm3/min
• If oxy/thm is 245 Nm3/thm then furnace is assumed to produce
1512/245 = 6.17 thm/minute = 370 thm/hour
1512/245 = 6.17 thm/minute = 370 thm/hour
• Aim PCI of 130 kg/thm. So coal flow setpoint calculated is
370 * 130 = 48140 kg/h
• If the furnace is racing so that it is only using 235 Nm3/thm:
• Actual production rate = 1512/235 * 60 = 386 thm/hour
• Actual coal rate = 48140/386 = 125 kg/thm (5 kg/thm short)
– Plus the effect of less coke to burn and higher direct reduction

42. 8h expected and actual charging rate on RTR1
• 8 hourly charging rate is the actual measured charges per 8 hours
• Expected charging rate is what the 8 hour charging rate would be if the furnace
was charging as expected. The expected make is calculated from the average blast
volume and oxygen enrichment over the past 8 hours and the oxy/thm factor. This
is divided by the current fe yield per charge to calculate the expected charging
rate.
• Actual v expected is the percentage difference between the actual charging rate
• Actual v expected is the percentage difference between the actual charging rate
and what it should have been.
• If the furnace is charging at least 5% faster than it should be, averaged over 8
hours, then this indicates a racing furnace. The colour of this row changes to red.
• If the charging rate is more than 2% above where it should be then the colour
changes to orange, as this is likely to lead to a cooling furnace if it persists.
• Note that when burdening for a change in PCI rate the expected rate will be more
accurate on the expert system as it allows for the change in coke volume in the
stack which occurs before the change in PCI rate.