How is aerodynamics important in Blast Furnace

1. Iron & Steel Making
MT32008
Spring, 2016-2017
Aerodynamics in Blast Furnace (BF)

2. Importance
 Higher Productivity requires higher
blast, coke throughput (Q) but lower
coke rate (K)
 Lower coke rate requires better
utilization of thermal and chemical
potential of the gas which in turn
demands more uniform bed
permeability
 Higher blast throughput also increases
pressure drop which may lead to
irregularities like Channeling in dry
zone and flooding/Hanging in wet
zone
P, THM/hr (Productivity) =
THMkgK
hrkgQ
/,
/,
.

3. Aerodynamics of Packed and Fluidized Bed
Reactor
Gas In
Gas out
P1
P2
• Pressure drop?
• Ergun Eq:
• Friction factor, ψ for packed bed
 
P
P
T
T
w
dH
P 0
0
2
0.03
...
1
. 






mRe
150
75.1 
 



1
..
Re 00 dw
m

4. 1. Calculate the pressure drop (mm of H2O/cm) in dry zone of
Blast Furnace (BF) based on the following data: BF gas at
1000oC, gas flow rate=4000 Nm3/m2/hr, gas density at STP
=1.5×10-3g/cc, viscosity at 1000oC = 480×10-6 g/cm-s, particle
diameter = 4 cm, voidage = 0.25; average pressure = 1.6 atm.
Tutorial

5.  For a given size, if
voidage decreases by
0.1, the pressure drop
increases by 3-5 times
and more at lower
voidage
Effect of voidage and particle
size on pressure drop
 The effect of particle
diameter is staggering
below 3-4 mm particle
size.

6. Effect of mixed layers and size ratios on pressure drop
• Figure represents data for
spherical particles
• Pressure drop increases several
folds for mixed bed compared to
layered one
• In mixed beds pressure drop
passes through a maximum with
increase in fine fraction
• Pressure drop is very sensitive to
size difference between fine &
course particles
• Considering burden range in BF,
pressure drop may be lowered by
reducing size difference between
coke and ore
• However, coke/ore size ratio is
maintained at 3-5, considering the
operation at lower part of the
furnace

7. Bed Instability & Fluidization
• Condition of incipient fluidization:
• Wen & Yu generated an universal
correlation between shape factor
and voidage at the condition of
incipient fluidization
• Wen & Yu Correlation:
  g
L
P
GS  

1
  2
,Re 33.7 0.0408 33.7D mf Ga  
 3
2
P S G Gd g
Ga
  



,Re mf P
D mf
v d



8. Bed Instability in Blast Furnace
 Densities of ore, limestone
and coke are 5, 2, and
1gm/cc, respectively.
 Coke will fluidize first
followed by limestone and
ore
 For a particular diameter,
critical gas velocity
increases at higher
temperature

9. Bed Instability in Blast Furnace
This monogram defines the
limit of productivity by
fluidization at the BF top
and the effect of top
pressure
– For productivity of 50
THM/m2/day without
HTP, ore size below
6mm and coke size
below 23mm will be
blown out

10. Channeling
 BF charge has non-uniformity both w.r.t. size, shape,
density and their distribution
 Critical gas velocity may be exceeded locally
 Lighter particles (coke) are blown out of those region and
deposited in regions of low velocity and the heavier ore
settles down preferentially (ore shift)
 The above phenomena contributes to compactness of
less permeable region and make the radial pressure
drop more uneven
 Gas then flows through a system of distinct channel,
called channeling
 Restoring blast rate to previous value - not a solution
(Hysteresis effect)

11. Channeling
• Furnace takes air without
increase in pressure drop
• High exit gas temperature
• High CO/CO2 ratio
• Higher flue dust emission
• Increased coke rate
• Precautions:
– Burden with better strength
– Narrow size distribution
without fines
– Optimum size
– HTP
Indications

12. Tutorial
In order to increase productivity, 40% more blast is injected in BF
per unit time (base case at 2400 Nm3/m2/hr). To restrict
channelling, it is intended to maintain the pressure drop (1.4 atm)
and hence linear velocity of gas through BF to be constant by
enhancing top pressure. What will be the top pressure to achieve
this. Assume, velocity exponent for pressure drop is at 1.75.

13. Liquid holdup and flooding in wet zone
 Higher productivity also limited by
liquid holdup and flooding
 In the bosh region, coke is the only
solid material through which gas flow
up and liquid moves down
 Pressure drop increases with
increase in gas throughput at fixed
liquid throughput or vice versa
 Increased resistance give rise to
loading followed by flooding
A - loading limit B – flooding limit.
(results from Hansen
based on countercurrent
gas liquid flow in packed
bed)

14. Flooding limit
2
, . 0.001or f k 
2.0
3
2
)( 


 l
SSF
g
v
factorfloodingf 
2
1
)( 






l
g
G
L
ratiofluidk


log 0.559log( ) 1.519f k  
L – superficial mass flow rate of the bosh
slag (kg/m2h)
G - superficial mass flow rate of the tuyer-
gas (kg/m2h)
Sherwood’s relationship of critical flooding
factor with fluid ratio

15. High Top Pressure (HTP)- a remedy for
Channeling
A furnace is operating with top and bottom pressures as 1.1 and 2.5
atm, respectively. If we intend to blow 30% more blast to enhance
coke burning rate and productivity, keeping pressure drop and
linear velocity constant under HTP. Calculate the required top
pressure.
 
P
P
T
T
w
H
P 0
0
2
0.03
...
1
. 






n
wTK
dH
PdP
0..
Ans: Top pressure: 2.1 atm