Post combustion in converter steelmaking

Oghbasilasie Haile Holappa Lauri

Helsinki University of TechnologyDepartment of Materials Science and Rock EngineeringLaboratory of Metallurgy Report TKK-V-B128.r - - .. ' _ - _ . -T .rsaaaEspoo 1997

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Helsinki University of TechnologyDepartment of Materials Science and Rock EngineeringLaboratory of Metallurgy

Post combustion in converter steelmaking

Oghbasilasie Haile Holappa Lauri

Research programme: SULA IIProject: Post combustion in converter process

Key words: post combustion, saving energy, smelting reduction,steelmaking, top blowing, bottom blowing, combined blowing

Vuorimiehentie 2 K FIN-02150 Espoo, Finland

ISSN 0785-5168 ISBN 951-22-3487-4

CONTENTS

ABSTRACT....................................................................................................................................4

1. INTRODUCTION..................................................................................................................... 5

1.1. Definition....................................................................................................................51.2. Post Combustion Ratio and Heat Transfer Efficiency......................................... 71.2.1. Measurement of Post Combustion Ratio............................................................81.2.2. Measurement of Heat Efficiency......................................................................... 9

2. PHYSICO-CHEMICAL BASES OF POST COMBUSTION......................................... 11

2.1. Thermodynamics of Post Combustion Reaction................................................ 112.1.1. Carbon-Oxygen Equilibrium..................................... 132.1.2. Influence of Iron on Post Combustion..............................................................16

2.2. Rate Phenomena.....................................................................................................172.2.1. Hydrodynamics.....................................................................................................172.2.2. Heat Transfer.........................................................................................................22

3. INFLUENCE OF DIFFERENT PROCESS PARAMETERSON POST COMBUSTION............................................................................................. 25

3.1. Post Combustion in LD process............................................................................ 253.2. Effect of Lance Height on Post Combustion........................................................283.3. Bottom Stirring........................................................................................................30

3.4. Special Lance Construction.................................................................................... 323.5. Refractory Wear /Vessel Volume..........................................................................34

3.6. Effect of Slag on Post Combustion Ratio............................................................ 37

4. CONTRIBUTION OF SMELTING REDUCTION........................................................... 40

4.1. Post Combustion Behaviour in In-bath Type Smelting Reduction...................40

2

4.2. Kinetics.....................................................................................................................424.3. Post Combustion in Different Smelting Reduction Processes...........................43

5. FUTURE...................................................................................................................................49

6. CONCLUSIONS................................................................................................... 50

REFERENCES............................................................................................................................. 51

3

ABSTRACT

The purpose of this work is to study the fundamentals of post combustion and the effect of different process parameters on the post combustion ratio (PCR) and heat transfer efficiency (HTE) in converter steelmaking process. The PCR and HTE have been determined under normal operating conditions. Trials assessed the effect of lance height, vessel volume, foaming slag and pellet additions on PCR and HTE.

Based on enthalpy considerations, post combustion of CO gas is regarded as one of the most effective means of increasing the heat supply to the BOP. The thermodynamic study of gas-metal-slag reactions gives the limiting conditions for post combustion inside the converter reactor.

Different process parameters influencing both thermodynamic equilibria and kinetic conditions can greatly affect the post combustion ratio.

Different features of converter processes as well smelting reduction processes utilizing post combustion have been reviewed.

4

1. INTRODUCTION

1.1. Definition

The off-gas of the oxygen converter contains considerable chemical energy which is being increasingly utilized with off-gas collection. From the economic point of view, it would be advantageous in the converter process to return at least a part of this energy to the process. With full combustion of CO to CO2 and the utilization of this heat in the converter, sufficient energy would be available to increase the scrap rate to about 55 % of the total charge. But it is known that complete combustion of CO to CO2 cannot be reached in contact with liquid iron because of the required high oxygen potential, which then would significanty oxidize iron.

The energy obtainable by post combustion can thus only be utilized if the post combustion

takes place in the gas space and the enormous amount of heat released thereby will be transferred from the gas phase back into the bath. If a larger portion of the post combustion

energy remained in the off-gas, the off gas temperature would increase so drastically that the refractory lining of the converter would be destroyed.

For example, the explicit aim of the rotary furnace processes to reduce the heat load of the vessel wall caused by the high off-gas temperature of the post combustion in such a way that the melt had constantly to cool the wall during the rotary movement. However, it is known that the rotary furnace although offering the highest energy utilization of the steel refining processes, has failed for insufficient life of the refractory lining /!/.

The post combustion ratio in the converter is dependent upon the balance between the oxidation of CO produced to CO2 and the reduction of CO2 to CO. It is considered that the post combustion ratio depends on the results of complicated heterogeneous reactions in a nonsteady state. Under the assumption that the main reactions of post combustion are the formation of CO2 by the reaction between the oxygen jet and CO in the atmosphere and the formation of CO by the reaction between CO2 produced as described above and carbon

5

contained in the steel, the post combustion ratio was studied by Hirai. M, Tsujino. R. et al. /2/ on the basis of a reaction model shown in Fig. 1.

•Lance nozzle

CO entrained by O2 jet - O2 jet Supersonic

jot core region

COo getting out of jet

Free jet region

CO by de- carburization reaction X

Region of COg' getting out

Fig. 1 Schematic diagram of post combustion 111

The velocity of the oxygen jet with increasing distance from the outlet of the lance nozzle changes from the supersonic jet core region to the free jet region through the transition jet region. In this model a jet of CO% is formed by the reaction between oxygen free jet region arid CO produced by the decarburization reaction, being entrained from the atmosphere into the oxygen free jet region (CO + 1/20% —> CO%). The main reactions are shown in table 1 /Based in Refer. 2 and 18/.

Table 1. Gibbs energies for the reactions of post combustion in converter.

Reactions AG°; cal / mol

rci + i/2o9(g) = co(g) - 33939 - 9.75T (1)

C0(g) + l/20?(g)-»C0?.(g) -67890 +21.10T (2)

C0?(g) + [C]-»2C0(g) 34580 - 30.95T (3)

CO?(g) + Fem -> (FeO) + CO(g) 7094 - 7.48T (4)

(FeO) + [C]->CO(g) + Fem 27486 - 23.47T (5)

6

Carbon dioxide at the surface layer of jet with the velocity below a certain critical value is

entrained into and dissipated with the flow of CO produced from the spot, and the rest of CO2 which is not dissipated and the oxygen jet reacts with carbon in the steel bath to form CO. It is assumed that the amount of CO2 dissipated is proportional to the flow rate with the velocity below a certain critical value in the free jet region, and that the post combustion ratio is

proportional to the ratio of the amount of CO2 dissipated to the amount of CO produced by the decarburization reaction /2/.

1.2. Post Combustion Ratio and Heat Transfer Efficiency

One of the most important process parameters in steelmaking processes is the post combustion ratio (PCR) obtained in the furnace. The post combustion ratio is defined as:

PCR%C02+%H20

%CO + %C02 + %H2 + %H20X 100 (6)

Higher post combustion ratios imply that more heat is generated in the vessel, this heat is necessary to heat scrap and carry out the steelmaking reactions. Unfortunately, not all the heat generated by the post combustion reactions is actually transferred to the bath, thus, a heat-

transfer efficiency (HTE) term is required. Conventionally, HTE is defined:

HTE =excess heat in off - gas heat available from PC

X 100 % (7)

According to this definition, the HTE will be 100 % if the gas leaves the vessel at the same temperature as the bath. In recent years, there has been a trend in conventional oxygen steelmaking to increase the degree of post combustion in the furnace gases and to efficiently transfer the heat back to the bath. The primary purpose is to increase the amount of scrap that

7

can be melted. There have been a number of different processes developed with one or more of the following features:

(a) special lances with secondary oxygen ports,(b) separate injection ports from the side of the vessel,(c) special soft-blowing practices

Generally, it has been found that as the PCR is increased, the HTE falls, thus, the net amount of heat transferred to the bath is limited. Additionally, at high post combustion ratios, the off-gas may be so hot that it damages the refractory in the vessel cone or the off-gas system /3/.

1.2.1. Measurement of Post Combustion Ratio

According to the study of Farrand B.L., Wood J.E. et al. /4/ the instantaneous value of PCR is plotted as a function of blowing time in Fig. 2. The average value of PCR for the complete heat was determined to be 16.1 % as shown in table 2. The shape of the PCR curve reflects the changes that occur in the decarburization rate showing clearly the stable period of maximum decarburization in the middle of the blow (Fig. 3).

Table 2. PCR and HTE for standard Blowing Practice /4/

Mean Standard Deviation

PCR(%) 16.1 1.3

HTE (%) 43.8 5.7

In the early part of the blow (0-5 min.) the PCR is high due to the low decarburization rate and a corresponding excess of oxygen. In the middle of the blow (6-14) the decarburization rate is

relatively constant and the PCR is steadily decreasing. In the final stages of the blow the decarburization rate decreases and the PCR increases again. The results illustrated in Fig. 3 only show the beginning and middle periods of the blow as data from the final minutes was highly erratic due to heat specific conditions. For the batch operation of EOF steelmaking the

8

stable period during the middle of the blow is the best period to show the effect of process changes on PCR /4/.

40Average N=39

*>@00*0000****

o 400-

3 200-

Average

5 10 15Blowing Time (min) Blowing Time (min)

Fig. 2 Instantaneous post combustion ratio Fig. 3 Decarburization rate for normal for normal KOBM blowing practice /4/ KOBM blowing practice /4/

The gradually decreasing PCR during stable decarburization can be related to the increase in gas velocity in furnace. As the bath temperature increases through the blow so does the gas

volume. Increased gas volume means decreased gas residence time and reduced available time for combustion of CO gas to CO% inside the vessel. Gas residence time is defined as the volume of gas generated per unit time divided by the free inner volume of the furnace /4/.

1.2.2. Measurement of Heat Efficiency

The average value of HTE for a heat was calculated by Farrand B.L., Wood J.E. et al. I A] to be 43.8% as shown in table 1. This result was verified by the observed correlation between tap temperature and PCR. The tap temperature increases 4.9 °C for each 1% increase in PCR. The theoretical value for 100% efficiency is 10.4°C for 1% PCR. Therefore the HTE based on actual tap temperature measurement is 47% which is in good agreement with the value of

9

43.8% calculated from the off-gas analysis. The shape of the HTE curve is analogous to the shape of the PCR curve described above and is shown in Fig. 4.

Average

5 10 15Blowing Time (min)

Fig. 4 Instantaneous heat transfer efficiency for normal blowing practice 14/

The HTE shows great variability at the beginning and end of the heat when the decarburization rate is increasing or decreasing. During the period of stable decarburization (6-14 min.) the

HTE steadily decreases. This decrease in HTE may be caused by two factors:

(i) The increasing gas velocity in the furnace

(ii) The increasing bath temperature.

The reduced gas residence time due to increased gas velocity reduces the time for heat transfer to the bath. The increasing bath temperature decreases the driving force for energy transfer from the gas phase to the steel bath. In the bath EOF process it is not possible to separate gas velocity from gas and bath temperature effects since both are increasing as the blowing time

10

increases. As a result, during these trials it was not possible to establish the exact reason for the decrease in PCR and HTE during the middle period of the blow.

2. PHYSICO-CHEMICAL BASES OF POST COMBUSTION

2.1. Thermodynamics of Post Combustion Reaction

When practising CO post combustion during decarburization, the basic idea is to input sufficient secondary O2 to convert the CO coming off the bath to CO%. This idea has roots in the Kaldo steelmaking process in which CO was combusted to CO% inside the furnace, allowing the use of 600 kg of scrap per ton of pig iron. As shown schematically in Fig. 5, the input of secondary oxygen in the BOF is achieved by inserting secondary lance ports and/or cone tuyeres in the BOF such that post combustion occurs in the upper part of the converter

above the bath /5/.

SECONDARY 02

--------POSTCOMBUSTION ZONE

DECARBURIZATION ZONE

Fig. 5 Decarburization and Post combustion zones in the converter 151

The model developed by Vensel D. et al. 151 for this study, is based primarily on the thermodynamics of BOF decarburization and subsequent CO post combustion; it is an iterative system of equations describing the equilibrium. Although carbon removal and CO conversion

11

may occur simultaneously in the BOF, for the purposes of this analysis, they can be considered to occur in different regions of the converter. For this reason, the model incorporates two independent reaction zones; a decarburization zone and a post combustion zone, as illustrated

in Fig. 5. Also, only those reactions involved in carbon removal and CO conversion wereincorporated in the model. That is, it was assumed that decarburization does not begin until all silicon has been removed and that iron oxidation, phosphorus removal and sulphur removal donot affect carbon removal.

Thermodynamic equilibrium in the post combustion zone is based on the reaction (CO + j^02 —> C02). It is shown in table 1.

The equilibrium constant for equation (2) can be written:

K P co 2Pco pl/2

r02(8)

Where PCo2,co, o2 mean the partial pressure of carbon dioxide, carbon monoxide and oxygen,

respectively.

Using Dalton's Law, equation (8) can be written in the therms of the equilibrium molar rates of CO, CO2 and O2. Setting hdco equal to the molar rate of CO from decarburization, and hvco, the

molar rate of CO conversion to CO2 by post combustion, the equilibrium molar rate of CO after post combustion is just (nd0 - ri£0). Similarly, the equilibrium molar rate of CO2 after

post combustion is + n£0) and that for O2 is (n£2 - jX hco). Thus, equation (8)

becomes:

K =(a d

CO.2 + A co )(n~d + hd co ^ nco2 Al(9)

Where K is thermodynamic equilibrium constant, n is molar rate (gram-moles/min), p is post combustion and d is decarburization.

12

The parameters hdco and hdco^ are a funcPtion of the blow time and may be calculated from the

decarburization zone model.

Since the partial pressures of CO and CO2 after decarburazation and the 0% flow, required for decarburization are functions of the blow time, hdco and hd0i are also a functions of the blow time. Hence for a given h^, the molar rate of CO conversion by post combustion, hpco, and thus the equilibrium Pco Pco^ and leaving the post combustion zone may be

determained as function of decarburization time 15/.

2.1.1. Carbon-Oxygen Equilibrium

The results obtained from the thermodynamic models of decarburization and post combustion provide some very interesting observations about the steelmaking process. Fig. 6 and 7

illustrate calculated trends in the equilibrium partial pressures of CO and CO% after decarburization and with no post combustion.

.125 j—

.100 -

3 .075 -

i -1 Q.

.025 -

oL> 0

BLOW-TIME, MIN

1.00

£3 -975i8 -950 Q.

.925

.900

' •;s ..09

/4.0 Y. CT7

2.65,/ A

------------ 1THERk ODYNAK 1C CONT lOL-y

.085 \11

M ASS TRA NSFER C ONTROL'y\ .

\T y.e

y

10 15 20BLOW TIME. MIN

25

MASS TRANSFER CONTROL

.015% C

THERMODYNAMIC CONTROL

4.0 %C^ M - V

Fig. 6 Equilibrium partial pressure of CO with carbon in the metal bath from the decarburization zone 15/.

Fig. 7 Equilibrium partial pressure of C0% with carbon in the metal bath from the decarburization zone /5/.

13

% *(s 00+00)/oo

They indicate that until the carbon content reaches about 0.3 weight percent, the CO-CO2 off­gas mixture remains fairly constant and is essentially all CO, but below 0.3 percent carbon there is a sudden and drastic increase in CO2. If thermodynamic control of decarburization had also

prevailed below 0.3 percent carbon, the increase in CO2 levels would be even more dramatic (Fig. 6 and 7). Clearly, this great shift in CO/CO2 is solely due to the decreased activity of carbon in metal bath.

A similar trend in CO/CO2 is generally observed in practice, however off-gas measurements at converter mouths indicate that the combustion has proceeded down to 85 to 90 percent CO. Thus, it might be concluded that the decarburization equilibrium is not achieved in practice. On the other hand, it can be postulated that decarburization is at equilibrium and that a certain amount of CO post combustion naturally occurs in the EOF, thereby lowering the CO content at the converter mouth. This may be verified when comparing measured off-gas compositions for top blown processes with 100 percent bottom blown processes, as in Fig. 8.

1 1OBM

%•

w

4 LD-AC

_____ 1____ i 1 1 1

0 20 40 60 80 100

OXYGEN BLOWN, %

100

16o

90

80

70

60

505 10 15 20

BLOW TIME, MIN

1

DEC

/I I

ARBORIZATION ONLY

' — ■“"•=5X

t t

~~n—'

15% i ;o post c OMBUSTIO |l

25

Fig. 8 ------------- for top and bottomco+co2blowing processes 15/

Fig. 9 ------------- as a function of blowingCO + CO 2

time for the decarburization zone with 15 percent CO post combustion 151

14

For bottom blowing, no post combustion is possible and it is seen that measured XF profiles, defined as (CO)/(CO + CO2) versus blow time, closely approach the equilibrium XF profiles obtained from the model for the decarburization zone. Thus, the decarburization equilibrium is achieved in the EOF and the post combustion of the gas above the steel bath, which is possible only in top blown converters, is responsible for the lower than predicted off-gas compositions

observed in practice. This is illustrated in Fig. 9 for the predicted XF from the combined decarburization and post combustion models for a stoichiometric oxygen input for decarburization only and for an excess oxygen input to achieve 15 percent CO post combustion. - - - -

In combined blowing, some analysis also suggests that when CO% is injected through tuyeres, it will be an effective coolant when the decarburization is thermodynamically limited (i.e., when percent C > 0.3). CO% injection during this part of the blow would also enhance the decarburization of the bath. During the latter part of the blow (i.e., percent C<0.3), CO2 is

more stable and therefore would not be as effective and excessive tuyere wear would occur unless higher flowrates are used.

% c.49 .09 .015

_ 600

02 (NmVMIN)

BLOW TIME. MIN

OXYGEN FLOWRATE DECREASE

675 — 510 NmVMIN

HEAT 4834

HEAT 4835

OXYGEN FLOWRATE INCREASE 575— 680NmVMIN

BLOWING TIME, MIN

Fig. 10 Effect of rate of oxygen input in aCO1801 vessel on the-------------as predicted

CO + C02from the decarburization zone and post combustion zone /5/.

Fig. 11 Effect of the rate of oxygen input

on the------------- 151.co+co,

15

Fig. 10 shows that if the decarburization rate is fixed the post combustion degree will increase (and XF will decrease) when oxygen flow rate is increased. This phenomenon has been observed also in practice (Fig. 11). However, this conclusion is somehow contradictory with the common experience of increasing decarburization rate with increasing oxygen flow rate.

The thermodynamic models also predicts some very interesting occurrences when the carbon content drops below 0.3 percent. For instance, as mentioned in the previously, the off-gas composition becomes much richer in CO2. This is a thermodynamic phenomenon; low carbon

activity must result increased CO2 content. Also, a similar phenomena occurs in the predicated XF profiles . According to Fig. 10 in the post combustion zone decreases drastically after 0.3 percent C. This corresponds to a marked increase in CO conversion even though there is less CO coming from the decarburization zone as there is no change in the total O2 input rate.

2.1.2. Influence of Iron on Post Combustion

As Liuyi et al. /6/ stated if an iron droplet in the slag emulsion is decarburized so far that, during its residence time, the X*:0i (* = equilibrium) content exceeds the value for the FeO/Fe

equilibrium, iron oxidation starts. Since the drop carbon content is very small at this moment, and since iron oxide covers the drop surface, it is assumed that carbon oxidation of the drop stops after the iron oxidation has started. Droplets of different sizes have different decarburization rates, the time t' (residence time) until the start of iron oxidation is not equal

for different drop sizes at the same CO2 content in the gas phase.Smaller droplets are decarburized faster than larger ones. Hence, the difference between the decarburization time t' i (i = class j droplets) and the total residence time t'. is longer, and iron

oxidation is more extensive at small droplets. Such droplets may be totally oxidized to FeO. Large droplets on the contrary, have a lower degree of decarburization. The X*co content mayremain smaller than the equilibrium value X*co FeG for the FeO/Fe equilibrium during the entire

residence time. In such case, no iron oxidation occurs. Thus, when iron is oxidized at small droplets, carbon oxidation proceeds simultaneously at large droplets.

Carbon dioxide generated by the post combustion reaction oxidizes carbon dissolved in the dispersed iron droplets according to reaction (3) which is C02(g) + [C] -> 2CO(g). At the

drop-gas interface, the Boudouard equilibrium can be written:

16

(10)KB ~(1 - JQ)'

Where [C]‘ is carbon concentration and XL is equilibrium CO2 concentration at drop surface.

During the oxidation process, the carbon concentration decreases, and, as a consequence of the

Boudouard equilibrium, the CO2 concentration at the drop surface increases. According to the reaction (4) which is CC>2(g) + Fcq —» (FeO) + CO(g) the value X^ Fe0 is given by the

equilibrium.

KFeO/Fe

y *A C02 FeO

1 _ V*1 A COz, FeO(11)

As the drop temperature changes during the oxidation process, X^ Fe0 is time dependent.

2.2. Rate Phenomena

2.2.1. Hydrodynamics

The speed of the combustible gas against the flame surface is generally known as the combustion speed. It varies from 10-1000 m/sec, depending on gas composition and its mixing ratio with air (oxygen), temperature and pressure.

Fig. 12 shows the combustion speed of CO gas in the case of a laminar flow under standard conditions. Gas combustion in an oxygen environment is about 10 times as fast as in air, and

17

turbulent flow is nearly double that of a laminar flow. The post combustion mechanism is considered to be diffusion combustion caused by the impact between the oxygen flow injected from the PC nozzle and the CO gas rising through the furnace. Therefore, stable combustion will be ensured by supplying oxygen at a speed equal to or lower than the combustion speed

HI.

When fluid flows out of a nozzle a jet is formed by its interaction with the surrounding fluid. There exists a potential flow core at the lower jet stream side of the nozzle outlet, where the

speed and concentration of fluid are the same as those in the nozzle.

Primary air ( 7.)

Fig. 12 Combustion rate of carbon monoxide gas in laminar flow 111

Outside this region is a free boundary zone where momentum and mass move in a direction perpendicular to the stream. While such phenomena vary with nozzle diameter as well as speed,

the distribution or intensity of the disturbance at the nozzle outlet, flow speed and concentration at a given point can be expressed as shown in Fig. 13. In this figure:

18

VOa

x(12)

vm do+ b

vvm

exp Ku| — (13)

where do = ipside diameter of nozzle (m), vo = initial delivery speed at nozzle outlet (m/sec), x = distance from nozzle outlet (m), vm = speed of core flow at x in jet (m/sec), r = distance from core flow (m) and a, b and Ku in the formulas are constant

7777) 7777777;

X (m)

Fig. 13 Schematic picture of jet stream with notations. Ill

Fig. 14 shows the schematic representation of post combustion lance, in the converter. Both gas jets from main and secondary nozzles involve the surrounding gas and impinge on metal bath.

19

CC>2 and 0% in the gas jet react with [C] at the interface between gas and molten steel, as the gas jet from the main nozzle is more intensive than that from the secondary nozzle /8/.

02Lance

Secondary Flow

Main Flow

Fig. 14 The main flow and secondary flow lance in the converter ISI

As the calculation result of Pei W., Yu Z., Axelsson C. I. et al. 191 the gas phase velocity vectors and temperature contours of one 2D simulation are shown in Fig. 15a and 15b. The results show that the secondary oxygen jet should have a strong impulse to get a good flame structure /9/.

20

H=0.5m

/ //

\ \ \ \

IIUUWn

l \ WWNNSs. s V

H=0.5in8=30°

PC%=33

6. 1920 S. 2030

4. 2140

3. 2240

2, 23S0

1. 2480

Fig. 15a Velocity vectors in the gas phase 191 Fig. 15b Temperature contours /9/

Fig. 16a and 16b show gas velocity vectors and these figures show the direction of flow with

arrows, artificially made the same length everywhere.

From this diagram, it is possible to see the major features of post combustion. The high velocity oxygen jet impinges on the liquid surface. It then spreads from the centre and pushes the hot combustion gases to the vessel wall. Most of the gas flows upwards along the vessel wall to the

mouth. Part of the upcoming gas forms a recirculatory loop which entrains the furnace gas into

21

(a) (b)

Fig. 16 Gas velocity vectors 131

the oxygen jet The gas which is entrained into the jet is primarily CO which reacts with the

oxygen near the outer boundary of the jet. The reaction is so rapid that there is only about 20 percent oxygen in the jet, leaving the impact zone beneath the lance; by the time flow reaches the vessel wall, there is virtually no free oxygen left At the impact zone, there is between 20

and 40 percent CO2 in the gas, which will react with the bath just as oxygen would /3/.

2.2.2. Heat Transfer

An increased amount of oxygen supplied to the furnace enhances the combustion rate, but does not directly lead to a temperature rise in the hot metal because the CO2 gas produced is consumed in the decarburizing process, thus causing an endothermic reaction, or the transfer of combustion heat to hot metal is insufficient because the post combustion zone is inappropriate. To increase the rate of heat transfer to the hot metal, it is important to form an optimum post

22

combustion zone, taking into account the converter, the gas recovery unit and lance tip wear.

The following should be taken into consideration when determining the optimum post combustion zone, from the viewpoint of a favourable heat transfer ratio and equipment protection /7 /:

(i) Heat should be transferred to slag on the hot metal face and to foaming slag on the sides of the furnace, directly by radiation, which is then transferred by condition through the boundary face of hot metal.

(ii) Heat should be directly transferred to the exposed hot metal face by radiation.

(iii) Heat should be radiated onto the furnace wall, and then transferred to the hot metal via the refractories. Since fusion damage to refractories requires special attention from the standpoint of protecting the converter itself, the combustion zone should be as far from the furnace wall as possible.

(iv) To protect the top lance tips from combustion heat, the combustion zone should not be formed near the nozzle tips.

Thus the appropriate zone for post combustion in converters is presumed to be just above the hot metal, more than lm from the refractory surface and a short distance from the top lance tips. This concept is shown in Fig. 17 /7/.

In order to accelerate heat transfer under moderate stirring conditions, acceleration of the circulation of carburization materials in the slag layer is favored. It also coincides with the acceleration of carburization.

In order to improve various operation performance at the same time, it is necessary to adjust the distribution of the metal droplets, carbonaceous materials and oxygen gas jet as follows:

(i) The oxygen jet should not enter in the lower part of the slag layer, where a large amount of metal droplets is suspended.

(ii) The amount of metal droplets in the upper part of the slag layer, where the oxygen jet enters, should be as low as possible.

(iii) A sufficient amount of carbonaceous materials enters the lower part of slag layer, and circulates.

23

Lance

combustion region"^Oxygen jet

olten

LD converter

Fig. 17 Mechanism of Heat Transfer 111

These conditions can be achieved by an appropriate combination of the amount of slag, the conditions of oxygen top blowing and bottom gas bubbling, and controlling the size of

carbonaceous materials.

24

3. INFLUENCE OF DIFFERENT PROCESS PARAMETERS ON POST COMBUSTION

3.1. Post Combustion in LD process

In the top blowing converter we have two problems, if the height of the lance is high the

stirring is low and if the height of the lance is low the stirring is high (Fig. 18) but the control of post combustion is very difficult (the amount of post combustion is low).

Oxygen

I

Fig. 18 LD Converter

25

The waste gases in a conventional BOF contain 80-90 % CO with the balance CO2 as a result

of the primary decarburization reaction which is the basis of the BOF process. This large volume of CO is a substantial potential source of energy if a suitable method can be developed

to combust this CO to CO% within the BOF vessel and transfer the heat liberated in the combustion reaction back into the metal bath.

According to the experimental work of John J., Repasch Jr., Michael Byrne, David W. Kern, et. al. /10/ two types of post combustion lance were tested:

(i) Type I with the secondary oxygen ports at the lance tip.(ii) Type U with the secondary oxygen ports located above the lance tip.

The various lance designs port sizes and oxygen flowrates tested are listed in Table 3. For

simplicity of design a single oxygen source was typically used. The total oxygen flowrate was increased and the ports were sized so that the total oxygen was partitioned between the primary and secondary ports in the ratio of 4:1 primary to secondary.

Table 3 Oxygen Lance Design Parameters for the 2 ton BOF /10/

Lance tvn<; Primary Nozzles Secondarv Nfozzles

standard Num­

ber

Diamete

r Inch

Angle to

vertical

Flow,

scfm

Num­

ber

Diameter

Inch

Angle to

vertical

Flow,

scfm

Total

Flow scfm

single hole 1 0.315 200 200

Three Hole 3 0.168 10° 200 200

Type I PC Low Port

3+3 3 0.168 10° 200 3 0.084 50 250

Dual

Circuit3 0468 10° 200 3 0.084 20-60° 50-60 200-300

Dual

Circuit

3 0468 10° 200 3 0.152 50 250

Type II PC High Port

Cu Pipes 3 0.168 10° 200 3 0.186 -20° 50 250High Port 3 0.168 10° 200 3 0.084 u> O 0 50 250

26

Initial post combustion experiments used Type I lances with the primary and secondary oxygen ports located at the lance tip. Comparison of standard lance heats with the Type I PC lance heats showed an apparent increase in scrap melting capability of 8 % absolute with the PC lance. However, turndown slag FeO levels were significantly higher for the PC lance heats

(31.4 % average FeO for the PC lances versus 19.1 % FeO for the standard lances) while turndown carbon levels were significantly lower than the aims (0.05 % C for the PC lances

versus 0.12 % C for the standard lances). Gas analysis results for a number of standard and

Type I PC lance heats are shown in Fig. 19 where the ratio CO/(CO+CO%) is plotted as a function of blowing time. This ratio expresses the CO content of the waste gas as a fraction of the total decarburization product, therefore combustion of CO to CO2 to any appreciable extent would result in lower values of this ratio.

^Oxygen1

10 15Blow Time, Minutes

Standard LancePC Lance

Blow Time, minutes

Fig. 19 CO/(CO+C02) ratios for standard Fig. 20 Change in 00/(00+00%) ratio for and Type I post combustion lances /16/ pseudo-Type II Lance (copper pipe) /10/

As is seen in Fig. 19 the bulk of the data lie within a scatter band of 80+/-5 % and the PC lance results are indistinguishable from those of the standard lance. Varying the secondary port angle from 15° to 60° form the vertical had no measurable effect on the 00/(00+00%) ratio. Experiments with the hard and soft blowing through the secondary ports using a specially

designed dual-oxygen-circuit lance also produced no measurable effect on the CO/(CO+CO%) ratio. Therefore concluded that the Type I lances are ineffective in promoting significant CO

27

combustion within the BOF vessel. Heat and mass balances of the turndown data confirmed

that the scrap increase seen was achieved at the expense of the high iron losses to the slag and the over blowing of the heats to low carbon levels. These calculations showed that only a small amount of post-combustion may have been achieved, on the order of increasing the CO2 content by 3 % absolute, which is within the scatter band for the measured gas compositions shown in Fig. 19. This result was subsequently confirmed in a limited number of Type I PC lance heats run in the Bethlehem Plant's 270 ton BOFs.

The next step in the experimental program was to move the secondary oxygen ports away from the lance tip to determine the effectiveness of combusting CO to CO2 with a Type II PC lance. To save time and to minimize the number of physical changes made to the oxygen lances, a standard lance was converted to a Pseud-Type II PC lance by attaching copper pipes to the outside surface of the standard lance and supplying the secondary oxygen through these copper pipes. The copper pipes terminated 36-42 inches above the lance tip and were inched at an angle of about 20° to the vertical. The termination point of the copper pipes defined the location of the secondary ports. Again the ratio of the oxygen flow was 4:1, primary to

secondary flow. It was recognized that these copper pipes tips were unlikely to survive an entire heat so no attempt was made to melt additional scrap during these runs; gas analysis results would determine the potential for this approach. The gas analysis results from one of these "copper pipe experiments" are shown in Fig. 20. Argon gas was used in the copper pipes during the early blow, switched to oxygen for several minutes then switched back to argon for the remainder of the blow. With argon flowing through the copper pipes the CO/(CO+CC>2) ratio was about 80 %, at eleven minutes into the blow the argon was switched to oxygen and the ratio fell to approximately 56 %, at fourteen and half minutes into the blow the oxygen was

switched back to argon with a resulting increase in the measured C0/(C0+C02) ratio. The copper pipe experiments were repeated a number of times and similar results were obtained each time. These results demonstrated that a secondary source located away from the primary

source could indeed be used to combust CO to CO2 /10/.

3.2. Effect of Lance Height on Post Combustion

The lance height is the distance between the lance tip and the liquid surface before blowing. According to experiment of Huin D., Landry J.M., Reboul.J.P. et al. /11/, an increase of lance height leads to a higher post combustion ratio.

28

It was observed by Gou H., Irons G.A. et al. /3/ that the oxygen lance height had a very significant influence on PCR and HTE. When the lance was raised from 4 to 5 m, the PCR increased from 16 to 23 %. In their trials, bath temperatures were not measured; the bath

temperatures quoted in Table 4 were measured in earlier trials in the same vessel. The blowing time in each heat lasted approximately 19 minutes; stable operation was achieved from the 5 to

15 minute marks. Three operational conditions were examined:

(i) a 4 m lance height early in the vessel campaign;(ii) a 4 m lance height late in the campaign when the inner dimensions of the vessel were

increased due to refractory erosion; and(iii) a 5 m lance height, also late in the campaign.

Table 4. Results from Dofasco's KOBM at 8 Minute Mark in blow /3/.

Lance Height No. of Heats DCR (kg/min) PCR % HTE % T T

4 m early campaign 10 740 15.0 63.4 13904 m late campaign 14 722 16.3 63.2 1390

5 m late campaign 16 678 22.6 55.5 1390

In most studies, similar increases in PCR and decreases in HTE with increasing lance height have been noted. The conventional interpretation of this phenomenon is that more CO is entrained into the oxygen jet to increase PCR. For a given blowing rate, if the DCR (Decarburization Rate) increases, more primary oxygen is used to generate carbon monoxide; consequently, less primary oxygen is available to post combust the carbon monoxide to carbon dioxide. This relationship is clearly shown in Fig. 21. The line through the data is simply generated from the oxygen mass balance for a constant blowing rate. Examination of the scales in Fig. 21 reveals that small changes in DCR result in substantial changes in the PCR /3/.

29

O MEASURED LH=4m • MEASURED LH=5m — CALCULATED

640 660 680 700 720 740 760DECARBURIZATION RATE (kg/min)

Fig. 21 Relationship between observed (DCR) and PCR from exit gas analyses in Dofasco's KOBM131.

Raising the lance height softens the blowing, so that less oxygen is transferred to the bath. This fundamental relationship has not been previously appreciated 131. The post combustion ratio is decreased by lowering a position of blowing of post combustion oxygen by elevating a position of the lance. Conversely, when the position of the lance is lowered, the post combustion ratio is decreased.

3.3. Bottom Stirring

Most oxygen converters in the world are equipped with a mean to inject gas through the

bottom (as LBE). Therefore it is important to study the effect of stirring flow rate on post combustion generation. As shown in Fig. 22 the post combustion is promoted by a low bottom stirring flowrate. An increase of the carbon dioxide reduction by a high stirring rate seems to be a reasonable explanation for this observation.

30

100

LPost combustion ratio (%)

PC (%)

Bottom stirring 0.36 0

1

.12 0.36

Flowrate (Nml/min.t)

-------------------i

0 5 10 15 Time (

Fig. 22 Influence of bottom stirring rate on PCR in a combined blowing 61 pilot LBE Converter /II /

Indeed, a large stirring rate leads to decrease of Fe20g content of slag and therefore to favourable condition for CO2 reduction by the bath /ll/. However, it must be noted that the

results in these pilot scale experiments are very different from general industrial LBE runs.

Schematic of a combined oxygen blowing process, i.e. to blow a converter from the top and through the bottom is shown in Fig. 23. The metal in converter is blown from the top with oxygen and through the bottom, with oxygen or argon (sometimes with nitrogen).

31

top blowing lance

or side tuyereslagformers post combustion

r=~fc— hydiT°" -hr carbonoxygen

N2and/or

Ar

Fig. 23 Post Combustion in Combined Blowing Converter 1121

In combined blowing converter processes the mixing effect of the gas flow from the converter bottom is so effective that the top lance can be kept in the higher position, which leads to a higher post combustion ratio, therefore to control the post combustion is not difficult in

combined blowing converter processes.

3.4. Special Lance Construction

Obviously, the post combustion method is the simplest, most economical and effective method in improving heat balance of smelting process as well as reasonable utilization of CO gas energy resource and increasing scrap ratio. A special oxygen lance has been widely used for post

32

combustion of CO gas in converters. Fig. 24a and 24b, show a typical post combustion oxygen lance used in nowadays.

(a) (b)

Fig. 24 Special lances /10/

The experimental work of John J., Repasch Jr., Michael Byrne, David W. Kern, et al. /10/, was aimed at optimising the post combustion lance design and quantifying the scrap increase that could be achieved with these lances. Varying ports, angles of the secondary ports and the ratio of primary to the secondary oxygen were studied. The optimization experiments led to a lance

design with three primary oxygen ports located at the lance tip and three secondary ports located 36 inches above the lance tip and inclined at 30°C to the vertical axis of the lance. In

the normal blowing position the secondary ports are located at approximately the same

33

elevation as the intersection of the barrel and cone of the 2 - ton BOF as shown schematically inFig. 24b.

The post combustion dual-flow oxygen lance is comprised of the primary and secondary oxygen lances. The constructional parameters of dual-flow oxygen lance involve nozzles,

configuration of nozzles, nozzle number and arrangement around the lance tip, angles between centre lines of nozzles and lance axis, the distance between the main and post combustion nozzles along the lance axis, etc.

The oxygen lance can be considered as a "burner" blowing pure oxygen in a carbon monoxide rich atmosphere. It appears that the carbon dioxide concentration decreases. In other words, carbon monoxide is entrained from the converter atmosphere in the oxygen jet and reacts with it to form carbon dioxide. This phenomenon is the way COg generation may be explained in the converter /11/.

3.5. Refractory Wear /Vessel Volume

The post combustion trials were conducted over a six months.period by Farrand B.L, Wood J.E., et al. /4/ that encompassed the last half of one campaign and the first half of the next. It was observed that the PCR increased with increasing furnace life (volume), See Fig. 25.

No. of Heats * 0-500•1000-1500 O2000-2500

K

5 10 15Blowing Time (min)

Fig. 25 Effect of Converter (wear) on post combustion ratio /4/.

34

The HTE did not change with furnace life. Over the course of a campaign the furnace inner diameter at the slag line increases from 5,7 to 7,0 metres contributing to a furnace volume increase of about 40 %. The increasing vessel volume increases the gas residence time in the

furnace thereby increasing the time available for the post combustion of the off gas.The increase in PCR was verified by the result of a heat and mass balance as shown in table 5. The difference in PCR at different periods in the campaign life are statistically significant This observation must be considered when comparing PCR results from different periods over the furnace life.

Table 5 Effect of Furnace Life on PCR /4/

Furnace Life (Heats) PCR (%)Off Gas Analysis Heat And Mass Balance

0-500 13.1 14.01000-1500 14.8 15.32000-2500 16.1 16.6

According to the study of John J., Repasch Jr., Michael Byrne, David W. Kem, and Michael R.

Deamer /10/ as more heats are made with the PC lances (Fig. 24 see on page 33), a problem

with high rates of refractory wear became obvious. Refractory wear could not be evaluated in the 2 ton BOF because of differences in lining configuration so a conservative approach was required to implement PC lance use in the operating furnaces at Bethlehem without incurring a refractory wear penalty. BOF refractory wear in the plant's 270 ton furnaces was measured with

wear monitors developed at Homer Research Laboratories. The typical BOF lining had two idle side wear monitors, one located in the mid-barrel region and the second located in the furnace cone. Typical working lining wear rates with conventional lances averaged 0.037 inches per heat to the first hole in the mid barrel region using tarbonded refractories, as shown in table 6.

35

Table 6 Bethlehem Plant BOF Mid-Barrel Lining Life Data /10/

Campaign Lining Thickness (inches)

Number of heats toFirst Hole

Wear Rate to First Hole(inches per heat)

70-1-82 30 990 0.030

80-1-82 36 790 0.045

70-1-82 30 778 0.039

80-1-83 30 714 0.042

70-1-83 27 863 0.031

80-2-83 30 1054 0.02870-1-84 30 966 0.03180-1-84 27 486 0.055

note: the bolded numbers indicate campaigns in which PC lances were used

Also shown in table 6 are wear data from early campaigns with limited post combustion lance use. The wear rate 0.055 inches per heat shown for campaign 80-1-84 was outside the normal scatter and was attributed to the relatively large number of consecutive post combustion heats (473) made during this campaign. The accelerated wear resulted in the early appearance of the first hole in the lining and in the premature termination of the campaign at 890 heats because of severe lining wear /10/.

** poet combustion

Basicity .

Fig. 26 MgO in slag vs basicity (CaO/Si02). /I3/

36

According to the trials of Kuusela P., Lindfors N.O., Jonsson L. /13/ a quite high MgO content in the slag was achieved. Those trials in which material (carbon) was injected through the bath showed the highest MgO contents of the slag and largest erosion of the refractory lining (e.g.

peat-injection-trials). The post combustion tests showed no tendency for increased MgO content in the slag. Fig. 26 shows the MgO content of slag as a function of basicity

(Ca0/Si02).

3.6. Effect of Slag on Post Combustion Ratio

According to the experiment of Hirai M., Tsujino R., et. al. /2/ Fig. 27 shows the relation between the height of slag foaming and the post combustion ratio in the test with a 250 t converter. The height of the slag foaming (distance from the bath surface to the top end of

foaming slag) was determined by measuring the change in the temperature of the sub lance.

Dual lance 1Main Oz : 33000 (ttarVh) Sub 02 : 3000

LG 1.8 (m)( (at 15 rain) ,

Slag foaming height (m)

Fig. 27.Effect of slag foaming on the post combustion ratio

It is observed that the post combustion ratio in the Q-BOP is zero, and that the oxygen

potential from the slag composition is not consistent with that determined from the post combustion ratio. From these facts, it can be said that the adverse effect of slag is not

37

attributable to the composition of slag but results from such physical factors as the interference for entrainment of CO into oxygen jet by slag foaming and the abstraction for dissipation of CO2 from the region of oxygen jet produced by combustion.

0 Normal N=39 • Foaming Slog

N=14

O Normal N=39 • Foaming Slag

60

Foaming Slag Period

Foaming Slag Period^

5 10 15Blowing Time (min)

5 10 ISBlowing Time (min)

Fig. 28 Effect of foaming slag on PRC 74/. Fig. 29 Effect of foaming slag on HTE /4/.

The effect of a foaming slag on PCR and HTE is shown in Fig. 28 and 29 and compared in Table 7. The PCR was observed to decrease during the foaming slag period. The PCR returned to the normal level when the foaming slag was destroyed.

Table 7 Normal and Foaming Slag Results (6-14 min. period of blow) 74/

Slag PCR (%) HTE PCR X HTE100

mean s mean s

Normal

(N=39)15.8 2.3 41.9 5.3 6.6

Foaming(N=14)

13.7 1.1 50.0 6.6 6.8

38

The HTE was observed to increase for the foaming slag over the non-foaming condition. From Fig. 29, the HTE for the normal non-foaming heat is shown to decrease from roughly 50 % to 20 % over the 6—14 minute period. With a foaming slag the HTE was more consistent in the 50

to 40 % range, though a decreasing trend was also noted. In the particular trials the increase in HTE resulting in no net increase in energy input to the furnace (i.e. PCR X HTE = constant)

/4/.

At Dofasco the lance has a single oxygen flow control for both the primary and secondary ports. It may be possible to take advantage of the foaming slag effect on by using a dual flow top lance design. In this case the PCR characteristics of the lance tip could be changed at different times during the blow. It could be increased when the slag was foaming and decreased when the slag collapsed. An attempt was made to increase the PCR in the foaming slag by raising the lance height (5 m compared to normal 4 m). This was unsuccessful because it was

not possible to maintain a stable foaming slag with the 5 m lance height

° Normal Heat • Foaming Slag

Foaming Slog Period

5 10 15

Blowing Time (min)

Fig. 30 Particulate emission rate during normal and foaming slag heats /4/.

During the foaming slag trials Dofasco's Environmental Control Department measured the particulate emission rate from the KOBM. There was a reduction in the particulate emission rate during the period of slag foaming by a maximum 59 % at the 6 minute point. See Fig. 30.

39

4. CONTRIBUTION OF SMELTING REDUCTION

In-bath smelting with post combustion is known as a concept for the energy-saving production of hot metal and steel. The main advantages to be expected from the smelting with post

combustion are:

(i) direct use of fine ore

(ii) use of coal instead of coke(iii) decrease of the primary fuel consumption

4.1. Post Combustion Behaviour in In-bath Type Smelting Reduction

At various pre-reduction degree of iron ore (PRD), Fig. 31 shows the influence of post combustion degree on coal consumption per ton hot metal produced and surplus energy

25

I?!iin

108642

Ore PRD Hpc*90%temp. f\o/

- 25*C v /e \ ZZZZ*. BF-processf 10% \ range

02500

2000

1500

1000

500r>

s jrjj/sj/ss /777J

PRD* * Pre-reduction degree 0%^ of iron ore

10% X.20%X. X. Target

—cr—,VT//777/7y7y/777T/^^S^^Z777^

■ ' ' i^i i°0 10 20 30 40 50 60

Ore Gas; i

Pre­reduction

J• fOre Gas

A(Smelting(reduction

R>st-combustion degree,0 D (%)

Fig. 31 Effect of post combustion degree in smelting reduction furnace on coal consumption rate and surplus energy evolved from the process /14/.

40

evolved from the smelting reduction process. Post combustion ratio here is represented as oxidation degree of offgas (OD) which is given off from the smelting reduction furnace (SRF).

The OD is equivalent to the post combustion ratio (PCR):

(14)

If higher OD than 50 % is obtained, lower coal consumption can be obtained than that for blastfurnace process with the equivalent surplus energy, even if PRD (Pre-Reduction Degree) is

lower than 30 % /14/.

Optimization of '-eding rate of

w materialsOptimization of oxygen lane

• Double flow lance• Ultra-soft blowing

• Depression of coal carry-over

• Control of coal feeding ratePost-combustion

in slag layer• Keeping slag level

high• Control of lance

Intensifying 1 I \bath stirring I Smelting Reduction Furnace

Fig. 32 Key factors to achieve efficient post combustion in the SRF /14/.

The combination of low OD (< 50 %) and high PRD (> 30 %) yields unnecessary excess

process gas energy, more than that required at the integrated steelworks. When higher PRD than 30 % is decided, OD should be kept lower than 30 % to secure reduction potential for the pre-reduction. This gives a substantial increase of required process gas for the PRF, much

larger than the energy required in the subsequent plants. Therefore, the combination of intensive post combustion (OD > 50 %) and the light pre-reduction is considered to be

reasonable as the alternative process /14/.

In order to obtain low coal consumption, it is important to achieve simultaneously high OD and high heat transfer efficiency (r\PC). Also, an increase of t\pc results in lowering offgas

temperature after post combustion, which may prevent the refractory damage. Fig. 32 shows

the primary conditions for efficient post combustion in the SRF /14/.

When the heat of post combustion is transferred from the high temperature gas to the iron bath, it is considered that slag may play an important role as heat transfer medium /14/.

4.2. Kinetics

The applicability of post combustion depends on the successful procedure of heat and mass transfer from gas to melt. In the melt itself, the reduction reaction proceeds, and this part of the

system is, therefore, governed by the kinetics of reduction. In order to describe heat and mass transfer and reduction kinetics quantitatively, an appropriate process model must be set up.For modelling the process of in-bath smelting it is suitable to characterize it by the following properties /15/

(i) The process is determined by a number of input flow rates, namely those of ore, coal, and oxygen, and by a number of output flow rates, namely those of iron, slag, and off-gas, which in most cases are constant in time.

(ii) From the constancy in time of the input and output flow rates it follows that the process is in a steady-state.

42

(iii) In accordance with the steady-state condition, the reaction system in the smelting reactor

has a number of state variables such as temperature, concentrations and others which are also constant in time.

(iv) In the reaction system there are at least four different phases present, namely gas, slag,

metal, and solid carbon.(v) The phases are intermingled with one another in a number of emulsions and suspensions.

Starting from these properties, the process kinetics can be divided into a number of steps. The first step is to formulate the equations of macro-kinetics within the in-bath smelting system. The macro-kinetic rate constants are obtained from a consideration of micro-kinetics. This is the second step of process kinetics, and it is that step which requires most of the labour in the entire procedure of modelling. Micro-kinetics describe the processes taking place within the single reaction sites which are structured as emulsions or suspensions of several single phases.

According to this structure, the single micro-kinetic processes include the chemical and thermal reactions at the interfaces between the dispersed phases, the physical processes of drop formation and of the establishment of the life times of drops and bubbles and of their size distribution functions as well as flow and mixing processes /15/.

4.3. Post Combustion in Different Smelting Reduction Processes

Beside bottom blowing in-bath smelting also top blowing smelting is object of intense research

and development work mainly in Japan and the United States. Fig. 33 gives a schematic view how the process of reduction and post combustion can proceed. Lumpy ore is fed from the top and is melted in a hot and foaming iron oxide containing slag, below which the metal melt is located. In many cases it is usual to inject fine grained coal directly into the slag as the reducing agent. Post combustion takes place above or within the top region of the slag, whereby heat is transferred to ejected slag droplets or to the fine dispersed slag in the gas-slag foam respectively. As the heat is transferred to the slag, reoxidation of carbon at this process is excluded. Oxidation of ferrous to ferric iron in the slag must, however, be considered, but its amount is much smaller than the direct oxidation of metallic iron by CO2. Thus, post combustion and heat transfer to slag is easier than to metal droplets.

43

The reduction reaction takes place in the region where the fine grained coal is injected into the slag. Droplets of metallic iron are produced here and carbon monoxide is evolved in consequence of the reduction reaction. The rising carbon monoxide bubbles generate acirculating flow within the slag (Fig. 33). It should have a pattern that the regions of reductionand of post combustion are locally separated from one another with respect to the produced

iron droplets which means that the flow of iron droplets is directed to the metal bath and,

hence, that the droplets do not contact the post combustion region where they would be oxidized. The slag carries out the complete recirculation, in order to transfer heat from the post combustion to the reduction, while the droplets remain deeper under the influence of gravity

/15A

N„(Q N,.(C)

Fig. 33 Top blowing process with (0%) and bottom stirring (N%) /15/

The process of technology of the bottom blowing reactor with coal injection and post combustion was developed, using the Savard-Lee tuyere, in so called OBM-S and KMS steelmaking process in order to attain high scrap rates /15/. It was then transferred to coal gasification and to the development of the HI smelt process for smelting reduction /15/. At this technology, pulverized coal is injected with inert gas through the reactor bottom into the melt.

44

Ore is fed from the top. The bottom injection of coal and the strong carbon monoxide evolution from the reaction of iron oxide with the dissolved carbon results in an intensive mixing of slag and metal accompanied by the ejection of metal and slag droplets into the gas space above the smelt Under these circumstances, the entire system may be considered as consisting of two reaction sites: the metal site including the emulsified slag and the gas site with ejected droplets, whereby the gas site is assumed to be completely mixed. A schematic view of the system is shown in Fig. 34 for a converter shape. It is assumed that the oxygen blown in from the top

through several nozzles oxidizes the carbon monoxide to carbon dioxide and produced CO2- CO mixture is homogeneously distributed over the gas site where the droplets are present.

oret

oxygen or pre-heated air

oxygen or pre-heated air

co*co2 /

t 1 Icoal

Fig. 34 Top blowing process with coal injection /IS/

As mentioned in chapter two, the heat liberated by the post combustion reaction is transferred from gas to droplets by convective and radiative heat transfer. The droplets are heated up and, by falling back, give their surplus heat content to the melt Simultaneously, the carbon dioxide oxidizes the carbon contained in the metal droplets and eventually the iron itself by convective

mass transfer. The convective heat and the convective mass transfer from the bulk gas phase to

45

the droplets surface are coupled with one another according to their physical similarity. Therefore, heat transfer is unavoidably accompanied by reoxidation when the heat is directly transferred via iron droplets.

According to the experiment of Hirata T., Ishikawa M. et al. /16/ the combined conditions of

bottom blown nitrogen and side blown oxygen affected PCR and HTE. Fig. 35a and 35b show the influence of bottom blown nitrogen on PCR and HTE in combination with top and side

blown oxygen (the ratio of side blown oxygen was 50 %). The increase in the metal stirring power decreased the PCR, while it increased the HTE.

Fig. 35a Top blowing process with side Fig. 35b Influence of metal stirring powerblowing oxygen gas and bottom stirring through bottom blown nitrogen on PCR

(N%) gas /16/ and HTE under the combined condition of top and side blow oxygen 1161.

On the other hand, with regard to the influence of side blown oxygen, the influence was different from that of the bottom blown nitrogen. Though the oxygen also changed PCR and HTE, it changed them in different ways according to the combined conditions of the side and bottom blown gas, as shown in Fig. 36. When the metal stirring power was rather weak (<0.7 kW/t), the side blown oxygen increased both PCR and HTE; when the power was rather strong (>0.9 kW/t), the oxygen decreased PCR, though it increased HTE. On the influence of bottom

blown nitrogen, the increase in the metal stirring power always decreased PCR, though the influence was small under the condition without side blown oxygen.

o'LlI\—X

£oco0_

O 50Ratio of side blown Oz

30 40 50 60 70PCR (%)

• Top and side Ozc Top 02

Fig. 36 Influence of side blown oxygen on Fig. 37 Relation between HTE and PCR PCR and HTE in conjunction with the under two oxygen blowing condition /16/influence of bottom blown nitrogen 716/

47

Fig. 37 shows the relationship between HTE and PCR. The following two conditions of gas blowing are compared in the figure: The one is the combination of top and side oxygen and

bottom nitrogen; another is that of top oxygen and bottom nitrogen. In the former condition the

increase in PCR resulted in the decrease in HTE, which corresponds to the changes in the Fig. 35. In the latter condition, HTE was lower than that in the former condition at the same PCR. The side blown oxygen was effective in improving HTE at the same PCR /16/.

48

5. FUTURE

In the BOF steelmaking process that uses the heat of carbon oxidation, the heat of reaction

from C to CO is mainly used. The heat of reaction from CO to CO2, however, is much greater than that from C to CO as given by Eqs. (1) and (2). If the secondary combustion step from CO to CO2 is utilized, the amount of scrap melting can be increased without prolonging the tap-to- tap time /12/.

In the normal BOF operation, the ratio of CO burned to CO2 (the post combustion ratio) is 5 —

10 It has been confirmed that the amount of scrap melting can be increased by increasing the post combustion ratio. For example, the scrap ratio can be increased by 3.4 % by increasing the post combustion ratio by 10 % /12/.

Functionally, it is doubtful if converters will change very much for the future. Gas collection, secondary lances and slag stoppers are standard, as are arrangements for bottom stirring and oxygen injection if called for. Post combustion is being developed and used more widely. It allows more scrap melting and thus reduces hot metal demand as well as saving energy.

Recent work on bath smelting processes showed that PCR of 50 % or higher with high heat transfer efficiency are possible. If higher levels of post combustion could be achieved in oxygen steelmaking (OSM) this extra energy can be used to melt more scrap, as is usually the case. However, the energy could be used to reduce iron oxide, such as is done in bath smelting. Adding ore to the BOF as a coolant has been done for many years but in the future, with high degrees of post combustion the amounts of oxide reduced can be significantly increased /17/.

49

6. CONCLUSIONS

The fundamentals of post combustion and the effect of different process parameters on the post combustion ratio (PCR) and heat transfer efficiency (HTE) in converter steelmaking process are studied in this report. From the economy view, post combustion saves the consumption of energy in the converter steelmaking processes.

One of the most important process parameters in steelmaking process is the post combustion

ratio obtained in the furnace. Higher post combustion ratios imply that more heat is generated in the vessel, this heat is necessary to heat scrap and carry out steelmaking reactions. Unfortunately, not all the heat generated by the post combustion reaction is actually transferred to the bath, thus, a heat transfer efficiency (HTE) term is required.

Post combustion ratio (PCR) in the converter is dependent upon the balance between the

oxidation of CO produced to CO% and the reduction of CO2 to CO. PCR is determined by the rate of oxygen supply, oxygen used for decarburization, and the remainder available for post combustion.

The location of the post combustion zone can be controlled, to a certain extent, by adjusting

the lance practice. Observations from steelmaking processes with post combustion show that an

increasing lance height increases the PCR.

The combination of top and side blowing oxygen and bottom blown nitrogen had higher HTE at the same PCR compared with that without the side blown oxygen. Under the appropriate condition the side blown oxygen is able to increase both HTE and PCR, a mere increase in the metal stirring power through the bottom blown nitrogen increased HTE, however it decreased PCR.

The appropriate combination of top and side blown oxygen and bottom blown nitrogen contributes to the favourable conditions for smelting reduction, improving the post combustion ratio, the heat transfer efficiency, and the reduction rate.

50

REFERENCES

1 Bogdandy L von., Brotzmann K., Fassbinder H.G. Post Combustion and Fuel Injection

in the Converter Process, Mixed Gas Blowing, Fourth Process Technology Conference, Iron

and Steel Society, Vol. 4, Chicago, HI., USA, 3-4 Apr. 1984, p.109 - 112.

2 Hirai M., Tsujino R., et al. Mechanism of Post Combustion in Converter, Transaction

ISU, Vol. 27,1987, p.805- 813.

3 Gou H., Irons G.A. and Lu W.K. Mathematical Modelling of Post-Combustion in a

KOBM Converter, Metallurgical Transactions B, Volume 24B, February 1993, p.179-188.

4 Farrand B.L, Wood J.E., Goetz F.J. Post Combustion Trials at Dofasco's KOBM furnace,

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