The Swedish and Finnish National
Committees of the International Flame
Research Foundation – IFRF
HOT STOVE OXYGEN-ENRICHED COMBUSTION IN AN
IRON-MAKING PLANT
Chuan Wang1,*, Andy Cameron2, Axel Bodén1, Jonny Karlsson3, Patrick Lawrence Hooey1,4
1,*
Centre for process integration in steelmaking
Swerea MEFOS, Luleå, Sweden
Chuan.Wang@swerea.se
2
Linde Gas,
The Priestley Centre, 10 Priestley Road
Guildford, Surrey
England
Andy.Cameron@linde.com
3
SSAB EMEA
Luleå
Sweden
Jonny.Karlsson@ssab.com
4
University of Oulu (Adjunct Professor)
Oulun Yliopisto
Finland
* corresponding author
ABSTRACT
The presented paper investigates the application of oxygen-enriched combustion in hot
stoves in an iron-making plant. The enriched oxygen is used to reduce the consumption of
the high calorific value gas of COG while maintaining the same flame temperature in the
hot stoves as the reference case. The investigation is carried by using a spreadsheet hot
stove model. Compared to the conventional oxygen-enriched combustion, higher stove
efficiency can be achieved when heat exchanger is installed to recover the sensible heat
in flue gas by preheating combustion air and BFG; higher stove efficiency can also be
achieved when parts of flue gas are recirculated to hot stoves. For the studied plant, it
indicates that heat exchanger has a better effect than flue gas recirculation in terms of
stove efficiency. However, it has been noticed that flue gas recirculation can help to
concentrate CO2 content in the flue gas, which will be essential for the carbon capture in
the BF iron-making process.
Keywords: oxygen-enriched; hot stoves; blast furnace (BF); flue gas recirculation
1. INTRODUCTION
Recent years energy intensive processes have shown their great interests on applying
oxygen-enriched/oxy-fuel combustion technology to industrial furnaces. Compared to airfuel combustion, oxygen enrichment requires less fuel for reheating in commercial
installations due to the reduction in heat losses to the fuel gas associated with reduction or
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elimination of nitrogen from the process gas stream. The theory of oxygen-enriched
combustion can be found in [1]. There are also some other advantages for using oxygenenrichment combustion technologies, such as lower NOx emissions, higher productivity,
improved temperature stability and heat transfer [2].
In process of iron-making, hot stoves are often used to preheat air which is used in the
blast furnace (BF). Hot stoves work as counter-current regenerative heat exchangers. The
preheated air is called hot blast. The hot stoves typically use low calorific blast furnace
gas (BFG) combined with higher calorific value cove oven gas (COG). BFG is generated
from BF when producing hot metal. COG is a valuable fuel being high in hydrogen (H2)
and methane (CH4). For an integrated steel plant, COG is often delivered from the coking
plant where coke is produced. Compared to the conventional air-fuel hot stoves, less N2
will be generated which will absorb less reaction heat from the combustion. This will lead
to a higher adiabatic flame temperature (AFT) with the same amount of fuel gas. As for
the hot stove, therefore, a higher blast temperature can be achieved. On the other hand,
the low caloric value fuel gas can also be combusted alone without mixing with
enrichment gas, such as COG, LPG or NG, to get the same flame temperature with the
use of oxygen enrichment instead. For the second option, Bisio et al. [3] made an analysis
on the basis of second laws of thermodynamics.
The presented paper is to investigate the potential of using oxygen enrichment in hot
stoves at an integrated steel plant. The stove efficiency and fuel consumptions by using
air-fuel and oxygen-enriched combustion are calculated and compared. In addition, a new
concept, oxygen-enriched combustion with flue gas recirculation into hot stoves under
specific conditions, is also presented and compared with conventional oxygen-enriched
combustion.
2.
Description of hot stove model
2.1
Hot stove – BF system at the studied plant
A simple structure of hot stove and blast furnace is presented in Figure 1. The hot stove
in the figure includes two separate parts, i.e. the combustion chamber and check chamber.
They work as a counter-current regenerative heat exchanger. The fuel gas is first
combusted in the combustion chamber. The flue gas passes through the check chamber
and heats it up, then leaves the stack to the ambient. This progress is often call on-gas
time. When it is ready, the blast time is started. During the blast time, the cold blast is
blown into the system in an opposite cycle, and is heated up by the check chamber. It
then passes through the combustion chamber. Before blowing into the blast furnace, it is
often mixed with cold blast to get the required and stable hot blast temperature.
BFG is a type of process gas with a low calorific value. It is often mixed with COG to get
a high heating value before entering into the combustion chamber. Traditionally the
combustion air is used in the hot stove for the fuel gases combustion.
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Figure 1. The structure of hot stove – blast furnace system at the studied plant.
2.2
Hot stove model
A spreadsheet-based blast furnace model has been developed at our research group [4].
The model is a static 1-dimensional heat and mass balance including three sub-models,
i.e. the blast furnace, hot stove and burden calculation. The three sub-models are
connected and balanced via iterative calculations.
Figure 2. The schematic diagram of hot stove model.
The hot stove model calculates fuel requirements for blast heating as well as maximum
blast temperature with the schematic shown in Figure 2. As shown in Figure 2, the user
can choose the heat exchanger (HEX) to heat up fuel gas and/or combustion air in the hot
stove model.
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Recently the hot stove model has been further developed to include the oxygen
enrichment. As shown in Figure 2, the enriched oxygen can be added either through the
combustion air or the flue gas recirculated into the hot stove. This can be freely chosen in
the model. The fuel gas requirements are calculated from blast furnace gas (imported
from the blast furnace model) and coke oven gas, as well as hot stove operating data input
listed in Table 1. The calculations can be constrained by hot stove flame temperature and
minimum temperature difference between the flame temperature and required blast
temperature. Additional adjustments for blast temperature increase with compression,
heat losses, and hot stove change-over are also included.
The flame temperature calculation is made iteratively according to Eq. (1).
AFT  ( Hi, fuel gas  H air and / or oxygen  H combustion  Hj, flue gas recirculation )
 Cp, k
T  FT
(1)
Where,
AFT  Adiabatic flame temperature, C
 Hi,
fuel gas
 sensible enthalpy of all gas of fuel gases (after heat exchanger
if heat exchanger is installed )
H
air and / or oxygen
 sensible enthalpy of air with oxygen if air is enriched with oxygen (after
heat exchanger if heat exchanger is installed )
 H
 Hj,
combustion
 enthalpy of combustion of fuel gases, e.g . BFG and / or COG )
flue gas recirculation and / or oxygen
 sensible enthalpy of gases of flue gas recirculated to hot stoves 
sensible enthalpy of oxygen enriched
 Cp, k
T  FT
 heat capacity of all gases after combustion of fuel gases at flame temperatur e.
The stove efficiency is an important parameter to evaluate the efficiency of a hot stove
for hot blast production. The stove efficiency is defined as Eq. (2). This efficiency is
often called “economic efficiency” for hot stoves.
Stove efficiency, % 
Energy in hot blast (GJ / hr )
 100
Chemical energy in fuel gases(GJ / hr )
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(2)
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Table 1. Input variables for hot stove model.
Parameter
Coke oven gas available
COG composition,
temperature
Flue gas temperature
Heat losses
Maximum flame temperature
Maximum achievable blast
temperature parameters
Minimum O2 in flue gas
Compressor adiabatic
efficiency
Air from atmosphere:
temperature, moisture
BFG gas: composition,
moisture
Flue gas recirculation only
Ratio of combustion air,
oxygen and flue gas
Unit
On/off
Notes
If off, hot stove must run on BFG gas only
°C
GJ/h
°C
Fixed or calculated
Fixed (& time based)
Fixed
Based on difference between hot stove flame
temperature and achievable blast temperature
Fixed
Affects cold blast temperature and compressor
power consumption
Imported from BF model
%
%
Imported from BF model
On/off
If on, the oxygen is added into recirculated flue
gas.
Adjustable for each part for step-wise oxygen
enrichment and flue gas recirculation.
2.3
Oxygen enrichment vs. flue gas recirculation
The conventional oxygen enrichment for the hot stove is to inject oxygen via the
combustion air, as shown in Figure 2. The economic analysis on the hot stove oxygenenriched combustion can be found in [5]. The oxygen enriched can be heated up together
with the combustion air if the heat exchanger is installed. A new concept is to add oxygen
into flue gas which recirculates back to the hot stove after combustion to replace the
combustion air. By doing so, parts of sensible heat in the flue gas can be recovered. In
addition, the heat exchanger can also be chosen for this new concept.
The conventional oxygen enrichment combustion often leads to a high flame temperature
or peak temperature. However, this could be better under the flue gas recirculation
oxygen-enriched combustion. As shown in Figure 3, the patterns of flame temperature
distributions for the flue gas recirculation oxygen enrichment combustion become more
flat due to the dilute effect of the recirclualted flue gas.
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Figure 3. The temperature distributions for conventional oxygen enrichement (left) and flue gas
recirculation oxygen enrichment combustion (right). Source: AGA.
2.4
Operating conditions at the studied plant
The fuel gases at the studied plant are process gases of COG and BFG, which are
produced from the coking plant and BF, respectively. Table 2 shows the analysis of the
process gases. For the reference period at the studied plant, the hot blast produced was
255.0 kNm3 per hour with a temperature of 1104 °C, which is required by the blast
furnace to produce hot metal with a production rate of 274.6 ton per hour, as shown in
Table 3.
BFG has a low heating value of 2.89 MJ/ Nm3. It was mixed with COG to achieve a high
heating value of 4.27 MJ/ Nm3, which can be seen in Table 3. The COG required was
around 10.29 kNm3/hr.
Table 3 also shows the flue gas generated and its temperature. The oxygen content in the
flue gas is 1.0%. There is no heat exchanger installed at the studied plant. However, a
part of flue gas (around 55 kNm3/hr) is used to dry pulverized coal (PCI drying) at the
moment. In addition, a small amount of COG is also combusted for PCI drying for the
current situation.
Table 2: Process gas analysis, %
CH4
C2.5H5
H2
CO
CO2
N2
O2
H 2O
Heating value (LHV), MJ/Nm3
COG
21.49
2.59
60.36
5.67
1.47
5.91
0.20
2.33
17.01
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BFG
0
0
3.69
19.78
23.37
45.86
0
7.29
2.89
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Table 3. Key operating parameters in hot stoves
Parameter
Blast temperature
Blast flow
Flame temperature
O2 in the combustion air
Combustion air
COG consumption
BFG consumption
Mixed gas heating value
Stove efficiency
Flue gas total
Flue gas temperature
Flue gas for PCI drying
O2 content in the flue gas
CO2 content in the flue gas
Unit
°C
kNm3/hr
°C
%
kNm3/hr
kNm3/hr
kNm3/hr
MJ/ Nm3
%
kNm3/hr
°C
kNm3/hr
%
%
Value
1104
255.0
1393.5
20.95
105.5
10.29
95.28
4.27
89.5
196.57
270
55
1.0
22.8
3. RESULTS
Oxygen enrichment combustion in the hot stoves has two main effects on the HS-BF
system. A higher flame temperature will be achieved with oxygen enrichment when
maintaining the same fuel gas rate, which will lead to a higher blast temperature to BF.
Thus, the reductant consumption, e.g. coke rate, can be reduced. With oxygen
enrichment, the low caloric value of BFG can also be combusted alone to get the same
flame temperature without mixing with COG. Thus COG saved can be used to replace the
fossil fuels at some other process units within the plant. The possibilities of using the
saved COG within the plant have discussed in our previous work [6].
In this work, the model is run to get the same flame temperature and hot blast amount as
the reference case. In addition, some other parameters such as heat loss, flue gas
temperature, flue gas amount to PCI drying, oxygen content in the flue gas are also kept
the same as the reference case. For the flue gas to PCI drying, the same flow rate as the
reference case is assumed for all modelling work. Although the flue gas temperature is
assumed the same for all cases, this will lead to a slight difference in energy due to a
small change with the heat capacity. The oxygen is injected via either combustion air for
the conventional oxygen enrichment combustion or recirculated flue gas for the flue gas
recirculation oxygen enrichment combustion to replace COG. The calculation results
show the hot stove operating parameters.
Compared to the reference case, with enriched oxygen amount of 13.38 kNm3/hr, Figure
4 shows that the combustion air flow rate is reduced from 105.5 in the reference case to
32.81 kNm3/hr, while BFG consumption is increased from 95.28 to 155.47 kNm3/hr.
When heat exchanger is considered to preheat the combustion air together with enriched
oxygen and BFG, BFG consumption will be reduced from 155.47 to 146.62 kNm3/hr.
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The enthalpy of BFG presented in Figure 4 includes both combustion enthalpy and
sensible enthalpy.
Figure 4. The calculation results of conventional oxygen enrichment combustion without HEX (left)
and with HEX (right).
For the case of flue gas recirculation oxygen enrichment combustion, the oxygen is
injected via the recirculated flue gas. Figure 5 shows the calculation results. Compared to
the reference case, the combustion air is completely replaced by oxygen. The flue gas
recirculated amount to hot stoves is 25.44 and 33.19 kNm3/hr respectively for the cases of
without HEX and with HEX. BFG consumption is also higher in the flue gas recirculation
oxygen enrichment combustion compared to the reference case. However, with the help
of HEX, BFG consumption will be lowered down from 152.29 to 145.16 kNm3/hr, as
shown in Figure 5.
Figure 5. The calculation results of flue gas recirculation oxygen enrichment combustion without
HEX (left) and with HEX (right).
The stove efficiency can be increased by recovering the sensible heat from the flue gas
via heat exchanger to preheat BFG and combustion air or flue gas recirculation directly
into hot stoves. For the studied plant, as shows in Figure 6, HEX’s effect is higher than
flue gas recirculation to hot stoves, 95.1% compared to 91.5%. The hot stove efficiency
will be up to 96.0% when both HEX and flue gas recirculation are included. The hot
stove efficiency is almost kept the same as the reference case if neither HEX or flue gas
recirculation is considered. Compared to the traditional air fuel combustion, N2 content in
the flue gas will be lower in the oxygen enrichment combustion, which consequently will
lead to a higher CO2 content in the flue gas. This can be seen in Figure 6. It has been also
noticed that CO2 content in flue gas is higher in flue gas recirculation oxygen-enriched
combustion than conventional oxygen-enriched combustion.
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98
46
44
96
CO2 content in flue gas, %
Stove efficiency, %
42
94
92
90
40
38
36
34
88
32
86
30
O2 enriched only
O2 enriched only
O2 enriched + HEX Flue gas recir. only Flue gas recir. + HEX
O2 enriched + HEX
Flue gas recir. only Flue gas recir. + HEX
Figure 6. Comparison of stove efficiency (left) and CO2 content (right) in the flue gas for different
cases.
Figure 7 shows the changes of combustion air and CO2 content in flue gas with enriched
oxygen flow rate without consideration of HEX. As shown in the figure, the combustion
air flow decreases while CO2 content increases with increased oxygen flow rate stepwise, from 22.8% to 36.6%. With the replacement of COG by BFG, the heating value of
fuel gas decreases from 4.27 to 2.89 MJ/Nm3.
4.5
35
4.3
30
80
25
20
60
15
40
10
Combustion air
20
CO2 content
0
0
2
4
6
8
10
Oxygen flow rate, kNm3/hr
12
Heating value, MJ/Nm3
Combustion air, kNm3/hr
100
40
CO2 content in flue gas, %
120
4.1
3.9
3.7
3.5
3.3
3.1
2.9
5
2.7
0
2.5
0
14
2
4
6
8
10
Oxygen flow rate, kNm3/hr
12
14
Figure 7. Left: The correlation between oxygen flow rate and combustion air, CO2 content in flue
gas; Right: The correlation between oxygen flow and fuel gas heating value.
The amount of flue gas recirculated to hot stoves can be controlled step-wise in the
model. The model is run to achieve the same goals as the previous cases, and there is no
COG consumption. This is continuation of end point in Figure 7 (left), the case of
conventional oxygen enrichment without HEX. As shown in Figure 8, the combustion air
is further reduced until it is completely replaced by oxygen, while CO2 content in flue gas
is increased from 36.6% to 43.3%. At the same time, the amount of flue gas recirculated
to hot stoves increases. Consequently, the hot stove efficiency increases step-wise due to
the increased sensible heat used by the hot stoves.
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92
42
91.5
25
20
37
15
32
10
27
Combustion air
5
CO2 content
0
13
14
15
Stove efficiency, %
Combustion air, kNm3/hr
30
47
CO2 content in flue gas, %
35
19
90.5
90
89.5
22
16
17
18
Oxygen flow rate, kNm3/hr
91
0
20
5
10
15
20
Flue gas recirculated, kNm3/hr
25
Figure 8. Left: The correlation between oxygen flow rate and combustion air, CO2 content in flue
gas; Right: The correlation between the amount of flue gas recirculated to hot stoves and stove
efficiency.
For an integrated steel plant, CO2 emissions from hot stoves account around one third of
the entire CO2 emissions. As shown in Figure 6 and 8, flue gas recirculation helps to
concentrate the CO2 in the flue gas which cannot be achieved by the conventional
oxygen-enriched combustion. Therefore, this concept provides a simple option to
generate a flue gas well suited to carbon capture without impacting the physics and
chemistry of the BF iron making process. It represents a low cost low risk alternative to
options such as TGRBF or HISARNA whilst improving stove efficiency regardless of
whether or not it is used in conjunction with a heat exchanger. In addition, the concept of
flue gas recycle secures a significant proportion of the stove efficiency improvements that
would otherwise require maintenance intensive and unreliable heat exchangers.
4. CONCLUSIONS
Oxygen-enriched combustion in hot stoves in an iron-making plant as an example has
been studied in this work. The enriched oxygen is used to reduce the consumption of the
high calorific value gas of COG while maintaining the same flame temperature in the hot
stoves as the reference case.
Conventionally, oxygen-enriched combustion is done by adding oxygen via the
combustion air. However, this could be also done via the flue gas recirculated back to hot
stoves. The latter is a new concept for oxygen-enriched combustion in hot stoves. For
both these two alternatives, heat exchangers can be installed. A hot stove model has been
further developed to calculate the hot stove efficiency and fuel gas combustion under
specific oxygen enrichment conditions. Compared to the conventional oxygen-enriched
combustion, higher stove efficiency can be achieved when heat exchanger is installed to
recover the sensible heat in flue gas to preheat combustion air and BFG; higher stove
efficiency can also be achieved when parts of flue gas are recirculated to hot stoves. For
the studied plant, it seems that heat exchanger has a higher effect than flue gas
recirculation for the stove efficiency. A highest stove efficiency will be achieved when
combining both HEX and flue gas recirculation.
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In principle, oxygen plant has a maximum oxygen production capacity. Exceeding this
capacity, liquefied oxygen has to be supplied externally, which will lead to a relative high
oxygen price compared the internal supply. The optimised oxygen level in the hot stoves
is most important from the economic point of view when balancing prices of COG and
other fuels used within an integrated steel plant, such as LPG, natural gas, oil or even coal
and coke rate in BF. The process gases network in the integrated steel plant nowadays is
often connected in between, thus, the total effects of oxygen enrichment combustion in
hot stoves within the whole plant has to be analysed.
The calculating results indicate high CO2 content in the flue gas for the oxygen
enrichment combustion, especially for the flue gas recirculation oxygen enrichment
combustion. Therefore, flue gas recirculation can help to concentrate CO2 content in the
flue gas, which will be essential for the carbon capture in the BF iron-making process.
However, compared to the oxy-fuel combustion, N2 content in the flue gas is higher due
to a high N2 content in BFG. How to separate CO2 from the flue gas is of great interest to
study.
Furthermore, the radiative heat transfer in the hot stove theoretically can be better due to
a high content of radiating gases of CO2 in the flue gas although convection heat is
dominated in the hot stoves. Therefore, it is interesting to investigate the radiation heat
changes due to the riched CO2 in the flue gas.
5. REFERENCES
[1] L. Zheng, Z.J. Kang, L. Zhang, Q.X. Zhao, Theory and application of oxygenenhanced combustion, Industrial Furnace 3 (2004) 10-14. (In Chinese).
[2] H. Kramer, The effect of oxygen enrichment on radiative heat transfer. Fuel
Efficiency and NOx Emissions, TOTeM-17:IFRF, 2000.
[3] G. Bisio, A. Bosio, G. Rubatto, Thermodynamics applied to oxygen enrichment of
combustion air, Energy Convention and Management 43 (2002) 2589-2600.
[4] P. Hooey, A. Boden, C. Wang, C. Grip and B. Jansson. Design and application of a
spreadsheet-based model of the blast furnace factory, ISIJ International 50 (2010)
924-930.
[5] G.Y. Zai., Hot stove oxygen-enriched combustion of economic analysis, Industrial
Furance 30 (2008) 30-33. (In Chinese)
[6] C. Wang, J. Karlsson, P.L. Hooey, A. Boden, Application of oxygen enrichment in
hot stoves and its potential influence on the energy system in an integrated steel
plant, Submitted to the international conference of WREC 2011, May 2011,
Linköping, Sweden.
6. ACKNOWLEDGEMENTS
This work is part of the research program at Centre for Process Integration in
Steelmaking (PRISMA) located at Swerea MEFOS AB in Luleå, Sweden. PRISMA is an
Institute Excellence Centre supported by the Swedish Agency for Innovation Systems,
the Knowledge Foundation, and eight industrial partners within the iron- and steel
industry.
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