OXYFUEL COMBUSTION OF LOW CALORIFIC BLAST FURNACE GAS FOR
STEEL REHEATING FURNACES
John Niska, Anders Rensgard, –MEFOS (Luleå, Sweden)
Tomas Ekman – AGA-Linde (Lindigö, Sweden)
john.niska@mefos.se, 0920-201986
ABSTRACT
Pilot trials at MEFOS have proven that a new S3 blast furnace gas (BFG)-oxyfuel
burner can give high performance, low NOx, low cost reheating for the steel industry.
The S3 burner has been developed by AGA-Linde based on REBOX® flameless
combustion technology with the optional use of a booster fuel. This burner was tested
in a series of trials in MEFOS chamber furnace using propane (LPG) as the booster fuel,
but natural gas could also be used. The trials investigated the environmental emissions
and reheating rates with and without the booster fuel. The NOx emissions were low and
there was not a problem with unburnt CO. Trials were made with up to 40% of the
energy from LPG. Steel reheating using BFG-oxyfuel combustion required a higher
energy input than LPG-air combustion, but this is compensated by the fact that BFG is a
low cost fuel.
This new multi-fuel oxyfuel burner provides industry with a system capable of
providing a wide range of performance from lower cost operation with pure BFGoxyfuel to higher performance and higher furnace productivity with propane-oxyfuel
when compared to typical conventional reheating furnaces using air combustion of
fossil fuels. The reduction of fossil fuel consumption with LPG-oxyfuel combustion
reduces the emission of greenhouse gases (carbon dioxide), and even greater reductions
in carbon dioxide emissions are possible when process gases are used which would
otherwise be flared.
Keywords: Oxyfuel, blast furnace gas, NOx, flameless, carbon dioxide, emissions, LPG
1
INTRODUCTION
The iron ore company LKAB operates an experimental blast furnace (EBF) in Luleå
together with MEFOS. The EBF supplies top gas or blast furnace gas (BFG) through a
new pipeline to the reheating furnaces at the Heating and Metalworking Department.
AGA designed and installed a dual fuel oxyfuel burner for firing the BFG with propane
boosting in the chamber furnace at MEFOS for trials within an EU RFCS project
[MALFA, 2008]. Some results from these chamber furnace trials are presented in this
report.
THEORY
Blast furnace gas is a low calorific fuel which has too low of a flame temperature to
directly replace fossil fuels in typical steel reheating furnaces with air combustion. The
flame temperature can be increased by oxyfuel combustion, plus by enrichment of the
BFG with another fuel, by preheating the BFG or by a combination of these techniques.
BFG preheating can be done with regenerative or recuperative heat recovery, but it is
normally a capital intensive alternative relative to fuel enrichment. Integrated steel mills
normally have an oxygen supply system for converting the raw iron to steel making
both oxygen, BFG and other gaseous fuels available for other uses. BFG enrichment
with LPG was the approach for using BFG which was investigated in these trials.
2500
Adiabatic Flame Temperature (C)
2200
LPG with 450C air
LPG with 20C cold air
1900
75% BFG + 25% LPG oxyfuel
1600
100% BFG oxyfuel
1300
1000
0
1
2
3
4
5
6
7
Excess Oxygen (% wet)
Figure 1. Flame temperatures for BFG-oxyfuel versus LPG with preheated air.
2
Steel reheating furnaces often use LPG (propane) or natural gas (methane) with the
combustion air preheated with a recuperator to save energy and to increase the furnace
productivity. The flame temperature with the use of oxyfuel combustion can be
calculated with a thermodynamics program. A comparison of LPG with the combustion
air preheated to 450°C versus BFG-oxyfuel is given in Figure 1, based on calculations
made with Hauck´s program E-Solutions for the average BFG composition given in
Table 1 [MARION, 2000]. Note that the flame temperature does not change very
quickly with an increase in the excess oxygen for the use of oxyfuel, as compared to air
combustion.
BFG-OXYFUEL COMBUSTION TRIALS
The primary fuel in the blast furnace gas is carbon monoxide (CO), which is present
together with nearly the same percentage of carbon dioxide (CO2) and small amounts of
hydrogen (H2) The composition of the BFG varied with the ranges given in Table 1. An
ABB Industrial IT Extended Automation System 800xA is used to control the
combustion in the chamber furnace together with a Siemens Simatic S7-400 to control
the fuel and oxygen flows to the new oxyfuel burner. The control system is designed to
monitor the lambda ratio and process temperatures to avoid unsafe process conditions.
The BFG composition was entered manually in the control system to compensate for
variation in the BFG quality and allow the system to maintain the desired burner power
while maintaining stable furnace temperatures, etc.
Table 1. Blast furnace gas composition
average
max
min
CO (%)
22.5
23.7
21.4
CO2 (%)
19.8
20.8
18.9
H2 (%)
2.6
2.8
2.4
Energy
3
(kW/Nm )
0.868
0.914
0.824
density
3
(kg/m )
1.363
1.370
1.357
The average lower heating value during these trials was 0.868 kW/Nm3 which is
equivalent to 3.12 MJ/ Nm3.
The BFG oxyfuel burner
AGA designed and built a flameless, dual fuel Type S3 oxyfuel burner based on the
REBOX® concept. A sketch of the burner design is given in Figure 2. A ceramic
burner block gives self-cooled operation. Startup with a cold furnace is possible with a
flame, then when the furnace is over 750°C the burner can be switched over to the
flameless mode for lower NOx with oxygen injection from the four holes around the
central fuel inlet. A view of the burner from the outside of the chamber furnace is given
in Figure 3. The burner is capable of operation up to about 400 kW with pure BFG, pure
propane (LPG) or a wide range of mixtures of these fuels. The burner could be also
converted to operate with natural gas instead of propane.
3
Figure 2. A sketch of the dual fuel Type S3 oxyfuel burner
Figure 3. A photo of the prototype S3 oxyfuel burner when mounted on the chamber
furnace
Burner emission trials
The furnace emissions were monitored by gas sampling from the exhaust duct using an
IR spectrometer for NOx, CO and CO2 concentrations. The Type S3 burner is capable
of a wide range of operational conditions, since there are so many parameters which can
be varied over a wide range. Higher NOx is expected for higher furnace temperatures,
more excess oxygen, a higher percentage LPG and degree of flameless operation (based
on the amount of oxygen to the flameless peripheral ports versus the central port). A set
of operational conditions were chosen as given in Table 2. The NOx levels varied from
about 10 to 20 mg/MJ for all the conditions tested as shown in Figure 4. The NOx was
always low for pure BFG oxyfuel, and slightly higher with LPG enrichment. There was
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some increase in the NOx with higher levels of excess oxygen, which can be related to
air leakage.
The increase in the NOx with the use of LPG enrichment can also be seen when plotting
the specific NOx emissions (in mg/MJ) versus the specific CO2 emissions (in kg/MJ in
Figure 5). The high concentrations of CO2 in the BFG give high levels of CO2
emissions for BFG oxyfuel, and lower specific CO2 emissions when using LPG
enrichment. The BFG is a process waste gas that needs to be used or flared, so there can
be a net decrease in the CO2 emissions if an integrated steel mill uses BFG to replace
fossil fuels. Oxyfuel combustion is also an energy efficient process, so BFG with or
without propane enrichment is a way to reduce fossil fuel consumption and net CO2
emissions.
Table 2. Parameters chosen for burner emission trials
Parameter
Range
Furnace temperature
1275, 1200, 1125 °C
Heat extraction power
High, Low
Central O2
30%, 15%, 0% of total O2
Fuel mixture
0%, 25%, 40% LPG (% of power)
%O2 dry (Lambda)
2%, 4%
Temperature (Alla)
25
Average of NOx mg/MJ
20
15
Fuel mix BFG/CPG
25%CPG
40%CPG
BFG
10
5
0.00
0.13
0.51
0.53
0.58
0.80
1.04
1.10
1.38
1.44
1.44
1.67
5.97
6.05
6.51
6.55
6.57
6.58
6.64
6.76
6.80
7.05
7.09
7.22
7.66
7.80
7.87
7.97
8.14
8.15
8.53
8.69
8.91
8.97
9.00
9.15
9.35
9.42
9.47
9.65
9.67
9.93
10.15
10.53
0
%Air in Flue gas
Figure 4. NOx emissions for BFG oxyfuel with and without LPG enrichment
5
25
NOx (mg/MJ)
20
15
10
5
0
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.3
CO2 (kg/MJ)
Figure 5. NOx in mg/MJ versus the specific carbon dioxide discharge for the range of
conditions given in Table 2.
The CO concentrations were insignificant for all the conditions tested, even for close to
stoichiometric combustion. Table 3 gives data for the fuels tested which were used to
compute the specific NOx emissions in mg/MJ.
Table 3. Fuel and combustion constants
Lower
Fuel
(% by energy)
LPG
40%LPG/BFG
25%LPG/BFG
BFG
Air LPG
Heating
Value
(MJ/Nm3)
92.56
5.09
4.12
3.124
92.56
O2
required
(Nm3/Nm3)
5.075
0.23
0.18
0.126
5.075
Density
of
mixture
(kg/Nm3)
2.047
1.378
1.371
1.363
2.047
BFG
LPG
Flue gas
(Nm3/GJ)
0.00
192.06
240.08
320.10
0
(Nm3/GJ)
10.80
4.32
2.70
0.00
10.80
(Nm3/Nm3)
7.10
1.13
1.07
1
31.38
Steel reheating trials
The reheating rate with BFG oxyfuel with and without LPG enrichment were evaluated
by charging the chamber furnace with two blooms equipped with thermocouples to
monitor the reheating rate. LPG-air trials with a power input of 300 kW were chosen for
comparison, so these trials were made with this maximum power even if the oxyfuel
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burner is capable of even higher power input. A constant furnace temperature set point
(1235°C) and percent excess oxygen (1%) were desired, but the constant furnace power
and furnace response times caused the reheating cycles to vary considerably (see Figure
6 for 75% BFG-25% LPG oxyfuel based on input kW).
Heating Trial with 75%BFG/25%CPG
Chamber furnace, March 17
1300
Temperature, Power, Oxygen [°C, kW, %*100]
1200
1100
1000
900
800
Furnace temperature
TC Specimen, surface
TC Specimen, centre
BFG Power
CPG Power
O2*100
700
600
500
400
300
200
100
0
08:00:01
08:10:01
08:20:01
08:30:01
08:40:01
08:50:01
09:00:01
09:10:01
09:20:01
09:30:01
Time [h:m:s]
Figure 6. Reheating trial conditions for 75% BFG- 25% LPG oxyfuel
A comparison of the reheating curves for pure BFG oxyfuel, 25% LPG enrichment,
40% LPG enrichment, pure LPG oxyfuel and LPG-air combustion for a reference is
given in Figure 7. The fastest reheating was for pure LPG oxyfuel as expected, since
the radiative heat transfer and the energy efficiency are the best for this case. The pure
BFG oxyfuel was the worst, since the energy efficiency was the worst for this case, and
the furnace temperature dropped far below the set point level. The LPG air trials were
made with two different set point temperatures, and gave results approximately the
same as 75% BFG-25% LPG oxyfuel. The furnace uses cold combustion air, so better
performance would be expected for LPG-air if the air was preheated. The trial with 60%
BFG-40% LPG was worse than with 25% LPG, which was unexpected. This can be
explained by poor furnace power control at the start of the trial (see Figure 8). The
power control with pure LPG oxyfuel did not vary smoothly, so even better
performance would be expected with better power control for this case also.
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Center Temperatures
1200
Temperature [°C]
1000
800
600
100%CPG
25%CPG
400
CPG air 1255
CPG air 1245
200
40%CPG
100%BFG
0
12:10:00 AM 12:20:00 AM 12:30:00 AM 12:40:00 AM 12:50:00 AM
1:00:00 AM
1:10:00 AM
1:20:00 AM
1:30:00 AM
Time [h:m:s]
Figure 7. The reheating rate curves for steel blooms reheated with various fuel
combinations
400
350
300
200
Power [kW]
250
150
100
100%BFG power
40%CPG Tot power
25%CPG Tot power
CPGair2
CPGair1
100%CPG power
50
0
12:10:00 AM 12:20:00 AM 12:30:00 AM 12:40:00 AM 12:50:00 AM
1:00:00 AM
1:10:00 AM
1:20:00 AM
1:30:00 AM
Time [h:m:s]
Figure 8. Chamber furnace power control during the reheating trials
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CONCLUSIONS
The Type S3 BFG-oxyfuel burner gave low NOx and low CO emissions in these
chamber furnace trials. The reheating rate with pure BFG-oxyfuel was slower than
propane-cold air combustion with a constant burner power, but propane boosting of the
BFG could be used to compensate as predicted by theoretical flame temperature
calculations. The fastest reheating and the highest productivity was available when the
burner was operated in the 100% propane-oxyfuel mode. Therefore the reheating
process in industrial furnaces can be optimized from low energy costs with a high
percentage of BFG oxyfuel to high productivity with exclusively propane oxyfuel
reheating. BFG oxyfuel should always have a booster fuel available like propane,
natural gas or coke oven gas when used in reheating furnaces.
ACKNOWLEDGEMENTS
This work was carried out with a financial grant from the Research Fund for Coal and
Steel of the European Community and financial support from MEFOS member
companies.
REFERENCES
MALFA, E., ET. AL, (2008), CO2 Reduction in reheating furnaces (CO2RED), TGS3, Technical report
4, Research DG RTD G5, Brussels, Belgium.
MARION, J., ET AL, (2000), Hauck E-Solutions, Hauck Mfg. Co. Lebanon, PA.
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