The Swedish and Finnish National
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COMBUSTION AND EMISSION CHALLENGES AT LKAB
Christian Fredriksson1*, Daniel Marjavaara2, Fia Lindroos2, Samuel Jonsson2, Stefan Savonen2, Neil Smith3
1
LKAB
Box 952
SE-97128 Luleå
Sweden
christian.fredriksson@lkab.com
2
LKAB
SE-98186 Kiruna
Sweden
3
FCT International
20 Stirling St. Thebarton, SA, 5031
Australia
neil.smith@fctinternational.com
* corresponding author
ABSTRACT
Increased production combined with stringent emission regulations and growing energy
prices has initiated a number of activities at LKAB to investigate and evaluate
alternatives to the company’s current combustion technology and fuels. Reduction of
NOx emissions is, for example, one of these activities. The NOx work at LKAB has been
going on for the last decade and has included both primary and secondary control
measures. Work with primary measures has comprised pre-studies, physical and
numerical modelling work, pilot scale and full scale trials in both Straight-Grate and
Grate-Kiln pelletizing plants. Secondary measures have consisted of both SCR and
SNCR studies. LKAB is also the first and only mining company in the world that have
installed an SCR system in an existing Grate-Kiln plant.
LKAB’s experimental combustion furnace, that is operated with high excess air ratios (n
≈ 5-6) and high combustion air temperatures (900 – 1300°C), has played a major role in
these NOx investigations. It has, for example, been used to evaluate options to reduce
NOx-emissions from Straight-Grate pelletizing plants during 2009. Specifically, different
combustion configurations were tested including a pre-combustor, secondary air, wateroil mixtures and gas fuel. Except for the use of gas fuels, all these configurations showed
a significant NOx-reduction compared to the current reference case. With the precombustor and a secondary air temperature of 450 °C the NOx-emissions could be
reduced by approximately 65 %. The NOx emissions can be reduced even further with
lower secondary air temperature but with the consequence of higher energy cost.
Keywords: Combustion, pelletizing plant, induration machine, fossil fuel, NOx
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1. INTRODUCTION
Luossavaara-Kiirunavaara AB (LKAB) is an international high-tech minerals group, one
of the world’s leading producers of upgraded iron ore products for the steel industry and a
growing supplier of industrial minerals products to other sectors. In the process to
produce iron ore pellets, the iron ore pellets (mainly magnetite) undergo heat treatment in
a pelletizing plant where they are dried, oxidized and sintered to obtain the correct
properties. The outgoing product is then used in blast furnaces or direct reduction
furnaces of steel plants. The heat in the pelletizing plant is today generated by
combustion of fossil fuels such as heavy fuel oil and coal.
Increased production combined with stringent emission regulations and growing energy
prices have initiated a number of activities at LKAB to investigate and evaluate
alternatives to the company’s current combustion technology and fuels. A division,
Energy and Emissions (TLE), was for example, started in 2006 to work with these
questions.
During the last decade LKAB has been working on both primary and secondary methods
to reduce NOx emissions from the company’s pelletizing plants. Compared to
conventional heat generation plants the specific emissions of NOx are higher due to the
high excess air ratios (n ≈ 5-6) and high combustion air temperatures (900 – 1300°C).
Work with primary measures has comprised pre-studies, physical and numerical
modelling work, pilot scale and full scale trials in both Straight-Grate and Grate-Kiln
pelletizing plants. As secondary measures LKAB installed 2008 the world’s first and the
only SCR system in a Grate-Kiln plant, which is still being evaluated. Another secondary
measure that has been tested in an existing plant is the SNCR technology, with an
instantaneous reduction level of 15% or less depending on temperature.
This paper will focus on the work that has been done to test and evaluate primary NOxreduction techniques for Straight-Grate plants. Specifically the paper will present the
method, result and conclusions from the pilot trials in LKAB’s experimental combustion
furnace (ECF).
1.1 Straight-Grate plants
In a Straight-Grate induration machine balled pellets made from a mixture of iron ore,
additives and water (so called green pellets or wet pellets) are transported throughout the
machine in order to be indurated, see Figure 1. The drying, oxidization and sintering in an
induration machine is typically done in four zones, i.e. the drying zone (UDD and DDD)
where the green pellets are dried, the preheating zone (PH) where most of the pellets are
oxidized, the firing zone (F) were the oxidized pellets are sintered and the cooling zone
(C and C2) where the pellets are cooled down before transportation to the harbour.
The external heat required for the process is generated in the preheating (PH) and firing
zone (F) by combustion of external fuels. It is also here most of the NOx is formed. The
large excess air ratios and high temperatures in the secondary air (from the cooling
section) that passes through the downcomers and combustion chamber affect the
magnitude of the formed NOx, see Figure 2. Hence, the design of the burner and
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combustion chamber is important to minimize the NOx emissions. Results from tests with
different modifications of the burner and combustion chamber system are presented in
this paper.
Figure 1. Sketch of an indurating machine in a Straight-Grate plant.
Figure 2: Cross-section of the induration machine in the burner section.
1.2 NOx emission reduction methods
NOx emissions from the oil burners in the MK3 grate kiln are mostly from the thermal
mechanism due to the high peak flame temperatures and high oxygen concentrations.
Flames are short and intense and of high temperature because the oil is sprayed directly
into a hot co-flowing air stream. Some fuel NOx is also formed. The maximum nitrogen
content of the heavy fuel oil is 0.3%. In the worst case scenario where all of the fuelbound nitrogen is oxidised into NOx, fuel NOx will contribute to about 20% of the total
current NOx emissions.
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Thermal NOx formation scales exponentially with temperature and has a weaker
dependence on oxygen concentration [1]. Consequently, modification of a combustion
process to reduce thermal NOx is most effective when peak flame temperatures are
reduced. This can be achieved by limiting the rate of mixing of air and fuel, and can also
include either fuel staging or air staging. Staging controls both fuel and thermal NOx by
reducing the availability of oxygen in the high-temperature regions in the flame.
Air staging generates lower temperatures in the initial fuel-rich stage than in flames with
excess air by limiting the rate of the combustion reactions due to the low availability of
oxygen. As a consequence, air staging is an effective means of reducing thermal NOx
emissions. Air staging is also commonly used for reduction of fuel NOx, where supply of
approximately 60% of stoichiometric air to the fuel rich stage has been shown to be the
optimal level to force fuel N reactions to produce molecular N2 rather than NO [2].
The aim of the current work is to quantify the NOx reductions that can be obtained in a
pilot scale model of an MK3 downcomer / burner arrangement by various methods.
These methods include air staging by burning some of the fuel in a pre-combustor with a
limited air supply, use of water to cool the flame and use of gaseous fuels instead of oil.
2. Method
2.1 LKAB experimental pilot scale furnace
Experiments with different NOx combustion configurations were conducted at LKAB’s
pilot scale facility ECF in Luleå. The ECF was originally designed to provide a scaled
down version of a Grate-Kiln plant and was designed with an 800 mm internal refractory
diameter and a length of 14 metres, see Figure 3. The ECF kiln is refractory lined with
200 mm thick refractory. Outside the refractory, a layer of insulation protects the outer
steel shell.
Figure 3: External and internal views of the original ECF.
Pilot scale experiments for the straight-grate plant MK3 was possible to perform with
some modifications of the original ECF hood (inlet of ECF kiln). Specifically, one of
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three different down scaled burner port configurations of a Straight-Grate plant could be
mounted on the ECF hood. Figure 4 shows the ECF hood with a downcomer, a
combustion chamber and a pre-combustor, compare with Figure 2. Hence most of the
ECF kiln only acted as an exhaust pipe. By changing the ECF hoods connected to the
ECF kiln, it was possible to test different configurations and compare it with the
reference configuration that modelled the existing full scale Straight-Grate plant MK3 in
Malmberget. The three ECF hood configurations that were trialled are:



Refractory insert for Reference Hood to model existing full-scale plant
Reference Hood with secondary air
Pre-combustor with secondary air register developed by FCT and LKAB
The configuration with reference hood and secondary air register was tested to investigate
if only addition of “cool” secondary air could reduce NOx emissions. In the precombustor, see Figure 4, the fuel was partially combusted with an air deficiency and was
then completely burned in the burner port and furnace. Besides the changes in hood
design and air supply, tests with water-oil mixture and gas fuel were conducted.
The oil burner was a down scaled version of the existing oil burner in MK3. It uses
compressed air for atomisation and primary air for flame stabilisation and cooling of the
burner nozzle. For tests with propane gas, a gas burner was used.
Figure 4: Drawing of the burner port with pre-combustor in the ECF.
The downcomer air flow-rate was 2300Nm3/hr at 900 °C and with 21% O2. The design
size of the oil burner was 400 kW. The downcomer air was heated by gas fired pebble
heaters. The air was provided from two refractory pipes, which enter at the top of the
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downcomer. The air path then follows two 90° bends designed to introduce the air into
the angled part of the downcomer with a velocity profile similar to that which would exist
at full-scale, see Figure 4. The downcomer, pre-combustor and combustion chamber were
384 mm in diameter. The existing 800 mm ECF pilot scale kiln was used as the exhaust
of the new 384 mm diameter combustion chamber, as seen in Figure 4.
When the secondary air register was used, the secondary air was supplied by a separate
fan and heater that could heat the secondary air up to 450°C. The reason for using heated
secondary air is to investigate possibilities to use heated process air instead of ambient air
in order to minimise the energy loss.
Some of the tests were conducted with water added to the oil via an emulsifier supplied
by LKAB, which is designed to mix the oil and water to a very fine scale to produce an
emulsion. The amount of water added is reported in water/oil mass ratio, it varied
between 0.29 and 1.34. Water was heated with an electric heating element prior to mixing
with oil. The water temperature could not be kept constant for all flow-rates and varied
between 44°C and 74°C, which is a negligible difference in energy terms as most of the
cooling effect of water comes from the latent heat of vaporisation and the sensible heat of
vapour.
The test campaign was conducted in May 2009 during approximately 10 days of
operation. Besides a shut down for burner hood changes, the pilot kiln was in continuous
operation. Each test condition was operated until thermal steady state conditions were
achieved which was monitored by the wall thermocouples and gas analysis.
2.2 Measurements
Type K thermocouples were embedded at several positions in the refractory of EFC kiln
in order to make general comparisons between flames. Other thermocouples were also
used to measure secondary air temperatures and gas temperatures inside the kiln. An
infra-red gas analyser was used for flue gas measurements which could also be connected
to a probe for in-flame traverses. The IR analyser had a regular purge / clean cycle which
can be observed in the data. In-flame gas, temperature and solids sampling were
performed for some flames, once they had approached steady-state conditions. Water
cooled probes were used. The gas probe was connected to the main gas analyser, so
whilst in-flame gas analysis was occurring, the flue gas was not being recorded. The
suction pyrometer used for in-flame temperature measurement consisted of a Type B
thermocouple (rated to 1750°C) inside a ceramic sheath, with a ceramic radiation shield
around the thermocouple junction. Solids samples (which may be soot and ash) were
taken with a dedicated probe, then dried and weighed.
2.3 NOx reporting method
NOx data in this paper is reported as the raw NOx value in ppmv with O2 (%). It is also
reported as g NO/MJ to allow comparisons between flames with different air and fuel
flow rates. The main NOx species is NO. It is not uncommon to also report NOx as g
NO2/MJ, because NO is gradually converted to NO2 in the atmosphere. To convert g
NO/MJ to g NO2/MJ, multiply by the ratio of molecular weights, i.e. 46/30. The infra-red
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analyser reported NOx in parts per million by volume (ppmv) which is equivalent to
molar ppm. The analyser NOx value was converted to g NO/MJ by assuming that the flue
gas volume and molar flow-rates are the same as the incoming air. NOx for some flames
was also reported in g NO/useful MJ. This is the NOx corrected after discounting the
energy lost to heat up the water and the secondary air introduced through the burner.
3. RESULTS
3.1 Reference Oil Flames
The reference oil burner was used to generate the 400 kW reference oil
flame. The Reference Oil Flame used 36 Nm3/hr of primary air and 6.3 Nm3/hr of
atomising air. It models full scale flames which use 150 Nm3/hr of primary air and 25
Nm3/hr of atomising air. The Reference flame was run a number of times. A NOx value
of 0.525 g NO/MJ was used for comparison with other flame throughout this report.
3.2 Effects of pre-combustor and pre-combustor stoichiometry
The effect of using the pre-combustor and varying the stoichiometry is shown in Figure 6.
The reference case (baseline flame), is shown on the y-axis. By varying the precombustor stoichiometry it was possible to find an optimum when approximately 70 % of
stoichiometric air was used.
0.6
Baseline f lame
0.5
gNO/MJ
0.4
0.3
0.2
0.1
Notes:
- Secondary air temperature = 450 C
- Outer to inner f low ratio = 1.7 except f or the lowest stoich. case
- Ref oil nozzle was used f or all runs
0
0
20
40
60
80
100
120
140
Pre-combustor Stoichiometry (%)
Figure 6: Effect of pre-combustor stoichiometry on NOx emissions.
3.3 Effects of secondary air temperature
The secondary air temperature was varied in the reference hood and the pre-combustor
hood. Figure 7 compares pre-combustor flames with pre-combustor stoichiometry of
74%, with flames in the reference hood with the same secondary air flow-rate. The
reference flame (Baseline) is also shown in the same figure. The reference oil burner was
used for all cases. The data in Figure 7 shows that reducing secondary air temperature has
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a pronounced effect on NOx. With a secondary air temperature of 450°C the reduction
was 65 % and with 50°C secondary air temperature the reduction was 77 %.
0.6
Reference hood
0.525
Pre-combustor
0.5
0.4
Notes:
- Pre-combustor stoichiometry = 74%
- Burner total stoichiometry in ref erence hood =
74%
- Ref oil nozzle was used f or all runs
gNO/MJ
0.324
0.3
0.233
0.2
0.155
0.121
0.1
0
0
Baseline
Baseline
Sec air temp = 450 °C
Sec air temp = 50 °C
Figure 7: Effect of secondary air temperature on NOx emissions.
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3.4 Water-oil mixtures
The effect of mixing water into the oil was assessed in the reference hood and the precombustor using the reference oil burner. The amount of water added is reported as
water/oil mass ratio, it varied between 0.29 and 1.34. The results of the emulsion trials
are shown in Figure 8. Water has a substantial effect on reducing NOx in all
circumstances. The reduction is linear with increasing water flow-rate, indicating it is a
direct flame cooling effect. For the flow-rates tested, the effect of using water in the
reference hood is not as significant as changing from the reference hood to the precombustor hood without water. Use of water in the pre-combustor hood produced the
lowest NOx of any of the flames tested in the trials. Note: the fuel flow was not increased
to compensate for the heat lost to the water. Further trials would be required at constant
heat release to obtain a more representative analysis.
0.6
Reference hood with quarl
Baseline f lame
Reference hood
0.5
Pre-combustor
gNOx/MJ
0.4
0.3
0.2
0.1
Notes:
- No secondary air was used in both ref erence hoods
- Ref oil nozzle was used f or all runs
- Pre-combustor stoichiometry = 74%
0
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
Water to oil mass ratio
Figure 8: The effect of oil/water emulsion on NOx emission in the reference hood and pre-combustor
3.5 Gas flames
Figure 9 shows a comparison between oil and propane gas flames. Flames in both the
reference hood and the pre-combustor hood supplied burner and register air to a total of
74% of the stoichiometric requirement. In all cases the secondary air temperature was
450°C. The results shown here demonstrate a significant increase in NOx from gas
flames over oil flames. However, it is worth noting that the propane gas flame in the precombustor produced 0.215 g NO/MJ compared to the baseline reference flame 0.525 g
NO/MJ (Figure 7). This is a 60% reduction, and although not as great as the reductions
produced by the oil flames is still very large.
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0.45
0.425
0.4
Reference hood
Precombustor
0.35
0.325
g NOx /MJ
0.3
0.25
0.215
0.2
0.151
0.15
0.1
0.05
0
Ref oil
Ref gas
Figure 9: Comparison of Oil and Gas flames.
4. CONCLUSIONS
Figure 10 shows a progression of NOx reductions that can be achieved by various
methods. Emissions were reduced from 0.525 g NO/MJ to 0.32 g NO/MJ by introducing
“cool” air (at 450°C) through an air register around the burner. Further reductions could
be achieved through water cooling by using water oil mixtures. However, a precombustor produces the greatest NOx reductions and is the favoured method of reducing
NOx. Figure 11 expresses the same NOx data in ppmv. Figure 12 shows the same flame
conditions as in Figure 10 after applying the NOx correction to account for the energy
lost to heat up the water and the secondary air. The general trend with the corrected NOx
does not change, however, a less NOx reduction is achieved with reference to the
baseline flame. On the basis of the results from these pilot scale tests, the method with
pre-combustor was chosen to be tested in full-scale trials.
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0.6
0.525
0.5
Burner total stoic.=74%
0.4
gNO/MJ
0.324
No Secondary air
Water/oil mass ratio = 1.34
0.3
0.207
PC stoic.=74%
PC stoic.=74%
Water/oil mass ratio = 1.0
0.2
0.155
0.096
0.1
0
Baseline
Ref erence hood
Ref erence hood+Emulsion
Pre-combustor
Pre-combustor+Emulsion
Figure 10: Summary of NOx emissions.
300
250
242.9
Burner total stoic.=74%
NOx (PPMV)
200
No Secondary air
Water/oil mass ratio = 1.34
150
136.9
97.9
100
PC stoic.=74%
PC stoic.=74%
Water/oil mass ratio = 1.0
65.4
50
41.4
O2 =17.7%
O2 =17.9%
O2 =17.6%
O2 =17.9%
O2 =17.9%
Baseline
Ref erence hood
Ref erence hood+Emulsion
Pre-combustor
Pre-combustor+Emulsion
0
Figure 11: Summary of NOx emissions, expressed in ppmv.
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0.6
0.525
0.5
Burner total stoic.=74%
gNO/ Useful MJ
0.4
0.380
No Secondary air
Water/oil mass ratio = 1.34
0.3
0.245
PC stoic.=74%
0.2
PC stoic.=74%
Water/oil mass ratio = 1.0
0.182
0.108
0.1
0.0
Baseline
Ref erence hood
Ref erence hood+Emulsion
Pre-combustor
Pre-combustor+Emulsion
Figure 12: Summary of NOx emissions corrected for heat lost for water and secondary air heating.
5. REFERENCES
[1] A.A. Westenberg, Kinetics of NO and CO in lean, premixed hydrocarbon flames.
Combust. Sci. and Technol. 4, (1971), 59.
[2] J.W. Glass and J.O.L. Wendt, Mechanisms governing the destruction of nitrogenous
species during the fuel rich combustion of pulverized coal. 19th Intl. Symp.
on Combn, The Combustion Institute, Pittsburgh, (1983), 1243-1251
6. Acknowledgements
LKAB express the company’s gratitude to FCT and Swerea Mefos for participating in
this work.
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