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. 2021 Jun 22;6(26):16905–16912. doi: 10.1021/acsomega.1c01564

Combustion Using Oxygen-Lancing in a Reheating Furnace

Cheol Woo Lee †, In Su Kim ‡, Jung Goo Hong †,*
PMCID: PMC8264849  PMID: 34250349

Abstract

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Fuel economy has been a primary issue in the steel industry because it uses large amounts of energy, such as the gaseous fuel of byproduct gas. Furthermore, reheating throughput capacity has been a key issue because it can improve furnace efficiency, leading to fuel economy. Many attempts have tried improving fuel economy using oxygen in a reheating furnace. Oxygen-lancing technology was developed to increase fuel economy and maintain the same level of NOx concentration simultaneously. Mechanisms that inject oxygen into flames locally causing flame quenching and at the same time suppressing the increase in NOx concentration due to recirculation of reheating furnace-burned gases are key to this study. Various oxygen concentrations for its lancing were used to investigate its effects on furnace temperature and NOx concentration in a test furnace. It was determined that 30% of oxygen was optimal regarding fuel economy and NOx concentrations. Oxygen was injected into the flame using two lancing pipes at 11° in a design capacity of 125 MW. The results showed a 3–5% increase in fuel economy and the same level of NOx concentration in the furnace.

1. Introduction

A heavy plate reheating furnace heats slabs to 1200 °C to roll them easily. The slab residence time in the furnace significantly influences both productivity and fuel economy expressed as fuel consumed per unit weight of heated slabs. Several combustion technologies using oxygen (O2) have been applied to industrial reheating furnaces to improve productivity and fuel economy simultaneously (Figure 1).

Figure 1.

Figure 1

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Various types of oxygen-enriched combustion (OEC) (photograph courtesy of Cheol Woo Lee. Copyright 2021).

Using 100% O2 as an oxidizer has been applied to batch-type furnaces for fuel economy. The 100% O2 using combustion technology, however, has some disadvantages such as the corrosion of a furnace perimeter wall or high NOx concentrations in the case of the surrounding air intrusion into the batch furnace. However, the O2 concentration in the combustion air is optimal near 30% because the gradient of adiabatic flame temperature becomes smaller for the premixed methane-air flame (Figure 2).1,2

Figure 2.

Figure 2

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Calculated adiabatic flame temperature at the premixed methane-air flame temperature.

As shown in Figure 2, the flame temperature increases as the O2 enrichment rate increases because the nitrogen content in the oxidizer decreases. This oxygen combustion may cause an increase in NOx due to the rise in flame temperature. The possibility of an increase in NOx concentration due to oxygen combustion was expected, so oxygen was injected locally in the middle of the flame, which would immediately cool down and suppress the rapid increase in thermal NOx. Unlike normal flame quenching, the reheating furnace temperature is about 1250 °C, so this flame quenching does not completely stop the combustion chemical reaction. Rather, the high-velocity oxygen injection results in the introduction of burnt gas to the flame side and recirculation to thicken the flame’s wrinkles, making the combustion zone wider. Therefore, oxygen is injected to restrain NOx formation to the instantaneous cooling effect, and subsequently combustion chemical reactions continue within high temperature in reheating furnace, resulting in a higher overall temperature than conventional air combustion. As a result of flame quenching, the flame is expected to stretch, which can also be expected to cause radiation transfer by flames in the long direction of the slab due to the flame stretch, and these effects are thought to be implemented as energy savings.

Meanwhile, the air combustion is 1800–1900 °C when used as an oxidizer, whereas the O2 (100%) is significantly higher in the 2600–2700 °C range. In this temperature range, the proportion of thermal dissociation, which was not considered in air combustion, increases the number of intermediate combustion chemical reaction products. Therefore, methyl radicals, such as CH3 and OH, are actively produced in the combustion chemical chain reaction compared to air combustion, which increases flame temperature. The oxidation of methane is a spectacular example to show how complex the seemingly simple reaction CH4 + 2O2 → CO2 + 2H2O can be. The following is a chain mechanism (Scheme 1).

Scheme 1. Chain Mechanism of CH4 + 2O2 → CO2 + 2H2O.

Scheme 1

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In high-temperature plasma, the recombination of O2 atoms in the presence of a third body M is prominent. The asterisk on M in the stoichiometric equation indicates that M is energized in the reaction.

Wu et al. conducted an experimental study to investigate the influence of 21–30% oxygen concentration on the heating rate, emissions, temperature distributions, and fuel (natural gas) consumption in heating and furnace temperature-fixing tests. An increase in the oxygen concentration led to a rapid heating rate and lesser fuel consumption because of lower levels of inert gas (N2).3 Aanjaneya et al. conducted numerical research for confined turbulent jets for homogeneous combustion with O2 enrichment. Homogeneous combustion and its variants (MILD, FLOX, and CDC) have emerged as attractive techniques to abate NOx emissions. The underlying theory is to arrest the Damköler number (Da) to values close to unity by intense dilution (internal or external) of the reactant streams.4 Fordoei et al. numerically studied the heat transfer characteristics, flame structure, and pollutants emission in the MILD methane-air, oxygen-enriched, and oxy-methane combustion. The results indicate that by replacing CO2 with N2, emission of visible light is reduced significantly and maximum temperature drops noticeably until flame reaches the nozzle-shaped section of the furnace. Moreover, it is found that MILD oxygen-enriched and oxy-methane combustion has regarded advantages compared to MILD methane-air combustion involving enhancing the temperature uniformity about 10–15%, more distributed reaction zone, and a significant reduction of NOx emission. CO emission shows different behavior and is unchanged under MILD oxygen-enriched regime, while it increases from 11 ppm in MILD methane-air to 140 ppm in MILD oxy-methane with an injection of 90% CO2 mass fraction.5 Chen al. experimentally investigated NOx emission based on combustion characteristics of biodiesel compared to diesel. Here, oxygenated biodiesel produces more reactive radicals than diesel, and these radicals will surely accelerate the combustion speed, improve the intensity of diffusion combustion, shorten the combustion duration, and increase the brake thermal efficiency (BTE). Consequently, the diffusion combustion temperature, especially the peak combustion temperature of biodiesel is higher than that of diesel. As a result, NOx emission of biodiesel is higher than that of diesel in most cases, except under the condition of low loads under low and medium speeds.6

This research proposes a combustion method, the s oxygen diluted partially premixed (ODPP)/oxygen enriched supplemental combustion (OESC), to reduce NOx generation by adjusting O2 concentrations. The relationships between the operating parameters were deduced. The effects of the ODPP/OESC on NOx and CO emissions were evaluated experimentally. Hagihara et al. studied an ultralow NOx O2-enriched combustion system using an oscillation combustion method. This study used a periodical combustion method to restrain NOx emissions at O2-enriched combustion. This method periodically changes the O2 concentration in the oxidizer and O2 ratio simultaneously by controlling the flow rate of air and O2 periodically to keep the same whole O2 ratio condition of the original stationary combustion.7 Guo et al. conducted experimental research and simulation analysis of regenerative O2-enriched combustion technology. As a type of high-temperature combustion technology, O2-enriched combustion is widely used in the glass industry, metallurgical industry, and thermal power engineering. To further increase the combustion temperature and save energy simultaneously, this study investigated and analyzed a combustion mode that combines regenerative combustion technology and O2-enriched combustion, and contrastively analyzed each combustion feature of regenerative O2-enriched combustion technology.8

Research on the O2-enriched combustion technology using various fuels continued. Alabas et al. experimentally investigated O2 enrichment in synthetic gas flames. H2 and CO were added to 30% methane gas in a premixed laboratory-scale model burner with an H2/CO ratio and medium. Swirl numbers (s = 0.6 and 1) were changed by keeping the equivalent ratios constant.9 Yilmaz et al. experimentally investigated the effect of O2 enrichment on flame stability and emissions during biogas combustion.10 Riahi et al. numerically investigated turbulent combustion with hybrid enrichment by hydrogen and oxygen. In this study, NG + H2/air + O2 turbulent flame is numerically investigated using the computational fluid dynamics (CFD) code. The results obtained show that hydrogen addition to natural gas improves the mixing between the reactants, reduces their residence time, and reduces the length and thickness of the flame. On the other hand, the hydrogen enrichment minimizes the CO2 and CO production and increases the NOx level.11

Recently, Pio et al. studied the safety parameters for O2-enriched flames. The heat flux burner was adopted for measuring the laminar burning velocity of methane in O2-enriched air at different equivalence ratios. Results were compared with numerical data obtained from detailed kinetic mechanisms.12

A few researchers, however, have studied the effects of O2-enriched combustion on premixed flames. In this study, therefore, the lancing-type O2-enrichment combustion was achieved through experiments, and based on this technology, it was applied to the site. Furthermore, there is a possibility of an improved fuel economy with a similar NOx level if O2 is directly injected into the flame in a furnace.

2. Results and Discussion

2.1. Oxygen-Lancing in the Test Furnace

Figure 3 shows the test results of O2-lancing on the furnace temperature, O2 concentration, NOx concentration, and CO for various fuel equivalence ratios. From Figure 3, the furnace temperature peaks at ϕ = 0.9, regardless of the enriched O2 concentration, which is a typical pattern in hydrocarbon–air combustion. The furnace temperature increases as the enriched O2 concentration increases, regardless of the fuel equivalence ratio. The furnace temperature peaks when the enriched O2 concentration is 30% and the fuel equivalence ratio is ϕ = 0.9. The furnace temperature at 30% O2 concentration combustion is higher by ∼8% than that of conventional combustion, which is 21% of O2 concentration combustion. Therefore, there is a significant possibility of fuel-saving only if O2 cost is low enough. The O2 concentration in the flue gas correlates well with the fuel equivalence ratio for all enriched O2 concentrations, meaning almost complete combustion, except for the stoichiometric ratio when considering the CO concentration. At the stoichiometric ratio, the CO concentration increases significantly as the enriched O2 concentration increases. It reaches ∼6000 ppm for 30% enriched O2 concentration. This could be related to the thermal dissipation of carbon dioxides near the stoichiometric ratio at high temperatures.13 This phenomenon, however, will not be discussed here because it is beyond the scope of this experiment. The NOx concentration increases linearly with the fuel equivalence ratio and enriched O2 concentration. The NOx concentration at ϕ = 0.9 remains similar, regardless of the enriched O2 concentration, whereas it significantly differs with a smaller ϕ. Therefore, ϕ = 0.9 is the optimal point for O2-lancing for fuel-saving, CO concentration, and NOx concentration.

Figure 3.

Figure 3

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Effects of enriched oxygen-lancing concentration on furnace temperature, oxygen concentration in the flue gas, NOx concentration, and CO concentration.

It is critical to investigate fuel-saving by comparing the furnace temperature for both conventional and O2-lancing combustion. Figure 4 shows the furnace temperature for 90% fuel consumption, 95 and 100% under 27% enriched O2-lancing combustion, and 100% under conventional combustion. From Figure 4, the furnace temperature of 90% fuel consumption under O2-lancing is higher than that of 100% fuel consumption under conventional combustion. Therefore, O2-lancing combustion is at least 10% superior to the conventional combustion for fuel economy. Given that the NOx concentration is similar for both O2-lancing and conventional combustion and fuel economy is better for O2-lancing combustion at the same fuel equivalence ratio, there is a possibility of low NOx concentration for O2-lancing combustion.

Figure 4.

Figure 4

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Comparison of furnace temperature.

The O2-lancing direction could be the most critical factor affecting fuel economy and NOx concentrations in the furnace. Figure 5 shows the furnace temperature and NOx concentrations for conventional combustion, 25% O2-lancing, and 30% O2-lancing with O2-lancing from the bottom, upper, and lateral sides. From Figure 5, the furnace temperature and NOx concentrations are similar for all three cases. When O2-lancing is supplied from the bottom side, the flame will be lifted toward the furnace ceiling because of the buoyancy effect of burned hot gases, resulting in inferior fuel economy. However, when O2-lancing is supplied from the upper side, the flame will be stretched toward the material, namely, the slabs, improving the fuel economy but increasing the possibility of flame contact with the slabs. There will be a minimal effect on the slabs and inner wall of the furnace. Therefore, O2-lancing from the lateral side is effective for fuel economy and operation stability. Note that the furnace temperature and NOx concentrations in Figure 5 are different from those in Figure 14 because the experiments were conducted over a long period to cool down the test furnace completely. Therefore, the experiment was conducted only once a day; thus, the atmospheric temperature was lower than in the earlier experiments, leading to different furnace temperatures and NOx concentrations. Hence, conventional combustion experiments were periodically conducted for reference.

Figure 5.

Figure 5

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Effect of oxygen-lancing direction on the furnace temperature and NOx concentrations.

The effect of O2-lancing velocity on the furnace temperature was investigated by changing the diameter of O2-lancing nozzles. From Figure 6, little differences exist between furnace temperatures for different O2-lancing velocities. It can be inferred that a certain velocity is good enough for O2-lancing when considering the furnace temperature and electricity consumption for increasing O2-lancing velocity. However, NOx concentrations for higher O2-lancing velocities are higher than those of lower O2-lancing velocities. This could be related to quick and intensive combustion, causing a luminous flame rather than mild combustion, partly with flameless combustion. Here, a summary of the mild and flameless combustions is given. Mild is an abbreviation for moderate or intense low-oxygen dilution. According to Cavaliere et al., a combustion process is mild when the inlet temperature of the reactant mixture is higher than the mixture self-ignition temperature, whereas the maximum allowable temperature increase regarding the inlet temperature during combustion is lower than the mixture self-ignition temperature (in Kelvin).14−16 Flameless combustion is one of the most active combustion technologies recently used by combustion researchers and equipment manufacturers for high efficiency and eco-friendliness. Flameless combustion is based on the exhaust gas recirculation phenomenon, where NOx formation is suppressed without compromising the thermal efficiency. In flameless combustion, the recirculated exhaust gas is defined as the exhaust gas that is recirculated and mixed into combustion air before the reaction.17−19

Figure 6.

Figure 6

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Effect of oxygen-lancing velocity on the furnace temperature and NOx concentrations.

The scale thickness and hardness are also critical factors for slab quality. Experiments for this test were conducted for the test furnace by heating it from room temperature to 1200 °C, with five specimens located equally at a 0.9 m interval. Figure 7 shows the results for temperature, scale thickness, and photos for all five specimens. The scales for specimens with O2-lancing have similar thicknesses or even a thinner thickness. This could be because the enriched O2 concentration in the flame on the slab surface forms a void in the scale layers.20,21 Therefore, enriched O2-lancing combustion contributes to better descaling of the slabs.

Figure 7.

Figure 7

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Comparison of scale thickness and shape (photograph courtesy of Cheol Woo Lee. Copyright 2021).

2.2. Oxygen-Lancing Application in the Furnace

It is essential to control O2 concentrations in the flue gas for both scale formation and fuel economy. Tunable diode laser spectroscopy (TDLS (Yokogawa TDL200)) was used to control O2 concentrations in the flue gas for the test furnace with feedback control.22−25 From Figure 8, the O2 concentrations in the flue gas are well maintained with feedback control, whereas it has big fluctuations without feedback control. Thus, it is better to apply TDLS to obtain a more precise O2 concentration control in the flue gas in real time when 25% of enriched O2-lancing was applied to the heavy plate furnace. From Figure 9, the fuel economy of O2-lancing combustion is better than that of conventional combustion in the real furnace. The improved specific fuel consumption is ∼10%, which is like that in the test furnace when abnormal working conditions are excluded (represented with a dashed ellipse).

Figure 8.

Figure 8

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Oxygen concentrations in the flue gas with and without TDLS control in the test furnace (coke oven gas (COG), 200 Nm3/h; ϕ = 0.9; oxygen enrichment = 25%).

Figure 9.

Figure 9

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Comparison of relative specific fuel consumption by oxygen-lancing in the real furnace.

3. Conclusions

O2-lancing was used to investigate its effect on fuel economy and NOx concentrations for both a test furnace and a real furnace by changing O2 concentrations in the oxidant. The furnace temperature was measured using k-type thermocouples, and CO and NOx concentrations were analyzed using an infrared gas analyzer. A 1-MW burner using COG as fuel was used for a test furnace. O2-lancing was injected from the bottom, upper, and lateral sides in a test furnace. O2-lancing was applied only to #1 and #2 heating zones to maximize its effect in the furnace. TDLS was used instead of a conventional zirconium-type O2 sensor to control O2 concentrations in the flue gas precisely. Comparisons were made for fuel economy by metering the fuel consumption rate under the same material conditions in the furnace. The results are as follows.

  • (1)

    Fuel economy was improved about 11% with O2-lancing in the flame for both the test furnace and the furnace.

  • (2)
    The NOx concentrations before and after application of O2-lancing were almost of the same level.
    • Before application, 60–70 ppm; after application, 60–75 ppm (at O2 11%)
  • (3)

    Furthermore, O2-lancing has a minimal effect on scale formation on the slab surface.

There is no special report for O2-lancing on slab quality from the milling and quality control departments.

4. Experimental Section

4.1. Experimental Apparatus

Experimental tests were conducted on a test furnace and a furnace for heavy plate mills. Figure 10 shows a schematic diagram of a test furnace. A burner (TENOVA) of 1 MW was used to evaluate fuel economy and NOx concentrations by changing O2 concentrations in combustion air.

Figure 10.

Figure 10

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Schematic of test facility (photograph courtesy of Cheol Woo Lee. Copyright 2021).

The fuel and combustion air supplied to the main burner are first described. Air for the main burner was supplied from an air blower of 500 mmH2O, with a rated power of 7.5 kW. It could change its quantity to control the fuel equivalence ratio (ϕ) by adjusting a damper in the airflow duct. Air was supplied at room temperature because this experiment compared the effect of O2 injection into the flame. Fuel as a coke oven gas (COG, byproduct gas) flowed into the burner from a branch line of a process COG supply line of 1200 mmH2O, which was intended for all combustion facilities in steelworks. The low calorific value of COG was 17.3 MJ/Nm3, and Table 1 presents its major composition.

Table 1. Major Composition of Coke Oven Gas (COG) Used in This Experiment.

element CO2 O2 CO H2 CH4 N2 C2H4 total
vol (%) 1.8 0.2 6.4 57.5 23.3 7.5 3.3 100
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Regarding the pilot burner, the fuel as liquefied natural gas (LNG) was supplied from the gas bomb, pressurized at 120 bar, and depressurized at 1 bar at the bomb exit, to the pilot burner through a gas pressure regulator (Parker) with a final pressure of 250 mmH2O. The low calorific LNG value was 42.6 MJ/Nm3, and Table 2 presents its major composition. An air compressor supplied air for the pilot burner (Hanshin Machinery, 4 HP, Republic of Korea) with a final pressure of 300 mmH2O. O2 for lancing was from a branch line of a process O2 supply line of 0.6 MPa, which was intended for all combustion facilities in the Dangjin.

Table 2. Major Composition of Liquefied Natural Gas (LNG) Used in This Experiment.

element CH4 C2H6 C3H8 N-C4H10 I-C4H10 N2 total
vol (%) 92.9 5.3 1.1 0.3 0.3 0.1 100
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Fuel and air quantities were controlled through mass flow controllers (Dwyer Instruments). The setting of O2 concentrations and lancing injection method was conducted as above. Finally, a tunable diode laser spectrometer (TDLS, Yokogawa TDL200) was installed to set the optimal O2 concentration in the test furnace, and feedback control was used to compare TDLS control ON and OFF. The feedback control was conducted considering the appropriate concentration ranges of O2 and CO (Figure 11), controlled by a programmable logic controller (Siemens S7).

Figure 11.

Figure 11

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Relation between heat loss and CO and O2 concentrations.

4.2. Experimental Setup

O2 was injected into a burner flame in the test furnace with three ways of lancing, namely, the upper, parallel, and lower positions (Figure 12). The O2-lancing pressure was maintained at ∼0.25 MPa to form delayed O2-mixing into the flame and promote burned gas recirculation into the flame simultaneously (Figure 13). It was considered that the flue gas recirculation effect within the test furnace will result in the inflow of combustion products, such as heat capacity, large specific heat CO2, and H2O into the flame, inhibiting thermal NOx. The role of inhibiting thermal NOx by injecting O2 around the burner is also considered, causing local quenching of flames. O2 was injected into the flame using lance pipes at an angle of 11° relative to the flame direction. The test furnace temperature was measured on top of the test furnace using k-type thermocouples (T/C), which were equally located at 450 mm. Flue gas was extracted from a flue gas duct to analyze each concentration of O2, CO, and NOx using an infrared gas analyzer (Fuji Electronics, Japan) on a dry basis. Table 3 shows the conditions used for the experiments.

Figure 12.

Figure 12

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Oxygen-lancing method (photograph courtesy of Cheol Woo Lee. Copyright 2021).

Figure 13.

Figure 13

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Imaginary diagram of lancing (photograph courtesy of Cheol Woo Lee. Copyright 2021).

Table 3. Experimental Conditionsa.

type COG flow rate (Nm3/h) combustion air flow rate (Nm3/h) oxygen flow rate (Nm3/h)
main burner 200 1024 ϕ = 1.0 0 (enrichment 21%)
817 44 (enrichment 25%)
736 60 (enrichment 27%)
635 82 (enrichment 30%)
* fuel equivalence ratio (ϕ): 0.7–1.0 (variables)
oxygen-lancing direction upper, lower, parallel (side)
oxygen-lancing injection velocity (m/s) 95, 190
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a

* Pilot burner: LNG 3.5 Nm3/h, air 40 Nm3/h (fixed condition).

4.3. Experimental Procedure

A burner with 1 MW heat capacity was used to invest the effect of O2 concentration on furnace temperature, fuel consumption, and NOx emissions in the test furnace for O2-lancing combustion. The temperature gradient by time passage for 1 h was measured for both air combustion and O2-enriched combustion under the test furnace condition. The fuel equivalence ratio was changed from 0.7 to 1.0 to investigate the best fuel equivalence ratio for both fuel economy and NOx concentration. Here, the fuel quantity was fixed and those of air and oxygen were changed. Two types of O2 injection velocity were selected by changing the size of the O2 injection hole to investigate the effect of fuel and oxidant mixing, surrounding fluid entrainment, and flame quenching.

O2-lancing was applied to 14 sets of burners for #1 & #2 heating zones, which had 77 MW heat capacity. O2-lancing application to #1 & #2 heating zones was reasonable because the temperature increases drastically in that zone, which means that a large possibility of fuel-saving exists in the #1 & #2 heating zones. Only 30% of the O2 concentration was used for the furnace because it had the most fuel-savings effect in the test furnace with low CO concentrations and with the same level of NOx concentrations. The fuel consumption rate was measured with and without O2-lancing for the same condition of material temperature and material quantity at the same time.

Glossary

Abbreviation List

COG

coke oven gas

LNG

liquefied natural gas

TDLS

tunable diode laser spectroscopy

The authors declare no competing financial interest.

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