Niska Paper

7/30/2019 Niska Paper

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OXYFUEL COMBUSTION OF LOW CALORIFIC BLAST FURNACE GAS FOR

STEEL REHEATING FURNACES

John Niska, Anders Rensgard, MEFOS (Lule, Sweden)Tomas Ekman AGA-Linde (Lindig, Sweden)

[email protected], 0920-201986

ABSTRACT

Pilot trials at MEFOS have proven that a new S3 blast furnace gas (BFG)-oxyfuelburner 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 testedin 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 emissionsand reheating rates with and without the booster fuel. The NOx emissions were low andthere was not a problem with unburnt CO. Trials were made with up to 40% of theenergy from LPG. Steel reheating using BFG-oxyfuel combustion required a higherenergy input than LPG-air combustion, but this is compensated by the fact that BFG is alow cost fuel.

This new multi-fuel oxyfuel burner provides industry with a system capable ofproviding a wide range of performance from lower cost operation with pure BFG-

oxyfuel to higher performance and higher furnace productivity with propane-oxyfuelwhen compared to typical conventional reheating furnaces using air combustion offossil fuels. The reduction of fossil fuel consumption with LPG-oxyfuel combustionreduces the emission of greenhouse gases (carbon dioxide), and even greater reductionsin carbon dioxide emissions are possible when process gases are used which wouldotherwise be flared.

Keywords: Oxyfuel, blast furnace gas, NOx, flameless, carbon dioxide, emissions, LPG


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INTRODUCTION

The iron ore company LKAB operates an experimental blast furnace (EBF) in Luletogether with MEFOS. The EBF supplies top gas or blast furnace gas (BFG) through anew 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 propaneboosting 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 thisreport.

THEORY

Blast furnace gas is a low calorific fuel which has too low of a flame temperature todirectly replace fossil fuels in typical steel reheating furnaces with air combustion. Theflame temperature can be increased by oxyfuel combustion, plus by enrichment of theBFG 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 isnormally a capital intensive alternative relative to fuel enrichment. Integrated steel millsnormally have an oxygen supply system for converting the raw iron to steel makingboth oxygen, BFG and other gaseous fuels available for other uses. BFG enrichmentwith LPG was the approach for using BFG which was investigated in these trials.

1000

1300

1600

1900

2200

2500

0 1 2 3 4 5 6 7

Excess Oxygen (% wet)

AdiabaticFlameTemperature(C)

LPG with 450C air

LPG with 20C cold air

75% BFG + 25% LPG oxyfuel

100% BFG oxyfuel

Figure 1. Flame temperatures for BFG-oxyfuel versus LPG with preheated air.


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Steel reheating furnaces often use LPG (propane) or natural gas (methane) with thecombustion air preheated with a recuperator to save energy and to increase the furnace

productivity. The flame temperature with the use of oxyfuel combustion can becalculated with a thermodynamics program. A comparison of LPG with the combustionair preheated to 450C versus BFG-oxyfuel is given in Figure 1, based on calculationsmade with Haucks program E-Solutions for the average BFG composition given inTable 1 [MARION, 2000]. Note that the flame temperature does not change veryquickly with an increase in the excess oxygen for the use of oxyfuel, as compared to aircombustion.

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 ofhydrogen (H2) The composition of the BFG varied with the ranges given in Table 1. AnABB Industrial IT Extended Automation System 800xA is used to control thecombustion in the chamber furnace together with a Siemens Simatic S7-400 to controlthe fuel and oxygen flows to the new oxyfuel burner. The control system is designed tomonitor the lambda ratio and process temperatures to avoid unsafe process conditions.The BFG composition was entered manually in the control system to compensate forvariation in the BFG quality and allow the system to maintain the desired burner powerwhile maintaining stable furnace temperatures, etc.

Table 1. Blast furnace gas composition

CO (%) CO2 (%) H2 (%)Energy(kW/Nm

3)

density(kg/m

3)

average 22.5 19.8 2.6 0.868 1.363

max 23.7 20.8 2.8 0.914 1.370

min 21.4 18.9 2.4 0.824 1.357

The average lower heating value during these trials was 0.868 kW/Nm3 which isequivalent to 3.12 MJ/Nm3.

The BFG oxyfuel burner

AGA designed and built a flameless, dual fuel Type S3 oxyfuel burner based on theREBOX concept. A sketch of the burner design is given in Figure 2. A ceramicburner block gives self-cooled operation. Startup with a cold furnace is possible with aflame, then when the furnace is over 750C the burner can be switched over to theflameless mode for lower NOx with oxygen injection from the four holes around thecentral fuel inlet. A view of the burner from the outside of the chamber furnace is givenin Figure 3. The burner is capable of operation up to about 400 kW with pure BFG, purepropane (LPG) or a wide range of mixtures of these fuels. The burner could be alsoconverted to operate with natural gas instead of propane.


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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 chamberfurnace

Burner emission trials

The furnace emissions were monitored by gas sampling from the exhaust duct using anIR spectrometer for NOx, CO and CO2 concentrations. The Type S3 burner is capableof a wide range of operational conditions, since there are so many parameters which canbe 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 (basedon the amount of oxygen to the flameless peripheral ports versus the central port). A setof operational conditions were chosen as given in Table 2. The NOx levels varied fromabout 10 to 20 mg/MJ for all the conditions tested as shown in Figure 4. The NOx wasalways 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 toair leakage.

The increase in the NOx with the use of LPG enrichment can also be seen when plottingthe specific NOx emissions (in mg/MJ) versus the specific CO2 emissions (in kg/MJ inFigure 5). The high concentrations of CO2 in the BFG give high levels of CO2emissions for BFG oxyfuel, and lower specific CO2 emissions when using LPGenrichment. The BFG is a process waste gas that needs to be used or flared, so there canbe a net decrease in the CO2 emissions if an integrated steel mill uses BFG to replacefossil fuels. Oxyfuel combustion is also an energy efficient process, so BFG with orwithout propane enrichment is a way to reduce fossil fuel consumption and net CO2emissions.

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%

0

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00

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1.

67

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97

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64

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76

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22

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66

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80

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87

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97

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91

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65

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93

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53

25%CPG

40%CPG

BFG

Temperature (Alla)

Average of NOx mg/MJ

%Air in Flue gas

Fuel mix BFG/CPG

Figure 4. NOx emissions for BFG oxyfuel with and without LPG enrichment


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0

5

10

15

20

25

0.16 0.18 0.2 0.22 0.24 0.26 0.28 0.3

CO2 (kg/MJ)

NOx(mg/MJ)

Figure 5. NOx in mg/MJ versus the specific carbon dioxide discharge for the range ofconditions given in Table 2.

The CO concentrations were insignificant for all the conditions tested, even for close tostoichiometric combustion. Table 3 gives data for the fuels tested which were used tocompute the specific NOx emissions in mg/MJ.

Table 3. Fuel and combustion constants

Fuel

(% by energy)

LowerHeatingValue(MJ/Nm3)

O2required(Nm3/Nm3)

Densityofmixture(kg/Nm3)

BFG

(Nm3/GJ)

LPG

(Nm3/GJ)

Flue gas

(Nm3/Nm3)

LPG 92.56 5.075 2.047 0.00 10.80 7.10

40%LPG/BFG 5.09 0.23 1.378 192.06 4.32 1.1325%LPG/BFG 4.12 0.18 1.371 240.08 2.70 1.07BFG 3.124 0.126 1.363 320.10 0.00 1Air LPG 92.56 5.075 2.047 0 10.80 31.38

Steel reheating trials

The reheating rate with BFG oxyfuel with and without LPG enrichment were evaluatedby charging the chamber furnace with two blooms equipped with thermocouples tomonitor 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(1235C) and percent excess oxygen (1%) were desired, but the constant furnace power

and furnace response times caused the reheating cycles to vary considerably (see Figure6 for 75% BFG-25% LPG oxyfuel based on input kW).

Heating Trial with 75%BFG/25%CPGChamber furnace, March 17

0

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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]

Temperature,Power,Oxygen[C,kW,%*100]

Furnace temperatureTC Specimen, surfaceTC Specimen, centreBFG PowerCPG PowerO2*100

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 isgiven in Figure 7. The fastest reheating was for pure LPG oxyfuel as expected, sincethe radiative heat transfer and the energy efficiency are the best for this case. The pureBFG oxyfuel was the worst, since the energy efficiency was the worst for this case, andthe furnace temperature dropped far below the set point level. The LPG air trials weremade with two different set point temperatures, and gave results approximately thesame 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 beexplained by poor furnace power control at the start of the trial (see Figure 8). Thepower control with pure LPG oxyfuel did not vary smoothly, so even betterperformance would be expected with better power control for this case also.


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Center Temperatures

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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]

Temperature[C]

100%CPG

25%CPG

CPG air 1255

CPG air 1245

40%CPG

100%BFG

Figure 7. The reheating rate curves for steel blooms reheated with various fuelcombinations

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]

0

50

100

150

200

250

300

350

400

Power[kW]

100%BFG power40%CPG Tot power25%CPG Tot powerCPGair2CPGair1100%CPG power

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 thesechamber furnace trials. The reheating rate with pure BFG-oxyfuel was slower thanpropane-cold air combustion with a constant burner power, but propane boosting of theBFG could be used to compensate as predicted by theoretical flame temperaturecalculations. The fastest reheating and the highest productivity was available when theburner was operated in the 100% propane-oxyfuel mode. Therefore the reheatingprocess in industrial furnaces can be optimized from low energy costs with a highpercentage of BFG oxyfuel to high productivity with exclusively propane oxyfuelreheating. 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 andSteel of the European Community and financial support from MEFOS membercompanies.

REFERENCES

MALFA, E., ET. AL, (2008), CO2 Reduction in reheating furnaces (CO2RED), TGS3, Technical report4, Research DG RTD G5, Brussels, Belgium.

MARION, J., ET AL, (2000), Hauck E-Solutions, Hauck Mfg. Co. Lebanon, PA.

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