Enhancement of Iron Ore Sinter Reducibility through Coke Oven Gas Injection into the Modern Blast Furnace

Abstract

Energy network within the integrated steel works should be used more efficiency to reduce the energy consumptions and CO₂ emissions. The injection of free resources of coke oven gas (COG), which is rich with hydrogen, into the modern blast furnace is one of such measures. In order to clarify the effect of COG injection on the reduction processes in the blast furnace; iron ore sinter was isothermally and non-isothermally reduced with different gas compositions at different temperature. The gas compositions were selected to simulate the conditions of middle (150 m³/tHM) and intensive (300 m³/tHM) injection of COG into the blast furnace. The results were compared to that obtained under typical blast furnace conditions without COG injection. The isothermal reduction at 900–1200°C indicated the enhancement of the reduction rate as COG injection increased. The non-isothermal reduction indicated the efficiency of intensive injection of COG in decreasing the direct reduction from 50% to only 5% at 1200°C. Reflected light microscopy, scanning electron microscopy and X-ray techniques were used to characterize the microstructure and the developed phases in the origin and reduced sinter. The rate controlling mechanism of sinter under different conditions was predicted from the correlation between apparent activation energy calculations and microstructure examination.

1. Introduction

The iron and steel industry is one of the main branches responsible for energy consumption and CO₂ emissions. Despite remarkable decrease in specific CO₂ emissions from the steelmaking in the industrial countries, the total amount of CO₂ emissions is growing across worldwide due to the continuous increasing of steel production reached to about 1.5 billion ton in 2011.¹⁾ Nowadays the steel industry faced increasing demand to minimize the energy consumption and gas emissions especially from ironmaking processes. The efficient use of byproduct gases is essentially important for the profitability of steel mill operation due to the high energy volumes and the costs involved.²⁾ The injection of COG into the modern blast furnace is one of effective measures for iron and steel industry to achieve low-carbon ironmaking, energy saving and emission reduction.³⁾ The specific amount of generated coke oven gas by conventional cokemaking is in the range from 410 to 560 Nm³/t of coke depending on the volatile matters in the coal charge.⁴⁾ In 2011, the worldwide coke production reached a new record of 641.4 million tonnes with COG amounted to be more than 310 billion Nm³.^5,6) The COG is currently used after its cleaning from tar, naphthalene, raw benzene, ammonia, and sulfur for heating of blast furnace stoves, ignition furnaces in sintering plant, heating furnace in rolling mills and electric power generation in power plant.^6,7) The estimations which carried out on optimizing the energy consumption in the integrated iron and steel works indicated that the utilization of COG for power generation is not always the optimal credits.⁴⁾ The COG is consisting of e.g. 58% H₂, 27% CH₄, 7% CO and small amount of CO₂, N₂, and other elements.⁹⁾ This composition of COG which is rich with hydrogen has attracting much attention in the recent years for its utilization in the reduction processes.^8,9,10,11) The flexibility of COG utilization in the integrated steel plant for DRI production through the addition of Midrex process is expected to be very efficient.¹²⁾ Also the injection of COG into the blast furnace contributes of decreasing the energy consumption and CO₂ emissions. The injection of COG into the blast furnace has influence on the raceway conditions and iron ore reduction. The mathematical modelling on the injection of COG into the blast furnace tuyeres indicated better combustion conditions and higher injection rate by using two injection lances compared to one lance.^13,14,15) The combustion of COG hydrocarbons in the front of tuyeres by blast oxygen results in a development of carbon monoxide and hydrogen gases which increase the potential of reducing gas on account of N₂.¹³⁾ The theoretical calculation and commercial trails which carried out on the replacement of natural gas with coke oven gas in blast furnace showed lower coke consumption and higher hot metal production.¹⁶⁾ The high efficiency of COG is due to the fact that it contains 3.5–4 times fewer hydrocarbons compared to that of natural gas.¹⁷⁾ This improves the combustion in the tuyere hearth, activate coke column, and increase gases utilization in the furnace. It has been noticed higher amount and higher heating value of blast furnace top gas could be generated through COG injection into the blast furnace.¹⁶⁾ Although the injection of COG into the blast furnace is practiced in some countries with different injection rate from about 30 to 280 m³/tHM but its effect for example on the sinter reducibility is not clear.^9,11,18,19) Furthermore, it was reported that the maximum level of COG injection at the blast furnace tuyere is thought to be 0.1 ton COG/t hot metal according to the thermochemical conditions while the replacement ratio is 0.98 ton of coke/t of COG.²⁰⁾

The current study aims at investigation of the influence of COG with different injection levels into the blast furnace on the reduction kinetics and mechanism of iron ore sinter. The gas compositions are selected to simulate experimentally the results of numerical analysis method based on raceway mathematical model, multi-fluid blast furnace model, and exergy analytical model.³⁾ In the base case, PC is injection into the blast furnace (145 kg/tHM). By COG injection, PCI is decreased while oxygen enrichment is increased to maintain a constant flame temperature. In order to clarify the influence of COG injection on the reduction processes; the results are compared with that obtained under typical blast furnace conditions without COG injection. The reduction has been carried out isothermally at temperatures in the range of 900–1200°C. On the other hand the non-isothermal reduction was carried out under continuous variation of gas compositions and heating rate simulates the blast furnace conditions with different amount of COG injections. The structure and morphological changes of original and reduced sinter were intensively studied and correlated with the reduction kinetics and mechanism.

2. Experimental Techniques

The reduction of industrial iron ore sinter has been carried out using a laboratory system as shown in Fig. 1. The system consisted of vertical tube Tammann furnace connected with an automatic sensitive balance. Alumina reaction tube was fitted inside the graphite heating tube where the heat transferred mainly by radiation to the sinter samples. The holey alumina crucible containing samples were hold by a Pt wire and connected to a balance for continuous measuring of the weight loss as a function of time. With a pneumatic cylinder, the crucible containing the sinter samples were lifted up and down within less than 1.0 second into the Tammann furnace. The temperature was measured with platinum thermocouple which fixed near to the sample. For isothermal reduction; purified N₂ with flow rate of 1.0 liter/min was purged in the reaction tube from the bottom during the heating up of the furnace to the pre-determined temperature. At the applied temperature, the sinter pieces with average size 8–10 mm and average weight 20 g were placed in the alumina crucible (diameter = 30 mm, length = 40 mm) and centered in the middle of hot zone in the furnace. After soaking the sample at this temperature for 10 minutes, a reducing gas simulated the base condition without COG injection, middle COG injection (150 m³/tHM), and intensive COG injection (300 m³/tHM) as given in Table 1 is purged into the reduction alumina tube with flow rate of 4.0 liter/min. The conditions of base, middle COG injection, and intensive COG injection will be referred hereafter as case 1, 2 and 3; respectively. These scenarios are selected based on the results of simulated blast furnace with COG injection.³⁾ The gas composition of case 1 corresponds to the typical blast furnace operation with about 145 kg/tHM PCI and about 3% oxygen enrichment. The gas composition of case 2 and 3 corresponds to the blast furnace operation with middle and intensive COG through the decreasing of PCI to 115 and 85 kg/tHM and increasing the oxygen enrichment to 19 and 38% respectively. The time was accounted for 150 min in all isothermal reduction tests. For non-isothermal reduction; similar flow rate, size and weight of sinter which used in the isothermal reduction was applied while the gas composition was changed for cases 1–3 as given in Table 2. The heating rate starting from room temperature up to 1200ºC was selected to simulate the blast furnace conditions and fixed for all cases. The total time was 240 min in all non-isothermal experiments.

Fig. 1.

Schematic diagram of laboratory reduction system.

Table 1. Composition of the applied reducing gases in isothermal trials.

	CO, vol.%	H2, vol.%	N2, vol.%	H2/CO	H2/ (H2 + CO)
Case 1: Normal condition	37	8	55	0.22	0.18
Case 2: Middle COG injection (150 m3/t HM)	45	20	35	0.44	0.31
Case 3: Intensive COG injection (300 m3/tHM)	55	35	10	0.64	0.39

Table 2. Gas composition and heating rate scenario of non-isothermal reduction.

Step No.	Temperature, ºC and heating rate, K/min.	Case 1, CO–H2–CO2–N2 Vol.%	Case 2, CO–H2–CO2–N2 Vol.%	Case 3, CO–H2–CO2–N2 Vol.%
1	RT-200ºC, 10 K/min.	0-0-0-100	0-0-0-100	0-0-0-100
2	200–400ºC, 10 K/min.	22-0-23-55	30-12-23-35	40-27-23-10
3	400–900ºC, 10 K/min.	27-3-15-55	35-15-15-35	45-30-15-10
4	900–1000ºC, 2 K/min.	30-5-10-55	40-15-10-35	50-30-10-10
5	1000–1200ºC, 5 K/min.	37-8-0-55	45-20-0-35	55-35-0-10
6	1200ºC, 0 K/min for 60 min.	37-8-0-55	45-20-0-35	55-35-0-10

During the reduction experiment, the weight loss was continuously recorded as a function of time. At the end of experiment, the reduced sinter was lifted up and putted in a closed chamber under high flow rate of Ar to avoid the reoxidation during cooling. For partial reduction, the oxygen weight loss required to achieve a certain reduction extent was pre-calculated and the reaction was stopped when the weight loss reached the predetermined value. The total reduction degree was determined depending on the calculation of oxygen represented in iron oxides of sinter.

The iron ore sinter was examined before and after reduction by reflected light microscope (RLM- Leica Aristomet) and scanning electron microscope-backscattered electron image (SEM-EDX/BSE, ZEISS DSM 962). The porosity of sinter was measured by poresizer (Micrometrics 9320). The formed phases and its quantitative analysis were identified by high performance X-ray diffractometers (Cu-Kα1 radiation).

3. Results and Discussion

3.1. Characterization of Raw Materials

The chemical analysis of iron ore sinter is given in Table 3. The X-ray diffraction analysis of the applied sinter exhibited that the sinter was composed of three main phases: hematite, calcium silicate, and calcium ferrites as given in Fig. 2. The microstructure of sinter sample was examined by RLM as given in Figs. 3(a)–3(d). The phases were analysed by SEM-EDX. Figure 3(a) showed dense structure of sinter with random distribution of pores. Figure 3(b) showed pale white dense grains (m) at the outer surface which is magnetite. Figure 3(c) exhibited hematite phase (h) and the presence of calcium ferrite phase (CF) appeared as flakes between and on the outer surface of hematite. Figure 3(d) illustrated the presence of calcium silicate phase (CS) distributed between calcium ferrites.

Table 3. Chemical analysis of iron ore sinter.

Element	Fe	FeO	Mn	SiO2	Al2O3	CaO	MgO	P	S	CaO/SiO2
wt%	57.0	5.09	0.47	4.722	1.134	11.5	0.99	0.041	0.018	2.43

Fig. 2.

XRD phases analysis of the applied iron ore sinter.

Fig. 3.

Microstructure of the applied iron ore sinter with different magnifications: (a) 50× (b) 200× (c) 500× (d) 1000×.

The total porosity of sinter in addition to the bulk and apparent density were measured by poresizer as given in Table 4 while the pore size distribution is shown in Fig. 4. It can be seen two different modes of pores in sinter; small pores which have diameter in range of 0.1–1.0 μm and large pores in the range of 10–100 μm.

Table 4. Porosity and density of applied sinter.

Measured item	Bulk density, g/mL	Apparent density, g/mL	Total porosity, %
Value	4.1158	4.5802	10.14

Fig. 4.

Pore size distribution of applied sinter.

3.2. Isothermal Reduction of Sinter

Typical reduction curves of sinter isothermally reduced by different gas mixtures at 900–1200ºC are given in Figs. 5(a)–5(c) for case 1–3 respectively. The reduction degree was calculated based on the oxygen of iron oxides in sinter.²¹⁾ In all cases the reduction degree increased with temperature. In each case the reduction rate started relatively fast at the initial stages and then converted to stable manner at different time according to the applied temperature till the end of reduction. In case 1, the reduction reached a maximum value of 54% at 1200ºC after 150 min while the lowest reduction value of 38% is obtained at 900ºC. The difference between reduction curves decreased with rising temperature and became unremarkable at 1100 and 1200ºC while this difference increased in going from case 2 to 3. The reduction degree increased in case 2 and 3 to become 1.5 (83%R) and 1.83 (99%R) times higher than that in case 1 at 1200ºC; respectively. The comparison between the reduction curves for case 1–3 at the same temperature is given in Figs. 6(a)–6(d). At all temperatures, the reduction showed the maximum value in case 3 and the minimum value in case 1. The reduction degree in case 3 is in range of 1.8–1.9 times higher than that in case 1 as the difference of reduction reached to 35–45%. This difference decreased between case 3 and 2 to become in the range of 7–16% which represented 1.1–1.2 times higher in case 3 compared to case 2. In general, the higher COG injection was resulted in higher potential of reducing gas (CO + H₂) and consequently higher reduction rate of sinter.

Fig. 5.

Isothermal reduction curves of sinter at 900–1200ºC with gas mixtures simulated: (a) Case 1: without COG (b) Case 2: 150 m³/tHM COG (c) case 3: 300 m³/tHM COG.

Fig. 6.

Comparison between the isothermal reduction curves for cases 1–3 at: (a) 900ºC (b) 1000ºC (c) 1100ºC (d) 1200ºC.

3.3. Reduction Kinetic and Mechanism

In order to clarify the positive effect of COG injection on the sinter reducibility, the reduction rate (dr/dt) at the initial stages (15%) and at moderate (45%) stages of reduction was calculated and plotted against the corresponding temperature, as indicated in Figs. 7(a) and 7(b) respectively. It can be observed that, at both the initial (15%) and moderate (45%) reduction stages, the reduction rate increased gradually with temperature for case 2 and 3 while its slowly increased at temperature ≥1100ºC in case 1. In addition, the difference between the rate of reduction in case 2 and 3 increased with temperature at both the initial and moderate reduction stages.

Fig. 7.

Effect of temperature on the reduction rate for sinter reduced with different gas mixtures at: (a) 15% reduction (b) 45% reduction.

The rate controlling mechanism at different reduction stages can be predicted from the apparent activation energy calculation and microstructure investigations.

The values of apparent activation energy were calculated from Arrhenius equation which is given in Eq. (1).   

$${\text{K}}_{\text{r}}={\text{ K}}_{\text{o}}{\text{e}}^{-\text{Ea}/\text{RT}}$$(1)

where,

K_r: reduction rate constant (s^–1),

K_o: frequency factor (s^–1),

Ea: apparent activation energy (kJ. mole^–1),

R: universal gas constant (8.314*10^–3 kJ mole^–1 K^–1), and

T: absolute temperature (K)

The relationship between the logarithm of reduction rate and the reciprocal of absolute temperature at 15% and 45% reduction is given in Figs. 8(a) and 8(b) respectively. The computed values of apparent activation energy from Arrhenius equation are given in Table 5. The relationship between the activation energy values and the rate controlling step is given in Table 6.²²⁾ These values indicated that at the initial stages (15%) the reduction is most likely controlled by a combined effect of gaseous diffusion and interfacial chemical reaction with more participation of chemical reaction in case 1. As the reduction proceeds (45%) the reduction mechanism is still combined effect of gaseous reduction and interfacial chemical reaction with more participation of gaseous diffusion in cases 2 and 3 as the activation energy values deceased.

Fig. 8.

Arrhenius plots for sinter reduced with different gas mixtures at: (a) 15% reduction (b) 45% reduction.

Table 5. Activation energy values for sinter reduced with different gas mixtures.

	Regression Equations		Ea, kJ/mol
	15% Reduction	45% Reduction	15% R	45% R
Case 1	log K = 1.7256 – 0.253 * 1/T	log K = 1.1133 – 0.2529 * 1/T	48.28	48.26
Case 2	log K = 1.3425 – 0.167 * 1/T	log K = 4.1869 – 0.1012 * 1/T	31.94	19.32
Case 3	log K = 1.63129 – 0.195 * 1/T	log K = 0.9022 – 0.145 * 1/T	37.21	27.82

Table 6. Relationship between activation energy values and the reduction mechanism.

Activation energy values (Ea), kJ/mole	Probable rate controlling step
8–16	Gaseous diffusion
29–42	Combined of gaseous diffusion and interfacial chemical reaction
60–67	Interfacial chemical reaction
≥ 120	Solid state diffusion

The microstructure examination of sinter reduced up to 45% with different gas mixtures was used to clarify the reduction mechanism suggested by apparent activation energy calculations as shown in Fig. 9. It can be seen that the outer layer consist mainly of metallic iron with some grains of wüstite which decreased in going from case 1 to case 3. On the other hand the middle layer and core of samples were consisted mainly of iron oxides with the presence of few grains of metallic iron which increased from case 1 to case 3. The presence of wüstite with metallic iron in relatively porous structure in case 1 indicated that the reduction mechanism was mainly chemical reaction while the disappearance of metallic iron in the core of sample indicated the participation of gaseous diffusion mechanism in the rate controlling step. In case 2 and 3, the wüstite grains in outer layer were few which give the chance for the reducing gas to diffuse into the middle and core layer. As the diffusion of reducing gas takes place the resistance increased especially for CO. The diffusivity of reducing gas molecule is inversely proportional to the square root of its molecular weight.²³⁾ The diffusivity of H₂/H₂O is 3–5 times faster than that of CO/CO₂ depending on the applied temperature.²⁴⁾ The disappearance of metallic iron grains in the core of sinter samples in case 1 and its increasing ingoing from case 2 to case 3 indicated that the diffusivity of reducing gas mixtures increased as the H₂/CO ratio increased as given in Table 1. The effect of temperature and gas composition on the microstructure of sinter reduced at 900 and 1200ºC for 150 min is shown in Figs. 10 and 11 respectively. Figure 10 showed few grains of metallic iron in the outer layer in case 1 and it is completely disappeared in the core of sample. On the other hand more metallic iron grains appeared in the outer layer of case 2 and 3 in a relatively pores structure. In the middle layer the metallic iron were segregated as the structure became dense while its appearance decreased in the core. Figure 11 showed the effect of higher temperature (1200ºC) on the structure of metallic iron. The metallic iron grains became denser and larger compared to that at 900ºC. The iron oxides in the form of hematite and wüstite were only appeared in case 1 and case 2 respectively.

Fig. 9.

Microstructure photomicrographs for sinter reduced up to 45% reduction at 1100ºC.

Fig. 10.

Microstructure photomicrographs for sinter reduced for 150 min at 900ºC.

Fig. 11.

Microstructure photomicrographs for sinter reduced for 150 min at 1200ºC.

3.4. Non-isothermal Reduction of Sinter

The non-isothermal reduction curves of sinter reduced under different conditions of temperature and reducing atmosphere are given in Figs. 12(a)–12(c). The reduction conditions were given in Table 2. It can be noticed that there was no reduction took place at temperature lower than 600ºC (up to 60 min) in all cases regardless the composition of the applied gas mixtures. After that (starting from 60 min), the reduction was sharply increased with different rate depending on the reduction potential of applied gas mixtures in each case. This remarkable increase of reduction took place as hematite is reduced to magnetite and magnetite to wüstite up to temperature of about 850ºC. At 850–1000ºC, the reducing speed shortened as the reduction of wüstite to metallic iron took place. At 1000–1200ºC, the changing of the gas mixtures in each case to higher reduction potential through stopping the flow of CO₂ in addition to the continuous increasing of temperature resulted in higher reduction rate with different extent. The comparison between the reduction degree for case 1, 2, and 3 is given in Fig. 13. It indicates that the reduction of sinter in case 1 (simulate normal blast furnace conditions) exhibited the smallest reduction degree which reached to only ~50% after 240 min. On the other hand the reduction degree in case 2 and 3 (simulate the conditions of 150 and 300 m³/tHM injection of COG) was reached to 75% and 95% respectively. The microstructure photomicrographs of reduced sinter are given in Fig. 14. The matrix structure consisted of metallic iron and lower oxides in case 1 and 2. In case 3 the iron oxides completely disappeared even at the core of the sample and the metallic iron became predominate allover the matrix.

Fig. 12.

Reduction curves of sinter non-isothermally reduced up to 1200ºC: (a) Case 1 (b) Case 2(3) Case 3.

Fig. 13.

Reduction curves of sinter non-isothermally reduced up to 1200ºC.

Fig. 14.

Microstructure photomicrographs of sinter non-isothermally reduced up to 1200ºC.

The qualitative and quantitative analysis of the phases developed in the sinter non-isothermally under different gas conditions has been carried out using high performance X-ray diffractometers as given in Fig. 15 and Table 7 respectively. It can be seen that the metallic iron is greatly increased from 16 wt.% in case 1 to more than 79 wt.% in case 3 on account on the reduction of wüstite which decreased from 75.4 wt.% to only 2 wt.%.

Fig. 15.

XRD analysis of sinter non-isothermally reduced up to 1200ºC: (a) Case 1 (b) Case 2 (c) Case 3.

Table 7. Quantitative analysis of the phases developed during the non-isothermal reduction of sinter.

	Fe, wt.%	FeO, wt.%	CaSiO4, wt.%
Case 1	16	75.4	8.6
Case 2	26.5	53.7	19.8
Case 3	79.2	2.0	18.2

These finding indicated that the reduction under conditions simulated the injection of 150 m³/tHM (case 2) and 300 m³/tHM (case 3) of COG was very effective in the enhancement of the reduction rate of sinter. The improving of the reduction degree of sinter especially at temperature lower than 1100ºC is very effective factor in decreasing the coke consumption in the blast furnace.^25,26,27) The coke consumption is related strongly to the direct reduction of wüstite which takes place at temperature of about 1100ºC as given in Eqs. (2), (3), (4). As the direct reduction decreases with COG injection, the coke consumption decreases. The results of non-isothermal reduction given in Fig. 13 indicates that the reduction degree at 1100ºC in case 3 (~55%R) was 1.7 times higher than that in case 1 (35%R) and this value was increased to 1.9 at 1200ºC. This proves that the direct reduction will decrease by 90% at temperature > 1200ºC in case 3 compared to that in case 1. Based on this estimation, only 5% direct reduction will take place at temperature higher than 1200ºC in case 3 compared to 50% in case 1.   

$$\text{FeO}+\text{CO}=\text{Fe}+{\text{CO}}_{\text{2}}\hspace{1em}\hspace{1em}\mathrm{\Delta }\text{H}°=-\mathrm{\hspace{0.17em} }\text{17}.\text{13}\mathrm{\hspace{0.17em} }\text{kJ}/\text{mol}$$(2)

  

$${\text{CO}}_{\text{2}}+\text{C}=\text{2CO}\hspace{1em}\hspace{1em}\mathrm{\Delta }\text{H}°=+\mathrm{\hspace{0.17em} }\text{172}.\text{47}\mathrm{\hspace{0.17em} }\text{kJ}/\text{mol}$$(3)

  

$$\begin{array}{c}\text{Overall reaction}:\hfill \\ \text{FeO}+\text{C}=\text{Fe}+\text{CO}\hspace{1em}\hspace{1em}\mathrm{\Delta }\text{H}°=+\mathrm{\hspace{0.17em} }\text{155}.\text{34}\mathrm{\hspace{0.17em} }\text{kJ}/\text{mol}\end{array}$$(4)

The increasing of the difference between the reduction curves as the time proceeded (steps 5 and 6) can be attributed not only to the higher reducing power of applied gas mixtures but also to the higher efficiency of H₂ as the temperature increased. From the thermal point, the reduction efficiency of H₂ becomes higher than that of CO at > 810ºC due to the endothermic reaction as given in Eq. (5). Although the reduction of wustite with H₂ is accompanied by heat absorption but this amount of heat is accounted for only 18% of that consumed in the direct reduction given in Eq. (4).   

$$\text{FeO}+{\text{H}}_{\text{2}}=\text{Fe}+{\text{H}}_{\text{2}}\text{O}\hspace{1em}\hspace{1em}\mathrm{\Delta }\text{H}°=+\mathrm{\hspace{0.17em} }\text{28}.0\text{1}\mathrm{\hspace{0.17em} }\text{kJ}/\text{mol}$$(5)

This finding clarifies well the enhancement of the reduction process of sinter under conditions simulate the COG injection into the blast furnace. The vital role of COG injection is expected to participate in decreasing the direct reduction and consequently the coke consumption in blast furnace. However the effect of hydrogen concentration in the gas mixtures on the water gas shift reaction and consequently the coke consumption will be studied in our next investigations.

4. Conclusions

In this study, the reduction behaviour of sinter was carried out under conditions simulated blast furnace operation with different injection level of COG. The reduction condition was simulated the typical blast furnace conditions (case 1), injection of 150 m³/tHM COG (case 2), and injection of 300 m³/tHM (case 3). The isothermal reduction was carried out at 900–1200ºC while the non-isothermal reduction took place from room temperature up to 1200ºC. The main finding can be summarised as follows:

(1) The isothermal reduction was greatly enhanced in case 3 and case 2 compared to that in case 1. The reduction degree increased in case 2 and 3 to become 1.5 and 1.83 times higher than that in case 1 at 1200ºC; respectively. The difference in reduction degree in case 2 and 3 is in the range of 7–16% while it is increased between case 1 and 2 to reach to 35–45%.

(2) A correlation between apparent activation energy and microstructure examination of reduced sinter were carried out to elucidate the reduction mechanism: at the initial stages (15% reduction) the reduction is most likely controlled by a combined effect of gaseous diffusion and interfacial chemical reaction with more participation of chemical reaction in case 1. At moderate reduction stages (45% reduction), the reduction mechanism is still combined effect of gaseous reduction and interfacial chemical reaction with more participation of gaseous diffusion in cases 2 and 3.

(3) The non-isothermal reduction exhibited the smallest value (~50% reduction) in case 1 while it increased to 75% and 95% in case 2 and 3 respectively. This indicates that only 5% direct reduction will take place at temperature higher than 1200ºC in case 3 compared to 50% in case1.

(4) The isothermal and non-isothermal results exhibited the efficiency of COG injection into the blast furnace in the enhancement of the reduction process and consequently the decreasing of coke consumption and CO₂ emissions.

Acknowledgements

The authors wish to thank Mr. J. S. Rathore for his cooperation and participation in the experiments of the current work. The authors gratefully acknowledge the financial support provided to the corresponding author of this research by Alexander von Humboldt Foundation in Germany.

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