Reduction of specific coke consumption in blast furnace by impact on thermal reverse zone

Introduction

The question of determining the rational parameters for combined blast during blast furnace smelting, in alignment with the developmental goals of ferrous metallurgy, remains pertinent [1 – 4]. The simultaneous utilization of reducing additives in the form of natural gas and pulverized coal fuel, with the ratio of their flow rates to that of the process oxygen ranging from 0.9 to 1.2, has contributed to the stabilization of theoretical combustion temperature and other process parameters. The degrees of direct and indirect reduction are notable factors in these processes [5]. Upon analyzing the operation of blast furnaces at PJSC “Magnitogorsk Metallurgical Plant” (MMK), the significant role of the reactivity index and coke strength after the reaction was further confirmed [6]. In the context of blast furnaces in Japan, it becomes imperative to estimate the initial gasification temperature of carbonaceous materials as the thermal reverse zone (TRZ) takes shape. The study presented in reference [7] delves into the injection of hydrogen as a reducing agent, replacing carbon, with the aim of mitigating CO₂ oxide emissions. Simultaneously, this approach enhances the efficiency of reduction through CO gaseous oxide. The experiment demonstrated that the reduction of CO₂ oxide emissions from the blast furnace is achieved within a hydrogen concentration range of up to 20 %.

The papers [8 – 11] underscore the vital role of mathematical models due to the scarcity of information on process parameters such as temperature, pressure, and reduction levels along height of the furnace. The processes that arise and evolve within the TRZ, leading to a substantial increase in iron reduction, are pivotal elements within these evolving mathematical models. These models are currently in development and implementation. The kinetic modeling, encompassing an evaluation of the thermal reverse zone’s impact on blast furnace operations, is congruent with this perspective [7]. An investigation was conducted to assess the influence of material temperature upon charging into the furnace, their reductibility, and gas pressure at the furnace’s mouth on the outcomes of blast furnace smelting. The findings revealed that as gases become more actively utilized within the furnace, the temperature at the onset of the thermal reverse zone decreases [9]. The paper [12] has illustrated that the specific consumption of coke diminishes with an increase in the duration of time the charged materials spend within the TRZ. A noteworthy reduction of 3.6 kg/t of cast iron was achieved [13] by mitigating heat outflow from the region where W_ch/W_g ≥ 1 to the area where W_ch/W_g < 1, with Wch and W_g representing the heat capacities of charge and gas flows, respectively. In the thermal reverse stage (W_ch ≈ W_g), the temperature registered a decline of 2.5 °C, while the temperature differential between the gas and the charge contracted by 1.3 °C.

An evaluation of rational approaches concerning blast furnaces at MMK has unveiled the substantial significance of coke reactivity index and post-reaction strength in relation to the adjustment of natural gas consumption [6]. This complements prior explorations of these phenomena [14 – 19]. Theoretical and experimental investigations have contributed to the recognition of the importance of temperature boundaries distinguishing between indirect, mixed, and direct reduction regions [14]. The values characterizing these boundaries exhibit interrelationships with the rates of oxide reduction by gases such as CO and H₂, as well as the rates of carbon gasification by gaseous reduction products, СО₂ and Н₂О [16].

In laboratory experiments, where the charge was maintained under identical temperature and time conditions as those in the upper section of the blast furnace shaft, a comparison was conducted between charcoal and coke concerning their influence on the degree of sinter reduction by a gas mixture comprising 29 % CO, 2 % H₂ and 60 % N₂ [16]. The results obtained were as follows:

Based to these data, at temperatures corresponding to intense heat exchange in the upper section of the furnace (up to 750 °C), the reduction degree exhibited only a marginal increase, with fuel reactivity nearly unchanged. Simultaneously, the flue gas composition contained approximately 10.4 – 10.8 % СО₂. At temperatures corresponding to the initiation and progression of the thermal reverse zone (above 750 °C), a portion of CO₂ oxide engaged in interactions with carbon from the fuel, leading to the formation of CO oxide. The formation of CO oxide by charcoal, a fuel with a higher reactivity index, contributed more significantly to the enhancement of sinter reduction compared to its formation by coke. This effect assumed considerable importance in the optimization of the blast furnace process as natural gas consumption increased [18].

Experiments that involved the examination of coke samples extracted from the pilot blast furnace unveiled four different reactivity pathways. The outcome of these pathways includes the potential reduction of coke-specific consumption and the augmentation of blast intensity. Furthermore, the ability of coke carbon to react with СО₂ oxide was also observed to increase [20 – 22]. In practical applications, these findings can be implemented by incorporating machine vision systems for both the upper and lower sections of the blast furnace [23 – 29]. Additionally, they align with concepts associated with the electronic theory of iron reduction from oxides [30; 31].

Materials and methods

Using the insights gathered from the works [1 – 31], we successfully reduced coke-specific consumption within a blast furnace with a production capacity of 1370 m³. This reduction was achieved by strategically manipulating the thermal reverse zone, primarily through an increase in natural gas-specific consumption, while concurrently adjusting the coke reactivity index (CRI) and coke strength after reaction (CSR) in opposite direction. The most important operational characteristics of the furnace are depicted in Fig. 1 – 4.

The vertical pressure behavior (active weight Р_а) of materials along both the height and the cross-section of the furnace is presented in Fig. 1 and 2.

Average temperature trends along the height of the blast furnace are depicted in Fig. 3, while those concerning temperature distribution by radius are illustrated in Fig. 4 and 5. These figures are constructed based on the findings and insights from the referenced papers [14; 17; 22].

When temperatures fall below the range of 850 – 900 °C, it is observed that the heat capacity of the gas flow surpasses that of the charge flow (W_g > W_ch), while the W_ch/W_g ratio remains within the range of 0.7 – 0.9. This surplus heat in the upper heat exchange phase, not absorbed by the charge, finds application in low-temperature processes without necessitating an increase in the quantity of coke introduced into the furnace. Notably, no additional coke is required for moisture evaporation.

Fig. 5 illustrates the portion of oxygen from all oxides extracted from the charged materials by various reducing agents. In the figure: 1 – accelerated reduction occurring in the upper part of the blast furnace, which is attributed to the low stability of the chemical bond of Fe₂O₃ and Fe₃O₄; 2 – reduction taking place in the conditions of the thermal reverse zone, which are highly conducive to reactions FeO + CO = Fe + CO₂ and FeO + H₂ = Fe + H₂O, with each unit of temperature change significantly influencing the reduction process; 3 – accelerated reduction due to the intensity of its direct progression (FeO + C = Fe + CO); 4 – slower reduction occurring as the process reaches its conclusion, with only a few unreduced oxides remaining; 5 – a decrease in the reduction degree due to the oxidation of smelting products in tuyere hearths; 6 – rapid reduction of elements, mainly iron, previously oxidized in tuyere hearths. The solid line in the figure corresponds to modern blast furnace smelting conditions, while the dashed line signifies potential advancements in blast furnace smelting technology.

Results and discussion

At the blast furnace of MMK, the reduction in coke-specific consumption was achieved by elevating the utilization of hydrogen and augmenting the degree of iron reduction from FeO oxide. This was accomplished by influencing the thermal reverse zone, leading to an increase in natural gas consumption exceeding 120 m³/t of cast iron. The study focused on two paired periods: I and II (the first pair) and III and IV (the second pair). The periods within each pair were contiguous, primarily spanning seven days each.

During period I of the first pair, under the initial conditions, the natural gas flow rate stood at 123.2 m³/t of cast iron. When the blast contained 27.2 % of oxygen, the ratio of gas and oxygen consumption equated to 0.43 (Table 1). In the second pair (base period III), these values increased to 135.8 m³/t of cast iron; 27.6 % oxygen and a ratio of 0.47, respectively.

In both pairs of periods, several key parameters experienced growth, including the mass of the gas-air mixture per tuyere, its kinetic energy of efflux, and the extent of the loose portion of the tuyere hearth. Notably, these parameters exhibited more pronounced changes in the first pair of periods.

The coke strength after reaction CSR under the baseline conditions of the first pair of periods amounted to 36.3 %, with a reactive index (CRI) of 38.4 %. During the renewed smelting mode, CRI increased by 0.9 % to reach 39.3 %. In the basic conditions of the second pair, CSR was 39.5 %, and CRI was 39.1 %, with a variation reducing CRI to 37.8 % (Table 2).

In period II, as compared to period I, CSR decreased from 36.3 to 34.6 %, and CRI increased from 38.4 to 39.3 %. This change resulted in an increase in the ratio of natural gas consumption to the total oxygen entering the furnace, rising from 0.43 to 0.45. The gas-specific consumption also increased from 123.2 to 133.5 m³/t of cast iron. Additionally, the oxygen content in the blast increased from 27.2 to 28.4 %. These alterations facilitated an increase in hydrogen reduction from 31.9 to 37.2 % (Table 3) and an enhancement in its utilization, from 45.3 to 48.8 %. Meanwhile, the length of the TRZ increased towards the blast furnace mouth by 1.9 %, while its position at the bottom remained unchanged. A significant reduction in carbon reduction, from 30.7 to 24.4 %, led to a decrease in heat consumption within the TRZ, resulting in an average temperature difference t_g – t_ch (Fig. 3) increasing by 36 °С.

In period IV, in comparison to period III, the main factors pertaining to smelting conditions underwent the following changes:

– an increase in natural gas consumption from 135.8 to 143.9 m³/t of cast iron;

– a decrease in the oxygen content in the blast from 27.6 to 27.0 %;

– an increase in the ratio of natural gas consumption to the total oxygen entering the furnace from 0.47 to 0.51;

– a decrease in CRI from 39.1 to 37.8 %;

– an increase in CSR from 39.5 to 40.2 %.

As a result, several changes in the studied processes during period IV, compared to period III, were observed:

– an increase in the length of the TRZ towards the blast-furnace mouth by 2.6 % and the distance from the tuyere hearth by 3.4 %;

– an increase in the degree of carbon reduction from 32.0 to 33.3 %;

– a slight, albeit insignificant, increase in the temperature difference t_g – t_ch (Fig. 3), averaging only 0.3 °С.

The key thermal performance characteristics accompanying these alterations are detailed in Table 4.

In terms of the recorded parameters, the temperature in the axial zone of the blast-furnace mouth decreased by 14 °С in period II when compared to period I, and conversely, it increased by 20 °C in period IV when compared to period III (Table 1).

The series of modifications within the processes, encompassing the ratios Т_per/Т_g, Т_per/Т_c, Т_c/Т_g, resulted in a reduction in coke-specific consumption in period II compared to period I from 434.9 to 430.2 kg/t of cast iron, while in period IV, in comparison to period III, from 437.5 to 435.9 kg/t of cast iron (Table 5). During the first pair of periods, furnace productivity increased by 27 t/day; however, the conditions and progression of the processes during the second pair led to a decrease in cast iron production by 41 t/day (Table 5).

The shift of the coke carbon gasification process in period II, in comparison to period I, led to a 6.9 % reduction in the factor of charge and its product’s resistance to gas movement in the lower part of the bosh (Fig. 1, the region of minimal P_a value). This shift also facilitated smelting with an 11.5 % increase in lift force of the gas flow (Table 1). The shift of the coke carbon gasification region in period IV compared to period III to the region of lower temperatures led to a decrease in the coke specific consumption and the processes rate, resulting in decline of the furnace output (Table 5).

Conclusions

The parameters, such as length, height location, temperature, and the temperature difference between rising gases and descending materials within the blast furnace’s TRZ, exert significant influence on coke-specific consumption. The formation of the TRZ is contingent on various factors, including the coke reactivity index (CRI), coke strength before reaction (CSR), natural gas consumption, and process oxygen consumption.

At the blast furnace of MMK, a series of smelting operations were conducted, categorized into two pairs of periods: the first pair serving as the base period and the second as the experimental period.

The first pair of periods witnessed an increase in natural gas consumption, rising from 123.2 to 133.5 m³/t of cast iron. The length of the TRZ extended towards the blast furnace mouth by 1.9 %, maintaining its position at the bottom unchanged. Simultaneously, the temperature gradient between the gas and materials surged by 36 °C. Conversely, in the second pair of periods, natural gas consumption increased from 135.8 to 143.9 m³/t of cast iron. The TRZ expanded towards the blast furnace mouth by 2.6 %, and the distance from the tuyere hearth extended by 3.4 %. However, the temperature difference between the gas and materials within the TRZ registered a negligible increase, averaging just 0.3 °С.

In the first pair of periods, a reduction in coke-specific consumption by 4.7 kg/t of cast iron was achieved, resulting in a notable increase in furnace productivity by 27 t/day. However, in the second pair, coke-specific consumption decreased by 1.6 kg/t of cast iron, leading to a decrease in cast iron production by 41 t/day.