Introduction

Steelwork, which is one of the hugest industries, faces two main problems, depletion of high-grade resources [21] and energy. Because the amounts of high-grade ore and coal are decreasing, it is presumed that the price will increase. However, critical technologies to utilize low-grade resources have not been established yet. Despite energy recovery techniques such as coke dry quenching and top gas recovery turbines being widely applied [2], a huge amount of exhaust heat is generated from steelworks.

As a technology to utilize low-grade resources, our group has proposed biotar ironmaking [11, 12, 16, 17] which allows the use of low-grade carbon resource and low-grade iron ore without degradation or decrease in the volume of the product. This process can simultaneously yield carbon-loaded iron or iron oxide and clean gas from low-grade iron ore and biomass. Low-grade iron ores such as limonite, including FeOOH, have rarely been used in steelworks, primarily because of their endothermic dehydration to form hematite (Fe2O3). The hematite produced by dehydration of limonite is a mesoporous solid with a high-specific surface area [11]. Hata et al. [11] exposed the mesoporous ore to the vapors of nascent volatiles produced by the pyrolysis of woody biomass at 773 K. The hematite was reduced to magnetite while tar was deposited in the mesopores, resulting in a carbon concentration up to 4 wt% of the resulting ore. Heating this carbon-loaded magnetite in an inert atmosphere up to 1173 K further reduced the ore, mainly by direct reduction, i.e., reduction by solid carbon; the ore thus produced had an O/Fe atomic ratio of 0.81. Conversely, iron oxide is an attractive option for eliminating or reforming tar. The catalytic performance of iron oxide in reforming tar from biomass has been demonstrated by Uddin et al. [22] and Nordgreen et al. [19], who suggested the importance of specific surface area and oxidation states in catalytic performance. Kudo et al. [17] continued the work in the field. They studied simultaneous production of carbon-loaded iron oxide and tar-free and reformed gas from low-grade natural iron ore and biomass. Recently, we have developed further the concept of biotar ironmaking to integrate the use of low- or high-grade coal [5, 7–9].

Many studies are reported to utilize high-temperature waste heat in steelworks. In particular, the high-temperature thermal energy of blast furnace slag is not always recovered despite its high potential energy [3]. Various other studies about the recovery of waste heat of slag were reviewed by Barati et al. [6]. Furthermore, it is well known that coke oven gas (COG) contains tar and that this tar causes clogging of the heat exchanger, making recovery of sensible heat from COG (over 1073 K) difficult. Therefore, the sensible heat of the COG and the chemical exergy of the tar are not utilized in conventional ironmaking. To recover such high-temperature waste heat, Akiyama et al. [3]. suggested a thermochemical method. They presented various theoretical endothermic reactions such as steam reforming and dry reforming reactions that could occur with the use of high-temperature wastes as a heat source; their results indicated that the thermal energy of high-temperature wastes could be recuperated as chemical energy of CO and H2, which would be available as reduction gas for ironmaking. Additionally, Kasai et al. [15] and Li et al. [18], Qin et al. [20] proposed processes of steam reforming of methane and coal gasification, respectively, using waste heat from molten slag.

In addition, it is important to design a waste heat recovery system for ironmaking that will reduce not only fossil fuel consumption but also raw material inputs, since the purpose of the steelmaking industry is to achieve the most efficient production of steel. However, there were few studies that focused on both viewpoints.

Given these circumstances, we propose a new system that simultaneously produces carbon-loaded iron oxide and reforms the coal tar and COG using low-grade iron ore, COG with tar, the sensible heat of the COG, and waste heat from slag. Figure 1 is a schematic diagram of the proposed system. First, limonite ore (which is low-grade iron ore with combined water) is dehydrated to obtain a mesoporous solid. Second, the tar contained in the COG is deposited during the chemical vapor infiltration (CVI), iron ore reduction, and reforming processes, leaving carbon in the mesopores of the ore. The ore is reduced by the COG and the tar is reformed by the combined sensible heat of the slag and COG, which acts as a heat source, with the ore as a catalyst. The proposed system is a type of thermochemical method from the viewpoint of waste heat recovery and can produce iron using the sensible heat of the COG and chemical exergy of the tar in combination with the limonite ore and waste heat of the slag. As a result, it could allow the realization of coproduction of carbon-loaded iron oxide and reformed COG and has great potential for reduction of the consumption of high-grade coal and ore.

Fig. 1
figure 1

Schematic diagram of the proposed system and problems in the conventional ironmaking process

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The purpose of this study is to estimate the feasibility of the proposed system by process simulation. The proposed process is analyzed on the basis of the following criteria: (1) the requirement for cokes, (2) the total chemical exergy, (3) the mass fraction of carbon deposited onto the carbon-loaded and prereduced iron ore, and (4) the supplies of heat from the coke oven system that are available to other systems under reasonable assumptions.

Details of System Analysis

We defined the boundary of the analysis to include the coke oven system and the ironmaking system. Conventional system: Case 1 and Proposed system: Case 2 including CVI process were analyzed.

Conventional System: Case 1

Figure 2 shows a process system diagram [14] for a conventional ironmaking system, without recovery of the tar, sensible heat of the COG, or waste heat of the slag. First, the coal is dry-distillated at about 1273 K and then hot coke is produced in the coke oven. Simultaneously, COG with tar is generated as a byproduct. Next, the sensible heat of the hot coke is recovered by coke dry quenching (CDQ) and, finally, the coke is supplied to the blast furnace. CDQ is effective for recovering waste heat and decreasing fossil fuel consumption. However, the hot COG with tar is quickly cooled and separated, so that the sensible heat of the COG and chemical exergy of the tar is wasted in conventional ironmaking systems.

Fig. 2
figure 2

Process system diagram of a conventional ironmaking system. Case 1: without recovery of the tar, sensible heat of the COG, and waste heat of the slag. Open circles processes; solid arrows flow of substances; dashed arrows flow of intermediary energy; dotted line system boundary

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Proposed System: Case 2

Figure 3 shows the process system diagram for the proposed ironmaking system, in which COG and dehydrated porous iron ore are provided for the CVI/Reduction/Reformer process, with hot slag as the heat source. The unique aspect of this ironmaking system is the use of the sensible heat of the COG, chemical exergy of the tar, limonite ore, and waste heat of the slag. First, limonite ore was dehydrated at 723 K [11] to obtain a mesoporous solid. The reaction proceeded as follows:

Fig. 3
figure 3

Process system diagram of the proposed ironmaking system, Case 2, in which COG and dehydrated, porous iron ore are provided for the CVI/reduction/reformer process, with hot slag as a heat source

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Dehydration of limonite

$$ 2{\text{FeOOH}} \to {\text{Fe}}_{2} {\text{O}}_{3} + {\text{H}}_{2} {\text{O}}\quad \Delta H \, = { 49}. 8 {\text{ kJ}}/ 1\;{\text{mol - Fe}}_{ 2} {\text{O}}_{ 3}\;{\text{at 723 K}} $$
(1)

Then, in the CVI/Reduction/Reforming process, the tar contained in the COG was deposited, leaving carbon in the mesopores of the ore. The ore was reduced by the COG, and the tar was reformed by the combined sensible heat of the slag and COG, which acted as a heat source, with the ore as a catalyst.

The main reactions in the process proceeded as follows:

Decomposition of tar

$$ {\text{Tar}} \to {\text{C}}\,{ + }\,{\text{C}}_{n} {\text{H}}_{m} + {\text{Other}}\,{\text{chemical}}\,{\text{species}} $$
(2)

Reduction of iron ore by reducing gas

$$ 1. 5 {\text{Fe}}_{ 2} {\text{O}}_{ 3} + {\text{ Reducing gas }} \to {\text{ Fe}}_{ 3} {\text{O}}_{ 4} + {\text{ a CO}}_{ 2} + {\text{ bH}}_{ 2} {\text{O}} $$
(3)

Reforming process

$$ a{\text{C}}_{n} {\text{H}}_{m} + b{\text{H}}_{2} {\text{O}} + c{\text{CO}}_{2} \to e{\text{CO}} + f{\text{H}}_{2} $$
(4)

The proposed system can use the sensible heat of the COG and chemical exergy of the tar in combination with the limonite ore and waste heat of the slag in an ironmaking system.

Calculation Method

We performed process simulations of the two cases under the following assumptions:

  1. (1)

    The numbered streams in Figs. 2 and 3 were explicitly analyzed. Table 1 [1, 12, 13] lists the initial operating data for the integrated steelworks assumed in the study. Here, Q coking was calculated based on coke being heated from 298 to 1273 K with 15 % heat loss. Q CDQ was calculated based on the sensible heat of coke recovered from a change from 1273 to 773 K. In the conventional system, the compositions of the COG were fixed in the ratio of COG to pig-iron of final product described in Table 1, and they were the same before and after the cooling and separation process. The temperatures of the COG and tar after the process were 298 K.

    Table 1 Initial operating data of the integrated steelworks assumed in the study
    Full size table
  2. (2)

    The compositions of the streams after the CVI process were equilibrium compositions that were determined using the Gibbs free energy minimization technique. This technique is based on the fact that the system is thermodynamically favorable when its total Gibbs free energy is at its minimum value and its differential is zero for a given temperature and pressure [4, 10]. The temperatures of the materials in each output stream were given as terminal temperatures, Tter, based on heat and mass balances. The following chemical species were considered: CH4, C2H4, C2H6, C6H6, C7H8, C8H10, C10H8, C6H6O, C20H12, C14H10, CO, CO2, H2, H2O, H2S, O2, N2, Fe2O3, and Fe3O4.

  3. (3)

    The components of the tar expected to form the carbon deposits were CH4, C2H4, C2H6, C6H6, C7H8, C8H10, C10H8, C6H6O, C20H12, C14H10, CO, CO2, H2, H2O, H2S, O2, and N2.

  4. (4)

    The deposited carbon was not considered in the equilibrium calculation, and direct reduction of ore by deposited carbon was not considered.

  5. (5)

    The chemical exergy and thermal exergy were considered. The total exergy of each stream was calculated by the following equation:

    $$ \varepsilon = \sum {n_{\text{i}} \varepsilon_{\text{c,i}} + \sum {n_{\text{i}} C_{\text{p,i}} \left( {T - T_{0} - T_{0} \,\ln \frac{T}{{T_{0} }}} \right)} } $$
    (5)

    where ε, n i, ε c,i, C p,i, T, and T 0 are total exergy of a stream, molar quantity of a chemical species, chemical exergy, specific heat, temperature, and reference temperature, respectively. Table 2 lists the standard chemical exergy of each chemical component.

    Table 2 Standard chemical exergy of each chemical component
    Full size table
  6. (6)

    The coke included 92 mass% of C and 8.0 mass% of ash. 1 mol of C can reduce 1.5 mol of O in iron ore, because (mol fraction of CO)/(mol fraction of CO2) of blast furnace gas is generally about 1.0.

  7. (7)

    1 kg of deposited carbon can reduce 1 kg of C, therefore, 1 kg of deposited carbon can reduce 25/23(=100/92) kg of coke.

  8. (8)

    To analyze and evaluate the systems, the following values were defined.

    1. (i)

      The mass of input porous iron ore: M ore,in

      M ore,in means the mass of input porous iron ore after dehydration of limonite ore. This analysis used M ore,in as a parameter.

    2. (ii)

      The ratio of deposited carbon: η C,dep

      $$ \eta_{\text{C,dep}} = \frac{{M_{\text{C,dep}} }}{{M_{\text{C,tar}} }} $$
      (6)

      where M C,dep and M C,tar are the total mass of deposited carbon and the total mass of carbon in the tar, respectively. In the analysis, η C,dep was set as a parameter

    3. (iii)

      The mass fraction of the deposited carbon on a carbon-loaded and prereduced iron ore, MFC,sup, was obtained as follows:

      $$ {\text{MF}}_{\text{C,sup}} = \frac{{M_{\text{C,dep}} }}{{M_{\text{ore,re}} + M_{\text{C,dep}} }} $$
      (7)

      where M ore,re is the total mass of prereduced iron ore. The sum of M ore,re and M C,dep, the denominator on the right side of the above equation, represents the total mass of carbon-loaded and prereduced iron ore. In the analysis, was an important value used to estimate whether or not the conditions of the proposed system were possible, by comparison with some reported value.

    4. (iv)

      The amount of coke required in the proposed system

      $$ \left( {M_{\text{coke}} \;{\text{in}}\;{\text{proposed}}\;{\text{system}}} \right) = \left( {M_{\text{coke}} \;{\text{in}}\;{\text{conventional}}\;{\text{system}}} \right) - \frac{25}{23}\left( {M_{\text{C,dep}} - \frac{1}{3}\frac{{m_{\text{w,C}} M_{{{\text{Fe}}_{ 3} {\text{O}}_{ 4} }} }}{{m_{{{\text{w,Fe}}_{ 3} {\text{O}}_{ 4} }} }}} \right), $$
      (8)

      where M coke, \( M_{{{\text{Fe}}_{ 3} {\text{O}}_{ 4} }} \), m w,C, \( m_{{{\text{w,Fe}}_{ 3} {\text{O}}_{ 4} }} \) are the amounts of coke required, mass of Fe3O4 after the CVI/reduction/reformer process, molar weight of C, and molar weight of Fe3O4, respectively. The second term on the right side reflects the reduction of coke by the deposited carbon based on the assumptions (6) and (7). The third term on the right side is the coke equivalent of producing Fe3O4 by reduction of Fe2O3, calculated on the assumptions (6) and (7), and the following equation:

      $$ \frac{3}{2}{\text{Fe}}_{ 2} {\text{O}}_{ 3} + \frac{1}{3}{\text{C}} \to {\text{Fe}}_{{_{ 3} }} {\text{O}}_{ 4} + \frac{1}{3}\left( {\frac{1}{2}{\text{CO}} + \frac{1}{2}{\text{CO}}_{ 2} } \right). $$
      (9)
    5. (v)

      Total chemical exergy: ε total

      In the conventional system:

      $$ \varepsilon_{\text{total}} = \varepsilon_{\text{COG}} $$
      (10)

      In the proposed system:

      $$ \varepsilon_{\text{total}} = \varepsilon_{\text{gas}} + \varepsilon_{\text{C,dep}} + \varepsilon_{\text{ore,re}} $$
      (11)

      where ε COG, ε gas, ε C,dep, and ε ore,re are the chemical exergy of the COG, reformed gas, deposited carbon, and reduced ore, respectively, after the CVI/reduction/reformer process.

    6. (vi)

      The available supply of heat from the coke oven system to the other systems, i.e., Q supply can be obtained as follows.

    In the conventional system:

    $$ O_{\text{supply}} = O_{\text{CDQ}} + O_{\text{COG}} - O_{\text{coking}} $$
    (12)

    In the proposed system:

    $$ Q_{\text{supply}} = Q_{\text{CDQ}} + Q_{\text{gas}} - Q_{\text{coking}} $$
    (13)

    where Q coking, Q CDQ, Q COG, and Q gas are the amount of heat required for coking and the available supplies of heat from the CDQ, the COG in the conventional process, and the reformed gas in the proposed process, respectively.

    The coke oven system is not only a coking system but also an energy supply system. Q supply in the proposed system must exceed the supply in the conventional system, because a smaller Q supply would cause an overall shortage of heat in the ironmaking system. Q supply strongly depends on the mass of coke produced per ton of pig-iron. Therefore, Q supply represents a constraint with respect to the effectiveness of the proposed system. The proposed system can be established only if the following condition is satisfied:

    $$ Q_{\text{supply}} \left( {\text{Proposed}} \right) \, \ge \, Q_{\text{supply}} \left( {\text{Conventional}} \right) $$
    (14)

Results and Discussion

Figure 4 shows the effects of M ore,inand η C,dep on amounts of (a) CH4, (b) H2, and (c) CO, and T ter of the reformed gas after the CVI/reduction/reformer process in the proposed system (Case 2). These parameters were most important in formulating an understanding of the system in terms of chemical reactions. T ter decreased as M ore,in increased, primarily because total heat contents of ore increased as M ore,in increased. The amount of CH4 had minimal value in each η C,dep, while amounts of H2 and CO had maximal value. These mean that steam reforming and dry reforming (such as in Eq. 4) occurred mainly for small values of M ore,in where reaction temperatures were high. Conversely, a methanation reaction occurred for large values of M ore,in where reaction temperatures were low. The methanation reaction can be described as follows.

Fig. 4
figure 4

Effects of M ore,in and η C,dep on amounts of a CH4, b H2, and c CO, and T ter of the reformed gas after the CVI/reduction/reformer process in the proposed system

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$$ 2 {\text{CO }} + {\text{ 2H}}_{ 2} \to {\text{CH}}_{ 4} + {\text{CO}}_{ 2} $$
(15)
$$ {\text{CO}} + 3{\text{H}}_{2} \to {\text{CH}}_{4} + {\text{H}}_{2} {\text{O}} $$
(16)

These results indicate that smaller values of M ore,in are advantageous for gas reforming.

Figure 5 illustrates the chemical exergy of gas, deposited carbon, and reduced ore for the production of 1 ton of coke. Here, M ore,in = 0 refers to the conventional process. Total chemical exergy in the proposed system was larger than that in the conventional system, although the values in each system were remarkably constant regardless of conditions. These results clearly show that the chemical exergy of coal can be used effectively by applying it in the proposed system. The chemical exergy of gas decreased as M ore,in and η C,dep increased, while the chemical exergy of the ore increased in proportion to increases in M ore,in. This is primarily because hematite in the ore was completely reduced to magnetite in the proposed system; as a result, the chemical exergy of the gas was converted to the chemical exergy of the magnetite. In the CVI/Reduction/Reformer process, the exergy of the waste heat of the COG and slag was regenerated with the reforming of tar, and part of the chemical exergy of the reducing gas was converted to the chemical exergy of the magnetite.

Fig. 5
figure 5

Chemical exergy of gas, carbon deposited on ore, and the reduced ore for production of 1 ton of coke under all conditions, calculated by the Gibbs free energy minimization technique and the heat and mass balances. M ore,in = 0 indicates the conventional process

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Figure 6 shows the effects of M ore,in and η C,dep on the M coke that was required to produce 1 ton of pig-iron in the proposed system. Clearly, the values under all conditions were lower than those for the conventional system. The values decreased in proportion to increases in both η C,dep and M ore,in. These decreases occurred because increases in η C,dep led directly to decreases in M coke and magnetite production increased in proportion to increases in M ore,in due to the reduction of hematite in the ore, as shown in Fig. 5. The minimum value of M coke in the analysis was 329 kg/ton-pig-iron, which was 14.5 % less than that required by the conventional system at η C,dep = 1.0 and M ore,in = 1300 kg. The results clearly show that the proposed system offers great potential for decreasing the amount of coke required to produce 1 ton of pig-iron.

Fig. 6
figure 6

Effects of M ore,in and η C,dep on M coke required to produce 1 ton of pig-iron in the proposed system (Case 2) with the value of the conventional system (Case 1) for comparison

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Figure 7 shows the effects of M ore,in and η C,dep on ε total per ton of pig-iron in the proposed system (Case 2) and the conventional process (Case 1). Values of ε total in the proposed system were larger than those for the conventional system under all conditions, even though M coke for the proposed system was smaller than for the conventional system. The exergy values decreased in proportion to increases of M ore,in and η C,dep. This occurred because M coke for the proposed system first decreased in proportion to increases in M ore,in and η C,dep and, thereafter, increases in M ore,in led to increases in the sensible heat of the ore and a decrease in the chemical exergy of the gas owing to a decrease in the heat for reforming. The maximum total chemical exergy per production of 1 ton pig-iron in the analysis was 5.00 GJ/ton-pig-iron, which was 14.2 % greater than that of the conventional system, at η C,dep = 0.20 and M ore,in = 100 kg.

Fig. 7
figure 7

Effects of M ore,in and η C,dep on ε total per ton of pig-iron in the proposed system (Case 2) with the value for the conventional process (Case 1) shown for comparison

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Figure 8 shows the effects of M ore,in and η C,dep on MFC,sup with a previously reported value for comparison [17]. The values were inversely proportional to M ore,in and increases in η C,dep. The results provided a minimum M ore,in value for each η C,dep by assessing whether the values were lower than previously reported. When the values were higher than the reported value, the system was classed as unrealistic because extremely high amounts of carbon were deposited and the values were impossible to achieve in real operations. The minimum More,in values for η C,dep = 0.20, 0.40, 0.60, 0.80, and 1.0 were 100, 108, 154, 200, and 246 kg, respectively. From the viewpoint of MFC,sup, the proposed system can be designed using a wide range of M ore,in for each η C,dep.

Fig. 8
figure 8

Effects of M ore,in and η C,dep on MFC,sup with a previously reported value [17] for comparison

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Figure 9 shows the effects of M ore,in and η C,dep on Q supply in the proposed system, also illustrating Q supply in the conventional process and the effect of dehydration on Q supply (Eq. 1). For each η C,dep, Q supply decreased in proportion to increases in M ore,in and η C,dep. These results can be explained by the decrease in M coke accompanying the increase in η C,dep and M ore,in, as shown in Fig. 6.

Fig. 9
figure 9

Effects of M ore,in and η C,dep on Q supply in the proposed system, shown with the value of Q supply for the conventional process and the effect of dehydration on Q supply (Eq. 1)

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The decrease in M coke for the proposed system indicates that the values for the following parameters per ton of pig-iron also decreased:

  1. (1)

    the generation of tar and COG,

  2. (2)

    the amount of heat required for coking, and

  3. (3)

    the amount of heat generated from CDQ.

This is because these values are proportional to the amount of coke required.

The maximum M ore,in for each η C,dep can be clarified, by checking whether the values were larger than the corresponding values for the conventional process (Eq. 14). From the viewpoint of Q supply, the maximum values of M ore,in for η C,dep = 0.20, 0.40, 0.60, 0.80, and 1.0 were 1300, 1162, 804, 438, and 0 kg, respectively. Furthermore, when the reaction heat of dehydration is considered, the maximum values of M ore,in for ηC,dep = 0.20, 0.40, 0.60, 0.80, and 1.0 were 808, 605, 419, 232, and 0 kg, respectively.

From Figs. 8 and 9, we can determine the range over which the proposed system would be effective, considering the value of M ore,in for each η C,dep.

Table 3 lists the ranges of M ore,in required to establish the proposed system—the minimum M coke and the maximum total chemical exergy, with or without the dehydration process. For η C,dep values between 0.20 and 0.80, the conditions necessary to establish the proposed system were satisfied. The ranges were wider for smaller values of η C,dep. The results indicate that dehydration processes are very important for establishing the proposed system. Since the temperature of dehydration is low, about 723 K, some exhaust heat can be applied without combustion of the reformed gas. The minimum M coke without considering dehydration was 348 kg, which was 9.62 % smaller than for the conventional system at M ore,in = 438 kg and η C,dep = 0.80. The minimum M coke for the proposed system considering dehydration was 351 kg, which was 8.78 % smaller than the conventional system, at M ore,in = 232 kg and η C,depo = 0.80. The maximum total chemical exergy was 5.00 GJ, which was 14.2 % larger than that for the conventional system, at M ore,in = 100 kg and η C,dep = 0.20, with or without consideration of dehydration.

Table 3 Range of M ore,in required to establish the proposed system, minimum coke requirement, and maximum total chemical exergy
Full size table

Conclusions

The purpose of this paper was to estimate the feasibility of the coproduction of carbon-loaded iron oxide and reformed COG using coal tar, limonite ore, and waste heat by process simulation, from the viewpoints of (1) the requirement for coke, M coke, (2) the total chemical exergy, ε total, (3) the mass fraction of deposited carbon on a carbon-loaded and prereduced iron ore, MFC,sup, and (4) available supplies of heat from the coke oven system to other systems, Q supply, under reasonable assumptions. The conclusions can be summarized as follows:

  1. (1)

    The proposed system can help reduce M coke: the values decreased in proportion to increases in ηC,dep and M ore,in.

  2. (2)

    The proposed system can help increase the total chemical exergy: the values decreased in proportion to increases in M ore,in and η C,dep.

  3. (3)

    Considering MFC,sup and Q supply as constraints, the required values of M ore,in were achieved when η C,dep values were between 0.20 and 0.80. The minimum M coke without considering dehydration was 348 kg, which was 9.62 % less than the corresponding value for the conventional system, at M ore,in = 438 kg and η C,dep = 0.80. The minimum M coke for the proposed system considering dehydration was 351 kg, which was 8.05 % less than the corresponding value for the conventional system, at M ore,in = 232 kg and η C,dep = 0.80. The maximum total chemical exergy was 5.00 GJ, which was 14.2 % higher than the corresponding value for the conventional process, at M ore,in = 158 kg and η C,dep = 0.20, with or without consideration of dehydration.

The results of the present work contribute in the sustainability of ironmaking process because the suggested process can be a simultaneous solution of resource and energy problems.