metals

Article
Development of Mass–Energy Balance Model Based on a New
Process of RSF with Hy-O-CR
Haifeng Li 1,2, * , Jingran Chen 2 , Zhiguo Luo 1,2 and Xiaoai Wang 3

1 Key Laboratory for Ecological Metallurgy of Multimetallic Mineral (Ministry of Education),

Northeastern University, Shenyang 110819, China
2 School of Metallurgy, Northeastern University, Shenyang 110819, China
3 HBIS Group Co., Ltd., Shijiazhuang 050023, China
* Correspondence: lihf@smm.neu.edu.cn

Abstract: At present, blast furnace (BF) ironmaking is still the main process for producing hot metal
in China and around the world. Under the constraint of the global goal of “double carbon”, it is
urgent to carry out hydrogen metallurgical innovation for the existing BF ironmaking process with
higher carbon emissions. In recent years, BF technology with hydrogen enrichment and pure oxygen
has made some progress, effectively reducing carbon emissions of hot metal per tons, but it is still
unable to break through the technical bottleneck of emission reduction of more than 30%. In view
of this, the authors put forward an ironmaking technology of a reduction smelting furnace (RSF)
that is hydrogen-rich and utilizes pure oxygen and carbon recycle (Hy-O-CR), which breaks through
the technical defect of traditional BF emission reduction of less than 30% by reshaping the furnace.
Firstly, the construction process of the mass and energy balance model for two main unit modules
in the new process (RSF with Hy-O-CR and top gas cycle) is introduced, and then the parameter
optimization under specific scenario conditions is analyzed, and the influence mechanism of several
key variables on the parameters in the furnace is obtained. Finally, the emission of CO2 in the whole
process is explored in the case of two typical operating parameters. The results show that after using
CCUS technology, the minimum value of direct CO2 emission is 215.93 kg/tHM, which is as high
as 84.58% compared with the traditional BF process. Even if the removed CO2 is counted in carbon
emissions, the minimum value of direct or indirect carbon emissions is 729.85 kg/tHM, and the
proportion of emission reduction can reach 47.87%. The research results show that the reconstruction
Citation: Li, H.; Chen, J.; Luo, Z.;
Wang, X. Development of
of Hy-O-CR technology can change the ratio of direct reduction and indirect reduction, which greatly
Mass–Energy Balance Model Based on breaks through the emission limit of the traditional BF and provides a new reference for hydrogen
a New Process of RSF with Hy-O-CR. metallurgy technology and a basis for further study of the optimization of RSF size.
Metals 2024, 14, 127. https://
doi.org/10.3390/met14010127 Keywords: mass–energy model; hydrogen metallurgy; top gas cycle; reduction smelting furnace;
CO2 emission
Academic Editors: Pasquale Cavaliere
and Antoni Roca

Received: 9 November 2023

Revised: 6 January 2024 1. Introduction
Accepted: 19 January 2024
Climate change is an urgent and formidable challenge facing human society today.
Published: 21 January 2024
Carbon dioxide (CO2 ), as an inevitable outcome of industrial society modernization, has
caused a serious greenhouse effect; therefore, reducing carbon-based gas emissions has
become a global goal. In September 2020, China’s president Xi Jinping proposed the “dual
Copyright: © 2024 by the authors.
carbon” strategy at the 75th United Nations General Assembly, which aims to reach the peak
Licensee MDPI, Basel, Switzerland. of CO2 emissions before 2030 and achieve carbon neutrality before 2060 [1,2]. At present,
This article is an open access article the carbon-emitting industries of China are making efforts to develop decarbonization
distributed under the terms and technologies, aiming to achieve carbon reduction transformation before the deadline set
conditions of the Creative Commons by the “dual carbon” targets. In this regard, the steel industry consumes approximately
Attribution (CC BY) license (https:// 13% of the national energy, while its carbon emissions account for around 15% of the total
creativecommons.org/licenses/by/ national emissions. Therefore, as a high-energy and high-emission sector, the steel industry
4.0/). urgently needs to undergo green and low-carbon transformation [3–5].

Metals 2024, 14, 127. https://doi.org/10.3390/met14010127 https://www.mdpi.com/journal/metals

Metals 2024, 14, 127 2 of 18

The blast furnace (BF) and basic oxygen furnace (BOF) process is still the main process
of iron and steel production in China, in which more than 90% of the CO2 emission and
70% of the energy consumption are from the BF ironmaking process (sintering, coking,
and BF). The entire process relies on reducing gas CO generated by coke for the reduction
of iron oxides, and the resulting CO2 is directly emitted into the atmosphere. Therefore,
implementing low-carbon transformation in the BF ironmaking process becomes a key
step in reducing emissions in the steel industry [6–9]. The traditional BF, due to its ad-
vantages such as high production capacity, efficiency, low cost, and mature technology,
will remain the mainstream steel production process for a considerable period of time in
the future. After extensive research, metallurgists believe that “hydrogen metallurgy”,
which utilizes H2 for reduction, is the primary direction for green and low-carbon devel-
opment in the steel industry. It is expected that the proportion of hydrogen metallurgy
will significantly increase year by year before 2060, becoming the mainstream process for
reducing CO2 emissions [10,11]. Currently, many steel companies both domestically and
internationally have made arrangements for hydrogen metallurgy, forming two feasible
technical pathways: the short-process route of green hydrogen direct reduction with an
electric arc furnace (EAF) and the long-process route of conventional BF reconstitution
with hydrogen. Among them, the EAF process utilizing scrap steel or direct reducing iron
(DRI) produced by hydrogen-based process as raw materials seems to be the most mature
technology route to date, capable of reducing 95% of CO2 emissions. However, it is limited
by scarce high-grade iron ore resources [12]. Considering the existing stock of BFs, it is
anticipated that BF ironmaking will remain the primary ironmaking method in China in
the short term. The hydrogen-rich ironmaking process in BFs has become more mature
through continuous research, but there are significant differences between hydrogen-based
and carbon-based gas reduction conditions. The hydrogen content in BFs remains at a
low level, not exceeding 15%, greatly limiting the extent of CO2 emission reduction in
BFs [11]. Exploring new methods of a high proportion of hydrogen to break through the
CO2 emission limit requires technical reconstruction of the hydrogen-rich process and
proposing new plans to provide theoretical and technical support for achieving the ultimate
emission reduction in ironmaking.
Northeastern University has studied the key factors limiting emissions reduction in
BFs and proposed a new ironmaking process of a reduction smelting furnace (RSF) that is
hydrogen-rich and utilizes pure oxygen and carbon recycle (Hy-O-CR) [13]. They have also
developed a mass–energy balance model for the new process in a previously published
paper of Ref. [14]. By upgrading technologies such as injecting high-temperature reducing
gas through the upper shaft tuyere and cooperating to control the gas distribution ratio
between the shaft zone and the hearth zone, the technical bottleneck that the metallization
rate in the upper part of the traditional BF is less than 70% has been solved. The two
technologies of decarbonization of the top gas and hydrogen production by electrolysis
of water are reconstructed to form a new technology of hydrogen-rich self-circulation of
decarbonized gas. This technology involves decarbonized gas and green hydrogen being
mixed, heated, and injected into RSF through a specific tuyere structure, which is a new
technique that can significantly reduce carbon emissions in the ironmaking process.
The process of RSF with Hy-O-CR is a transformation and upgrade based on processes
such as Fink [15], NKK [16], ULCOS top gas recycling blast furnace process [17], and shaft
injection blast furnace [18–20]. The RSF process of full oxygen injection instead of the
BF process with hot air injection can avoid the problem of nitrogen (N2 ) accumulation
in the top gas circulation process cycle. The oxygen blast furnace (OBF) process often
encounters the problem of excessive cooling in the upper zone [21,22]. Although injecting
a high proportion of hydrogen-based gas from the shaft tuyere can significantly reduce
carbon emissions, the endothermic reaction during the reduction with hydrogen-based
gas exacerbates this problem [23]. Mehdi [24] addressed the issue of insufficient heat in
the upper zone by adding auxiliary fuel combustion to release heat. Additionally, high-
temperature reducing gases were injected into the furnace shaft to promote the reduction of
Metals 2024, 14, 127 3 of 18

FeO. The authors proposed a new process that also utilizes the injecting of a high-proportion
hydrogen-based gas into the furnace shaft to achieve maximum emission reduction. The
issue of insufficient heat in the upper zone was solved by excess high-temperature gas
being injected into the furnace shaft, which significantly increased the metallization rate
in the upper zone and minimized direct reduction in the lower zone. The heat generated
from the combustion of auxiliary fuel sources (such as natural gas, coke oven gas, coal
or coke, etc.) was sufficient to meet its internal heat consumption for direct reduction in
the lower zone, and the excess gas flowed upstream and carried the waste heat from the
lower zone to the upper furnace shaft area. This inevitably led to a decrease in the gas
utilization rate inside the furnace, but the gas generated from the combustion of auxiliary
fuel can be fully utilized by means of the top gas circulation, which improves the overall
gas utilization.
The top gas circulation model is a sub-module of the new RSF process. It is a new
technology for hydrogen-rich recycling of the top gas treated by CO2 removal equipment.
The main components of the circulating gas are CO and H2 . Considering the carburization
phenomenon of the mixture of CO and H2 in the heating process, the circulating gas process
has been modified and upgraded in order to better fit the new process. Mapelli C [25]
researched the future scenarios for reducing emissions and consumption in the Italian
steelmaking industry. The amount of CO2 emissions, water and electricity consumption,
and soil exploitation of the main steel production routes (integrated cycle, scrap recycling,
and direct reduction) were analyzed applying three possible future scenarios: use of
carbon capture and storage (CCS); use of green hydrogen in substitution of natural gas;
and use of biomethane. Foo S Y [26] proposed that introducing a certain amount of
CO2 can suppress the carburization reaction of CO and H2 in the furnace. Therefore, in
this model, the CO2 removal rate of the top gas is not 100%, and the remaining CO2 is
introduced into the furnace along with CO and H2 . This not only inhibits the carburization
phenomenon of the circulating gas but also reduces the decarburization cost. By combining
CO2 capture, utilization, and storage technologies (CCUS), the overall process achieves
maximum emission reduction. In addition, in order to fully utilize the chemical heat of the
top gas, a certain proportion of the top gas is used as the combustion gas source to heat and
elevate the temperature of the circulating reducing gas injected into the furnace shaft. The
electrolytic water device is used to supply the required H2 for the hydrogen-rich process
of reducing gas, and the byproduct oxygen (O2 ) can also serve as a direct source of O2
injected through the lower tuyere into the RSF [27]. Although the power consumption and
cost of the process flow after using electrolytic water for hydrogen production are higher
compared to traditional BFs, with the reduction of the cost of hydrogen production, the
optimization of the process, and the development of CCUS technology in the future, the
new process will have a very broad development prospect in the future low-carbon era.

2. Model of RSF with Hy-O-CR

The schematic diagram of the new process of RSF with Hy-O-CR is shown in Figure 1.
The top gas produced in the RSF is purified by a dust collector and then divided into two
parts, of which a small part of the gas is used as a combustion gas source for combustion
and heating, and most of the gas goes into the dryer for H2 O removal and the CO2 separator
for CO2 separation. The removed H2 O can be recycled and used as a water source for the
electrolytic water process. The extracted CO2 enters a CO2 storage tank for enhanced oil
recovery or chemical raw materials. The resulting reducing gas is divided into two parts for
further treatment, a small part of which is directly stored in the gas tank without heating as
a backup gas source. In addition, most of the reducing gas (CO, H2 , and a small amount of
CO2 ) is mixed with a certain amount of hydrogen, enters into the heating exchanger, and is
heated to form high-temperature (>900 ◦ C) reducing gas, which is injected into the RSF from
the furnace shaft tuyere to achieve decarbonization reduction gas recycling. The reason for
not fully removing CO2 is that the inclusion of a certain amount of CO2 effectively prevents
carburization reactions during the heating process [28,29]. The electrolytic water device
 is injected into the RSF from the furnace shaft tuyere to achieve decarbonization reduction
Metals 2024, 14, 127 gas recycling. The reason for not fully removing CO2 is that the inclusion of a certain
4 of 18
amount of CO2 effectively prevents carburization reactions during the heating process
[28,29]. The electrolytic water device supplies sufficient H2, which is also supplemented
into the heat exchanger. The byproduct O2 can be injected into the lower tuyere for full
supplies sufficient H2 , which is also supplemented into the heat exchanger. The byproduct
oxygen
O can be injecting.
injected into the lower tuyere for full oxygen injecting.
2

Figure 1. Schematic diagram of the new process of RSF with Hy-O-CR.

Figure 1. Schematic diagram of the new process of RSF with Hy-O-CR.

The RSF is
The RSF is aaphysical
physicaland andchemical
chemicalreaction
reactionprocess
process conducted
conducted in ainclosed
a closed
andand com-
complex
plex system, and it is impossible to obtain key process parameters
system, and it is impossible to obtain key process parameters inside the furnace through inside the furnace
through measurements.
measurements. Therefore, Therefore,
constructingconstructing a mathematical
a mathematical model formodel
process forcalculation
process calcu- can
lation can quantitatively
quantitatively obtain the
obtain the changes changes
in mass andin mass inside
energy and energy
the RSF,inside the RSF,
providing providing
data support
data support for
for subsequent subsequent
parameter parametercapacity
optimization, optimization, capacity
expansion, expansion,
or increasing or increasing
the CO 2 emission
the CO
reduction limit. A reaction model of RSF was established based on the principle ofon
2 emission reduction limit. A reaction model of RSF was established based the
mass
principle of mass and energy conservation, with the production per
and energy conservation, with the production per ton of hot metal (HM or molten iron) ton of hot metal (HM
or
as molten iron) asThe
the reference. themodel
reference. The model
obtained obtained
the mass of rawthe mass ofconsumed
materials raw materials consumed
per ton of HM,
per ton of HM,
the volume and the volume and
composition composition
of reducing of reducing
gas, the mass andgas, the mass and
composition composition
of corresponding
of corresponding
by-products such as by-products
slag, and the such as slag,
volume and
and the volume
composition ofand composition
the top gas. of the top
gas. The process of establishing the mathematical model and its calculation process for the
The be
RSF will process
describedof establishing the mathematical
in the following model andmathematical
section. The established its calculationmodel processcanforbe
the RSF will be described in the following section. The established
used to analyze and determine the volume of reducing gas and top gas required per ton of mathematical model
can
HMbe in used to analyze
the RSF, providingand input
determine the volume
and output of reducing
conditions gastop
for the andgastopcirculation
gas requiredmodel per
ton
andof HM in the
facilitating theRSF, providing
selection input anddecarburization
of subsequent output conditions for the top
equipment gas circulation
capacity on-site.
model and facilitating the selection of subsequent decarburization equipment capacity on-
2.1. Boundary Conditions of RSF with Hy-O-CR Model
site.
The compositions of the iron-containing raw materials, namely, coke and flux in this
2.1.
paper,Boundary Conditions
are listed in Tables of RSF
1–3. with
The Hy-O-CR
sulfur and Model
silicon compositions in the hot metal are
set according to conventional
The compositions values, as shown
of the iron-containing rawinmaterials,
Table 4. The othercoke
namely, components
and flux in inthis
the
HM are obtained based on empirical formulas. The formula for the
paper, are listed in Tables 1–3. The sulfur and silicon compositions in the hot metal are set carbon content is
[C] = 1.34 + 2.54 × 10 −3 × T − 0.35[P] − 0.30[Si] − 0.54[S] + 0.04[Mn], where T
HM
according to conventional values, as shown in Table 4. The other components in the HM HM
represents
are obtained thebased
temperature
on empirical metal in ◦The
of hotformulas. C and the element
formula for the symbols ([P], [Si],
carbon content [S], =[Mn])
is [C] 1.34
represent their content in hot metal in %.
+ 2.54 × 10−3 × THM – 0.35[P] – 0.30[Si] – 0.54[S] + 0.04[Mn], where THM represents the tem-
perature of hot metal in °C and the element symbols ([P], [Si], [S], [Mn]) represent their
Table 1. Composition of iron ore raw materials (mass fraction), %.
content in hot metal in %.
Composition TFe FeO CaO MgO SiO2 Al2 O3 MnO P2 O5 TiO2
Sinter 57.45 8.64 9.37 1.41 5.26 1.92 0.21 0.12 0.11
Pellet 67.30 0.52 0.32 0.69 2.40 0.47 0.09 0.03 0.26
Lump 62.80 0.38 0.10 0.90 3.48 1.58 0.07 0.13 0.06
Metals 2024, 14, 127 5 of 18

Table 2. Composition of coke (mass fraction), %.

Ash Volatile
Composition H2 O C(Fix)
Fe2 O3 CaO MgO SiO2 Al2 O3 H2 CO2 N2 CH4
Coke 0.43 87.3 0.76 0.45 0.11 5.32 4.03 0.12 0.28 0.64 0.02

Table 3. Composition of flux (mass fraction), %.

Composition TFe CaO MgO SiO2 Al2 O3 MnO SO3 P2 O5 CO2

Dolomite 0.14 30.54 20.06 3.52 0.22 0.26 0.02 0 46.06
Limestone 0 55.36 0.90 1.00 0.14 0 0.10 0 44.49
Silica 0 0.34 0.08 94.95 2.31 0 0.10 0.02 0.36

Table 4. Composition of hot metal (mass fraction), %.

Composition Si S
Mass fraction in hot metal, % 0.5 0.025

In addition to the boundary conditions above, referring to the relevant previous

work [30–36], the process parameters are assumed based on the calculation results of a
hydrogen-rich blast furnace, hydrogen-based shaft furnace, and so on. The following
assumptions are used in the model:
(1) Within the RSF, a gas temperature of 1000 ◦ C is used as the boundary between the
upper and lower zone, which are divided into the upper low-temperature reduction
zone and the lower high-temperature smelting zone, assuming that the boundary
temperature for the solid burden entering the lower high-temperature zone is 950 ◦ C,
and the boundary temperature for gas entering the upper low-temperature zone
is 1000 ◦ C.
(2) Sulfur in the RSF goes into the top gas, hot metal, and slag. The distribution ratio of
sulfur in the top gas (ω s_top ) is 5% [30], the content of sulfur in the hot metal is 0.025%,
and the others enter the slag.
(3) The methane production ratio (φCH4 ) is 1% of the total carbon consumption in the
lower zone [31].
(4) The temperature of the hot metal and slag are set to 1500 ◦ C and 1550 ◦ C, respectively.
(5) In order to ensure the fluidity of the slag, the binary basicity (R2 ) of the slag is set
to 1.15 [30,31].
(6) According to the operation experience of the shaft furnace, the reduction potential (E)
of the top gas is not less than 1.22 [32].
Under the constraints of the aforementioned boundary and assumption conditions,
the model investigated the impact of different top gas temperatures (Ttop_gas ), metallization
rates [33,34] (MR) in the bottom of the shaft, water–gas reaction rates [35,36] (φwater_gas ),
and composition (φCO_redu ) and temperature (Tredu ) of the reducing gas on key parameters
of the RSF process.

2.2. Mass Balance

According to the corresponding parameter settings under different operating condi-
tions, based on the production of 1 ton of hot metal, the initial value of the coke mass is
set. Through element mass balance in the RSF, the consumption of raw materials, mass of
the slag, and composition and flow rate of gas in different parts of the RSF, as well as the
flow rate of hot air can be obtained. The input parameters of the overall model for the RSF
include iron ore, coke, flux, and the composition of reducing gas entering the RSF shaft
tuyere. The output parameters include the consumption of raw materials (ore, flux, and gas
circulation) for the production of 1 ton of hot metal, as well as the mass and composition
of the top gas and slag produced. Mass balance calculations can quantitatively analyze
the impact of operational parameters on the mass of coke consumption and important
Metals 2024, 14, 127 6 of 18

indicators, such as CO2 emissions, while providing basic data for heat balance calculations.
The coke consumption can be obtained through iterative calculations based on heat balance.

2.2.1. Mass of Iron Ore Consumption

Through the Fe element balance equation corresponding to the input material and the
output product, it is possible to derive the consumption of iron ore. The main input sources
of Fe element are iron ore, with a small amount or partial contribution from coke and flux;
the output form of the Fe element is mainly in the form of HM, with a small amount or
partial distribution in the slag or the dust of the top gas. Taking the production of 1 ton of
HM as a reference, the consumption of iron ore, denoted as more , is calculated according to
Formula (1).
∑ mFe_i − mFe_ j
more = (1)
ωFe_ore
112
mFe_coke = mFe2 O3 _coke × (2)
160
where i represents HM, slag, and dust; j represents coke and flux; mFe_i and mFe_j represent
the mass of Fe in the HM, slag, dust, coke, and flux, kg/tHM; ω Fe_ore represents the
mass fraction of Fe in mixed iron ore, %; and mFe2 O3 _coke represents the mass of Fe2 O3 in
coke, kg/tHM.

2.2.2. Mass of Flux Consumption

From the element conservation of oxide CaO and SiO2 and the binary basicity equation
of slag, it is possible to derive the amount of flux consumption. Slag is a byproduct of
the ironmaking process, and its basicity affects its fluidity, thereby affecting the smooth
operation of the RSF. The basicity of the slag in this model is represented by the binary
basicity (R2 ). The CaO and SiO2 in the slag mainly come from the flux, with a small amount
coming from coke and raw materials. A small amount of SiO2 in the output product is
reduced by direct reduction and is distributed in hot metal in the form of [Si]. The mass of
the flux can be derived according to Formula (3).

∑ mi × ωCaO_i
60
= R2 (3)
∑ mi × ωSiO2 _i − 1000 × ωSi_HM × 28

where i represents ore, coke, and flux; mi represents the mass of the iron ore, coke, and
flux, kg/tHM; ω CaO_i and ω SiO2 _i represent the mass fraction of CaO and SiO2 in the input
burden of iron ore, coke, and flux, %; and ω Si_HM represents the mass fraction of Si in the
hot metal, %.

2.3. Energy Balance

Taking the gas temperature in the shaft at 1000 ◦ C as the limiting condition, the
reduction zone is divided into the upper low-temperature reduction zone and the lower
high-temperature reduction zone, and the energy balance of the high-temperature zone and
the low-temperature zone are calculated respectively. By establishing the energy balance
equation for the lower high-temperature zone, the consumption of coke can be solved to
meet the minimum heat requirements for direct reduction in the lower zone and the heat
required for slag-iron melting. Through the establishment of the energy balance equation
in the upper low-temperature region, the amount of reducing gas and the utilization rate of
gas needed in the iron ore reduction process are calculated according to the thermodynamic
equilibrium constant, and through the double constraints of heat demand and minimum
reduction potential requirements, the amount of shaft reduction gas that needs to be added
in the reduction process can be determined. It should be noted that the decisive temperature
of the thermodynamic equilibrium constant is 950 ◦ C for the solid burden entering the
lower high-temperature zone. By using the step-by-step reduction method, the reduction
Metals 2024, 14, 127 7 of 18

zone in the upper part of RSF is divided into three stages, and the main chemical reactions
and heat changes are shown in Table 5.
From Table 5, it can be seen that in the reduction process of iron oxides by H2 in
the upper zone, except for the reaction 3Fe2 O3 + H2 = 2Fe3 O4 + H2 O, the other reactions
are endothermic. If the proportion of H2 is too high, it will inevitably cause a problem
of insufficient heat in the upper zone. Therefore, the authors used the combustion of
auxiliary fuel (coke was used in the basic case) in the lower zone to solve this problem, and
the supplementary combustion fuel only provides heat. The minimum consumption of
coke was obtained by solving the energy balance equation of the high-temperature zone.
Under the double constraints of heat balance and minimum reduction potential of the
top gas (E > 1.22) [30], the total amount of reducing gas injected into the shaft tuyere that
meets the conditions above was calculated for the upper low-temperature zone. Finally, the
composition and flow rate of the top gas were obtained through the C-H-O element balance.

Table 5. Chemical equation and ∆H (kJ/mol).

Chemical Reactions ∆H Chemical Reactions ∆H

3Fe2 O3 + H2 = 2Fe3 O4 + H2 O −2.07 3Fe2 O3 + CO = 2Fe3 O4 + CO2 −43.23
Fe3 O4 + H2 = 3FeO + H2 O 60.45 Fe3 O4 + CO = 3FeO + CO2 19.29
FeO+H2 = Fe + H2 O 30.23 FeO + CO = Fe + CO2 −10.93
CaCO3 = CaO + CO2 178.87 MgCO3 = MgO + CO2 116.94
C + 1/2O2 = CO −110.54 H2 O(l) = H2 O(g) 44.77

2.3.1. Calculation of Coke Ratio

In order to compensate for the insufficient heat in the upper zone during reduction,
the process adopts two optimization methods: injecting high-temperature reducing gas
into the shaft tuyere and utilizing the high-temperature gas generated by coke combustion
in the lower zone to achieve counterflow heat transfer. The heat matching issue between
the upper and lower zones is achieved by adjusting two key operational parameters: the
coke ratio and reducing gas volume. The coke ratio, represented by mcoke , is divided into
two parts: the mass of coke involved in the direct reduction process (mredu_coke ), the mass
of coke for hot metal carburization, and the mass of coke used for combustion heating
(mburn_coke ). The mass of coke is obtained by heat balance of the high-temperature zone.
The heat input terms (Qin_low ) of the high-temperature zone include the heat generated by
coke combustion at the tuyere (Qburn_coke ), the sensible heat brought into the lower zone by
high-temperature solid materials of coke (Qcoke ) and DRI (QDRI ). The heat output terms
(Qout_low ) include the heat carried away by hot metal (QHM ) and slag (Qslag ) leaving the
lower zone, the heat carried away by gas rising into the upper zone (Qgas_low ), the heat
consumed by the direct reduction of elements such as Fe, Si, and Mn in the lower zone
(Qredu_low ), and the heat loss in the lower zone (Qloss_low ).
The mass of coke consumption (mcoke ) is calculated according to Formulas (4)–(6).

Qin_low = Qburn_coke + Qcoke + QDRI (4)

QDRI = more + mflux − mLoss_O_up × qDRI (5)

Qout_low = QHM + Qslag + Qgas_low + Qredu_low + Qloss_low (6)

where Qburn_coke represents the combustion heat of coke in high temperature zone, MJ/tHM;
mloss_O_up represents the losing O amount of iron ore in the upper zone after indirect
reduction, kg/tHM; and qDRI represents the specific heat capacity of DRI or sponge iron in
the high-temperature (950 ◦ C) zone, MJ/kg.

2.3.2. Calculation of Reducing Gas Volume

The upper zone of the RSF needs to satisfy two constraints, namely, the heat balance
and minimum reduction potential of the top gas. The reasonable flow rate of the reducing
Metals 2024, 14, 127 8 of 18

gas is obtained under the two constraints above, in which the composition of the reducing
gas is CO:H2 = 3:7 in the basic case. The following points need to be explained. In view of
the successful experience of MIDREX, the authors suggest that the CO:H2 of the reducing
gas composition should be set to 3:7, which is only a reference case, and the effects of
different components on the RSF process in the following part will be analyzed. The heat
input terms (Qin_up ) in the upper zone include the heat released by Fe2 O3 through CO
and H2 reduction (QCO_Fe2 O3 , QH2 _Fe2 O3 ), the heat released by FeO through CO reduction
(QCO_FeO ), the sensible heat of high-temperature reducing gases injected into the shaft
tuyere (Qgas_shaft ), the heat released by the water–gas reaction (QH2 O_CO ), and the phys-
ical heat carried by the counterflow rising gas from the lower zone (Qgas_low ). The heat
output terms (Qout_up ) in the upper zone include the heat absorbed by FeO through H2
reduction (QH2 _FeO ), the heat absorbed by Fe3 O4 through CO and H2 reduction (QCO_Fe3 O4 ,
QH2 _Fe3 O4 ), the heat carried by coke and DRI entering the lower zone (Qcoke , QDRI ), the heat
carried away by the top gas (Qgas_top ), the heat absorbed from carbonate decomposition
(QCaCO3 _MgCO3 ), and the heat loss in the upper zone (Qloss_up ).
The required reducing gas volume is calculated by Formulas (7)–(9).

Qin_up = Qi + Qgas_shaft + Qgas_low (7)

Qout_up = Q j + Qcoke + QDRI + Qgas_top + Qloss_up (8)

VCO_top + VH2 _top
=E (9)
VCO2 _top + VH2 O_top
where i represents CO_Fe2 O3 , H2 _Fe2 O3 , CO_FeO, and H2 O_CO; j represents H2 _FeO,
CO_Fe3 O4 , H2 _Fe3 O4 , CaCO3 , and MgCO3 ; and V CO_top , V H2 _top , V CO2 _top , and V H2 O_top
represent the volume of CO, H2 , CO2 , and H2 O, respectively, in the top gas, Nm3 .

2.4. Calculation Flow Chart of the RSF Model

Under the same boundary conditions, the model studies the effects of process parame-
ters such as the temperature of the top gas, MR, water–gas conversion rate, reducing gas
temperature, and composition (the ratio of CO:H2 ). The program calculation flowchart
is shown in Figure 2. According to the constraint conditions of the model, the results
satisfying the process balance of the whole process under different operation parameters
are calculated, and through iterative calculations, the results of the mass and heat balance
for the low-temperature and high-temperature zones inside the RSF are obtained.
The steps of the program calculation process are as follows:
First of all, the mass of iron ore and flux is obtained by a mass balance equation
and basicity equation. The iterative calculation is carried out under the constraints of the
high-temperature zone heat balance, low-temperature zone heat balance, and the minimum
reduction potential of the top gas, and the values of the operating variables, such as coke
consumption and reducing gas volume injected from the shaft tuyere, are obtained.
Secondly, the mass and composition of the slag produced by the RSF are obtained
by calculating the incoming material through the mass conservation of the element of the
raw material.
Finally, under the limitation of the metallization rate and water–gas reaction ratio, the
gas composition and volume of different stages of the process of iron ore reduction are
deduced, and the calculation results are preserved in the form of the total volume of the
top gas and the proportion of each component. In addition, the model can also obtain the
oxygen consumption needed for combustion in the lower high-temperature area, which
provides data support for the follow-up top gas cycle model.
 Finally, under the limitation of the metallization rate and water–gas reaction ratio,
the gas composition and volume of different stages of the process of iron ore reduction
are deduced, and the calculation results are preserved in the form of the total volume of
the top gas and the proportion of each component. In addition, the model can also obtain
Metals 2024, 14, 127
the oxygen consumption needed for combustion in the lower high-temperature 9area, of 18
which provides data support for the follow-up top gas cycle model.

Start

Operating parameters: Tgas_top,

Tredu, MR, φwater_gas, φCO_redu
Energy balance
in upper zone mcoke

Energy balance
Vredu
in lower zone

Calculation of flux amount (mflux)

Calculation of ore consumption (more)

No
|Qin_low − Qout_low| <= 10−2

Yes
The energy balance in the upper area and the limitation of
gas reduction potential (E) on the RSF top gas

max{E >= 1.22, |Qin_up − Qout_up| <= 10−2} No

Yes
Calculation of mslag, Vgas_top

End

Figure2.2. Program
Figure Programcalculation
calculationflow
flowchart.
chart.

3.
3. Model
Model of of Top
TopGasGasRecycling
Recycling
The
The carbon recycling of
carbon recycling of the
the top
top gas
gas can
can reduce
reduce the theconsumption
consumption of of fossil
fossil fuels
fuels andand
help to achieve carbon neutrality around the world as soon as possible.
help to achieve carbon neutrality around the world as soon as possible. In the traditional In the traditional
BF
BF ironmaking
ironmaking process,
process, hot
hot air
air isis injected
injected into
into the
the lower
lower tuyere
tuyere zone,
zone, resulting
resulting in in aa high
high
concentration of N (vol. > 45%) in the top gas. The circulation of
concentration of N22 (vol. > 45%) in the top gas. The circulation of gas inevitably leadsgas inevitably leadsto
to
N2Naccumulation
2 accumulation issues.
issues. Therefore,
Therefore, this study
this study proposesproposes
a newaironmaking
new ironmaking
processprocess
reactor
reactor
called thecalled the
RSF, RSF, which
which adoptsadopts
a fully aoxygen
fully oxygen
injectinginjecting
designdesign
to avoidto or
avoid or eliminate
eliminate the N2
the N accumulation issues. The top gas is divided into two parts:
accumulation issues. The top gas is divided into two parts: one part is used as a combus-
2 one part is used
as a combustion
tion gas source for gasheating
sourcethefordecarburization
heating the decarburization
gas, while thegas, otherwhile
part the other
serves as apart
gas
serves as a gas source for generating decarburization gas. As a gas source
source for generating decarburization gas. As a gas source for producing decarburization for producing
decarburization gas, it needs
gas, it needs to undergo to undergo
purification purification
treatment due to the treatment
presencedue to the presence
of non-reducible of
gases
non-reducible gases (such as H 2
(such as H2O and CO2) in its composition. O and CO ) in its composition. The H O
2 H2O and CO2 are removed2 through a dryer
The and CO 2 are
removed through a dryer and a CO 2 separator device (pressure swing
and a CO2 separator device (pressure swing adsorption unit), respectively. The removed adsorption unit),
respectively. The removed H2 O can be recycled as a water source of an electrolytic device,
H2O can be recycled as a water source of an electrolytic device, and the removed CO2 can
and the removed CO2 can be utilized as a gas source for oil recovery or as a raw material
be utilized as a gas source for oil recovery or as a raw material for chemical products. The
for chemical products. The remaining decarburization gas, which has a high reduction
remaining decarburization gas, which has a high reduction potential (vol. (CO + H2) >
potential (vol. (CO + H2 ) > 95%), is reheated and re-injected into the furnace through a
specific tuyere for recycling. The gas treatment process described above can maximize the
use of gas resources and improve the overall energy efficiency of the process.

3.1. Boundary Conditions of Top Gas Recycling Model

The initial composition of the circulating gas source is obtained by real-time iterative
updating of the top gas composition obtained from the RSF process calculation model. The
following assumptions were made during the model establishment process.
(1) The heat loss of the top gas is neglected when it is cleaned and recycled.
Metals 2024, 14, 127 10 of 18

(2) The dedusting rate of the dust collator is set at 100%, and the dewatering rate of the
dryer is set at 100%.
(3) The adsorption rate of CO2 separator is 95%, and it has no effect on the adsorption of
other gas components.
(4) The heat exchange rate of the heat exchanger is set at 60%.

3.2. Calculation of the Top Gas Recycling Model

In order to give full play to the reduction effect of decarbonized gas in the RSF, it is
necessary to raise its temperature to 950 ◦ C, which can be achieved by electric heating or
combustion heat exchange. However, using electric heating would increase the overall
energy consumption of the system. Therefore, this model uses the method of burning
part of the top gas for heating in order to improve the thermal efficiency of the system.
The amount of gas required for combustion is related to the heat required for raising the
temperature of the decarburization gas and can be calculated using Equation (10).

Vi_redu × qi
Vburn = (10)
ω j_top × Q j

where Vi_redu represents the volume of i component of the reducing gas, Nm3 /tHM;
i represents CO, CO2 , or H2 ; qi represents the sensible heat per unit volume i at 950 ◦ C,
MJ/Nm3 ; ω j _top represents the molar amount of j, mol/tHM; j represents CO or H2 ; and Qj
represents the heat released by the combustion of H2 or CO, MJ/mol.

4. Results and Discussion

The establishment of a comprehensive mathematical model enables the understanding
of quantitative relationships between various parameter variables, facilitating the adjust-
ment of input parameters to improve the operational performance indicators of the RSF.
Under specific boundary conditions, this study utilizes the developed mathematical model
to analyze in-depth the effects of different variables on key smelting parameters inside
the RSF. These include the temperature and composition (CO:H2 ) of the decarburization
circulating reducing gas injected from the RSF shaft tuyere, which impact process energy
consumption and carbon emissions; the variation in the metallization rate of iron ore
affecting the flow rate of decarburization circulating reducing gas and the coke ratio; and
the influence of the water–gas reaction ratio parameters on the utilization rates of CO and
H2 in the top gas. In addition, the carbon and hydrogen footprints can be clearly drawn
through the model in this paper, which provides a direction for the further optimization of
the process.

4.1. Effect of Decarbonization Reducing Gas on Process Energy Consumption and Carbon Emissions
The composition and temperature of the reducing gas injected from the RSF shaft
tuyere have a significant impact on the thermal balance inside the RSF and are also one
of the key factors in optimizing the RSF with the Hy-O-CR process. The total energy
consumption of the RSF can be obtained from the overall process heat balance model,
which includes the carbon oxidation heat, high-temperature reducing gas sensible heat,
hydrogen oxidation heat, methane generation heat, and slag formation heat. The model
analyzes the effects of the composition (different CO ratios, φCO_redu ) and temperature
(Tredu ) of the reducing gas on the total energy consumption and CO2 emissions inside the
RSF, with the calculation results shown in Figure 3.
 includes the carbon oxidation heat, high-temperature reducing gas sensible heat, hydro-
gen oxidation heat, methane generation heat, and slag formation heat. The model analyzes
the effects of the composition (different CO ratios, φCO_redu) and temperature (Tredu) of the
Metals 2024, 14, 127 reducing gas on the total energy consumption and CO2 emissions inside the RSF, with the
11 of 18
calculation results shown in Figure 3.

(a) (b)
Figure 3.
3. The
The influence
influence of
of φ
φCO_redu and Tredu on energy consumption (a) and CO2 emissions (b).
Figure CO_redu and T redu on energy consumption (a) and CO2 emissions (b).

In Figure
In Figure 3a,3a, it
it can
can bebe observed
observed that that as
as the
the COCO ratio
ratio increases
increases in in the
the injected
injected reducing
reducing
(with a corresponding
gas (with corresponding decrease of the H22 ratio), the heat released from carbon oxida-
tion gradually
tion gradually increases,
increases, whilewhile thethe heat
heat released
released fromfrom hydrogen
hydrogen oxidation
oxidation decreases.
decreases. The The
total amount
amount of injected
injected reducing
reducing gas decreases,
decreases, resulting in a reduction reduction in in the
the sensible
sensible
brought in
heat brought in by
by the
the high-temperature
high-temperature reducing reducing gas, gas, but
but the
the total
total energy
energy consumption
consumption
increases in
increases in the
the whole
whole process.
process. The research
research findings
findings indicate
indicate thatthat increasing
increasing the the COCO ratio
ratio
in the injected reducing
in reducing gas from from 5% to 40% leads to a 1% increase increase in energyenergy consumption,
consumption,
approximately 93.8 MJ/tHM. MJ/tHM.InInother otherwords,
words, forfor
every
every1%1% increase
increase in the COCO
in the ratio, energy
ratio, en-
ergy consumption
consumption increases
increases by 2.68 byMJ/tHM.
2.68 MJ/tHM.
The resultsThe results
above areabove basedareon based on hydrogen
hydrogen produc-
production
tion from clean fromenergy
clean power
energygeneration,
power generation,
which does which does notany
not produce produce
carbonany carbon
emissions.
emissions.
Therefore, the Therefore,
energy the energy consumption
consumption of hydrogenofproduction
hydrogen production
by electrolysis by electrolysis
is not consid- is
not
eredconsidered
in the process in the process
above, so theabove, so the
influence of influence of different
different hydrogen hydrogentechnologies
production production
technologies on energy consumption
on energy consumption is also one of is also one of the restrictive
the restrictive factors to be factors to be considered
considered in this new in
this new process in the future. Additionally, from Figure 3a, it
process in the future. Additionally, from Figure 3a, it can be observed that as the temper- can be observed that as the
temperature of the injected
ature of the injected reducing reducing gas increases,
gas increases, there is there is a slight
a slight decrease decrease in total
in total energyenergy
con-
consumption,
sumption, which is due to the increase of sensible heat carried by the reducing gas and
which is due to the increase of sensible heat carried by the reducing gas and
leads
leads toto the
the decrease
decrease of of the
the amount
amount of of reducing
reducing gas. gas. Under
Under thethe condition
condition of of satisfying
satisfying thethe
reduction
reduction potential,
potential, it it will
will inevitably
inevitably leadlead toto the
the increase
increase of of the
the COCO utilization
utilization rate
rate of
of the
the
top gas, and then the total energy consumption
top gas, and then the total energy consumption will be reduced. will be reduced.
In
In addition,
addition,the thecomprehensive
comprehensiveprocess processmodel modelshould
should also
also consider
consider thethe
proportion
proportion of
residual gas after the circulation of the top gas, which is a crucial
of residual gas after the circulation of the top gas, which is a crucial indicator for assessing indicator for assessing
the
the overall
overall utilization
utilization efficiency
efficiency of of the
the new
new process.
process. WhenWhen the the proportion
proportion of of residual
residual gasgas
after
after circulation is equal to 0, it indicates that the high reduction potential gas formed after
circulation is equal to 0, it indicates that the high reduction potential gas formed after
the
the top
top gas
gas treatment
treatment is is used
used inin the
the cycle, which not
cycle, which only ensures
not only ensures the maximum utilization
the maximum utilization
of
of the
the gas
gas cycle,
cycle, but
but also
also does
does notnot need
need external equipment for
external equipment for gas
gas storage
storage and and supply.
supply.
However, the metallization rate (MR)is one of the main factors
However, the metallization rate (MR)is one of the main factors affecting the process above, affecting the process above,
and the model analyzes the influence of the metallization rate on the circulating gas volume,
and the model analyzes the influence of the metallization rate on the circulating gas vol-
as shown in Figure 3a. It can be seen in the figure that when the iron ore is moved down
ume, as shown in Figure 3a. It can be seen in the figure that when the iron ore is moved
to the bottom of RSF shaft and the MR is 90%, the φCO_redu of the reducing gas at around
down to the bottom of RSF shaft and the MR is 90%, the φCO_redu of the reducing gas at
45% can meet the requirement for the complete utilization of the top gas circulation. While
around 45% can meet the requirement for the complete utilization of the top gas
the MR is 95%, the φCO_redu of the reducing gas is approximately 35%, which can meet the
requirement for the complete utilization of the top gas circulation.
At the same time, as the CO ratio of the injected reducing gas increases, the emission
of CO2 also significantly increases, as shown in Figure 3b. The calculations of the model
indicate that, while satisfying the constraints of thermal balance inside the RSF, for every 1%
increase in the CO ratio of the reducing gas, the emission of CO2 increases by approximately
Metals 2024, 14, 127 12 of 18

7.53 kg/tHM. The temperature of the injected reducing gas has a certain influence on CO2
emissions at low CO concentrations, but this influence diminishes as the CO concentration
increases. This is because, with the increase in CO concentration, the effect of H2 becomes
weaker, resulting in a weakened reduction in CO2 emissions.
Overall, it is currently difficult to simultaneously reduce energy costs and achieve low
CO2 emissions. At present, this process has limited CO2 emissions as much as possible
while considering cost control. However, with the reduction of energy costs and the
development of “green hydrogen” technology in the future, there is potential for further
optimization and adjustment of the process parameters to break through the upper limit of
emission reduction and achieve even greater emission reductions.

4.2. Effect of MR on Decarbonization Reduction Gas and Coke Ratio

In traditional BFs, the MR has consistently remained below the upper limit of 70%
(that is, the direct reduction degree by carbon is higher than 30%) as the burden materials
descend to the furnace shaft. This is primarily due to the lower rate of the reduction agent
CO compared to H2 (the proportion of H2 in BF is less than 5%). This new process employs
the method of injecting reducing gas from the RSF shaft tuyere to achieve integrated
technological upgrades, such as high temperature and high hydrogen enrichment in the
RSF shaft. As a result, the MR in the upper zone can be increased to around 90%. The
improvement in the MR in the upper zone inevitably reduces the demand for direct
reduction processes in the lower zone, which can greatly reduce the demand for direct
reduction heat, and then greatly reduce the coke demand for direct reduction in the lower
region, so as to reduce the consumption of fossil energy. At the same time, it is necessary to
increase the amount of reduction gas at the furnace body in order to meet the demand of
indirect reduction in the upper region.
The MR is related to the coke ratio and the amount of reducing gas injected into the
RSF shaft. Under the conditions of φCO_redu = 30%, Tredu = 950 ◦ C, and φwater_gas = 30%,
the relationship between these parameters was analyzed using the model above, and
the results are shown in Figure 4. It can be seen that for every 1% increase in the MR
of the iron ore at the RSF shaft, the consumption of coke (coke ratio) is reduced by ap-
Metals 2024, 14, x FOR PEER REVIEWproximately 5.95 kg/tHM, and the amount of reducing gas is increased by approximately 13 of 19
2.45 Nm3 /tHM, which corresponds to a ratio of about 0.2%. These results provide a basis
for adjusting the process parameters.

(a) (b)
Figure
Figure 4.
4. The
Theinfluence
influence of
of MR
MR on
on coke
coke consumption
consumption (a)
(a) and
and the
the volume
volume of
of shaft
shaft reducing
reducing gas
gas (b).
(b).

As
As we
we all
all know,
know, the
the MR
MR of
of the
the iron
iron ore
ore at
at the
the RSF
RSF shaft
shaft is
is also
also related
related to
to parameters
parameters
such
such as
as the
the composition,
composition,temperature,
temperature,andandflow
flowrate
rateof
ofthe
thereducing
reducinggasgasat
atthe
theRSF
RSFshaft.
shaft.
The uniformity of the radial MR is also a key factor to consider. Currently, this model
simplifies the analysis of the influence of different MR on process parameters. In future
work, various dynamics will be incorporated to study the effects of these variables on the
MR and the level of radial MR uniformity. This will provide references for the RSF size
design.
Metals 2024, 14, 127 13 of 18

The uniformity of the radial MR is also a key factor to consider. Currently, this model
simplifies the analysis of the influence of different MR on process parameters. In future
work, various dynamics will be incorporated to study the effects of these variables on
the MR and the level of radial MR uniformity. This will provide references for the RSF
size design.

4.3. Effect of Water–Gas Reaction Ratio on Gas Utilization Ratio

In the RSF, both CO and H2 are the main reducing agents involved in the reduction
process of iron oxides. The gas utilization rate of CO or H2 represents the efficiency
of gas utilization inside the furnace, which also represents the degree of reaction of the
reducing gas inside the furnace. Previous in-depth research has been conducted on this
topic. Japanese scholars [35–39] found that when the proportion of H2 in the BF is high, the
product H2 O in the reduction process from FeO to Fe will inevitably undergo a water–gas
shift reaction with CO (H2 O + CO = H2 + CO2 ) during the gas upward movement, and the
higher the proportion of H2 , the higher the proportion of the reaction with CO. It should be
noted that the formula for calculating the utilization rate of CO and H2 used in this paper
is the same as that in the previous literature [31–33], namely, the utilization of CO(η CO ) is
equal to CO2 /(CO + CO2 ), and the utilization of H2 (η H2 ) is equal to H2 O/(H2 + H2 O). In
addition, the water–gas reaction ratio is equal to H2 O**/H2 O*, in which H2 O* represents
the generation amount of H2 O in the reduction process from FeO to Fe by H2 , and H2 O**
represents the consumption amount of H2 O during the gas’s upward movement.
Based on the research above, this model introduces the variable of the proportion of
the water–gas shift reaction with CO to consider the impact of different water–gas reaction
ratios on the gas utilization rate inside the RSF. The results of the model calculation are
shown in Figure 5. It can be observed that as the ratio of the water–gas shift reaction
increases, the utilization rate of CO increases, while the utilization rate of H2 decreases.
Specifically, for every 1% increase in the water–gas reaction ratio, the utilization rate
of CO increases by approximately 0.27%, while the utilization rate of H2 decreases by
approximately 0.01%. This is because the water–gas shift reaction (CO + H2 O = CO2 + H2 )
Metals 2024, 14, x FOR PEER REVIEWproceeds in the forward direction, producing CO2 and H2 as products. A higher proportion 14 of 19
of water–gas shift reaction results in a decrease in the proportion of CO and an increase in
the proportion of H2 in the top gas, leading to an increase in the CO utilization rate and a
decrease in the H2 utilization rate. However, it should be noted that in the actual process,
H
H22plays
playsthe
therole
roleof
ofaacatalyst.
catalyst.Without
Withoutthethepresence
presenceof ofHH22, ,the
thedesired
desiredhigh
highMR MRwould
wouldnotnot
be
be achieved.
achieved. Therefore,
Therefore,there
there is
is also
also an
an upper
upper limit
limit for
for the
the amount
amount of H22,, as
of H as excessive
excessive H
H22
would only lead to resource waste.
would only lead to resource waste.

Figure
Figure5.
5.The
Theinfluence
influenceof
ofφφwater_gas on gas utilization rate.
water_gas on gas utilization rate.

4.4. Analysis of Carbon–Hydrogen Footprints

The reduced melting furnace process described in this article adopts a carbon–hydro-
gen dual reducing agent coupling method to reduce iron oxides in the furnace. Due to its
excellent performance in the BF, coke or carbon-based gases are still extensively used in
this model. Through mathematical modeling of different zones, a quantitative analysis
 Metals 2024, 14, 127 14 of 18

4.4. Analysis of Carbon–Hydrogen Footprints

The reduced melting furnace process described in this article adopts a carbon–hydrogen
dual reducing agent coupling method to reduce iron oxides in the furnace. Due to its ex-
cellent performance in the BF, coke or carbon-based gases are still extensively used in this
model. Through mathematical modeling of different zones, a quantitative analysis can be
conducted on the forms and footprints of carbon-containing substances at various inlet and
outlet points in the RSF. The carbon footprint flow can provide a better analysis of the usage
status of carbon-based materials and CO2 emissions, enabling a more accurate assessment
of the new process in terms of carbon reduction requirements and national policies, in order
to establish suitable operational systems. Additionally, a high proportion of hydrogen in the
reducing gas of the RSF is the main driving force in the reduction process. By establishing
the hydrogen footprint flow, the utilization status of hydrogen-based materials in the RSH
can be clarified. In a relatively short period, due to the current technological limitations,
there is a higher cost for hydrogen production. However, it is possible to lower the over-
all process cost by appropriately increasing the hydrogen recycling ratio and enhancing
water reuse.
Figure 6 shows the carbon and hydrogen footprints in two different scenarios. The
main sources of carbon in the new process flow are coke and carbon-containing reducing
gas (CO and CO2 ) injected into the RSF shaft tuyere. The main expenditures include the
form of [Fe3 C] entering the carbon-containing hot metal and carbon-containing gases (CO
and CO2 ) discharged from the top of the furnace in gas form. After being processed by
different top gas purification devices, a portion of the carbon-containing gas of the top gas
is burned and directly released into the atmosphere in the form of CO2 , while the remaining
Metals 2024, 14, x FOR PEER REVIEW 15 of 19
gas is captured by the CO2 separator and used as a kind of gas source for oil displacement
underground. The rest of the gas is recycled in the form of CO.

(a) Carbon footprints in scenario 1 (b) Hydrogen footprints in scenario 1

(c) Carbon footprints in scenario 2 (d) Hydrogen footprints in scenario 2

Figure 6. Footprint analysis of carbon and hydrogen in different scenarios.
Figure 6. Footprint analysis of carbon and hydrogen in different scenarios.

In Figure 6a,b of scenario 1, the boundary conditions were the temperature of the
reducing gas (Tredu = 950 °C), the CO ratio (φCO_redu = 30%) of the reducing gas, the temper-
ature of the top gas (Ttop_gas = 200 °C), the water–gas reaction ratio in the RSF (φwater_gas =
20%), and the metallization rate (MR = 90%) of iron ore in the bottom of the RSF shaft. In
Figure 6c,d of scenario 2, the boundary conditions were Tredu = 950 °C, φCO_redu = 20% Ttop_gas
= 200 °C, φwater_gas = 30%, and MR = 95%. The data presented in the figure refer to the mass
of C or H elements expressed in different ways of CO2, CO, C, H2O, and H2, kg/tHM.
Metals 2024, 14, 127 15 of 18

There are two main sources of hydrogen in the new process flow. In addition to the
moisture in the raw materials and volatiles from coke, the vast majority comes from green
hydrogen produced by the electrolysis of water. After passing through the RSF, the product
is water, a portion of which is released into the atmosphere after combustion for heat
generation, while the rest is removed from the dryer and stored for recycling.
In Figure 6a,b of scenario 1, the boundary conditions were the temperature of the
reducing gas (Tredu = 950 ◦ C), the CO ratio (φCO_redu = 30%) of the reducing gas, the
temperature of the top gas (Ttop_gas = 200 ◦ C), the water–gas reaction ratio in the RSF
(φwater_gas = 20%), and the metallization rate (MR = 90%) of iron ore in the bottom of the
RSF shaft. In Figure 6c,d of scenario 2, the boundary conditions were Tredu = 950 ◦ C,
φCO_redu = 20% Ttop_gas = 200 ◦ C, φwater_gas = 30%, and MR = 95%. The data presented in
the figure refer to the mass of C or H elements expressed in different ways of CO2 , CO, C,
H2 O, and H2 , kg/tHM.
Through a comparative analysis of the two cases, it can be observed that with the vari-
ation of the operating parameters, the carbon content in the circulating gas, CO, decreased
from 170.76 kg/tHM to 113.79 kg/tHM, and the carbon content in CO2 also decreased
accordingly, from 22.76 kg/tHM to 21.47 kg/tHM. The mass of hydrogen in circulation
increased from 32.99 kg/tHM to 35.35 kg/tHM, but the amount of hydrogen supplemented
through the electrolysis of water also increased from 33.39 kg/tHM to 40.47 kg/tHM.
The emission of CO2 is a crucial parameter for measuring and evaluating the new
process model. Compared to traditional BF processes, this new process results in lower
direct CO2 emissions. Based on an analysis of the two cases, the direct CO2 emissions
decreased from 58.89 Nm3 /tHM (equivalent to CO2 emissions of 215.93 kg/tHM) to
48.85 Nm3 /tHM (equivalent to CO2 emissions of 179.12 kg/tHM). In comparison to the CO2
emissions from traditional BFs (1400 kg/tHM), the proportion of direct emission reduction
increased from 84.58% to 87.21%. These analyses are based on the CCUS (carbon capture,
utilization, and storage) technology in the new process. Alternatively, without CCUS
technology, the CO2 emissions would decrease from 729.85 kg/tHM to 660.73 kg/tHM,
resulting in an increase in emission reduction proportion from 47.87% to 52.81%. This target
value significantly surpasses existing technologies, such as the BF with hydrogen-rich or
oxygen-enriched processes, providing a new direction for the technological development
of hydrogen metallurgy. It also offers data support for subsequent parameter optimization
of this new model.

5. Conclusions
This article systematically introduces the modeling process and assumptions of RSF
with Hy-O-CR. Starting from the key factors limiting the reduction of emissions from
BFs, this article investigates new low-carbon processes that explore the limits of energy
consumption and extreme carbon reduction through integrated technologies, such as top
gas recycling at a high temperature and high reducing potential gas injection into the RSF.
After systematic research, the following conclusions were obtained.
1. The reduction gas injected into the RSF shaft contains a certain proportion of hydrogen.
Due to the endothermic reaction of hydrogen reduction of the iron oxides, the thermal
balance distribution in the existing BF is disrupted. The new process requires addi-
tional heat input to compensate for the heat deficiency during the reduction process.
This is achieved by supplying high-temperature reducing gas through the top gas
recycling injection in the upper zone and reducing the direct reduction heat in the
lower zone (promoting upward heat transfer of high-temperature gas). In the case
of a high proportion of hydrogen, calculations and analyses using a thermal balance
model in the lower-temperature upper zone and the higher-temperature lower zone
confirm that the process can still achieve a balanced process in both the upper and
lower zones when the distribution ratio of direct and indirect reduction is changed.
2. Under the set constraints, as the CO ratio of the injected reducing gas increases, the
total energy consumption of the process shows an upward trend. However, due
Metals 2024, 14, 127 16 of 18

to the use of top gas recycling heating technology in this process, the operating
parameters can be optimized with the goal of achieving 100% utilization of recycled
gas. Surplus or insufficient recycled gas will cause waste of resources or increased
energy consumption. Therefore, the optimal solution is based on operating conditions
obtained from 100% recycling of the top gas.
3. Compared to traditional BFs, the new process achieves a significant reduction in CO2
emissions through a high proportion of hydrogen and CCUS technology. Without
CCUS technology, the minimum CO2 emissions throughout the process can reach
660.73 kg/tHM, resulting in a 52.81% reduction compared to traditional BFs. With the
implementation of CCUS technology, the direct CO2 emissions of the process can be
reduced to 179.12 kg/tHM, resulting in an impressive 87.21% reduction compared to
traditional BFs.
This hydrogen metallurgy ironmaking process provides a new and exploratory direc-
tion for low-carbon processes. The model results can serve as a theoretical reference for
future semi-industrial trials of the new process.

Author Contributions: Conceptualization, H.L.; data curation, J.C.; funding acquisition, H.L.; in-
vestigation, Z.L.; methodology, H.L.; project administration, H.L.; resources, X.W.; software, H.L.;
supervision, H.L.; writing—original draft, J.C.; writing—review and editing, H.L. All authors have
read and agreed to the published version of the manuscript.
Funding: Financial support was provided by the National Key R&D Program of China (No. 2022YFE0208100)
and the Fundamental Research Funds for the Central Universities (N2225022).
Data Availability Statement: Data are contained within the article.
Conflicts of Interest: Author Xiaoai Wang was employed by HBIS Group Co., Ltd. The remaining
authors declare that the research was conducted in the absence of any commercial or financial
relationships that could be construed as a potential conflict of interest.

Nomenclature

BF Blast furnace
tHM Tonne of hot metal
HM Hot metal
tuy Tuyere
MR Metallization ratio, 85–95, %
more The mass of ore consumption, kg/tHM
mcoke The mass of coke consumption, kg/tHM
mslag The mass of slag production, kg/tHM
mHM The mass of hot metal, 1000 kg
mflux The mass of flux consumption, kg/tHM
mdust The mass of dust production, kg/tHM
mi_j Mass of i in j, mi_j = mdust × ω i_j , kg/tHM
Mass fraction of i in j; I is the composition from Table 1 to Table 4, j is ore, coke,
ω i_j
slag, HM, flux, and dust, %
qDRI Calorific value of ore at 950 ◦ C, MJ/kg
qcoke Calorific value of coke at 950 ◦ C, MJ/kg
qCO Calorific value of CO at Ttop_gas , MJ/Nm3
qCO2 Calorific value of CO2 at Ttop_gas , MJ/Nm3
The heat released by reaction of CO_Fe2 O3 , H2 _Fe2 O3 , CO_FeO, C_O2 , H2 O_CO,
Qi
H2 _O2 and CO_O2 , MJ
R2 Binary basicity, R2 = 1.15
Ttop_gas Temperature of top gas, 150–250 ◦ C
Tredu Temperature of reducing gas, 800–950 ◦ C
φCO_redu Proportion of CO in reducing gas, 10–50%
φwater_gas Proportion of water–gas reaction, 10–50%
φCH4 Proportion of CH4 , 1%
Metals 2024, 14, 127 17 of 18

φco_redu Proportion of CO in reducing gas

Hburn_tuy The amount of heat burned in the tuyere, MJ
V CO_top Volume of CO in furnace top gas, Nm3
V H2 _top Volume of H2 in furnace top gas, Nm3
V CO2 _top Volume of CO2 in furnace top gas, Nm3
V H2 O_top Volume of H2 O in furnace top gas, Nm3
Vi_redu Volume of components in reducing gas; I = CO, H2 , CO2 , and H2 O, Nm3
The heat of material, including coke, DRI, HM, slag, gas_low, redu_low,
Qmaterial
gas_up, in_low, out_low, MJ
Qloss_low The loss of heat, 400 MJ
qH2 Calorific value of H2 at Ttop_gas , MJ/Nm3
qH2 O Calorific value of H2 O at Ttop_gas , MJ/Nm3
The heat absorbed by reaction of C_O2 , H2 _FeO, CO_Fe3 O4 , H2 _Fe3 O4 , CaCO3 ,
Qj
and MgCO3 , MJ

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