Mathematical Modeling and Analyses of Integrated Process with Blast Furnace Iron Making and Co-gasification of Coal-COG-BF Top Gas

Abstract

For the blast furnace iron-making process which depends on the coke seriously and is the largest CO₂ emission source in iron and steel industry, a novel technology integrated the blast furnace process and co-gasification of coal-COG-BF top gas is investigated and examined its potential of energy conservation and CO₂ emission reduction. The mathematical model of the whole integrated process is established based on mass and heat balance principles. And typical operation analyses for the integrated process are demonstrated by this model. The results indicate that it takes about 4 times for the unsteady process caused by the recycling of BF top gas to reach steady. Compared with conventional blast furnace process, the coke ratios in two integrated process cases selected decrease by 33.5 kg/t_HM and 81.3 kg/t_HM obviously, and the fuel ratios decrease 1.1% and 8.2% due to the increase of coal ratios by 27.9 kg/t_HM and 39.1 kg/t_HM. The energy consumption decreases by 8.3% and 10.8%. And the CO₂ emission decreases by 37.0% and 42.9%.

1. Introduction

Iron and steel production processes are highly intensive in energy consumption and CO₂ emission.¹⁾ Over 50% of energy is consumed and 70% of CO₂ is emitted in the blast furnace (BF) iron-making process.²⁾ Coke is the key primary energy carrier in BF. To reduce the coke is an on-going and important research direction. The most effective way is to inject the hydrocarbons as the substitution of coke into BF.

To date, the pulverized coal injection (PCI) is a widely used technology.³⁾ The PC could replace about 40%–50% of coke,⁴⁾ and the PC ratio of up to 250 kg/t_HM (ton hot iron) reported can be achieved.⁵⁾ The chemical properties of PC, however, have significant impact for the operation of BF,⁶⁾ and cause problems for the combustion in raceway to affect the gas permeability and dirty the deadman zone, as a consequence, the productivity decreases.^7,8)

To achieve low coke ratio and CO₂ emission reduction, the injection of non-fossil fuel such as natural gas (NG), biomass, coke oven gas (COG), pure H₂ and BF top gas has already received an increasing interest.^{9,10,11,12,13)} The major advantage of this topic is the high quantities of H₂ and CO can partly substitute for the carbon of fossil fuel to reduce the ferrous oxides.¹⁴⁾ Various theoretical researches and industrial tests have proven the feasibility and progressiveness.^{15,16,17,18,19)} The utilization for COG and BF top gas as the by-products of iron-making process is consistent with the concept of optimized allocation for resources.²⁰⁾ However, the cracking reaction of methane in COG may introduce the carbon deposition to cause the deterioration for permeability in BF, and lower the in-furnace temperature.²¹⁾ The BF top gas with removal CO₂, as the hot reducing gas (HRG), is recycled, which is united with oxygen enrichment technology, and called top gas recycling oxygen blast furnace (TGR-OBF).²²⁾ How to effectively and economically capture and disposal the CO₂ of BF, however, is the limitation for this process development and application.²³⁾

The coal gasification, as the most potential clean and efficient coal conversion technology, produces the gases which are essentially the mixture of CO and H₂. This process is based on several factors include the composition of feedstock, the amount or types of oxidizer (O₂, air) and moderator (steam, CO₂, NG or combination), and the gasifier temperature/pressure.²⁴⁾ The gasification technology has been used broadly in the combined system of chemical engineering or electric power.²⁵⁾ The maturity and stability make gasification technology suitable for using in the iron-making process.

Thus, this paper proposes a novel iron-making technology integrated BF process and coal gasification. In the process, the COG and BF top gas with the coal occur the co-gasification, then the gasified gas completely instead of PC injects into BF. On the one hand, the CO₂ of top gas could be effectively converted into CO to carry out the recycling of top gas without CO₂ capture, and the COG is heated and reformed. On the other hand, this technology could overcome the disadvantages of PCI and simplify the BF operation to improve productivity. A conceptual design and feasibility studies about this integrated process are put forward in this paper to provide the theoretical basis for further research and implementation of the process.

2. Methodology

2.1. Process Description

The schematic diagram of the integrated process proposed is shown in Fig. 1. The four units of BF, gasifier, heater and hot stove are considered in the whole process. At first, the industrial oxygen and conventional BF top gas are sent into gasifier to react with coal. Due to the high carbon content in the coal and BF top gas, the H₂ to CO (H₂/CO) ratio of gasified gas is low. With the merit of adjusting H₂/CO ratio, the COG is used as auxiliary gasifying agent. Then, the clean gasified gas after heating is injected into the raceway zone of BF, and reacts with blast, part of coke. The bosh gas generated goes up and increases in volume with the direct reduction product gas. After passing through the indirect reduction, the new top gas is obtained. Part of new top gas after dedusting is recycled into the gasifier to take part in the gasification as the context described. The other part of top gas is used as fuel supplied to the heater, hot stove and other users in the plant.

Fig. 1.

Schematic diagram of the integrated system. (Online version in color.)

Following the above analysis, the gaseous product content of gasifier is affected by the change of recycled BF top gas. And the content of different regional gases, the direct reduction degree in BF also change with the variety of gasified gas injected. Apparently, the BF and gasifier interact as both cause and effect, which results in the unsteady operation of the whole process.

Therefore, how to carry out the integrated coupling of co-gasification and BF iron-making process, and analyse the change in operation of the whole process is an important theoretical investigation. The mathematical model for integrated process based on the mass and thermal balances is constructed.

2.2. Model Formulation

2.2.1. Assumptions

To analyse the complicated coal gasification and BF process in integrated system presented, the following assumptions for model based on the previous work are made.

1) coal gasification unit

Due to the coal of high-volatile utilized, the fluidized bed gasification is selected as the case study to model the coal gasification process. The gasification temperature and pressure are fixed as 950°C and 0.1 MPa,²⁶⁾ respectively. The tar free gasified gas is mainly made up of CO and H₂, accompanying with N₂, CO₂, H₂O, CH₄ and H₂S, and traces of other components are ignored.²⁷⁾ Thus, the gasification reaction can be written as:   

$$\begin{array}{l}\mathrm{c}\mathrm{o}\mathrm{a}\mathrm{l}+\mathrm{i}\mathrm{n}\mathrm{d}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{\hspace{0.17em} }{\mathrm{O}}_2+\mathrm{B}\mathrm{F}\mathrm{\hspace{0.17em} }\mathrm{t}\mathrm{o}\mathrm{p}\mathrm{\hspace{0.17em} }\mathrm{g}\mathrm{a}\mathrm{s}+\mathrm{C}\mathrm{O}\mathrm{G}\to \\ \mathrm{C}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{N}}_2+\mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{S}+\mathrm{a}\mathrm{s}\mathrm{h}\end{array}$$(1)

The 20% of S in coal is set to convert into the ash.²⁸⁾ The heat loss in gasification process is settled as 3% of the input energy.

2) BF unit

(1) The ferrous loss ratio of burden is 0.3%, 50% of Mn and 100% of P dissolve in the pig iron. 50% of CaCO₃ decomposes in the high temperature.²²⁾

(2) The BF is mainly divided into three major zones: the upper furnace (UF), low furnace (LF) and combustion area (raceway).⁵⁾ The UF and LF is different by the boundary in which the gas temperature is nearly the same as the charge temperature.²⁹⁾ Based on the previous studies,³⁰⁾ the gas temperature is chosen 1000°C. The materials and main chemical reactions occur in different region are shown in Fig. 2. The shaft efficiency usually is 95%.³¹⁾

Fig. 2.

Gasified gas-injection BF. (Online version in color.)

(3) The total heat loss is settled as 8%, whereby the heat loss in LF accounts for 80% of the total heat loss. And the raceway is regarded as adiabatic area to calculate the theoretical combustion temperature.

2.2.2. Calculation

(1) Coal Gasification Unit

The coal gasification calculation was developed based on the Gibbs free energy minimization method (non-stoichiometric equilibrium) which is suitable for process studies and estimate of gasified gas on the influence of the parameters.³²⁾ By taking into account the global reaction in gasifier mentioned, starting from the initial mass of coal (m_{coal_0}), gasifying relative BF top gas/coal ratio (F_tc), COG/coal ratio (F_cc) and oxygen/coal ratio (F_oc), the composition of gasified gas could be calculated, assuming m_{coal_0}=1 kg, with the following chemical equilibrium.

1) Chemical equilibrium

Firstly, we could obtain the chemical balances representing the molar balance for each element respectively.

Carbon balance   

$$\begin{array}{l}{\displaystyle \frac{\omega {(C)}_{coal}}{12}}+{\displaystyle \frac{F_{tc}\times ({\phi }_{CO\_t}+{\phi }_{C{O}_2\_t}+{\phi }_{C{H}_4\_t})}{22.4}}\\ +{\displaystyle \frac{F_{cc}\times ({\phi }_{CO\_c}+{\phi }_{C{O}_2\_c}+{\phi }_{C{H}_4\_c})}{22.4}}\\ ={\displaystyle \frac{V_{gasif\_0}\times ({\phi }_{CO\_g}+{\phi }_{C{O}_2\_g}+{\phi }_{C{H}_4\_g})}{22.4}}\end{array}$$(2)

Oxygen balance   

$$\begin{array}{l}{\displaystyle \frac{\omega {(O)}_{coal}}{16}}+{\displaystyle \frac{\omega {(H_{2}O)}_{coal}}{18}}+{\displaystyle \frac{F_{oc}\times \alpha }{11.2}}+\\ {\displaystyle \frac{F_{tc}\times ({\phi }_{CO\_t}+2{\phi }_{C{O}_2\_t}+{\phi }_{H_{2}O\_t})}{22.4}}+{\displaystyle \frac{F_{cc}\times ({\phi }_{CO\_c}+2{\phi }_{C{O}_2\_c})}{22.4}}\\ ={\displaystyle \frac{V_{gasif\_0}\times ({\phi }_{CO\_g}+2{\phi }_{C{O}_2\_g}+{\phi }_{H_{2}O\_g})}{22.4}}\end{array}$$(3)

Hydrogen balance   

$$\begin{array}{l}\omega {(H)}_{coal}+{\displaystyle \frac{\omega {(H_{2}O)}_{coal}}{9}}+\\ {\displaystyle \frac{F_{tc}\times ({\phi }_{H_2\_t}+2{\phi }_{C{H}_4\_t}+{\phi }_{H_{2}O\_t})}{11.2}}+{\displaystyle \frac{F_{cc}\times ({\phi }_{H_2\_c}+2{\phi }_{C{H}_4\_c})}{11.2}}\\ ={\displaystyle \frac{V_{gasif\_0}\times ({\phi }_{H_2\_g}+2{\phi }_{C{H}_4\_g}+{\phi }_{H_{2}O\_g}+{\phi }_{H_{2}S\_g})}{11.2}}\end{array}$$(4)

Nitrogen balance   

$$\begin{array}{l}{\displaystyle \frac{\omega {(N)}_{coal}}{14}}+{\displaystyle \frac{F_{tc}\times {\phi }_{N_2\_t}+F_{cc}\times {\phi }_{N_2\_c}+(1-\alpha )F_{oc}}{11.2}}\\ ={\displaystyle \frac{V_{gasif\_0}\times {\phi }_{N_2\_g}}{11.2}}\end{array}$$(5)

Sulfur balance   

$$\frac{0.8\omega {(S)}_{coal}}{32}=\frac{V_{gasif\_0}\times {\phi }_{H_{2}S\_g}}{22.4}$$(6)

  

$$\phi {}_{CO\_g}+\phi {}_{C{O}_2\_g}+\phi {}_{H_2\_g}+\phi {}_{C{H}_4\_g}+\phi {}_{N_2\_g}+\phi {}_{H_{2}S\_g}+\phi {}_{H_{2}O\_g}=1$$(7)

For solving the eight unknowns include the yield and composition of gasified gas (V_{gasif_0}, φ_{CO_g}, ${\phi }_{C{O}_2\_g}$, ${\phi }_{H_2\_g}$, ${\phi }_{H_{2}O\_g}$, ${\phi }_{N_2\_g}$, ${\phi }_{C{H}_4\_g}$ and ${\phi }_{H_{2}S\_g}$) with only six balance equations, two more equations based on independent reactions in equilibrium are required. The chemical reactions of R1–R4 are most descriptive of the gasification process.³³⁾ And the water gas-shift reaction (R3) is a combination of the water-gas (R1) and Boudouard (R2) reactions. Thus, the chemical equilibrium constants from R3 and R4 are applied.   

$$C+H_{2}O\iff CO+H_2$$(R1)

  

$$C+C{O}_2\iff 2CO$$(R2)

  

$$CO+H_{2}O\iff C{O}_2+H_2$$(R3)

  

$$CO+3H_2\iff C{H}_4+H_{2}O$$(R4)

The chemical equilibrium constants for R3 and R4 are given as follows. And the constants could be obtained by the Gibbs function equation.   

$$K_3=\frac{P_{C{O}_2\_g}\cdot P_{H_2\_g}}{P_{CO\_g}\cdot P_{H_{2}O\_g}}=\frac{{\phi }_{C{O}_2\_g}\cdot {\phi }_{H_2\_g}}{{\phi }_{CO\_g}\cdot {\phi }_{H_{2}O\_g}}$$(8)

  

$$K_4=\frac{P_{C{H}_4\_g}\cdot P_{H_{2}O\_g}}{P_{CO\_g}\cdot P_{H_2\_g}^3}=\frac{{\phi }_{C{H}_4\_g}\cdot {\phi }_{H_{2}O\_g}}{{\phi }_{CO\_g}\cdot {\phi }_{H_2\_g}^3}{\left(\frac{P}{P^{\theta }}\right)}^{-2}$$(9)

  

$$K=exp(-\mathrm{\Delta }{G}_T^{\theta }/RT)=exp\left(-\sum v_i{G}_{T,i}^{\theta }/RT\right)$$(10)

The Gibbs free energy for single product and reactant could be calculated using the NASA polynomial available in the literature.^34,35) Then we obtain a system of non linear equations include Eqs. (2), (3), (4), (5), (6), (7), Eqs. (8) and (9) which could be solved by the Newton-Raphson method.

2) thermal balance

According to the first thermodynamic principle, the thermal balance equation of gasification process is formulated as Eq. (11).   

$$\begin{array}{l}\sum _{in}{H}_{f,j}+\sum _{in}{n}_j{\overline{C}}_{p,j}(t_{in}-25)=\\ \sum _{out}{H}_{f,j}+\sum _{out}{n}_j{\overline{C}}_{p,j}(t_{gasif}-25)+Q_{loss}\end{array}$$(11)

Where, the enthalpy H_f,j could be calculated by standard formation enthalpy. The formation ehthalpies for gases are obtained from Stull et al.³⁶⁾ The formation enthalpy of coal is estimated using the method given by Syen et al.,³⁷⁾ presented in Eq. (12). The specific heat for gaseous specie ${\overline{C}}_{p,gas}$ could be calculated by Eq. (13)³⁸⁾ in which the values of heat capacity coefficients a_i, b_i, c_i, d_i are adopted from the thermochemical tables.³⁹⁾   

$$h_{f,coal}^0=HHV+\frac{\omega {(H)}_{coal}}{2}\times h_{f,H_{2}O}^0+\frac{\omega {(C)}_{coal}}{12}\times h_{f,C{O}_2}^0$$(12)

  

$$\begin{array}{l}{\overline{C}}_{p,gas}=\\ \sum _i^n{\phi }_i{\displaystyle \frac{{\int }_{298}^{t+273}(a_i+b_i\times {10}^{-3}T+c_i\times {10}^{\text{5}}{T}^{-2}+d_i\times {10}^{-6}{T}^2)dT}{t\text{-}2\text{5}}}\end{array}$$(13)

In the integrated process, the composition of gasified gas obtained has important impact on the BF operation. The reducing gas (CO and H₂) as effective composition of gasified gas is beneficial to BF. Thus, before injected into BF, the gasified gas with highest content of reducing gas needs be selected by optimizing the BF top gas/coal ratio F_tc.

Finally, to obtain setting volume of gasified gas (V_gasif) injected into BF for per ton pig iron production, the required coal consumption, corresponding volumes of BF top gas, COG and industrial oxygen could be calculated as the following:   

$$\begin{array}{l}{m}_{coal}={\displaystyle \frac{V_{gasif}}{V_{gasif\_0}}},\mathrm{\hspace{0.17em} }{V}_{TG}={\displaystyle \frac{F_{tc}\times V_{gasif}}{V_{gasif\_0}}},\mathrm{\hspace{0.17em} }{V}_{COG}={\displaystyle \frac{F_{cc}\times V_{gasif}}{V_{gasif\_0}}},\\ \hspace{1em}{V}_{O_2}={\displaystyle \frac{F_{oc}\times V_{gasif}}{V_{gasif\_0}}}\end{array}$$(14)

(2) Blast Furnace Unit

As the V_gasif of gasified gas after heated to t_{gasif_hearth} is injected into the tuyere of BF, the process parameters of BF are calculated.

1) Mass calculation of ore, flux and slag

Based on the iron element balance and binary basicity of the slag, the following could be obtained.   

$$m_{ore}\times \omega {(TFe)}_{ore}+m_{flux}\times \omega {(TFe)}_{flux}=m_{TFe\_other}$$(15)

  

$$\begin{array}{l}[R_2\times \omega {(Si{O}_2)}_{ore}-\omega {(CaO)}_{ore}]\times m_{ore}+\\ [R_2\times \omega {(Si{O}_2)}_{flux}-\omega {(CaO)}_{flux}]\times m_{flux}\\ =m_{CaO\_other}-R_2\times m_{Si{O}_2\_other}\end{array}$$(16)

Where,   

$$\begin{array}{l}{m}_{TFe\_other}=[Fe]\times 10/0.997+m_{dust}\times \omega {(TFe)}_{dust}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}-m_{coke}\times \omega {(TFe)}_{coke}\end{array}$$(17)

  

$$m_{CaO\_other}=m_{coke}\times \omega {(CaO)}_{coke}-m_{dust}\times \omega {(CaO)}_{dust}$$(18)

  

$$\begin{array}{l}{m}_{Si{O}_2\_other}=m_{coke}\times \omega {(Si{O}_2)}_{coke}-m_{dust}\times \omega {(Si{O}_2)}_{dust}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}-[Si]\times 10\times \frac{60}{28}\end{array}$$(19)

By the simultaneous equations (Eqs. (15) and (16)), the mass of ore and flux could be calculated.

Then, the mass of the slag is as follows.   

$$m_{slag}=\sum _i{\lambda }_i\times m_j\times \omega {(i)}_j$$(20)

Where j is ore, coke, flux, dust. i is the FeO, SiO₂, CaO, Al₂O₃, MgO, MnO, S. And λ_i is the distribution rate of i into the slag.

2) Parameters in raceway zone

(1) Volume of blast

The C–O balance and the carbon of coke combusted in raceway zone could be described as follows.   

$$\begin{array}{l}{\displaystyle \frac{V_{blast}\times (2{\phi }_{O_2\_b}+{\phi }_{H_{2}O\_b})}{22.4}}+{\displaystyle \frac{V_{gasif}\times {\phi }_{H_{2}O\_g}}{22.4}}\\ ={\displaystyle \frac{m_{C\_b}}{12}}+{\displaystyle \frac{V_{gasif}\times {\phi }_{C{H}_4\_g}}{22.4}}\end{array}$$(21)

  

$$m_{C\_b}=m_{coke}\times \omega {(C)}_{coke}-m_{C\_d}-m_{C\_HM}-m_{C\_g}-m_{C\_dust}$$(22)

Where,   

$$\begin{array}{l}{m}_{C\_d}=10\times \\ \left\{[Fe]\times r_{d0}\times \frac{12}{56}+[Si]\times \frac{24}{28}+[Mn]\times \frac{12}{55}+[P]\times \frac{60}{62}\right\}\end{array}$$(23)

  

$$m_{C\_HM}=10\times [C]$$(24)

  

$$m_{C\_g}=12\times \left(\frac{V_{gasif}\times {\phi }_{C{O}_2\_g}}{22.4}+\frac{0.5m_{flux}\times \omega {(Ig)}_{flux}}{44}\right)$$(25)

  

$$m_{C\_dust}=m_{dust}\times \omega {(C)}_{dust}$$(26)

Therefore, the volume of blast could be calculated by the direct reduction degree given as r_d0.

(2) Components of the bosh gas

The bosh gas formed in tuyere includes CO, H₂ and N₂. The calculation for each component is following.   

$$\begin{array}{l}{V}_{CO\_bosh}=V_{blast}\times (2{\phi }_{O_2\_b}+{\phi }_{H_{2}O\_b})+V_{gasif}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\times ({\phi }_{CO\_g}+2{\phi }_{C{O}_2\_g}+{\phi }_{H_{2}O\_g})\end{array}$$(27)

  

$$\begin{array}{l}{V}_{H_2\_bosh}=V_{blast}\times {\phi }_{H_{2}O\_b}+V_{gasif}\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\times ({\phi }_{H_2\_g}+2{\phi }_{C{H}_4\_g}+{\phi }_{H_{2}S\_g}+{\phi }_{H_{2}O\_g})\end{array}$$(28)

  

$$V_{N_2\_bosh}=V_{blast}\times {\phi }_{N_2\_b}+V_{gasif}\times {\phi }_{N_2\_g}$$(29)

  

$$\begin{array}{l}{V}_{bosh}=V_{blast}\times (1+{\phi }_{O_2\_b}+{\phi }_{H_{2}O\_b})+V_{gasif}\\ \hspace{1em}\hspace{1em}\times (1+{\phi }_{C{O}_2\_g}+{\phi }_{C{H}_4\_g}+{\phi }_{H_{2}O\_g})\end{array}$$(30)

3) Parameters of LF zone

(1) Gaseous components in the LF zone

The each component of high temperature gas in LF zone is calculated as follows   

$$V_{CO\_LF}=V_{CO\_bosh}+\left(\frac{m_{C\_d}}{12}+\frac{m_{flux}\times \omega {(Ig)}_{flux}}{44}\right)\times 22.4$$(31)

  

$$V_{H_2\_LF}=V_{H_2\_bosh}$$(32)

  

$$V_{N_2\_LF}=V_{N_2\_bosh}$$(33)

  

$$V_{LF}=V_{bosh}+\left(\frac{m_{C\_d}}{12}+\frac{m_{flux}\times \omega {(Ig)}_{flux}}{44}\right)\times 22.4$$(34)

(2) Calculation of the direct reduction degree

The calculation of the direct reduction degree is based on Rist operation diagram. Due to the gasified gas with high CO and H₂, the hydrogenation operation diagram used is shown in Fig. 3.

Fig. 3.

Rist operation diagram of gasified gas injection BF. (Online version in color.)

At point E,   

$$x_E=0,\mathrm{\hspace{0.17em} }{y}_E=-(y_f+y_b+y_{\theta })$$(35)

Where,   

$$y_b=\frac{V_{blast}\times ({\phi }_{O_2\_b}+{\phi }_{H_{2}O\_b})/11.2}{[Fe]\times 10/56}$$(36)

  

$$y_{\theta }=\frac{V_{gasif}\times \sum _i({\phi }_{i\_g}^O+{\phi }_{i\_g}^{H_2})/22.4}{[Fe]\times 10/56}$$(37)

  

$$\begin{array}{l}{y}_f=\\ (4\times [Si]+1.02\times [Mn]+4.52\times [P]+0.13\times m_{flux}\times \omega {(Ig)}_{flux})/[Fe]\end{array}$$(38)

In Eq. (37), ${\phi }_{i\_g}^O$ and ${\phi }_{i\_g}^{H_2}$ denote the content of i contains O and H₂ in gasified gas, respectively.

According to the step-by-step reduction of ferric oxides,⁴⁰⁾ the reduction of low valence ferric oxides is the restrictive. And the chemical reactions of low valence iron oxide are as follows.⁴¹⁾   

$$\mathrm{F}\mathrm{e}\mathrm{O}+\mathrm{C}\mathrm{O}=\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)+\mathrm{C}{\mathrm{O}}_2;\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }lg{K}_{CO}=1\mathrm{\hspace{0.17em} }190.79/T-1.27$$(R5)

  

$$\mathrm{F}\mathrm{e}\mathrm{O}+{\mathrm{H}}_2=\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O};\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }lg{K}_{H_2}=-1\mathrm{\hspace{0.17em} }223.69/T+0.84$$(R6)

Therefore, the coordinates of point W, the restriction point of above reaction equilibrium, are following.   

$$\begin{array}{l}{x}_W=1+{\displaystyle \frac{K_{CO}}{1+K_{CO}}}\times {\displaystyle \frac{V_{CO\_LF}}{V_{CO\_LF}+V_{H_2\_LF}}}+{\displaystyle \frac{K_{H_2}}{1+K_{H_2}}}\\ \mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\mathrm{\hspace{0.17em} }\times {\displaystyle \frac{V_{H_2\_LF}}{V_{CO\_LF}+V_{H_2\_LF}}},\mathrm{\hspace{0.17em} }{y}_W=1.056\end{array}$$(39)

In Fig. 3, the shaft efficiency is further described as Eq. (40).⁴²⁾   

$${\eta }_{shaft}=\frac{GZ}{GW}=\frac{(y_d+y_i)-y_Z}{(y_d+y_i)-y_W}=0.95$$(40)

Thus,   

$$y_Z=0.05(y_d+y_i)+0.95\times 1.056$$(41)

Where, y_d+y_i =$\sum _j\left[\frac{3\times F{e}_2{O}_{3j}}{160}+\frac{Fe{O}_j}{72}\right]/\sum _j\frac{TF{e}_j}{56}$, the j is same define in Eq. (20).

Based on the coordinates of point G (1, y_d+y_i) and W, the abscissa of point Z is calculated. Finally, the linear equation of Rist operation line (AE) can be obtained by the coordinates of point E and Z. The iterated direct reduction degree r_d1, the ordinate of point B, is calculated. This new r_d1 will be input to the given r_d0 again and again until the equation $\left|r_{d1}-r_{d0}\right|\le 0.0001$ is satisfied. Then, the calculation for burden into LF zone is as follows.   

$$\begin{array}{l}{m^{\prime }}_{ore}=m_{ore}\left[1-\frac{16\omega {(F{e}_2{O}_3)}_{ore}}{160}\right]-16(1-r_d)\times [Fe]\times \frac{10}{56}\\ \hspace{1em}\hspace{1em}\hspace{0.5em}-(m_{dust}-m_{C\_dust})\end{array}$$(42)

  

$${m^{\prime }}_{flux}=m_{flux}[1-0.5\omega {(Ig)}_{flux}]$$(43)

  

$${m^{\prime }}_{coke}=m_{coke}\times [1-\omega {(vol)}_{coke}-\omega {(H_{2}O)}_{coke}]-\frac{m_{C\_dust}}{\omega {(C)}_{coke}}$$(44)

4) Parameters of UF zone

(1) Utilization ratio of the reducing gas

Based on the linear equation of AE obtained, the coordinates of point A (x_A, y_d+y_i) are calculated. The definition of x_A is as follows.   

$$X_A=1+\frac{V_{CO\_LF}\times {\eta }_{CO}+V_{H_2\_LF}\times {\eta }_{H_2}}{V_{CO\_LF}+V_{H_2\_LF}}$$(45)

With the Bogdandy formula ${\eta }_{H_2}$= 0.88η_CO+0.1,²²⁾ the utilization ratios of CO and H₂ in indirect reduction could be calculated.

(2) Calculation of the top gas

The formula for each component in top gas is as follows.   

$$V_{CO\_top}=V_{CO\_LF}\times (1-{\eta }_{CO})+\frac{22.4}{28}{m}_{coke}\times \omega {(CO)}_{coke}$$(46)

  

$$V_{H_2\_top}=V_{H_2\_LF}\times (1-{\eta }_{H_2})+\frac{22.4}{2}{m}_{coke}\times \omega {(H_2)}_{coke}$$(47)

  

$$V_{N_2\_top}=V_{N_2\_LF}+\frac{22.4}{28}{m}_{coke}\times \omega {(N{}_2)}_{coke}$$(48)

  

$$\begin{array}{l}{V}_{C{O}_2\_top}=V_{CO\_LF}\times {\eta }_{CO}+\frac{22.4}{44}\\ [m_{coke}\times \omega {(C{O}_2)}_{coke}+0.5m_{flux}\times \omega {(Ig)}_{flux}]\end{array}$$(49)

  

$$V_{H_{2}O\_top}=V_{H_2\_LF}\times {\eta }_{H_2}+\frac{22.4}{18}{m}_{coke}\times \omega {(H_{2}O)}_{coke}$$(50)

  

$$V_{C{H}_4\_top}=\frac{22.4}{16}{m}_{coke}\times \omega {(C{H}_4)}_{coke}$$(51)

The total volume of top gas is:   

$$\begin{array}{l}{V}_{top}=V_{C{O}_2\_top}+V_{CO\_top}+V_{H_2\_top}+V_{N_2\_top}\\ \hspace{1em}\hspace{1em}+V_{H_{2}O\_top}+V_{C{H}_4\_top}\end{array}$$(52)

The calorific value of dry top gas is:   

$$\begin{array}{l}{q}_{top}=12\mathrm{\hspace{0.17em} }645\times {\phi }_{CO\_t\_de{H}_{2}O}+10\mathrm{\hspace{0.17em} }800\times {\phi }_{H_2\_t\_de{H}_{2}O}\\ \hspace{1em}\hspace{1em}+35\mathrm{\hspace{0.17em} }818\times {\phi }_{C{H}_4\_t\_de{H}_{2}O}\end{array}$$(53)

The heat exchange efficiency is settled as 85%,⁴³⁾ therefore, the volumes of top gas consumed as fuel are as follows.   

$$V_{top\_stove}={\displaystyle \frac{V_{blast}\times {\overline{C}}_{p\_blast}\times (t_{blast}-25)}{0.85\times 22.4\times q_{top}}}$$(54)

  

$$\begin{array}{l}{V}_{top\_heater}=\\ {\displaystyle \frac{V_{gasif}\times \left[{{\overline{C}}^{\prime }}_{p\_g}\times (t_{gasif\_hearth}-25)-{\overline{C}}_{p\_g}\times (t_{gasif}-25)\right]}{0.85\times 22.4\times q_{top}}}\end{array}$$(55)

And the top gas for other users is as Eq. (56).   

$$V_{top\_out}=V_{top}-V_{TG}-V_{top\_heater}-V_{top\_stove}-V_{de{H}_{2}O}$$(56)

5) Mass and thermal balance of BF

(1) Mass balance

Mass input and output of the BF are presented in Eqs. (57) and (58).   

$$m_{input}=m{}_{ore}+m{}_{flux}+m_{coke}+m_{blast}+m_{gas}$$(57)

  

$$m_{output}=m_{HM}+m_{slag}+m_{topgas}+m_{dust}+m_{loss}$$(58)

(2) Thermal balance

① overall heat balance

The heat balance of BF considers the real chemical reactions. The input energies include the sensible enthalpies of blast and gasified gas, the gasification enthalpies of carbon in tuyere and in direct reduction, CO and H₂ combustion enthalpies in indirect reduction. The output energies include the decomposition heat of flux, oxides, sulfur deprivation, H₂O and CO₂, the sensible enthalpies of hot metal, slag, dust and top gas, and the heat loss. The heat balance equation is formulated as Eq. (59).   

$$\begin{array}{l}{Q}_{blast}+Q_{gasif}+Q_{C\_raceway}+Q_{C-CO}+Q_{CO-C{O}_2}+Q_{H_2-H_{2}O}=\\ Q_{flux\_decomp}+Q_{oxide\_decomp}+Q_{H_{2}O+C{O}_2\_decomp}+Q_S+Q_{slag}+\\ Q_{HM}+Q_{top}+Q_{dust}+Q_{loss}\end{array}$$(59)

② heat balance of LF and UF zones

The energy of LF zone is very critical for the operation of BF. If the heat carried by ascending high temperature gas of LF zone, the burden is inadequately preheated, then the indirect reduction in UF will be affected seriously. Therefore, to ensure the reasonable distribution of energy, the calculations for the heat balance of LF and UF zones is carried out, which could be presented as Eqs. (60) and (61).   

$$\begin{array}{l}{Q}_{blast}+Q_{gasif}+Q_{burden}+Q_{C\_raceway}+Q_{C-CO}=0.5Q_{flux\_decomp}+\\ Q_{FeO,Si{O}_2,MnO,P_2{O}_5\_decomp}+Q_{H_{2}O+C{O}_2\_decomp}+Q_S+Q_{slag}+Q_{HM}+\\ Q_{gas\_LF}+0.85Q_{loss}\end{array}$$(60)

  

$$\begin{array}{l}{Q}_{gas\_LF}+Q_{CO-C{O}_2}+Q_{H_2-H_{2}O}=0.5Q_{flux\_decomp}+\\ Q_{ironoxide\_decomp}+Q_{burden}+Q_{top}+Q_{dust}+0.15Q_{loss}\end{array}$$(61)

③ theoretical combustion temperature in raceway zone

In raceway area, the theoretical combustion temperature could be estimated as follows.   

$$\begin{array}{l}{t}_f=\\ {\displaystyle \frac{Q_{blast}+Q_{gasif}+Q_{coke\_raceway}+Q_{C\_raceway}-Q_{H_{2}O+C{O}_2\_decomp}}{V_{bosh}{\overline{C}}_{p\_bosh}/22.4}}\end{array}$$(62)

2.2.3. Calculation Process

The flowchart of calculation process described above is presented in Fig. 4.

Fig. 4.

Calculation flow of integrated process. (Online version in color.)

3. Results and Discussion

The calculation model established would be used for the integrated process under typical conditions. The ultimate analysis of bituminous coal used in gasifier is listed in Table 1, and the composition of COG provided is shown in Table 2.¹¹⁾ The process data of 2580 m³ BF in China are used. The chemical composition of raw materials and coke are presented in Tables 3 and 4. The usage ratio of the sinter, pellet and lump ore are 70%, 20% and 10% respectively. The designed pig iron components are listed in Table 5. The oxygen content in the blast is 22.7%, and the blast temperature is 1200°C. The composition of traditional BF top gas obtained to inject into the gasifier initially is listed in Table 6.

Table 1. Ultimate analysis of coal.

C	H	N	S	O	Ash	Moisture
75.10	6.89	1.00	0.28	8.19	5.39	3.15

Table 2. Composition of COG.

H2	CO	N2	CO2	CH4
60.5	7.2	4.7	1.3	26.3

Table 3. Compositions of raw material and dust.

Material	TFe	FeO	SiO2	CaO	MgO	Al2O3	MnO	S	P2O5	C	Ig
Sinter	57.95	7.05	4.66	8.49	3.03	1.54	0.13	0.01	0.14	–	–
Pellet	61.35	0.24	9.52	0.3	0.77	1.55	0.14	0.01	0.10	–	–
Lump	67.09	2.94	1.98	0.65	0.14	1.38	0.24	0.05	0.05	–	–
Flux	1.39	–	0.85	52.9	0.86	0.88	–	–	–	–	42.52
Dust	40.59	10.88	6.27	5.95	4.11	1.07	0.09	0.31	0.06	25.37	–

Table 4. Composition of coke.

C-fixed	Ash						Volatile					S	Moisture
	SiO2	Al2O3	CaO	MgO	FeO	P2O5	CO2	CO	CH4	H2	N2
84.91	5.69	4.15	0.54	0.14	0.4	0.05	0.11	0.2	0.05	0.27	0.33	0.48	2.68

Table 5. Chemical composition of pig iron.

Si	Mn	P	S	C	Fe
0.42	0.09	0.09	0.03	4.71	94.66

Table 6. Composition of top gas from conventional BF.

CO	H2	N2	CO2	CH4	H2O
21.12	3.77	50.23	20.59	0.02	4.27

The fixed operation conditions of system selected is shown in Table 7. Expect for the COG/coal ratio in gasifier, the volume and temperature of gasified gas injected into BF in case 2 also are more than that in case 1.

Table 7. Operation conditions selected in the integrated system.

Process	Gasifier	BF
	Fcc/m3·kg−1	Vgasif/m3·tHM−1	tgas_hearth/°C
Case 1	0.3	600	1100
Case 2	0.6	700	1250

3.1. Optimization Analysis for Composition of Gasified Gas

The composition of gasified gas is the key to the coupling of gasification and BF iron making. The impact of BF top gas/coal ratio on reducing gas content in gasified gas is plotted in Fig. 5. With higher BF top gas/coal ratio, the CO content gradually decreases, the H₂ content is the result of two opposite processes, thus, the content of reducing gas inclines to increase and then decrease. Compared with the results as 0.3 m³/kg of COG/coal ratio, the H₂ content increases while CO content decreases, and the total reducing gas content increases when the COG/coal ratio is 0.6 m³/kg.

Fig. 5.

Effect of BF top gas/coal ratio on the reducing gas in different COG/coal ratio. (Online version in color.)

Due to the initial use of traditional BF top gas in gasifier, the optimum BF top gas/coal ratio should select 0.84 m³/kg (F_cc=0.3 m³/kg), as the content of reducing gas that consists of 52.43%CO+33.88%H₂ reaches maximum. And the maximum content of reducing gas with 49.19%CO+39.11%H₂ is obtained as the BF top gas/coal ratio is 0.74 m³/kg (F_cc=0.6 m³/kg). With the recycling of BF top gas in integrated process, the optimum BF top gas/coal ratio will change due to the varying composition of BF top gas.

3.2. Operation of the Whole Integrated Process

The unsteady operation for the whole integrated process is characteristic by the number of cycles. Firstly, the evolution of key parameters, input and output materials of gasifier and BF units in case 1 are presented in Tables 8, 9, 10. During the transition of top gas from traditional BF to integrated system, the optimum BF top gas/coal ratio decreases from initial 0.84 m³/kg to 0.72 m³/kg, and the reducing gas composition of gasified gas increases from 52.43%CO+33.88%H₂ to 54.12%CO+36.37%H₂. To produce 600 m³/t_HM of gasified gas applied to BF, the required BF top gas decreases from 143.7 m³/t_HM to 127.2 m³/t_HM, while the coal ratio increases from 171.1 kg/t_HM to 177.9 kg/t_HM.

Table 8. Evolution of key parameter and input materials required for 600 m³/t_HM gaisifer gas during unsteady process.

Times	Key parameters /m3·kg−1		Main composition of gasified gas/%				Required raw materials
	Ftc	Foc	CO	H2	N2	CH4	mcoal/kg·tHM−1	VO2/m3·tHM−1	VTG/m3·tHM−1	VCOG/m3·tHM−1
1	0.84	0.57	52.43	33.88	12.99	0.32	171.1	97.3	143.7	51.3
2	0.74	0.56	53.94	36.05	9.22	0.41	176.6	99.4	129.8	53.0
3	0.72	0.56	54.08	36.35	8.77	0.43	177.5	99.8	127.8	53.2
4	0.72	0.56	54.12	36.37	8.69	0.47	177.9	99.9	127.2	53.4
5	0.72	0.56	54.12	36.37	8.70	0.47	177.9	99.9	127.2	53.4

Table 9. Evolution of input and output materials of BF during unsteady process.

Times	Input materials				Output materials
	more/kg·tHM−1	mflux/kg·tHM−1	mcoke/kg·tHM−1	Vblast/m3·tHM−1	mslag/kg·tHM−1	mdust/kg·tHM−1	Vtop_fuel/m3·tHM−1	Vtop_out/m3·tHM−1	VH2O/m3·tHM−1
1	1606.1	12.8	340.1	714.4	281.9	20.0	366.4	1011.2	99.3
2	1606.1	12.0	332.5	700.9	280.6	20.0	349.7	1001.7	117.3
3	1606.1	11.9	331.4	699.7	280.5	20.0	347.5	1000.3	119.6
4	1606.1	11.8	331.0	698.9	280.4	20.0	347.4	1000.1	120.5
5	1606.1	11.8	331.0	698.9	280.4	20.0	347.4	1000.1	120.5

Table 10. Evolution of composition of BF top gas.

Times	CO	H2	N2	CO2	H2O	CH4
1	25.11	7.26	38.06	21.97	7.59	0.01
2	25.41	7.78	36.54	22.14	8.11	0.01
3	25.43	7.86	36.34	22.16	8.19	0.01
4	25.43	7.88	36.32	22.15	8.21	0.01
5	25.43	7.89	36.31	22.15	8.21	0.01

Due to the increase in effective composition of gasified gas, the coke ratio in BF reduces from 340.1 kg/t_HM to 331.0 kg/t_HM. Simultaneously, the blast required changes from 714.4 m³/t_HM to 698.9 m³/t_HM, which leads to the decrease in fuel gas. The contents of CO, H₂, H₂O and CO₂ in top gas increases while the N₂ decreases. Then, they are stabilized at 25.43%, 7.89%, 8.21%, 22.15% and 36.31%, respectively. The increase of H₂O and CO₂ results in the decrease of BF top gas/coal ratio in Table 8 to maintain the reaction temperature of gasifer. On the whole, it takes about cyclic 4 times for the unsteady process to reach the steady state.

The evolution of key parameters, input and output materials of gasifier and BF units in case 2 are listed in Tables 11, 12, 13. After cyclic 4 times, the values of that in the whole process do not change any more. As shown in Table 11, the appropriate BF top gas/coal ratio decreases from the 0.74 m³/kg to steady 0.60 m³/kg, thus the BF top gas for 700 m³/t_HM gaisifer gas produced decreases to 112.5 m³/t_HM, the coal ratio gradually increases to 189.1 kg/t_HM. The reducing gas composition of gasified gas finally stabilized at 50.28%CO+42.39%H₂. The higher effective composition, volume and temperature of gasified gas injected than that in case 1 result in the decrease of coke ratio to 283.2 kg/t_HM.

Table 11. Evolution of key parameter and input materials required for 700 m³/tFe gaisifer gas during unsteady process.

Times	Key parameters /m3·kg−1		Main composition of gasified gas/%				Required raw materials
	Ftc	Foc	CO	H2	N2	CH4	mcoal/kg·tFe−1	VO2/m3·tHM−1	VTG/m3·tHM−1	VCOG/m3·tHM−1
1	0.74	0.62	49.19	39.11	10.90	0.41	181.5	112.2	134.3	108.9
2	0.62	0.61	50.19	41.99	6.91	0.51	187.9	114.8	115.6	112.7
3	0.60	0.61	50.27	42.35	6.46	0.54	188.8	115.2	113.1	113.3
4	0.60	0.61	50.28	42.38	6.40	0.57	189.1	115.3	112.5	113.5
5	0.60	0.61	50.28	42.39	6.39	0.57	189.1	115.3	112.5	113.5

Table 12. Evolution of input and output materials of BF during unsteady process.

Times	Input materials				Output materials
	more/kg·tHM−1	mflux/kg·tHM−1	mcoke/kg·tHM−1	Vblast/m3·tHM−1	mslag/kg·tHM−1	mdust/kg·tHM−1	Vtop_fuel/m3·tHM−1	Vtop_out/m3·tHM−1	VH2O/m3·tHM−1
1	1606.4	7.9	295.0	602.2	274.3	20.0	336.5	962.5	146.1
2	1606.5	6.8	285.0	583.9	272.6	20.0	316.3	946.1	155.6
3	1606.5	6.6	283.7	581.6	272.4	20.0	314.0	944.1	156.9
4	1606.5	6.6	283.2	580.9	272.3	20.0	313.5	943.4	157.2
5	1606.5	6.6	283.2	580.8	272.3	20.0	313.5	943.4	157.2

Table 13. Evolution of composition of BF top gas.

Times	CO	H2	N2	CO2	H2O	CH4
1	24.67	9.69	34.00	21.52	10.11	0.01
2	24.89	10.55	31.93	21.64	10.98	0.01
3	24.90	10.66	31.69	21.65	11.09	0.01
4	24.90	10.69	31.64	21.65	11.12	0.01
5	24.90	10.69	31.64	21.65	11.12	0.01

The Fig. 6 shows the comparison for mass flow of two typical steady integrated system with the conventional BF process. The reform of top gas and COG makes the integrated system need higher amount of coal to produce the same quality of hot metal. Thus, the coal ratios of integrated process in case 1 and case 2 are 27.9 kg/t_HM and 39.1 kg/t_HM greater than that of conventional BF with PCI, the coke ratios, however, decrease by 33.5 kg/t_HM and 81.3 kg/t_HM respectively. As a result, the decrease of fuel ratios are 1.1% and 8.2%.

Fig. 6.

Mass flow comparison of conventional BF process and steady integrated processes. (Online version in color.)

3.3. Techno-economic Analysis of Integrated Process

3.3.1. Energy Consumption

Based on the mass flows of conventional BF process and integrated processes in case 1 and case 2, the energy balances of these three cases are calculated by using the energy equivalent values,⁴⁴⁾ as shown in Fig. 7. The energy balances of the integrated processes are different from the conventional BF process. With the supplement by the coal, COG and recycled BF top gas in integrated process, the energy of coke that is as the primary energy input, accounts for 53.2% (case 1) and 46.2% (case 2) of the total, which is less than that of 59.2% in conventional BF process. And the blast power consumption trends to decrease due to the decrement of blast. Overall, compared with conventional BF process, the total energy input in these two integrated process cases slightly changes. However, with the higher of the energy output of BF top gas as the secondary energy, the energy consumption of integrated process is 8.3% (case 1) and 10.8% (case 2) less than the traditional BF process.

Fig. 7.

Energy balance of integrated system and traditional BF. (Online version in color.)

3.3.2. CO₂ Emission

Based on carbon balance, the carbon flows in these three cases for one ton of hot metal produced are presented in Fig. 8. As illustrated, the total carbon input of these three cases is 430.7 kg/t_HM, 426.6 kg/t_HM and 404.9 kg/t_HM, respectively. Compared with the traditional BF process, the total carbon input of the integrated process in case 2 is reduced by 6.0% due to the obvious reduction of coke. In the integrated process, the carbon of top gas accounting for 8.0% in case 1 and 7.4% in case 2 is recycled. With decrease in the blast, the carbon fixed in the flue gas by the combustion of top gas in integrated process is 40.1% (case 1) and 45.4% (case 2) less than that of traditional BF process. Then, the carbon in top gas exported for other users, as the main carbon output, increases by 27.7% and 21.7%, thus, the utilization of exported BF top gas is very important for the reduction of CO₂ emission.

Fig. 8.

Carbon flow comparison of conventional BF and steady integrated processes. (Online version in color.)

The calculation for the direct CO₂ emission is based on the carbon flow, and the carbon in product and by-product is not included.⁴⁵⁾ Because the hot metal and BF top gas for other users are the product and by-product in the process respectively, thus, the formula is as follows.   

$$PC{E}_{direct}=\left(\sum _{in}{C}_i-C_{HM}-C_{top\_out}\right)\times 44/12$$(63)

And the indirect CO₂ emission mainly is from the electricity consumption used for blast in process. The calculation is following.   

$$PC{E}_{indirect}=\sum _{i}E{F}_{ele}\times D_i$$(64)

Where, EF_ele is the emission factor, kgCO₂/kWh. It uses the world average value according to IEA.⁴⁶⁾ D_i is the blast power consumption in process, kWh/tFe.

The CO₂ emission is composed of the direct and indirect CO₂ emission. The CO₂ emission of these three cases are shown in Fig. 9. Compared with the traditional BF process, the direct CO₂ emission of integrated process reduces by 38.8% (case 1) and 44.0% (case 2) due to the decrement of total carbon input and increment of carbon in top gas for users. With the decrease of total blast, the indirect CO₂ emission trends to decrease. Thus, the net CO₂ emission of integrated process is 37.0% (case 1) and 42.9% (case 2) less than that of traditional BF process.

Fig. 9.

CO₂ emission of conventional BF and steady integrated processes. (Online version in color.)

4. Conclusions

(1) The mathematical model for the integrated process of BF iron making and co-gasification of coal-COG-BF top gas proposed is constructed. And the operation of integrated process is analyzed. It takes about 4 times for the whole unsteady process to reach steady.

(2) The BF in integrated system has higher reducing gas concentration than the conventional BF, thus the coke ratios in case 1 and case 2 decrease by 33.5 kg/t_HM and 81.3 kg/t_HM, respectively. Although the coal consumption is 27.9 kg/t_HM and 39.1 kg/t_HM more than that of conventional BF with PCI due to the heating and reform of COG and BF top gas, the decrease of fuel ratios still are 1.1% and 8.2%.

(3) Compared with the conventional BF, the total energy input in these two integrated process cases slightly changes due to the increase in the energy input of coal and COG. The energy consumption of integrated process, however, decreases by 8.3% (case 1) and 10.8% (case 2) with the higher energy output of top gas as the secondary energy.

(4) The total carbon input of integrated process in case 2 is 6.0% less than that of traditional BF process due to the notable reduction of coke. And the carbon in top gas for other users in integrated process increases. Thus, compared with the traditional BF prcess, the direct CO₂ emission of integrated process reduces by 38.8% (case 1) and 44.0% (case 2). And the indirect CO₂ emission trends to decrease, then the net CO₂ emission decreases by 37.0% and 42.9%.

Acknowledgement

Thanks are give to the financial supports from the key Program of National Nature Science Foundation of China (U1360205), the Basic Research Program of National Nature Science Foundation of China (52004096) and the China NSF projects (19150223E & E2019209314 & E2021209023).

Nomenclature

m_{coal_0}: initial mass of coal (kg)

ω(i): mass fraction of i

F_tc: BF top gas/coal ratio (m³/kg)

F_cc: COG/coal ratio (m³/kg)

F_oc: oxygen/coal ratio (m³/kg)

α: concentration of O₂ in industrial oxygen

φ_{i_t}: content of i in top gas

φ_{i_c}: content of i in COG

V_{gasif_0}: yield of gasified gas obtained by the per kilogram coal (m³/kg)

φ_{i_g}: content of i in gasified gas

H_f,j: formation enthalpy of j in gasifier (kJ)

n_j: mole of j in reactant and product (mol)

${\overline{C}}_{p,j}$: specific heat (J/mol·°C)

$h_{f,coal}^0$: standard formation enthalpy of coal (kJ/kg)

HHV: high heating value of coal (kJ/kg)

V: volume of gas (m³/t_HM)

m: mass of substance (kg/t_HM)

Ig: burning loss

t: temperature of substance (°C)

η: utilization ratio of CO or H₂

Q: heat of material (kJ)

PCE: process CO₂ emission (kg/t_HM)

Subscripts

C_b: carbon combusted in tuyere of BF

C_d: carbon consumed in the directed reduction

C_HM: carbon in hot metal

C_g: carbon consumed in the boudouard reduction

C_g: carbon in dust

C_i, C_{top_out}: carbon input and and carbon fixed in top gas for users

Conflicts of Interest

All authors declared that no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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