metals

Article Multi-Objective Optimization of Cost Saving and Emission Reduction in Ironmaking Process

Shun Yao, Shengli Wu *, Bo Song, Mingyin Kou , Heng Zhou and Kai Gu

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (S.Y.); [email protected] (B.S.); [email protected] (M.K.); [email protected] (H.Z.); [email protected] (K.G.) * Correspondence: [email protected]; Tel.: +86-010-6233-4989

 Received: 20 October 2018; Accepted: 17 November 2018; Published: 23 November 2018 

Abstract: Due to the increasing environmental pressures, one of the most direct and effective way to achieve emission reduction is to reduce the CO2 emissions of the blast furnace process in the and industry. Based on the substance conservation and energy conservation of ironmaking process and the engineering method, the carbon loss model was firstly established to calculate the amount of solution loss. Based on this model, the blast furnace emission reduction optimization mathematical model with the cost and CO2 emissions as objective functions was then established using the multiple-objective optimization method. The optimized results were obtained by using the GRG (Generalized Reduced Gradient) nonlinear solving method. The optimization model was applied to the B# blast furnace of BayiSteel in China. The optimization model was verified by comparing the optimized results with the actual production data. The optimization model was then applied to analyze the effects of coke ratio, coal rate, blast temperature and other factors on the cost, CO2 emission and solution loss, and some measures to save cost, reduce emissions and reduce solution loss have been proposed.

Keywords: blast furnace; cost; CO2 emissions; solution loss; multi-objective optimization

1. Introduction

Global anthropogenic CO2 emissions grew by 1.4% in 2017, reaching a historic high of 32.5 gigatonnes (Gt), a resumption of growth after three years of global emissions remaining flat [1]. And the CO2 emissions is expected to grow to 37 Gt in 2020 [2]. Since 2012, China has become the world’s largest carbon emitter, and its CO2 emissions accounted for 29% of the world [3]. In 2010, the CO2 emissions from the iron and steel industry accounted for 15% of China’s total CO2 emissions [4–6]. In China, CO2 emissions of ironmaking system, including sintering, coking and blast furnace, account for nearly 90% of the iron and steel industry. The CO2 emissions of blast furnaces process account for more than 70% of ironmaking system [3,4,7–9]. In China, the main CO2 emission source is blast furnace process that almost entirely uses coke and pulverized coal as fuel [10]. Therefore, the most direct and effective way to achieve emission reduction is to reduce the CO2 emissions of the blast furnace process in the iron and steel industry. The CO2 emissions calculation method of the World Steel Association (WSA) is adopted in this paper. In this method, CO2 emissions equals the direct emission plus the indirect emission and minus the deductible carbon emission. The direct emission is the CO2 emissions caused by the consumption of fuels and fluxes in the production process. The indirect emission is the CO2 emissions emitted by raw materials and energy in their own production processes. The deductible carbon emission is the

Metals 2018, 8, 979; doi:10.3390/met8120979 www.mdpi.com/journal/metals Metals 2018, 8, 979 2 of 14

amount of CO2 emissions deducted from the sale or reuse of by-products. The emission factors set by the WSA is an international average value, and some emission factors are not applicable to Chinese enterprises. Therefore, the measured emission factors of target blast furnace are used in this model, and the CO2 emissions of blast furnaces process and the CO2 emissions of ironmaking system are then calculated respectively. The CO2 emissions of ironmaking system includes the indirect emission of the upstream processes such as coking and sintering [9,11]. Metallurgical coke is the most important and inevitable raw material in the blast furnace. It is a fuel, a reductant, a carburetant and a permeable medium in the blast furnace. As coke sustains the passages of liquid metal and slag toward the lower part and the high temperature reducing gas toward the upper part, it is regarded as an indispensable material in blast furnace. Besides, coke is the only solid phase in the high temperature zones adjacent to the blast furnace raceways. Therefore, coke quality affects the gas permeability, liquid permeability of the burden and state of hearth [12]. Due to the increase of blast furnace volume, environmental pressures and the increasing scarcity of good quality coking coal, injecting pulverized coal is developed to decrease reliance on coke. With the decrease of coke ratio, the ore to coke ratio increases in the burden, so a lesser amount of coke has to maintain sufficient permeability and performance of blast furnace. The residence time of coke in the blast furnace becomes larger. Coke is deteriorated by the mechanical, thermal and chemical effects when it moves toward lower part of blast furnace. With the prolonging of the residence time of coke, coke suffers more mechanical, thermal and chemical effects than ever before, resulting in increased deterioration of coke. Coke is gasified in the blast furnace shaft by the so-called solution loss reactions with CO2 and H2O. Solution loss reaction is the main factor that causes the decrease of coke strength in the blast furnace, except for the inevitable mechanical and thermal effects [13–17]. There are extensive and deep research on the solution loss and deterioration of coke. For example, Wang et al. [16] investigated the effect of solution loss reaction on coke degradation. The results showed that solution loss reactions reduce CSR and degradation coke, and the coke degradation is decreased with increase of ore prereduction rate. Haapakangas et al. [17] studied that the effects of H2 and H2O on coke reactivity in a range of temperatures, the effects of H2 and H2O on threshold temperature of gasification, and determining activation energies for CO2 and H2O. Xing et al. [18] studied the degradation of coke under simulated temperature and gas composition conditions in the blast furnace. Both gasification and decreased the mechanical strength of coke. Compared with annealing at 1673 K, gasification at the same temperature caused larger degradation of all three cokes, and the effect was more significant on the more reactive coke. Fang et al. [19] studied the high temperature compressive strength of coke. It was suggested that the compression strength of coke is in a linear relationship with its carbon loss rate in the scope of real reactions in blast furnace. And the influence of carbon loss rate on the compressive strength is reduced with the increasing temperature. Carbon loss rate is about 30% outside the combustion zone in blast furnace. n experimentally researched the influence of temperature and solution loss reaction to the high temperature compressive strength of coke in laboratory conditions. The results showed that the high temperature compressive strength of coke decreases with the increase of temperature, CO2 concentration and solution loss reaction extent. Guo et al. [21] researched the solution loss kinetics behavior on coke strength after reaction. The results showed that the temperature of gradient reaction brings the most serious degradation to three cokes are different duo to different reactivity, and the temperature of the high reactive coke is about 1100 ◦C which is lower than another two cokes. Liu et al. [22] investigated kinetics of coke gasification with CO2 by non-isothermal thermogravimetry. The results showed that when the gasification temperature is higher than 1200 K, both of the carbon conversion rate and gasification reaction rate increase significantly. When the gasification temperature reaches about 1500 K, the gasification reaction rate attains the maximum. These researches are good for guiding ironmaking process. However, the amount of solution loss in the blast furnace has been seldom reported. Linear programming, which has been studied early and matured quickly with a well-developed research method, is a basic branch in operations research with a wide range of applications. In linear Metals 2018, 8, 979 3 of 14 programming, mathematical model is the most commonly used and effective way to obtain optimal solutions, which means it can make target value achieve maximum or minimum under restrained conditions. When solving the multi-objective optimization issues, the multi-objective programming problem is usually transformed into a single-objective programming problem, so that a solution can be obtained that best meets the requirements. The commonly used methods include a constraint method, a sequential single object method, and an evaluation function method. The method of linear programming can also be used to study the blast furnace process [23,24]. When using the constraint method to complete the objective function, an important index is selected as the objective function, and the remaining indicators are taken as constraints, and the value cannot exceed the one of the single-objective optimizations. When using the sequential single-objective approach, each goal is sorted according to the degree of importance, and then an optimal solution is sought for each goal in a sub-set of the constraint region. When using the evaluation function method, a single-objective evaluation function needed to be constructed according to the actual characteristics of the problem, and then an optimal solution should be found according to the single objective [25]. In order to decrease CO2 emissions in the iron and steel industry, several technologies have been investigated, such as blast furnace with top-gas recycling (TGR-BF), new reduction process (HIsarna), carbon capture and storage (CCS) [26]. In addition to the technology development endeavors, interest in the utilization of renewable energy sources in ironmaking and processes to replace part of the fossil-based reductants and fuels has increased recently. Biocarbon is a promising alternative to fossil-based reductant for reducing greenhouse gas emissions and increasing sustainability of the metallurgical industry. It has been proposed in the scientific literature that biocarbon could replace a small portion of the top-charged coke or all injected reducing agents in the blast furnace (BF) [26,27]. In the next study, we will investigate the impact of biocarbon fuel instead of coke and pulverized coal on the material balance and energy balance in the blast furnace. In this study, based on the substance conservation and energy conservation of ironmaking process and the engineering method, the carbon loss calculation model is firstly established to calculate the amount of solution loss. And then, based on the carbon loss calculation model, the blast furnace emission reduction optimization mathematical model is established by using the multiple-objective optimization method. And then, this optimization model is used to seek the optimal solution of the model in order to obtain the best burden structure, operation parameters and product quality parameters. Moreover, this optimization model is used to investigate the effects of main operation parameters on the cost, CO2 emission and carbon loss of B# blast furnace in Bayisteel, which provides a theoretical basis for the stable operation of blast furnace, emission reduction and cost savings.

2. Modeling

2.1. Blast Furnace Carbon Loss Calculation Model The calculation methods of theoretical coke ratio mainly include combined calculation method, Rist operating line calculation method, regional heat balance calculation method and engineering method. The engineering method is widely used in actual production. The principle of this method is simple, and the calculation is simple. Moreover, this method can accurately reflect the actual situation of energy utilization in the blast furnace by adopting the second whole furnace heat balance, which calculates heat consumption according to the actual oxidation–reduction process in the blast furnace. However, there are some shortcomings in engineering method, such as ignoring the effects of blast humidity and pulverized coal and using the assumed direct reduction degree and unchangeable empirical specific heat capacity. Therefore, the shortcomings of engineering method have been corrected firstly. Besides, carbon loss of direct reduction is calculated by this model based on conservation of carbon. Furthermore, the amount of carbon loss of coupled direct reduction can be calculated by the result of carbon loss of direct reduction. This model assumes the following Metals 2018, 8, 979 4 of 14 conditions: all pulverized coal is burned with oxygen; all combustion of carbon and oxygen is incomplete combustion; the distribution ratio of iron to hot metal and slag is 0.999:0.001.

2.1.1. Calculation of Direct Reduction Degree According to the principles of ironmaking, the formula of direct reduction degree is shown as:

ωFe(DR) rd = (1) ωFe(HM) where, rd represents the direct reduction degree; ωFe(DR) represents the amount of Fe that is generated by direct reduction, kg/t; ωFe(HM) represents the amount of Fe in hot metal, kg/t. On the basis of Fe-O-C balance, the amount of Fe element produced by direct reduction could be calculated by the carbon loss of direct reduction. The calculating formula is shown as follows:

56 ω = × ω (2) Fe(DR) 12 C(DR) where, ωC(DR) is the carbon loss of direct reduction, kg/t. According to the conservation of carbon, the formula of carbon loss of direct reduction is shown as:

ω = ω − ω − ω − ω − ω − ω − ω (3) C(DR) C(total) C(Vad) C([C]) C(XO) C(H2O) C(combustion) C(dust) where, ωC(total) is the total amount of carbon in coke, kg/t; ωC(Vad) is the amount of carbon in volatiles of coke, kg/t; ωC([C]) is the amount of carbon consumed by hot metal carburization, kg/t; ωC(XO) is the amount of carbon consumed by non-ferrous oxide direct reduction, kg/t; ωC(H2O) is the amount of carbon consumed by oxygen of blast moisture, kg/t; ωC(combustion) is the amount of carbon consumed by combustion of coke in raceway, kg/t; ωC(dust) is the amount of carbon in dust, kg/t.

2.1.2. Calculation of Carbon Loss In blast furnace, according to different reaction modes, carbon loss of coke direct reduction reaction can be divided into carbon loss of coupled direct reduction reaction and carbon loss of molten direct reduction reaction. The direct reduction reaction, which is coupled by solution loss reaction and indirect reduction reaction, is regarded as coupled direct reduction reaction. Therefore, the amount of carbon consumed by solution loss reaction is the same as coupled direct reduction reaction. Moreover, molten direct reduction reaction is caused by direct reduction of coke and molten slag. In this model, the reduction zone of iron oxide in blast furnace is divided into 3 parts as follows:

(1) Indirect reduction zone. In this region, the direct reduction reaction has not occurred. The indirect reduction reaction equations as follows:

FeO(s) + CO(g) = Fe(s) + CO2(g) (4)

FeO(s) + H2(g) = Fe(s) + H2O(g) (5)

(2) Coupled direct reduction zone. In this region, solution loss reaction and indirect reduction reaction can be coupled into direct reduction reaction. The solution loss reaction equations are shown as: C(s) + CO2(g) = 2CO(g) (6)

C(s) + H2O(g) = CO(g) + H2(g) (7)

Equation (4) + Equation (6) and Equation (5) + Equation (7) can obtain direct reduction reaction equation as follows: FeO(s) + C(s) = Fe(s) + CO(g) (8) Metals 2018, 8, 979 5 of 14

(3) Molten direct reduction zone. In this region, the indirect reduction reaction has not occurred. And coke is the only material that remains a solid phase, the direct reduction reaction takes place between coke and molten slag. The molten direct reduction reaction equation is shown as:

FeO(l) + C(s) = Fe(l) + CO(g) (9)

The coke consumption in blast furnace is shown in the following ways: combustion in raceway; hot metal carburization; non-ferrous oxide direct reduction; iron oxide direct reduction; volatile matter of coke volatile; coke fines are discharged with gas into dust. The iron oxide direct reduction reaction includes coupled direct reduction reaction and molten direct reduction reaction, and the coke carbon loss of direct reduction includes carbon loss of coupled direct reduction and carbon loss of molten direct reduction. The following assumptions are made for the calculation of coupled direct reduction carbon loss: the coke carbon loss rate is 30% before coke falling into the raceway [19,28]; before coke falling into raceway, the coke carbon loss is caused by coupled direct reduction, hot metal carburization, coke volatiles and coke fines that is discharged into dust; the carbon loss of molten direct reduction is ignored before coke drops into raceway; the hot metal carburization can be completed 80% before hot metal droplets drop into raceway[29,30]. The formula for carbon loss of coupled direct reduction is as follows:

ωC(CDR) = φLC × ωC(total) − ωC(Vad) − ωC(dust) − φCC × ωC([C]) (10) where, ωC(CDR) is the amount of carbon loss of coupled direct reduction, kg/t; φLC is the coke carbon loss rate before coke falling into the raceway, it is 0.3 in assumptions; φCC is the completed ratio of hot metal carburization, it is 0.8 in assumptions. The formula for carbon loss of molten direct reduction is as follows:

ωC(MR) = ωC(DR) − ωC(CDR) (11) where, ωC(MR) is the amount of carbon loss of molten direct reduction, kg/t.

2.2. Blast Furnace Emission Reduction Optimization Mathematical Model Based on the carbon loss calculation model, the blast furnace emission reduction optimization mathematical model is established by using the multiple-objective optimization method. And then, this optimization model is used for single-objective and multi-objective optimization of cost and CO2 emissions.

2.2.1. Optimization Variables

There are many factors that affect the cost and CO2 emissions of blast furnace process. In order to facilitate calculation and investigate the impact of some variables on the model, the oxygen enrichment rate, blast temperature and blast humidity are fixed values when seeking the optimal solution. The optimization variables of the model are selected based on the raw material structure, fuel structure, operation parameters and product quality parameters of the B# blast furnace in Bayisteel, as shown in Table1. Metals 2018, 8, 979 6 of 14

Table 1. Optimization variables and optimum solution of single and multiple objective and actual value of blast furnace.

Single Objective Optimization Multi-Objective Actual Parameter Variable CO2 Emissions of CO2 Emissions of Cost Blast Furnaces Ironmaking Optimization Value Process System

Sinter consumption (kg/t) x1 1518 1005 1005 1337 1309 Pellet 1 consumption (kg/t) x2 115 605 605 202 211 Pellet 2 consumption (kg/t) x3 0 0 0 0 65 Pellet 3 consumption (kg/t) x4 0 0 0 0 50 Ore consumption (kg/t) x5 56 0 0 150 28 Flux consumption (kg/t) x6 0 0 0 0 4 Coke consumption (kg/t) x7 400 410 410 401 425 Pulverized coal x 152 97 97 149 110 consumption (kg/t) 8 3 Blast volume (m /t) x9 1394 1268 1268 1390 1371 Oxygen enrichment rate (%) x10 0 0 0 0 0 3 Blast humidity (g/m ) x11 3 3 3 3 3 ◦ Blast temperature ( C) x12 1112 1112 1112 1112 1112 3 Gas volume (m /t) x13 1863 1684 1684 1856 1676 Fe content (%) x14 93.871 93.911 93.911 93.871 94.23 Pig iron C content (%) x15 5.2 5.2 5.2 5.2 4.80 Pig iron Si content (%) x16 0.4 0.4 0.4 0.4 0.43 Pig iron P content (%) x17 0.077 0.077 0.077 0.077 0.097 Pig iron Mn content (%) x18 0.32 0.28 0.28 0.32 0.308 Pig iron S content (%) x19 0.047 0.047 0.047 0.047 0.02 Pig iron Ti content (%) x20 0.085 0.085 0.085 0.085 0.12 Theoretical combustion — 2073 2141 2141 2077 2133 temperature (◦C) Cost (RMB/t) — 1586 1657 1657 1589 1650 CO emissions of blast 2 — 1146 1112 1112 1145 1158 furnaces process (kg/t) CO emissions of ironmaking 2 — 1665 1600 1600 1645 1672 system (kg/t) Carbon loss of coupled direct — 59.00 61.80 61.80 59.37 68.68 reduction (kg/t) Carbon loss of coupled direct reduction as a percentage of — 17.13 17.51 17.51 17.17 18.77 total carbon in coke (%)

2.2.2. Objective Functions In this model, the cost of blast furnace ironmaking is the net cost of the consumption of raw materials, fuel, air blast, and oxygen enrichment consumed for producing 1 ton of hot metal, after deducting the revenue from the recovery of blast furnace gas. The objective function of cost can be described as follows n P = ∑ pixi (12) i=1 where, P is the cost of blast furnace ironmaking, and pi is the unit price of variable xi. The calculation of CO2 emissions in the blast furnace process is based on the conservation of carbon. The carbon emission is that the amount of input carbon minus the amount of deductible carbon emission in the products and by-products. CO2 emissions of blast furnaces process equals the CO2 equivalent amount at the input end minus the amount of deductible CO2 emissions at the output end. The calculation of CO2 emissions of ironmaking system should add the indirect emission of sinter, pellets, coke, and power medium at the input of blast furnace process emissions. The objective function of CO2 emission can be written as follows

n n n C = Cd + ∑ ciixi = ∑ cdixi + ∑ ciixi (13) i=1 i=1 i=1 where, C is the CO2 emissions of ironmaking system, Cd is the CO2 emissions of blast furnace process, cdi is the direct emission factors of variable xi and cii is the indirect emission factors of variable xi. Metals 2018, 8, 979 7 of 14

2.2.3. Constraint Conditions In order to ensure that the operating parameters of the blast furnace are within a reasonable range during the optimization process, the constraint conditions are indispensably established. The constraint conditions can be described as follows

Cni ≤ Ci(x) ≤ Cmi(i = 0, 1, . . . , m) (14) where, Ci(x) indicates that there are i constraint conditions, Cmi is the upper limit of the i th constraint condition, and Cni is the lower limit of the i th constraint condition. Constraint conditions include balance constraints, process constraints and specific constraints. Material balance and heat balance are the most basic constraints of the model as the establishment of a model must conform to the conservation of matter and conservation of energy. The process constraints are the constraints on the process parameters, representing the control of the blast furnace ironmaking process. The specific constraints are the constraints on the relationship between the different parameters based on the production statistics. The balance constraints include material balance, heat balance, element balance, etc. Process constrains include the basicity constraint of slag, product parameter constraints, etc. Specific constraints include theoretical combustion temperature constraint, bosh gas volume constraint, etc.

3. Results and Discussions

3.1. Different Objective Optimization Results In the optimization model, the Generalized Reduced Gradient solving method is used in the single objective optimization, and the evaluation function method is used in the multi-objective optimization. Firstly, the single-objective optimization of cost, CO2 emissions of blast furnaces process, and CO2 emissions of ironmaking system are performed respectively. And then, multi-objective optimization is solved by the evaluation function method. Finally, the optimization results are compared with the actual production data to verify the reliability of multi-objective optimization. After multi-objective optimization of cost and CO2 emissions, the results show that the cost is decreased by 60.94 RMB/t (RMB is the currency of China), the CO2 emissions of blast furnaces process is reduced by 12.80 kg/t, the CO2 emissions of ironmaking system is reduced by 27.16 kg/t and the carbon loss of coupled direct reduction is reduced by 9.31 kg/t, as shown in Table1.

3.2. Analysis of Main Influence Factors In order to investigate the influence of main factors such as coke ratio, coal rate, blast temperature, blast humidity, oxygen enrichment rate and burden metallization ratio on the cost, carbon loss and CO2 emissions in the blast furnace production process, the control variables method is used for major factors respectively based on the blast furnace optimization model under the same conditions of other constraints, and multi-objective optimization is conducted to obtain the impact of major factors on costs, carbon loss of coupled direct reduction, CO2 emissions of blast furnace process, and CO2 emissions of ironmaking system.

3.2.1. Coke Ratio and Coal Rate

Figure1a shows the relationship between the coke ratio and the cost and CO 2 emissions. It can be seen that with the increase of coke ratio, there is an increase of cost, and both CO2 emissions slightly increase. Figure1b shows the relationship between the coke ratio and carbon loss of coupled direct reduction and its percentage of total carbon in coke. This figure shows that as coke ratio increases, the carbon loss of coupled direct reduction and its percentage of total carbon in coke increase. Metals 2018, 8, x FOR PEER REVIEW 8 of 14 Metals 2018, 8, x FOR PEER REVIEW 8 of 14 reduction and its percentage of total carbon in coke. This figure shows that as coke ratio increases, Metalsreductionthe2018 carbon, 8, 979and loss its of percentagecoupled direct of total reduction carbon and in cokeits percentage. This figure of totalshows carbon that asin coke increase.ratio increases, 8 of 14 the carbon loss of coupled direct reduction and its percentage of total carbon in coke increase.

(a) (b) (a) (b) Figure 1. (a) Effect of coke ratio on cost and CO2 emissions; (b) Effect of coke ratio on carbon loss of FigureFigurecoupled 1. (1.a direct )(a Effect) Effect reduction of of coke coke ratioand ratio its on onpercentage cost cost and and CO ofCO total22 emissions;emissions; carbon in ( (b bcoke.)) Effect Effect of of coke coke ratio ratio on on carbon carbon loss loss of of coupledcoupled direct direct reduction reduction and and its its percentage percentage of of total total carboncarbon inin coke.

Figure 2a shows the relationship between the coal rate and the cost and CO2 emissions. It can be FigureFigure2a 2a shows shows the the relationship relationship between between the the coal coal rate rate andand thethe costcost and CO 2 emissions.emissions. It Itcan can be be seen from the figure that as coke ratio increases, the cost and CO2 emissions decrease,2 and both the seen from the figure that as coke ratio increases, the cost and CO2 emissions decrease, and both the seendecrement from the of figure CO2 thatemissions as coke are ratio small. increases, Figure 2b the shows cost and the CO effect2 emissions of coal rate decrease, on carbon and loss both of the decrementdecrementcoupled ofdirect COof CO2 reductionemissions2 emissions and are are small.its small.percentage Figure Figure2b of shows 2btota showsl thecarbon effect the ineffect of coke. coal of rateThis coal on figurerate carbon on shows carbon loss that of loss coupled with of directcoupledincrease reduction ofdirect coal and reductionrate, its the percentage carbon and its loss ofpercentage totalof coupled carbon of direct tota in coke.l reductioncarbon This in figure andcoke. its shows Thispercentage figure that with ofshows total increase thatcarbon with of coalin rate,increasecoke the decrease. carbon of coal loss rate, of the coupled carbon direct loss of reduction coupled direct and its reduction percentage and of its total percentage carbon of in total coke carbon decrease. in coke decrease.

(a) (b) (a) (b) FigureFigure 2. (2.a )(a Effect) Effect of of coal coal rate rate on on cost cost and and CO CO22 emissions;emissions; ( (bb)) Effect Effect of of coal coal rate rate on on carbon carbon loss loss of of coupledFigurecoupled direct 2. direct (a) reductionEffect reduction of coal and and rate its its percentageon percentage cost and of ofCO total total2 emissions; carbon carbon inin ( b coke.coke.) Effect of coal rate on carbon loss of coupled direct reduction and its percentage of total carbon in coke. SinceSince coke coke can can be be partly partly replaced replaced by by injection injection ofof pulverizedpulverized coal, coal, and and the the price price of of coke coke is ismuch much higherhigher thanSince than thatcoke that of can of pulverized pulverized be partly coal,replaced coal, so so the theby increase injectionincrease ofof coalcoalpulverized rate can coal, reduce reduce and the thethe cost. cost.price With Withof coke the the decreaseis decrease much higher than that of pulverized coal, so the increase of coal rate can reduce the cost. With the decrease of cokeof coke ratio ratio and and the the increase increase of of coal coal rate, rate, thethe volumevolume of bosh gas increases, increases, and and the the proportion proportion of of of coke ratio and the increase of coal rate, the volume of bosh gas increases, and the proportion of reducingreducing components components in in the the bosh bosh gasgas increases, resulting resulting in in deterioration deterioration of the of thekinetic kinetic conditions conditions of reducing components in the bosh gas increases, resulting in deterioration of the kinetic conditions of of thethe solutionsolution loss reaction. Hence, Hence, with with the the decrease decrease of of coke coke ratio ratio and and the the increase increase of ofcoal coal rate, rate, the the the solution loss reaction. Hence, with the decrease of coke ratio and the increase of coal rate, the carboncarbon loss loss of of coupled coupled direct direct reduction reduction decreases. decreases. However, coke coke is is indispensable indispensable material material in inthe the carbonblast furnace, loss of ascoupled it sustains direct the reduction passages decreases.of liquid metal However, and slag coke toward is indispensable the lower part material and of in high the blastblast furnace, furnace, as as it it sustains sustains the the passages passages ofof liquidliquid metal and slag slag toward toward the the lower lower part part and and of ofhigh high temperaturetemperature reducing reducing gas gas toward toward the the upper upper part. part. Hence, Hence, coke coke ratio ratio cannot cannot be be too too small.small. IfIf cokecoke ratio is temperatureis too small, thereducing smooth gas operation toward theof blast upper furnace part. Henc may e,not coke be ensured.ratio cannot Besides, be too in small. order If to coke maintain ratio too small, the smooth operation of blast furnace may not be ensured. Besides, in order to maintain isthe too theoretical small, the combustion smooth operation temperature of blast within furnace a reas maonabley not be range, ensured. the Besides,coal rate incannot order be to increasedmaintain thethe theoretical theoretical combustion combustion temperature temperature withinwithin aa reasonablereasonable range, range, the the coal coal rate rate cannot cannot be be increased increased without limit. Although the coke ratio and coal rate have little effect on CO2 emissions, reducing coke withoutwithout limit. limit. Although Although the the coke coke ratio ratio and and coalcoal raterate have little effect effect on on CO CO2 2emissions,emissions, reducing reducing coke coke ratio and increasing coal rate can also slightly reduce CO2 emissions. Therefore, it is necessary to reduce coke ratio and increase coal rate as much as possible within a reasonable range. According to actual production statistical data of B# blast furnace in Bayisteel, the reasonable range of the coke ratio is about 370–500 kg/t, and the reasonable range of coal rate is about 20–170 kg/t. Metals 2018, 8, x FOR PEER REVIEW 9 of 14

ratio and increasing coal rate can also slightly reduce CO2 emissions. Therefore, it is necessary to reduce coke ratio and increase coal rate as much as possible within a reasonable range. According to Metals 2018 8 actual ,production, 979 statistical data of B# blast furnace in Bayisteel, the reasonable range of the coke9 of 14 ratio is about 370–500 kg/t, and the reasonable range of coal rate is about 20–170 kg/t. 3.2.2. Blast Temperature 3.2.2. Blast Temperature Figure3a shows the effect of blast temperature on the cost and CO 2 emissions. As can be seen Figure 3a shows the effect of blast temperature on the cost and CO2 emissions. As can be seen from the figure, with the increase of blast temperature, the cost and CO2 emissions reduce. Figure3b from the figure, with the increase of blast temperature, the cost and CO2 emissions reduce. Figure 3b showsshows the the relationship relationship between between the the blast blast temperaturetemperature and carbon loss loss of of coupled coupled direct direct reduction reduction andand its its percentage percentage of of total total carboncarbon in in coke. coke. It Itcan can be beseen seen from from the figure the figure that with that the with increase the increase of blast of blasttemperature, temperature, the thecarbon carbon loss lossof coupled of coupled direct direct reduction reduction and its and percentage its percentage of total of carbon total carbonin coke in cokereduce. reduce. AsAs the the blast blast temperature temperature increases, increase thes, physicalthe physical heat broughtheat brought by the by blast theincreases, blast increases, so the chemicalso the heatchemical emitted heat by carbonemitted combustionby carbon combustion decreases accordingly,decreases accordingly, and the carbon and the emission carbon emission also decreases. also In addition,decreases. increasingIn addition, the increasing blast temperature the blast temper also providesature also conditions provides conditions for increasing for increasing the amount the of pulverizedamount of coal. pulverized And injection coal. And of pulverized injection of coal pulver canized replace coal partcan replace of coke. part As of the coke. blast As temperature the blast increases,temperature the amount increases, of carbonthe amount loss ofof coupled carbon loss direct of reductioncoupled direct decreases. reduction With decreases. the increase With of the blast temperature,increase of theblast coke temperature, ratio decreases, the coke and ratio the decr proportioneases, and of the reducing proportion components of reducing in components the bosh gas increases,in the bosh resulting gas increases, in deterioration resulting of in the deterioration kinetic conditions of the ofkinetic the solutionconditions loss of reaction. the solution Therefore, loss reaction. Therefore, increasing the blast temperature can reduce the cost, CO2 emissions and carbon increasing the blast temperature can reduce the cost, CO2 emissions and carbon loss of coupled directloss reduction. of coupled direct reduction.

(a) (b)

FigureFigure 3. (3.a) ( Effecta) Effect of of blast blast temperature temperature on on cost cost and and COCO22 emissions; ( (bb)) Effect Effect of of blast blast temperature temperature on on carboncarbon loss loss of of coupled coupled direct direct reduction reduction and and its its percentage percentage ofof totaltotal carbon in coke. coke.

3.2.3.3.2.3. Blast Blast Humidity Humidity

FigureFigure4a shows4a shows the the effect effect of blast of blast humidity humidity on theon the cost cost and and CO CO2 emissions.2 emissions. The The figure figure shows shows that as thethat blast as the humidity blast humidity increases, increases, the cost andthe COcost2 emissionsand CO2 emissions increase. Figureincrease.4b showsFigure the4b relationshipshows the betweenrelationship the blast between humidity the andblast carbon humidity loss and of coupled carbon directloss of reduction coupled anddirect its reduction percentage and of totalits carbonpercentage in coke. of It total can becarbon seen in from coke. the It figure can be that seen with from the the increase figure of that blast with humidity, the increase the carbon of blast loss of coupledhumidity, direct the carbon reduction loss and of coupled its percentage direct ofreduct totalion carbon and its in cokepercentage increase. of total carbon in coke increase.With the increase of blast humidity, the heat consumption in raceway increases, so carbon consumptionWith the increases increase accordingly, of blast humidity, and the the carbon heat emissionconsumption also in increases. raceway Moreover,increases, so as thecarbon blast humidityconsumption increases, increases the theoreticalaccordingly, combustion and the carbon temperature emission also decreases. increases. In Moreover, order to maintainas the blast the theoreticalhumidity combustion increases, the temperature theoretical within combustion a reasonable temperature range, decreases. it is necessary In order to reduce to maintain the amount the theoretical combustion temperature within a reasonable range, it is necessary to reduce the amount of pulverized coal and increase the amount of coke. As the coke ratio is increased, the specific of pulverized coal and increase the amount of coke. As the coke ratio is increased, the specific surface surface area of coke is increased, resulting in an increase in carbon loss of coupled direct reduction. area of coke is increased, resulting in an increase in carbon loss of coupled direct reduction. Accordingly, decreasing the blast humidity can reduce the cost, CO2 emissions and carbon loss of coupled direct reduction.

Metals 2018, 8, x FOR PEER REVIEW 10 of 14 Metals 2018, 8, x FOR PEER REVIEW 10 of 14

MetalsAccordingly,2018, 8, 979 decreasing the blast humidity can reduce the cost, CO2 emissions and carbon loss10 of 14 Accordingly,coupled direct decreasing reduction. the blast humidity can reduce the cost, CO2 emissions and carbon loss of coupled direct reduction.

(a) (b) (a) (b) FigureFigure 4. (4.a) (a Effect) Effect of of blast blast humidity humidity on on cost cost and and COCO22 emissions; ( (bb)) Effect Effect of of blast blast humidity humidity on on carbon carbon lossFigureloss of coupledof coupled4. (a) Effect direct direct of reduction blast reduction humidity and and its onits percentage costpercentage and CO of of2 emissions; total total carboncarbon (b ) inin Effect coke. of blast humidity on carbon loss of coupled direct reduction and its percentage of total carbon in coke. 3.2.4.3.2.4. Oxygen Oxygen Enrichment Enrichment Rate Rate 3.2.4. Oxygen Enrichment Rate FigureFigure5a shows5a shows the the effect effect of of oxygen oxygen enrichment enrichment rate rate on on the the costcost andand CO22 emissions.emissions. The The figure figure Figure 5a shows the effect of oxygen enrichment rate on the cost and CO2 emissions. The figure showsshows that that as as the the oxygen oxygen enrichment enrichment rate rate increases, increases, thethe costcost decreases and and CO CO2 2emissionsemissions decreases decreases slightly.showsslightly. Figurethat Figure as5 theb 5b shows oxygen shows the enrichmentthe relationship relationship rate betweenincreases, between the the oxygencostoxygen decreases enrichmentenrichment and CO rate rate2 emissions and and carbon carbon decreases loss loss of of slightly. Figure 5b shows the relationship between the oxygen enrichment rate and carbon loss of coupledcoupled direct direct reduction reduction and and itsits percentage of of total total carbon carbon in incoke. coke. It can It canbe seen be seenfrom fromthe figure the figurethat coupled direct reduction and its percentage of total carbon in coke. It can be seen from the figure that thatwith with the the increase increase of of oxygen oxygen enrichment enrichment rate, rate, the the carbon carbon loss loss of of coupled coupled direct direct reduction reduction and and its its with the increase of oxygen enrichment rate, the carbon loss of coupled direct reduction and its percentagepercentage of totalof total carbon carbon in in coke coke decrease. decrease. percentage of total carbon in coke decrease. AsAs the the oxygen oxygen enrichment enrichment raterate increases, the the th theoreticaleoretical combustion combustion temperature temperature increases. increases. In As the oxygen enrichment rate increases, the theoretical combustion temperature increases. In In orderorder toto maintainmaintain the theoretical combustion combustion temper temperatureature within within a areasonable reasonable range, range, it itis isnecessary necessary order to maintain the theoretical combustion temperature within a reasonable range, it is necessary to increaseto increase the the amount amount of of pulverized pulverized coal, coal, leading leading toto decreasedecrease the amount amount of of coke, coke, thereby thereby reducing reducing tothe increase cost. As the the amount coke ratio of pulverized is decreased, coal, the leading specific to surface decrease area the of amount coke is of decreased, coke, thereby resulting reducing in a the cost. As the coke ratio is decreased, the specific surface area of coke is decreased, resulting in a thedecrease cost. Asin carbon the coke loss ratio of coupled is decreased, direct thereduction. specific Therefore, surface area increasing of coke theis decreased, oxygen enrichment resulting inrate a decreasedecrease in carbonin carbon loss loss of of coupled coupled direct direct reduction. reduction. Therefore, increasing increasing the the oxygen oxygen enrichment enrichment rate rate can reduce the cost, CO2 emissions and carbon loss of coupled direct reduction. cancan reduce reduce the the cost, cost, CO CO2 emissions2 emissions and and carbon carbon lossloss ofof coupled direct reduction. reduction.

(a) (b) (a) (b) Figure 5. (a) Effect of oxygen enrichment rate on cost and CO2 emissions; (b) Effect of oxygen FigureFigureenrichment 5. 5.(a )(a Effectrate) Effect on ofcarbon of oxygen oxygen loss enrichment ofenrichment coupled direct raterate reduction onon costcost andand and COits 2percentage emissions;emissions; of ( btotal (b) )Effect Effect carbon of of inoxygenoxygen coke. enrichmentenrichment rate rate on on carbon carbon loss loss of of coupled coupled direct directreduction reduction andand its percentage of of total total carbon carbon in in coke. coke.

3.2.5. Burden Metallization Ratio

Figure6a shows the effect of burden metallization ratio on the cost and CO 2 emissions. The figure shows that as the burden metallization ratio increases, the cost and CO2 emissions decreases gradually. Figure 6b shows the relationship between the burden metallization ratio and carbon loss of coupled direct reduction and its percentage of total carbon in coke. It can be seen from the figure that with the Metals 2018, 8, x FOR PEER REVIEW 11 of 14

3.2.5. Burden Metallization Ratio

Figure 6a shows the effect of burden metallization ratio on the cost and CO2 emissions. The Metalsfigure2018 ,shows8, 979 that as the burden metallization ratio increases, the cost and CO2 emissions decreases11 of 14 gradually. Figure 6b shows the relationship between the burden metallization ratio and carbon loss of coupled direct reduction and its percentage of total carbon in coke. It can be seen from the figure increasethat with of burden the increase metallization of burden ratio, metallization the carbon rati losso, ofthe coupled carbon loss direct of reductioncoupled direct and itsreduction percentage and of totalits carbonpercentage in coke of total decrease. carbon in coke decrease. TheThe method method of of increasing increasing thethe burden metallization metallization rate rate is to is add to add scrap scrap iron ironto blast to blastfurnace. furnace. As Asthe the burden burden metallization metallization rate increases, rate increases, the amount the of amount scrap iron of that scrap does iron not thatundergo does the not oxidation– undergo thereduction oxidation–reduction reaction in the reaction blast furnace in the increases, blast furnace resulting increases, in the decrease resulting in cost, in the CO2 decrease emissions in and cost, COcarbon2 emissions loss of and coupled carbon direct loss ofreduction. coupled directIn addition, reduction. the melting In addition, of scrap the iron melting will ofabsorb scrap a iron large will absorbamount a large of heat, amount so the of burden heat, so metalli the burdenzation metallizationrate should not rate be too should large. not be too large.

(a) (b)

FigureFigure 6. 6.(a )(a Effect) Effect of of burden burden metallization metallization ratioratio onon costcost and CO 22 emissions;emissions; (b ()b )Effect Effect of of burden burden metallizationmetallization ratio ratio on on carbon carbon loss loss of coupledof coupled direct direct reduction reduction and and its percentageits percentage of totalof total carbon carbon in coke. in coke. 3.2.6. Ore Consumption 3.2.6. Ore Consumption Figure7a shows the effect of ore consumption on the cost and CO 2 emissions. The figure shows that as theFigure ore 7a consumption shows the effect increases, of ore consumption the cost significantly on the cost reduces and CO first,2 emissions. and then The the figure cost shows slightly increases.that as the When ore consumption the ore consumption increases, isthe 60 cost kg/t, sign theificantly cost isreduces the lowest. first, and It canthen bethe seen cost thatslightly with theincreases. increase When of ore the consumption, ore consumption CO2 emissions is 60 kg/t, ofthe blast cost furnacesis the lowest. process It can increases be seen firstly,that with and the the increase of ore consumption, CO2 emissions of blast furnaces process increases firstly, and the increment of CO2 emissions becomes smaller after the amount of ore exceeds 60 kg/t. And, it can be increment of CO2 emissions becomes smaller after the amount of ore exceeds 60 kg/t. And, it can be seen that with the increase of ore consumption, the CO2 emissions of ironmaking system increases seen that with the increase of ore consumption, the CO2 emissions of ironmaking system increases first and then decreases, the CO2 emissions of ironmaking system is the maximum when the ore consumptionfirst and then reaches decreases, 60 kg/t. the CO Figure2 emissions7b shows of theironmaking relationship system between is the themaximum ore consumption when the ore and consumption reaches 60 kg/t. Figure 7b shows the relationship between the ore consumption and carbon loss of coupled direct reduction and its percentage of total carbon in coke. It can be seen from carbon loss of coupled direct reduction and its percentage of total carbon in coke. It can be seen from the figure that with the increase of ore consumption, the carbon loss of coupled direct reduction and the figure that with the increase of ore consumption, the carbon loss of coupled direct reduction and its percentage of total carbon in coke remains basically unchanged. its percentage of total carbon in coke remains basically unchanged. Because the ore is cheaper than other iron-bearing raw materials, the use of ore can significantly Because the ore is cheaper than other iron-bearing raw materials, the use of ore can significantly reducereduce costs. costs. However, However, the the use use of of ore ore with with a a relativelyrelatively low iron grade grade will will result result in in an an increase increase in inthe the slagslag ratio. ratio. When When the the ore ore consumption consumption reaches reaches 6060 kg/t,kg/t, the the slag slag ratio ratio reaches reaches the the upper upper limit limit of ofthe the relativerelative constraint constraint condition. condition. As As the the ore ore consumptionconsumption is further increased, increased, in in order order to to make make the the slag slag ratioratio meet meet the the constraint constraint conditions, conditions, it it is is necessary necessary toto reduce the the consumption consumption of of sinter sinter and and increase increase thethe consumption consumption of of pellet, pellet, resulting resulting in in a a slight slight increase increase inin costcost and a a decrease decrease in in the the CO CO2 emissions2 emissions of of ironmakingironmaking system. system. In In addition, addition, the the use use ofof largelarge amountsamounts of of ore ore can can cause cause a adecrease decrease in inslag slag basicity, basicity, whichwhich may may affect affect the the performance performance of of blast blast furnace, furnace, soso thethe oreore consumption should should not not be be too too large. large.

Metals 2018, 8, 979 12 of 14 Metals 2018, 8, x FOR PEER REVIEW 12 of 14

(a) (b)

FigureFigure 7. (7.a) ( Effecta) Effect of of ore ore consumption consumption on on cost cost and and CO CO22 emissions;emissions; ( (bb)) Effect Effect of of ore ore consumption consumption on on carboncarbon loss loss of coupledof coupled direct direct reduction reduction and and its its percentage percentage ofof totaltotal carbon in coke. coke.

4. Conclusions4. Conclusions BasedBased on on the the conservation conservation of of material material andand energyenergy ofof ironmakingironmaking process process and and the the engineering engineering method,method, the the blast blast furnace furnace carbon carbon loss loss calculation calculation modelmodel isis firstlyfirstly established.established. A A multi-objective multi-objective optimizationoptimization mathematical mathematical model model isis thenthen establishedestablished withwith cost and CO CO22 emissionsemissions as as objective objective functionsfunctions based based on on the the carbon carbon loss loss calculation calculation model. model. TheThe optimal solutions solutions of of blast blast furnace furnace burden burden structurestructure and and operating operating parameters parameters forfor singlesingle objectobject and multiple multiple objects objects are are solved solved respectively. respectively. AfterAfter the the multi-objective multi-objective optimization,optimization, the the optimal optimal solution solution shows shows that thatthe co thest is cost reduced is reduced by 60.94 by RMB/t, the CO2 emissions of blast furnaces process is reduced by 12.80 kg/t, and the CO2 emissions 60.94 RMB/t, the CO2 emissions of blast furnaces process is reduced by 12.80 kg/t, and the CO2 emissionsof ironmaking of ironmaking system is system reduced is by reduced 27.16 kg/t. by 27.16Based kg/t. on the Based optimization on the optimizationmodel, the influence model, of the influencemain factors of main such factors as coke such ratio, as coal coke rate, ratio, blast coal temperature, rate, blast blast temperature, humidity, oxygen blast humidity, enrichment oxygen rate and burden metallization ratio on the cost, CO2 emissions and carbon loss are analyzed. The measures enrichment rate and burden metallization ratio on the cost, CO2 emissions and carbon loss are analyzed. to reduce cost, reduce emissions and reduce carbon loss in blast furnace production are as follows: The measures to reduce cost, reduce emissions and reduce carbon loss in blast furnace production are reduce the coke ratio and increase the coal rate within a suitable range; increase the blast temperature; as follows: reduce the coke ratio and increase the coal rate within a suitable range; increase the blast minimize the blast humidity; appropriately increase the oxygen enrichment rate and burden temperature; minimize the blast humidity; appropriately increase the oxygen enrichment rate and metallization rate. burden metallization rate. Author Contributions: Conceptualization, Shengli Wu; Data curation, Shun Yao; Formal analysis, Shun Yao; Author Contributions: Funding acquisition, Conceptualization,Shengli Wu, Mingyin S.W.; Kou Dataand Heng curation, Zhou; S.Y.; Investigation, Formal analysis, Shun Yao; S.Y.; Methodology, Funding acquisition, Shun S.W., M.K. and H.Z.; Investigation, S.Y.; Methodology, S.Y.; Project administration, S.W. and B.S.; Resources, B.S.; Supervision,Yao; Project M.K.; administration, Validation, S.Y.Shengli and K.G.;Wu and Visualization, Bo Song; S.Y.;Resources, Writing–original Bo Song; Su draft,pervision, S.Y.; Writing–review Mingyin Kou; & editing,Validation, M.K. Shun Yao and Kai Gu; Visualization, Shun Yao; Writing – original draft, Shun Yao; Writing – review & editing, Mingyin Kou. Funding: This work was supported by the National Natural Science Foundation of China (Grant Number 51804027),Funding: the This China work Postdoctoral was supported Science by Foundation the National (Grant Natural Number Science 2017M610769) Foundation andof China Fundamental (Grant Number Research Funds51804027), for the Centralthe China Universities Postdoctoral (Grant Science Number Foundati FRF-IC-18-010).on (Grant Number 2017M610769) and Fundamental ConflictsResearch of Interest:Funds forThe the Central authors Univer declaresities no conflict(Grant Number of interest. FRF-IC-18-010). Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

rd direct reduction degree ωC(H2O) the amount of carbon consumed by oxygen of blast moisture, kg/t

ωFe(DR) the amount of Fe that is generated by ωC(combustion) the amount of carbon consumed by direct reduction, kg/t combustion of coke in raceway, kg/t ωFe(HM) the amount of Fe in hot metal, kg/t ωC(dust) the amount of carbon in dust, kg/t

ωC(DR) the carbon loss of direct reduction, kg/t ωC(CDR) the amount of carbon loss of coupled direct reduction, kg/t ωC(total) the total amount of carbon in coke, kg/t ϕLC the coke carbon loss rate before coke falling into the raceway

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Nomenclature rd direct reduction degree ωFe(DR) the amount of Fe that is generated by direct reduction, kg/t ωFe(HM) the amount of Fe in hot metal, kg/t ωC(DR) the carbon loss of direct reduction, kg/t ωC(total) the total amount of carbon in coke, kg/t ωC(Vad) the amount of carbon in volatiles of coke, kg/t ωC([C]) the amount of carbon consumed by hot metal carburization, kg/t ωC(XO) the amount of carbon consumed by non-ferrous oxide direct reduction, kg/t ωC(H2O) the amount of carbon consumed by oxygen of blast moisture, kg/t ωC(combustion) the amount of carbon consumed by combustion of coke in raceway, kg/t ωC(dust) the amount of carbon in dust, kg/t ωC(CDR) the amount of carbon loss of coupled direct reduction, kg/t φLC the coke carbon loss rate before coke falling into the raceway φCC the completed ratio of hot metal carburization ωC(MR) the amount of carbon loss of molten direct reduction, kg/t

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