Study on CO2 gasification reactivity and structure characteristics of

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Catalysis and Kinetics

Study on CO2 gasification reactivity and structure characteristics of carbonaceous materials from Corex furnace Zhengjian Liu, Guangwei Wang, Jianliang Zhang, JuiYuan Lee, Haiyang Wang, Minmin Sun, and Chuan Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00072 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Study on CO2 gasification reactivity and structure characteristics of carbonaceous materials from Corex furnace Zhengjian Liu a, Guangwei Wang a, *, Jianliang Zhang a, Jui-Yuan Leeb, Haiyang Wang a, Minmin Suna , Chuan Wang c,d a

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing,

Beijing 100083, China b

Department of Chmeical Engineering and Biotechnology, National Taipei University of Technology, 1,

Sec.3, Zhongxiao E. Rd., Taipei 10608, Taiwan c

Swerea MEFOS, SE-971 25 Luleå, Sweden

d

Thermal and Flow Engineering Laboratory, Åbo Akademi University, Åbo, FI-20500 Finland.

*Correspondence authors. Tel.:+86 10 62332550; fax:+86 10 62332364. E-mail addresses: [email protected](G.Wang) ABSTRACT: The gasification reactivities of four pretreated carbonaceous materials from the Corex furnace, including one coal char, two metallurgical cokes and one recycling dust, was investigated by thermogravimetric analyzer (TGA), in which the recycling dust came mainly from two metallurgical coke and accounted for a mass fraction of 71.3 %. The physical physicochemical properties of the different samples were tested systematically. The results showed that the recycling dust had a similar gasification reactivity with the coal char and the value of which was higher than those of the two metallurgical cokes. The structure analysis ascertained that the main factor which affected the gasification reactivity was the carbonaceous structure. Moreover, to characterize the reactive behavior of the different samples, three nth-order typical gas-solid reaction models were employed in this study. It has been found that the RPM was the best one. The activation energies of different chars were in the range of 181.1-202.3 kJ/mol. Key words: Corex furnace; Recycling dust; Gasification; Kinetic model. 1. Introduction In recent 30 years the iron and steel industry in China had great achievement, and the annual output of pig iron reached 703 million tons in 2016, accounting for 51% of the total world production [1]. However, the largest proportion of pig iron still came from the blast furnace process, which was a long process route with large investment, serious pollution and low flexibility, especially the good 1

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quality coke indispensable. Due to the scarce of coking coke resource, the development of new iron making processes were increasingly on the agenda [2]. The non-blast furnace iron making process contains direct reduction process and smelting reduction processes. The direct reduction iron making process includes two methods, one is gas based (e.g. natural gas), and the other is coal based. The gas based method is more complicated but widely used with a proportion over 90% of the total world direct pig iron production [3]. However, natural gas resource is limited in China with its high cost of production, and it mainly supplies for residents use. The development of gas based direct reduction process is thus restricted. For the smelting reduction process, the pure oxygen, pulverized coal and lump ore are the main raw materials, and it is more suitable for China to develop because coal is the main energy resource in China[4]. Many smelting reduction technologies have been developed, including Corex, Finex, Hismelt and Dios. Among them, Corex process in South Africa, India and China has realized industrial production. In South Korea, the Finex process developed from Corex also approached to industrial implementation [5]. Compared with traditional blast furnace process, Corex is a short process route with low investment, low cost and low pollution; moreover, the quality of the hot metal is close to that from blast furnace processes. The device of Corex furnace contains up-reduction furnace and low gasification smelting furnace. Iron ore is pre-reduced to a certain metallization level in up-reduction furnace and then lowered into low gasification smelting furnace through screw conveyer lowering for further reduction and smelting to produce hot metal with the aimed temperature and composition [6]. The charged lump coal, the coke and iron ore pulverized seriously under the condition of intense thermal stress in the gasification smelting furnace. A large number of small particles are formed and part of them rises into riser tube. Temperature in the riser tube can reach 1323 K, and a high temperature cyclone dust removal device is set between the gasification smelting furnace and the up-reduction furnace. The dust collected by the cyclone dust removal is a main index reflecting the pulverization character of raw materials. The dust contains metallic iron, iron oxide, carbon and gangue. And among them the metallic iron, iron oxide and gangue are from the iron ore, whereas the carbon is from the pulverization of lump ore and coke [7]. In order to consume the dust within the process, a row of burner nozzles containing dust and oxygen 2 ACS Paragon Plus Environment

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nozzles are set at the top of the gasification smelting furnace. Oxygen and the recycling dust are injected from the dust nozzle to gasify the carbon in the dust forming CO, then entering into the top gas. At the same time the iron oxide is reduced and entered into the hot metal. Oxygen is injected from the oxygen nozzle to adjust both the gas composition and temperature at the top of the gasification smelting furnace [8]. One advantage of Corex compared to the blast furnace process is that it could lower the outputs of pollutant and dust. However, up to now, the study on the reaction behavior of recycling dust from the gasification smelting furnace is still limited. In order to more effectively utilize the recycling dust, the physicochemical properties and gasification characteristics of the recycling dust are necessary to be thoroughly investigated. Several reports have been published to investigate the reaction characteristics of the recycling dust. Du et al. [9] investigated the performance of recycling dust in the freeboard of Corex melter gasifier. The results showed that the shape of recycling was irregular and granulometric distribution was uneven, the combustion of carbon in the recycling dust mainly takes place in front of the dust burner, and the combustion ratio almost reach 100 %. Wang et al. [10] analyzed the combustion characteristics of the recycling dust from the Corex melter gasifier under high oxygen content atmosphere. The combustion rate curve of the recycling dust had two significant peaks, corresponding to the combustion peaks of coal char and coke fine. The observation was in accordance with the conclusion that the carbon from the unconsumed coal and coke fine entered the recycling dust. Meanwhile, Berger et al. [11] studied the combustion performance of the recycling dust in front of the dust burner with different numerical approaches. The influence of the burner activity for the dome gas, on the base of the change of temperature and species composition, was studied. However, the gasification characteristic, conversion and recycling of the dust in the melter gasifier has received scant attention. While, such investigation is crucial to deeply understand its essential characteristics including physical-chemical structures and reaction kinetics behaviors, which can provide some fundamental supports for its efficient and reasonable utilization. Therefore, there is still a strong need to conduct some more studies on the topic. Some work has been performed on the physicochemical properties and combustion characteristics but for the dust from blast furnace. Chang et al. [12] have carried out tests to have an in-depth 3 ACS Paragon Plus Environment

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understanding on steel plant metallurgical dusts by using laser particle size analyzer, chemical analysis, XRD, SEM-EDS and FTIR. Zhao et al. [13] studied the carbon constitution and micro structure of acid-washing dust from blast furnace by applying SEM, laser particle size analyzer, XRD, thermogravimetric analysis and polarization microscopy. Results indicated that the carbonaceous compositions of the dust were mainly came from the coke fines, especially the carbonaceous compositions in the bag dust of the hop pocket, which had a more disordered crystalline structure and thus had a high reactivity. Gupta et al. [14] investigated three kinds of blast furnace dust through chemical analysis and found that for the particle size over 90 µm most of the carbonaceous structure was coke, whereas for that with a size over 250 µm it was totally coke. Machado et al. [15] quantified the percentages of the carbonaceous materials from the blast furnace dust using XRD associated with chemical analysis, and a simplified quantification method using only three size fractions was proposed. Raman spectrum characteristic values of the standard coal and coke blended samples were obtained by Yu et al. [16]. Through comparing Raman spectrum characteristic values of the different samples with standard function, the content of unburned coal and coke in the blends were distinguished. Wu et al. [17] applied the microscopic and chemical analysis, further determined the content of the unconsumed fine coke and pulverized coal in the blast furnace dust (bag dust and gravitational dust of hop pocket). These analysis methods can offer a great guidance for the study on the dust from Corex. In the current research, in order to more detailed study the physicochemical properties and gasification characteristics of carbonaceous materials of the recycling dust in more detail, four samples of coal char, coke and recycling dust were firstly treated by acid-washing to eliminate the influence of mineral elements on constitution and structure. The physicochemical properties of the samples were characterized by the element analysis, SEM, Raman spectrum and Petrographic analysis. The gasification reactivities of these four samples at the dome temperature were investigated by the thermogravimetric analyzer and the kinetic characteristics during the gasification process were studied using the non-linear fitting method. This research is expected to facilitate understand the gasification process of recycling dust and offer necessary information for the design and operation of the Corex process.

4


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2. Material and methods 2.1 Material preparation and analysis One original coal, two metallurgical cokes (Coke-1, Coke-2) and one recycling dust from the Corex melter gasifier were collectted from a steel plant. All samples were pulverized and sieved to RC > MC-2 > MC-1. From above results, a conclusion can be drawn that the total pore volume and surface area of both MC-1 and MC-2 are respectively far higher than those of the CC and the RC. The pore volume distribution of the different pore size ranges is an important parameter to characterize the porous structure of different chars. The Pore volume distribution results with different pore size ranges are shown in Fig.2. It is observed that 7 ACS Paragon Plus Environment

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the values of MC-1 and MC-2 are obviously larger than those of the CC and the RC, respectively. This is likely due to the removal of a large quantity of volatile matter while long time coking process, resulting in an increasing porosity. According to Fig.2(a), the pores are divided into two classifications: meso-macro-pore larger than 10 nm and micro-meso-pores less than 10 nm, and the pore volume ratio (RV= micro-meso-pores/ meso-macro-pore) between the two ones are calculated, results are shown in Fig.2(b). As shown, values of the MC-1 and MC-2 are obviously smaller than those of the CC and the RC, which indicates that compared with the CC and the RC mesoporous and macroporous in MC-1 and MC-2 contribute more in total pore volume area and total specific surface. The carbonaceous structures of four samples were analyzed by Raman spectrum analyzer, as illustrated in Fig.3. It could be found that the samples have similar Raman spectra with two peaks appear at a shift of ~1600 and ~1350 cm-1, which referring to G and D bands [19]. The Raman spectra were separated into five peaks to further obtain the parameters (peak position, bandwidth, integrated intensity ration, etc.) [20]. Positions of D1, D2, D3, D4, G peaks were located at about 1340-1358, 1600-1620, 1500-1550, 1180-1200, and 1590-1610 cm-1, respectively. The fitting results of the separated Raman spectra peak are depicted in Fig.3 with a good agreement. The value of specific parameters for CC、MC-1、MC-2 and RC are listed in Table 3. It indicated that for CC and MC-2, the intensity of D1, D3 and D4 peaks are lower than that of MC-1 and RC, which represents that the content of the disorder structure like sp2、sp3 and C=C in CC and MC-2 is relatively low. Meanwhile, comparing to the peaks of D2 and G, the intensity for the samples of MC-1 and RC is obviously higher than that in CC and MC-2, which represents that the graphitic crystalline structure in the samples of MC1 and RC is high. By these analysis, conclusion could be drawn that the ordering degree of different samples could not be ascertained only based on the intensity of D1, D2, D3, D4 and G. According to work of Sheng et al. [21,22], the values of ID3+D4/IG and IG/IAll could be used to represent the ordering degree of different samples. High ID3+D4/IG value represented low ordering degree and high IG/IAll value represents high ordering degree. The comparison of ID3+D4/IG and IG/IAll for different samples ascertains that the ordering degree from high to low is MC-1, MC-2, CC and RC. After a long 8


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time and high temperature coking process, the volatilization is complete, and thus the aromatization and graphite degree in MC-1 and MC-2 are high. The CC char was prepared under laboratory condition with a coking temperature and time as 1473 K and 30 min, and the branched structure was destroyed totally. Therefore, the order of the carbonaceous structure graphite degree for CC is higher than that of RC. The pyrolysis condition in laboratory is different from that in Corex, and the influences of heating rate, temperature, time and atmosphere on pyrolysis process need to be considered in detail. The graphite degree of RC is obviously lower than those for two cokes, and this is due to existence of unreacted pulverized coal in RC, as shown in SEM pictures. On the basis of the above analysis, it could be ascertained that the carbonaceous materials in the recycling dust are from the unconsumed coal and coke fine. However, the ratio of different materials in RC cannot be obtained from the SEM and Raman analysis. It is common to use the petrographic analysis, the XRD quantitative analysis and the Raman spectra analysis to ascertain the origin of the carbonaceous materials for the blast furnace dust [13,14,17,23]. For the XRD and Raman methods, the standard contrast sample needed to be prepared, associating with a serious producing condition and high cost, and they were not widely used yet. The petrographic analysis method is easy to carry out, and it has become a usual method to analyze the origin of the carbonaceous materials [17]. Fig.4(a), (b) and (c) are the common microstructure for the unconsumed coal, which marked as A, B and C. For Fig.4(a), in the early stage of combustion, the pyrolysis proceeds quickly, and cracks and zigzag edge appear on the particle surface. In Fig.4(b), many large cracks and erosion pores could be observed, and the cracks are produced by total evaporation of moisture while the pores are produced by incomplete combustion of carbon. As shown in Fig.4(c), after volatilization, many pores are produced. On the surface and the inner wall of pores with reaction proceeding, the honeycomb structure is produced which is with low density as one tenth of coal. (d), (e), (f), (g), (h), and (i) are important microstructures in coke, and (d), (e), (f), (g) are anisotropy, while (i) is isotropy and all of them can be marked as D, E, F, G, H and I. Fig.4(d) and (e) are the typical mosaic structure constituted by euhedral and unhedral grains. Fig.4(f) is the fluid structure that could be judged from river like curve on the surface. Fig.4(g) is inertia grain, and it would not change in coking process. Fig.4(h) is silk like carbon, 9 ACS Paragon Plus Environment

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and the grains show net array. Fig.4(i) is block structure, the carbon grains show random directional irregular array for which the optical characteristics would not change with direction, and under optical microscope the surface is smooth with same color. The area ratio of the above mentioned nine micro constituents to the total surface area is shown as follows: A-16.14%, B-2.64%, C-23.78%, D-18.6%, E-28.1%, F-2.54%, G-0%, H-3.29%, I-2.92%. With ascending of coal gas, the consumption rate of unconsumed coal and coke fine in dust is different. Densities of these two carbonaceous materials are different, and so amending factors should be introduced when using the volume fraction presenting their mass fraction. The specific surface area of the carbonaceous material is amended using empirical formula. According to Wu et al. [17,24], the amending factors were ascertained as α=0.9, β=0.6, γ=0.1. Since the content of gangue and the iron oxide in the recycling dust after acid washing are very low and the dust is with very fine spherical shape, the area ratio is close to the volume ratio. Subsequently, the residual mass fraction of the unconsumed coal and coke fine could be calculated according to Eqs. (2) and (3). The mass fraction of unconsumed coal is 28.7 % and it is 71.3 % for the coke fine.

α × (AD +AE +AF +AG +AH +AI ) α × (AD +AE +AF +AG +AH +AI )+AA +β × AB +γ × AC AA +β × AB +γ × AC wUPC = α × (AD +AE +AF +AG +AH +AI )+AA +β × AB +γ × AC wUCK =

(2)

(3)

where, w UCK and wUCP are the mass percentage of the coke fine and unconsumed coal, Ai ,i =A-I is the area percentage of each constituent, and α, β, γ are density amending factors with values of 0.9, 0.6 and 0.1, respectively. 3.2 Thermogravimetric analysis The gasification curves of different chars under various temperatures are depicted in Fig.5. The type of reaction curves is the same, and the reaction rates first increased and then decreased. According to study of Sahimi et al. [25,26], in the early stage of reaction, the solid mass was consumed and the pores were enlarged, leading to the increase of specific surface area and then increase of reaction rate. With the reaction proceeding, the micro pores in the particle diminish and the adjacent pores were joined together, leading to a decrease of specific surface area and further decreases reaction rate. At the 10


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temperature of 1273 K, the total gasification times are 2000 s for CC and RC, 3000 s for MC-1 and 5000 s for MC-2, which illustrates that the gasification reactivity of CC and RC is the best, MC-1 follows, and MC-2 is the lowest. The reaction rate increases as the gasification temperature increases, and the gasification time required for reaching certain conversion ratio is shortened. Taking the sample CC for an example, the time required to achieve the complete carbon conversion is around 5000 s at 1223 K, whereas this time is shortened to 700 s at 1373 K. At higher temperatures, the molecule energy is increased, and thus the carbon bond is easier to break. Meanwhile, the gasification of carbon is endothermic, and increasing the temperature is beneficial for the reaction proceeding [27]. In order to illustrate the gasification difference for different samples quantitatively, according to Takarada et al. [28], the reaction index Rs was introduced. Rs=0.5/t0.5 where t0.5 was time needed to reach a conversion ratio of 50%. The Rs of different samples under different temperatures is summarized in Table 4, which indicated that as gasification temperature increases, values of Rs increase quickly. The comparison of Rs for different samples shows that the gasification reactivity of RC is the best, CC follows and MC-2 is the lowest. The gasification reactivity of these four chars is obviously different. After acid-washing, the content of volatile and ash in samples are very low, and the influences on the gasification process are negligible. The samples were sieved with high criteria to control the particle size concentrated in range of 20-60 µm. Therefore, it could be ascertained that the volatile content, the ash and the particle size were not the primary factors determining the gasification reactivity. Thus, the material porosity and carbonaceous structure are the focus of this work. According to Table 2, the specific surface areas for different samples from high to low are MC-2, MC-1, RC, CC, and the order for the total porosity is the same. Generally, the higher the porosity, the easier it is to react. Yuan et al. [29] got the same conclusion when studying the relationship between coal char gasification reactivity and porosity structure. However, according to above study, the gasification reactivity for these four samples from high to low is RC, CC, MC-2, MC-1, and this order is in contradiction with the specific surface area. It implies that there must be some other factors determining the gasification reactivity for the studied samples. From Table 3, it could be found that the ID3+D4/IG value for different samples from high to low 11 ACS Paragon Plus Environment

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is RC, CC, MC-2, MC-1, and the order for IG/IAll is reverse. Based on this, the ordering degree from high to low for these four samples is MC-1, MC-2, CC and RC. In general, the higher the ordering graphitic crystalline degree, the lower the reactivity. This agrees with the observation in this work. Literatures have reported the relationship between the gasification reactivity and Raman spectroscopy. Wu et al. [29] applied the Raman spectroscopy to ascertain the carbon crystalline structure of the residual carbon in gasification slags and coal chars. Results showed that the gasification activity was closely related to the active sites with sp2 bond and sp2-sp3 bond, corresponding to ID3 /IG and ID3+D4/IG, respectively. In the previous researches [30,31], the same phenomenon was also observed that the gasification reactivity was mainly determined by its carbonaceous structure determined by ID/IG or IV/IG. 3.3 Kinetic model During the char reaction in the gasifier, the following main reactions may be considered: C s + CO 2 → 2CO

o ∆H 298 K = + 159.7 kJ mol

C s + O 2 → CO 2

∆H

o 298K

(4)

= −405.9 kJ mol

(5) Reaction (4) is an endothermic reaction, and it is generally considered to be the most important in a gasification process. Oxidation reaction (5) could provide the energy required for the promotion of reaction (4) to adjust the content of gasifier top gas. The reaction rate can be described by:

dx dt = k ( T ) f ( x )

(6)

where k ( T ) represents the apparent gasification reaction rate constant, f ( x ) is a conversion function that represents the reaction model, T is the absolute temperature. Replacing the Arrhenius equation, k ( T ) can be expressed by: k = k0e −E

where k0 represents the pre-exponential factor,

RT

(7) E represents the activation energy, R represent

sthe universal gas constant. In this study, three reaction models are applied to study the course of reaction, which are the volumetric model (VM), the random pore model (RPM) and the unreacted core model (URCM). The RPM model was initially proposed by Bhatia and Perlmutter [32,33]. When the gasification reaction is controlled by the interface chemical reaction, the gasification rate can be calculated by: 12


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dx dt = kRPMe− E RT (1 − x ) 1 −ψ ln (1 − x )

(8)

where ψ is a dimensionless parameter indicating the pore structure when conversion x is equal zero. During reaction, sample’s structure changes are not considered in VM model, thus it is the simplest model [34,35]. The kinetic expression for the reaction rate is as follow:

dx dt = kVMe−E RT (1− x)

(9) The URCM was came up by Szekely and Evans [36], which supposes that the reaction started from the surface of particle and formed ash layer, and then the unreacted core reduced continuously with the increase of reaction time. The overall reaction could be expressed as follow:

dx dt = kURCMe− E RT (1 − x )

23

(10)

Eqs.(8), (9) and (10) are three explicit formulae to calculate the gasification conversion rate dx dt , the conversion x , temperature T , under the condition of the reaction control. By applying nonlinear least-squares fitting methods, experimental data can be used to calculate kinetic parameters, such as E ,

k0 and ψ The objective function can be expressed as: N

(

OF = ∑ ( dx dt )exp,i − ( dx dt )calc,i i =1

where

( dx dt )exp,i

is experiment data;

( dx dt )calc,i

)

2

(11)

is value calculated by model; N is the number of

data points. Gasification test is conducted under the isothermal condition. Though the gasification process is endothermic, the heat compensation is existed during experiment in thermogravimetric device, thus, the temperature could be treated as stable. Eq.(8) then could be transformed and integrated as:   k0 ⋅ψ  −E     −E   x = 1 − exp  −  k 0 ⋅ exp  ⋅ exp   ⋅ t  ⋅ 1 +  ⋅ t  4 RT      RT     

(12)

In the same way, Eqs. (9) and (10) can be transformed into:  k  −E   x = 1 −  1 − 0 ⋅ exp  ⋅t 3  RT   

3

(13)

  −E   (14) x = 1 − exp  −k0 ⋅ exp  ⋅t  RT    x was calculated by Eqs. (12), (13) and (14) with the previously obtained k0 , E and ψ . The calculation of x was implemented to validate the reliability of the kinetic models and their capacity 13


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to describe both dx dt and x . The kinetic model may be further tested and validated by contrasting the calculated and experimental x values. The deviation (DEV) among the calculated and experimental curves was calculated by following equations:

 N  ∑ ( dx dt ) exp,i − ( dx dt ) calc,i DEV ( dx dt )( % ) = 100 ×  i =1 max ( dx dt )exp

(

)

2

12

 N 

(15)

12

2  N   ∑ ( xexp,i − xcalc,i ) N   DEV ( x )( % ) = 100 ×  i =1 max ( x )exp

where DEV ( x )( % ) is relative error; xexp,i and

( dx dt )calc,i

( dx dt )exp,i

(16)

are experiment data; xcalc,i and

are value calculated by the model; max ( x )exp and max ( dx dt )exp represent the highest

absolute values in the experimental curves. 3.4 Kinetic parameters The relationship between conversion and reaction rate under various gasification temperatures are depicted in Fig.6. It is observed that the conversion rate first increases and then decrease. The gasification kinetics of four type of samples were investigated by RPM, VM and URCM models. The value of kinetic parameters ( E , k0 and ψ ) which calculated by the data gained under the temperatures of 1223 K, 1273K, 1323K and 1373K are shown in Table 5. The dx dt curves calculated from Eqs.(8)-(10) by the parameters gained from the data under various heating rates are shown in Fig.6. The RPM fitting result is in a great consist with the experimental data. However, the fitting results of the other two models, which were decided by the physical assumptions of the models, showed that the gasification rate decreased with the increase of the conversion. The RPM could predict the maximum reaction rate of the gasification because it considers the changes of pore structure with conversion. As shown in Table 5, correlating coefficient for RPM is all over 0.9794, illustrating that the assumption made above is rational. Structure factor ψ is related with the specific surface area S0 , total pore length

L0 and porosity ε0 . The higher is ε0 the lower is ψ, and the lower is S0 the higher is ψ. Influence of S0 on structure factor ψ is in secondary power and is higher than that of L0 and ε0 . Through comparison of ψ and changing rules of pore structure for the different samples, it can be concluded that the specific 14


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surface area of MC-2 is the highest as 18.53 m2/g with the lowest ψ as 0.89. The specific surface area of CC is the lowest as 2.53 m2/g with the highest ψ as 90.82. This rule is in accordance with the former analysis and it further illustrates the adaptability of RPM in describing gasification process of carbonaceous materials. In addition, it can be perceived that the RPM model does not capture the trend of conversion rate increasing at first, then reaching a peak, followed by a decreasing conversion rate. The main reason was that there were experimental errors in the tests. In the experimental design, the reaction gas switching process required a certain amount of time to complete, which caused some of the reaction gas and the protective gas to be mixed in the furnace. Dilution of the reaction gas concentration, which in turn affected the reaction rate at the start of gasification. And its effect became more significant as the gasification temperature increased.

For different models the order of activation energy for different samples is the same, from high to low as CC, MC-2, MC-1 and RC. Normally for higher activation energy samples higher temperature is needed to activate molecule and to reach certain reaction rate. Through comparison of relationship between the reaction rate and the activation energy for different samples, the activation energy order is not in accordance with reaction activity order, which could be attributed to the compensation effect. In Arrhenius equation apparent gasification reaction rate is contained activation energy E and pre-exponential factor k0. Pre-exponential factor k0 implies the mass of active molecule with effective collision, and with increase of E the reaction becomes hard to happen, but with increase of k0 the reaction rate quickens. Therefore, in gasification process there is a compensation effect between E and k0. In the previous studies [30,31], there also existed a significant compensation effect between E and k0 in the gasification process of different chars. According to the research of Xie et al. [37], the reason might be that gasification reactions have similar mechanisms, particularly for the oxygen-containing surface complexes C(O). Also, it was more possible for the free active-site of carbon to contact with CO2 and generate C(O) for the reactions with low E values. Meanwhile, stronger bond between C(O) formed and the carbon structure became more stable, which limited the movement of C(O) and resulted in a lower value of k0. The gasification activation energy of CC sample in this research is 202.3kJ/mol. In prior studies [38], activation energy value of coal char was tested in a drop tube furnace in CO2 atmosphere, and the value was 180.12 kJ/mol. Micco et al. [39] acquired activation energy values of Argentinean subbituminous char prepared by different pyrolysis temperatures and the value ranged 15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

from 158 kJ/mol to 165 kJ/mol. It indicates that small differences exist among the literatures and this research about the value of the activation energy, and activation energies showed in references are slightly lower than our calculations. This could be explained by the conditions (high pyrolysis temperature and long pyrolysis time) applied in this study, which are 1473 K and 30 min. Activation energy in the gasification process for two cokes and the recycling dust in this study is in range 181.1-195.4 kJ/mol. Wang et al. [40] investigated the CO2 gasification kinetics of metallurgical coke, and their results showed that the activation energy of M-coke reacting with CO2 was 165.5 kJ/mol. Guo et al. have studied the activation energy of metallurgical coke [41,42] and the result was 126.7-171.5 kJ/mol. The activation energies of coke in reference were smaller than those in this study. The main reason is that the coke in the tests was undergone an acid-washing treatment to furthest remove inorganic mineral. Activation energy values of unburned carbon in both coarse and fine slags under CO2 atmosphere have been calculated by Xu et al. [38], and results show that the values were within the range of 141.7 kJ/mol and 210.2 kJ/mol. The activation energy of dust may be resulted from the differences in forming temperature and the fuel properties themselves. However, in general, the activation energies calculated in this paper are in agreement with those in the literatures. Furthermore, the kinetic parameters of gasification process for different samples are listed in Table 5 to further demonstrate the fitting reliability of three models, Eqs.(12), (13) and (14) were used to further calculate the gasification conversion rate and results are shown in Fig.7. The dots are the experimental results and line represents calculated data. The experimental data of different samples are in great accordance with calculated values. To quantify the errors generated by the kinetic models when predicting conversion values, Eq.(15) was used to calculate the deviation (DEV) between calculation data and experimental value. This procedure was also employed in dx/dt curves using Eq.(16). The related results of all samples were obtained from all three models and results are shown in Table 6. Result obtained by RPM model shows the lowest deviation, which indicates that RPM model is the most suitable model for describing samples’ reactivity. Based on above analyses, a conclusion could be drawn that when used in the gasification process, RPM model shows a better accuracy. Through the above analysis, it can foresee that this research result will provide useful information for the establishment of mathematical model in COREX-3000 iron-making process.

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4. Conclusions Characteristics of three solid fuels and one recycling dust in the Corex process were systematically investigated in this study and the thermogravimetric analysis method was used to study their gasification characteristics and kinetics under CO2 atmosphere. Results show that the recycling dust in the Corex process is mostly from coke with a mass fraction of 71.3 %. The gasification reactivity of recycling dust is similar to that of coal char and far higher than the two cokes. Microstructure of carbonaceous structure is the main factor which affects the gasification reactivity. Kinetics study shows that the most suitable model is the RPM, which can represent the gasification process of the four samples. According to the RPM, the activation energy for CC, MC-1, MC-2 and RC are 202.3 kJ/mol, 184.6 kJ/mol, 195.4 kJ/mol and 181.1kJ/mol, respectively. The compensation effect is existed in all samples during the process of gasification. Acknowledgements This work was supported by the Young Elite Scientists Sponsorship Program By CAST (2017QNRC001)， the Funding for Academic Collaboration between USTB and NTUT (TW2018003) and financial support from the NTUT–USTB Joint Research Program (NTUT-USTB-107-09) References [1] Wang, W. A Review of China's Iron and Steel Technical and Economic Indicators in 2016. World Metals 2017-02-28 (B02). [2] Wang, X. Ferrous Metallurgy (Ironmaking). Beijing: Metallurgical Industry Press, 2013. [3] Liu, S.; Bai, C. Technology research and development trend of direct reduction. J. Iron Steel Res. 2011, 23, 1-5. [4] Guo, P.; Zhao, P.; Pang, J.; Cao, C. Technical analysis on smelting reduction ironmaking process. Iron Steel Vanad Titan 2009, 30,1-9. [5] Zhang, J.; Liu, Z.; Yang, T. Non-blast furnace ironmaking. Beijing; Metallurgical Industry Press, 2015 [6] Ziebik, A.; Lampert, K.; Szega, M. Energy analysis of a blast-furnace system operating with the Corex process and CO2 removal. Energy 2008,33,199-205. [7] Xu, R.; Zhang, J.; Wang, G.; Zuo, H.; Li, P.; Wang, H.; Li, H.; Liu, S. Isothermal kinetic analysis on fast pyrolysis of lump coal used in Corex process. J. Therm. Anal. Calorim. 2016, 123,773-783. [8] Zhan, W.; Wu, K.; Fu, P.; Qu, J.; Wang, C.; Liu, Q.; Shao, T. Establishment and application of the Rist operating line for the Corex melter gasifier. J Univ Sci Technol B 2013, 35,448-453.

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[9] Du, K.; Wu, S.; Zhang, Z.; Chang, F.; Liu, X. Analysis on inherent characteristics and behavior of recycling dust in freeboard of Corex melter gasifier. ISIJ Int. 2014, 54, 2737-2745. [10] Wang, H.; Zhang, J.; Wang, G.; Zhao, D.; Guo, J.; Song, T. Research on combustion characteristics and kinetic analysis of the recycling dust for Corex furnace. Energies 2017, 10, 255-267. [11] Berger, K.; Welb, C.; Kepplinger, W. L. CFD simulation of the carbon dust combustion in the Corex meter gasifier. Steel Res. Int. 2008, 79, 579-585. [12] Chang, F.; Wu, S.; Zhang, F.; Lu, H.; Du, K. Characterization of sintering dust, blast furnace dust and carbon steel electric arc furnace dust. Charact. Min. Met. Mater. 2015, 20, 83-90. [13] Zhao, D.; Zhang, J.; Wang, G.; Alberto, N. C.; Xu, R.; Wang, H.; Zhong, J. Structure characteristics and combustibility of carbonaceous materials from blast furnace flue dust. Appl. Therm. Eng. 2016, 108, 1168-1177. [14] Gupta, S.; Sahajwalla, V.; Chaubal, P.; Youmans, T. Carbon structure of coke at high temperatures and its influence on coke fines in blast furnace dust. Metall. Mater. Trans. B 2005, 36, 385-394. [15] Machado, A. S.; Mexias, A. S.; Vilela, A. C. F.; Osorio, E. Study of coal, char and coke fines structures and their proportions in the off-gas blast furnace samples by X-ray diffraction, Fuel 2013, 114, 224-228. [16] Yu, J.; Sun, L.; Xiang, J.; Hu, S.; Wang, Y. New method of quantitative determination of the carbon source in blast furnace flue dust. Energy Fuels 2014, 28, 7235-7242. [17] Wu, K.; Ding, R.; Han, Q. Research on unconsumed fine coke and pulverized coal of BF dust under different PCI rates in BF at Capital Steel Co. ISIJ Int. 2010, 50, 390-395. [18] Wang, G.; Zhang, J.; Shao, J.; Liu, Z.; Wang, H.; Li, X.; Zhang, P.; Geng, W.; Zhang G. Experimental and modeling studies on CO2 gasification of biomass chars. Energy 2016, 114, 143-154. [19] Wang, G.; Zhang, J.; Chang, W.; Li, R.; Li, Y.; Wang, C. Structural features and gasification reactivity of biomass chars pyrolyzed in different atmospheres at high temperature. Energy 2018, 147, 25-35. [20] Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessnera, R.; Pöschl, U. Raman microspectroscopy of soot and related carbonaceous materials: spectral analysis and structural information. Carbon 2005, 43, 1731-1742. [21] Sheng, C. Char structure characterized by Raman spectroscopy and its correlation with combustion reactivity. Fuel 2007, 86, 2316-2324. [22] Gong, X.; Guo, Z.; Wang, Z. Effects of Fe2O3 on pyrolysis reactivity of demineralized higher rank coal and its char structure. Ciesc. J. 2009, 60, 2321-2326. [23] Machado, A. D. S.; Mexias A, S.; Vilela, A. C. F.; Osorio, E．Study of coal，char and coke fines structures and their proportions in the off-gas blast furnace samples by X-ray diffraction. Fuel 2013, 114, 224-228. [24] Zheng, T.; Wu, K.; Xu, W.; Zhu, R.; Jiang, W. Ratio Between Unconsumed pulverized coal and coke in BF Dust. Iron Steel 2006, 41, 20-24. [25] Sahimi, M.; Tsotsis, T. T. Statistical modeling of gas-solid reaction with pore volume growth: Kinetic regime. Chem. Eng. Sci. 1988, 43, 113-121.

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[26] Ding, L.; Gong, Y.; Wang, Y.; Wang, F.; Yu, G. Characterization of the morphological changes and interactions in char, slag and ash during CO2 gasification of rice straw and lignite. Appl. Energy 2017, 195, 713-724. [27] Jing, X.; Wang, Z.; Fang, Y. Steam re-gasification properties and kinetics of coal char fines derived from fluidized bed gasifier. J. Fuel Chem. Technol. 2013, 41, 400-406. [28] Takarada, T.; Tamai, Y.; Tomita, A. Reactivities of 34 coals under steam gasification. Fuel 1985, 64,1438-1442. [29] Wu, S.; Huang, S.; Ji, L.; Wu, Y.; Gao, J. Structure characteristics and gasification activity of residual carbon from entrained-flow coal gasification slag. Fuel 2014, 122, 67-75. [30] Wang, G.; Zhang, J.; Zhang G.; Ning, X.; Li, X.; Liu, Z.; Guo, J. Experimental and kinetic studies on co-gasification of petroleum coke and biomass char blends. Energy 2017, 131, 27-40. [31] Wang, G.; Zhang, J.; Hou, X.; Shao, J.; Geng, W. Study on CO2 gasification properties and kinetics of biomass char and anthracite char. Bioresour. Technol. 2015, 177, 66-73. [32] Bhatia, S. K.; Perlmutter, D. D. A random pore model for fluid-solid reactions: І Isothermal kinetic control. AIChE J. 1980, 26, 379-385. [33] Bhatia, S. K.; Perlmutter, D. D. A random pore model for fluid-solid reactions: ІІ Diffusion and transport effects. AIChE J. 1981, 27, 247-254. [34] Kasaoka, S.; Sakata, Y.; Tong, C. Kinetic evaluation of reactivity of various coal chars for gasification with carbon dioxide in comparison with stream. Int. Chem. Eng. 1985, 25, 160-175. [35] Shang, J.; Eduardo, E. W. Kinetic and FTIR studies of the sodium catalyzed steam gasification of coal char. Fuel 1984, 63, 1640-1649. [36] Szekely, J.; Evans, J. W. A structural model for gas-solid reactions with a moving boundary. Chem. Eng. Sci. 1970, 25, 1091-1107. [37] Xie, K. Coal structure and its reactivity. Beijing: Science Publishing House, 2002. [38] Xu, S.; Zhou, Z.; Gao, X.; Yu, G.; Gong, X. The gasification reactivity of unburned carbon present in gasification slag from entrained-flow gasifier. Fuel Process. Technol. 2009, 90, 1062-1070. [39] Micco, G. D.; Nasjleti, A.; Bohe, A. E. Kinetics of the gasification of a Rio Turbio coal under different pyrolysis temperatures. Fuel 2012, 95, 537-543. [40] Wang, P.; Zhang, Y.; Li, J.; Long, H.; Meng, Q.; Yu, S. Effects of CO2 and H2O on solution loss reaction of coke. The Chinese J. Process. Eng. 2016, 16, 138-143. [41] Guo, W.; Xue, Q.; Liu, Y.; Guo, Z.; She, X.; Wang, J.; Zhang, Q.; An, X. Kinetic analysis of gasification reaction of coke with CO2 or H2O. Int. J. Hydrogen Energy 2015, 40, 13306-13313. [42] Li, K.; Zhang, J.; Liu, Z.; Ning, X.; Wang, T. Gasification of graphite and coke in carbon-carbon dioxide-sodium or potassium carbonate systems. Ind. Eng. Chem. Res. 2014, 53, 5737-5748.

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Table 1 Proximate and ultimate analyses and high heating value of different samples Sample Coal Coke-1 Coke-2 Recycling dust CC MC-1 MC-2 RC a

Proximate analysis (wt,%) FCda Ad Vd 60.51 10.54 28.95 83.31 11.96 4.73 82.80 12.23 4.97 27.18 68.22 4.60 84.39 3.27 12.34 91.37 3.83 4.80 90.15 4.84 5.01 91.46 4.01 4.53

Cd 81.82 85.02 83.96 28.50 87.67 92.76 91.32 92.59

Ultimate analysis (wt,%) Hd Od a Nd 1.69 4.52 0.81 0.13 2.22 0.25 0.24 2.76 0.34 0.07 2.73 0.19 1.45 6.36 0.73 0.17 2.58 0.20 0.28 2.87 0.31 0.11 2.80 0.17

Calculated by difference. FC, fixed carbon; A, ash; V, volatile matter; d, dry basis.

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Sd 0.62 0.42 0.47 0.29 0.52 0.46 0.38 0.32

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Table 2 Pore characteristic parameters of different chars Sample St (m2/g) Vt (cm3/g) Da (nm) CC 2.53 0.016 15.67 MC-1 13.89 0.086 5.42 MC-2 18.53 0.093 5.79 RC 8.64 0.042 8.62 Note: St, total surface area; Vt, total pore volume; Da, average pore diameter.

21 ACS Paragon Plus Environment

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Table 3 Microstructure parameters of carbonaceous materials in different chars Sample CC MC-1 MC-2 RC

ID1 12200 18110 12570 24910

ID2 1667 1513 980 5566

ID3 2653 2369 3315 4318

ID4 2698 3391 2746 5683

IG 3488 5593 4239 5341


ID3+D4/IG 0.192 0.153 0.156 0.246

IG/IAll 0.158 0.182 0.178 0.117

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Table 4 Gasification characteristic parameters of different chars Rs CC MC-1 MC-2 RC

1223 2.12 1.15 1.60 3.25

Temperature，K 1273 1323 5.26 10.59 2.44 4.58 3.97 8.16 7.66 12.35


1373 15.87 8.16 13.70 18.32

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Table 5 Kinetic parameters of various models by fitting kinetic data of CO2 gasification of different chars Sample CC MC-1 MC-2 RC

RPM E(kJ/mol) k0(min-1) 202.3 3.0E+04 184.6 1.2E+04 195.4 4.9E+04 181.1 1.7E+04

VM Ψ R E(kJ/mol) k0(min-1) 90.82 0.9794 196.5 8.6E+04 1.83 0.9985 182.9 1.5E+04 0.89 0.9984 194.6 5.5E+04 3.19 0.9843 178.4 2.2E+04 2


URCM R E(kJ/mol) k0(min-1) 0.9494 199.0 9.5E+04 0.9943 184.9 4.7E+03 0.9977 196.0 5.1E+04 0.9794 181.2 2.3E+04 2

R2 0.9636 0.9980 0.9968 0.9831

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Table 6 Deviation between the experimental and calculated conversion and reaction rate data Samples CC MC-1 MC-2 RC

PRM 1.75 1.23 1.31 1.36

DEV(x)% VM 3.56 2.86 2.25 2.67

URCM 3.18 1.63 1.32 1.42

RPM 3.98 2.53 2.62 2.75


DEV(dx/dt)% VM 9.17 5.69 4.91 5.36

URCM 8.01 3.48 2.67 2.98

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Fig.1. SEM photographs of different chars: (a)CC, (b)MC-1, (c)MC-2, (d)RC


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Fig.4. Petrographic analysis of the unconsumed fine coke and pulverized coal in the recycling dust. (a):Un-deformed coal, (b):Deformed coal, (c):Residue coal, (d):Coarse grain mosaic structure, (e):Fine grain mosaic structure, (f): Flowing structure, (g) Macrinite structure, (h):Hemophilic silk carbon, (i):Block structure


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Study on CO2 gasification reactivity and structure characteristics of

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