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Assessment on the Application of Commercial Medium Grade Charcoal as a Substitute for Coke Breeze in Iron Ore Sintering Zhiyun Ji, Xiaohui Fan, Min Gan, Xuling Chen, Qiang Li, and Tao Jiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01876 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on November 14, 2016
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Assessment on the Application of Commercial Medium Grade Charcoal as a Substitute for Coke Breeze in Iron Ore Sintering Zhiyun Ji, Xiaohui Fan*, Min Gan, Xuling Chen, Qiang Li, Tao Jiang Affiliation: School of Minerals Processing & Bioengineering, Central South University Mailing address: No.932, South Lushan Road, Yuelu District, Changsha, Hunan, 410083, P.R.China Corresponding author: Prof. Xiaohui Fan Contact details:
[email protected], Tel/Fax: +86-731-88877952 E-mail address of other co-authors: Zhiyun Ji:
[email protected] Min Gan:
[email protected] Xuling Chen:
[email protected] Qiang Li:
[email protected] Tao Jiang:
[email protected] 1 ACS Paragon Plus Environment
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ABSTRACT: The application characteristics of cost-effective commercial medium grade charcoal (MG-charcoal) in sinter-making process were assessed in this paper. The results showed that compared with high grade charcoal (HG-charcoal), MG-charcoal characterized lower fixed carbon content, higher volatile content, and higher porosity and specific surface area, which led to its even greater difference in thermochemical behaviors with coke breeze than HG-charcoal. This property of MG-charcoal resulted in faster sintering speed, lower bed temperature and shorter holding time above 1200 oC for adhesive minerals melting, which restrained its proper replacement percentage at 20 %, while it was 40 % for HG-charcoal. To achieve similar sintering performance as the HG-charcoal, the combustion behavior of MG-charcoal and coke breeze were regulated simultaneously by coating fine-grained iron ore concentrate and adhering CaO respectively. It was found that coating concentrate was useful to reduce the combustion rate of MG-charcoal for reduced probability to contact air directly, while adhering CaO helped to accelerate the combustion rate of coke breeze due to potential catalytic effect, which finally exhibited a relatively matched combustion speed. Sinter pot trials verified that after pretreating MG-charcoal particles and coke breeze through pre-granulation process, MG-charcoal replacing 40 % coke breeze showed comparable sintering performance to the case using 100 % coke breeze. The emissions of CO2, SO2 and NOx achieved an obvious reduction by 21.70%, 27.75%, and 18.31% respectively. Keywords: iron ore sintering; medium grade charcoal; thermochemical behavior; sintering performance; combustion regulation
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1. INTRODUCTION The output of crude steel in China has been ranking first all over the world since 1996. As steelmaking is an energy-intensive process, enormous amounts of fossil energy are consumed annually. During its assumption process, high levels of greenhouse gases mainly the CO2 were emitted. It was found that the CO2 emitted from iron&steel industry took up about 9.2 % of its total emission amount in China and about 30% of total amount in Chinese industrial sectors including iron&steel, non-metallic minerals, chemicals and petrochemicals, etc. Besides, about 51% of the global CO2 emissions came from Chinese steel industry. 1 Li et al. 2 estimated that for reaching the goal of decreasing the emissions of CO2 per unit of GDP by 40-45% compared with that of 2005, China will reduce carbon emission by 1651Mt of in 2020, with even slowing down its economy growth rate. Iron ore sintering process is an important thermal-treatment stage of the whole steelmaking chain, with its purpose to convert fine-grained iron-bearing materials (-8 mm) into sinter cake, which characterized high mechanical strength, great thermal and reducing behaviors, and could meet the gas permeability needed for efficient blast furnace operations. 3-5 In China, iron ore sinter accounts for more than 75 % of the total iron-containing burden in blast furnace for ironmaking.6, 7 However, the sinter-making process typically consumes 9–12 % of the total energy for steelmaking, which is mainly provided by the combustion of solid fossil fuels such as coke breeze and anthracite.
8
Previous research verified that the CO2 generated by the combustion of fossil fuels in sinter-making process took up more than 10% of the total amount of steel production. 9
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Therefore, reducing the emission of CO2 from this process effectively is meaningful to control the emission of greenhouse gas in the whole iron & steel sector. Factually, the application of clean and renewable bio-energy, which was considered completely carbon neutral, in iron ore sintering process has been widely regarded as a promising strategy to reduce CO2 emission. The researchers from Corus investigated the use of olive residues and sunflower husk pellets in the sintering process, and they found the positive effects of biomass fuels replacing fossil fuels on reducing both CO2 and gaseous pollutants like SOx and NOx emissions due to their lower sulphur and nitrogen contents.
10,11
However, volatile matters in the uncarbonized biomass fuel would be
pyrolyzed before the approach of combustion front in sintering process, which led to poor fuel utilization and exerted potential hazard to electrostatic precipitators. Therefore, much more research was focused on using charcoal or other carbonized biomass fuels as substitutes for coke breeze. The reports from Australian CSIRO
12-15
, Brazil
16
showed
that charcoal was a potentially appropriate substitute to coke breeze for the sake of controlling both greenhouse gas and SOx, NOx, etc. However, charcoal characterized higher chemical activity than coke breeze, which would damage the sinter quality when fossil fuels were replaced at a higher proportion. For addressing this problem, some measures such as adding more reactive iron ores, using coarse charcoal, adjusting granulation methods to coat charcoal particles into the granules etc., were researched. 14, 17, 18
Our group has been engaging in the application of charcoal-based biomass fuel into iron ore sintering process. We also found the great potential of reducing CO2, SOx and
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NOx emissions when coke breeze was partially or completely replaced by charcoal. 18-20 But the charcoal in our previous research was a typical kind of high grade one with fixed carbon content close to 90 % and volatile content less than 8 %. However, high-grade charcoal (HG-charcoal) was usually prepared from multi-stage and high-temperature carbonization process to guarantee its high quality, the production cost of which was much higher than medium grade charcoal (MG-charcoal) with an average volatile content of 23.5 %.
21-23
Taking the feasibility of industrial-scale application of
charcoal into consideration, cost must be the bottleneck problem. Therefore, investigating the application of MG-charcoal in iron ore sintering is of great significance.
Unfortunately,
systematic
research
about
the
characteristics
of
MG-charcoal, its influences on sintering performance and strengthening measures are still scarce. In this research, an all-round evaluation on the primary physicochemical properties and thermochemical properties of MG-charcoal was conducted, and a laboratory-scale sinter pot was used to evaluate its influences on sintering performance. Relevant measures for improving the sintering performance of MG-charcoal to the comparable level of HG-charcoal were also taken into consideration.
2. EXPERIMENTAL SECTION 2.1 Materials The raw materials used to produce sinter consisted of iron ores, three typical kinds of fluxes (quick-lime, dolomite, and limestone and), solid fuels (usually coke breeze and anthracite) and return fines (sinter 6.3mm after testing in a tumbler, %. The examination usually based on the methods in ISO3271 (2007). As the mass of sample (15 kg) required in this standard was hard to reach in our sinter pot trials, reducing the width of the tumbler to its half or one fifth was also effective. In this investigation, we reduced the width to tis half, and 7.5kg sinter samples were used for tumbler index examination.
2.3 Calculations 2.3.1 Charcoals replacing coke breeze During sintering process, the role of solid fuels is to provide heat for minerals melting. Replacing coke breeze based on the carbon content and heat balance is the usually adopted method according to the references 17, 27. As the heat is mainly generated by the combustion of carbon-related components with the exception of carbonate, the content of carbon can nearly reflect the amount of heat the fuels could supply. In this
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investigation, we chose the principle of charcoal provided equal amount of heat to the coke breeze replaced could supply. Though we did not use the ratio of carbon content of charcoal to the carbon content of the coke breeze it replaced, but we also took the sinter quality into account. Based on the analyses, Equation (3) was applied to calculate the mass of charcoals used to replace coke breeze for each case. mb =
mc rb Qc Qb
(3)
Where mb refers to the mass of charcoal (including the ash), kg; mc refers to the mass of coke breeze used in base case (including the ash), kg; rb refers to the substitute proportion of charcoal to coke breeze; Qc and Qb refer to the calorific value of coke breeze, charcoal respectively, MJ/kg.
2.3.2 Combustion efficiency Equations (4) and (5) were used to calculate the real-time and average combustion efficiencies of fuels for each sintering trials. Rt =
[CO2 ]t ⋅ 100% [CO2 ]t + [CO ]t
Rave =
1 t e − ti
∫
te
ti
(4)
Rt dt
(5)
Where Rt and Rave refer to the real-time and average combustion efficiency, respectively, %; [CO2]t and [CO]t refer to emission concentrations of CO2, CO respectively in flue gas, ppm; ti and te refer to the initiating time and ending time of sintering, s.
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2.3.3 Melting zone During sintering process, the quality of sinter is closely linked to the thermal condition in sintering bed. To evaluate the thermal characteristics quantitatively, two important indicators including duration time of temperature for minerals melting (DTMT) and melting zone were obtained from the time-sintering bed temperature profile, as shown in Figure 1. DTMT was the duration time of temperature above 1200 oC during sintering, which was the initial melting temperature of adhesive minerals like calcium ferrite. It was obtained from the interval between the time that the sintering bed temperature initially reached 1200 oC and the time that the bed temperature started to drop below 1200 oC, as shown in Figure 1. Sufficient DTMT was the precondition to generate enough amount of melting phase during sintering process. Melting zone is defined as the area that the sintering bed temperature is above 1200 o
C in the time-sintering bed temperature profile (Figure 1). It was a comprehensive
indicator to reflect the capacity of sintering bed to generate melting phase. Larger melting zone indicated that more melting phase would be formed, thus contributing to well filling the voids between separate particles and combined them together after cooling. Equation (6) was used to calculate the value of melting zone. t2
A = ∫ (T − T0 )dt
(6)
t1
Where A is the value of melting zone, oCs; T0 is the initiating temperature of melting process, oC; T is the temperature of sintering bed at time t, oC; t1 and t2 are the initiating time and ending time of melting process respectively, s .17
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3 RESULTS AND DISCUSSION 3.1 Thermal behavior of MG-charcoal 3.1.1 Combustion characteristics Figure 2a-c shows the combustion characteristics of coke breeze and charcoals, where can be clearly found that the Ts and Te of MG-charcoal were 336 oC, 558 oC respectively, which were much lower than that of HG-charcoal. Additionally, the DSC curves formed during the burning process of coke breeze and HG-charcoal showed similar plain and sustained heat-releasing characteristics, which indicated that the combustion of these fuels proceeded in a relatively uniform manner. However, a sharp peak formed during the burning process of MG-charcoal, which stood for an intensive and rapid combustion process. Qmax and Vmax observed from DSC and DTG curves further verified that bigger gap existed between the combustibility of MG-charcoal and coke breeze. Figure 2d gives the relation of combustion intervals between charcoals and coke breeze. Obviously, MG-charcoal showed a considerably lower overlapping degree of 8.9 % to coke breeze than HG-charcoal, which showed that the combustion process of MG-charcoal was hard to match that of coke breeze. The differences in terms of microstructures between charcoals and coke breeze potentially served as the explanation to the phenomenon stated above. As shown in
Figure 3a, the surface of coke breeze particles only comprized some bigger pores, which seemed to be denser than charcoals. Compared with HG-charcoal (Figure 3b) , MG-charcoal characterized abundantly porous structure with numerous micropores widely distributed (Figure 3c-d). This structure was closely related to the porosity and
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specific surface area of fuels, as shown in Table 3. Therefore, MG-charcoal could provide more opportunities for carbon materials to react with O2 which makes it able to burn off rapidly than HG-charcoal.
3.1.2 Gasification characteristics The capability of fuels reacting with CO2 is another important indicator to reflect their thermochemical behaviors. Figure 4 demonstrates the comparison of gasification characteristics between coke breeze and charcoals. It could be clearly observed that the Ts and Te of coke breeze in Figure 4a were higher than that of charcoals, which showed the higher gasification reactivity of charcoals, similar to the results of Wei et al.
28
Compared with HG-charcoal, the Ts and Te of MG-charcoal were considerably lower. The Qmax and Vmax shown in Figure 4c were -23.90 mW/mg and -6.99 %/min respectively, the absolute values of which were considerably higher than that of coke breeze and HG-charcoal. Figure 4d further indicated that compared with HG-charcoal, MG-charcoal showed much lower overlapping degree with the gasification temperature interval of coke breeze. These phenomena indicated that MG-charcoal had higher chemical activity to react with CO2 than HG-charcoal and coke breeze. Higher porosity and specific surface area of MG-charcoal served as the reason to its higher gasification speed. Additionally, it was easy to find that the TG and DTG curves shown in Figure 4b and Figure 4c contained two obvious peaks. Taking the volatile contents of HG-charcoal and MG-charcoal into consideration, it was easy to speculate that the first peak appeared on the lower temperature stage represented the volatile removing process. During this process, new pores and surface areas would form according to our previous
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research.
29
As the volatile content of MG-charcoal was much higher than that of
HG-charcoal, more pores and surface areas would be formed. This property of MG-charcoal further accelerated its velocity of gasification.
3.2 Influences of MG-charcoal on sintering performances 3.2.1 On quality of sinter product The influences of charcoals replacing coke breeze on sintering quality were investigated, and the results are given in Table 4. It was found that HG-charcoal replaced 20 %-40 % coke breeze increased sintering speed, while decreased yield, tumbler index and productivity compared with the base case. But the sintering quality still remained comparable to the base case. When replacement percentage was improved further to 60 %, sintering speed continued increasing, while other indexes all appeared obvious decrease. Consequently, the proper proportion for HG-charcoal replacing coke breeze could reach 40 %. When MG-charcoal replaced 20 % coke breeze, sintering index could reach a comparable level to the base case. After improving the replacement percentage to 40 %, sintering speed was further accelerated, while all other indexes were obviously weakened. Consequently, the proper proportion of MG-charcoal replacing coke breeze was hard higher than 20 %.
3.2.2 On process property of sintering The influences of charcoals replacing coke breeze on sintering quality were closely linked to the changes of sintering process. Figure 5a and Figure 5b respectively demonstrate the influences of charcoals replacing 40 % coke breeze on real-time combustion efficiency (Rt) and average combustion efficiency (Rave) for each case. It
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was found that the Rt and Rave of the case using coke breeze only exhibited higher value than the case using charcoals. Compared with HG-charcoal, the values of Rt and Rave for MG-charcoal was relatively lower, which indicated that MG-charcoal could not burn off as sufficiently as HG-charcoal. Therefore, higher amount of potential heat was wasted in the form of unburned CO for the case using MG-charcoal. This phenomenon was tightly related to the gasification characteristics of fuels as introduced in section 3.1.2. MG-charcoal exhibited higher gasification speed than HG-charcoal, which also showed its higher reaction speed with CO2 in sintering bed.
Figure 6 shows the influences of using charcoals to replace coke breeze on the temperature of sintering bed. As can be observed from Figure 6a, charcoals replacing 40 % coke breeze could put the temperature-rising process in sintering bed forward, especially for the case using MG-charcoal. This phenomenon was caused by the rapid combustion speed of charcoals as introduced in section 3.1.1. During the combustion process of charcoals in sintering bed, it could burn ahead of coke breeze, and therefore accelerated the flame front speed and damaged the match of flame front speed and heat transfer front. This property directly led to the decrease of the highest bed temperature as could be observed from Figure 6a for unmatched two fronts were adverse to the effective accumulation of heat to create necessary high temperature. 30 As can be found in Figure 6b, charcoals replacing 40 % coke breeze reduced the values of DTMT and melting zone. Especially when MG-charcoal replaced 40 % coke breeze, the DTMT and melting zone were decreased from 202 s and 12129 oCs to 36 s and 2148 oCs respectively. During sintering process, DTMT and melting zone played an
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important role in the generation of liquid calcium ferrite, which was the main adhesive mineral to combine the distributed fine ore particles together. Sufficient DTMT and melting zone contributed to forming enough liquid calcium ferrite, which therefore guaranteed higher mechanical strength for sinter.
31, 32
From Figure 7 it could find that
large amount of adhesive minerals like calcium ferrite formed for the case using coke breeze only. However, charcoals replacing 40% coke breeze led to the decrease of the amount of calcium ferrite. Especially for the case using MG-charcoal, the absolute content of calcium ferrite was decreased by 10.42 %, which was adverse to produce sinter with high mechanical strength. Therefore, it was not hard to comprehend the apparently reduced tumbler index and yield when MG-charcoal replacing 40 % coke breeze.
3.3 Strengthening sintering performance through fuels combustion regulation According to the research stated above, the huge gap of the combustion speed between MG-charcoal and coke breeze was the key factor to limit the replacement percentage of MG-charcoal. For addressing this problem, bridging the unmatched combustibility of coke breeze and MG-charcoal was a potentially effective strategy. Based on the thermal behavior of these fuels, measures for slowing down the combustion speed of MG-charcoal and accelerating the combustion speed of coke breeze were taken simultaneously to achieve a matched combustion behavior in sintering process.
3.3.1 Influences of concentrate-coating on MG-charcoal combustion For slowing down the combustion speed of MG-charcoal in sintering bed, restraining its
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direct exposure to air is a potentially effective approach.
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17, 18
To achieve this goal,
fine-grained iron ore concentrate was selected to coat the surface of MG-charcoal. Detailed method was described as 500g iron ore concentrate (hematite with size range
