- Email: [email protected]
Variability of metallurgical coke reactivity under the NSC test conditions
Fuel 241 (2019) 519–521
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Variability of metallurgical coke reactivity under the NSC test conditions Lukas Koval , Richard Sakurovs ⁎
T
CSIRO, Mineral Resources, 1 Technology Court, Pullenvale, QLD 4069, Australia
ARTICLE INFO
ABSTRACT
Keywords: Coke reactivity NSC test Coke gasification
Three coke samples with significant quality differences were selected for reactivity measurements following the NSC test procedure using a specially designed sample holder that allowed each individual coke particle to be weighed before and after the test. The mass loss of individual particles varied between 5 and 50% for each of the three cokes. Thus a major source of variability in NSC reactivity test results is caused by variability in reactivity of the individual coke particles.
1. Introduction Quality of metallurgical coke strongly affects the performance of the blast furnace, thus it must be strictly controlled. Mechanical strength and propensity of coke to gasification by carbon dioxide are probably the most important coke quality paramaters. Several test methods have been developed to asses the coke quality in the past. Yet the procedure developed by researchers at the Nippon Steel Company (NSC) has become the most widely accepted internationally and is prescribed by various standards [1–4]. However, if the repeatability of the test could be improved, then finer discrimination between coke qualities can be made. Nonetheless, the fundamental reasons for the current limits to the repeatability of NSC test results are not fully understood. Thus, a number of approaches have been investigated in the past to identify the critical test parameters. While it was shown that coke particle size of +19 to 21 mm or +19 to 22.4 mm does not affect the coke reactivity [5], differences in the equipment (thermocouple position, retort diameter), radial and axial temperature gradients in the reaction vessel and temperature deviations during gasification bias the coke reactivity results significantly [6,7]. However, in this study we present results obtained using an original approach to asses the repeatability of the metallurgical coke reactivity test: tracking the mass loss of individual coke particles after the NSC reactivity test. 2. Experimental 2.1. Coke samples and carbonisation conditions Three coke samples were chosen for this study based on their quality parameters as shown in Table 1. All coke samples were prepared ⁎
from single coal and carbonised in pilot scale moveable wall coke oven using the standard coal charge bulk density, carbonisation temperatures, quenching and stabilisation procedures. Standardised crushing and sizing procedures were used to prepare relevant coke size particles for testing. 2.2. Gasification conditions Coke reactivity index (CRI) and coke strength after reaction (CSR) were determined following the ISO standard [8]. However, custom made sample holder was used to arrange the coke particles in separated segments and layers as shown in Fig. 1. Thus the order of particles within a layer was fixed and first particle in each layer was constantly in the same relative position in the furnace. Moreover, each individual coke particle was weighed before and after the test to determine the mass loss of each individual coke particle after the gasification. Sum of the determined mass loss of individual coke particles was expressed as experimental coke reactivity index (CRI EXP) as shown in Table 1. No significant differences were found between the two methods and results were well within the repeatability limits of the ISO standard [8]. 3. Results and discussion Fig. 2 shows the mass loss of individual coke particles of the premium quality coke with lowest reactivity (coke A) in different layers from bottom (layer 1) to top (up to layer 8) of the reaction vessel. Mass loss of the individual coke particles of coke A was found in the range of 1.6–52.4 wt%. Similar trends were observed for other two coke samples studied. The variation in mass loss was unexpectedly large and indicates significant variability in coke reactivity on the ∼20 mm size scale. It is
Corresponding author. E-mail address: [email protected] (L. Koval).
https://doi.org/10.1016/j.fuel.2018.12.053 Received 17 October 2018; Received in revised form 29 November 2018; Accepted 11 December 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
Fuel 241 (2019) 519–521
L. Koval, R. Sakurovs
could be affected by the temperature gradient (radial∼5 °C, axial∼10 °C) within the reaction vessel [6], since gasification is strongly dependent on temperature [7,10] and reaction regime [11]. We exclude this possibility in our study as we found no correlation between coke mass loss and horizontal position in the sample holder. This also excludes possible variation due to gas channelling effects. The differences in coke reactivity could be related to the coke crushing and fissuring. Even though the coke crushing process is standardised and it was previously concluded that coke on 20 mm size scale could be considered homogenous [12] and free of fissures [13,14], recent research shown that coke crushing introduces micro-fractures into the coke structure even down to 10 nm in size [15] (although in that study the coke was crushed extensively (75–125 µm and 125–250 µm)). It is known that coke strength after reaction (CSR) varies significantly from oven wall to centre within a given coke oven charge [16–18]. Although the studies focused on differences in coke strength after reaction (CSR) rather than differences in coke reactivity (CRI), it is well known that both indices are highly correlated. Thus variation in coke reactivity from wall to centre would be expected as well. Nevertheless, it is also expected that differences will be much smaller than the variation seen here. Thus, we believe that these differences in mass loss of individual coke particles reflect inherent variability in metallurgical coke reactivity for coke samples used in the NSC test. Moreover, a recent study using micro-CT imaging technique has found that some 20 mm-sized coke particles react preferentially at the surface and some react through the coke structure upon gasification with carbon dioxide at 1100 °C [19–21], which is consistent with our findings.
Table 1 Coke quality parameters. Coke sample
CSR ISO
CRI ISO
CRI EXP
A B C
70.1 54.4 34.1
22.6 30.3 44.8
23.1 30.4 46.5
Notes: CRI ISO – CRI index obtained by ISO standard procedure. CRI EXP – CRI index obtained using custom holder.
unlikely that the factors which were previously found to affect coke reactivity by others such as thermocouple position, reaction vessel diameter and temperature deviations during the gasification [6,7] can produce such a variation in loss of mass of individual particles. All coke samples were reacted by one operator using the same equipment. Moreover, gasification temperature was controlled within the strict limits of ISO standard ( ± 3 °C) at all times. Thus, these effects could be ruled out. Inspection of Fig. 2 also shows that there was a gradient in mass loss with from bottom to top of the reaction vessel which was irrespective of coke quality. Difference between the average mass loss of coke particles from the bottom and top layer of the reaction vessel for high, medium and low reactivity coke was 32, 15 and 12 wt% respectively. Such results were expected, since carbon dioxide is converted to carbon monoxide while passing up through the sample bed and the inhibiting effect of carbon monoxide on coke gasification is well known [9]. Previous research also shown that coke reactivity during the test
Fig. 1. Schematic diagram of the sample holder and coke particle arrangement in the reaction vessel. 520
Fuel 241 (2019) 519–521
L. Koval, R. Sakurovs
60 50
Mass loss [wt%]
Layer 1 - Bottom 40
Layer 2 Layer 3
30
Layer 4 Layer 5
20
Layer 6 10
Layer 7 Layer 8 - Top
0 0
7
14
21
28
35
42
49
56
Coke particle Fig. 2. Mass loss of individual coke particles in coke layers (coke A).
These results show that the variability in the reactivity of individual 20 mm coke particles may be a major factor in determining the repeatability of the NSC test. If the variation is due to micro-fissuring occurring during the crushing step, current metallurgical coke crushing processes would require re-evaluation. Obviously, further research is required to confirm and explain the cause of such extreme variability in metallurgical coke reactivity.
coke strength after reaction (CSR); 2018, ISO. p. 25. [5] British Carbonization Research Association C., U.S.N.T.I. Service. The Evaluation of the Nippon Steel Corporation Reactivity and Post-reaction-strength Test for Coke; 1980. [6] Arendt P, Huhn F, Kuhl H. CRI CSR survey. Cokemaking International. 2001;1:4. [7] Reifeinstein A. C12004 The Coke Reactivity Test Critical Parameters. Australia: ACARP; 2003. p. 62. [8] ISO_18894-2006. In: Coke – Determination of coke reactivity index (CRI) and coke strength after reaction (CSR). ISO; 2006, p. 20. [9] Sakurovs R, Burke L. Influence of gas composition on the reactivity of cokes. Fuel Process Technol 2011;92(6):1220–4. [10] Kashiwaya Y, Ishii K. Kinetic analysis of coke gasification based on non-crystal/ crystal ratio of carbon. ISIJ Inter 1991;31(5):440–8. https://doi.org/10.2355/ isijinternational.31.440. [11] Jayasekara AS, Monaghan BJ, Longbottom RJ. The kinetics of reaction of a coke analogue in CO2 gas. Fuel 2015;154:45–51. [12] Andriopoulos N, Dukino R, Sakurovs R. C9060 The Strength Controlling Properties of Coke and Their Relationship to Tumble Drum Indices and Coal Type. Australia: ACARP; 2002. p. 105. [13] Bennett P, et al. C22039 Implications of Coking Conditions on CSR. Australia: ACARP; 2015. p. 91. [14] Koval L, Sakurovs R. I600 from the CSR test as a measure of pilot oven coke strength. Ironmaking Steelmaking 2018:16. [15] Sakurovs R, et al. Nanostructure of cokes. Int J Coal Geol 2018;188:112–20. [16] Patrick JW, Stacey AE. The strength of industrial cokes: Part 1. Variability of tensile strength in relation to fissure formation. Fuel 1972;51(January):7. [17] Anamoto K, Coke,. strength development in the coke oven: 1. Influence of maximum temperature and heating rate. Fuel 1997;76(1):5. [18] Nyathi MS, et al. Impact of Oven Bulk Density and Coking Rate on Stamp-Charged Metallurgical Coke Structural Properties. Energy Fuels 2013;27(12):7876–84. [19] Jenkins DR, et al. Examination of coke reactivity using micro-ct analysis. Linz, Austria: ECIC; 2016. [20] Jenkins DR, et al. Effect of Coke Reactivity upon Coke Strength with Focus on Microstructure. Australia: ACARP; 2017. C24053. [21] Sakurovs R. C25050 Sakurovs R, editor. Overview of Outcomes of Research Supported by ACARP and NERDDC on the Utilisation of Coking Coals, 1978–2014. Australia: ACARP; 2018. p. 168.
Conflict of interest None. Acknowledgement The authors are grateful to BHP for the provision of the coke samples. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2018.12.053. References [1] BS 4262:1984 Method of specifying the technical quality of coke for use in blast furnace; 1984. p. 16. [2] ASTM D5341/D5341M-17a. In: Standard Test Method for Measuring Coke Reactivity Index (CRI) and Coke Strength After Reaction (CSR). [3] AS 1038.13-1990. In: Section 3: Determination of coke reactivity index and coke strength after reaction. 2013;201:29. [4] ISO_18894-2018_2. In: Coke – Determination of coke reactivity index (CRI) and
521
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Short communication
Variability of metallurgical coke reactivity under the NSC test conditions Lukas Koval , Richard Sakurovs ⁎
T
CSIRO, Mineral Resources, 1 Technology Court, Pullenvale, QLD 4069, Australia
ARTICLE INFO
ABSTRACT
Keywords: Coke reactivity NSC test Coke gasification
Three coke samples with significant quality differences were selected for reactivity measurements following the NSC test procedure using a specially designed sample holder that allowed each individual coke particle to be weighed before and after the test. The mass loss of individual particles varied between 5 and 50% for each of the three cokes. Thus a major source of variability in NSC reactivity test results is caused by variability in reactivity of the individual coke particles.
1. Introduction Quality of metallurgical coke strongly affects the performance of the blast furnace, thus it must be strictly controlled. Mechanical strength and propensity of coke to gasification by carbon dioxide are probably the most important coke quality paramaters. Several test methods have been developed to asses the coke quality in the past. Yet the procedure developed by researchers at the Nippon Steel Company (NSC) has become the most widely accepted internationally and is prescribed by various standards [1–4]. However, if the repeatability of the test could be improved, then finer discrimination between coke qualities can be made. Nonetheless, the fundamental reasons for the current limits to the repeatability of NSC test results are not fully understood. Thus, a number of approaches have been investigated in the past to identify the critical test parameters. While it was shown that coke particle size of +19 to 21 mm or +19 to 22.4 mm does not affect the coke reactivity [5], differences in the equipment (thermocouple position, retort diameter), radial and axial temperature gradients in the reaction vessel and temperature deviations during gasification bias the coke reactivity results significantly [6,7]. However, in this study we present results obtained using an original approach to asses the repeatability of the metallurgical coke reactivity test: tracking the mass loss of individual coke particles after the NSC reactivity test. 2. Experimental 2.1. Coke samples and carbonisation conditions Three coke samples were chosen for this study based on their quality parameters as shown in Table 1. All coke samples were prepared ⁎
from single coal and carbonised in pilot scale moveable wall coke oven using the standard coal charge bulk density, carbonisation temperatures, quenching and stabilisation procedures. Standardised crushing and sizing procedures were used to prepare relevant coke size particles for testing. 2.2. Gasification conditions Coke reactivity index (CRI) and coke strength after reaction (CSR) were determined following the ISO standard [8]. However, custom made sample holder was used to arrange the coke particles in separated segments and layers as shown in Fig. 1. Thus the order of particles within a layer was fixed and first particle in each layer was constantly in the same relative position in the furnace. Moreover, each individual coke particle was weighed before and after the test to determine the mass loss of each individual coke particle after the gasification. Sum of the determined mass loss of individual coke particles was expressed as experimental coke reactivity index (CRI EXP) as shown in Table 1. No significant differences were found between the two methods and results were well within the repeatability limits of the ISO standard [8]. 3. Results and discussion Fig. 2 shows the mass loss of individual coke particles of the premium quality coke with lowest reactivity (coke A) in different layers from bottom (layer 1) to top (up to layer 8) of the reaction vessel. Mass loss of the individual coke particles of coke A was found in the range of 1.6–52.4 wt%. Similar trends were observed for other two coke samples studied. The variation in mass loss was unexpectedly large and indicates significant variability in coke reactivity on the ∼20 mm size scale. It is
Corresponding author. E-mail address: [email protected] (L. Koval).
https://doi.org/10.1016/j.fuel.2018.12.053 Received 17 October 2018; Received in revised form 29 November 2018; Accepted 11 December 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.
Fuel 241 (2019) 519–521
L. Koval, R. Sakurovs
could be affected by the temperature gradient (radial∼5 °C, axial∼10 °C) within the reaction vessel [6], since gasification is strongly dependent on temperature [7,10] and reaction regime [11]. We exclude this possibility in our study as we found no correlation between coke mass loss and horizontal position in the sample holder. This also excludes possible variation due to gas channelling effects. The differences in coke reactivity could be related to the coke crushing and fissuring. Even though the coke crushing process is standardised and it was previously concluded that coke on 20 mm size scale could be considered homogenous [12] and free of fissures [13,14], recent research shown that coke crushing introduces micro-fractures into the coke structure even down to 10 nm in size [15] (although in that study the coke was crushed extensively (75–125 µm and 125–250 µm)). It is known that coke strength after reaction (CSR) varies significantly from oven wall to centre within a given coke oven charge [16–18]. Although the studies focused on differences in coke strength after reaction (CSR) rather than differences in coke reactivity (CRI), it is well known that both indices are highly correlated. Thus variation in coke reactivity from wall to centre would be expected as well. Nevertheless, it is also expected that differences will be much smaller than the variation seen here. Thus, we believe that these differences in mass loss of individual coke particles reflect inherent variability in metallurgical coke reactivity for coke samples used in the NSC test. Moreover, a recent study using micro-CT imaging technique has found that some 20 mm-sized coke particles react preferentially at the surface and some react through the coke structure upon gasification with carbon dioxide at 1100 °C [19–21], which is consistent with our findings.
Table 1 Coke quality parameters. Coke sample
CSR ISO
CRI ISO
CRI EXP
A B C
70.1 54.4 34.1
22.6 30.3 44.8
23.1 30.4 46.5
Notes: CRI ISO – CRI index obtained by ISO standard procedure. CRI EXP – CRI index obtained using custom holder.
unlikely that the factors which were previously found to affect coke reactivity by others such as thermocouple position, reaction vessel diameter and temperature deviations during the gasification [6,7] can produce such a variation in loss of mass of individual particles. All coke samples were reacted by one operator using the same equipment. Moreover, gasification temperature was controlled within the strict limits of ISO standard ( ± 3 °C) at all times. Thus, these effects could be ruled out. Inspection of Fig. 2 also shows that there was a gradient in mass loss with from bottom to top of the reaction vessel which was irrespective of coke quality. Difference between the average mass loss of coke particles from the bottom and top layer of the reaction vessel for high, medium and low reactivity coke was 32, 15 and 12 wt% respectively. Such results were expected, since carbon dioxide is converted to carbon monoxide while passing up through the sample bed and the inhibiting effect of carbon monoxide on coke gasification is well known [9]. Previous research also shown that coke reactivity during the test
Fig. 1. Schematic diagram of the sample holder and coke particle arrangement in the reaction vessel. 520
Fuel 241 (2019) 519–521
L. Koval, R. Sakurovs
60 50
Mass loss [wt%]
Layer 1 - Bottom 40
Layer 2 Layer 3
30
Layer 4 Layer 5
20
Layer 6 10
Layer 7 Layer 8 - Top
0 0
7
14
21
28
35
42
49
56
Coke particle Fig. 2. Mass loss of individual coke particles in coke layers (coke A).
These results show that the variability in the reactivity of individual 20 mm coke particles may be a major factor in determining the repeatability of the NSC test. If the variation is due to micro-fissuring occurring during the crushing step, current metallurgical coke crushing processes would require re-evaluation. Obviously, further research is required to confirm and explain the cause of such extreme variability in metallurgical coke reactivity.
coke strength after reaction (CSR); 2018, ISO. p. 25. [5] British Carbonization Research Association C., U.S.N.T.I. Service. The Evaluation of the Nippon Steel Corporation Reactivity and Post-reaction-strength Test for Coke; 1980. [6] Arendt P, Huhn F, Kuhl H. CRI CSR survey. Cokemaking International. 2001;1:4. [7] Reifeinstein A. C12004 The Coke Reactivity Test Critical Parameters. Australia: ACARP; 2003. p. 62. [8] ISO_18894-2006. In: Coke – Determination of coke reactivity index (CRI) and coke strength after reaction (CSR). ISO; 2006, p. 20. [9] Sakurovs R, Burke L. Influence of gas composition on the reactivity of cokes. Fuel Process Technol 2011;92(6):1220–4. [10] Kashiwaya Y, Ishii K. Kinetic analysis of coke gasification based on non-crystal/ crystal ratio of carbon. ISIJ Inter 1991;31(5):440–8. https://doi.org/10.2355/ isijinternational.31.440. [11] Jayasekara AS, Monaghan BJ, Longbottom RJ. The kinetics of reaction of a coke analogue in CO2 gas. Fuel 2015;154:45–51. [12] Andriopoulos N, Dukino R, Sakurovs R. C9060 The Strength Controlling Properties of Coke and Their Relationship to Tumble Drum Indices and Coal Type. Australia: ACARP; 2002. p. 105. [13] Bennett P, et al. C22039 Implications of Coking Conditions on CSR. Australia: ACARP; 2015. p. 91. [14] Koval L, Sakurovs R. I600 from the CSR test as a measure of pilot oven coke strength. Ironmaking Steelmaking 2018:16. [15] Sakurovs R, et al. Nanostructure of cokes. Int J Coal Geol 2018;188:112–20. [16] Patrick JW, Stacey AE. The strength of industrial cokes: Part 1. Variability of tensile strength in relation to fissure formation. Fuel 1972;51(January):7. [17] Anamoto K, Coke,. strength development in the coke oven: 1. Influence of maximum temperature and heating rate. Fuel 1997;76(1):5. [18] Nyathi MS, et al. Impact of Oven Bulk Density and Coking Rate on Stamp-Charged Metallurgical Coke Structural Properties. Energy Fuels 2013;27(12):7876–84. [19] Jenkins DR, et al. Examination of coke reactivity using micro-ct analysis. Linz, Austria: ECIC; 2016. [20] Jenkins DR, et al. Effect of Coke Reactivity upon Coke Strength with Focus on Microstructure. Australia: ACARP; 2017. C24053. [21] Sakurovs R. C25050 Sakurovs R, editor. Overview of Outcomes of Research Supported by ACARP and NERDDC on the Utilisation of Coking Coals, 1978–2014. Australia: ACARP; 2018. p. 168.
Conflict of interest None. Acknowledgement The authors are grateful to BHP for the provision of the coke samples. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2018.12.053. References [1] BS 4262:1984 Method of specifying the technical quality of coke for use in blast furnace; 1984. p. 16. [2] ASTM D5341/D5341M-17a. In: Standard Test Method for Measuring Coke Reactivity Index (CRI) and Coke Strength After Reaction (CSR). [3] AS 1038.13-1990. In: Section 3: Determination of coke reactivity index and coke strength after reaction. 2013;201:29. [4] ISO_18894-2018_2. In: Coke – Determination of coke reactivity index (CRI) and
521