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Issue
Metall. Res. Technol.
Volume 116, Number 6, 2019
Article Number 610
Number of page(s) 10
DOI https://doi.org/10.1051/metal/2018086
Published online 30 August 2019

© EDP Sciences, 2019

1 Introduction

The Iron and Steel Industry has undergone a radical change, and processes have been developed continually for the faster rate of production. The technological development has also been made over the last two decades towards conventional coke ovens concerning emission reduction. The modern coke ovens are large units which use good quality of refractories and high capital expenditure. The quality of refractories plays a vital role in prolonging the coke oven life and its operational performance. The sustainability of the coke oven battery life also depends on expert construction, quality of bricks, careful initial heating, proper operation and overall efficient maintenance.

The productivity of coke ovens was continuously increased through constructive changes, including but not limited to the enlargement of coke oven dimensions, reduction of linear brick thickness, and widening the oven chamber. The design complexities of the structure and the need to ensure not less than a 25-year lifetime, the quality standard for silica coke oven bricks had changed accordingly [14]. The heat transfer in the heating wall takes place by conduction. The heat supplied to the charge is determined by the temperature gradient in the heating wall, which depends on the thermal conductivity of the material of construction. Because of the thermal conductivity of the ceramic material is defined as the amount of heat flow through a unit area, in a direction normal to the surface area, in a specified time, with a known temperature gradient. Thermal conductivity depends on the chemical and mineralogical composition, porosity and the application temperature. The high thermal conductivity of refractory is desirable when heat transfer through brickwork is required, for example in recuperators, regenerators, muffles, etc. Low thermal conductivity is helpful when the heat has to be conserved.

Coke strength depends on to a more considerable extent on thermal conditions prevailing during coal carbonization. The thermal conditions are influenced by the operating parameters of oven viz. flue temperature, wall temperature, carbonization time, bulk density and moisture content, etc. The flue temperature and coking time are dependent variables with an inverse relationship.

The heating rate is associated with the pattern of heat supply during the carbonization, which influences the strength and fissuring of the coke. A proper heat requirement and the pattern of heat supply should be achieving optimal coke quality and also optimize total heat consumption. The plastic layer of coal is a highly complex medium, where intricate physical and chemical equilibria between solid, liquid and gaseous components make the study difficult [5]. During the coking, plastic layer of coal charge, substances of organic coal mass are present in the softening stage. It is agreed that the formation of a plastic layer of coal during coking is a pledge to obtain a coking residue, and the gas pressure arising in the layer predetermines magnitude of coking pressure. The zones vary in the viscosity [6], the magnitude of the force to perforate the layer [7], which can be separated from the plastic layer. The literature reported that the study of coal plastic layer is a ‘passive’ function, which reduces resolidification of its mass at the temperature of half coke formation and to baking in it of mineral admixture and the inert compound of the inorganic mass of coal. A relatively small amount of the work is connected with the investigation of the ‘active’ role of a coal plastic layers which are consequences of this interaction [8]. It has been reported that all coals irrespective of their rank, can be de-volatilized without showing any swelling behaviour, provided the heating rate is sufficiently slow [9].

The present investigation describes the scope of using high thermal conductivity of silica bricks in coke oven battery, to impart the desired heating rate even at the lower centre mass temperature and thereby reduce the energy consumption.

1.1 Influence of heating rate on cokemaking

The heating rate depends on the initial temperature, the heat capacity of the oven on charging and oven temperature increment per unit time. In general, higher the oven temperature, the rapid is the initial rate of heating, plastic temperature range, plasticity and volatile evolution characteristics of the coal.

The thermal coefficient of coke oven batteries is determined by the operational parameters and thermal efficiency of the battery. The heat transfer into the coal cake takes place mainly by radiation with the side brick wall, which functions as the secondary heating surface. During coking, the coal mass is progressively carbonized from the surface of coal mass towards the center of the charge from both sides of the conjugated wall of the oven. Identical carbonization rates from both the sides can be achieved by appropriate heat input from the side walls. The uniform coking rate from both sides will result in an even carbonization and hence improvement in coke quality and productivity [10].

The principle difference in coking conditions of slot oven (0.45 and 0.55 m wide) is the difference in the maximum rate of heating of the coal blend (3.4 and 2.7 °K/min). The heating rate as mentioned above with the dimensions of °K /min is termed as ‘temperature gradient’. The heating rate is not correct since the temperature gradient is a derivative of the geometric coordinate in the direction of most rapid temperature change having the dimensions of °K/min. However, in the absence of the mass transfer, at the fixed point, the temperature changes with time at a constant rate reaches to the external values of 3.4 °K/min for 0.45 m wide and 2.7 K/min for an oven 0.55 m wide [11]. The temperature change difference between coking processes in ovens which differ only in width and is of great importance since it discloses the physical meaning of the effect of the chamber width on the coking process.

1.2 Parameters influencing the heat transfer in the charge

The thermal conductivity of the bricks: literature reported that the temperature is one of the most critical parameters of coking process. In coke oven battery, coking chamber alternate with combustion chambers, so that one each side of the coking chamber there is a combustion chamber from which heat is conducted to the coking chamber through the walls of the combustion chamber. The higher thermal conductivity of silica bricks will increase the heat transfer rate and consequently, the output of the oven will increase the throughput. It has been reported that temperature of the combustion chamber, wall thickness and width of coking chamber remaining the same, increase in thermal conductivity of refractory from 1.08–1.21 Btu/Ft/hr° F, increase the throughput by approximately 10% [1].

To maintain stable heat transportation, their thermal conductivity in the heating walls has been improved by reducing the porosity of the bricks [2]. Literature also reported that an increase in the throughput for a given temperature of the heating flue, there is more even heat transfer into coke, diminution of energy consumption for the underfiring forgiven throughput of the coke oven battery, decrease the heating flue temperatures and NOx concentration of the flue gas for a given throughput [3].

The moisture content throughout the charge affects the heating rate; on the one hand, a significant amount of heat is required for the evaporation of water, and thermal effects arise from the condensation of water. Also, the distribution of moisture primarily controls the bulk density distribution within the charge.

The bulk density distribution in the chamber controls the local heat demand throughout the coke bed. This distribution can also be measured in operational ovens. It is more than difficult to measure the effective thermal conductivity of coal /coke beds as a function of temperature because the material undergoes conversion phases; the results of various investigations show a significant scatter of data [12]. Furthermore, the thermal conductivity also depends on the bulk density.

The operating temperature of the oven is necessary to produce the desired quality of coke at optimal coking time with minimal consumption of energy and Nox and CO2 emission.

2 Experimental

The experimental work was conducted in 7 kg electrically heated test oven (internal dimension between door shall: length 370 mm; width 115 mm; height 305 mm). The free space temperature of the oven should be maintained 900 ± 10 °C before charging coal/coal blend into the oven for desired coking rate and coke quality. The time taken for the centre of the charge to reach 900 °C should be considered as the principal criterion of carbonization, and the wall temperature is adjusted to give the required heating rate. Table 1 shows the typical properties of charged coal which is used in this study. The operating conditions like coal moisture (10%), coal cake density (1150 kg/m3 on wet basis), and crushing fines (90% below 3.2 mm) were maintained constant for all tests. The cross-sectional view of the 7 kg carbolite oven is shown in Figure 1.

Table 1

Typical properties of coal/coal blend.

thumbnail Fig. 1

Cross-sectional view of electrically heated 7 kg carbolite oven.

2.1 Measurement of temperature profile and heating rate

In the present study, three different ceramic castables (LC-65, SiC-30% and LC-85 castable) materials were used in the lining the laboratory oven. The charging temperature and the coking time were varied to achieve desired heating rate and coke end temperature.

In each experiment, a set of six thermocouples denoted T1-T6 were inserted directly into the charged coal cake to a depth of 200 mm via holes in the ram side temporary oven door for measuring the temperature patterns. The measurement of temperature profiles during the carbonization were recorded at four different heights (viz., 32 mm × 42.5 mm, 32 mm × 42.5 mm, 47.5 mm × 82.5 mm, 37.5 mm × 127.5 mm, 37.5 mm × 127.5 mm and 47.5 mm × 172.5 mm) of coal cake with the help of thermocouples throughout the coking cycle for all experimental oven. With the help of recorded temperature profile, coking rate/heating rates were calculated for all experimental ovens. The locations of thermocouples inside the coal cake used in the experiments are shown in Figure 2.

thumbnail Fig. 2

Location of the thermocouple inside the coal cake.

2.2 Determination of coke quality

After completion of coking / carbonization time (5 hrs), the red-hot coke pushed from oven manually and quenched with water. The dry coke samples were tested for determination of coke reactivity index (CRI) and coke strength after reaction (CSR).

3 Results and discussion

In this study, an attempt has been made to understand the impact of three different thermal conductivity ceramic castable (LC-65, LC-85 castable and SiC-30%) on heating rate, coking time and coke quality in a laboratory oven while maintaining similar operating conditions. The thermal conductivity of these ceramic castables (LC-65, LC-85 and SiC-30%) is 1.55 W/mK at HF, 2.11 W/mK at HF and 4.3 W/mK at HF respectively. The typical properties of coal / coal blend which are used in this study are shown in Table 1.

The temperature profiles were measured with the help of six thermocouples, but in some cases, few temperature profiles are missing due to the breakage of coal cake while inserting the thermocouple inside the coal cake resulted recorded temperature is also abnormal, and therefore, these temperature profiles are missing in some places to avoid the confusion. Table 2 depicts the initial charging temperature of free space, heating rate, coking time for all experimental ovens. The variation may be due to the effect of initial free space temperature and thermal conductivity of the oven throughout the cycle. It appears that heating rate measured at different heights of coal cake coincide with the passage of the plastic layers because temperature distribution in different transformation phases is almost same. Therefore, the rate of carbonization is crucial parameter to decide the adequate heat input and coke quality. The measured temperature profile during carbonization with LC-65, SiC-30%, and LC-85 castable are shown in Figures 3, 4 and 5 respectively. The relationship between heating rate and coke end temperature with LC-65, LC-85, and SiC-30% castable are shown in Figures 68 respectively. Similarly, the relation between average heating rate and coke quality are shown in Figures 911 for LC-65, LC-85, and SiC-30% castable respectively.

Table 2

Free space temperature, heating rate and coking time at different ceramic castable.

3.1 Effect of thermal conductivity on heating rate

Figure 3 shows the measured temperature profile at different operating conditions with LC-65 castable. The results depict that heating rate varies in the range of 2.68–5.11 °C/min. It is observed from results that higher heating rates lead to higher coke end temperature as compared to lesser heating rates. Results indicate that < 3 °C/min heating rate is not adequate to achieve desired coke end temperature after the end of 5 hrs coking cycle (Fig. 3a) in laboratory scale oven. This is due to less heat penetration during conversion of coal to coke. The results indicate that higher heating rate (5.11 °C/min) leads to higher coke end temperature (929 °C) as compared to lesser heating rate (3.51 °C /min), which produces less coke end temperature (874 °C). Therefore, an adequate heating rate is an important parameter to decide the desired coke end temperature within the desired coking time.

Figure 4 shows the measured temperature profile at the different operating condition with LC-85 castable. The results indicate that heating rate varies in the range of 2.55–6.38 °C/min. Results show that higher initial charging temperature (free space temperature) leads higher heating rate. Results indicate that with the increase in heating rate, the coke end temperature increased with LC-85 castable. In this study, the lowest coke end temperature was 853 °C with lowest heating rate 2.55 °C/min while at the highest coking rate of 6.38 °C/min, the coke end temperature is 1036 °C.

Figure 3b shows that at 900 °C initial charging, the heating rate was 3.51 °C/min while in the case of LC-85 castable (Fig. 4c and d) the same heating rate could be achieved at 740 °C and 782 °C initial charging temperature respectively. Therefore, LC-85 castable having higher heat transfer rate as compared to LC-65 castable produced same coke end temperature even at 100 °C lower initial charging temperature.

The study also confirmed that the thermal conductivity of ceramic castable/bricks plays a significant role in heat transfer rate and coke end temperature. Figure 4g and h depict that maintaining same initial charging temperature with LC-85 castable as compared to LC-65 castable (Fig. 3bd) reached higher heating rate and higher coke end temperature. The heating rate is ∼2 °C/min higher, and the coke end temperature is ∼100 °C higher. It indicates that LC-85 castable is having higher heat transfer rate and the coke end temperature reached 925 °C (Fig. 4k) and 957 °C (Fig. 4l) within 4.0 hrs instead of 5.0 hrs standard carbonization time.

Figure 5 shows the measured temperature profile at the different operating condition with SiC-30% castable. The results indicate that heating rate varies in the range of 3.57–9.56 °C/min. Results show that higher initial charging temperature (free space temperature) leads higher heating rate. Results indicate that with the increase in heating rate, the coke end temperature increased with SiC-30% castable. In this study, the lowest coke end temperature was 875 °C with lowest heating rate 3.57 °C/min while at the highest coking rate of 9.56 °C/min, the coke end temperature is 1100 °C.

Figure 3b show that at 900 °C initial charging, the heating rate was 3.51 °C/min while in the case of SiC-30% castable (Fig. 5a) the same heating rate could be achieved at 725 °C initial charging temperature. Therefore, SiC-30% castable having higher heat transfer rate as compared to LC-65 castable produced same coke end temperature even at 175 °C lower initial charging temperature.

The study also confirmed that the thermal conductivity of ceramic castable/bricks plays a significant role in heat transfer rate and coke end temperature. Figure 5d depicts that maintaining same initial charging temperature with SiC-30% castable as compared to LC-65 castable (Fig. 3bd) reached higher heating rate and higher coke end temperature. The heating rate is ∼4 °C/min higher, and the coke end temperature is ∼150 °C higher. It is also evident from Fig. 5d and Fig. 5e that the initial charging temperature is ∼175 °C lower as compared to Figure 3b but maintains the similar heating rate and coke end temperature.

It indicates that SiC-30% castable is having higher heat transfer rate and the coke end temperature reached 995 °C (Fig. 5i) and 1100 °C (Fig. 5j) within 3.0 hrs and 4.0 hrs instead of 5.0 hrs standard coking/carbonization time.

thumbnail Fig. 3

Measured temperature profile during carbonization of coal (LC-65 castable).

thumbnail Fig. 4

Measured temperature profile during carbonization of coal (LC-85 castable).

thumbnail Fig. 5

Measured temperature profile during carbonization of coal (SiC-30% castable).

3.2 Effect of heating rate on coke end temperature

It is evident from Figures 68 that initial charging temperature is in good agreement with the average heating rate and coke end temperature from LC-65, SiC-30% and LC-85 with the correlation coefficient 0.80, 0.79 and 0.86 respectively. Therefore, it indicates that heating rate of an oven depends not only on the initial charging temperature but also on thermal conductivity of the refractory bricks. Hence, it is possible to reduce 50% carbonization time or coking time with maintaining same heat input with the use of SiC-30% castable instead of LC-65 and on the other hand, reduce the initial charging temperature ∼175 °C to maintain the same heating rate. Similarly, it is possible to reduce 20% carbonization time with keeping same heat input with the use of LC-85 castable instead of LC-65 and on the other hand, reduce the initial charging temperature ∼100 °C to maintain the same heating rate.

thumbnail Fig. 6

Relation between heating rate and coke end temperature for LC-65 castable.

thumbnail Fig. 7

Relation between heating rate and coke end temperature (LC-85 castable).

thumbnail Fig. 8

Relation between heating rate and coke end temperature (SiC-30% castable).

3.3 Effect of heating rate on coke CSR

In general, it is crucial that a consistent pushing schedule should be maintained to achieve rated production of the coke oven. Figure 9 shows the relationship between heating rate and coke quality especially coke CSR with LC-65 castable. It is noted from the result that, the heating rate for coal varies from 2.68–5.11 °C/min. From the experimental result cited in Figure 9, a good relationship was observed between the average heating rate and coke CSR with a correlation coefficient 0.75. Also, results depict that the increase in average heating rates leads to an increase in coke CSR.

Figure 10 shows the relation of heating rate and coke CSR in laboratory scale oven with LC-85 castable. Results indicate that a linear relationship between average heating rate and coke CSR, heating rate increases coke CSR increases. Results show that the coke CSR varies in the range of 35.9–46.7 with a correlation coefficient value of 0.69. The figure indicates that minimum 2.55 °C/min heating rate produces the lowest coke CSR, i.e. 35.9 and a higher coke CSR of 46.7 is observed at the heating rates of 6.20 °C/min.

Figure 11 shows the relation of heating rate and coke CSR in laboratory scale oven with SiC-30% castable. A correlation coefficient value of 0.68 was observed. The figure indicates that minimum 3.69 °C/min heating rate produces the lowest coke CSR, i.e. 39.5 and a higher coke CSR of 47.2 is followed at the heating rates of 8.90 °C/min. It is also noted that the higher heating rate produces higher coke CSR while maintaining the other operating parameters constant. It is also found that the higher heating rate produces higher coke CSR while maintaining the other operating parameters constant. Therefore, it may be concluded from the results that the heating rate is an essential parameter for achieving the desired coke end temperature and coke quality.

thumbnail Fig. 9

Relation between average heating rate and coke CSR (LC-65 castable).

thumbnail Fig. 10

Relation between average heating rate and coke CSR (LC-85 castable).

thumbnail Fig. 11

Relation between average heating rate and coke CSR (SiC-30% castable).

4 Conclusion

The laboratory study confirmed that replacement of LC-65 castable with higher thermal conductivity castable SiC-30% reduces coking time by 50% whereas LC-85 decreases by 20% even at same coke end temperature and coke CSR. The requirement of free space temperature of the laboratories oven is different for using different thermal conductivity of the ceramic castable to maintain same heating rate and coke end temperature during coking. Results confirmed that increasing the thermal conductivity of ceramic castable decrease in the free space temperature to maintaining the constant heating rate (∼4 °C/min) and coke end temperature (∼925 °C). Also, it is confirmed that the use of SiC-30% and LC-85 castable, the input temperature should be reduced by ∼200 °C and ∼100 °C to maintain the same heating rate and coke end temperature as compared to LC-65 castable. It is also concluded that the use of SiC-30% and LC-85 castable, the heating rate is higher by ∼4 °C/min and ∼2 °C/min to maintain the same input temperature as compared to LC-65 castable.

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Cite this article as: Hari Prakash Tiwari, Aman Kumar, Vijay Kumar Tiwary, Pratik Swarup Dash, Atanu Ranjan Pal, Influence of thermal conductivity of ceramic materials on heating rate/coking time and coke quality in cokemaking, Metall. Res. Technol. 116, 610 (2019)

All Tables

Table 1

Typical properties of coal/coal blend.

Table 2

Free space temperature, heating rate and coking time at different ceramic castable.

All Figures

thumbnail Fig. 1

Cross-sectional view of electrically heated 7 kg carbolite oven.

In the text
thumbnail Fig. 2

Location of the thermocouple inside the coal cake.

In the text
thumbnail Fig. 3

Measured temperature profile during carbonization of coal (LC-65 castable).

In the text
thumbnail Fig. 4

Measured temperature profile during carbonization of coal (LC-85 castable).

In the text
thumbnail Fig. 5

Measured temperature profile during carbonization of coal (SiC-30% castable).

In the text
thumbnail Fig. 6

Relation between heating rate and coke end temperature for LC-65 castable.

In the text
thumbnail Fig. 7

Relation between heating rate and coke end temperature (LC-85 castable).

In the text
thumbnail Fig. 8

Relation between heating rate and coke end temperature (SiC-30% castable).

In the text
thumbnail Fig. 9

Relation between average heating rate and coke CSR (LC-65 castable).

In the text
thumbnail Fig. 10

Relation between average heating rate and coke CSR (LC-85 castable).

In the text
thumbnail Fig. 11

Relation between average heating rate and coke CSR (SiC-30% castable).

In the text

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