Regular Article
Hot Strength of Coke Prepared by Briquetting and Carbonization of Lignite
2022 Volume 62 Issue 12 Pages 2511-2515
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2022 Volume 62 Issue 12 Pages 2511-2515
The hot strength of coke prepared from an acid-washed lignite by briquetting-carbonization before and after CO2 gasification was, for the first time, investigated in this work. The hot strength at 1000°C before gasification was higher than coke strength measured at a room temperature. CO2 gasification resulted in a linear decrease of the hot strength with the reaction time. The lignite-coke showed faster decrease in the strength during gasification, compared to conventional cokes derived from caking coal or non-or slightly caking coal, due to the high reactivity caused by its porous structure and catalytic metal species remaining even after the acid-washing, while it showed superior hot strength at the initial stage of gasification.
Strength is one of the most important properties of metallurgical coke. The depletion of caking coal suitable for the high-strength coke as feedstock has requested development of methods for utilizing low-grade coals. Lignite, a type of lowest grade coals, has not been used as the feedstock for metallurgical coke despite the abundant reserves.
Cokes are generally high-porous materials, having cracks and large pores generated during a retorting process. There have been many studies that investigated the relationship between the strength and pore structure of cokes.1,2,3) In industry, drum index (DI) has been used for evaluating the coke strength. Since the measurement of DI needs a considerable amount of the coke, a compression strength measured with a diametrical-compression test is often employed as a type of index in academic research. A good correlation between DI and compression strength has been found by Sakai et al.4) On the other hand, it has been well known, as reported by Xing et al.,5) that the strength of coke substantially decreases upon heating in the presence of CO2 due to its gasification. In industry, coke strength after reaction (CSR) is used as the index for representing this trend.
For using lignite as the feedstock for coke, Mori et al.6) reported that a high-strength coke with the tensile strength of over 30 MPa was available from a Victorian lignite by its binder-free briquetting and the subsequent carbonization. The strength was much higher than that of metallurgical coke commonly used in industry (<6 MPa).7,8) The research group also showed that the strength increased by pretreatment of the lignite such as hydrothermal treatment, acid treatment, and fine pulverization.9,10,11,12,13) Song et al. reported the effect of lignite addition to caking coal on DI14) and CSR15) of produced coke. Thus, the strength of coke produced from lignite as feedstock or as additives for coke has been investigated. However, all the studies evaluated the strength at ambient temperatures (cold strength). Moreover, because the lignite-cokes produced by briquetting-carbonization have been prepared only by lab-scale experiments, they have not been tested for the CSR that requires a large quantity of the sample, and their strength after CO2 gasification is unknown.
The hot strength and its change during gasification are important for understanding the strength under conditions of their practical use in a blast furnace. Nevertheless, there have been only a few studies related to the coke’s hot strength, and the difference between hot strength and cold strength is currently not well understood. Ono et al.16) carried out a three-point bending test for a thin coke chip with a load of 200 g, and measured the displacement until the breakage. The sample was heated under N2 and then gasified by CO2 or H2O before the test. The displacement increased along with the gasification, but the hot strength during the gasification was unknown. Recently, Saito and Tsukamoto17,18) carried out a uniaxial compression test of cokes at a high temperature. They found that the hot strength was decreased by CO2 gasification. Interestingly, the hot strength was higher than the cold strength, and the strength after CO2 gasification at 1100°C for 60–80 min was maintained at levels similar to or higher than that of the original cold strength. The result indicated a possibility that the coke strength was affected by the temperature. These reports regarding hot strength used cokes produced from caking and non-or slightly caking coals.
The previous studies have revealed a potential of lignite to be converted to a strong coke. While, the lignite-coke is thought to be highly reactive to CO2 because it is rich in catalytic metals including alkali and alkaline earth metallic species (AAEMs) and micropores contributing to an enlargement of reactive surface. Therefore, quantitative evaluation of the decrease in its strength at high temperatures and during gasification is vital for its industrial application. In this study, a lignite-coke was prepared from an Indonesian lignite by the briquetting and carbonization method. After the assessment of gasification reactivity using a thermogravimetry, the fracture strength at a high temperature before and during gasification was measured by the uniaxial compression test.
An Indonesian lignite was used as the feedstock. The lignite had an elemental composition of C/H/N/O+S = 66.2/4.8/0.9/28.1 wt% on a dry and ash-free basis and volatile matter/fixed carbon/ash contents of 49.9/46.9/3.3 wt% on a dry basis. As shown later, the reactivity of coke derived from this lignite was too high to measure its hot strength during CO2 gasification at temperatures appropriate for the test. To decrease the coke reactivity, acid-washed lignite was prepared and used for the coke preparation. After pulverization to sizes below 106 μm, the fine powder of raw lignite was soaked and stirred in 1.0 M HCl at 60°C for 24 h. The slurry was filtrated using a PTFE membrane filter having a pore size of 0.45 μm. The solid residue over the filter was washed repeatedly with hot water until pH of the filtrate became neutral and then vacuum-dried to recover as the acid-washed lignite. The acid treatment is generally used for the removal of AAEMs without influencing organic portion of the lignite.10) As shown in Table 1, acid washing altered the ash content and composition so that the reactivity of coke was decreased by the removal of catalytic metals such as K, Ca, and Fe. The ash composition was analyzed with a X-ray fluorescence (XRF) spectrometer (Malvern Panalytical, Epsilon 1).
| Raw | Acid-washed | |
|---|---|---|
| Ash content (wt%) | 3.35 | 0.68 |
| Composition (wt%) | ||
| SiO2 | 7.19 | 47.29 |
| SO3 | 7.45 | 2.52 |
| K2O | 0.07 | |
| CaO | 17.22 | 3.00 |
| TiO2 | 0.17 | 0.61 |
| MnO | 0.88 | 0.07 |
| Fe2O3 | 65.09 | 43.53 |
| CuO | 1.07 | 1.53 |
| Others | 0.87 | 1.46 |
Details for the preparation of lignite-coke by briquetting and carbonization can be found elsewhere.6,9,10,11,12,13,19) Briefly, 2.0 g of acid-washed lignite was loaded in a mold having the diameter of 14 mm, heated to 130°C, and then pressed at 64 MPa for 8 min. The briquetting produces a briquette having the diameter and height of 14.0 mm and 8.0 mm, respectively. The prepared briquette was quickly transferred to a reactor for the carbonization and heated under N2 stream at 5°C/min to 1000°C, where the temperature was kept for 10 min. During carbonization, the briquette shrank to the sizes of 11.0 mm × 7.0 mm, being a high-strength coke with the apparent density of 1160 kg/m3. The prepared lignite-coke is hereafter denoted by Coke D.
2.2. Thermogravimetric Analysis (TGA)For investigating the reactivity, CO2 gasification of Coke D was performed on a thermogravimetric analyzer (Hitachi Hi-Tech Science, STA7200). A small piece of Coke D (3 mg) obtained after the compression test at room temperature was loaded on a platinum crucible, placed in the analyzer, heated to 1000°C (kept for 10 min) under 700 mL/min of N2, and then gasified under 700 mL/min of 50% CO2/N2 at 900, 950, or 1000°C until the mass loss was reduced to a negligible level.
The result of TGA was kinetically analyzed with a random pore model19) described by the following equation:
According to the method reported by Saito and Tsukamoto,17,18) the uniaxial compression test was carried out at Japan Testing Laboratories, Inc. on a testing machine equipped with an electric furnace. Coke D was heated at 35°C/min to 1000°C under a flow of N2. After 10 min, the gas was switched to CO2 to gasify the coke for a prescribed time, followed by the compression test at the same temperature. Including the test under gasification condition, the strength measurement was carried out at four different conditions: room temperature under air, 1000°C under N2, 1000°C under CO2 for 6 min, and 1000°C under CO2 for 9 min. Two coke samples were used at each condition. The hot strength of Coke A and Coke C, reported in our previous study,17,18) was used as a reference. For Coke A, the hot strength after 30 min of gasification by CO2 was additionally measured in this study.
The reaction rate of Coke D during CO2 gasification is plotted in Fig. 1. By the model fitting of experimental data, Ψ was calculated to be 89.8. This indicated that the porosity of Coke D significantly increased during the gasification. Slight deviations of experimental data from the calculation were possibly caused by remaining catalytic metals or suitability of the gasification model. By the Arrhenius plot of reaction rate (Fig. 2), activation energy (E) and frequency factor (k0) were calculated to be 236 kJ/mol and 9.68 × 107 min–1, respectively. Using the model equation and kinetic parameters, the CO2 gasification at a desired temperature can be predicted as shown in Fig. 3. Though not shown in the figure, the gasification reactivities of cokes prepared from the acid-washed lignite and unwashed lignite were compared. The gasification at 900°C was completed at 150 min and 23 min, respectively, showing a significant influence of the removal of catalytically active metallic species in the ash. Nevertheless, the reactivity of coke was still high even after the ash removal. In fact, the conversion of a blast furnace coke, employed for a comparison purpose, at 150 min of the gasification under the same conditions was well below 0.2. The high reactivity was attributed to catalytic activity of remaining metallic species and the porous structure. The lignite-cokes prepared by the briquetting-carbonization method does not have large pores like those in conventional blast furnace cokes, but instead they are rich in micropores. The micropore surface area, measured by CO2 ad/desorption at 0°C of the coke from this lignite was 317 m2/g. In the uniaxial compression test at high temperatures, a high temperature, such as 1100°C that is generally used in CSR measurement, was desirable to simulate the hot strength in industrial furnaces, but that temperature was difficult to employ in this work. Therefore, we determined to use the gasification temperature of 1000°C. The hot strength was investigated at 6 min and 9 min during the gasification. Based on the kinetic calculation, conversion of the coke under gasification conditions during TGA was 37% and 61% at 6 min and 9 min, respectively.
Gasification profiles of Coke D. Plots show the experimental data, and lines show the calculation data obtained by fitting with the random pore model. (Online version in color.)
Arrhenius plot of CO2 gasification of Coke D for determining E and k0.
CO2 gasification of Coke D at different temperatures. Plots show the experimental data, and lines show data calculated with the model equation and kinetic parameters. (Online version in color.)
Figure 4 shows fracture stress of Coke D. The hot strength measured under N2 (high temp., 0 min) was higher than the cold strength (room temp.). This trend agrees with that reported for cokes produced from caking and slightly caking coals,17,18) indicating the possibility that the strength of coke increases when heated regardless of the coal type, although the reason and mechanism have yet to be clarified.21)
Fracture stress of Coke D. The number of samples was 2. The error bars present actual fracture stress of samples. (Online version in color.)
The hot strength of Coke D was substantially decreased by the gasification with CO2. The strength decreased from 55.2 MPa to 43.4 MPa and 40.6 MPa at 6 min and 9 min, respectively. As indicated by the high value of Ψ in TGA, CO2 gasification caused a growth of pores in Coke D, which contributed to the degradation in hot strength because large pores could be origin of the fracture under mechanical pressure. The plots were expressed well by a linear function of gasification time. This was in contrast to the non-linear function (random pore model) of gasification rate observed in TGA. It should be noted that, in the uniaxial compression test, the mass-based conversion was not available because of the difficulty in a complete recovery of unreacted coke. The results may show a difference in the gasification system between uniaxial compression test and TGA. In TGA, the gasification occurred under a reaction-controlled regime. This was apparent from the fact that the kinetics was not altered by reaction temperature. While, in the former test, the supply of CO2 was limited to side surface of the cylindrically-shaped coke because the top and bottom surface faced pressing bars of the tester. In other words, the rate of gasification in the uniaxial compression test could be slower, compared to TGA. However, it was believed that the gasification of Coke D also occurred in a reaction-controlled regime. Numazawa et al.22,23) investigated reaction mechanisms of coke degradation by CO2, and they found that the rate-determining step changed from reaction rate to gas diffusion at 1100°C. Ono et al.16) showed that pore structure inside a coke after CO2 gasification at 1100°C was uniform. The gasification temperature of the present study was much lower than these previous works. Therefore, CO2 supposedly diffused inside the coke to gasify uniformly, resulting in the linear decrease in the strength.
Xing et al.5) reported that the cold strength of three types of coke was decreased by 1% or increased by 3.8% in 2 h of CO2 gasification at 1400°C. In the present study, the decrease was up to 6.4% only in 3 min (from 6 to 9 min of gasification). This comparison indicates that the cold strength is unsuitable for a quantitative representation of coke degradation during gasification, although the direct comparison of these results is not necessarily appropriate due to the difference in coal types and reaction conditions.
Figure 5 compares time-dependent changes in the strength during CO2 gasification of cokes reported in this work and our previous study.17,18) All the cokes lost their strength with time. The previous work reported the hot strength of Coke A (from caking coal) and Coke C (from non-or slightly-caking coal), produced in an industrial chamber oven, after gasification at 1100°C for 80 min and 60 min, respectively. When the plot of 30 min was added for Coke A in this work, a linear correlation between the strength and reaction time was found. Assuming the linearity for the three cokes, Coke D showed the highest slope, followed by Coke A and then Coke C. A rapid degradation of Coke D was evident. When 1100°C is applied to the gasification of Coke D, the slope will be much steeper. On the other hand, Coke D had higher initial strength than Coke A and Coke C. The lignite-derived coke is, thus, not suitable for the use at high temperatures for a long time, but it can show the strength higher than conventional cokes for the initial stage of gasification. Consequently, improvement of the initial strength is valuable for the lignite-coke in this regard, and the improvement is in fact possible in terms of lignite type, pretreatment, and briquetting-carbonization conditions as shown in our recent studies.6,9,10,11,12,13)
The hot strength of coke prepared from acid-washed lignite, Coke D, before and during CO2 gasification was investigated using the uniaxial compression tester equipped with the furnace. TGA showed the reactivity of Coke D was much higher than conventional cokes prepared from caking coal and non-or slightly caking coal because of the porous structure with the micropore surface area of over 300 m2/g and remaining catalytic metals. The kinetic analysis indicated that porous structure further developed during the gasification. The hot strength before gasification was higher than the cold strength. The experimental result quantitatively showed that Coke D had a high initial strength, compared to Coke A and Coke C, but it rapidly lost the strength during CO2 gasification in a linear manner as a function of time due to the growth of porous structure. The results demonstrated the sufficient strength of Coke D at the initial stage of gasification and the importance of having the initial strength as high as possible.
This work was supported by the 29th ISIJ Research Promotion Grant. Coke A sample was supplied by Nippon Steel Corp. The authors gratefully acknowledge the Cooperative Research Program of “Network Joint Research Center for Materials and Devices” for the support.
