How-to, Tips and Articles: Using HyperCoal as a Binder to Improve the Compressive Strength of Coal Briquettes Prepared via Hot-pressing

Abstract

HyperCoal was prepared from low-rank coal via high-temperature solvent extraction with N-methylpyrrolidone as an extraction solvent and a liquid-to-solid ratio of 50 mL/g in a high-temperature and high-pressure reactor. When HyperCoal was used as a binder and pulverized coal was used as the raw material, the compressive strength of the hot-pressed briquettes (each with a diameter of 20 mm and mass of 5 g) under different conditions was studied using a hot-pressing mold and a high-temperature furnace. The compressive strength of the hot-pressed briquettes was substantially improved and reached 436 N when the holding time period was 15 min, the hot-pressing temperature was 673 K, and the HyperCoal content, was 15wt%. Changes in the carbonaceous structure, as reflected by the intensity ratio between the Raman G- and D-bands (IG/ID), strongly affected the compressive strength of hot-pressed briquettes prepared at different hot-pressing temperatures. Compared with cold-pressed briquettes, hot-pressed briquettes have many advantages, including high compressive strength, low ash content, high moisture resistance, and good thermal stability; thus, we expect that hot-pressed briquettes will have broad application prospects.

1. Introduction

Large deposits of low-rank coal are available in locations such as Australia, eastern Europe, North America, Germany, and China [1]. However, low-rank coal is difficult to use directly because of its high ash content, high moisture content, high sulfur content, high oxygen content, and low heating value [2‒5]. Rapid industrialization has led to an energy crisis. In the coal coking industry, in particular, coking coal has gradually become scarce. Making full use of existing low-rank coal has become a key to solving future energy problems.

Organic solvent extraction is a new approach for extracting special chemical products from low-rank coal [6‒8]. Organic solvent extraction results in conditions that improve the use value of low-rank coal. First developed in Japan, HyperCoal, which has an ash content of less than 0.2%, can be obtained through organic solvent extraction. It has attractive properties, including a low ash content, high calorific value, excellent thermoplasticity, high reactivity, good softening behavior, and good melting characteristics [9‒11]. HyperCoal can be used in, for example, advanced fuels, raw materials for gasification and liquefaction, coal blending of coking, electrode materials, and pitch-based carbon fibers [12‒17].

In our study, HyperCoal was used as a binder to produce hot-pressed briquettes using weakly adhesive pulverized coal as raw materials. In the metallurgical industry, lump coal with high volatility is the traditional fuel for providing heat and reducing gas. However, pulverized coal is inevitably produced as a byproduct during the production, transport, and use of lump coal [18]. Because of its small particle size, pulverized coal does not meet the requirements for various industrial processes and causes a huge waste of fuel [19‒21]. Making pulverized coal into coal briquettes is a good solution to this problem. Compared with cold-pressed briquettes, hot-pressed briquettes made with HyperCoal as the binder exhibit better qualities, such as a higher compressive strength, lower ash content, better water resistance, and greater thermal stability. In addition to its use in industrial boilers, kilns, gasifiers, and coke ovens, HyperCoal is suitable for use in COREX as a lump coal substitute. High quality (in terms of properties such as mechanical strength and thermal properties) is necessary for coal briquettes, and ingredients with low toxicity and ash content are also important [22‒24].

2. Experimental

2.1. Properties of samples

Two kinds of coal were used in the experiments. One was KL, which was used to prepare HyperCoal. The other was GD, which was used as a raw material for hot-pressed briquettes. Samples were dried in a drying oven at 378 K for 4 h to remove surface moisture. Afterwards, the KL coal and GD coal were crushed and sieved to less than 0.074 mm and 1 mm, respectively. The KL HyperCoal was obtained from the KL coal via a high-temperature extraction. Table 1 shows the proximate and ultimate analysis of the samples, as determined according to Chinese standards GB/T 212-91 and GB/T 476-91, respectively.

Table 1. Proximate analysis and ultimate analysis of samples

Table 2 shows the adhesion index and fluidity of the samples. The adhesion index was measured according to standard GB 5447―85, and the fluidity was based on the measurement of the maximum penetration distance [25].

Table 2. Adhesion index and fluidity of samples

2.2. Preparation of HyperCoal

Fig. 1 shows the device used for the extraction of HyperCoal. First, 8 g of KL coal and 400 mL of N-methyl-2-pyrrolidone (NMP) were mixed in a liquid-to-solid ratio of 50 mL/g in a high-emperature and high-pressure (HTHP) reactor. High-purity argon was then introduced into the HTHP reactor for 15 min at a gas flow rate of 400 mL/min to purge the air. The HTHP reactor was then heated from room temperature to 623 K and maintained at this temperature for 1 h. During the heating process, stirring was continued to keep the solid and liquid phases in contact; the stirring rate was 100 r/min. The solid–liquid mixture was removed after the reactor was allowed to naturally cool to 323 K.

Fig. 1. Schematic of the extraction device. 1-Temperature control device; 2-Gas cylinder; 3-Heating jacket; 4-Thermocouple; 5-Air outlet; 6-HTHP reactor; 7-Pressure gauge; 8-Mechanical stirrer.

Next, suction filtration was used to separate the solid–liquid mixture. The liquid phase was passed through a rotary evaporator to recover the NMP solvent and to obtain the solid product, which was washed repeatedly with alcohol and deionized water and dried in a drying oven at 378 K for 8 h. Performing multiple experiments was necessary to produce a uniformly mixed sample and to prepare for the subsequent hot-pressing experiments.

2.3. Preparation of coal briquettes

Fig. 2 shows a schematic of the hot-pressing device. The preparation was conducted as follows: Different proportions of HyperCoal and GD coal were weighed and mixed well firstly. Samples (5 g) were then taken into the hot-pressed mold (diameter: 20 mm), and the mold was placed in the high-temperature furnace finally. After the set temperature was reached, it was maintained for various holding time periods. Coal briquettes were compacted at a pressure of 30 MPa with a pressing rate of 4 mm/min and a holding time period of 1 min; they were removed from the furnace after it had cooled to room temperature.

Fig. 2. Schematic of the hot-pressing device. 1-Universal testing machine; 2-Indenter; 3-High-temperature furnace; 4-Hot-pressing mold; 5-Bracket; 6-Computer; 7-Temperature control device; 8-Table.

Cold-pressed briquettes were prepared in the different manner with the hot-pressed briquettes. Before pressing, the binders were subjected to different treatments. Bentonite was directly mixed with deionized water and pulverized coal; starch was mixed with pulverized coal after gelatinization; molasses was mixed with pulverized coal after dilution. Afterwards, samples (5 g) were taken into the mold and pressed at a pressure of 30 MPa with a pressing rate of 4 mm/min and a holding time period of 1 min, after which, they were removed and dried in a drying oven at 378 K for 4 h.

2.4. Detection methods

The compressive strength of the coal briquettes was measured according to the method described in Chinese standard M/T 748-2007. With the side of the briquettes facing the force application surface, coal briquettes were placed one by one onto the center of the test machine. One-way force was applied at a displacement speed of 1 mm/min. The compressive strength of the coal briquettes was recorded and averaged. A high resolution Raman spectrometer (HORIBA LabRAMHRE, France) was used to analyze the carbonaceous structure of the coal briquettes.

3. Results and discussion

3.1. Compressive strength

Fig. 3 shows the compressive strength of hot-pressed briquettes at different hot-pressing temperatures with and without HyperCoal. In the absence of HyperCoal, the hot-pressing treatment barely improved the compressive strength of the briquettes. However, the compressive strength increased from 43 to 273 N at higher temperatures when HyperCoal was added as a binder. Therefore, the compressive strength of coal briquettes could be improved by adding HyperCoal as a binder under hot-pressing conditions.

Fig. 3. Compressive strength of hot-pressed briquettes at different hot-pressing temperatures with and without HyperCoal (HyperCoal content: 0 or 10wt%; holding time period: 30 min).

Various hot-pressing conditions may have different effects on the compressive strength of the hot-pressed briquettes. Fig. 4 shows the compressive strength of hot-pressed briquettes with different holding time periods. When the holding time period was zero, the compressive strength was very low. With an increase in holding time period, the compressive strength reached a maximum at a holding time period of 15 min. At a longer holding time period, the compressive strength gradually decreased. Therefore, the ideal holding time period for hot-pressed briquettes is 15 min.

Fig. 5 shows the compressive strength of hot-pressed briquettes at different hot-pressing temperatures. With an increase in hot-pressing temperature, the compressive strength increased in three stages. The first stage was 648–673 K; in this stage, the compressive strength increased dramatically. The second stage was 673–748 K, during which the compressive strength exhibited little change. The third stage was 748–773 K, where the compressive strength increased again. Therefore, the most economical hot-pressing temperature is 673 K.

Fig. 4. Compressive strength of hot-pressed briquettes with different holding time periods (HyperCoal content: 10wt%; hot-pressing temperature: 673 K).

Fig. 5. Compressive strength of hot-pressed briquettes at dif- ferent hot-pressing temperatures (HyperCoal content: 10wt%; holding time period: 15 min)

Fig. 6 shows the compressive strength of hot-pressed briquettes with different HyperCoal contents. The HyperCoal content has a significant effect on the compressive strength of hot-pressed briquettes. When the HyperCoal content was 5wt%, the compressive strength was only 70 N. However, it was as high as 436 N when the HyperCoal content was 15wt%, and the compressive strength had little change when the HyperCoal content exceeded 15wt%. Therefore, the ideal HyperCoal content is 15wt%.

3.2. Raman analysis

Changes in the carbonaceous structure may lead to differences in the compressive strength of hot-pressed briquettes prepared at different hot-pressing temperatures. Fig. 7 shows the Raman spectra of hot-pressed briquettes prepared at different hot-pressing temperatures.

Fig. 6. Compressive strength of hot-pressed briquettes with different HyperCoal contents (holding time period: 15 min; hot-pressing temperature: 673 K).

Fig. 7. Raman spectra of hot-pressed briquettes prepared at different hot-pressing temperatures.

Raman spectroscopy is commonly used to analyze the structure of a substance because it does not require the sample to be pretreated and is a nondestructive way to test samples [26‒28]. Two bands appear in the Raman spectra of these samples: the G-band, with a Raman shift from 1580 to 1600 cm−1, which is related to amorphous carbon, and the D-band, from 1350 to 1380 cm−1, which represents graphitized carbon. The intensity ratio of G-band to D-band (IG/ID) represents the degree of disorder and is commonly used to characterize the carbonaceous structure of different coal chars. Dividing the Raman spectrum into five peaks is a common practice to reduce errors associated with peak fitting [29‒30]. Fig. 8 shows the peak-fitting results of hot-pressed briquettes prepared at different hot-pressing temperatures. The fitted data are consistent with the original data, which indicates that the fit is excellent. Table 3 shows the characteristic peak-fitting parameters of hot-pressed briquettes prepared at different hot-pressing temperatures.

Fig. 8. Peak-fitting results of hot-pressed briquettes prepared at different hot-pressing temperatures: (a) 648 K; (b) 673 K; (c) 698 K; (d) 723 K; (e) 748 K; (f) 773 K; (g) 798 K.

Table 3. Characteristic peak-fitting parameters of hot-pressed briquettes prepared at different hot-pressing temperatures

With an increase in temperature, the IG/ID ratio gradually increased, which represented a decrease in the defect concentration and an increase in the degree of graphitization. For coal pyrolysis, the reactions from 648 to 823 K are dominated by depolymerization and decomposition, which produce most of the volatile matter. At 623 K, bituminous coal begins to soften and then melts and bonds; it becomes semi-coke at 773 K. As pyrolysis progresses, the carbon structure of the hot-pressed briquette changes, exactly matching the changes in the ratio IG/ID and the compressive strength.

3.3. Performance comparison

Compared with cold-pressed briquettes prepared with common binders, hot-pressed briquettes prepared with HyperCoal have numerous advantages. Fig. 9 shows the compressive strength of cold-pressed briquettes prepared with different binders. The bonding effect of bentonite and starch was low; the highest compressive strength was 128 N when the bentonite content was 20wt% and 141 N when the starch content was 15wt%. Thus, the low compressive strength limits the use of bentonite and starch as binders. However, the binding effect of molasses was completely different; when the molasses content was 15wt%, the compressive strength reached 403 N. This compressive strength fully meets the COREX requirements for entry into a furnace. Therefore, companies such as South Korea’s Pohang and China’s Baosteel use molasses as a binder for industrial briquettes [31]. However, as a hydrophilic binder, molasses has poor moisture resistance. Its high viscosity also causes difficulty in molding. In contrast, hot-pressed briquettes prepared with HyperCoal do not suffer such defects.

Because coal briquettes need to be used at high temperatures, they should exhibit good thermal stability. Fig. 10 shows the compressive strength of hot-pressed briquettes and cold-pressed briquettes at room temperature after reheating. An increase in temperature was obviously unfavorable for cold-pressed briquettes prepared with molasses. With an increase in temperature, the bonding structure was destroyed and the lowest value of compressive strength was only 116 N. In contrast, an increase in temperature was favorable for hot-pressed briquettes prepared with HyperCoal. The compressive strength was consistently greater than 330 N and was as high as 1265 N at 873 K. Hot-pressed briquettes prepared with HyperCoal have many advantages over cold-pressed briquettes, and they will have broad market prospects.

Fig. 9. Compressive strength of cold-pressed briquettes prepared by different binders.

Fig. 10. Compressive strength of hot-pressed briquettes and cold-pressed briquettes at room temperature after reheating

4. Conclusions

Because of its excellent bonding and flow performance, HyperCoal can be used as a binder to improve the compressive strength of coal briquettes prepared via hot-pressing. The factors that affect the compressive strength of hot-pressed briquettes were studied. The ideal holding time period for hot-pressed briquettes was 15 min, the economical hot-pressing temperature was 673 K, and the ideal HyperCoal content was 15wt%. Under these conditions, the compressive strength reached 436 N. According to the Raman analysis, changes in carbonaceous structure determined the compressive strength of hot-pressed briquettes under different hot-pressing temperatures. With an increase in hot-pressing temperature, the IG/ID ratio gradually increased, which matched the changes in the compressive strength of hot-pressed briquettes. Compared with cold-pressed briquettes prepared with common binders, the hot-pressed briquettes prepared with HyperCoal showed numerous advantages, such as high compressive strength, low ash content, high moisture resistance, and good thermal stability. There are broad market prospects for such briquettes.

Source: Yajie Wang - University of Science and Technology Beijing, Hai-bin Zuo - University of Science and Technology Beijing, Jun Zhao - University of Science and Technology Beijing

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Wednesday, March 18, 2020

Using HyperCoal as a Binder to Improve the Compressive Strength of Coal Briquettes Prepared via Hot-pressing