Micromorphology and safety properties of meager and meager-lean coal for blast furnace injection

Currently, the shortage of coke resources is a strategic problem that restricts the development of steel industry in China. Blast furnace injection (BFI), in which pulverized coal is used instead of coke in a furnace to provide heat and reducing agent [1], is a major technological revolution in modern blast furnace smelting, which largely reduces the cost of iron production [2–4].

To further reduce the production cost and promote systematic safety, considerable studies have been conducted on the utilization of economical low-rank coal, such as meager or meager-lean coal, as BFI coal [5]. The explosivity of injection coal is a main factor that affects the safety of coal pulverizing systems [6–7], which is a part of BFI. Song et al. [6] investigated the explosion characteristics of modified coal, and the results indicated that a higher specific surface area and the developed pore structure could strengthen the explosivity of coal particles. By using a 1.2 L Hartman tube, Bi and Wang [8] investigated the effects of the physical properties of pulverized coal on its explosivity and found that the particle size and volatile content affected the explosion safety of coal particles considerably. Likewise, Li et al. [9] investigated the effects of particle size and size dispersity on the explosivity of coal dust by using a 20 L spherical explosion vessel and found that the explosion severity parameters of coal dust increase as the particle size and size dispersity decrease. The combustion performance of pulverized coal is also important to improve the efficiency and safety of blast furnaces [10]. By conducting proximate and ultimate analysis and the Rock-Eval test, Raygan et al. [2] studied four kinds of coals and found that the mixing of coals could improve the combustion properties of pulverized coals. Osório et al. [11] evaluated the combustion behavior of coal blends by utilizing a thermogravimetric analyzer, and the results demonstrated that low-rank coal influenced the ignition temperatures of blends, whereas high-rank coals affected their burnout behavior. To investigate the burnout behavior of some coals and coal blends, Du et al. [12] simulated the reactions of BFI in a blast furnace and found that the combustion efficiency of the coal could not be improved when the particle size was reduced to 200–325 mesh. Although intensive studies have been conducted on the explosion and combustion of pulverized coal, the combustion safety of injecting low-rank coal such as meager or meager-lean coal has not been widely investigated.

On the basis of the above findings, this paper starts with the safety requirements for coal injection in the blast furnace according to the coal situation in China. Several types of meager and meager-lean coal and anthracite were selected for microstructure and composition analyses. Then, relevant experiments on safety properties were conducted, including the Hardgrove grindability index (HGI), explosive test, and minimum ignition temperature test of mixed coal with different proportions of meager and meager-lean coal. Lastly, we innovatively provided a systematic analysis of the safety of injecting meager and meager-lean coal. This study is of great significance to the rational use of meager and meager-lean coal resources for BFI in China.

The meager and meager-lean coal selected in this paper were from the Changcun and Zhangcun coal mines of the Lu’an Group and the Nos. 4 and 6 coal mines of the Hebi Coal Industry Group, China. These four coal samples were between bituminous coal and anthracite in terms of their properties, with no coking core and slightly higher volatile content and better combustion and reactivity than anthracite. Considering the abundant reserves and balanced properties of meager and meager-lean coal in China, applying these coal samples to BFI is more suitable.

Scanning electron microscopy (Hitachi SU8000, Japan) was used to observe and analyze the microstructure of the coal samples. The macerals of coal can be divided into organic macerals, which are converted from the organic matter of plants, and inorganic macerals, which are the minerals in coal. The organic macerals of humic coal converted from higher plants are divided into four groups, namely, gelatinous microcomponent (vitrinite), fusinized microcomponent (inertinite or fusinoid group), stable component (exinite), and transitional maceral (semi-vitrinite and semi-fusinoid group). The sources of inorganic macerals include the inorganic components (minerals) in coal-forming plants and the minerals mixed during the coal-forming process. The latter is the main source of minerals. Common minerals include clay minerals, sulfides, oxides, and carbonates [13].

On the basis of the analysis of the macerals of the selected coal samples, the content and distribution of each component in different coal samples were evaluated. Then, preliminary and qualitative inferences on the grindability, explosivity, and structural strength of the coal samples were derived [14].

The volatilization, ash content, solid carbon content, water content, and sulfur content of pulverized coal are usually the critical parameters in the process of BFI safety research. These parameters affect, to some extent, the combustion rate, displacement ratio, and other vital properties in the process of pulverized coal injection. In the current coal industry in China, large enterprises generally adopt automatic industrial analyzers to conduct composition analysis of the coal that they mine. Thus, the composition data of the selected coal samples can be inquired from the enterprise database [15].

The grindability of pulverized coal indicates the degree of difficulty in pulverized coal grinding. The commonly used HGI is obtained by grinding coal samples with a special coal mill [16–17]. The HGI of coal injection is between 65 and 100 [18]. A small HGI corresponds to increased hardness of the coal, thereby presenting some difficulties to powder-making and increasing power consumption. The high hardness of the coal powder will also speed up coal injection equipment wear (especially the spray gun) and shorten its service life [19].

The explosivity of coal samples can be measured according to the maximum explosive pressure, the rate of the pressure rise, the lower limit of the explosion, and the length of the return flame. To optimize the test and analysis process and meet the needs of routine explosive inspection at the injection site, this paper determined the explosivity of the coal sample by evaluating the length of the return flame.

As shown in Fig. 1, the length of the return flame measuring instrument of pulverized coal consists of a long tube test system, an injection system, a dust removal system, a flame length photoelectric testing system, and an automatic control system [20]. The highlighted area in Fig. 1 is the ignition source. The stainless-steel pipe at the far end of the ignition source is the injection system, and the fiber-reinforced plastic part at the near end of the ignition source is the long tube test system and flame length photoelectric test system. In engineering applications, if the length of the return flame formed by the detonation of the coal powder is more than 600 mm, the coal powder has a strong explosivity. If the length of the return flame is between 400 and 600 mm, the coal powder has a medium-intensity explosivity, and if it is less than 400 mm, the coal powder has a weak explosivity. If only a few or no sparks occur at the fire source, the coal powder is considered non-explosive coal.

Figure 1.  Measuring instrument for length of return flame.

In the BFI process, pulverized coal is dispersed in the blast furnace cavity at a certain concentration. When the ignition source increases the temperature of the pulverized coal to reach the ignition temperature, the pulverized coal will combust or explode. From the viewpoint of reaction control and safety, the ignition temperature of pulverized coal must be determined accurately [21]. In this paper, we designed and manufactured the equipment to measure the ignition temperature of pulverized coal by referring to the Goldberg–Grewalder furnace, as recommended by the International Electrotechnical Commission.

During the test, a certain amount of coal powder to be evaluated was placed in the powder storage chamber, and the two-way valve of the air compressor was opened. When the air storage chamber is inflated to the required diffusion pressure, the valve is closed. When the furnace temperature rises and remains constant at the set temperature, the solenoid valve is opened. The high-pressure gas will pulverize the coal in the furnace. If pulverized coal combustion is present in the furnace, the furnace from the lower end can be observed from the flame and bright light. The lowest ignition temperature of pulverized coal is the lowest temperature of the inner surface of the furnace tube when the pulverized coal in the furnace tube is not ignited. The test equipment is shown in Fig. 2. The cylindrical barrel-shaped equipment on the right is the Goldberg–Grewalder furnace.

Figure 2.  Instrument of minimum ignition temperature test.

Vitrinite is the main part of coking coal, with a small amount of fusinite, semi-fusinite, and ash, as shown under the microscope. The uniform structure of vitrinite and the heterogeneity of optical analysis indicate that the coal has a high degree of metamorphism. No other components are present in most of the coal particles, as shown in Fig. 3(a). A small number of vitrinite particles are mixed with filaments, semi-filaments, or ash, as shown in Figs. 3(b) and 3(c).

Figure 3.  Microstructure of anthracite in Jiaozuo, China: (a) vitrinite particles, (b) fusinite particles, and (c) clay in the vitrinite.

From the microscopic observation of meager-lean coal in the Zhangcun coal mine, vitrinite was mostly found, followed by fusinite, semi-fusinite, and a small amount of ash. Vitrinite is distributed in irregular lamellar patterns with fusinite and semi-fusinite, as shown in Fig. 4(a). Vitrinite has few single particles, and the frosting phenomenon is weak, indicating that the metamorphism of the coal is low. Ash consisted of clay and carbonate, which are mainly distributed in the pores of the fusinite (Fig. 4(b)) and vitrinite.

Figure 4.  Microstructure of cleaned coals: (a) vitrinite and semi-fusinite and (b) fusinite and carbonates of meager-lean coal in Zhangcun coal mine; (c) vitrinite in meager-lean coal in Changcun coal mine containing fusinite.

The microscope observation revealed that the coal is mainly vitrinite with a small amount of fusinite, semi-fusinite, and ash. The vitrinite particles are gray, and the optical heterogeneity is weak, indicating that the coal has low metamorphism. Most of the coal particles do not contain other components, and a small number of particles contain fusinite and semi-fusinite, as shown in Fig. 4(c). The main component of ash is carbonate, which fills the holes of fusinite.

The microscope revealed that the coal is mainly vitrinite with a small amount of fusinite, semi-fusinite, and ash. The vitrinite particles are gray, and the optical heterogeneity is weak, indicating that the coal has low metamorphism. A small number of particles contain fusinite and semi-fusinite, sometimes in a layered distribution, as shown in Figs. 5(a) and 5(b). Ash consists of clay and carbonate, as shown in Fig. 5(c).

Figure 5.  Microstructure of cleaned meager-lean coal in the No. 4 coal mine of Hebi mining group company: (a) vitrinite containing semi-fusinite; (b) vitrinite containing fusinite; (c) carbonate.

The microscope revealed that vitrinite is the main part of the coal, followed by fusinite and a small amount of ash. The vitrinite particles are gray, and the optical heterogeneity is weak, indicating low metamorphism. Most of the particles are not mixed with other components, and a few of them are mixed with fusinite and semi-fusinite, sometimes in a layered distribution, as shown in Figs. 6(a) and 6(b).

Figure 6.  Microstructure of cleaned meager-lean coal in the No. 6 coal mine of Hebi mining group company: (a) vitrinite and fusinite; (b) vitrinite containing fusinite.

The contents of vitrinite, fusinite, and semi-fusinite in the coal samples were estimated through microscopic observation, as shown in Table 1. In accordance with the microstructure of coal, vitrinite with a uniform structure is the main component of anthracite in Jiaozuo, and its proportion reaches 90vol%. Fusinite and semi-fusinite account for only 5vol%–8vol%. The remaining groups are divided into ash and impurities [22].

The microstructures of coal indicated that vitrinite accounts for most of the four types of meager-lean coal. Simultaneously, some fusinite and semi-fusinite should belong to the meager-lean coal, as indicated by the analysis of the coal’s metamorphism degree. The microstructure determined that all four types of meager-lean coal have good grindability, and the powder should have weak explosivity. Some differences exist in the microstructures of the four types of coal; these include the lower vitrinite contents of meager-lean coal in the Zhangcun coal mine and No. 6 coal mine of the Hebi mining group company, and the higher contents of fusinite and semi-fusinite (see Table 1). Therefore, the structural strengths of meager-lean coal in the Changcun coal mine and No. 4 coal mine of the Hebi mining group company are higher, their grindabilities are slightly lower, their explosivities are weaker, and the minimum ignition temperatures should be higher. These results are consistent with the performance of the test structure [23].

Table 2 shows the chemical compositions of the five types of coal. According to the “Classification for volatile matter of coal” MT/T849-2000, the four types of meager and meager-lean coal all have low volatility, and the anthracite in Jiaozuo is ultra-low-volatile coal. According to the “Classification for quality of coal – Part 1: Ash” GB/T15224.1-94, the four types of meager and meager-lean coal and the anthracite in Jiaozuo are low ash coal. According to “Classification for quality of coal – Part 2: Sulfur content” GB/T15224-94, all five coal samples belong to ultra-low- sulfur coal [24]. From the analysis of the five coal samples, the following results can be obtained:

(1) Fixed carbon content.

The fixed carbon of coal is the residual organic material after the volatile content is ascertained. In a certain range, a high content of fixed carbon corresponds to a great calorific value and thus higher favorability for coal injection. Therefore, the content of fixed carbon in coal used for coal injection is generally required to be between 70wt% and 87wt%. Research shows that when the content of fixed carbon is greater than 87wt%, the calorific value decreases as the fixed carbon increases. Five types of coal used in the experiment, with fixed carbon contents between 75wt% and 80wt%, are suitable for BFI [25].

(2) Volatility and explosivity.

The volatile contents of the five coal samples range from 5.13wt% to 14.64wt%, which belong to low-volatile-content and weakly explosive coal. Compared with anthracite, the four types of meager and meager-lean coals have better combustion performance and higher injection combustion rates, which are conducive to improving the replacement ratio.

During the injection process, nitrogen can be used only in high-pressure parts such as the silo pump and injection tank to control the oxygen content within 12vol%. This setting can significantly simplify the process of powder-making and injection, reduce the investment for equipment, and has great advantages.

(3) Ash and ash fusibility.

When the blast furnace is injected, the ash content of the coal should be as low as possible. When the ash content is high, the theoretical combustion temperature and combustion efficiency and the replacement ratio of coke will be reduced. In general, the ash content and the coke proportion increase by 2wt% and 1wt%, respectively.

(4) Sulfur content.

Sulfur is a harmful substance in coal. SO₂ and SO₃ produced by sulfur combustion could endanger human health and cause air pollution. Sulfur causes metal oxidation and decarburization in the heating furnace, and sulfide corrodes the coking equipment during the coking process. The sulfur in coke and pulverized coal also affects the quality of pig iron and steel. If the sulfur content in steel is greater than 0.07wt%, hot brittleness will be produced. The steel cannot be used as a result. Additional limestone must be added to the blast furnaces and steel furnaces to remove the sulfur from steel. However, this approach will increase the cost and reduce the production capacity. The sulfur contents of coking and pulverized coal need to be controlled; a sulfur content of <1.0wt% is generally required.

The total sulfur contents of the five coal samples are 0.20wt%–0.4wt%, which belong to ultra-low-sulfur coal and are beneficial for blast furnace smelting and pig iron quality. Table 2 shows that the four types of meager and meager-lean coal are all high-quality coals with low volatile contents, low ash contents, and low sulfur contents. The meager-lean coal in the No. 4 coal mine of the Hebi mining group company has the lowest ash content of 8.74wt%. The analysis results show that the coal quality and ash and sulfur contents of meager-lean coal from the No. 4 coal mine of the Hebi mining group company are the lowest, thereby making it the best choice of meager and meager-lean coal sources. Anthracite in Jiaozuo has a low ash and sulfur content, thus making it suitable for coal injection.

To conduct further research on coal sample performance, this paper retrieved various types of bituminous coal samples from the main domestic coal-producing areas. We prepared these standard samples for the grindability, explosive, and minimum ignition temperature tests, and conducted a comparative analysis with the experimental results of pulverized coal samples. Table 3 shows the chemical compositions of the bituminous coal samples.

The grindability test standard sample was composed of the coals in Table 3. The anthracite in Jiaozuo was respectively mixed with the four types of meager and meager-lean coal in various mass percentages, and then the grindability tests were conducted, as shown in Fig. 7.

According to the measurement, the HGI values of the five types of coal was as follows: anthracite in Jiaozuo with HGI = 59; meager-lean coal in Changcun coal mine with HGI = 108; meager-lean coal in Zhangcun coal mine with HGI = 115; meager-lean coal in the No. 4 coal mine of the Hebi mining group company with HGI = 102; meager-lean coal in the No. 6 coal mine of the Hebi mining group company with HGI = 122. The mixing of different coal samples will cause different changes in their microstructures. From Fig. 7, when anthracite in Jiaozuo is mixed with the meager-lean coal from the No. 4 or No. 6 coal mine of the Hebi mining group company, the change in the microstructure has a great influence on the hardness; if the proportion of the meager-lean coal in the mixed coal sample exceeds 90wt%, the grindability index will decrease instead. However, when anthracite in Jiaozuo is mixed with the meager-lean coal from the Changcun coal mine or Zhangcun coal mine, the microstructure of the mixed coal sample has no effect on the hardness change; the grindability of the mixed coal sample will increase with the increase in the proportion of the meager-lean coal.

Figure 7.  Comparison chart of grindability test data for the mixed pulverized coals.

The sample data in Fig. 7 indicate that when the proportion of meager and meager-lean coal is different, the ability to improve the grindability of mixed coal is different. When the proportion of meager and meager-lean coal is 10wt%, the mixed coal sample with meager-lean coal in Zhangcun coal mine has the best grindability. When the proportion of meager and meager-lean coal is 20wt%, the mixed coal sample with meager-lean coal in the Zhangcun coal mine has the best grindability. When the proportion of meager and meager-lean coal is 30wt%, the mixed coal sample with meager-lean coal in the No. 4 coal mine of the Hebi mining group company has the best grindability.

The above five types of coals were screened, and the particle size of coal powder less than 200 purposes was taken to construct standard samples. The anthracite in Jiaozuo was respectively mixed with four types of meager and meager-lean coals in different mass percentages, and the mixed samples were used for explosive tests. The length of the return flame of the pulverized coal explosion was used as the measurement standard, as shown in Fig. 8.

Figure 8.  Comparison chart of explosive test data of the mixed pulverized coal.

The experimental data in Fig. 8 show that the length of the return flame of anthracite in Jiaozuo is 0 mm, thereby indicating non-explosivity. The lengths of the return flame of meager-lean coals in the Changcun coal mine, the Zhangcun coal mine, and the Nos. 4 and 6 coal mines of the Hebi mining group company are 10, 13, 8, and 14 mm, respectively. Thus, these four types of meager and meager-lean coal are weakly explosive coals.

On the basis of the non-explosive anthracite, the four types of low-explosive meager and meager-lean coals were respectively added. When the proportion of meager and meager-lean coal in the mixed sample is less than or equal to 40wt%, the length of the return flame is 0 mm, thus indicating non-explosivity. When the proportion of meager and meager-lean coal reaches 50wt%, the mixed coal has weak explosivity. With the increase in the meager-lean coal proportion, the explosivity of mixed coal gradually increases. In general, if the proportion of meager and meager-lean coal is less than or equal to 40wt%, the mixed coal will not be explosive during the injection process.

The same standard samples of mixed coal used in the explosion test were also used in this test.

The data in Fig. 9 indicate that the minimum ignition temperature of anthracite in Jiaozuo is 393°C. Given the grindability and non-explosivity discussed above, the 30wt% proportion of meager-lean coal in mixed coal samples was chose as a reference. From Fig. 9, the minimum ignition temperatures of mixed coals with 30wt% meager-lean coal in the Changcun coal mine, Zhangcun coal mine, and the Nos. 4 and 6 coal mines of the Hebi mining group company are 39, 47, 48, and 29°C lower than that of pure anthracite, respectively. Thus, the mixed coal sample with 30wt% meager-lean coal in the No. 4 coal mine has the most obvious reduction in the minimum ignition temperature, followed by the Zhangcun coal mine and then the Changcun coal mine; the poorest reduction is found in the mixed coal with meager-lean coal in the No. 6 coal mine. A lower minimum ignition temperature of the mixed pulverized coal means that it is easier to ignite, therefore the meager-lean coal in the No. 4 coal mine of the Hebi mining group company is the preferred choice for blast furnace coal injection.

Figure 9.  Comparison chart of experimental data of minimum ignition temperatures of the mixed pulverized coals.

In the BFI process, the grindability of coal directly affects the difficulty of making pulverized coal, and the coal sample with poor grindability will increase the power consumption, thus raising production costs. In addition, the coal sample with poor grindability, due to its high hardness, will accelerate the wear of the injection equipment and shorten the service life of the equipment.

Explosions in the blast furnace must be avoided to ensure production safety in the BFI process. The maximum mixing ratio that can be used without explosion was determined through the explosive experiment of mixed coal samples. In combination with the minimum ignition temperature test, the optimum mixing ratio at which the ignition energy can be minimized without explosion can be obtained.

In the BFI process, different coal samples have different energy requirements. A low ignition temperature of the coal sample corresponds to a low required energy supply; the energy consumption required for BFI with this coal sample is also low.

Therefore, the optimal proportion scheme for BFI with meager and meager-lean coal can be found by measuring the HGI, the length of the return flame, and the minimum ignition temperature. On the premise of ensuring production safety, the quantity of coal injection and the replacement ratio of coal injection in blast furnace are increased, and the production cost is reduced at the same time.

The microstructures, compositions, and safety of the selected meager and meager-lean coal and high-quality anthracite were analyzed. On the basis of the experiment results, the coal sample that is suitable for BFI was determined, and the following conclusions were drawn:

(1) An analysis of the microstructure and composition of the coal samples shows that the four types of meager and meager-lean coal are high-quality coal with low volatile, low ash, and low sulfur contents. Anthracite in Jiaozuo is high-quality coal with low ash and sulfur contents. Therefore, jetting these two types of pulverized coal together is feasible.

(2) The grindabilities of the five types of coal are as follows: anthracite in Jiaozuo with HGI = 59; meager-lean coal in Changcun coal mine with HGI = 108; meager-lean coal in Zhangcun coal mine with HGI = 115; meager-lean coal in the No. 4 coal mine of the Hebi mining group company with HGI = 102; meager-lean coal in the No. 6 coal mine of the Hebi mining group company with HGI = 122. When the proportion of meager-lean coal in the mixed coals is different, the grindability of mixed coal is different. The grindability gradually improves as the proportion of meager-lean coal increases at a certain scale.

(3) The anthracite in Jiaozuo has no explosivity, while the four types of meager-lean coal all have weak explosivity. When the proportion of meager-lean coal in the mixed coal exceeds 40wt%, the explosivity of the mixed coals increases gradually as the proportion of meager-lean coal increases. When the proportion of meager-lean coal does not exceed 40wt%, the mixed coals will not explode during the process of pulverizing and injection.

(4) The anthracite in Jiaozuo with the minimum ignition temperature of 393°C was used as a basis in the ignition performance test of mixed coal samples. As the proportion of meager-lean coal increases, the minimum ignition temperature decreases gradually.