Quality indicators and requirements for coke
Based on the behavior and function of coke in the blast furnace, the following quality indicators are generally required for coke in production:
1. Particle size
A key indicator of smooth blast furnace operation is the permeability of the burden layer within the furnace, which is related to the uniformity of the blast furnace charge. Therefore, it is generally required that the coke particle size should not be significantly larger than the ore particle size; the two should be as close as possible to maintain uniformity. The average particle size of metallurgical coke is generally around 50mm, and can be controlled within the range of 25-70mm. In particular, the content of 40-60mm particle sizes should be increased. Currently, the particle sizes of metallurgical coke exported from China are mostly in the 30-80mm and 40-90mm ranges.
2. Ash content
Coke contains minerals, which leave residues during combustion to form ash. High ash content negatively impacts coke. When coke burns in the high-temperature environment of a coke oven, the increased ash content damages the internal structure of the coke, leading to more cracks. This not only reduces the coke’s strength but also increases its surface area. The increased cracks allow CO2 to diffuse more easily into the coke’s interior, further deteriorating its thermal properties.
Ash is an impurity and inert substance in coke, mainly composed of acidic oxides such as SiO2 and Al2O3. Due to its high melting point, it can only be reacted with fluxes such as CaO during ironmaking to form low-melting-point compounds that can be discharged from the blast furnace as slag. Therefore, metallurgical coke is required to have as low an ash content as possible.
3. Sulfur
Sulfur is a harmful component in coke. Of the sulfur brought into the blast furnace by the furnace charge, only 5% to 20% escapes with the blast furnace gas; the remainder participates in the in-furnace sulfur cycle and can only be discharged through the slag. High sulfur content in coke will increase the sulfur content of pig iron, reducing its quality, or increase slag basicity, causing a decline in blast furnace operating parameters. Therefore, it is necessary to select low-sulfur coal as much as possible during coke production to reduce the sulfur content of the coke.
4. Mechanical strength and thermal strength
The strength indicators of coke are divided into mechanical strength (shrapnel resistance and abrasion resistance) and thermal strength (reactivity and post-reaction strength).
The mechanical strength of coke is an important indicator of its ability to support the furnace charge and ensure normal blast furnace operation. The thermal strength of coke reflects its ability to resist chemical attack and protect the furnace charge in the blast furnace; it is a primary indicator for comprehensively evaluating the thermal stability of coke and is more important than its mechanical strength.
The rotary drum test can determine both the crush resistance and abrasion resistance of coke. Although it cannot fully reflect the actual conditions of coke at high temperatures within the blast furnace, it can reflect the transportation of coke before it enters the blast furnace and the mechanical destructive forces it experiences within the furnace, thus it is widely used. Related research shows that for every 1% increase in the strength index of coke, the coke ratio produced in the blast furnace can decrease by 0.7% to 1.5%, while pig iron production can increase by 0.5% to 1.5%, demonstrating a significant impact. Therefore, higher coke strength is desirable, and the requirements for metallurgical coke are: a high crush resistance (M40) and a low abrasion resistance (M10).
Coke reactivity refers to the chemical reactivity of coke with the gases it comes into contact with during use. The reaction between coke and CO2 is the most significant reaction in a blast furnace; therefore, this indicator generally refers to the coke’s ability to react with CO2 at high temperatures (1100℃). The post-reaction strength of coke refers to the “residual” strength measured using a specially designed Type I small drum after the coke has reacted with CO2 at the aforementioned temperature for a certain period. During use, the porosity of coke inevitably changes significantly after reacting with CO2, resulting in a marked decrease in strength, but a certain level of strength must still be maintained for continued use. China’s latest metallurgical coke quality standards stipulate that Grade 1 metallurgical coke has a CRI ≤ 30.0% and a CSR ≥ 60.0%, while Grade 2 metallurgical coke has a CRI ≤ 35.0% and a CSR ≥ 55.0%.
5. Volatile matter
Volatile matter is an important indicator of coke maturity. Generally, the volatile matter content of metallurgical coke should be below 1.8%; a high volatile matter content indicates immature coke. At the same time, volatile matter is also one of the indicators for pollution control in coking plants. If the volatile matter content of coke increases, the amount of dust emitted during coke pushing will increase significantly. Therefore, it is generally advisable to ensure that the volatile matter content of coke does not exceed 2%.
6. Phosphorus content
Since high phosphorus content in coke will increase the cold brittleness of pig iron, the phosphorus content of blended coal should be kept as low as possible in coking production, generally below 0.030%, in order to control the phosphorus content in coke.
Factors affecting coke quality
As coke’s roles as a heat source, reducing agent, and carburizing agent are increasingly replaced, its skeletal structure is becoming more crucial, thus placing higher demands on coke strength. Key factors influencing coke quality are coking coal and the coking process.
1.1 Coking coal quality
Coking coal is the most important factor affecting coke quality, and different types of coal play different roles in the coke-making process. Gas coal is a low-ranking coking coal with poor caking properties and a narrow plastic temperature range. When coked alone, it experiences significant semi-coke shrinkage and a high rate of gaseous release during semi-coke solidification. Therefore, coke obtained from single-layer coking has high porosity and low strength. Due to poor tensile stress resistance within the layers, the coke develops numerous longitudinal cracks, resulting in long, thin, and brittle coke lumps. One-third coking coal, falling between gas coal and bituminous coal, is a coking coal with high volatile matter and caking properties. During heating, it produces a large amount of plastic material, and its thermal stability is better than gas coal. It can be coked alone to obtain coke with a certain size and strength. Bituminous coal is a strongly caking coking coal that produces a large amount of plastic material during carbonization. It has a wide plastic temperature range and good thermal stability, but when coked alone, interlayer shear stress causes numerous transverse cracks in the coke, resulting in high porosity and brittle coke. Coking coal has moderate volatile matter and good caking properties. When coked alone, it produces coke with fewer longitudinal and transverse cracks, larger lump size, high mechanical strength, and good abrasion resistance. Lean coal has a high degree of metamorphism, fewer pyrolysis products, and poor melting properties. During coking, it experiences less shrinkage, resulting in a lower crack rate in the coke. Therefore, when coked alone, it produces coke with larger lump size and fewer cracks, but lower strength. Different degrees of coal metamorphism and different contents of microscopic components lead to different quantities and qualities of plastic bodies produced during coking, directly affecting the structure of the coke and resulting in differences in strength and lump size. 1/3 coking coal, bituminous coal, and coking coal have relatively good coking properties.
The quality of blended coal is influenced by coal type and coking process. Generally, blended coal indicators are expressed using moisture, ash, volatile matter, sulfur, caking index (G), maximum plastic layer thickness (Y), Oya expansion (b), and Gibbs freeness (MF). However, given the current prevalence of coal blending, it is necessary to combine coal petrography equipment to detect the microscopic components and vitrinite reflectance of the coal for blending and coking. According to coal petrography theory, different coal types have different petrographic compositions, and different coal particles have active and inert components. Active components act as binders, while inert components act as the framework for coking centers. The active-inert ratio is a fundamental factor affecting coke quality. The microscopic component content also varies with different coal particle sizes; therefore, the active-inert ratio can be adjusted by controlling particle size through the coking process to improve coke quality.
1.2 Coking Process
Factors affecting coke quality in coal blending and coking processes include bulk density, moisture content, particle size, and thermal regime.
1) Bulk density
On the one hand, increased bulk density allows coal particles to easily fill the pores, resulting in more thorough contact. This allows the same amount of colloidal material produced during coal pyrolysis to bind more particles.
On the other hand, increased bulk density leads to tighter coal contact, making it harder for gases generated during pyrolysis to decompose and escape, thus increasing the expansion pressure between coal particles. Increased resistance during gas escape further hinders gas release, resulting in poor permeability of the colloidal material, which promotes better bonding between deformed coal particles. The high expansion pressure and poor permeability both increase the residence time of gases within the colloidal material, widening the temperature range within the material. This facilitates the stabilization of free radicals generated during coal pyrolysis, increasing the amount of liquid products and producing a more stable colloidal material. The increased quantity and stability of the colloidal material both contribute to better bonding between coal particles, thereby improving coke strength. However, increased bulk density strengthens the bond between adjacent layers, reducing the relaxation effect of shrinkage stress during coking, leading to higher shear stress between adjacent layers and a greater susceptibility to transverse cracks. Methods to increase bulk density include adjusting moisture content, particle size, and coal compaction. In industry, coal pretreatment involves heating, crushing, and tamping to increase bulk density.
2)Moisture
The moisture content of coal fed into the coke oven affects its bulk density. Data shows that as moisture increases, the bulk density gradually decreases, reaching its minimum at 8%. Afterward, due to the lubricating effect of moisture, it increases again with further increases in moisture. However, increased moisture requires more heat energy to evaporate in the coke oven and prolongs the coking time. Furthermore, the moisture brought into the coke oven by coking coal is a major source of coking wastewater, which not only affects coking speed but also wastes energy. Therefore, reducing the moisture content of coking coal can reduce coking energy consumption and coking wastewater. Studies have shown that controlling the moisture content of coal fed into the oven at around 6% results in coke with ideal cold and hot strength. The moisture content of coal fed into the oven can be controlled through coal preheating, drying, and moisture conditioning techniques. There are numerous reports both domestically and internationally on controlling the moisture content of coking coal fed into the oven. Bao Junfang et al. studied different moisture contents of coal fed into the oven, and their results showed that reducing the moisture content of coal fed into the oven increases the bulk density, which not only improves production capacity but also accelerates the heating rate, making the coke structure more compact.
3) Particle size
The particle size and particle size distribution of coal directly affect the bulk density of the coal charged into the furnace. Different distributions of the microscopic components in the coal lead to varying contact states during coking, affecting the coal’s caking properties and consequently impacting coke quality and subsequent chemical product systems. Currently, coking plants primarily use fineness indicators (the content of particles <3mm) to control the particle size of the coal charged into the furnace. If the coal particle size is too large or too small, the gaps between coal particles increase, reducing the bulk density. Furthermore, if the particle size is too large, weakly caking coal and minerals, acting as inert substances, cannot be evenly distributed, resulting in uneven heat transfer, poor interfacial contact, and hindering the development of the mesophase. This leads to more coke cracks and reduced strength. Conversely, if the coal particle size is too small or too large, not only does the bulk density decrease and the coal’s caking properties decrease, but the excessive dust content also deteriorates the operating environment, causing severe coal dust entrainment in the raw coal gas, easily clogging pipelines and increasing tar residue, creating difficulties for the chemical product recovery system.
Generally, the fineness requirement for blended coal during top-loading is 72% to 80%, approximately 85% for briquetted coal coking, and over 90% for compacted coking. Studies show that different coal particle sizes have different microscopic compositions, resulting in varying contact states between active and inert components during coking, directly affecting the quality and quantity of the plastic mass. Therefore, controlling the optimal particle size and distribution of various coals can improve the coking and agglomerating properties of the coal, maximizing the role of each coal type in blending. To achieve optimal fineness and particle size distribution, industrial grinding techniques are employed, with pre-grinding and selective grinding being commonly used depending on the coal type.
4) Thermal system
Coking temperature and coking time are among the factors affecting coke properties. Increasing the final coking temperature and extending the oven-keeping time allows for more complete pyrolysis and higher condensation in the later stages of coking, which is beneficial for the breakage and precipitation of small molecules. This results in a denser coke structure, improved pore and pore wall structure, thus increasing the mechanical strength and post-reaction strength of the coke, and reducing its reactivity. Japan uses the “fire-drop management” method, which involves continuing to heat the coke cake after the “fire-drop” process until it reaches the specified requirements before pushing the coke. This principle is similar to our traditional method of increasing the coking speed and extending the oven-keeping time. Zhao Lishu et al. found that increasing the standard coking temperature by 30℃ and extending the coking time by 1 hour improved both abrasion resistance and post-reaction strength. Dry quenching is commonly used in industry to improve coke quality.
Analysis of vitrinite properties of coking coal with different metamorphic degrees
The main organic microscopic components in coal include vitrinite, inertinite, and chrysrinite, and different components have different binding and coking properties.
In the coking process, based on the differences in their caking and coking properties, the microscopic components in coal are divided into two major categories: active components and inert components. The former mainly includes vitrinite and chrysrinite, while the latter mainly includes inertite.
The active components can soften and melt during the coking process, which is the main reason why coal powder binds into lump coke. The magnitude and quantity of their binding ability are necessary conditions for determining the properties of coke.
Coking coal contains relatively little husk matter, generally less than 3% (mass fraction). Coking coal with a higher degree of metamorphism basically contains no husk matter, indicating that the role of husk matter in the coking process is limited and can be ignored.
Vitrin is generally considered to be the main factor determining the caking properties of coking coal, and it is often used as a parameter for predicting coke quality.
However, a higher vitrinite content does not necessarily mean better caking ability or better quality coke produced. Different coking coals have different vitrinite properties, so understanding the vitrinite caking ability is of great significance for guiding coking coal blending.
Vitrinite is a major microscopic component in coal. Domestic scholars have conducted extensive experimental research on suitable methods for separating microscopic components, which is of guiding significance for optimizing coal blending structure.
Coal petrographic analysis can determine the content of vitrinite in coal. However, the binding capacity of vitrinite in coal (i.e., the mass of active components) is affected by many factors such as the degree of coal metamorphism and reduction.
To further determine the binding properties of vitrinite, researchers used density liquid enrichment separation to isolate vitrinite from coking coal. Different proportions of inert substances were added to the vitrinite, and its binding capacity was measured. Furthermore, the coke produced in a small coke oven was compared with the coke produced from the raw coal to further differentiate the characteristics of vitrinite in coking coal, providing a scientific basis for optimizing coal blending.
The research structure indicates that:
1. Compared with coke produced from the original coking coal, the content of isotropic, filamentous, and fragmented materials in the optical composition of coke produced from the vitrinite group of each coking coal is significantly reduced, while the content of medium and coarse-grained mosaic structure is significantly increased, and the content of fluid and lamellar structures is slightly increased.
2. The microscopic strength and structural strength of coke produced from the vitrinite group of coking coal are lower than those produced from the original coking coal. The particle coke reactivity of coke produced from the vitrinite group of coking coal is lower than that of coke produced from the original coking coal. As the degree of metamorphism increases, the decreasing trend of particle coke reactivity gradually decreases.
