I. Classification of Coal
01 By processing method
★★Raw Coal
Raw coal refers to the product produced by coal mines that has not been washed, screened, or processed, but has only undergone manual removal of gangue and impurities.
Raw coal is usually delivered in large pieces and contains impurities such as stones and iron pieces, requiring processing before being sent to the coal hopper in front of the furnace. Raw coal processing includes several steps such as crushing, screening, and magnetic separation, and for pulverized coal furnaces, coal grinding is also required.
According to data released by the National Bureau of Statistics in January, China produced a total of 3.445 billion tons of raw coal in 2017, an increase of 3.2% year-on-year.
★★Coal
Raw coal undergoes washing to remove gangue, transforming it into high-quality coal suitable for specific applications; this is known as refined coal. Refined coal can be divided into metallurgical coking refined coal and other coking refined coal.
Coal is further divided into three grades according to its intended use:
Metallurgical coking coal: particle size <100(50)mm, Ad≤12.5%;
Other coking coal: particle size <100(50)mm, Ad=12.51~16.00%;
Pulverized coal for injection: Particle size <50(25)mm, Ad≤14.00%.
★★Particle-sized coal
Granular coal refers to coal that has been washed or screened to remove some impurities and gangue, and has been classified into different particle sizes with a lower limit of 6 mm and an ash content of less than or equal to 40%.
★★ Coal Washing
Washed coal refers to coal that has undergone washing and processing to remove most impurities and gangue, with particle size classification limits of 100, 50, 25, 20, 13 and 6 mm or less.
Coal washing can be classified into washed raw coal, washed mixed coal, mixed coal, washed mixed fine coal, mixed fine coal, washed fine coal, fine coal, washed pulverized coal, and pulverized coal. Except for washed mixed coal, which requires an ash content of less than or equal to 32%, all others require an ash content of less than or equal to 40%.
★★Low-quality coal
Coal with low carbon content, low calorific value, high ash content, and poor ignition is generally referred to as low-quality coal.
The physical and chemical properties of low-quality coal are: high moisture content (total moisture as high as 20%-50%, and some reach 60%), high oxygen content (accounting for about 20% of organic matter), and low calorific value: generally below 4500 kcal/kg.
Currently, low-quality coal is mainly utilized through the following methods:
1. After drying, it can be used directly as fuel.
2. After mild pyrolysis and gasification, the fractions are separated and then utilized separately.
3. After drying, it is directly fed into the gasification furnace to produce syngas.
4. After drying, it is directly fed into the liquefaction unit to produce oil products.
02 By Purpose
★★ Thermal Coal
Thermal coal refers to coal used as a power source. In a narrow sense, it generally refers to coal used for thermal power generation. In a broader sense, any coal used to generate power for purposes such as power generation, locomotive propulsion, and boiler combustion falls under the category of thermal coal.
The quality requirements for coal used for power generation are lower than those for any other type of coal. For example, coal gangue with a lower heating value greater than 4186.8 kJ/kg can be used as fuel for fluidized bed boilers. However, considering the economic benefits of power processes, coal used for power generation should still meet certain quality requirements.
First, the calorific value is considered; second, the ash melting point and the ease of slagging are taken into account. For pulverized coal boilers, the grindability coefficient must also be considered; and for chain grate boilers, the lump content of the coal and other indicators need to be taken into account.
★★Metallurgical Coal
Metallurgical coal is a type of bituminous coal with medium to low volatile matter content and medium to strong caking properties. It is a term used for bituminous coals with a high degree of coalification and good coking properties. It is also known as prime coking coal.
Coking coal is divided into two categories. The first category of coking coal has a dry ash-free volatile matter (Vdaf) > 10%–28%, a caking index (G) > 65, and a maximum plastic layer thickness (y) ≤ 25 mm. This type of coal has particularly good coking properties and can be used to produce qualified blast furnace coke on its own.
Another type of coking coal has a dry ash-free volatile matter (Vdaf) of >20%–28% and a caking index (G) of >50–65, and its coking properties are worse than the former.
★★Lignite
Lignite, also known as coking coal, is the least coalified type of mineral coal. It is a brownish-black, dull, low-grade coal that falls between peat and bituminous coal.
With high-quality coal almost exhausted, lignite has become the main type of coal used in my country. However, due to the low degree of coalification of lignite, a large amount of black ash is released into the air when it is burned. If it is not washed and refined, the large-scale use of inferior lignite will lead to an increasingly serious smog problem in my country.
★★Bitcoin coal
Bituminous coal is widely distributed, being the most abundant and widespread type of coal in nature. It is mainly concentrated in countries such as the United States, the former Soviet Union, China, Europe, Australia, and South Africa. In China, bituminous coal is primarily distributed in the northern provinces (autonomous regions), with North China accounting for over 60% of the country's total bituminous coal reserves.
★★Anthracite
Anthracite, also known as white coal or red coal, is the most highly coalified type of coal. Anthracite has a high fixed carbon content, low volatile matter yield, high density, high hardness, high ignition point, and burns without smoke.
It is black, hard, and has a metallic luster. It does not stain when rubbed with grease; its fracture is conchoidal; and it burns with a short flame and little smoke. It does not char.
Generally, it contains over 90% carbon and less than 10% volatile matter. It has no thick plastic layer. Its calorific value is approximately 6000-6500 kcal/kg. Sometimes, coal with exceptionally high volatile matter content is called semi-anthracite; coal with exceptionally low volatile matter content is called high-grade anthracite.
II. Basic Coal Indicators
01Moisture
Moisture content has a significant impact on the processing and utilization of coal. When it is burned, it turns into steam and absorbs heat, thus reducing the calorific value of the coal.
Moisture in coal can be divided into external moisture and internal moisture, with internal moisture generally used as an indicator of coal quality. The lower the degree of coalification, the larger the internal surface area of the coal, and the higher the moisture content.
02 Ash Content
Ash content is the solid residue left after coal is completely burned, and it is an important indicator of coal quality. Ash content mainly comes from non-combustible minerals in coal.
When minerals burn and ashed, they absorb heat, and the large amount of ash discharged carries away heat. Therefore, the higher the ash content, the lower the thermal efficiency of coal combustion; the more ash content, the more ash and slag produced by coal combustion, and the more fly ash emitted. Generally speaking, high-quality coal and washed coal have relatively low ash content.
03 Volatile Matter
Under certain temperature and conditions, the combustible gas produced by the thermal decomposition of organic matter in coal is called "volatile matter," which is a mixture of various hydrocarbons, hydrogen, carbon monoxide, and other compounds.
Volatile matter is also a major coal quality indicator, and it has important reference value when determining the coal processing and utilization pathways and technological conditions.
04 Fixed Carbon
Fixed carbon is one of the quality indicators for coal or coke. It is obtained by subtracting the mass of volatile matter and ash from the mass of dry coal. It is usually expressed as a mass percentage.
In the laboratory, approximately 1g of pulverized coal is typically placed in a covered standard crucible and heated at 900℃ for 7 minutes to remove moisture and volatiles. The remaining mass is then subtracted from the ash content to obtain the final product. This can serve as a reference for selecting coal in industrial applications.
05 sulfur
This refers to the sulfur content in coal. Sulfur, phosphorus, fluorine, chlorine, and arsenic are all harmful components in coal, with sulfur being the most significant.
When coal is burned, most of the sulfur is oxidized into sulfur dioxide (SO2), which is emitted with the flue gas, polluting the atmosphere, harming the growth of animals and plants and human health, and corroding metal equipment. When coal with high sulfur content is used in metallurgical coking, it also affects the quality of coke and steel. Therefore, sulfur content is one of the important indicators for evaluating coal quality.
06 Calories
The calorific value of coal, also known as coal kilocalories, is the heat (calorific value, kilocalories) generated when coal burns in an oxygen bomb. Based on the input of sulfur, hydrogen, total moisture, and water analysis, the instrument automatically calculates the higher calorific value, lower calorific value, and lower calorific value on an as-received basis of coal.
