Coal quality refers to the physical and chemical properties of coal and its applicability. Its main indicators include ash content, total moisture, analyzed water, sulfur content, calorific value, volatile matter, fixed carbon, coke slag characteristics, coking properties, and cohesiveness. It refers to the sum of characteristics or features that coal products possess during their formation, mining, and processing, which can meet the needs of different users. According to the quality characteristics and uses of coal products, the quality of coal can generally be represented by certain quality indicators (or standards).
Different types of coal have different chemical compositions, properties, and processing characteristics. Analyzing and testing coal to fully understand it has important guiding significance for reducing resource waste, achieving dual carbon emissions reduction, and comprehensively utilizing coal. We will introduce the basic indicators, definitions, and impacts of coal quality:
1、 Moisture (M)
Moisture is included in the processes of mining, transportation, storage, and washing. The external moisture attached to the surface of coal particles and the large capillary pores, as well as the internal moisture adsorbed or condensed in the capillary pores inside the coal particles, can completely evaporate the internal moisture in the coal when the temperature exceeds 100 ° C
Excessive moisture in coal is not conducive to processing, transportation, thermal stability, and heat conduction, directly affecting the quality of coke, reducing coke production capacity, prolonging coking time, increasing energy consumption in the coking process, and reducing coke yield
The moisture content of coal is divided into two types: internal water (Minh) and external water (Mf) The presence of moisture is extremely detrimental to the utilization of coal. As a fuel, the moisture in coal becomes steam, which consumes heat during evaporation; In addition, the moisture content of clean coal also has a certain impact on coking. Generally, for every 2% increase in moisture content, the calorific value decreases by 100kcal/kg (kcal/kg); For every 1% increase in moisture in refined coal smelting, the coking time is extended by 5-10 minutes.
2、 Ash content (A)
Ash content refers to the residue left after complete combustion of coal, and minerals from coal are harmful substances in coal.
The residue left after complete combustion of coal is called ash content, and the higher the inherent ash content, the poorer the selectivity of coal. High ash content indicates that the combustible components in coal are low and the calorific value is low. During coking, it will reduce the strength of coke, increase the consumption of blast furnace volume, and reduce the production of pig iron
Ash content is a harmful substance. As the ash content in thermal coal increases, the calorific value decreases, the slag discharge increases, and the coal is prone to slagging; For every 2% increase in ash content, the calorific value decreases by about 100kcal/kg, ash content increases by 1%, coke strength decreases by 2%, blast furnace production capacity decreases by 3%, and limestone consumption increases by 4%.
3、 Volatile matter (V)
Volatile matter refers to the products of heating and decomposition of organic matter and some minerals in coal, and its size is related to the degree of coal metamorphism. The higher the degree of coal metamorphism, the lower the volatile matter. When coal is heated under high temperature and isolated air conditions, the gas and liquid products discharged are called volatile matter, which affects the coking rate, coke oven gas, tar yield, and coke quality. Coal with high volatile matter has a high yield of chemical by-products during dry distillation, making it suitable as a low-temperature dry distillation or gasification raw material; Bituminous coal with moderate volatile matter has good cohesiveness and is suitable for coking. In coal blending and coking, the volatile matter is used to determine the coal blending ratio, and the volatile matter of the mixed coal is controlled within the appropriate range of 25% -31%. The main components of volatile matter are methane, hydrogen, and other hydrocarbons. It is one of the important indicators for identifying the type and quality of coal. Generally speaking, as the degree of coal metamorphism increases, the volatile matter of coal decreases. Lignite and gas coal have higher volatile matter, while lean coal and anthracite have lower volatile matter
4、 Sulfur content (S)
The harmful elements in coal, including organic sulfur and inorganic sulfur, have a relatively broad requirement for sulfur in thermal coal, with coking coal requiring less than 1%. The sulfur in thermal coal has a serious corrosive effect on boilers, and the sulfur dioxide produced after combustion pollutes the atmosphere. The sulfur in coking coal is the main source of sulfur in furnace materials. High sulfur in coke leads to high sulfur in pig iron, high thermal brittleness, and reduced quality. In steelmaking pig iron, a sulfur content greater than 0.07% is considered waste. The synthesis of ammonia uses coal to produce semi water gas, and the hydrogen sulfide contained in the gas poisons and renders the catalyst ineffective.
4、 Fixed carbon mass (FC)
Fixed carbon content refers to the residue after removing moisture, ash, and volatile matter, and it is an important indicator for determining the use of coal. The difference between the moisture, ash content, and volatile matter of coal subtracted from 100 is the fixed carbon content of coal. Based on the benchmark used to calculate volatile matter, the fixed carbon content of different benchmarks such as dry basis and dry ash free basis can be calculated. When measuring the volatile matter of coal, the remaining non-volatile substances are called coke residue, and the coke residue minus the ash content is called fixed carbon. It is a non-volatile solid combustible material in coal, which can be calculated using (100-M-A-V) and is a mixture of high molecular weight compounds. The higher the fixed carbon content, the generally lower the volatile matter. The fixed carbon of coal is the main source of coal thermal energy, and the higher the fixed carbon of coal used as fuel, the better
5、 Bonding index (G)
The ability of bituminous coal to bond with specialized anthracite after heating under specified conditions is one of the important criteria for coal classification and an important indicator for smelting clean coal. The higher the bonding index, the higher the coking property. Adhesive property refers to the ability of coal particles to bond together into blocks during the dry distillation process due to the decomposition and melting of organic matter in coal. It is commonly measured by the maximum thickness Y value of the gelatinous layer and the adhesive index G value. The larger the two, the better the adhesive property. The adhesive property of coal is the main basis for evaluating whether bituminous coal can be used for coking, and it is also an important basis for evaluating low-temperature dry distillation, gasification, and power coal. The cohesiveness of coal is a necessary condition for coking, and coal with good coking properties must have good cohesiveness. When the temperature of coal with adhesive properties rises to 300C, gaseous products begin to precipitate, and as the temperature increases, tar continuously precipitates, forming a gelatinous body. The quantity and quality of gelatinous body are the key factors affecting the quality of coke.
6、 Maximum thickness of glial layer (Y)
The maximum thickness of the gelatinous layer formed by bituminous coal after being heated to a certain temperature is the maximum difference between the upper and lower layers of the gelatinous body measured by a probe in the determination of the gelatinous layer index of bituminous coal. It is one of the important criteria for coal classification. The thickness of the thermal coal gel layer is large and prone to coking; Smelting clean coal has clear requirements for the thickness of the gelatinous layer.
7、 Coal ash melting temperature (ash melting point)
The coal ash melting deformation temperature (DT), softening temperature (ST), flow temperature (FT), and commonly used softening temperature (ST) obtained under specified conditions that vary with heating temperature. The higher the coal ash melting temperature, the less likely the coal ash is to form slag. Due to different boiler designs, the requirements for ash melting temperature are also different. The melting temperature of coal ash directly affects the performance of coal as fuel and gasification raw material. Low melting temperature of coal ash makes it easy to form slag, which increases the difficulty of slag discharge. Especially for boilers with solid slag discharge and gasification furnaces with moving beds, high melting temperature of coal ash is required.
8、 Ha's Grindability Index (HGI)
The Hasse grindability index is an important indicator reflecting the grindability of coal. The grindability of coal refers to the difficulty of grinding a certain amount of coal into powder while consuming the same amount of energy. The higher the grindability index, the easier it is to grind coal into powder. The grindability index is an important indicator for quality evaluation of coal used in coal-fired boilers and blast furnaces for power generation.
9、 Crucible Expansion Number (CSN)
The crucible expansion number is the number of the expansion program obtained by heating coal in a crucible under specified conditions to characterize the expansion and plasticity indicators of coal. The size of the crucible expansion number depends on the coal ash fusibility, gas evolution during the formation of the gum, and the impermeability of the gum.
This article is shared by Henan Shanhe Precision Industry, a coal testing instrument manufacturer, hoping to be helpful to everyone. If you have any questions, please let us know in the comment section or private message!