The industrial classification of coal is mainly based on coal rank, and the classification of coal rank is based on coal quality indicators, especially some key coal quality indicators. Some coal quality indexes, i.e. coalification degree indexes, have been introduced before. This section will briefly introduce the acquisition of some coal rank indexes around the industrial classification of coal (coal quality and coal chemistry are independent disciplines).
"Basis" means that the test results are based on the state of the coal sample. The commonly used "bases" in coal quality analysis include air drying base, dry base, receiving base, dry ash-free base and dry mineral-free base. In the old and new standards, the symbols used for "base" are different (Table 7- 1).
Table 7- 1 Comparison of symbols used in various foundations in new and old standards
Air-dried base refers to coal in balance with air humidity, and the symbol is ad (air-dried base); Dry basis refers to coal in an assumed anhydrous state, and the symbol is D (dry basis); The received base refers to the coal in the received state as the benchmark, and the symbol is ar (received); Dry ash-free base refers to the coal in the assumed water-free and ash-free state, and the symbol is daf (dry ash-free); Mineral-free dry basis refers to coal in an imaginary water-free and mineral-free state, and the symbol is dmmf (mineral-free dry basis).
It can be seen that the new "Ji" is represented by the first letter of an English noun, while the old "Ji" is represented by the prefix of Chinese Pinyin.
Second, the main indicators of coalification
The coalification index is obtained through the industrial analysis of coal. Industrial analysis is also called technical analysis or practical analysis, including the determination of moisture, ash and volatile matter in coal and the calculation of fixed carbon. The industrial analysis of coal is the main index to understand the characteristics of coal quality and the basic basis to evaluate coal quality. According to the determination results of industrial analysis, we can preliminarily judge the properties and types of coal, the processing and utilization effects of various coals and their industrial uses.
Coal rank index is also called coal rank index. Because coalification is a complex process, the significance of various indexes changes is different in different coalification stages, so there are often different indexes of coalification degree for a certain coalification stage (Table 7-2).
Table 7-2 Changes of Common Coal Rank Indicators in Different Coal Rank Stages
Note: ① The change range of the measured values of each index is arranged according to the rising direction of coal rank; ② The regularity is poor. (According to Yang Qi et al. 1988)
The chemical composition and elements of coal have been introduced in Chapter 6. This paper briefly introduces some key indexes in coal quality analysis, such as moisture, ash, volatile matter, vitrinite reflectivity and so on.
1. moisture
Moisture is an important coal quality index, which plays an important role in the basic theoretical research and processing and utilization of coal.
In modern coal processing and utilization, sometimes high moisture content is a good thing. For example, moisture in coal can be used as a hydrogen donor for hydro-liquefaction and hydro-gasification. In coal quality analysis, coal moisture is the basic data for conversion of coal quality analysis results of different foundations. According to the moisture content of coal, the metamorphic degree of coal can be roughly inferred.
Step 2: Grey
The ash content of coal is not the inherent component of coal, but the residue after coal is completely burned under specified conditions. It is formed by a series of complex reactions such as decomposition and combination of minerals in coal under certain conditions, and is a derivative of minerals in coal. It is different from minerals in composition and quality, but there is a certain correlation between ash yield and mineral content in coal, so ash can be used to estimate mineral content in coal. There are three sources of minerals in coal, one is "primary minerals", that is, inorganic elements contained in coal-forming plants; The second is "secondary minerals", that is, minerals mixed or associated with coal in the process of coal formation, and the third is "foreign minerals", that is, minerals mixed in the process of coal mining and processing. The minerals in coal mainly include clay or shale, calcite (calcium carbonate), pyrite or white iron ore and other trace components, such as inorganic sulfate, chloride and fluoride.
Ash content in coal is another important index to study the characteristics and utilization of coal quality. In the study of coal quality, ash has different dependence on carbon content, calorific value, slagging, activity, grindability and other characteristics, which can be used to study the above characteristics. Because coal ash is a derivative of minerals in coal, it can be used to calculate the mineral content in coal. In addition, because the determination method of ash in coal is simple and its distribution in coal is uneven, it is generally used to evaluate the accuracy and precision of the method in the study of coal sampling and sample preparation methods. In the study of coal washing technology, the ash content of coal is generally used as an index to measure the washing efficiency.
3. Volatiles
Under the specified conditions, the volatile yield of coal samples after heating in isolation from air and moisture correction is volatile.
The volatiles of coal are mainly composed of water, hydrocarbon oxides and hydrocarbons (mainly CH4), but the physically adsorbed water (including external water and internal water) and mineral carbon dioxide in coal are not volatile.
The volatiles determined in industrial analysis are not the original volatiles in coal, but the thermal decomposition products of coal when heated under strict conditions. Changing any test conditions will have different degrees of influence on the determination results.
The main factors that affect the results of volatile matter determination are heating temperature, heating time and heating speed. In addition, the type and size of the test equipment, the material, shape and size of the sample container and the support of the container will all affect the determination result, that is, the determination result depends on the specified test conditions, so it is a highly standardized test item.
The volatile yield of coal is closely related to the degree of coal metamorphism-with the deepening of metamorphism, the volatile content gradually decreases (Table 73), so the type of coal can be estimated according to the volatile yield of coal. In China, the former Soviet Union, the United States, Britain, French, Polish and international coal classification schemes, volatile matter is the first classification index.
Table 7-3 Relationship between Volatile Matter and Coal Deterioration Degree
According to the volatile yield and char characteristics after volatile determination, the processing and utilization ways of coal can be preliminarily determined. For example, high volatile coal has a high yield of chemical by-products, which is suitable for low-temperature carbonization and gasification. Bituminous coal with moderate volatile matter has good cohesiveness and is suitable for coking. In coal blending and coking, volatile matter should be used to determine the coal blending ratio, so as to control the volatile matter of mixed coal in the appropriate range of 25% ~ 365438 0%. In addition, the yields of coke, gas, tar and crude benzene can be estimated according to the volatile matter. When burning coal, combustion equipment or coal source suitable for specific equipment can be selected according to volatile matter. The volatile matter can also be used as a reference for the selection of gasification and liquefaction process conditions. In environmental protection, volatiles are also used as the basis for formulating tobacco laws.
In addition, volatile matter has a good correlation with other coal characteristics, such as calorific value, carbon and hydrogen content. The calorific value and hydrocarbon content of coal can be calculated by volatile matter.
4. The vitrinite reflectance of coal
The vitrinite reflectance of coal is an index that can reflect the degree of coalification without being affected by the rock composition content of coal. The vitrinite reflectance of coal increases with the increase of carbon content in organic components and decreases with the increase of volatile yield. That is to say, the same maceral has different reflectivity in different metamorphic stages, which can better reflect the metamorphic degree of coal. Therefore, vitrinite reflectance is a promising coal classification index. Especially, the sensitive classification of anthracite is an ideal index to distinguish old anthracite, typical anthracite and young anthracite. At present, many countries in the world adopt vitrinite reflectance as the classification index of coal. In addition, the vitrinite reflectance of coal is of great significance for evaluating coal quality and coal processing and utilization. For example, Japan, West Germany and other countries use vitrinite reflectance to guide coking coal blending to control coal quality, which is also valuable in petroleum and geological exploration research.
Reflectivity refers to the percentage value of reflected light intensity and incident light intensity during vertical reflection, which is generally expressed by R.
Geology of coal
In order to measure the vitrinite reflectance of coal, a standard thin slice and a coal sample (vitrinite) with known reflectance are placed under a microscope. Under the incident light with a certain intensity, the weak optical flow reflected by them is converted into current by photomultiplier tube and amplified into strong electrical signal, which is then output and fed into the recording device. According to the reflected light intensity value of the standard sheet and the reflected light intensity value of the vitrinite of coal read on the dial of the recording device, the vitrinite reflectance of coal is calculated by the following formula:
Geology of coal
Where: R mirror is vitrinite reflectance of coal; I is the reflected light current intensity of standard film; I mirror is the current intensity of light reflected by vitrinite of coal; R is marked as the reflectivity of the standard film.
The reflectivity R standard of standard film is calculated according to the following formula:
Geology of coal
Where: n is the refractive index of the standard sheet; N0 is the refractive index of the medium between the sample and the objective lens, air is 1, and aromatic asphalt is1.515 ~1.518.