Coal

The classification is designed for an overall assessment of coal. To denote the varieties of coal, destined for use in power system, the following scheme is used: Grade = brand + class.
For example, coal grade DR. This means: by volatile- long-flame, and by fraction size – ordinary (piece size; 0-300 mm). In this case, the percentage of specific grades is not regulated. This may be coal, which consists of 90% of coarse waste and 10% rubble, and vice versa. If coal is marked as the KDP, it is – long-flame coal, sorted, with the size of pieces from 50 (40) 200 (300) mm. The content of small (less than 50mm) debris shall not exceed 15%.

QUALITY OF COAL:

MOISTURE IN COAL (W)

All coals contain a certain amount of moisture. In this case, depending on its state (confinedness) distinguish surface moisture (moisture wetting). This water is located on the surface of grains and lumps of coal. It can be easily removed by drying in air. The remainder (after removal of the surfacial) moisture characterizes the moisture content of coal, which is characteristic for his material, petrographic and grade composition and is specified as the maximum water-holding capacity (Wmax). Free moisture on the surface of the pieces of coal and grain and moisture, confined to cracks, voids and capillaries (Wex) in total specify such concept as an external moisture (Wex). In the laboratory, it is defined by drying in the drying oven: at 40 SB - for hardcoal and 50 ° C - for brown.
Moisture of air-dried coal, or analytic (WA), mainly represented by the adsorptive -bound water. It is determined by drying at a temperature of 105-110 ° C (using an accelerated method at 160 ° C). In Total these two types of water define the concept of general moisture (Wt) or working (Wrt). Moisture content (working and analytical) depends primarily on the degree of metamorphism (grade composition of) coal. Moisture in coal is not only ballast; it reduces its calorific value as it requires additional heat input on its evaporation. Therefore popular experience of wetting of coal before combustion, in fact, is incorrect. On the other hand, the wetting of coal dust leads to its pelletizing and rise of permeability to gases, generated by thermal decomposition of coal. But this technique is used primarily in despair - when using coal, not intended for fuel-bed firing. The increased content of external moisture also leads to increased adherence of coal fines, caking and freezing coal.

ASH CONTENT (A)

Ash content or dirt (not flammable) content is the main quality indicator. Mineral impurities - is largely neutral ballast, to a lesser extent - the source of harmful chemical elements that influence the technological characteristics of coal, and in the thermal engineering the degree of environmental contamination. The content of mineral impurities depends only on the conditions of peat accumulation, which means it may be different for different grades of coal. Distinguish internal, related to organic part of coal and an outer layers, which is composing the petrean layers, ash. Content of the first, usually small (less than 10%), but practically is irremovable during enrichment. Free ash, especially related to low carbon petrean layers can be easily removed by all types of enrichment.
Requirements to ash in various areas of industry are significantly different. In the thermal engineering are used brown and black coals, mostly with AD till 35%, with higher ash content, they require special types of combustion. There are many fuel gradations in the degree of ash content. But in the context of our work the most appropriate classification will be the one, based on the account, so-called reduced ash - ash ratio (in%) to the calorific value of wet ash-free coal (in MJ / kg, 1 MJ = 239 kcal)
Therefore, the ash requirements for low-metamorphosed, which means low-calorie coal (brown, long-flame) should be more stringent than to high-metamorphosed (thin, anthracite) ash. Devolatilization (Vdaf): organic mass of coal by thermal influence breaks into two derivatives: the volatile matter and non-volatile residue. The volatile matter consists of primary tar (in brown coal), coal tar (in hardcoal) and gases: carbon monoxide, hydrogen, methane, light hydrocarbons and their homologues. The content of volatile depends on the petrographic composition of coals: vitrinite (brilliant) varieties contain them more than fusain (matt).
This indicator is very important because it determines the behavior of coal in its use. So, high level of gaseous (volatile) component in the composition of the combustible matter of coal determines its high reactivity (i.e., inflammation occurs at lower temperatures), domination of convective type of heat transfer over radiant. But at the same coals with high volatile matter have lower rates of combustion heat, thermal stability. They usually have a higher percentage of incomplete combustion.

SULFUR CONTENT(STD)

Sulfur in coal is a harmful impurity. When coal is used in metallurgy, sulfur passes into the metal and degrades its quality. By the fuel combustion sulfur forms sulfides, which react in the atmosphere with water vapor, form sulfuric acid, which drops as the so-called acid rain. The sulfur content in coals of the pools of Kuznetsk, Kansk-Achinsk, Minusinsk ranged 0,2-0,6%. This indicator is most important for coals consumption value assessing, especially those used in the thermal engineering.

CALORIFIC VALUE (Q)

Distinguish between higher calorific value, translated to a dry ash-free condition of fuel (Qdaf). This indicator is used for comparison and classification of coals. Lower calorific value (Qr) characterizes the fuel in its natural state, i.e. for specific values of moisture and ash content.

Coal characteristics