About Possibility to Use Industrial Coal-Rank Classification to Reveal Coal Layers Hazardous Characteristics

Possibility Abstract The research is concerned with all the fossil coal metamorphism series based on their rank classification. Currently, based on the genetic characters of metamorphic coal conversions, no proved regulatory definition of the coal layer hazardous characteristics is available. For mining forecasts, the regulatory framework uses a limited number of the classificatory parameters with properties, which have not been determined for the whole multiplicity of coals metamorphic transformation. Special features of each coal rank shall be specified using their proxy parameters. It proves that they specify different aspects of the metamorphic coal conversions, as well as other classificatory parameters. For proved forecasting the coal layer hazardous characteristics, it is necessary to use some combinations of the classificatory parameters. It is testified that any alterations of coal properties due to their inner structure conversions may be characterized by maximum and minimum values of different parameters for the same coal rank. The limit for possible alteration of the K rank coals volatile mass yield has been established in lower than normal level. It is noted that coals with the same consumer performance may in some cases be characterized by different values of the classificatory parameters, and in other cases—the coals with different values of the classificatory parameters may have the same processing behavior. It is proposed, when establishing any hazardous properties of the coal layers in addition to the industrial classificatory parameters, one should use alterations of the organic part elemental composition, mineral impurities and moisture in the whole range of metamorphism series.


Abstract
The research is concerned with all the fossil coal metamorphism series based on their rank classification. Currently, based on the genetic characters of metamorphic coal conversions, no proved regulatory definition of the coal layer hazardous characteristics is available. For mining forecasts, the regulatory framework uses a limited number of the classificatory parameters with properties, which have not been determined for the whole multiplicity of coals metamorphic transformation. Special features of each coal rank shall be specified using their proxy parameters. It proves that they specify different aspects of the metamorphic coal conversions, as well as other classificatory parameters. For proved forecasting the coal layer hazardous characteristics, it is necessary to use some combinations of the classificatory parameters. It is testified that any alterations of coal properties due to their inner structure conversions may be characterized by maximum and minimum values of different parameters for the same coal rank. The limit for possible alteration of the K rank coals volatile mass yield has been established in lower than normal level. It is noted that coals with the same consumer performance may in some cases be characterized by different values of the classificatory parameters, and in other cases-the coals with different values of the classificatory parameters may have the same processing behavior. It is proposed, when establishing any hazardous properties of the coal layers in addition to the industrial classificatory parameters, one should use alterations of the organic part elemental composition, mineral impurities and moisture in the whole range of metamorphism series.

Introduction
Worldwide, issues of the coal layer mining activities [1] continue to be challenging currently. It is proved by the routine hard-hitting accidents in coal mines [2,3]. In many cases, natural disasters are the consequence of the coal layer hazardous characteristics manifestation, when mining [4]. Such characteristics are gas bearing capacity [5] and layer liabilities to gas-dynamic phenomena [6], coal ignitability [7], and its ability to dust production [8] and its explosibility [9].
When mining, the regulatory framework uses a limited number of the coal metamorphism series classificatory parameters to forecast their gas emission [10], occurrence of spontaneous fires [11] as evaluation criterion for hazardous characteristics of coals. Only 4 classificatory parameters are currently used in the regulatory documents [12] to assess the possibility of coal layer hazardous characteristics during the coal metamorphic conversions. The main one is their volatile mass yield during the coal thermal decomposition in an anaerobic environment (V daf ). In some degree, V daf specifies the organic matter elemental composition of bituminous coals only. In addition, to describe the anthracites, the volatile volume yield ( ) daf v V or specific electrical resistivity logarithm (lgρ) is used. When establishing the coal layer liability to gas-dynamic phenomena, jointly with V daf and lgρ parameters, the parameter of their plastic layer thickness (у) shall be used. The main criterion of their liability to ignitability shall be their ranking as bituminous coals of anthracites. When mining, assessment of all the possible manifestations of coal layer hazardous characteristics is limited by this list.
To assess the characteristics of coals appeared during their metamorphic conversions, more than 20 classificatory parameters are known [13,14]. Nevertheless, up to now, the challenging need to unify the coals for different purposes [15] exists. There is no doubt in relevance of such researches to improve the mining regulatory framework. Now, the classification of coals by their genetic and process parameters [16] is the most perfect. In addition to V daf , daf v V , lgρ, and у, it uses 6 more classificatory parameters are applied. The main of them is an average reflectance of vitrinite. As distinct from V daf , daf v V , lgρ, and у, the limits of its alterations are established for the whole range of metamorphism series of fossil coals as per their ranking.
The purpose of the study is to find interrelation between the fossil coal qualification parameters and possibility of their use to forecast the coal layer hazardous characteristics, when mining.

Materials and Methods
Exception for vitrinite (R о ), all the qualification parameters in the coal metamorphism series have their quantitative values in its separate ranges only. Having determined qualification parameters in these ranges, we may suppose any possible manifestations of the hazardous characteristics of coals of some metamorphism series. When developing the international GOST for classification of coals as per their genetic and process parameters [16], 10 parameters: daf v V ; V daf ; R о ; maximum water capacity for ash free state ( max daf W ); gross calorific value for wet ash ( daf s Q ); total fusainized components for clean coal (∑OK); semi-coking resin yield for dry ash free state ( daf SK T ); plastic layer thickness (у), free swelling index (SI); and vitrinite reflectance anisotropic index (A R ). By ten classificatory parameters, the coals are divided by types, ranks, grades, sub-grades, orders, sub-orders.
Ranking the fossil coals by types shall be performed as per different combination of three parameters R о , daf s Q , and V daƒ : As per main (R о ) and proxy parameters, 17 coal ranks have been specified (B, D, DG, G, GZh, OG, Zh, Zh, KZh, K, KO, KSN, KS, OS, TS, SS, T, A). In turn, the coal ranks have been divided by 27 grades, 44 sub-grades, and 49 classes.
To establish any possible ranges of the proxy parameters alteration as per [16], their dependencies on Rо have been considered, which has its qualitative evaluation of the coal metamorphism series based on their rank classification.
Classification concept supposes the metamorphic conversion intensification along with Rо increasing and arrangement of coals in their ordered series by rank classification between B and A. Having determined their qualitative composition of the main and proxy parameters, one may find the special factors, which determine the identity of rank under consideration. The parameters established in such a way may be used for detailed elaboration of the coal layer hazardous characteristics manifestation forecast.

Results
Analysis of the regulation [16] has shown absence of certain recommendations to select the value of ∑OK parameter, upon determination of the coal category. For all the ranks, grades, and sub-grades are recommended to be assumed less than 39 % or more than 40 %. For this reason, to reveal any hazardous characteristics of coal layers using ∑OK parameter is impossible as per [13]. Similarly, when establishing the order of brown coals and anthracites, it is recommended to use their maximum water capacity for ash free state ( max daf W ) and volatile value is more than 60 %; for sub-grades 2BV and 2BF-30÷50 %; and for 3B grade (sub-grades 3BV and 3BF)-less than 30 %. The same uncertainty of gradation use to be observed upon determination of the anthracites grades and sub-grades by their volatile volume yield. For AV sub-grade, it is more than 200 cm 3 /g; for sub-grades 1AF, 2AV, 2 AF-100÷150 cm 3 /g and more; and for 3А grade (sub-grades 3АВ and 3АФ)-150÷200 cm 3  on R о as per [16]. Independent of the big quantity of sub-grades (44), they have no distinct boundaries determined by the parameters Civil Engineering and Architecture 9(2): 507-511, 2021 509 R о and V daf .
In many cases, no lower and upper boundaries of V daf alteration have been established. The experimental data statistical processing [16,17,19] has allowed to fix this disadvantage. To make it possible to use in analysis V daf values along with R о , their alteration boundaries have been established using the coal ranks (Table 1). Almost all the values of V daf lower and upper alteration limits as per [16] are within area located between the boundaries (Fig. 1, curves 1 and 2). The experimental data analysis [15,[17][18][19] has revealed the abnormal low limit (2.7 %) of possible V daf alteration for K rank coals. Plastic layer thickness parameter (у) shall be used to establish the ranks, grades, orders, and sub-orders of the bituminous coal. No definite correlation of this value on R o is observed (Fig. 2, a).  [16] for fossil coals upon transition from B and K (0.40-0.59 %) and from K to A (2.20-2.59 %) accordingly; ×, + -data of V daf and R o as per [17,19] Only lower (1) and upper (2) alteration boundaries of у may be considered depending on R o . Within alteration range R o =0.80÷1.60 % value of у increases from 6 to 28 mm [16]. Its minimum values, including those equal to 0, are within alteration range R o =2.00÷2.60 %. Thus, depending on nature of y, one may suppose presence of several possible ranges R o , where the metamorphic conversions led to different consequences in the elemental composition and properties of the organic matter of coals. This is evidenced by increasing of у from 6 to 22 mm with values of R o =0.40÷0.80 % (Fig. 2, а). Upon alteration of R o within 0.80÷1.60 % interval, у parameter is within limits of 6÷12 mm. Nature of у range alteration at R o =1.60÷2.20 % is similar to у alterations at R o =0.40÷0.80 %. Obviously, it is related to the approximately same conversion of organic constituents by chemical composition and different restructuring the inner structure of coals. This is evidenced by the different alteration ranges R o equal to 0.60÷0.80 % and 1.60÷2.20% respectively.
The identical nature of alteration of interrelation between у and R o is established as [15]; and for the plastic layer-from the volatile yield (Fig. 2, b). Maximum у value of 37 mm is achieved at V daf equal to 32 %. It is little more than the maximum values of у (26 mm) by the classification. V daf alteration boundaries (Fig. 1) require to be specified due to as per [16], the minimum possible value of V daf is about 10 %. And the experimental data diagram, dependence of V daf on daf v V evidences that minimum V daf values are within range of 1.0÷4.0 % (Fig.  3).
Free swelling index value (SI) exists for some coal ranks only. They include D, TS, SS, and T coal ranks.

Discussion
The studies performed have confirmed the conclusions that the rank classification has narrow separation of K rank coals by their consumer properties. It is obviously that such an approach to the coal gradation using V daf and R o will not be sufficiently accurate for establishing the coal layer hazardous characteristics as well. It is confirmed by availability of the transition zones (3,4) ( Fig.  1) of alteration ranges V daf and R o upon transition from the brown to bituminous coals and from the bituminous coals to anthracites. All these zones include almost all the coal metamorphism series by their ranks.
As classificatory parameters to establish the coal layer hazardous characteristics, it is not possible to use daf SK T and A p as per [16] due to no indications of their certain alteration limits. For the coals of 1B grade and 2BV, 3BV sub-grades, daf SK T is less and more than 20 %. For 2 BF sub-grade, the recommended parameter value shall be less than 20 %; and for 3BF-less than 15 %.
The alteration range of R o =2.20÷2.60 % corresponds to transition from the bituminous coals to anthracites. In this R o alteration interval, у values are less than 6 mm. It is supposed that the same its values may be specific for the anthracites as well.
The composition and characteristics of anthracites within their alteration range of R o =2.60÷4.50 % and more are the least researched from the point of view of coal layer hazardous characteristics appearance. This alteration range of R o is almost a half of size of the whole fossil coal metamorphic conversion series by their ranks. The proxy parameters are insufficiently used to characterize the consequences of such conversions.

Conclusions
The top-down analysis of the current industrial coal classification has allowed establishing its particularities and identifying its peculiar provisions to establish the coal layer hazardous characteristics, when mining:  no recommendations on selection of the classificatory parameter certain limits for volatile yield, maximum water capacity for ash free state, etc. does not allow their use to establish the coal layer hazardous characteristics;  possible range boundaries of the mass volatile yield alterations have been determined; nonlinear relationship of the reflectance of vitrinite with the mass volatile yield has been noted, which evidences that they characterize different aspects of the coal metamorphic conversions;  the established abnormally low limit for the fossil coal mass volatile yield alterations has shown that coal ranks do not allow making the coal layer alterations of the anthracite properties during their geological conversions have been the least understood. At the maximum width of range of the vitrinite reflectance alteration, no certain alteration limits for proxy parameters are available;  availability of the free swelling index for coal ranks in diametrically opposite parts of the ordered series shows presence of the same properties with their different degree of metamorphic conversion;  for reliable establishing the coal layer hazardous characteristics manifestation, when mining, it is necessary to consider the indices, which may directly characterize such properties in addition to the industrial classification. They should include the elemental composition alterations of mineral impurities and moisture in the whole coal metamorphism series.