Influence of the natural content of mineral impurities and moisture on the manifestation of hazardous properties of coal seams

It is not possible to determine the type of coal in terms of reducibility from thermal decomposition without access to air for volatile maters to dry ash-free mass, which provides establishing the ratio between oxygen content and other essential components of organic mass. The hazards of coal mine operations depend to a large extent on the ratio of the main components, both in organic matter and in mineral impurities. Methods for determining the yield of volatile maters make it impossible to assess the effect of the main components of organic matter and mineral impurities on the hazard characteristics of coal seams. The volatile maters index reflects only the sum of the thermal decomposition gases: hydrogen, methane, carbon monoxide and carbon dioxide without considering the ratio between them. The determination of the hazard properties of a seam in relation to metamorphic transformations factor should include a case-by-case examination of the relationship between the major components of fuel both the organic and mineral, including the moisture content, for each coal seams.


Introduction
All current Ukrainian regulations on safe mining [1][2][3][4][5][6][7][8] do not use indicators that directly characterize changes in the elemental composition of coals due to metamorphic processes.For this reason, the influence of coal elemental composition on hazard expression remains largely unknown [9,10].Changes in the elemental composition of organic (combustion) mass during geological processes are established for most coal seams in Donetsk and Lviv-Volyn basins when determining the consumer properties of coal [11,12].The organic (combustible) mass, which characterizes consumer qualities, undergoes preliminary preparation, which consists in enriching coal and removing external moisture from it.
All types of solid fuels contain, in addition to the organic (combustion) mass, a mixture of mineral substances.When the fuel is burned, its organic mass is removed as CO 2 and H 2 O, and the mineral components undergo a series of transformations to form ash. Ash is an inorganic residue from the complete combustion of coal [13].The purpose of enrichment is to reduce the yield of ash.Coal is typically enriched with more than 10 % ash.
External moisture is a part of the total moisture of the fuel that is removed when it is dried to an air-dry state.The "total moisture" term is determined as the total external moisture content and the total moisture content of the air-dry fuel.The term is used to describe the humidity of coal as a whole (humidity content of coal).When drying coal in the air, free moisture from the outer surface of the pieces and capillary moisture from open cracks and pores (external moisture) shall be removed.Capillary moisture of closed pores, adsorption and hydrate moisture remain in the dry coal.When drying crushed coal at 105°C, the capillary moisture and adsorption moisture is removed from the pores opened by grinding [13].In order to properly evaluate the results of the coal analysis in order to establish its consumption characteristics, it is necessary to know the humidity of the sample in which the indicators are measured.To this end, the concept of "analytical sample moisture" has been introduced, i.e. a moisture content in a sample of less than 212 µm (0.2 mm).
From the analysis reviewed, it is clear that the indicators in the handbooks [11,12] are not directly relevant to the hazard characteristics of coal seams in underground mining, given the methods used to define them.They characterize only one part of the dry ash-free organic (combustible) component of coal.The proportion of mineral impurities, taking into accounts the ash yield, may exceed 40% [11,12] in some cases, which can certainly affect the hazards of coal seams.

Research procedure
The elemental composition of the organic (combustible) mass is determined without considering the moisture content on which the physico-mechanical and physico-chemical properties of coal mine miners largely depend.One of the main elements that can influence the hazard behaviour of miners is oxygen.It is present in both organic and mineral mass of fuels and is largely responsible for coal restorability.
The methodology is based on the fact that the consumer qualities of solid fuels are determined on the basis of analytical samples brought to an air-dry state (analytical state).The results, which are expressed in other fuel conditions are calculative.Recalculation is made on the basis of ash content, humidity and mineral mass [13].
In the present case, the formulae for conversion are(%): • from a dry, ash-free state: • organic mass: where O c , H c -oxygen and hydrogen content in the combusted mass, %; O r , H r -oxygen and hydrogen content for the operating condition of the fuel, %; W r t -total moisture for the operating condition of the fuel, %; A r -ash content of the sample, %; M M r -mineral mass for the working condition, %.
The total moisture for the working condition of the fuel for each coal seam is given in the references [11,12].There is also information on the ash content of the seam (A r s ) and enriched (A r e ) samples.The calculation of oxygen content to the operating state of the fuel O r according to equation ( 2) is difficult due to the lack of a reliable method for determining mineral impurities in coals [6].
For all coal seams listed in the guide [11], using data on the ash content of enriched (A r e ) and seam (A r s ) samples and the total moisture content (W r t ), the oxygen content was calculated according to equation (1) for enriched (O c e ) and seam (O c s ) samples respectively.Similar calculations have been made for coal seams in the guide [12], but only for the sum of oxygen and nitrogen for enriched ( O c e , N c e ) and formation ( O c s , N c s ) samples respectively.The influence of coal enrichment and analytical sampling on changes in oxygen content on the operating state of the fuel was determined by comparing the graphs for enriched (figure 1) and formation (figure 2) samples, respectively.
The enrichment process contributes to the removal of some mineral impurities, which improves the quality of coal consumption properties.The ideal hypothetical enrichment option is to remove almost all (100 %) mineral impurities.The bisectrics (2) of the coordinate grid (figure 1) correspond (O r e = O c ) to this case.The mutual location of the bisectress (2) and the average line (1), as well as the regression coefficient (0.93) and the free term (0.04) of equation ( 1) indicate that the mineral impurities residue after enrichment causes a relative decrease of oxygen content on average from 3 to 7.0 %.This relative decrease increases with the oxygen content of the combustible mass.At absolute value O r e = 1.0 %, the average relative oxygen value drops by 3 % and at O r e = 15.0 % -by 6.7 %.With an absolute oxygen content of [11] in the combusted mass of 0.3 to 14.6 % in most cases, the maximum absolute reduction for individual coal seams is in limits beetwin 0.02÷0.98%.These figures are derived from the position of the bisectress (2), the average line (1) and the standard deviations of the individual data (σ = 0.27%) from the average line under the "three sigma" rule (figure 1).This accuracy in the determination of oxygen content is quite satisfactory when the organic (combusted) mass element composition is determined to a dry, ash-free state in order to clarify the consumer properties of coals.
An entirely different error is obtained when the oxygen content is recalculated to conditions close to the mining (working r) conditions under which the hazard properties of coal seams occur.For this case, the trendline (1) is also characterized by a high correlation coefficient (0.98), but it is much further away from the bisectress (2) of the coordinate grid (figure 2).This follows from the values of the regression coefficient of equation 1 (0.81) and the absence of a free term.
The influence of the natural content of mineral impurities and total moisture on the relative  average decrease in oxygen content when converted to the working condition of coal is estimated on average to be 19.0 %.For the considered sample of coal seams (734), the absolute oxygen content of the combusted mass was in the range of 0.3 ÷ 14.6 %.The reduction in its content, when measured by the working condition, taking into account the actual discharge of ash and total moisture in the coals of each mine, is in absolute terms within the range of 0.00÷10.66%.This follows from the resulting dependency (1) and related graphs (figure 2).
The ideal hypothetical enrichment option is to remove almost all (100 %) mineral impurities.The bisectrics (2) of the coordinate grid (figure 3) correspond (H r e = H c ) to this case.The mutual location of the bisectress (2) and the average line (1), as well as the regression coefficient (0.85) and the free term (0.09) of equation (1) indicate that the mineral impurities residue after enrichment causes a relative decrease of hydrogen content on average from 6 to 13.5 %.This relative decrease increases with the hydrogen content of the combustible mass.
At absolute value H r e = 1.0 %, the average relative hydrogen value drops by 6 % and at H r e = 6.0 % -by 13.5 %.With an absolute hydrogen content of in the combusted mass of 1.2 to 5.9 % in most cases, the maximum absolute reduction for individual coal seams is in limits beetwin 0.51 ÷ 1.71 %.These figures are derived from the position of the bisectress (2), the average line (1) and the standard deviations of the individual data (σ = 0.15 %) from the average line under the "three sigma" rule (figure 3).This accuracy in the determination of hydrogen content is quite satisfactory when the organic (combusted) mass element composition is determined to a dry, ash-free state in order to clarify the consumer properties of coals.An entirely different error is obtained when the hydrogen content is recalculated to conditions close to the mining (working r) conditions under which the hazard properties of coal seams occur.For this case, the trendline (1) is also characterized by a high correlation coefficient (0.95), but it is much further away from the bisectress (2) of the coordinate grid (figure 4).This follows from the values of the regression coefficient of equation 1 (0.68) and its free term (0.15).
The influence of the natural content of mineral impurities and total moisture on the relative average decrease in hydrogen content when converted to the working condition of coal is  estimated from 17.0 to 29.5 %.For the considered sample of coal seams (557), the absolute hydrogen content of the combusted mass was in the range of 1.2÷5.9%.The reduction in its content, when measured by the working condition, taking into account the actual discharge of ash and total moisture in the coals of each seam, is in absolute terms within the range of 1254 (2023) 012064 IOP Publishing doi:10.1088/1755-1315/1254/1/0120646 1.25÷2.85%.This follows from the resulting dependency (1) and related graphs (figure 4).

Discussion
A comparison of the graphs (figure 1, figure 3) and (figure 2, figure 4) shows that pre-enrichment and consideration of the elemental composition for dry ash-free coal samples predetermines a practical functional dependency from oxygen and hydrogen content.The removal of some random quantity of mineral impurities and completely external moisture has largely systematized the ratio between the oxygen and hydrogen content of the combustible mass and its reconsidered values by the working condition.However, the reference state of the samples taken for calculation does not correspond to the natural state of coal in the work area, as the actual mineral impurities and moisture content are not taken into account.For this reason, despite the almost functional dependancy (1) and the low standard deviations (figure 1 and figure 3), there is no reason to use the recalculated results for this case to predict the hazards of coal seams.
The presence of natural mineral impurities and total moisture in the seam's samples significantly influences the results of recalculation to working condition (figure 2 and figure 4).Primarily, this caused large deviations of individual values from the trend line (1), which makes it impossible to use the average oxygen and hydrogen values, recalculated by the working state of coal, to determine the hazards of a particular coal seam.
The total moisture content is one of the main indicators of the degree of coals' metamorphism, but it is generally not used in regulations to predict the hazards of miners [14].The dependence of mineral impurities on the degree of metamorphic transformation is not generally established, but their presence in seam samples of coals of individual coal seams to be folowing a pattern [13].This indicates that it is possible to individually characterise a coal seam by oxygen and hydrogen content, calculated by the working condition, taking into account the values of total moisture and total ash.The oxygen content of the organic (combustible) mass at a dry, ash-free state is an auxiliary indicator of one side of its metamorphic transformations.The carbon content in organic (combustible) mass is one of the main indicators of metamorphic transformation [14] because it functionally controls the sum of the other main components (oxygen, hydrogen, nitrogen, sulphur).The dependence of individual organic matter components on carbon is less clear.This applies in particular to the oxygen content of the organic (combustible) mass.This dependence is purely non-linear and is characterised by a high deterministic coefficient (R 2 =0.87).The co-dependence of oxygen content with carbon, which is calculated as working-level ash output and total moisture content in enriched samples, is substantially lower.
For this case r = −0.64 and doubled the standard deviation of the individual values from the trend line (σ = 2.21).No specific relationship of oxygen content to carbon can be established in case of recovery from seam samples.It is characterized by a practical lack of correlation (r = −0.34)and a large standard deviation of the individual values (σ = 2.39).The reduction of mineral impurities (A r < 10.0 %) after coal enrichment and their joint consideration with seam moisture has led to a significant reduction in the correlation between considered components when converted to working condition.The weakening of this dependence is caused by the partial enrichment of coals (the presence of a random residue of mineral impurities) and total moisture's presence, which are individually different for each mine.Consideration of the ratio between the components after they have been brought to working condition on the basis of the actual ash content and total humidity has led to an unpredictable redistribution of the influence of factors on oxygen content [15].The natural accidental presence of mineral impurities and the different values of total moisture in the seam's samples have eliminated the monopolistic metamorphic ratio of oxygen in the organic, dry, ash-free mass to carbon.This resulted in a redistribution of the influence of factors on the components of O r and C r .Consideration of seam's samples with mineral inpurities and total moisture content has increased the number of influencing factors.

Conclusions
The formation and manifestation of dangerous properties of coal seams, judging by the requirements of current regulatory documents, are determined by the degree of metamorphic transformations of fossil coal.The use of indicators of mass (V d af ) and volume (V d V af ) output of volatile substances, which are set for dry ashless combustible (organic) mass, excludes the possibility of analyzing the influence of moisture and mineral impurities on the manifestation of dangerous properties of coal seams.The use of these indicators for the prediction of dangerous properties of coal seams, instead of indicators for the working condition (r), can lead to errors in their determination by 50 and 45 %, respectively.The conducted research made it possible to reveal the important role of mineral impurities in the formation of dangerous properties of coal seams.This is caused both by the significant possible content of mineral impurities in fossil coal, and by the simultaneous presence of the main components that determine the dangerous properties of coal seams (carbon, hydrogen, sulfur, oxygen and moisture), both in the organic and mineral parts of fossil coal.The influence of the natural content of mineral impurities and total moisture on the relative average decrease in oxygen content when converted to the state of coal is estimated at 19.0 % on average.

Figure 1 .
Figure 1.Oxygen Recalculation Results in combusted mass for working conditions of the fuel for enriched O r e samples of coal: 1 -the average trend lines; 2 -the bisectors of coordinate grids; 3 -a straight line defining the possible limit of maximum deviations of individual values from the trendlines (1) according to the "three sigma" rule; r, σ -correlation coefficients and standard deviations, respectively; × -experimental data [11] on the combustible mass of oxygen (O c ) converted to working condition according to [13].

Figure 2 .
Figure 2. Oxygen Recalculation Results in combusted mass for working conditions of the fuel for seam O r s samples of coal: 1 -the average trend lines; 2 -the bisectors of coordinate grids; 3a straight line defining the possible limit of maximum deviations of individual values from the trendlines (1) according to the "three sigma" rule; r, σ -correlation coefficients and standard deviations, respectively; × -experimental data [11] on the combustible mass of oxygen (O c ) converted to working condition according to [13].

Figure 3 .
Figure 3. Hydrogen Recalculation Results in combusted mass for working conditions of the fuel for seam H r e samples of coal: 1 -the average trend lines; 2 -the bisectors of coordinate grids; 3a straight line defining the possible limit of maximum deviations of individual values from the trendlines (1) according to the "three sigma" rule; r, σ -correlation coefficients and standard deviations, respectively; × -experimental data [11] on the combustible mass of hydrogen (H c ) converted to working condition according to [13].

Figure 4 .
Figure 4. Hydrogen Recalculation Results in combusted mass for working conditions of the fuel for seam H r s samples of coal: 1 -the average trend lines; 2 -the bisectors of coordinate grids; 3a straight line defining the possible limit of maximum deviations of individual values from the trendlines (1) according to the "three sigma" rule; r, σ -correlation coefficients and standard deviations, respectively; × -experimental data [11] on the combustible mass of hydrogen (H c ) converted to working condition according to [13].