Air pollution by coal dust as a factor of ecological compatibility for coal mining enterprises

Coal mining enterprises (there are more than 130 in the Kemerovo region), are powerful sources of pollutant emissions (P) into the atmosphere. They account for almost a third of the total mass of the cumulative annual emissions in the region. A significant number of coal mining enterprises are located in the vicinity of the boundaries of residential settlements of Kuzbass. At the same time, emissions of coal mining enterprises are mainly carried out from unorganized sources of air pollution (SAI), which do not have treatment facilities. All this leads to pollution of the surface air layer by the coal mining industry. Currently, the category of the enterprise is determined only by its economic activity associated with the production of a product. In particular, all coal-mining enterprises, including the extraction and enrichment of coal, anthracite and brown coal, are classified as category 1 enterprises, i.e. enterprises with significant negative environmental impact (NEI) and related to the areas of application of the best available technologies (BAT). However, this approach to the definition of the category does not take into account the individual characteristics of the enterprise and the level of ecological compatibility of its production.


Introduction
Coal mining enterprises have a number of characteristic production processes that generate identical emissions into the atmosphere [1][2][3][4][5][6]. Its include drilling and blasting operations, coal loading and transportation, dusty coal warehouses and dumps, ventilation installations, quarry transport. In all these processes, coal dust (code 3749) is the most characteristic and unique P for coal mining enterprises. Therefore, we assume that the pollution of the atmosphere by coal dust can be an indicator of the level of ecological compatibility in the coal mining enterprise as a whole. For the integral evaluation of all the characteristics of atmospheric pollution with coal dust offers the option Sp (ha/tons) representing the area S (ha/kt) the zone of influence of emissions of the plant of coal dust divided by the annual output M (kt) coal company. To determine this indicator, it is enough to include coal dust in the composition of the calculated SV and determine the area of the zone of influence.
According to [2] the zone of influence of emissions at the enterprise for a given P is defined as an area within which Cs>0.05 Mpcr, where Cs (mg/m3) is the maximum (in terms of wind speeds and directions) single concentration of this P, and PCs (mg/m3) -the corresponding single maximum permissible concentration (for coal dust PCs = 0.3 mg/m3). The distribution of Cs on the territory is determined by the calculation of air pollution on the basis of the normative methodology MRR-2017 [5] using unified programs for calculating air pollution [2].
The meaning of the indicator of ecological compatibility of the Sp is that it characterizes the size of the area with significant pollution by coal dust per 1000 tons of coal production. When determining the size of the zone of influence is taken into account the power of SAI emissions of the enterprise, their parameters and location on the industrial site, climatic parameters of the territory and the coefficient of relief. This takes into account almost all the parameters that determine the process of air pollution with coal dust. It is also taken into account how from an environmental point of view (dust suppression, humidification) the various stages of mining are organized.
Naturally, the proposed indicator Sp can be considered informative, if the inventory and the task of the calculated parameters of coal dust for different enterprises are carried out correctly and uniformly. The basis for the inventory of emission sources of coal mining enterprises is the industry methodology [6].
Note one important feature for determining the zone of influence associated with the setting of the settling coefficient of particles for the calculation of [5]. As a rule, in most calculations, project developers do not pay attention to the justification of the settling coefficient F, which takes into account the rate of settling of P (gases and aerosols, including solid particles) in the atmospheric air. However, as figure 1 shows, at a small distance (up to 1.5 km) from an unorganized areal source, the results of calculating the maximum surface concentration vary significantly.

Materials and Methods
Let us consider in more detail the procedure for selecting the settling coefficient F before carrying out the calculated estimates of atmospheric pollution by the method [5].
In the framework of the method [5], a dimensionless coefficient F is used in the calculation of atmospheric pollution, taking into account the rate of subsidence of P (gases and aerosols, including solid particles) in the atmospheric air. When developing projects, in most cases, task F is used on the basis of table 1 taken from Annex 2 to the methodology [5]. According to this table, all dust particles released into the atmosphere without treatment have a settling coefficient F=3. In fact, when particle size information is available, the value Of the F coefficient is set individually for each source, depending on the dimensionless ratio Vg/Um, where Vg is the gravitational settling velocity of the particles.  Um -dangerous wind speed at which the surface concentration for a given source reaches a maximum (p. 5. 10. from [2]). In this case, the recommendations of Annex 2 of [2] can be summarized in table 2. Table 2. Dependence of the coefficient F on the ratio Vg/Um.

Vg/Um
Sedimentation coefficient F  0,015 1,0 0,015 < Vg/Um  0,03 1,5 > 0,03 2,0-3,0 taking into account the degree of purification given in table 1 If the diameter of the particles D does not exceed 100 microns, the settling rate of the particles Vg can be found [7] using the Stokes formula: , 18 where Vgo is a gravitational subsidence velocity, cm / s; g ≈ 981 is an acceleration of gravity, cm / S2; Ro is a particle density, g / cm3; η is a dynamic viscosity of air, g/cm•s; dg is an aerodynamic diameter of particles, microns. In the future, instead of Vgo, we consider Vg = 0.01 Vgo, which has a dimension of m/s. The value of η varies slightly within the range of air temperature changes typical for atmospheric pollution calculations (from -20°C to +20 ° C) [9]. This change in practice can be neglected and, taking into account that most calculations in accordance with [5] are carried out on summer conditions, it is possible to assume a dynamic viscosity of the air η = 1,8•10 -4 (g/cm•s), which corresponds to a temperature of +20ºC.

Results and Discussion
Most sources of air pollution by dust particles in coal mining enterprises are unorganized, and their emissions do not have an initial flare rise due to the initial velocity and overheating of the gas-air mixture. In particular, the source describing the technological explosion is approached in the calculation by an unorganized source formed after the explosion cloud rises to a height where it has lost its initial dynamic characteristics. Any unorganized source, both linear and areal, has a dangerous velocity Um = 0.5 m / s, which is independent of the deposition coefficient F.
The density of coal varies from 1.20 to 1.5 g / cm3, and the density of the rock from 2.5 to 4.5 g / cm3. From tables 3 and 4 it is possible to determine the value of the settling coefficient F for coal particles of different diameters and densities. Table 3. Ratio of deposition velocity Vg (m/s) to hazardous wind velocity Um =0.5 m/s for an unorganized source depending on particle size D (µm) and coal density Ro (g / cm 3 ).  Using data from the classification table for automatic partitioning of total dust emissions by fractional composition [3], we determined the dispersed composition of dust particles emitted into the atmosphere for the main processes of coal mining (table 5).
A comparison of table 5 with tables 3 and 4 shows that 98% of the particles are less than 10 microns in size when blown off the dumps, so the settling coefficient can be taken for coal as F=1.0 and for rock F=1.5. During blasting, 70% of particles with a diameter of less than 10 microns are carried outside the cut (large particles settle in the quarry), so it is advisable to represent the explosion emission in the form of two sources, where the first (70% of the emission) has a coefficient F=1.0, and the second (30% of the emission) F=2.0. For rock dust coefficient F should be taken 1.5 and 2.5, respectively. The above estimates with the calculation of Vg according to Stokes law refer to the settling of particles in the resting air (in the research chamber or indoors). In the real atmosphere, dynamic and thermal factors are always present at the boundary with the underlying surface, where the propagation of P occurs, generating turbulence, under the influence of which the rate of subsidence can change. However, it is quite difficult to estimate the real limits of the change, because the data of standard meteorological observations are absolutely insufficient for the calculation of turbulence parameters.
Since 2012, the authors have been able to measure meteorological parameters using the acoustic mobile meteorological complex AMK-03. AMK-03 measures the following parameters: the projection of wind speed; wind direction; temperature; pressure; humidity; air density. In addition, the complex calculates the turbulence parameters: u* -friction velocity, m/s; L -Monin -Obukhov scale, m; H -vertical heat flow, cal/m2s. Thus, it is possible to specify the settling velocity Vg. First, use the real density and dynamic viscosity of the air in the Stokes formula (1), from which Vg is calculated. First, use the real density and dynamic viscosity of the air in the Stokes formula (1), from which VG is calculated.
Thus, the data of the mobile complex AMK-03, in combination with the laws of dependence of the parameters of the boundary layer of the atmosphere, allow reasonably enough to calculate the rate of deposition of particles of different sizes Vdn on the underlying surface. A special research module embedded in the ERA software package (www.lpp/EN), automatiseret calculation Vdn in the acquisition of archives of observations of metalocomplex AMK-03. When performing calculations for a specific time, it is possible to calculate the Vdn and set the coefficient F for these weather conditions.
The analysis of the calculations of dust particles deposition from atmospheric emissions with AMK-03 data (during which Vdn is calculated for each observation) shows that for Kuzbass conditions Vdn may differ from Vg by 30% upwards and by 60% downwards. This can be taken into account in determining the actual subsidence factor F in tables 3 and 4.
Zone influence determination of coal mining enterprises on air pollution with coal dust was carried out on the basis of the new methodology MRR-2017 [5]. The deposition rates for sources associated with mining processes in table 5 have been introduced uniformly for all enterprises in accordance with tables 1 to 4.
By results of generalization of calculations of the indicator of Sp for the coal mining enterprises of Kuzbass it is possible to allocate the following categories of the enterprises which influence coal mining on atmospheric air. Category 1 (low impact) Sp < 1.5; Category 2 (moderate exposure) 1.5 < Sp < 2.5; Category 3 (severe impact) 2.5 < Sp < 3.5; Category 4 (hazardous effects) 3.5 < Sp. Enterprises of the 3rd and especially the 4th category use environmentally imperfect coal mining technologies. On them first of all atmospheric protection actions (dust suppression, humidification, the closed points of loading, etc.), at the organization of processes of production, transportation, storage and shipment of coal have to be introduced. Let

Conclusion
The results of the calculation of the SP indicator are presented in table 6. It is easy to see that for 6 of the 8 enterprises considered, the values of Sp are quite close and lie in the range from 2.0 to 2.5, which classifies them as moderate impacts on natural complexes and the population.