Research of geomigration processes during underground coal gasification and combustion

Dniprop. Univer. Bull. Ser.: geol., geogr., 2016, 24 (2) Вісник Дніпропетровського університету. Серія: геологія, географія. 24 (2), 2016, 34–39. Vìsnik Dnìpropetrovs’kogo unìversitetu. Serìâ Geologìâ, geographìâ Dnipropetrovsk University Bulletin. Series: geology, geography. 24 (2), 2016, 34–39. Doi: 10.15421/1116230 http://geology-dnu.dp.ua _______________________________________________________________________________________


Дослідження геоміграційних процесів під час підземних газифікації та спалювання вугілля С. В. Жолудєв
Дніпропетровський національний університет імені Олеся Гончара, ggf2009@ukr.netIntroduction.In the process of underground gasification and coal combustion numerous chemical substances are emitted.They can leave the underground burning zone and move to the subsoil hydrosphere and polluting it in depends on water saturated above coal and sub coal water-bearing horizons.That is why it is necessary to prevent or limit their penetration into water-bearing horizons during and after the operation of underground burning.But at the stage of active operations, the risk of penetration by pollutants remains high.To some extent, this problem can be solved by estimating the possible distribution of pollutants from the area of underground generator.Presentation of the general material.The literature data provides us with the approximate content of pollutants in the products and wastes of underground gasification and underground coal combustion (table 1), which makes it possible to predict the extent and character of chemical pollution of the groundwater around the underground generator using methods of mathematical modeling.
Theoretical description of geomigrating processes with consideration of hydro-chemical transformations should use methods of thermodynamic modeling.Such approach is used for modeling contamination processes when considering conditions of technogenic pollution, the sources of which are in water-bearing rocks and enter the water supply during changes of thermodynamic condition in groundwater.
The determinant in the study of geomigrational process is mass transfer, which is the process of transferring certain ground water components (migrants).The importance of studying the mass transfer processes is related with high mobility of water solutions in the lithosphere.
Usually, the key component of the ground water filtration flow is convective transfer, which proceeds hydraulically with filtrating water.A single mass flow of convective mass transfer j к which is the amount of the migrant which passes convectional through a single area of flow per unit time, will be , (1) where j к -the flow of convective mass transfer; C -migrant concentration; V -velocity of filtration, related to actual velocity of the current u 0 by correlation u 0 = V/n 0 , where n 0 -active porosity of a rock.
Also the process of mass transfer includes different forms of dispersion, which cause the scattering of migrants in space.There are considered processes of micro dispersion which take place at inner porous (inner fault) levels, and macro dispersion, which takes place at the levels of aggregates and blocks of rocks.
At the molecular level, micro dispersion is caused, first of all, by the process of molecular diffusion, which creates a flow of migrant, described in Fick's law.

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(2) where -single mass diffusion flow (the amount of the substance which diffuses through a single area of flow per unit time); D m -coefficient of molecular diffusion.
Coefficient of molecular diffusion characterizes the sinuosity of filtration routes in porous environments, and according to experimental data, equals 0.5-0.7 for incoherent sands, and 0.25-0.5 for coherent sands.
According to the results of laboratory studies, the value of the diffusion coefficient for clayey rocks has the order of 10 -5 m 2 /day.At the same time, the magnitude D м can significantly decrease after sealing of rocks, and the magnitude D м significantly depends upon moisture in incomplete water saturation.
Fick`s law in the form (2) is reasonable for isothermal processes and in independent diffusion of components of a solution.Otherwise, more complicated phenomena of non-isothermal multi-component diffusion occur.

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(3) The generalization of experimental data shows the possibility of disordered structure of dependency for sandy-gravel rocks (3).
In the non one dimension flow of transfer, transversal hydro dispersion occurs, which causes transversal flow of the migrant, which is also defined by the Fick`s law, where D m is changed by the coefficient of transversal dispersion D т .Their rough approximation can be represented as , (4) whereparameter of transversal dispersion, which has typical meaning for close sand, equals In a homogenous environment, the model of transfer includes description of convective transfer and micro dispersion.The model of convective dispersion uses a scheme of displacement by piston, where it is considered that all areas of water move in each section with the same velocity.In such conditions, let us define the equation for transfer velocity of the dividing edge (front of displacement) of migrant solutions, which divide area of concentrations with С 0 for condition of instant increase in sorption equilibrium (i.e.not considering sorption kinetics), making balance equation of migrant in infinitesimal element of current dl, which is the border section of the migrant solutions for time (t) , (5) where Q -debit of the flow on the current line; N і N 0 -content of sobbed migrant (occluded) per unit volume of rock; С і С 0 -concentration of the solution; -area of transversal section of the current line.The equation ( 5) is solved by integration towards the trajectory current, should be based upon the geofiltration assessments, made for a general case using methods of numerical modeling.
After superposing convectional and dispersive transfer, the total unit mass current j м will be as follows where j к is defined with (1), and j d for micro dispersion is defined by the equation ( 2), where D m is substituted with hydro dispersion coefficient D l in filtration flow.
Theoretical description of such process was made for one-dimensional transfer in a filtration current with filtration velocity V in direction l written (7) in form , ( 7) Balance equation for a neutral migrant in an infinitesimal element with length and unit area of transversal section: where n 0 -active porosity of a rock.
Solution (10) in condition = C 0 at the edge l = 0 of half limited current is as follows Transformation from (11) to original in С 0 = соnst gives Calculations according to (12) show that after a certain time after the process had started, three main migration zones are formed: -the zone of dismissing migrant (with relative concentration = 1), -transitional -initial content of the migrant = 0. Another element of equation ( 12а) is small and can be neglected, so a simplified expression for relative concentration can be used., Analysis of solving fundamental task shows the peculiarities of convective and dispersion transfer forms manifestations.The equation ( 13) shows that, during piston displacement, defined only by convective transfer, where is ( ), the position of piston displacement front corresponds to the middle of transitional zone with average concentration between the content which displaces and which is being displaced.
To describe the transversal macro dispersion, it is reasonable to use the model of "sieving" small parts through a net of large grains, which filtrate.Using this model for point transfer of small particles, the division of their concentration in flat flow was defined where с 0 -initial concentration; x і у -coordinates in the direction of sieving (filtration) and perpendicular to it; d -diameter of grains which filtrate, corresponding to size of blocks.
Let us compare (14a) with the expression obtained by solving the task of convective-dispersion transfer, provides division of concentration in the flow, which moves with velocity V without extended dispersion, but with transversal dispersion, which is characterized by coefficient D т , with constant intensity P = V *d*C 0 at the beginning of the source`s coordinates.Such solution provides the following expression for a migrant`s concentration , (14б) where D y = D т -coefficient of transversal dispersion (in the (у) direction).
After comparing equations ( 14а) and (14б), we can see their similarity, and that they equally coincide if we consider that , , If the shape of blocks is considered cubic and the diameter of grains is defined by the size of block (for sand clays and loams -0.1 m, for sands -1 -10 m), the magnitude can be connected to diffusion mass transfer.Considering (f d = 36) for cubic blocks, the correlation will be , Using these methods, the calculations of horizontal and vertical migration of contaminant components (see table. 1) were made.The parameters of calculation are given in table 2.
There are many methods for calculation of hydro dispersion coefficient, but in this case, initial meanings allow to use Averianov`s method where V -velocity of ground water, which is calculated .