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Вюник Дншропетровського унiверситету. CepÍH: геологiя, географiя. 24 (2), 2016, 34-39. Visnik Dnipropetrovs'kogo universitetu. Seria Geología, geographia Dnipropetrovsk University Bulletin. Series: geology, geography. 24 (2), 2016, 34-39.
Doi: 10.15421/1116230 http://geology-dnu.dp.ua
UDC 622.278 + 662.73
Research of geomigration processes during underground coal gasification and combustion S. V. Zholudiev
Dnipropetrovs'k National University named after Oles Honchar, [email protected]
Today one of the main problem for areas of intense technological activities related to the production, processing and use of fossil fuels is management and environmental protection. With the current technology development difficult to achieve effective results in terms of further improving the technical and economic performance and in terms of environmental protection. Fundamentally new in the transition to underground coal gasification (UCG) and coal combustion (UCC) is the transition to non-waste technology. The possibility of smoke emission elimination through recycling. directing the underground gasification oxides of sulfur, nitrogen and other toxic components of smoke emissions. In underground water is possible their destruction to non-toxic state. Other harmful for the biosphere chemicals substances (arsenic, strontium) also remain in the underground gas generator. Also in the underground space may direct solid waste for laying out space voids possible reduction surface subsidence and preserve the landscape. Reducing the volume of drilling and laying out space allows you to transfer temporarily useless agriculture territory. Underground space of gas generators with hot rocks can be used for commercial purposes. Utilization of physical warm rocks by pumping cold and getting hot coolant can increase efficiency. But with all the above-mentioned, the environmental justification for building the station of underground coal gasification and coal combustion necessarily involves the analysis and forecast of man-made impact on the geological environment. In terms of the studied region of brawn coal Dnieper basin the prospects of widespread introduction of underground coal gasification and coal combustion technology inherent protection from problem resolution is actively used in the household groundwater from chemical contamination of underground coal combustion, disposal and coolant waste.
Keywords: underground coal gasification, underground coal combustion, geomigrationalprocesses, geotechnical system
Дослщження геомпграцшних процес1в пщ час пщземних газифнкацп та спалювання вугшля
С. В. Жолудев
Днтропетровський нацiональний yuieepcumem шет Олеся Гончара, [email protected]
На сучасному еташ розвитку суспшьства проблема ращонального використання i охорони довкшля входить в число прюритетних i особливо гостро вона проявляеться в районах штенсивно!" техногенно!" дй, пов'язано!" Í3 видобутком, переробкою i використанням горючих корисних копалини. При кнуючш технолога розробки все важче домагатися ефективних результата як з точки зору подальшого полшшення технiко-економiчних показниюв, так i з точки зору охорони природи. Принципово новим при переходi на тдземш газифжацй (111В) та спалювання вугшля (ПСВ) е пе-рехвд на безввдходну технолопю. Ввдкриваеться можливкть лжввдацй димових викидiв шляхом !х рециркуляцй. спря-мування в тдземний газогенератор оксиди арки, азоту i шших токсичних елементи димових викидiв. За наявносл вологи можливе 1х руйнування до нетоксичного стану. Iншi шкiдливi для бшсфери хiмiчнi сполуки (миш'як, стронцш) також залишаються в шдземному газогенераторь Також в тдземний газогенератор можливо спрямовувати твердi ввдходи для закладки порожнеч виробленого простору, що дозволяе ктотно зменшити освдання поверхш i зберегти ландшафт. Скорочення об'ему бурових робгг i закладання виробленого простору дозволяе передавати сшьському гос-подарству тимчасово ввдчужеш землi для рекультиваци. ПШдземний проспр ввдпрацьованих газогенераторiв з гарячи-ми породами можна використовувати в господарських цшях. Уткщзащя фiзичного тепла порвд шляхом закачування холодного i отримання гарячого теплоноая дозволяе тдвищити ефектившсть. Але при всьому вищеперерахованому, екологiчне обгрунтування будiвництва станцй ПГВ i ПСВ обов'язково передбачае i прогнозний аналiз техногенно!" ди на геологiчне середовище. В умовах дослвджуваного регiону Днiпровського басейну питання перспектив широкого впровадження технологш ПГВ i ПСВ неввд'емне вiд виршення проблеми захисту активно використовуваних в гос-подарствi пгдземних вод ввд хiмiчного забруднення продуктами пiдземного горшня вугiлля, утилiзацil теплоносiя i вiдходiв.
Ключовi слова: тдземна газифжащя, тдземне спалювання, геом^рацтт процеси, геотехтчна система
Introduction. 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 which is the amount of
the migrant which passes convectional through a single area of flow per unit time, will be
k = c-v,
(i)
where jK - the flow of convective mass transfer;
C - migrant concentration;
V - velocity of filtration, related to actual velocity of the current u0 by correlation u0 = V/ng, where n- 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.
id = Degrade, (2)
where jdJd - single mass diffusion flow (the amount of the substance which diffuses through a single area of flow per unit time);
D - coefficient of molecular diffusion.
m
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 m2/day. At the same time, the magnitude Dm can significantly decrease after sealing of rocks, and the magnitude Dm 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.
Table 1
The content of chemicals in products and waste PGV and PSV [1, 2]
№ Name of substance The substance content in products and waste, mg/dm3 Maximum permissible concentration (MPC) of chemical substances, mg/dm3
1 Ammonia 1,9 - 3,0 2,0
2 Benzene hydrocarbons 1,0 - 2,2 0,5
3 Pyridine bases 0,04 - 0,68 0,2
4 Hydrogen sulfide 0,03 - 0,32 0,003
5 Naphthalene 0,0004 - 0,1 0,01
6 Acetylene 0,000003 - 0,015 0,0015
7 Prussic acid 0,000006 - 0,008 0,0035
8 Phenol 0,0013 0,001
Extension of hydro dispersion (along the flow's direction) is described by Fick's law, where Dm is changed by the coefficient of hydro dispersion extension, which depends on filtration velocity. The results of laboratory tests for homogenous sands show linear dependency of Dt on V
Dt = Dm + StV, (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 Dm is changed by the coefficient of transversal dispersion Dm. Their rough approximation can be represented as
where St - parameter of transversal dispersion, which has typical meaning for close sand, equals 8t = 0.06 - 0.2 mm.
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 C0 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)
1it Q}\. C-C0 J
(5)
substituted with hydro dispersion coefficient Dj in filtration flow.
Theoretical description of such process was made for one-dimensional transfer in a filtration current with filtration velocity V in direction I written (7) in form
(7)
Balance equation for a neutral migrant in an infinitesimal element with length dl and unit area of transversal section:
dj;
— + no- = 0 dl 0 dt
(8)
where n0 - active porosity of a rock.
After substituting expression (7) for jM, in (8), we will receive differential equation of one-dimensional convective dispersion transfer
n0- + V- = DL — , (9)
u dt 31 1 312
Transformed (9), and inserting Laplace-Carson integral transformation C =L(c)
n0p(C-C0) + V^ = D[^ (1°)
OJ di £ dlz where p - parameter of transformation.
Solution (10) in condition C = C0 at the edge / = 0 of half limited current is as follows
C-£Tn
-.v
a =
Ù
oj
H, (11)
cL-c0
Transformation from (11) to original in C0 = const gives
C-Cr.
where Q - debit of the flow on the current line;
N i N0 - content of sobbed migrant (occluded) per unit volume of rock;
C i C0 - concentration of the solution;
fit - area of transversal section of the current line.
The equation (5) is solved by integration towards the trajectory current, should be based upon the geo-filtration assessments, made for a general case using methods of numerical modeling.
After superposing convectional and dispersive transfer, the total unit mass current jM will be as follows
(6)
where jK is defined with (1), and jd for micro dispersion is defined by the equation (2), where D is
0,5 (erfcÇ + Herfen (12a)
<
nal-vt
f
■n0l*Vt
VI
,ü = - (126) ^n0D.t Di
2\ nc Dlt
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 C = 1).
-transitional (l > C > 0)
- initial content of the migrant C = 0.
Another element of equation (12a) 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 convec-
i _ i _ II
tive transfer, where is (' — ' о ~ ), the position of
n0
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 Cn / У2 \
■ехЛ-чгЛ (14а)
с =
; TT.v
where cg - initial concentration;
x i y - 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, which provides division of concentration in the flow, which moves with velocity V without extended dispersion, but with transversal dispersion, which is characterized by coefficient Dm, with constant intensity P = V *d*C0 at the beginning of the source's coordinates. Such solution provides the following expression for a migrant's concentration
p ( vy* .
c = —exy \ — 77—:f, (146)
2 JiDyXV
4 D
yXj
where D = D - coefficient of transversal dispersion
y m r
(in the (y) direction).
After comparing equations (14a) and (146), we can see their similarity, and that they equally coincide if we consider that
d
DT
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 = 36) for cubic blocks, the correlation will be
(16)
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
D =
VI
2 In
Ço'
(17)
(15)
Initial data
where V - velocity of ground water, which is calcu-
Conclusions. For calculating horizontal mass transfer, we used (12a), (126) and (17). The results of calculations are given in fig. 1. The significant difference between the results in above coal and sub coal water bearing horizons is related to differences of filtration parameters. The largest radius of pollution was obtained by ammonia, although the difference between initial and maximum permissible concentrations was one of the smallest. At the same time, the difference of concentration of hydrogen sulfide is higher and the pollution radius is one of the smallest. So, the extent of pollution does not depend on the differences of concentrations, and should be defined separately for each contaminant, and defining the true regularities of pollution spreading in a horizontal direction requires additional studies.
For calculation of vertical mass transfer, we used (4.14a), (4.146) and (4.15). The calculation shows similarity of pollution by all substances. Both water-bearing horizons will have the same spread of
Table 2
for calculation
№ Name of substance Above coal horizon Sub coal horizon
1 Coefficient of filtration, Kf, m / day 3-6 6-12
2 Active porosity 0,175
3 Flows gradient 0,014
4 The path length of filtration, L, m 5000
5 Period of time, t, days 182,5
6 The diameter of the grains, d, m 10
Fig. 1. Diagram of spreading of different pollution contaminants in a horizontal direction
Fig. 2. Diagram of vertical spreading of contaminant substances
pollution spreading with all contaminants. The greatest pollution will be hydrogen sulfide pollution and will equal 57 m in a section, the least (2.5 m) - ammonia and phenol. Such a pattern clearly represents the correlation between the difference of concentration and radius of vertical pollution. The results of calculations are given in fig. 2.
References
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Radziwill A.J. (Eds), 1987. Dneprovskiy burougolniy basseyn [Dnieper brown coal basin]. K .: Publishing "Naukova Dumka", Kyiv. 328 p. (in Russian).
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Zholudiev S. V., 2014. Doslidzhennya geomigraci-ynih processiv pri pidzemnih gazifikaciyi ta spaluvanni vugillya [Research of geomigration processes during underground coal gasification and combustion]. Bulletin of Dnepropetrovsk National University: Geology - Geography, 16, 175-181. (in Ukraine).
Hadiurnna do pedmneai 10.11.16
