importance of the gas factor in the process

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Importance of the Gas Factor in the Process

of Formation of Outburst Zones in Coal VeinsKhojayev Rustam, Gabaidullin Ravgat, Filatov Igor, and Vlasova LyudmilaResearch Centre GeoMark LLC, Karaganda, Kazakhstan

Abstract. Both the theoretical research and the field studies were conducted in

order to discover regularities and criteria of origin of outbursts stipulated by the

kinetics of the methane desorption from coal in the process of its destruction.

The results of the research conducted have provided the basis for the

calculation procedure for gas emitted in the process of outburst and its pressure in

the gas collecting main in respect to the size range of the bulk of coal within it.

Keywords: Sudden outburst of coal and gas, desorption of methane from coal,

rock pressure, critical fractional composition.

It is widely accepted that one of the main factors determining the origin and

development of coal and gas outburst is a gas-dynamic characteristics of the coal

bed.

We have carried out theoretical studies and field observations [1] in order to

establish laws and parameters of coal and gas outbursts due to the kinetics of

desorption of methane from coal at their destruction.

It was established [2] that the amount of gas (cm3/g), which can be released

from coal during its destruction, depends not only on its fracturing, but also on itssorption properties.

This relation is expressed by the equations

Q

q bi

i i 100, (1)

q t a i , (2)

( ) t e r

1

6 12

122

, (3)

r

D

R A

i ti

2

2 , (4)

612 K. Rustam et al.

where Qi - amount of methane that is desorbed from the coal at a pressure drop on

its surface from the original to atmospheric, m3;



qi - amount of methane that is desorbed from one ton of coal being composed

of gas particles with the mean radius Ri , m3/t;

bi - percentage of fractions of mean radius Ri in the coal, it is determined by

the screen test of coal, %;

δ (t) - ratio of gas that is desorbing in a time t to sorption capacity of coal, it

is calculated on the basis of sorption characteristics of coal as determined by

experiment;

ν - sequential term of series;

Di - average coefficient of methane diffusion in coal for this change in

pressure at the sorption unit IGD (Institute of Mining, Moscow) [3], m2/sec;

A - adsorption coefficient of methane on coal:

A a / Q ; 0 (5)

Q П p p 0 0 / , (6)

a - sorption capacity of coal (m3/t), it is determined at the sorption unit;

П - coal porosity (m3/t), it is determined from true and apparent specific

gravities of coal;

p/p0 - original pressure of methane in cola taken relative to atmospheric;

α - compression degree of methane.

Accordingly, the amount of methane that can be released from the coal bed in

the boundaries of formed gas collector on seam failure and simultaneous drop of

gas pressure on the surface of the fractured coal from the original to atmospheric

(m3/t), can be calculated by the formula

Q mQV m i i уг i / , (7)

where Q i - amount of methane desorbing from 1 ton of coal from each split with

the thickness m i , m3;

m i - thickness of separate splits of coal making the bed, m;

V у г - volume of desorbing coal in the collector boundaries, m3 ;

- volume density of coal, t/m3.

However, in the calculation of the amount of gas in the gas collector, in

addition to methane desorbing in on the coal failure, it is necessary to account for

the free gas being in the pores of coal and taking part in the expansion:

Importance of the Gas Factor in the Process of Formation of Outburst Zones 613Qобщ Q Q0 , (8)

where Q0 - volume of free gas releasing from coal is calculated by the formula

(6), m3.

The volume of free gas Q0 can also be set as equal ~10÷15% of the potential

methane retention capacity (at PГ = 5,0 MPa).

The pressure of desorbing gas from the coal seam P in the zone of influence of

the rock drift and accumulating in the formed cavities of the gas collector

primarily depends on the amount of gas liberated Qtotal , and the total volume of

cavities (pores and cracks) in the boundaries of the gas collector.

To calculate the pressure of the free gas in the collector we use an equation that

combines the gas laws Boyle-Mariotte and Gay-Lussac:

PV

T PV

T

П 0 0

0

, (9)

where V 0 - gas space under the pressure P 0 =101,325*10-3 MPa and at the

temperature T 0 = 2730 K,

V П - gas space under the pressure P at the temperature T , we accept T = 3030К.

The methane volume V 0 under normal conditions ( P 0 and T 0 ) is calculated by



the Mendeleev-Clapeyron equation:

PV

m

M

RT , (10)

where m - substance weight, g

m m Qобщ 0 . (11)

m 0 - volume density of gas under P 0 and at T 0, m0 CH4 = 0,717 g/m3 ;

M - molar mass of substance, M CH4 = 16 g/mol;

R – universal gas constant, R = 8,314 J /mK.

Thus,

V

mRT

MP 0

0

0

, m3 (12)

Substitution the known values m0 CH4 = 0,717 g/m3 , M CH4 = 16 g/mol, R = 8,314

J /mK, P 0 =101,325*10-3 MPa, T 0 = 2730К, into the equation (12), we obtain the

dependence of methane space V 0 , under normal conditions on its amount Qtotal :

V 0 Qобщ 1,004 , m3 (13)614 K. Rustam et al.

In subsequent calculation the coefficient thrown away, assuming that V 0 =

Qtotal .

Knowing that the coal volume in the boundaries of the gas collector V уг, we get

free gas space, i.e. the volume of pores and cracks V П . As mentioned above, the

porosity of the fractured, prepared coal can be up to 0,1 m3/t and more. Therefore,

in the calculation we can accept V П = 0,1V уг . Translating the formula (9) and

substituting the known values of variables, we get the pressure of free gas in the

gas collector that was generated in the coal seam in the zone of influence of the

rock drift:

P

PV TV П Т

0 0

0

, (14)

Considering that we know the numerical values P 0 =101,325*10-3 MPa, T 0 =

2730К, T = 3030К, substituting them in (14), we get:

P

wQ

mSобщ

i K

, (15)Where mi - thickness of coal split (layer), m

S K - area of gas collector in the plan, m2;

w - gas pressure factor, accounting for the change in V and t 0, for

methane w = 11,25*10-3,MPa.

The obtained dependences made it possible to determine the gas pressure in the

gas collector on coal samples that were selected in the lower layer of the bed d6 of

the Eastern wing of the V.Lenin mine. The total weight was 3450 g, the amount of

factions – nine. The calculation results are shown in the Table 1.

These studies formed the basis of calculation methods of the gas volume



liberated and pressure under which the gas was in the collector, with the fractional

composition of the coal mass in the collector.

According to the the analysis of the research results and calculations made it

was found that the fractional composition of coal has a great impact on gas

emission of coal and as a consequence on the pressure. On this basis we made the

series of calculations of the specific energy per unit volume of the rock mass with

different fractional composition. The Table 2 below shows the results of

calculation of the parameters of the forming gas-coal collector with the critical

fractional composition: d = 0,25 mm (content 57%), d = 0,5 mm (content 15%), d

= 0,8 mm (content 15% ), d = 1,6 mm (content of 5%) at the rock volume of 200

m3 of various porosities and conditions of collector existence during 10 minutes.

The figure 1 shows aerodynamic characteristics of the parameters of the gas

collector formed by the joint influence of rock pressure and gas factor.Importance of the Gas Factor in the Process of Formation of Outburst Zones 6150

50

100

150

200

250

300

350

400

0 1 2 3 4 5 6 7 8P,

Q, 3 /

0,2 0,5 0,25 0,3 0,4

*10-1,

P Fig.1 Aerodynamic characteristics of the parameters of gas collector616 K. Rustam et al.

Table.1 Results of screen test of the coal samples of the lower layer of the bed d 6 of the Eastern wing of V. I.

Lenin mineFraction size, m < 0,00025 0,00025-0,0005

0,0005-0,001 0,001-0,0020,002-

0,0030,003-

0,0050,005-

0,0070,007-0,01> 0,01R i, mm 0,125 0,375 0,75 1,5 2,5 4,0 6,0 8,5 20,0g % g % g % g % g % g % g % g % g %

bi 238 6,9 693 20,1 1016 29,4 282 8,2 207 6,0 255 7,4 139 4,0 337 9,8 283 8,2

(10) 0,121 0,0668 0,066 0,0584 0,058 0,0579 0,05789 0,05787 0,05786

qi bi 17,12 26,85 35,28 9,5776 6,96 8,569 4,631 11,343 9,488

(20) 0,124 0,0668 0,0624 0,059 0,0583 0,058 0,0579 0,0579 0,0578

qi bi 17,120 26,853 36,685 9,676 6,992 8,586 4,634 11,346 9,489

(60) 0,283 0,105 0,071 0,061 0,059 0,058 0,058 0,058 0,058qi bi 39,087 42,229 41,798 10,048 7,091 8,634 4,646 11,36 9,491

(600) 0,726 0,297 0,156 0,089 0,070 0,063 0,060 0,059 0,058qi bi 100,155 119,450 91,785 14,599 8,377 9,270 4,800 11,550 9,52

(104) 1,000 0,866 0,548 0,303 0,189 0,123 0,090 0,075 0,061qi bi 138,0 348,08 322,14 49,640 22,637 18,174 7,215 14,663 10,010Calculation of gas pressure P(t) in the collector depending over time t, sec.:P(10) = 6,1 MPa;P(20) = 6,13 P;

P(60) = 6,76 P;P(600) = 9,61 P;P(104) = 17,82 P.



Importance of the Gas Factor in the Process of Formation of Outburst Zones 617

Table 2 Results of gas emission and specific power consumption calculationPorosity

, 3/ Free gas volume

Q0, m3

Coefficient of methaneadsorption, Total gas emission

Q, m3

Gas pressure , atm

Specific power consumption

, J/m3

0,086 2,88 6,94 2659 86,4 0,18

0,100 3,37 5,94 3250 105,5 0,28

0,150 5,05 3,96 3890 126,3 0,400,200 6,73 2,97 4796 155,7 0,61

0,250 8,47 2,38 5442 176,7 0,79

0,300 10,10 1,98 5666 184,0 0,85

Table 3 Basic data for selection of the gas-dynamic parameters of simulated outburstt, sec d = 0,0002 m d = 0,00025m d =0,0003 m d = 0,0004 m d = 0,0005 m, P Q, m3/s , P Q, m3/s , P Q, m3/s , P Q, m3/s , P Q, m3/s500 9,5 5,844 1,6 0,972 3,0 1,834 15,7 9,666 15,2 9,356

750 16,4 6,755 9,3 3,827 0,96 0,395 1,55 0,637 15,6 6,421

1000 20,6 6,345 13,9 4,292 7,2 2,214 4,10 1,250 0,36 0,111

1250 23,4 5,774 17,1 4,210 11,4 2,818 1,38 0,392 2,1 0,511

1500 25,5 5,240 19,4 3,985 14,6 2,991 3,60 0,743 3,7 0,755

2500 30,4 3,750 24,9 3,068 22,0 2,708 12,5 1,662 5,0 0,6173500 32,9 2,899 27,8 2,448 25,9 2,081 18,8 1,650 11,6 1,019

4500 34,3 2,352 29,7 2,030 28,4 1,948 22,1 1,549 15,8 1,082

5500 35,2 1,971 30,9 1,731 30,2 1,694 24,5 1,375 18,8 1,055

6500 35,6 1,691 31,8 1,507 31,6 1,497 26,4 1,250 21,1 1,001

7500 35,9 1,4771 32,4 1,331 32,6 1,340 27,8 1,143 22,9 0,942

618 K. Rustam et al.

The analysis of the results shown in the Table 2 and characteristics in the

Figure 1 confirms that with a fraction of less than 0.0003 m and their total content

in the gas-coal medium of the collector of not less than 57%, the potential energy

can be converted into kinetic, whereupon there is coal and gas outburst. Under

these conditions, the energy density of coal failure makes more than 0.79 MJ/m3,

what is consistent with the researches of other scientists and confirms the

appropriateness of the problem statement and the studies made by us of the

theoretical foundations of the mechanism of unloosening coal and gas outburst.

Furthermore, the results of the studies clearly indicate that a certain fractional

composition causes the occurrence of crushing waves that in turn alters the

fractional composition and consequently the process can reach the critical level atwhich the gas-dynamic conditions occur.

Conclusions1. The crushing wave has an essential role in forming of ourburst-prone situations

and this wave is characterized by speed and breaking shear stresses.

2. The minimum and maximum speeds of crushing waves at certain values of

porosity are set.

3. On the basis of the theoretical researches carried out and field studies we

determined the common factors and the parameters of occurrence of coal and

gas outbursts, driven by the methane desorption kinetics due to the coal failure.

4. We determined the pressure of the free gas in the gas collector formed in the

coal seam in the zone of influence of the rock drift and aerodynamic

characteristics of the collector formed as a result of the impact of mining

pressure and gas factor.5. It is found that with certain fractional composition of coal the potential energy

can pass into kinetic energy that causes the coal and gas outburst.

importance of the gas factor in the process

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