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Modeling temperature distribution upon liquid-nitrogen injection into a self heating coal mine goaf
Accepted Manuscript Title: Modeling temperature distributions upon liquid-nitrogen injection into a self heating coal mine goaf Authors: Guo-Qing Shi, Pengxiang Ding, Zhixiong Guo, Yan-ming Wang PII: DOI: Reference:
S0957-5820(18)31470-8 https://doi.org/10.1016/j.psep.2019.03.033 PSEP 1716
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
31 December 2018 26 March 2019 26 March 2019
Please cite this article as: Shi G-Qing, Ding P, Guo Z, Wang Y-ming, Modeling temperature distributions upon liquid-nitrogen injection into a self heating coal mine goaf, Process Safety and Environmental Protection (2019), https://doi.org/10.1016/j.psep.2019.03.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Modeling temperature distributions upon liquid-nitrogen injection into a self heating coal mine goaf
1College
of Safety Engineering, China University of Mining and Technology,
2Department
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Xuzhou, 221008, China
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Guo-Qing Shi1,3, Pengxiang Ding1, Zhixiong Guo2, Yan-ming Wang*
of Mechanical and Aerospace Engineering,
Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology,
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3State
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Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
M
A
Xuzhou 221008, China)
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Highlights
Both field test and simulation show occurrence of coal oxidation in workface #3418
Injection of liquid N2 forms a cooling zone suppressing coal oxidation
Injection of N2 reduces maximum goaf temperature preventing spontaneous combustion
Cooling zone and effectiveness are strongly affected by injection location and rate
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Abstract: To prevent spontaneous combustion in coal mines, liquid N2 was injected into goaf to decrease goaf temperature during mining. A mathematical model for calculating temperature distributions in goaf was first developed and validated by field measurements without inert gas injection. Then the model was adopted to simulate the time development of temperature distributions in a goaf with liquid N2 injection at six different locations. Simulation results show that the high temperature region is about 35-45m behind the workface on the air-return and 1
air-intake sides in the goaf without injection of liquid N2. With injection of liquid N2 the temperature contour in the goaf varies. The low temperature region influenced by the liquid N2 gradually enlarges at beginning. However, this region becomes relatively stable with continuous injection after 90 min. Further, it was found that the low temperature region (Temperature ≤300K ) influenced by liquid N2 injection depends on its injection location and
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injection flow rate. When the N2 injection entrance was located at 35m after workface on the air-return side, the final volume of low temperature region is the smallest. Meanwhile, when the injection entrance was located at 35m
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after the workface on the air-return side, the volume of region temperature <300k is the largest. Compared with the location of liquid N2 injection entrance, flow rate of liquid N2 flow has a more significant effect on the distribution
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of cryogenic area. Simulation result show that the volume of cryogenic space increases significantly with the
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increase of perfusion flow rate. And, the reduction range of maximum temperature and its influence factors was
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also be discussed in this paper. We find out that the reduction of the maximum temperature increased with the
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distance from workface in goaf. Compared with N2 injection on air-intake side, reduction extent of maximum
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temperature is larger when the injection entrance is on air-return side. This study provides a quantitative assessment
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for preventing coal oxidation and spontaneous combustion using liquid N2 technology.
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Keywords: Temperature distribution; Liquid N2 injection; Numerical simulation; Goaf; Coal spontaneous combustion.
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Nomenclature
a0
Attenuation rate in the tendency direction
a1
Attenuation rate in the strike direction
b0
Adjusting parameters in the strike direction
2
b1
Adjusting parameters at the tendency
C
Mass concentration (%)
cp
Specific heat capacity, (J/(kg℃)) Vector of gravity (m/s-2)
H
kf
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Height (m)
Initial caving coefficient
K p,min
Coefficient of bulk increase
k
2 Permeability (m )
k0
Base permeability (m2)
N
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K p,max
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heat transfer coefficient (W/(m·K))
q
Thermal source (J/m3S)
ST
Time (s)
u
Velocity vector
x, y, z
Spatial coordinates
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t
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3 SO 2 Source term of oxygen (mol/m S)
M
A
heat release when coal oxidation consumed one molar oxygen (J/mol )
Greek symbols
Porosity
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x
Adjusting parameter
r
Density of the gas mixture (kg/m3)
1. Introduction 3
Spontaneous combustion of coal is a severe issue that threatens the development of coal mining industry worldwide
[1,2].
Among China’s state-owned collieries, 56% of the mines have been jeopardized by spontaneous
combustion, and the combustion incidents in these mines account for 90–94% of all coal mine fires. Coal loss caused by coal spontaneous combustion in China is about 200 million tons per year, and coal spontaneous
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combustion also easily causes gas explosion, which poses a serious threat to workers’ lives and safety [3]。 In the US, Indian, and Australian coal mines, most fires were also caused due to spontaneous combustion [4-9].
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The occurrence of spontaneous combustion of coal is a complex physicochemical process[10,11]. As we know oxygen and heat accumulation are two necessary conditions for coal spontaneous combustion [12,13], so there are two
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common ways to prevent or delay coal spontaneous combustion. One way is to control and reduce oxygen
[14,15].
The other way is to decrease the goaf temperature and keep it always
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combustion would not occur easily
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concentration. If the oxygen were not enough, coal oxidation which is the initial stage of coal spontaneous
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below coal ignition point, it also can delay the occurrence of coal spontaneous combustion[16].
injected into a goaf
[17-19].
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To realize simultaneous reduction of both oxygen concentration and coal temperature, nitrogen could be In order to improve the efficiency of fire prevention and extinguishing by inert gas
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injection in to mine goaf, many experts and scholars have studied the variation of oxygen concentration in the coal
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spontaneous combustion zone when nitrogen is injected. However, little is known about the relationship between cooling effect of goaf and filling position or injection flow rate. The influence zone with injection of liquid N2 in goaf is quantitatively undetermined. In order to achieve a better cooling effect and prevent spontaneous combustion
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of coal, it is necessary to study the distribution law of goaf temperature when liquid nitrogen is injected into the goaf. In this paper, we preset temperature sensor to test the distribution of temperature in the mine goaf, and obtained the law of temperature distribution in goaf. And then we developed a 3-D mathematical model to
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investigate quantitatively the distribution of goaf temperature with injection of liquid N2 into a mine goaf. In this model, the coupling between the chemical reactions in the coal seam and O2 gas transport through the adjacent rocks was taken into account. The distributions of temperature in the goaf were obtained though simulation. We also examined the injection location and flow rate effects of temperature distribution in the goaf. The results were
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of significance for assessing the usefulness of liquid N2 injection on the prevention of spontaneous combustion.
Air return
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2. Field Measurement
#1measur i ng poi nt
Thermodetector Wire
#4 measur i ng poi nt
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Air intake
N
#3 measur i ng poi nt
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#2 measur i ng poi nt
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Fig. 1 Schematic diagram of field experiment
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Field measurement was carried out at 3418 workface in Liangbaosi coal mine in Shandong Province. It is a fully mechanized top coal caving workface, the mining elevation level of 3418 workface varies from -691m to
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-871m. The thickness of the coal seam is between 1.3~9.2m, with an average thickness of 6.35m. The mining height was 3.0m, average height of the top coal caving was 3.35 m. The length and width of the workface are
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2113m and 100m, respectively. The slope angle of the coal seam varies from 4°to 15°, with an average angle of 8°. The coal seam was low metamorphic bituminous coal, with easy occurrence of spontaneous combustion. The air quantity of mining workface is about 800m3/min. with U-type ventilation mode. And the ambient temperature is around 27℃, 300k. In order to study the goaf temperature distribution, AD590 was used as temperature senor, located at four 5
measuring points(#1-#4), #1 is located at air-return side, #4 is located as air intake side, #2 and #3 measuring point is 33m away from air-return and air- intake side. Thermoscope was placed at air intake laneway, temperature senor and thermoscope were connected by wires. The system of temperature measuring was showed as fig.1.
3. Mathematical Models of Heat Transfer in goaf
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In mine goaf, the coal absorbed oxygen and release heat[20]. Meanwhile, the air leak takes away some heat, temperature distribution in goaf is a very complicated problem, influenced by air flow and heat conduction[21-23].
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Usually, there are three basic ways of heat transfer in porous media area like mine goaf, such as heat conduction, convection and radiation[24,25]. However, there is no severe oxidation of coal in the early period of self-heating, and
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the difference of temperature between the caving body and its surrounding environment is not significant.
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Therefore, only heat conduction and convective heat transfer are considered in the study of heat transfer mode in
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the early period of self-heating, and heat radiation is not considered in this model. It is assumed that the porous
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media in the goaf are isotropic and satisfy local thermal equilibrium, i.e., the temperature of solids and fluids is the
cp
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same. The energy conservation equation for gas in goaf is[25]
T c p (Tu) k f (T ) ST t
ST
cp
is specific heat capacity, J/(kg℃), T is the temperature, k f is the thermal conductivity,
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In this equation,
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(4)
is thermal source, which contains the energy releases by coal oxidation, ST is described by,
ST q So 2
(5)
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Where q is thermo release by coal oxidation when coal oxidation consumed unit mass oxygen, So 2 is the
negative source term of oxygen which is caused by coal oxidation. It is could be described by the chemical reaction equation[26,27]. The definite solution condition of the energy conservation is that (1) Initial condition, 6
T t 0 t0 And t0 is the temperature of surrounding rock, 300K. (2) Boundary condition Using Neumann boundary condition which is specified by boundary heat flux as follow
t q n
( x, y, z )
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k f
The mass conservation equation, momentum conservation equation of gas movement in goaf and the
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expression of gas source term, Momentum source term can be seen in literature[27].
Permeability is the main parameter for solving the flow and heat transfer problems in mine Goaf. According to
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Wolf and Bruining, the permeability and porosity of goaf are the main factors that affect the gas flow and gas
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concentration distribution in goaf, and then affecting coal oxidation rate, heat generation and loss in goaf[28]. When
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the gas transfers in goaf, it is hindered with the inertia resistance and viscous resistance which are influenced by
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goaf permeability. According to the Carman-Konzeny equation, permeability of goaf can be described as the
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following equation:
Where
k0
(7)
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k0 x3 k= × 0.241 (1- x )2
is the base permeability of the broken rock at the maximum porosity and it was taken as 1x10-3m2,
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which places it the “open jointed rock” range according to Hoek and Bray [29]. The porosity model of a goaf is[27]
ìï é -ea x+b é x = í1- ê K p,min + ( K p,max - K p,min ) ´ expê-a1 ´ ( y + b1 ) ´ 1- e ( ) ë ïî ë
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(
0
0
)
-1
ùù ûúúû
üï z ý ´ (1- ) h ïþ
(8)
Where K p,max is the initial caving coefficient of bulk increase and its value is 1.6; K p,min is the coefficient of bulk increase in compaction and its value is 1.1; a0 and a1 are the attenuation rate in the tendency and strike direction, respectively, and their values are 0.0368 and 0.268;
e 1is the adjusting parameter and its value is 0.233, b0 and b1 7
are the adjusting parameters in the strike direction and at the tendency, and their values are 0.8 and 15; H is the height of goaf model.
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4. Physical model
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Fig. 2 The goaf model used in the present simulation
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According to the field situation of #3418 workface, the simulation model is established for the present
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simulation. The goaf dimension is 180m long (L), 100m wide (W), and 40m high (H). The cross section size of the laneway is 4m × 4m, and the cross section size of the workface is 4m × 8m. The length of the laneway is 30m. The
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coordinate origin is located at the junction of the workface and the goaf on the air-intake-side. Liquid N2 injection
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was considered from one of the six injection ports from either the air-intake or air-return side, with distance from the workface varying from 15, 25, to 35m, respectively. The serial number of injection entrance are P1, P2, P3 at
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air-intake side, and P4, P5, P6 at air-return side, showed by fig.2. Since it is a fully mechanized top coal caving workface, we assumed that 30% of the coal was left in goaf, that is to say that the thickness of coal remaining in the
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goaf is about 1.9m. The air quantity of the mining workface is set as 800m3/min, O2 molar concentration in the fresh airflow is 20.95% (equals to mass concentration at 23%). The initial temperature of goaf is set to ambient temperature 300K (27℃).
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Fig.3 Flow chart of liquid nitrogen injection Fig.3 is a process flow chart for preventing and extinguishing fire with liquid nitrogen injection into
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underground goaf. The nitrogen injection process is as follows: the liquid outlet of the storage tank, the liquid nitrogen delivery pipeline,, and the injection port. The liquid nitrogen reservoir is close to the outlet in goaf, which can effectively increase the perfusion rate of liquid nitrogen and make nitrogen perfuse into the goaf with low
N
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temperature, thus significantly improving the cooling effect in the goaf. In the present simulation, the liquid N2 is
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gasified in the air-intake or air-return laneway, and then the low temperature N2 gas was injected into goaf from the
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steel pipe (the pipe length is about 50m). Since N2 gasification temperature is about 80K(-193℃), after 50m pipe flowing the temperature would increase to about 156.8K. Thus, the temperature of N2 gas when it is injected into
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the goaf was set at 156.8K in the simulation.
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The simulation model was meshed by tetrahedron grids, with a total meshes of 543,874. The goaf model under this mesh sizes gives a satisfactory convergence.
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5. Results and Discussion 5.1 Field measured data
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In order to obtain the temperature distribution, and also make verification to the following simulation. With the temperature measuring system, temperature of #1 - #4 measuring point was measured every day during mining process of this workface, and the variation of temperature in goaf with the mining distance is obtained, temperature data showed by Tab.1 and Fig. 4. Table 1 Observation data of temperature at each measuring point
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4#
31 34 38 41 45 46 49 47 45 44 41 39 40 39 37 36 34 34 34 33 32 32 32 30 30 30
34 39 40 44 47 48 52 49 44 41 39 36 34 33 32 31 30 30 28 28 28 29 29 29 29 28
30 31 31 32 34 38 44 51 46 41 36 34 33 33 32 34 33 33 34 33 33 33 32 31 31 31
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123.8 128.3 132.8 138.0
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144.0 55
45
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#1 #2 #3 #4
40
A
Temperature(°C)
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50
N
A
76.5 82.9 88.5 94.9 100.5 106.9 113.3 118.5
31 30 32 30 30 32 32 31 30 29 28 29 29 32 31 31 29 30 29 29 28 29 29 29 29 29
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4.5 9.8 15.8 21.8 27.8 33.8 39.8 45.8 51.8 55.5 60.0 65.3 70.5
35
30
25 0
20
40
60
80
100
120
140
160
Distance from workface(m)
Fig.4 Temperature at measuring point variations with distance from workface 10
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3#
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Distance from Temperature/℃ workface/m 1# 2#
Fig.4 shows the temperature variations with the distance from workface. We can find that temperature at #3 and #1 is higher than temperature at the other two points. At beginning, the temperatures of measuring points are basically the same with environmental temperature except #3. Temperature at #3 point is higher than temperatures at other point, it means that the location of #3 measuring point is conducive to coal oxidation and thermal storage.
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At point #1, though temperature is same as environmental temperature, as the distance from the workface increasing, the temperature increases faster, it is only slightly lower than the temperature at point 3. Temperature at
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#2 and #4 is lower than the temperature at #1 and #3 testing point.
From the curve, we also can find out that the phenomenon of coal oxidation self-heating in goaf is very
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obvious, no matter which point, the temperature increases with the distance from the working face, and then
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gradually decreases to the surrounding rock temperature. The reason is that air leakage intensity decreases as the
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distance from the working face increases. Initially, the strength of the air leakage is sufficient to provide oxygen
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needed for the oxidation of coal in goaf, when the air leakage intensity decreases, the heat carried away by the air
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leakage decreases and the thermal storage environment becomes better, so the temperature of the goaf keeps rising at beginning. And then air leakage decreased continues to decrease. Oxygen provided by air leakage couldn’t
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maintain coal oxidation, and the intensity of heat generation decreases. Affected by heat dissipation, the
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temperature gradually decreases to the surrounding rock temperature. At point #3, when the distance between measuring points from 0m increased to 39.8m, temperature increases from 34℃ to 52℃, this the maximum value of temperature. At point #1, temperature increased to 49℃ when the distance from workface is 39.8m. For point #4,
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temperature increased from 30℃ to about 40℃ when the distance increased to about 45.8m. At measuring point #2, the temperature didn’t change significantly. It means that the degree of coal oxidation near this measuring point #2 is not obvious.
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5.2 Simulation of temperature distributions without liquid N2 injection
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Fig. 5 Simulated temperature distribution in the goaf 311
324 322 310
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320
314
N
316 140
160
310 308
#4 #3 #2 #1
306 304
300 -20
0
20
ED
302
40
60
80
180
A
312
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Temperature(K)
318
100
120
140
160
180
200
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Distance from workface(m)
Fig.6 Simulation temperature results at four measuring points
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As Fig.5 and Fig.6, simulation shows that the temperature increases at first and then decreases as the distance from the workface increases. The high temperature area in the goaf is 20m behind the workface, with the highest
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temperature points distributed on the air-return side and air-intake side. The reason is that temperature increase is determined by the heat release from the coal oxidation and the heat dissipation caused by the air leaking flow. In the workface or nearby, since the air flow is strong, though coal oxidation is quick, the heat dissipation caused by the air flow is also strong, and thus, the temperature is not so high. With increasing distance from the workface, the air leaking flow turns weak, heat dissipation would be weak, the temperature would be a little higher. At the area 12
where the distance is far from the workface that the leaking air flow is weak and could not provide sufficient oxygen for coal oxidation, there is no heat releasing and the temperature will turn down gradually. And the figure also show that the temperature on both the air-return and air-intake sides is higher than it is in the middle part. The reason is that, affected by supporting action of surrounding coal wall, the porosity on the air-return and air-intake
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sides is larger than that in the middle . The air leaking is strong so the oxygen concentration is higher, and the speed
325
320
320
Simulation data
310
Field test data
310
305
300 40
60
80
100
120
140
160
0
PT
315
310
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305
20
40
60
Simulation data
100
Field test data
315
120
140
160
Simulation data
310
305
Field test data
300 80
100
120
140
160
0
20
Distance from workface(m)
(c) #3 Measuring point
80
320
300
0
60
Distance from workface(m)
325
Temperature(K)
320
40
(b) #2 Measuring point
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(a) #1 Measuring point
325
20
M
A
20
Distance from workface(m)
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Field test data
300 0
Temperature(K)
Simulation data
N
305
315
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315
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325
Temperature(K)
Temperature(K)
of coal oxidation is high. Thus, the heat release would be strong.
40
60
80
100
120
140
160
Distance from workface(m)
(d) #4 Measuring point
Fig.7 Comparison between simulation and test data for temperature at different location The temperature measured at #1, #2, #3 and 4# measuring point were compared to the calculated value from the simulation, shown in Fig. 7. Though there is some difference between simulation result and field measuring
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data, especially at #2 point, the reason is that the stochastic distribution of porosity caused by irregular caving of top rocks which is difficult to express by mathematical model was not considered during this simulation model. In general, the variation trend of simulated temperature field is consistent with the measured data. Such as, the high temperature regions are located in the intake and return air sides in the goaf, respectively, and mainly distribute
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30m to 45m after workface. The numerical model is then validated and used for extensive parametric studies involving injection of liquid N2. For all the simulations thereafter, we kept the same airflow conditions as specified
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before. The volume flow rate of the liquid N2 injection is 12m3/min, 720m3/h.
5.3 Evolution of temperature and oxygen distributions with liquid N2 injection
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Field measurement and numerical simulation result all show that there are high temperature zones in the mine
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goaf caused by coal self-heating. The high temperature reached more than 50℃, it is very close with the critical
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temperature of coal spontaneous combustion which is generally from 60 to 80℃. It is very necessary to adopt the
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cooling technology to reduce the goaf temperature and prevent the coal from oxidizing and spontaneous
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combustion.
(b) 50min
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(a) 5min
(c) 90min
(d) Steady state
Fig. 7 shows the time development of the temperature distribution at the ground level with continuous liquid
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N2 injection using injection entrance P1. It is found that liquid N2 the injection changes the temperature distribution in the goaf. At the beginning, the goaf area where temperature is influenced by liquid N2 was small. It enlarges gradually. The temperature distribution at 90 min（fig.7 c）of continuous injection is very similar to the steady-state solution（fig.7 d）. That means the temperature distribution becomes stable 1.5 hours after the continuous injection.
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The simulation result also shows that the highest temperature decreased from 326.2k( 53.2℃) to 324.9k (51.9℃), the reason is that first heat released by coal oxidation was took away by low temperature. Another reason is that
intensity of heat generation. As result the high temperature decreased.
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A
N
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5.4 Cooling effects of different injection locations
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nitrogen reduces the concentration of oxygen in the goaf, and slows down the coal oxidation rate, reduces the
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(a) 15m after workface at air-intake side (Steady state)
(b) 25m after workface at air-intake side (Steady state)
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A
N
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(c) 35m after workface at air-intake side (Steady state)
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PT
ED
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(d) 15m after workface at air-return side (Steady state)
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(e) 25m after workface at air-return side (Steady state)
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(f) 35m after workface at air-return e side (Steady state)
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Fig. 9 Steady state temperature distributions when liquid N2 was injected from different locations
Fig. 9 shows the contour pictures of temperature distribution when the liquid N2 was injected from different
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locations with steady state. From the contrasts of these pictures we can see that no matter where the injection
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locations is, injection of liquid N2 can always decrease the temperature surrounding the injection location. The
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temperature distribution is not the same for different injection entrances. N2 injection would reduce the local
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injection temperature as well as the area of the spontaneous combustion dangerous zone. However, the temperature
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in the workface is not much influenced by the liquid N2 injection locations studied with flow rate of liquid N2 is
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3
Volume of region where temperature <300K(m )
720m3/h.
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2000
1500
1000
500
0
P1
P2 P5 P3 P4 Location of the injection entrance
P6
Fig. 10 Volume of region where temperature <300k in the goaf with different injection entrance
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When the liquid N2 was used for coal spontaneous combustion prevention and controlling, the goaf temperature and oxygen distributions influenced by the injection should be inspected. In order to exam the cooling effect due to liquid N2 injection, the volume of region where temperature <300k which is the temperature of underground environment in the coal mine was discussed. Since the coal was just in the bottom with a height of 3m,
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we just discuss the volume of region where the temperature is below 300K at the coal area. Fig. 10 plots the volume of region where temperature <300k at the goaf coal area with different liquid N2
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injection locations. It is seen that when the location of entrance at air return side, the volume of region where temperature below 300k is bigger than that when location is at air-intake side. The reason is that the air leakage
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intensity in the goaf is relatively high on the air-intake side, and after the air leakage is cooled down, it flows back
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to the working face again, which may affects the cooling effect in goaf.
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From the comparison of simulation result we also can find out that the N2 injection entrance was located at
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35m (Injection entrance P3) after workface on the air-return side, the final volume of region with temperature<300k
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is the smallest. Meanwhile, when the injection entrance was located at 35m (Injection entrance P6) after the workface on the air-return side, the volume of region temperature <300k is the largest. In other words, its cooling
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effect is the best among the cases studied.
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In fact, as well as the volume of low temperature area, the reduction range of maximum temperature is equally important for characterizing the cooling effect of liquid nitrogen in goaf and preventing the occurrence of coal spontaneous combustion. Fig. 11 is the highest temperature value in goaf when liquid nitrogen is injected into goaf
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with different positions. From the data in the figure, it can be seen that no matter where the injection entrance is the highest temperature in goaf will all be reduced by liquid nitrogen injection. Through the data bar on the left side of the cloud picture, we can see that the maximum temperature in the goaf are 324.9K, 324.4K, 324.3K, 324.3K, 323.8K and 322.6k when liquid N2 was injected with P1-P6 port
18
respectively, these values are below the temperature without nitrogen injection. It can be seen that by the influence of endothermic cooling and the slowdown of chemical reaction caused by the decrease of oxygen concentration, the maximum temperature in goaf decreases slightly, and also the possibility of spontaneous combustion of coal decreases. By comparing the maximum temperature in different simulation condition, we can find that the
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reduction of the maximum temperature increased with the distance from workface in goaf. Compared with N2 injection on air-intake side, reduction extent of maximum temperature is larger when the injection entrance is on
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air-return side.
5.4 Cooling effects with different injection flow rate
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In order to discuss the influence by liquid N2 injection volume on cooling effects, we simulated the
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temperature distribution when liquid N2 injection with different volume. Table.2 and Fig. 11 shows the volume
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value of region where the temperature below 300k when the liquid N2 was injected with different volume at
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injection entrance P1-P6, with flow rate of N2 are 360m3/h, 720m3/h, 1080m3/h and 1440m3/h, respectively.
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Table.2 Volume of region where the temperature below 300k 360m3/h
720m3/h
1080m3/h
1440m3/h
P1 P2 P3 P4 P5 P6
465.614 265.996 102.701 772.288 930.597 208.133
1001.86 892.665 733.62 1882.79 2000.48 2014.19
1655.04 1613.56 1657.78 2153.57 2728.19 3373.41
2155.86 2255.3 2413.45 2712.57 3231.63 4434.41
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Injection entrance
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P1 P2 P3 P4 P5 P6
4000
3000
2000
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1000
0 200
400
600
800
1000
1200
1400
1600
3
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3
Volume of region where tamperature below 300k(m )
5000
Flow rate of injection(m /h)
Fig. 11 Volume of region where temperature <300k with different injection entrance and flow rate
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From the data in Fig. 11 and Table 2, it can be seen that the volume of low temperature region in goaf is
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significantly affected by the perfusion flow rate, and the influence has a strong regularity. It shows that the volume
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of cryogenic space increases significantly with the increase of perfusion flow rate. When the flow rate of low temperature N2 is increased from 360 m3/h to 1440 m3/h using P6 injection entrance, the volume of low
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temperature zone（below 300K） in the goaf increases from 208.1 to 4434.41 m3, increased more than 20 folds.
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Through these simulations, we also can found out that when the injection entrance is located in the high temperature zone of coal oxidation in goaf, the volume of low temperature space（below 300K） is very small. For
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example, when the port P6 is used for injection with flow rate of 360m3/h, the volume of space where temperature is below 300 K is only 208.133 m3. When the port P3 is used for filling with flow rate of 360m3/h, the volume of
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space where temperature is below 300 K is only 102.70m3. This is because the outlet of perfusion is near the core high temperature area of the whole goaf. After low temperature nitrogen comes out of the pipeline, the temperature rises faster and the cooling range (low temperature space) is smaller.
6. Conclusions Simulations of gas and temperature distribution in a goaf were carried out. The following main conclusive 20
remarks can be drafted: (1) Influenced by coal oxidation, the high temperature of goaf could be more than 50℃, the high temperature area is found to be 35-45m behind the workface in the goaf without N2 injection. (2) Liquid N2 injection changes the local temperature distribution around the injection port. The temperature influential area due to liquid N2 injection enlarges gradually. After 90 min of continuous injection, however, the influential zone becomes stable. That is, the temperature distribution in the goaf stabilizes 1.5 hours after the
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continuous injection.
(3) The low temperature region (Temperature ≤300K ) influenced by liquid N2 injection depends on N2
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injection location, when the N2 injection entrance was located at 35m after workface on the air-return side, the final volume of low temperature region is the smallest. Meanwhile, when the injection entrance was located at 35m after the workface on the air-return side, the volume of region temperature <300k is the largest among the cases studied
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in the paper.
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(4) Low temperature region distribution was also influence by liquid N2 injection flow rate. The volume of
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cryogenic space increases significantly with the increase of perfusion flow rate. When the flow rate of low temperature N2 is increased from 360 m3/h to 1440 m3/h, the volume of low temperature zone（below 300K） may
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increase more than 20 folds.
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(5) The reduction range of maximum temperature and its influence factors was also be discussed in this paper. We find out that the reduction of the maximum temperature increased with the distance from workface in goaf. Compared with N2 injection on air-intake side, reduction extent of maximum temperature is larger when the
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injection entrance is on air-return side.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51774274) and the
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Fundamental Research Funds for the Central Universities (2017XKQY026).
21
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S0957-5820(18)31470-8 https://doi.org/10.1016/j.psep.2019.03.033 PSEP 1716
To appear in:
Process Safety and Environment Protection
Received date: Revised date: Accepted date:
31 December 2018 26 March 2019 26 March 2019
Please cite this article as: Shi G-Qing, Ding P, Guo Z, Wang Y-ming, Modeling temperature distributions upon liquid-nitrogen injection into a self heating coal mine goaf, Process Safety and Environmental Protection (2019), https://doi.org/10.1016/j.psep.2019.03.033 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Modeling temperature distributions upon liquid-nitrogen injection into a self heating coal mine goaf
1College
of Safety Engineering, China University of Mining and Technology,
2Department
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Xuzhou, 221008, China
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Guo-Qing Shi1,3, Pengxiang Ding1, Zhixiong Guo2, Yan-ming Wang*
of Mechanical and Aerospace Engineering,
Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology,
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3State
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Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
M
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Xuzhou 221008, China)
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Highlights
Both field test and simulation show occurrence of coal oxidation in workface #3418
Injection of liquid N2 forms a cooling zone suppressing coal oxidation
Injection of N2 reduces maximum goaf temperature preventing spontaneous combustion
Cooling zone and effectiveness are strongly affected by injection location and rate
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PT
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Abstract: To prevent spontaneous combustion in coal mines, liquid N2 was injected into goaf to decrease goaf temperature during mining. A mathematical model for calculating temperature distributions in goaf was first developed and validated by field measurements without inert gas injection. Then the model was adopted to simulate the time development of temperature distributions in a goaf with liquid N2 injection at six different locations. Simulation results show that the high temperature region is about 35-45m behind the workface on the air-return and 1
air-intake sides in the goaf without injection of liquid N2. With injection of liquid N2 the temperature contour in the goaf varies. The low temperature region influenced by the liquid N2 gradually enlarges at beginning. However, this region becomes relatively stable with continuous injection after 90 min. Further, it was found that the low temperature region (Temperature ≤300K ) influenced by liquid N2 injection depends on its injection location and
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injection flow rate. When the N2 injection entrance was located at 35m after workface on the air-return side, the final volume of low temperature region is the smallest. Meanwhile, when the injection entrance was located at 35m
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after the workface on the air-return side, the volume of region temperature <300k is the largest. Compared with the location of liquid N2 injection entrance, flow rate of liquid N2 flow has a more significant effect on the distribution
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of cryogenic area. Simulation result show that the volume of cryogenic space increases significantly with the
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increase of perfusion flow rate. And, the reduction range of maximum temperature and its influence factors was
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also be discussed in this paper. We find out that the reduction of the maximum temperature increased with the
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distance from workface in goaf. Compared with N2 injection on air-intake side, reduction extent of maximum
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temperature is larger when the injection entrance is on air-return side. This study provides a quantitative assessment
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for preventing coal oxidation and spontaneous combustion using liquid N2 technology.
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Keywords: Temperature distribution; Liquid N2 injection; Numerical simulation; Goaf; Coal spontaneous combustion.
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Nomenclature
a0
Attenuation rate in the tendency direction
a1
Attenuation rate in the strike direction
b0
Adjusting parameters in the strike direction
2
b1
Adjusting parameters at the tendency
C
Mass concentration (%)
cp
Specific heat capacity, (J/(kg℃)) Vector of gravity (m/s-2)
H
kf
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Height (m)
Initial caving coefficient
K p,min
Coefficient of bulk increase
k
2 Permeability (m )
k0
Base permeability (m2)
N
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K p,max
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heat transfer coefficient (W/(m·K))
q
Thermal source (J/m3S)
ST
Time (s)
u
Velocity vector
x, y, z
Spatial coordinates
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t
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3 SO 2 Source term of oxygen (mol/m S)
M
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heat release when coal oxidation consumed one molar oxygen (J/mol )
Greek symbols
Porosity
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x
Adjusting parameter
r
Density of the gas mixture (kg/m3)
1. Introduction 3
Spontaneous combustion of coal is a severe issue that threatens the development of coal mining industry worldwide
[1,2].
Among China’s state-owned collieries, 56% of the mines have been jeopardized by spontaneous
combustion, and the combustion incidents in these mines account for 90–94% of all coal mine fires. Coal loss caused by coal spontaneous combustion in China is about 200 million tons per year, and coal spontaneous
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combustion also easily causes gas explosion, which poses a serious threat to workers’ lives and safety [3]。 In the US, Indian, and Australian coal mines, most fires were also caused due to spontaneous combustion [4-9].
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The occurrence of spontaneous combustion of coal is a complex physicochemical process[10,11]. As we know oxygen and heat accumulation are two necessary conditions for coal spontaneous combustion [12,13], so there are two
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common ways to prevent or delay coal spontaneous combustion. One way is to control and reduce oxygen
[14,15].
The other way is to decrease the goaf temperature and keep it always
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combustion would not occur easily
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concentration. If the oxygen were not enough, coal oxidation which is the initial stage of coal spontaneous
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below coal ignition point, it also can delay the occurrence of coal spontaneous combustion[16].
injected into a goaf
[17-19].
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To realize simultaneous reduction of both oxygen concentration and coal temperature, nitrogen could be In order to improve the efficiency of fire prevention and extinguishing by inert gas
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injection in to mine goaf, many experts and scholars have studied the variation of oxygen concentration in the coal
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spontaneous combustion zone when nitrogen is injected. However, little is known about the relationship between cooling effect of goaf and filling position or injection flow rate. The influence zone with injection of liquid N2 in goaf is quantitatively undetermined. In order to achieve a better cooling effect and prevent spontaneous combustion
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of coal, it is necessary to study the distribution law of goaf temperature when liquid nitrogen is injected into the goaf. In this paper, we preset temperature sensor to test the distribution of temperature in the mine goaf, and obtained the law of temperature distribution in goaf. And then we developed a 3-D mathematical model to
4
investigate quantitatively the distribution of goaf temperature with injection of liquid N2 into a mine goaf. In this model, the coupling between the chemical reactions in the coal seam and O2 gas transport through the adjacent rocks was taken into account. The distributions of temperature in the goaf were obtained though simulation. We also examined the injection location and flow rate effects of temperature distribution in the goaf. The results were
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of significance for assessing the usefulness of liquid N2 injection on the prevention of spontaneous combustion.
Air return
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2. Field Measurement
#1measur i ng poi nt
Thermodetector Wire
#4 measur i ng poi nt
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Air intake
N
#3 measur i ng poi nt
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#2 measur i ng poi nt
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Fig. 1 Schematic diagram of field experiment
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Field measurement was carried out at 3418 workface in Liangbaosi coal mine in Shandong Province. It is a fully mechanized top coal caving workface, the mining elevation level of 3418 workface varies from -691m to
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-871m. The thickness of the coal seam is between 1.3~9.2m, with an average thickness of 6.35m. The mining height was 3.0m, average height of the top coal caving was 3.35 m. The length and width of the workface are
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2113m and 100m, respectively. The slope angle of the coal seam varies from 4°to 15°, with an average angle of 8°. The coal seam was low metamorphic bituminous coal, with easy occurrence of spontaneous combustion. The air quantity of mining workface is about 800m3/min. with U-type ventilation mode. And the ambient temperature is around 27℃, 300k. In order to study the goaf temperature distribution, AD590 was used as temperature senor, located at four 5
measuring points(#1-#4), #1 is located at air-return side, #4 is located as air intake side, #2 and #3 measuring point is 33m away from air-return and air- intake side. Thermoscope was placed at air intake laneway, temperature senor and thermoscope were connected by wires. The system of temperature measuring was showed as fig.1.
3. Mathematical Models of Heat Transfer in goaf
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In mine goaf, the coal absorbed oxygen and release heat[20]. Meanwhile, the air leak takes away some heat, temperature distribution in goaf is a very complicated problem, influenced by air flow and heat conduction[21-23].
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Usually, there are three basic ways of heat transfer in porous media area like mine goaf, such as heat conduction, convection and radiation[24,25]. However, there is no severe oxidation of coal in the early period of self-heating, and
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the difference of temperature between the caving body and its surrounding environment is not significant.
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Therefore, only heat conduction and convective heat transfer are considered in the study of heat transfer mode in
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the early period of self-heating, and heat radiation is not considered in this model. It is assumed that the porous
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media in the goaf are isotropic and satisfy local thermal equilibrium, i.e., the temperature of solids and fluids is the
cp
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same. The energy conservation equation for gas in goaf is[25]
T c p (Tu) k f (T ) ST t
ST
cp
is specific heat capacity, J/(kg℃), T is the temperature, k f is the thermal conductivity,
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In this equation,
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(4)
is thermal source, which contains the energy releases by coal oxidation, ST is described by,
ST q So 2
(5)
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Where q is thermo release by coal oxidation when coal oxidation consumed unit mass oxygen, So 2 is the
negative source term of oxygen which is caused by coal oxidation. It is could be described by the chemical reaction equation[26,27]. The definite solution condition of the energy conservation is that (1) Initial condition, 6
T t 0 t0 And t0 is the temperature of surrounding rock, 300K. (2) Boundary condition Using Neumann boundary condition which is specified by boundary heat flux as follow
t q n
( x, y, z )
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k f
The mass conservation equation, momentum conservation equation of gas movement in goaf and the
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expression of gas source term, Momentum source term can be seen in literature[27].
Permeability is the main parameter for solving the flow and heat transfer problems in mine Goaf. According to
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Wolf and Bruining, the permeability and porosity of goaf are the main factors that affect the gas flow and gas
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concentration distribution in goaf, and then affecting coal oxidation rate, heat generation and loss in goaf[28]. When
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the gas transfers in goaf, it is hindered with the inertia resistance and viscous resistance which are influenced by
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goaf permeability. According to the Carman-Konzeny equation, permeability of goaf can be described as the
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following equation:
Where
k0
(7)
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k0 x3 k= × 0.241 (1- x )2
is the base permeability of the broken rock at the maximum porosity and it was taken as 1x10-3m2,
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which places it the “open jointed rock” range according to Hoek and Bray [29]. The porosity model of a goaf is[27]
ìï é -ea x+b é x = í1- ê K p,min + ( K p,max - K p,min ) ´ expê-a1 ´ ( y + b1 ) ´ 1- e ( ) ë ïî ë
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(
0
0
)
-1
ùù ûúúû
üï z ý ´ (1- ) h ïþ
(8)
Where K p,max is the initial caving coefficient of bulk increase and its value is 1.6; K p,min is the coefficient of bulk increase in compaction and its value is 1.1; a0 and a1 are the attenuation rate in the tendency and strike direction, respectively, and their values are 0.0368 and 0.268;
e 1is the adjusting parameter and its value is 0.233, b0 and b1 7
are the adjusting parameters in the strike direction and at the tendency, and their values are 0.8 and 15; H is the height of goaf model.
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4. Physical model
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Fig. 2 The goaf model used in the present simulation
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According to the field situation of #3418 workface, the simulation model is established for the present
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simulation. The goaf dimension is 180m long (L), 100m wide (W), and 40m high (H). The cross section size of the laneway is 4m × 4m, and the cross section size of the workface is 4m × 8m. The length of the laneway is 30m. The
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coordinate origin is located at the junction of the workface and the goaf on the air-intake-side. Liquid N2 injection
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was considered from one of the six injection ports from either the air-intake or air-return side, with distance from the workface varying from 15, 25, to 35m, respectively. The serial number of injection entrance are P1, P2, P3 at
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air-intake side, and P4, P5, P6 at air-return side, showed by fig.2. Since it is a fully mechanized top coal caving workface, we assumed that 30% of the coal was left in goaf, that is to say that the thickness of coal remaining in the
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goaf is about 1.9m. The air quantity of the mining workface is set as 800m3/min, O2 molar concentration in the fresh airflow is 20.95% (equals to mass concentration at 23%). The initial temperature of goaf is set to ambient temperature 300K (27℃).
8
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Fig.3 Flow chart of liquid nitrogen injection Fig.3 is a process flow chart for preventing and extinguishing fire with liquid nitrogen injection into
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underground goaf. The nitrogen injection process is as follows: the liquid outlet of the storage tank, the liquid nitrogen delivery pipeline,, and the injection port. The liquid nitrogen reservoir is close to the outlet in goaf, which can effectively increase the perfusion rate of liquid nitrogen and make nitrogen perfuse into the goaf with low
N
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temperature, thus significantly improving the cooling effect in the goaf. In the present simulation, the liquid N2 is
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gasified in the air-intake or air-return laneway, and then the low temperature N2 gas was injected into goaf from the
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steel pipe (the pipe length is about 50m). Since N2 gasification temperature is about 80K(-193℃), after 50m pipe flowing the temperature would increase to about 156.8K. Thus, the temperature of N2 gas when it is injected into
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the goaf was set at 156.8K in the simulation.
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The simulation model was meshed by tetrahedron grids, with a total meshes of 543,874. The goaf model under this mesh sizes gives a satisfactory convergence.
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5. Results and Discussion 5.1 Field measured data
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In order to obtain the temperature distribution, and also make verification to the following simulation. With the temperature measuring system, temperature of #1 - #4 measuring point was measured every day during mining process of this workface, and the variation of temperature in goaf with the mining distance is obtained, temperature data showed by Tab.1 and Fig. 4. Table 1 Observation data of temperature at each measuring point
9
4#
31 34 38 41 45 46 49 47 45 44 41 39 40 39 37 36 34 34 34 33 32 32 32 30 30 30
34 39 40 44 47 48 52 49 44 41 39 36 34 33 32 31 30 30 28 28 28 29 29 29 29 28
30 31 31 32 34 38 44 51 46 41 36 34 33 33 32 34 33 33 34 33 33 33 32 31 31 31
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123.8 128.3 132.8 138.0
PT
144.0 55
45
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#1 #2 #3 #4
40
A
Temperature(°C)
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50
N
A
76.5 82.9 88.5 94.9 100.5 106.9 113.3 118.5
31 30 32 30 30 32 32 31 30 29 28 29 29 32 31 31 29 30 29 29 28 29 29 29 29 29
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4.5 9.8 15.8 21.8 27.8 33.8 39.8 45.8 51.8 55.5 60.0 65.3 70.5
35
30
25 0
20
40
60
80
100
120
140
160
Distance from workface(m)
Fig.4 Temperature at measuring point variations with distance from workface 10
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3#
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Distance from Temperature/℃ workface/m 1# 2#
Fig.4 shows the temperature variations with the distance from workface. We can find that temperature at #3 and #1 is higher than temperature at the other two points. At beginning, the temperatures of measuring points are basically the same with environmental temperature except #3. Temperature at #3 point is higher than temperatures at other point, it means that the location of #3 measuring point is conducive to coal oxidation and thermal storage.
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At point #1, though temperature is same as environmental temperature, as the distance from the workface increasing, the temperature increases faster, it is only slightly lower than the temperature at point 3. Temperature at
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#2 and #4 is lower than the temperature at #1 and #3 testing point.
From the curve, we also can find out that the phenomenon of coal oxidation self-heating in goaf is very
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obvious, no matter which point, the temperature increases with the distance from the working face, and then
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gradually decreases to the surrounding rock temperature. The reason is that air leakage intensity decreases as the
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distance from the working face increases. Initially, the strength of the air leakage is sufficient to provide oxygen
M
needed for the oxidation of coal in goaf, when the air leakage intensity decreases, the heat carried away by the air
ED
leakage decreases and the thermal storage environment becomes better, so the temperature of the goaf keeps rising at beginning. And then air leakage decreased continues to decrease. Oxygen provided by air leakage couldn’t
PT
maintain coal oxidation, and the intensity of heat generation decreases. Affected by heat dissipation, the
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temperature gradually decreases to the surrounding rock temperature. At point #3, when the distance between measuring points from 0m increased to 39.8m, temperature increases from 34℃ to 52℃, this the maximum value of temperature. At point #1, temperature increased to 49℃ when the distance from workface is 39.8m. For point #4,
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temperature increased from 30℃ to about 40℃ when the distance increased to about 45.8m. At measuring point #2, the temperature didn’t change significantly. It means that the degree of coal oxidation near this measuring point #2 is not obvious.
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5.2 Simulation of temperature distributions without liquid N2 injection
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Fig. 5 Simulated temperature distribution in the goaf 311
324 322 310
U
320
314
N
316 140
160
310 308
#4 #3 #2 #1
306 304
300 -20
0
20
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302
40
60
80
180
A
312
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Temperature(K)
318
100
120
140
160
180
200
PT
Distance from workface(m)
Fig.6 Simulation temperature results at four measuring points
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As Fig.5 and Fig.6, simulation shows that the temperature increases at first and then decreases as the distance from the workface increases. The high temperature area in the goaf is 20m behind the workface, with the highest
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temperature points distributed on the air-return side and air-intake side. The reason is that temperature increase is determined by the heat release from the coal oxidation and the heat dissipation caused by the air leaking flow. In the workface or nearby, since the air flow is strong, though coal oxidation is quick, the heat dissipation caused by the air flow is also strong, and thus, the temperature is not so high. With increasing distance from the workface, the air leaking flow turns weak, heat dissipation would be weak, the temperature would be a little higher. At the area 12
where the distance is far from the workface that the leaking air flow is weak and could not provide sufficient oxygen for coal oxidation, there is no heat releasing and the temperature will turn down gradually. And the figure also show that the temperature on both the air-return and air-intake sides is higher than it is in the middle part. The reason is that, affected by supporting action of surrounding coal wall, the porosity on the air-return and air-intake
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sides is larger than that in the middle . The air leaking is strong so the oxygen concentration is higher, and the speed
325
320
320
Simulation data
310
Field test data
310
305
300 40
60
80
100
120
140
160
0
PT
315
310
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305
20
40
60
Simulation data
100
Field test data
315
120
140
160
Simulation data
310
305
Field test data
300 80
100
120
140
160
0
20
Distance from workface(m)
(c) #3 Measuring point
80
320
300
0
60
Distance from workface(m)
325
Temperature(K)
320
40
(b) #2 Measuring point
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(a) #1 Measuring point
325
20
M
A
20
Distance from workface(m)
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Field test data
300 0
Temperature(K)
Simulation data
N
305
315
U
315
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325
Temperature(K)
Temperature(K)
of coal oxidation is high. Thus, the heat release would be strong.
40
60
80
100
120
140
160
Distance from workface(m)
(d) #4 Measuring point
Fig.7 Comparison between simulation and test data for temperature at different location The temperature measured at #1, #2, #3 and 4# measuring point were compared to the calculated value from the simulation, shown in Fig. 7. Though there is some difference between simulation result and field measuring
13
data, especially at #2 point, the reason is that the stochastic distribution of porosity caused by irregular caving of top rocks which is difficult to express by mathematical model was not considered during this simulation model. In general, the variation trend of simulated temperature field is consistent with the measured data. Such as, the high temperature regions are located in the intake and return air sides in the goaf, respectively, and mainly distribute
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30m to 45m after workface. The numerical model is then validated and used for extensive parametric studies involving injection of liquid N2. For all the simulations thereafter, we kept the same airflow conditions as specified
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before. The volume flow rate of the liquid N2 injection is 12m3/min, 720m3/h.
5.3 Evolution of temperature and oxygen distributions with liquid N2 injection
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Field measurement and numerical simulation result all show that there are high temperature zones in the mine
N
goaf caused by coal self-heating. The high temperature reached more than 50℃, it is very close with the critical
A
temperature of coal spontaneous combustion which is generally from 60 to 80℃. It is very necessary to adopt the
M
cooling technology to reduce the goaf temperature and prevent the coal from oxidizing and spontaneous
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combustion.
(b) 50min
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(a) 5min
(c) 90min
(d) Steady state
Fig. 7 shows the time development of the temperature distribution at the ground level with continuous liquid
14
N2 injection using injection entrance P1. It is found that liquid N2 the injection changes the temperature distribution in the goaf. At the beginning, the goaf area where temperature is influenced by liquid N2 was small. It enlarges gradually. The temperature distribution at 90 min（fig.7 c）of continuous injection is very similar to the steady-state solution（fig.7 d）. That means the temperature distribution becomes stable 1.5 hours after the continuous injection.
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The simulation result also shows that the highest temperature decreased from 326.2k( 53.2℃) to 324.9k (51.9℃), the reason is that first heat released by coal oxidation was took away by low temperature. Another reason is that
intensity of heat generation. As result the high temperature decreased.
ED
M
A
N
U
5.4 Cooling effects of different injection locations
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nitrogen reduces the concentration of oxygen in the goaf, and slows down the coal oxidation rate, reduces the
A
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PT
(a) 15m after workface at air-intake side (Steady state)
(b) 25m after workface at air-intake side (Steady state)
15
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A
N
U
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(c) 35m after workface at air-intake side (Steady state)
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PT
ED
M
(d) 15m after workface at air-return side (Steady state)
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(e) 25m after workface at air-return side (Steady state)
16
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(f) 35m after workface at air-return e side (Steady state)
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Fig. 9 Steady state temperature distributions when liquid N2 was injected from different locations
Fig. 9 shows the contour pictures of temperature distribution when the liquid N2 was injected from different
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locations with steady state. From the contrasts of these pictures we can see that no matter where the injection
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locations is, injection of liquid N2 can always decrease the temperature surrounding the injection location. The
A
temperature distribution is not the same for different injection entrances. N2 injection would reduce the local
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injection temperature as well as the area of the spontaneous combustion dangerous zone. However, the temperature
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in the workface is not much influenced by the liquid N2 injection locations studied with flow rate of liquid N2 is
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3
Volume of region where temperature <300K(m )
720m3/h.
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2000
1500
1000
500
0
P1
P2 P5 P3 P4 Location of the injection entrance
P6
Fig. 10 Volume of region where temperature <300k in the goaf with different injection entrance
17
When the liquid N2 was used for coal spontaneous combustion prevention and controlling, the goaf temperature and oxygen distributions influenced by the injection should be inspected. In order to exam the cooling effect due to liquid N2 injection, the volume of region where temperature <300k which is the temperature of underground environment in the coal mine was discussed. Since the coal was just in the bottom with a height of 3m,
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we just discuss the volume of region where the temperature is below 300K at the coal area. Fig. 10 plots the volume of region where temperature <300k at the goaf coal area with different liquid N2
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injection locations. It is seen that when the location of entrance at air return side, the volume of region where temperature below 300k is bigger than that when location is at air-intake side. The reason is that the air leakage
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intensity in the goaf is relatively high on the air-intake side, and after the air leakage is cooled down, it flows back
N
to the working face again, which may affects the cooling effect in goaf.
A
From the comparison of simulation result we also can find out that the N2 injection entrance was located at
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35m (Injection entrance P3) after workface on the air-return side, the final volume of region with temperature<300k
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is the smallest. Meanwhile, when the injection entrance was located at 35m (Injection entrance P6) after the workface on the air-return side, the volume of region temperature <300k is the largest. In other words, its cooling
PT
effect is the best among the cases studied.
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In fact, as well as the volume of low temperature area, the reduction range of maximum temperature is equally important for characterizing the cooling effect of liquid nitrogen in goaf and preventing the occurrence of coal spontaneous combustion. Fig. 11 is the highest temperature value in goaf when liquid nitrogen is injected into goaf
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with different positions. From the data in the figure, it can be seen that no matter where the injection entrance is the highest temperature in goaf will all be reduced by liquid nitrogen injection. Through the data bar on the left side of the cloud picture, we can see that the maximum temperature in the goaf are 324.9K, 324.4K, 324.3K, 324.3K, 323.8K and 322.6k when liquid N2 was injected with P1-P6 port
18
respectively, these values are below the temperature without nitrogen injection. It can be seen that by the influence of endothermic cooling and the slowdown of chemical reaction caused by the decrease of oxygen concentration, the maximum temperature in goaf decreases slightly, and also the possibility of spontaneous combustion of coal decreases. By comparing the maximum temperature in different simulation condition, we can find that the
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reduction of the maximum temperature increased with the distance from workface in goaf. Compared with N2 injection on air-intake side, reduction extent of maximum temperature is larger when the injection entrance is on
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air-return side.
5.4 Cooling effects with different injection flow rate
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In order to discuss the influence by liquid N2 injection volume on cooling effects, we simulated the
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temperature distribution when liquid N2 injection with different volume. Table.2 and Fig. 11 shows the volume
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value of region where the temperature below 300k when the liquid N2 was injected with different volume at
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injection entrance P1-P6, with flow rate of N2 are 360m3/h, 720m3/h, 1080m3/h and 1440m3/h, respectively.
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Table.2 Volume of region where the temperature below 300k 360m3/h
720m3/h
1080m3/h
1440m3/h
P1 P2 P3 P4 P5 P6
465.614 265.996 102.701 772.288 930.597 208.133
1001.86 892.665 733.62 1882.79 2000.48 2014.19
1655.04 1613.56 1657.78 2153.57 2728.19 3373.41
2155.86 2255.3 2413.45 2712.57 3231.63 4434.41
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Injection entrance
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P1 P2 P3 P4 P5 P6
4000
3000
2000
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1000
0 200
400
600
800
1000
1200
1400
1600
3
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3
Volume of region where tamperature below 300k(m )
5000
Flow rate of injection(m /h)
Fig. 11 Volume of region where temperature <300k with different injection entrance and flow rate
N
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From the data in Fig. 11 and Table 2, it can be seen that the volume of low temperature region in goaf is
A
significantly affected by the perfusion flow rate, and the influence has a strong regularity. It shows that the volume
M
of cryogenic space increases significantly with the increase of perfusion flow rate. When the flow rate of low temperature N2 is increased from 360 m3/h to 1440 m3/h using P6 injection entrance, the volume of low
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temperature zone（below 300K） in the goaf increases from 208.1 to 4434.41 m3, increased more than 20 folds.
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Through these simulations, we also can found out that when the injection entrance is located in the high temperature zone of coal oxidation in goaf, the volume of low temperature space（below 300K） is very small. For
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example, when the port P6 is used for injection with flow rate of 360m3/h, the volume of space where temperature is below 300 K is only 208.133 m3. When the port P3 is used for filling with flow rate of 360m3/h, the volume of
A
space where temperature is below 300 K is only 102.70m3. This is because the outlet of perfusion is near the core high temperature area of the whole goaf. After low temperature nitrogen comes out of the pipeline, the temperature rises faster and the cooling range (low temperature space) is smaller.
6. Conclusions Simulations of gas and temperature distribution in a goaf were carried out. The following main conclusive 20
remarks can be drafted: (1) Influenced by coal oxidation, the high temperature of goaf could be more than 50℃, the high temperature area is found to be 35-45m behind the workface in the goaf without N2 injection. (2) Liquid N2 injection changes the local temperature distribution around the injection port. The temperature influential area due to liquid N2 injection enlarges gradually. After 90 min of continuous injection, however, the influential zone becomes stable. That is, the temperature distribution in the goaf stabilizes 1.5 hours after the
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continuous injection.
(3) The low temperature region (Temperature ≤300K ) influenced by liquid N2 injection depends on N2
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injection location, when the N2 injection entrance was located at 35m after workface on the air-return side, the final volume of low temperature region is the smallest. Meanwhile, when the injection entrance was located at 35m after the workface on the air-return side, the volume of region temperature <300k is the largest among the cases studied
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in the paper.
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(4) Low temperature region distribution was also influence by liquid N2 injection flow rate. The volume of
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cryogenic space increases significantly with the increase of perfusion flow rate. When the flow rate of low temperature N2 is increased from 360 m3/h to 1440 m3/h, the volume of low temperature zone（below 300K） may
M
increase more than 20 folds.
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(5) The reduction range of maximum temperature and its influence factors was also be discussed in this paper. We find out that the reduction of the maximum temperature increased with the distance from workface in goaf. Compared with N2 injection on air-intake side, reduction extent of maximum temperature is larger when the
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injection entrance is on air-return side.
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Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51774274) and the
A
Fundamental Research Funds for the Central Universities (2017XKQY026).
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