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Process Safety and Environmental Protection 126 (2019) 278–286

Contents lists available at ScienceDirect

Process Safety and Environmental Protection

journa l homepage: www.e lsev ier .com/ locate /psep

odeling temperature distribution upon liquid-nitrogen injectionnto a self heating coal mine goaf

uo-Qing Shi a,c, Peng-xiang Ding a, Zhixiong Guo b, Yan-ming Wang a,∗

College of Safety Engineering, China University of Mining and Technology, Xuzhou, 221008, ChinaDepartment of Mechanical and Aerospace Engineering, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USAState Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Xuzhou, 221008, China

r t i c l e i n f o

rticle history:eceived 31 December 2018eceived in revised form 26 March 2019ccepted 26 March 2019vailable online 28 March 2019

eywords:emperature distributioniquid nitrogenumerical simulation

a b s t r a c t

Liquid N2 could be injected into goaf to decrease temperature and prevent spontaneous combustionduring mining. A working face at Liangbaosi coal mine was adopted for study. Firstly, a mathematicalmodel for calculating the temperature field in goaf was developed and field tests without injection ofliquid N2 were conducted to validate the model. Comparable development trends and hot zones betweenthe simulation and field measurements were found. The hot zones were located about 35–45 m behindthe workface on the air-return and air-intake sides in the goaf. Then the model was employed to simulatethe time development of temperature distribution in the goaf with injection of liquid N2 from differentlocations. It was observed that a low-temperature cooling zone (<300K) due to injection of liquid N2

gradually grew and became relatively stable 90 min after continuous injection. The size of the cooling

oafoal spontaneous combustion

zone depends on the injection location and flow rate. The cooling zone was smaller when the N2 wasinjected from the air-intake side than from the air-return side. The largest cooling zone was found whenthe N2 was injected 35m behind the workface from the air-return side. The cooling zone increases withincreasing N2 perfusion rate. This study provides a quantitative assessment for preventing coal oxidationand spontaneous combustion using the liquid N2 technology.

© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

. Introduction

Spontaneous combustion of coal is a severe issue thathreatens the development of coal mining industry worldwideKrishnaswamy et al., 1996; Stracher and Taylor, 2004). Amonghina’s state-owned collieries, 56% of the mines have been jeopar-ized by coal spontaneous combustion which accounts for 90–94%f all coal mine fires. Coal loss caused by coal spontaneous combus-ion in China is about 200 million tons per year. Coal spontaneousombustion also easily causes gas explosion, which poses a serioushreat to workers’ lives and safety (Li, 1998). In the US, Indian, andustralian coal mines, most fires were also caused by spontaneousombustion (Carras et al., 2009; Cliff et al., 2000; Dzonzi-Undi andi, 2015; Nimaje and Tripathy, 2016; Shamsi et al., 2004; Singh et al.,007).

The occurrence of coal spontaneous combustion is a complexhysicochemical process (Kuenzer et al., 2007; Yuan and Smith,009). As we know oxygen and heat accumulation are two nec-

∗ Corresponding author.E-mail address: CUMTwangym@163.com (Y.-m. Wang).

ttps://doi.org/10.1016/j.psep.2019.03.033957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights

essary conditions for coal spontaneous combustion (Chandra andPrasad, 1990; Rosema et al., 2001). Thus, there are two commonways to prevent or delay coal spontaneous combustion. One wayis to control and reduce oxygen concentration. If the oxygen werenot enough, coal oxidation which is the initial stage of coal spon-taneous combustion would not occur easily (Deng et al., 2018a, b).The other way is to decrease goaf temperature and keep it belowthe coal ignition point (Deng et al., 2015).

To realize simultaneous reduction of both oxygen content andcoal temperature, liquid nitrogen could be injected into a goaf(Adamus, 2001; Qin et al., 2016; Singh and Singh, 2004). Many stud-ies have been carried out on the variation of oxygen concentrationin the coal spontaneous combustion zone in order to understandthe efficiency of fire prevention and extinguishing by inert gasinjection into mine goaf (Liu et al., 2016). However, little is knownof the relationship between the goaf temperature cooling effectand the filling position and/or perfusion rate. The goaf tempera-ture influence zone with injection of liquid N2 was quantitatively

undetermined. In order to achieve a better cooling effect and toeffectively prevent spontaneous combustion of coal, it is neces-sary to study the influences of liquid nitrogen injection on goaftemperature development.

reserved.

G.-Q. Shi et al. / Process Safety and Environm

Nomenclature

a0 Attenuation rate in the tendency directiona1 Attenuation rate in the strike directionb0 Adjusting parameters in the strike directionb1 Adjusting parameters at the tendencyC Mass concentration (%)cp Specific heat capacity, (J/(kg·K))→g Vector of gravity (m/s−2)H Height of the goaf (m)kf Heat transfer coefficient (W/(m·K))Kp,max Initial caving coefficientKp,min Coefficient of bulk increasek Permeability (m2)k0 Base permeability (m2)L Length of the goaf (m)q Heat release due to coal oxidation (J/mol)ST Thermal source (W/m3)SO2 Source term of oxygen (mol/(m3·s))t Time (s)T Temperature (K)u Velocity vectorW Width of the goaf (m)x, y, z Spatial coordinates

Greek symbolsε Adjusting parameter� Porosity

dA3ffitiiwpsiip

2

Lmvviw14mofa

a

k = k0

0.241· �3

(1 − �)2, (5)

� Density of the gas mixture (kg/m3)

The objective of this study is to investigate quantitatively theistribution of goaf temperature under the injection of liquid N2.

working face at Liangbaosi coal mine in China was adopted. A-D mathematical model was utilized to simulate the heat trans-er and gas transport in the goaf. To examine the simulation model,eld tests were carried out and comparisons between the measured

emperatures and simulations were conducted. To incorporatenjection of liquid N2, the coupling between the chemical reactionsn the coal seam and O2 -gas transport through the adjacent rocks

as considered. The influences of liquid N2 injection location anderfusion rate on the temperature distribution in the goaf werecrutinized. The development of low-temperature zone due to thenjection of inert gas was emphasized. The results were of signif-cance for assessing the usefulness of liquid N2 injection on therevention of coal spontaneous combustion.

. Field measurement

Field measurements were carried out at workface #3418 iniangbaosi coal mine in Shandong Province, China, which is a fullyechanized top coal caving workface. The mining elevation level

aries from −691 m to −871 m. The thickness of the coal seamaries in 1.39̃.2 m, with an average thickness of 6.35 m. The min-ng height was 3.0 m, and the average height of the top coal caving

as 3.35 m. The length and width of the workface are 2113 m and00 m, respectively. The slope angle of the coal seam varies from◦ to 15◦, with an average angle of 8◦. The coal seam was lowetamorphic bituminous coal, characterized by easy occurrence

f spontaneous combustion. The air quantity of the mining work-3

ace is about 800 m /min with U-type ventilation mode. And the

mbient temperature is around 300 K.In order to measure the goaf temperature distribution, temper-

ture sensors (AD 590) were located at four different measuring

ental Protection 126 (2019) 278–286 279

points: with #1 at the air-return side, #4 at the air-intake side,#2 and #3 at 33 m away from the air-return and air-intake sides,respectively. A thermoscope was placed at the air-intake laneway.The temperature sensors and thermoscope were connected bywires. Fig. 1 sketches the temperature measuring arrangement.

3. Heat transfer modeling

In a mine goaf, the coal absorbs oxygen and releases heat(Banerjee, 1985). Meanwhile, the air leakage takes away some heat.The temperature development in a goaf is a very complicated prob-lem, influenced by air flow, chemical reactions, and heat transfer(Arisoy and Akgün, 1994; Beamish et al., 2000; Yuan and Smith,2008). Usually, heat transfer in a mine goaf incorporates all threemodes, i.e., heat conduction, convection, and radiation (Chen et al.,2015; Ejlali et al., 2011). However, there is no severe oxidation ofcoal in the early period of self-heating, and the difference of temper-ature between the caving body and its surrounding environment isnot significant. Therefore, only heat conduction and convection areconsidered in the present modeling of heat transfer. It is assumedthat the porous media in the goaf are isotropic and at local thermalequilibrium, i.e., the temperatures of solid and fluid are the same.The energy conservation equation for gas in a goaf is (Chen et al.,2015):

�cp∂T∂t

+ �cp∇ · (Tu) = ∇ · (kf · ∇T) + ST (1)

In which, � is the gas mixture density, cp is the specific heat capacity,T is the temperature, kf is the thermal conductivity, u is the velocityvector, ST is the thermal source, which is the energy release by coaloxidation and described by:

ST = q · � · So2, (2)

Where � is the porosity, q is the heat release due to coal oxidationper molar oxygen, and So2 is the negative source term of oxy-gen consumption that can be described by the chemical reactionequation (Liu et al., 2016; Shi et al., 2015).

The initial condition is

T∣∣t=0

= T0. (3)

In which, T0 is the initial temperature of the surrounding rock.Neumann boundary condition with a specified heat flux is adoptedas:

−kf∂T∂n

= q (x, y, z) ∈ � (4)

The present authors have studied the mass conservation and gastransport in the goaf in a previous study (Liu et al., 2016); and thus,such information is not described here.

According to Wolf and Bruining (2007), the permeability andporosity of goaf are two main factors that affect the gas transportand oxygen concentration distribution in goaf, influencing coal oxi-dation rate, heat generation and loss. When the gas transports inthe goaf, it is hindered with the inertia resistance and viscous resis-tance which are influenced by the goaf permeability. Accordingto the Carman-Konzeny equation, the permeability of goaf can bedescribed as:

where k0 is the base permeability of the broken rock at the maxi-mum porosity and it was taken as 10−3 m2 in this study, placing itthe “open jointed rock” range according to Hoek and Bray (1981).

280 G.-Q. Shi et al. / Process Safety and Environmental Protection 126 (2019) 278–286

Fig. 1. Schematic diagram of field temperature measurement.

sed in

�

wvadabth

4

mThclolew

Fig. 2. The goaf model u

The goaf porosity distribution is given as (Liu et al., 2016):

={

1 −[Kp,min +

(Kp,max − Kp,min

)

×exp [−a1 (y + b1) (1 − e−εa0(x+b0))]]−1} × (1 − z

H), (6)

here Kp,max is the initial caving coefficient of bulk increase and itsalue is 1.6; Kp,min is the coefficient of bulk increase in compactionnd its value is 1.1; a0 and a1 are the attenuation rate in the ten-ency and strike direction, respectively, and their values are 0.0368nd 0.268; ε is the adjusting parameter and its value is 0.233; b0 and1 are the adjusting parameters in the strike direction and at theendency, and their values are 0.8 and 15, respectively; H is theeight of the goaf.

. Physical model

According to the field situation of workface #3418, a simulationodel showing in Fig. 2 is established for the present simulation.

he goaf dimension is 180 m long (L), 100 m wide (W), and 40 migh (H). The cross-section size of the laneway is 4 m × 4 m, and theross-section size of the workface is 4 m × 8 m. The length of theaneway is 30 m. The coordinate origin is located at the junction

f the workface and the goaf on the air-intake side. Injection of

iquid N2 was considered from one of the six injection ports fromither the air-intake or the air-return side, with distance from theorkface varying from 15, 25, to 35 m, respectively. The reason for

the present simulation.

choosing such injection conditions is that one cannot lay a very longor very short nitrogen pipeline into a goaf. If the pipeline is too long,it has a risk of being broken easily; if it is too short, it may causenitrogen to flow back to the working face. Thus, it is reasonable toset the filling entrance between 15–35 m behind the workface.

The serial number of injection entrance is marked as P1(15 m),P2(25 m), P3(35 m) at the air-intake side, and P4(15 m), P5(25 m),P6(35 m) at the air-return side, respectively. Since the workface isa fully mechanized top coal caving workface, we assumed that 30%of the coal was left in the goaf, i.e., the thickness of the remain-ing coal in the goaf is about 1.9 m. The air quantity of the miningworkface is set as 800 m3/min, O2 molar concentration in the freshairflow is 20.95% (equals to a mass concentration at 23%). The initialtemperature of goaf is set as the ambient temperature 300 K.

Fig. 3 is a process flow chart for injecting liquid N2 into an under-ground goaf. The injection process is that the liquid nitrogen froma storage tank is delivered through the pipeline and released at theinjection port. The liquid nitrogen reservoir is placed close to theoutlet of the goaf, which can effectively increase the perfusion rateof N2 into the goaf under low temperature, significantly improvingthe cooling effect for the goaf. In the present simulation, liquid N2is gasified in the air-intake or air-return laneway, and then the low-temperature N2 gas was injected into the goaf from the steel pipe

(length is about 50 m). Since N2 gasification temperature is about80.2 K, after flowing in the pipe the gas temperature would increaseto about 156.8 K. Thus, the injection temperature of N2 gas into thegoaf was set at 156.8 K in the present simulation.

G.-Q. Shi et al. / Process Safety and Environmental Protection 126 (2019) 278–286 281

Fig. 3. Flow chart for liquid N2 injection.

Table 1The main properties of coal and rock.

Parameter Coal particle density Rock particle density Heat capacity of coal

Value 1380 kg/m3 1600 kg/m3 1000 J/(kg K)Parameter Heat capacity of rock Thermal conductivity of coal Thermal conductivity of rockValue 1000 J/(kg K) 0.Parameter Activation energy of coal HValue 82 kJ/mol 24

a

uttta

5

5

iawtow#attit#dt

Fig. 4. Temperatures measured with the advance of mining.

The main properties of the coal and rock in the present studyre listed in Table 1.

The commercial software package, FLUENT (ANSYS 14.5), wassed in the present simulations. The goaf model was meshed byetrahedron grids, with a total meshes of 543,874 in all the simula-ions below. We have tried four different sets of meshes and foundhat 543,874 meshes gave a satisfactory convergence balanced byn acceptable computational cost.

. Results and discussion

.1. Field measured data

In order to obtain the field temperature distribution and ver-fy our simulation model, field measurements were conducted tocquire the temperature information at #1 - #4 measuring pointsithout injection of liquid N2. The measurement was done daily as

he mining of workface #3418 processed. Fig. 4 shows the variationf the measured temperatures in the goaf vs. the distance from theorkface. It is seen that the temperature variations at #1, #3 and4 are greater than that at #2. At the beginning, the temperaturest the measuring points are just a little bit above the environmen-al ambient temperature (300 K) except at #3. The initially higheremperature at #3 point indicates that the location of #3 measur-ng point is conducive to coal oxidation and thermal storage. As

he distance from the workface increases, the temperature at both1 and #3 increases quickly, the temperature at #4 has an initialelay and then an abrupt increase. At about 35–45 m away fromhe workface, the three temperatures start to drop. At the long dis-

1998 W/(m K) 0.1998 W/(m K)eat of reaction Pre-exponential factor0 kJ/mol O2 2.9 × 106K/s

tance (>130 m from the workface), the difference among the fourmeasured temperatures is not very appreciable, i.e., the tempera-tures gradually decrease to the surrounding rock temperature. Suchtemperature curve variations indicate clearly the occurrence of coaloxidation self-heating in the goaf along measuring lines #1, #3,and #4. The air leakage intensity decreases as the distance fromthe working face increases. Initially, the strength of the air leakageis sufficient to provide oxygen needed for the oxidation of coal inthe goaf. When the air leakage intensity decreases, the heat car-ried away by the air leakage decreases, and the temperature of thegoaf rises at the beginning. As the air leakage continues to decrease,the oxygen provided by the air leakage couldn’t maintain the coaloxidation, and the intensity of heat generation decreases. Affectedby the heat dissipation, the temperature gradually decreases to thesurrounding rock temperature. The relatively smaller variation atmeasuring point #2 indicates that the degree of coal oxidation nearthis measuring point is not obvious.

5.2. Simulated temperature distribution without liquid N2injection

Simulation under the same field test conditions was run withassumed permeability and porosity profiles in Eqs. (5) and (6),respectively, and assumed uniform coal and rock properties.Comparisons of the simulated temperatures with the measuredtemperatures are shown in Fig. 5. Though there are some differ-ences between the simulation results and the field measuring data(#1, #3, and #4), in general, the variation trend and temperaturerange of the simulated temperature distributions are consistentwith the measured data. The differences could be mainly attributedto the stochastic distribution of coal and rock porosity caused byirregular caving of top rocks which is difficult to express accuratelyby the mathematical model. The high-temperature hot zones inboth the simulation and field measurements are located in thedistance of 30 m–45 m behind the workface. The irregularity andnon-obvious coal oxidation at measuring point #2 are not capturedby the simulation. This is the limitation of modeling with uniformmedium properties assumptions for the whole goaf. It should benoticed that such an irregularity was not discovered in our previousstudy (Liu et al., 2016) evidencing that the simulated oxygen distri-butions were in excellent agreement with field measurements. Thisis because heat transfer modeling depends strongly on the accuracyof the assumed thermal properties, while such a dependence for gas

transport modeling is relatively weak.

Fig. 6 compares the simulated temperature distributions amongthe four measuring points. It shows that the temperature increasesat first and then decreases as the distance from the workface

282 G.-Q. Shi et al. / Process Safety and Environmental Protection 126 (2019) 278–286

Fig. 5. Comparison between sim

itsoh

showed that there are high-temperature hot zones in the mine goaf

Fig. 6. Simulated temperature distribution at the four measuring points.

ncreases. The temperatures at long distance almost converge. Suchendency qualitatively matches with the field measurements. Fig. 7

hows the whole goaf temperature field through simulation with-ut injection of liquid N2. Both Figs. 6 and 7 demonstrate that theot zone in the goaf is located in the range of 20–60 m behind

ulation and field test data.

the workface. Higher temperature is found on the air-intake andair-return sides. These are consistent with the findings in Wanget al. (2012). The reason is that, affected by supporting action of thesurrounding coal wall, the porosity is larger on the air-return andair-intake sides than in the middle. The air leakage with greaterporosity is stronger so the oxygen concentration is higher, andthe speed of coal oxidation is higher. Thus, the heat release wouldbe stronger. The temperature increase is determined by the heatrelease from the coal oxidation and the heat dissipation causedby the air leakage flow. Nearby the workface, since the air flowis strong, though the coal oxidation is quick, the heat dissipationcaused by the air flow is also strong; and thus, the temperatureis not high. With increasing distance from the workface, the airleakage flow turns weak, heat dissipation would weaken, and thetemperature would increase. At the area where the distance is farfrom the workface such that the air leakage flow is very weak andcould not provide sufficient oxygen for coal oxidation, there is noheat releasing and the temperature will turn down gradually.

5.3. Evolution of temperature distribution with liquid N2 injection

The above field measurement and numerical simulation results

caused by coal self-heating. The maximum temperature reached325.2 K, which is close to the critical temperature of coal sponta-neous combustion (varying between 333 and 353 K in general). For

G.-Q. Shi et al. / Process Safety and Environmental Protection 126 (2019) 278–286 283

Fig. 7. Simulated temperature contour in the goaf.

temp

tboicuit

tertfttwi(rs

Fig. 8. Time development of the

his type of goaf, if some conditions affecting spontaneous com-ustion of coal change, fires caused by spontaneous combustionf coal may occur. Therefore, it is necessary to adopt the cool-

ng technology to reduce the goaf temperature and to prevent theoal from oxidizing and spontaneous combustion. For all the sim-lations thereafter, we will investigate the influence of liquid N2

njection, keeping same airflow conditions as specified in subsec-ion 5.2.

Figs. 8(a–d) show the time development of the temperature dis-ribution at the ground level with continuous N2 injection fromntrance P1 at four different time instants. The N2 perfusion flowate is 720 m3/h. It should be mentioned that the temperature spec-ra are different in the figures, with lowest maximum temperatureor the steady state (Fig. 8d) and highest maximum temperature forhe contour at 5 min (Fig. 8a). The injection of liquid N2 changes theemperature distribution in the goaf. At the beginning, the goaf areahere temperature is influenced by the N2 is small. The influenc-

ng zone enlarges gradually. The temperature distribution at 90 minFig. 8c) of continuous injection is very similar to the steady-stateesult (Fig. 8d). It means that the temperature distribution becomestable 1.5 h after continuous injection. The simulation result at

erature field with N2 injection.

steady-state also shows that the maximum temperature decreasesfrom 326.2 K at 5 min to 324.9 K. This can be attributed to two facts:(1) the heat released by coal oxidation is taken away by the low-temperature gas flow; and (2) the injection of nitrogen reduces theoxygen concentration in the goaf, slowing down the coal oxidationrate, and suppressing the heat generation.

5.4. Cooling effect by injection location

Figs. 9(a–f) show the contour pictures of temperature distribu-tion under six different injection locations. The N2 perfusion flowrate is 720 m3/h and the temperature spectra for each figure aredifferent. It is observed that the temperature in the workface isnot much influenced by the injection locations studied. Injection ofN2 always decreases the temperature zone surrounding the injec-tion port. The low-temperature influencing zone (<300 K, which isthe temperature of underground environment in the coal mine)

will suppress potential spontaneous combustion zone as the maxi-mum temperature under N2 injections is always below that withoutN2 injection. This is due to endothermic cooling and slowdown ofchemical reaction caused by the decrease of oxygen concentration

284 G.-Q. Shi et al. / Process Safety and Environmental Protection 126 (2019) 278–286

tribution for different injection locations.

u3fcpit

otItilmitai

5

iN

Fig. 9. Steady-state temperature dis

nder the injection of N2. The maximum temperature in the goaf is24.9 K, 324.4 K, 324.3 K, 324.3 K, 323.8 K and 323.6 K, respectivelyor injection from P1 to P6, all below the critical temperature ofoal spontaneous combustion. By comparing the maximum tem-eratures, it is found that the maximum temperature reduces with

ncreasing distance from the workface. The reduction of maximumemperature is larger when the injection is from the air-return side.

The size of the cooling zone (<300 K) depends on the locationf injection. Fig. 10 plots the volume of the cooling zone whereemperature <300 K at the goaf coal area vs. N2 injection location.t is seen that the cooling zone is bigger when the injection is fromhe air-return side (P4-P6) than from the air-intake side (P1-P3). Thiss because the air leakage is stronger on the air-intake side. After theeaking air is cooled down, it flows back to the working face, which

ay affect the cooling effect in the goaf. When N2 injection entrances located at 35 m (P3) behind the workface on the air-return side,he cooling zone is the smallest; while at the same distance from their-return side (P6), the cooling zone is the largest. In other words,ts cooling effect is the best among the cases studied.

.5. Cooling effect of injection flow rate

Fig. 11 plots the cooling zone size vs. N2 perfusion rate withnjection from six different ports P1-P6. The injection flow rate of

2 is 360 m3/h, 720 m3/h, 1080 m3/h and 1440 m3/h, respectively.

Fig. 10. Cooling zone size for different injection ports.

It is seen that the cooling zone size is significantly affected by the

perfusion rate of N2 injection. It increases nearly linearly with theperfusion rate. Using P6 as an example, when the flow rate of N2 isincreased from 360 m3/h to 1440 m3/h (an increase of 4 times), thecooling zone (<300 K) in the goaf enlargens from 208.1 to 4434.4 m3,

G.-Q. Shi et al. / Process Safety and Environm

afltcttfltt

6

gpscmf

•

•

•

•

Fig. 11. Cooling zone size vs. perfusion rate.

n increase > 20 folds. An interesting phenomenon is that, when theow rate is small (i.e., 360 m3/h), the cooling zone is smaller whenhe injection is from P3 or P6 (i.e., the high-temperature zone ofoal oxidation in goaf) than from other ports. This is because theemperature of nitrogen rises fast when it comes out of the high-emperature area when the flow rate is not appreciable. When theow rate is large, e.g. at 1440 m3/h, the cold nitrogen will suppresshe hot zone effectively and its cooling zone is larger for P3 or P6han its counterparts.

. Conclusions

Field tests and simulations have been carried out to investi-ate the temperature distributions in a mining goaf. Emphases arelaced on the validation of the modeling through comparison ofimulation with the field measurement and the examination of theooling effect of liquid N2 injection on the suppression of maxi-um temperature to prevent coal spontaneous combustion. The

ollowing conclusive remarks can be drafted:

Both field test and simulation without liquid N2 injection showedthat the maximum temperature in #3418 workface goaf couldreach to 325.2 K, with high-temperature hot zone located around35–45 m behind the workface in the goaf. It indicates the occur-rence of coal oxidation.The heat transfer simulation is consistent with the field tem-perature measurement qualitatively. The difference betweenmeasurement and simulation could be mainly attributed to theuniform assumption of medium thermal properties in simulation,while irregularity exists in the real goaf. The gas transport mod-eling was in good agreement with the field oxygen measurementas shown in our previous study (Liu et al., 2016).Liquid N2 injection forms a cooling zone around the injection portand reduces the maximum temperature in the goaf. The reductionmagnitude of maximum temperature increases as the injectionport is placed closer to the hot zone of the goaf. Reduction isalso more efficient when the injection is from the air-return side.When N2 injection is from P3 (35 m behind the workface on theair-return side), the cooling zone is the smallest; while at thesame distance from the air-return side (P6), the cooling zone isthe largest among the cases studied.

The cooling zone size is significantly influenced by the liquid N2injection flow rate. When the perfusion rate of N2 is increasedfrom 360 m3/h to 1440 m3/h, the size of the cooling zone isincreased by more than 20 folds for injection port P6. The cooling

ental Protection 126 (2019) 278–286 285

zone enlarges gradually and becomes stable 90 min after contin-uous injection.

Acknowledgements

This work was supported by the China National Key R&DProgram (Grant No. 2018YFC0808100), National Natural ScienceFoundation of China (No. 51774274) and the Fundamental ResearchFunds for the Central Universities (2017XKQY026).

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