CN105136837A - Liquid nitrogen circulating freeze-thawing permeability-increasing simulation test system and method for coal rock sample - Google Patents

Abstract

The invention discloses a coal rock sample liquid nitrogen cycle freeze-thaw permeability simulation test system and method, the test system includes a freeze-thaw device, a data acquisition system and a coal rock sample, the freeze-thaw device includes a liquid nitrogen freeze-thaw test box and a self-pressurized liquid nitrogen tank, the coal rock sample is placed in the liquid nitrogen freeze-thaw test box, the liquid nitrogen freeze-thaw test box is connected to the self-pressurized liquid nitrogen tank through a liquid nitrogen supply pipeline, the liquid nitrogen supply pipeline includes a three-way joint, the three-way joint is connected to the liquid nitrogen freeze-thaw test box, the self-pressurized liquid nitrogen tank and the liquid nitrogen pressurization pipe, the data acquisition system includes a high-frequency pressure sensor, a low-temperature strain gauge and a temperature sensor probe; the test method repeats the liquid nitrogen freeze-thaw cycle for many times, and examines the strain and temperature influence rules of the coal rock sample under different freeze-thaw variable conditions. The present invention can simulate the process of liquid nitrogen cycle freeze-thaw permeability coal or rock mass, and provides a feasible experimental platform for liquid nitrogen cycle freeze-thaw permeability coal rock mass extraction of gas or shale gas.

Description

A simulation test system and method for liquid nitrogen cycle freezing and thawing anti-reflection of coal rock samples

technical field

The invention relates to a freeze-thaw test system and method, in particular to a liquid nitrogen cycle freeze-thaw anti-reflection simulation test system and method for coal rock samples.

Background technique

According to statistics, my country's high-gas mines and mines with gas outburst hazards account for about 30% of the total number of mines in the country, and the high frequency of heavy and extremely large malignant gas accidents has not been fundamentally resolved, which will seriously hinder the future. my country's coal mine safety production work is going on smoothly. Therefore, coal mine gas drainage work is imperative. However, most of the coal seams in my country are high-gas and low-permeability coal seams, and gas extraction is difficult, and the concentration of gas extraction is low, most of which are lower than 20%. At present, methods such as hydraulic fracturing, hydraulic slotting and pre-splitting blasting are mostly used to increase the gas permeability of coal seams. With the continuous increase of mine depth and mining intensity, the conventional coal seam anti-permeability drainage method has a small cracking and anti-permeability range, and the coal seams The body cannot form a large-scale gas drainage fracture network, which makes the gas drainage rate low and the gas control effect is not ideal. Proposing a high-efficiency anti-permeability coal seam drainage method is the key point to speed up the prevention and control of deep coal mining disasters.

Freezing and thawing is a common physical geological action and phenomenon in nature, especially in structures with relatively large temperature differences, such as the Qinghai-Tibet Plateau, roads and buildings in the northern region. Freeze-thaw erosion is due to the volume expansion of the soil and its parent material pores or rock cracks when the water is frozen, which makes the cracks increase and increase, resulting in the fragmentation of the whole piece of soil or rock. After ablation, its corrosion resistance is stable. The property is greatly reduced, and the rock and soil move downward along the slope under the action of gravity. The alternating freezing and thawing of the moisture contained on the surface and inside of the structural member is called a freeze-thaw cycle. Repeated freeze-thaw cycles cause severe damage to the object's structure.

Under normal pressure, the temperature of liquid nitrogen can reach -196°C, and the latent heat of vaporization is 5.56kJ/mol. ^1m3 of liquid nitrogen can expand to 696m3 of pure gaseous nitrogen at 21°C, which can absorb a large amount of heat around it when vaporized. Liquid nitrogen has the advantages of simple preparation and wide source of raw materials. Liquid nitrogen can be used as an efficient refrigeration and anti-reflection medium in the freeze-thaw cycle of coal.

In view of the shortcomings of the current gas extraction technology, the phase change of liquid nitrogen can be used to cyclically freeze and thaw the coal seam to provide a more efficient coal seam permeability enhancement method for high-gas coal seams. A method based on horizontal directional drilling liquid nitrogen is proposed. In the gas extraction method of cyclic freezing and thawing anti-permeability coal seams, the coal body is under the joint action of water phase change frost heaving force, liquid nitrogen gasification expansion force and microporous liquid flow osmotic pressure, which promotes the expansion and connection of macroscopic and microscopic fractures in low permeability coal seams. Connect the gas drainage fracture network to increase the air permeability of the coal seam. Since the liquid nitrogen cycle freeze-thaw anti-permeability coal seam drainage method is affected by many factors, how to conduct research on the law of various influencing factors on the coal and rock mass anti-permeability in the laboratory, for the liquid nitrogen cycle freeze-thaw anti-permeability coal seam drainage gas It is an urgent scientific problem to provide scientific basis and theoretical basis for engineering application, and to determine the optimal value of each influencing variable.

Contents of the invention

Purpose of the invention: The purpose of this invention is to provide a simulation test system and method for freezing and thawing anti-permeability simulation of coal rock samples, aiming at providing scientific basis and theoretical basis for the engineering application of liquid nitrogen cyclic freezing and thawing anti-permeability coal seam drainage gas .

In order to achieve the above object, the present invention adopts the following technical scheme: a liquid nitrogen cycle freeze-thaw anti-reflection simulation test system for coal rock samples, including a freeze-thaw device, a data acquisition system and a coal rock sample, the freeze-thaw device includes liquid Nitrogen freeze-thaw test chamber and self-pressurized liquid nitrogen tank, the coal rock sample is placed in the liquid nitrogen freeze-thaw test chamber, the liquid nitrogen freeze-thaw test chamber is equipped with a heater and a liquid level sensor, and the heater is connected through the temperature controller Connect the temperature controller, the temperature controller is connected to the computer, the liquid level sensor is connected to the liquid level display set outside the liquid nitrogen freeze-thaw test chamber, the top of the liquid nitrogen freeze-thaw test chamber is equipped with a sealing cover of the liquid nitrogen freeze-thaw test chamber, and the liquid nitrogen The sealing cover of the freeze-thaw test chamber is equipped with a safety pressure relief valve, a low-temperature pressure gauge and an exhaust valve, and the bottom of the liquid nitrogen freeze-thaw test chamber is connected to a self-pressurized liquid nitrogen tank through a liquid nitrogen supply pipeline;

The liquid nitrogen supply pipeline includes a three-way joint, the first end of the three-way joint is connected to the bottom of the liquid nitrogen freeze-thaw test chamber, the second end of the three-way joint is provided with a liquid inlet valve/drainage valve, and the third end of the three-way joint is provided with a There is a booster valve, the self-pressurized liquid nitrogen tank is equipped with a liquid nitrogen tank shut-off valve, the liquid nitrogen tank shut-off valve is connected to the liquid inlet valve/drain valve through a low-temperature resistant metal hose, and a liquid nitrogen tank is installed outside the liquid nitrogen freeze-thaw test chamber. Nitrogen booster tube, the upper end of the liquid nitrogen booster tube is connected to the inside of the liquid nitrogen freeze-thaw test chamber, and the lower end of the liquid nitrogen booster tube is connected to the booster valve;

The data acquisition system includes a high-frequency pressure sensor, a low-temperature strain gauge, and a temperature sensor probe. The low-temperature strain gauge and the temperature sensor probe are respectively arranged at different positions of the coal and rock samples. The low-temperature strain gauge is connected to the strain gauge through a strain gauge connection line, and the temperature sensor probe is The temperature sensor is connected through the temperature sensor cable, the strain gauge and the temperature sensor are connected to the computer, the high-frequency pressure sensor is arranged inside the liquid nitrogen freeze-thaw test chamber, and the high-frequency pressure sensor is connected to the computer through the USB data cable.

Further, one side of the sealing cover of the liquid nitrogen freeze-thaw test chamber is connected with the liquid nitrogen freeze-thaw test chamber through a hinge, and the other side of the seal cover of the liquid nitrogen freeze-thaw test chamber is provided with a seal cover handle and fastening bolts. There is a sealing groove at the position corresponding to the bottom surface of the sealing cover of the freeze-thaw test chamber and the top of the liquid nitrogen freeze-thaw test chamber.

Further, the liquid nitrogen freeze-thaw test chamber contains polyurethane heat insulation layer.

The liquid nitrogen cycle freeze-thaw anti-reflection simulation test method for coal rock samples of the present invention: in the liquid nitrogen freeze-thaw test, first open the liquid inlet valve/drain valve and the stop valve of the liquid nitrogen tank, and the liquid nitrogen is injected into the liquid by a low-temperature resistant metal hose. The nitrogen freeze-thaw test chamber controls the amount of liquid nitrogen injected through the liquid level display. When the pressure in the freeze-thaw freeze-thaw test chamber exceeds the rated pressure, the safety pressure relief valve automatically releases the pressure to ensure the safety of the test. After freezing for a certain period of time, open the liquid inlet valve/drain valve to discharge the liquid nitrogen, set the melting temperature by the temperature controller and heat it through the heater, the coal rock sample begins to melt, and complete a liquid nitrogen freeze-thaw cycle; then repeat it several times The above-mentioned liquid nitrogen freeze-thaw cycle was used to investigate the influence of different freeze-thaw variables on the strain and temperature of coal and rock samples, and the test data were recorded in the computer for processing and analysis.

Further, when investigating the effects of different freeze-thaw variables on the strain and temperature of coal samples, firstly, by setting different liquid nitrogen freezing times, different melting temperatures, different sample water contents, different liquid nitrogen expansion pressures and different cycle times , in order to obtain multiple coal rock samples under different freezing and thawing variables, and then number each coal rock sample, and measure the mechanical characteristic changes of each coal rock sample through triaxial/uniaxial compression test and torsional shear test Through nuclear magnetic resonance technology, ultrasonic technology, acoustic emission technology, electron microscope scanning technology, CT scanning technology to test the rock mass pore characteristics and microscopic shape change law of each coal rock sample, and finally through the analysis of the mechanical characteristic change law of each coal rock sample, Quantitative analysis of rock mass pore characteristics and microscopic shape changes, to find out the best liquid nitrogen freezing time and melting temperature, as well as the influence of sample moisture content and liquid nitrogen freeze-thaw cycle times on the process of liquid nitrogen freeze-thaw anti-permeability coal rock mass .

Further, during the test, the liquid nitrogen expansion pressure in the liquid nitrogen freeze-thaw test chamber is jointly controlled by the booster valve, the liquid nitrogen booster tube and the low-temperature pressure gauge. The control process is as follows: first open the booster valve, and the liquid nitrogen freeze-thaw test The liquid nitrogen in the box enters the liquid nitrogen booster tube, which is a single-layer heat-conducting copper tube, in which the liquid nitrogen absorbs heat and rapidly expands and vaporizes into nitrogen gas, and the nitrogen gas enters the liquid nitrogen freeze-thaw test chamber, which is displayed by the low-temperature pressure gauge The pressure value jointly controls the switch of the booster valve to control the liquid nitrogen expansion pressure in the liquid nitrogen freeze-thaw test chamber. The liquid nitrogen expansion pressure in the liquid nitrogen freeze-thaw test chamber is applied to the coal rock sample through liquid nitrogen, that is, during the test process The size of the confining pressure on coal rock samples.

Beneficial effects: the invention can simulate the process of liquid nitrogen circulation freeze-thaw anti-permeability coal or rock mass, and provides a realizable experimental platform for liquid nitrogen circulation freeze-thaw anti-permeability coal and rock extraction gas or shale gas, It can quantitatively study the basic parameters of liquid nitrogen freeze-thaw anti-permeability coal and rock mass, and provide scientific basis and theoretical basis for on-site liquid nitrogen freeze-thaw experiments.

Description of drawings

Figure 1 is a schematic diagram of a simulation test system for freezing and thawing of coal and rock samples in liquid nitrogen cycles;

Fig. 2 is a perspective view of a liquid nitrogen freeze-thaw box;

Fig. 3 is a flow chart of the simulated test method for freezing and thawing of coal and rock samples in liquid nitrogen cycles.

In the figure: 1-computer, 2-temperature controller, 21-heater, 22-thermostat connection line, 3-high frequency pressure sensor, 31-USB data line, 4-strain gauge, 41-low temperature strain gauge, 42-strain gauge connection line, 5-temperature sensor, 51-temperature sensor probe, 52-temperature sensor connection line, 6-liquid nitrogen freeze-thaw test chamber, 61-safety pressure relief valve, 62-low temperature pressure gauge, 63-row Empty valve, 64-sealing cover of freeze-thaw box, 65-sealing groove, 66-sealing cover handle, 67-fastening bolt, 68-liquid level indicator, 69-liquid level sensor, 610-liquid nitrogen booster tube, 611- Booster valve, 612-tee joint, 613-inlet valve/drain valve, 7-self-pressurized liquid nitrogen tank, 71-stop valve of liquid nitrogen tank, 72-low temperature resistant metal hose, 8-coal rock test Sample.

Detailed ways:

The present invention will be further explained below in conjunction with the accompanying drawings.

As shown in FIGS. 1 and 2 , the liquid nitrogen cycle freeze-thaw anti-reflection simulation test system for coal rock samples of the present invention includes a freeze-thaw device, a data acquisition system and a coal rock sample 8 .

The freeze-thaw device includes a liquid nitrogen freeze-thaw test chamber 6 and a self-pressurized liquid nitrogen tank 7, and a coal rock sample 8 is placed in the liquid nitrogen freeze-thaw test chamber 6. The liquid nitrogen freeze-thaw test chamber 6 contains a polyurethane insulation layer, and a heater 21 and a liquid level sensor 69 are arranged in the liquid nitrogen freeze-thaw test chamber 6. The heater 21 is connected to the temperature controller 2 through a thermostat connection line 22, and the temperature control The device 2 is connected to the computer 1, and the liquid level sensor 69 is connected to the liquid level indicator 68 arranged on the outside of the liquid nitrogen freeze-thaw test chamber 6.

The top of the liquid nitrogen freeze-thaw test chamber 6 is provided with a liquid nitrogen freeze-thaw test chamber sealing cover 64, and the liquid nitrogen freeze-thaw test chamber sealing cover 64 is provided with a safety pressure relief valve 61, a low temperature pressure gauge 62 and an exhaust valve 63, and the liquid nitrogen One side of the seal cover 64 of the freeze-thaw test chamber is connected with the liquid nitrogen freeze-thaw test chamber 6 through a hinge, and the other side of the seal cover 64 of the liquid nitrogen freeze-thaw test chamber is provided with a seal cover handle 66 and fastening bolts 67, which are used for freezing and thawing in liquid nitrogen. A sealing groove 65 is provided at the position corresponding to the bottom surface of the test box sealing cover 64 and the top of the liquid nitrogen freeze-thaw test box 6 . Close the sealing cover 64 of the liquid nitrogen freeze-thaw test chamber by fastening the bolt 67, and ensure the heat insulation of the test environment through the sealing groove 65. When the pressure in the liquid nitrogen freeze-thaw test chamber 6 exceeds the rated pressure, the safety pressure relief valve 61 automatically releases the pressure, To ensure the safety of the test, when sampling under pressure is required, first open the exhaust valve 63 to unload the nitrogen pressure before taking out the sample.

The bottom of the liquid nitrogen freeze-thaw test chamber 6 is connected to the self-pressurized liquid nitrogen tank 7 through a liquid nitrogen supply pipeline. The liquid nitrogen supply pipeline includes a three-way joint 612, and the first end of the three-way joint 612 is connected to the liquid nitrogen freeze-thaw test chamber. 6 Bottom, the second end of the three-way joint 612 is provided with a liquid inlet valve/drain valve 613, the third end of the three-way joint 612 is provided with a booster valve 611, and the self-pressurized liquid nitrogen tank is provided with a liquid nitrogen tank shut-off valve 71, The stop valve 71 of the liquid nitrogen tank is connected to the liquid inlet valve/drain valve 613 through a low temperature resistant metal hose 72, and a liquid nitrogen booster pipe 610 is arranged outside the liquid nitrogen freeze-thaw test chamber 6, and the upper end of the liquid nitrogen booster pipe 610 is connected to the liquid nitrogen tank. The nitrogen freeze-thaw test chamber 6 is internally connected, and the lower end of the liquid nitrogen booster pipe 610 is connected to a booster valve 611 . After the pressurization valve 611 is opened, the liquid nitrogen inside the liquid nitrogen freeze-thaw test chamber 6 enters the liquid nitrogen pressurization pipe 610 to exchange heat with the outside and generate vaporization, and the vaporized nitrogen enters the liquid nitrogen freeze-thaw test chamber 6 for pressurization. The pressure value displayed by the low temperature pressure gauge 62 is combined with the switch of the booster valve 611 to control the liquid nitrogen expansion pressure in the liquid nitrogen freeze-thaw test chamber 6, and the liquid nitrogen expansion pressure in the liquid nitrogen freeze-thaw test chamber 6 is applied to the On the coal rock sample 8, that is, the magnitude of the confining pressure on the coal rock sample 8 during the test.

The data acquisition system includes a high-frequency pressure sensor 3, a low-temperature strain gauge 41, and a temperature sensor probe 51. The low-temperature strain gauge 41 and the temperature sensor probe 51 are respectively arranged at different positions of the coal rock sample 8, and the low-temperature strain gauge 41 is connected through a strain gauge connection line. 42 is connected to the strain gauge 4, the temperature sensor probe 51 is connected to the temperature sensor 5 through the temperature sensor connection line 52, the strain gauge 4 and the temperature sensor 5 are connected to the computer 1, and the high-frequency pressure sensor 3 is arranged inside the liquid nitrogen freeze-thaw test chamber 6. The frequency pressure sensor 3 is connected to the computer 1 through a USB data line 31. The high-frequency pressure sensor 3 is used to record the internal pressure change of the liquid nitrogen freeze-thaw test chamber 6, that is, the change of the confining pressure of the coal rock sample 8, and the strain gauge 4 is used to record the lateral and horizontal changes of the coal rock sample 8 during the freeze-thaw cycle process. For the change data of the longitudinal strain, the temperature sensor 5 is used to record the data of the surface and internal temperature changes of the coal rock sample 8 .

As shown in Figure 3, the coal rock sample liquid nitrogen cycle freeze-thaw anti-reflection simulation test method adopting the above-mentioned test system of the present invention: in the liquid nitrogen freeze-thaw test, first open the liquid inlet valve/drain valve 613 and the liquid nitrogen tank cut-off Valve 71, liquid nitrogen is injected into the liquid nitrogen freeze-thaw test chamber 6 through the low-temperature resistant metal hose 72, and the amount of liquid nitrogen injected is controlled by the liquid level indicator 68. During the test, the liquid nitrogen expansion pressure in the liquid nitrogen freeze-thaw test chamber 6 is increased by The pressure valve 611, the liquid nitrogen booster pipe 610 and the low temperature pressure gauge 62 are jointly controlled, and the control process is as follows: firstly, the booster valve 611 is opened, and the liquid nitrogen in the liquid nitrogen freeze-thaw test chamber 6 enters the liquid nitrogen booster pipe 610, and the liquid nitrogen The booster tube 610 is a single-layer heat-conducting copper tube, in which the liquid nitrogen absorbs heat and rapidly expands and vaporizes into nitrogen gas, and the nitrogen gas enters the liquid nitrogen freeze-thaw test chamber 6, and the pressure value displayed by the low-temperature pressure gauge 62 jointly controls the switch of the booster valve 611, To control the liquid nitrogen expansion pressure in the liquid nitrogen freeze-thaw test chamber 6, the liquid nitrogen expansion pressure in the liquid nitrogen freeze-thaw test chamber 6 is applied to the coal rock sample 8 through liquid nitrogen, that is, the coal rock sample 8 is surrounded by the test process. Compression size. When the pressure in the freeze-thaw test chamber 6 exceeds the rated pressure, the safety pressure relief valve 61 automatically releases the pressure to ensure the safety of the test. After the coal rock sample 8 is frozen in liquid nitrogen for a certain period of time, the liquid inlet valve/drainage valve is opened The valve 613 discharges the liquid nitrogen, and the melting temperature is set by the temperature controller 2. After being heated by the heater 21, the coal rock sample 8 starts to melt, and a liquid nitrogen freeze-thaw cycle is completed. The law of the influence of the strain and temperature on the coal rock sample 8 under the variable conditions of freezing and thawing, the test data is recorded in the computer 1 for processing and analysis.

When investigating the influence of different freezing and thawing variables on the strain and temperature of coal sample 8, firstly, by setting different liquid nitrogen freezing time, different melting temperature, different water content, different liquid nitrogen expansion pressure and different cycle times, to obtain Coal rock samples 8 that have undergone anti-permeability fracturing under different freeze-thaw variables, and then number each coal rock sample 8, and measure the mechanical characteristic changes of each coal rock sample 8 through triaxial/uniaxial compression tests and torsional shear tests According to the law, through the nuclear magnetic resonance technology, ultrasonic technology, acoustic emission technology, electron microscope scanning technology, CT scanning technology to test the rock mass pore characteristics and microscopic shape change law of each coal rock sample 8, and finally through the mechanical characteristic change of each coal rock sample 8 Quantitative analysis of the law, rock mass pore characteristics and micro-morphological changes, to find out the best liquid nitrogen freezing time and melting temperature, as well as the water content of coal and rock samples 8 and the number of liquid nitrogen freeze-thaw cycles on the effect of liquid nitrogen freeze-thaw anti-permeability coal rock mass The law of influence in the process.

The above is only a preferred embodiment of the present invention, it should be pointed out that, for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications can also be made. It should be regarded as the protection scope of the present invention.

Claims (6)

1. the anti-reflection simulation experiment system of coal petrography sample liquid nitrogen cycle freeze thawing, comprise freeze thawing device, data acquisition system (DAS) and coal petrography sample (8), it is characterized in that: described freeze thawing device comprises frozen-thawed chamber (6) and from supercharging liquid nitrogen container (7), coal petrography sample (8) is placed in frozen-thawed chamber (6), well heater (21) and liquid level sensor (69) is provided with in frozen-thawed chamber (6), well heater (21) connects temperature controller (2) by temperature controller connecting line (22), temperature controller (2) connects computing machine (1), liquid level sensor (69) is connected with the liquid level display (68) being arranged on frozen-thawed chamber (6) outside, frozen-thawed chamber (6) top is provided with frozen-thawed chamber gland bonnet (64), frozen-thawed chamber gland bonnet (64) is provided with safety relief valve (61), low-temperature pressure table (62) and exhaust-valve (63), frozen-thawed chamber (6) bottom is connected from supercharging liquid nitrogen container (7) by liquid nitrogen feeding pipe,

Described liquid nitrogen feeding pipe comprises three-way connection (612), three-way connection (612) first end connects liquid nitrogen freezing-thawing test case (6) bottom, three-way connection (612) second end is provided with liquid feed valve/tapping valve (613), three-way connection (612) the 3rd end is provided with pressure charging valve (611), liquid nitrogen container stop valve (71) is provided with from supercharging liquid nitrogen container, liquid nitrogen container stop valve (71) connects liquid feed valve/tapping valve (613) by low temperature resistant metal hose (72), liquid nitrogen ascending pipe (610) is provided with in frozen-thawed chamber (6) outside, liquid nitrogen ascending pipe (610) upper end is communicated with frozen-thawed chamber (6) inside, liquid nitrogen ascending pipe (610) lower end connects pressure charging valve (611),

Described data acquisition system (DAS) comprises high-frequency pressure sensor (3), low temperature foil gauge (41) and temperature sensor probe (51), low temperature foil gauge (41) and temperature sensor probe (51) are arranged in coal petrography sample (8) diverse location, low temperature foil gauge (41) connects strainmeter (4) by strainmeter connecting line (42), temperature sensor probe (51) connects temperature sensor (5) by temperature sensor connecting line (52), strainmeter (4) is all connected computing machine (1) with temperature sensor (5), high-frequency pressure sensor (3) is arranged in frozen-thawed chamber (6) inner side, high-frequency pressure sensor (3) connects computing machine (1) by USB data line (31).

2. the anti-reflection simulation experiment system of a kind of coal petrography sample liquid nitrogen cycle freeze thawing according to claim 1, it is characterized in that: described frozen-thawed chamber gland bonnet (64) side is connected with frozen-thawed chamber (6) by hinge, frozen-thawed chamber gland bonnet (64) opposite side is provided with gland bonnet handle (66) and fastening bolt (67), is provided with seal groove (65) in the position that frozen-thawed chamber gland bonnet (64) bottom surface is corresponding with frozen-thawed chamber (6) top.

3. the anti-reflection simulation experiment system of a kind of coal petrography sample liquid nitrogen cycle freeze thawing according to claim 1, is characterized in that: described frozen-thawed chamber (6) is containing polyurethane adiabatic layer.

4. according to the anti-reflection simulation experiment method of coal petrography sample liquid nitrogen cycle freeze thawing of pilot system described in any one in claims 1 to 3 item, it is characterized in that: in frozen-thawed test, first liquid feed valve/tapping valve (613) and liquid nitrogen container stop valve (71) is opened, liquid nitrogen injects liquid nitrogen freezing-thawing test case (6) by low temperature resistant metal hose (72), injection liquid nitrogen quantity is controlled by liquid level display (68), when in freeze thawing freezing-thawing test case (6), pressure exceedes rated pressure, safety relief valve (61) releasing pressure automatically, the security of warranty test, after coal petrography sample (8) freezes certain hour in liquid nitrogen, open liquid feed valve/tapping valve (613) and discharge liquid nitrogen, heat through well heater (21) after setting melt temperature by temperature controller (2), coal petrography sample (8) starts to melt, complete a frozen-thawed circulation, repeatedly repeat the circulation of above-mentioned frozen-thawed subsequently, investigate to the strain of coal petrography sample (8) and temperature affecting laws under different freeze thawing Variable Conditions, test figure is recorded in computing machine (1) interior Treatment Analysis.

5. the anti-reflection simulation experiment method of coal petrography sample liquid nitrogen cycle freeze thawing according to claim 4, it is characterized in that: time under the different freeze thawing Variable Conditions of investigation to the strain of coal petrography sample (8) and temperature affecting laws, first by arranging the different liquid nitrogen freezing time, different melt temperature, different sample water percentage, different liquid nitrogen turgor pressures and different cycle index, to obtain the coal petrography sample (8) of anti-reflection fracturing under multiple different freeze thawing variable, then each coal petrography sample (8) is numbered, the mechanical characteristics Changing Pattern of each coal petrography sample (8) is recorded by three axles/uniaxial compression test and torsion shear test, pass through nuclear magnetic resonance technique, ultrasonic technology, acoustic emission, electron-microscope scanning technology, the rock mass pore character of each coal petrography sample (8) of CT scan technical testing and microscopic pattern Changing Pattern, finally by the mechanical characteristics Changing Pattern to each coal petrography sample (8), rock mass pore character and the quantitative test of microscopic pattern Changing Pattern, find out best liquid nitrogen freezing time and melt temperature and sample water percentage and frozen-thawed cycle index to the affecting laws in the anti-reflection coal and rock process of frozen-thawed.

6. the anti-reflection simulation experiment method of coal petrography sample liquid nitrogen cycle freeze thawing according to claim 5, it is characterized in that: in process of the test, the interior liquid nitrogen swelling pressure of frozen-thawed chamber (6) are by pressure charging valve (611), liquid nitrogen ascending pipe (610) and low-temperature pressure table (62) jointly control, control procedure is as follows: first open pressure charging valve (611), liquid nitrogen in frozen-thawed chamber (6) enters liquid nitrogen ascending pipe (610), liquid nitrogen ascending pipe (610) is individual layer heat-conducting copper pipe, liquid nitrogen rapid expansion of absorbing heat wherein is vaporizated into nitrogen, nitrogen enters frozen-thawed chamber (6), the switch that force value jointly controls pressure charging valve (611) is shown by low-temperature pressure table (62), control the liquid nitrogen turgor pressure in frozen-thawed chamber (6), the interior liquid nitrogen swelling pressure of frozen-thawed chamber (6) are applied on coal petrography sample (8) by liquid nitrogen, the i.e. suffered confined pressure size of coal petrography sample (8) in process of the test.