We report the first results of a light weakly interacting massive particles (WIMPs) search from the CDEX-10 experiment with a 10 kg germanium detector array immersed in liquid nitrogen at the China ...Jinping Underground Laboratory with a physics data size of 102.8 kg day. At an analysis threshold of 160 eVee, improved limits of 8×10^{-42} and 3×10^{-36} cm^{2} at a 90% confidence level on spin-independent and spin-dependent WIMP-nucleon cross sections, respectively, at a WIMP mass (m_{χ}) of 5 GeV/c^{2} are achieved. The lower reach of m_{χ} is extended to 2 GeV/c^{2}.
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As a non-aqueous medium for increasing permeability in coal seams, liquid nitrogen fracturing has been widely studied. The changes of the pores in coal fractured by liquid nitrogen have important ...effects on coalbed methane (CBM) migration. It is difficult to thoroughly characterize the pore structure in coal using a single method. Therefore, this study carried out a detailed study of the pores in coal samples fractured by liquid nitrogen using both nitrogen adsorption and mercury intrusion. The results show that combining these methods can accurately determine pore sizes and specific surface areas in the samples tested. The maximum liquid nitrogen adsorption capacity and the injected mercury volume in the samples were positively correlated with the freezing times and freeze–thaw cycles. This indicated that the number of pores in the coal gradually increased. Cumulative total pore and seepage pore volumes in the samples showed a positive exponential correlation with freezing time. The volume increases also correlated with the number of freeze–thaw cycles and this increase followed a quadratic function. The cumulative specific surface areas also varied with freezing time but the areas first rose and then fell as the freeze–thaw cycles increased. Liquid nitrogen freezing time significantly affects the micropores and specific surface area. However, freezing time has only a minor effect on the larger seepage pores and the total pore volume. Liquid nitrogen freeze–thaw cycles help the formation of connections between micropores and larger pores and thus promote the development of fracture networks. This provides favorable conditions for CBM production.
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•The effect of liquid nitrogen is related to coal rank.•Different functional groups have different changing laws.•The contact angle of coal affects the gas adsorption capacity.
...Fracturing coal with liquid nitrogen (LN2) has a positive effect on coalbed methane (CBM) extraction. The purpose of this study is to investigate the gas adsorption capacity of coals of different ranks after the coals are frozen with LN2. For this purpose, gas adsorption tests, contact angle measurements, and Fourier transform infrared spectroscopic analyses were conducted on both non-frozen samples of lignite, bituminous coal, and anthracite and on identical samples after the samples had been frozen by immersion in LN2. The test results show that after LN2 freezing for 180 min, the maximum amounts of gas that can be adsorbed by the three types of coal are reduced, the coals’ contact angles are smaller, and the proportions of several surface functional groups on the coals change. The effects of LN2 freezing on the coals are related to the coal’s rank. With the increase of coal rank, the influence on the gas adsorbed capacity of coal first increases and then decreases. The gas adsorption capacities of the coals are linearly correlated with proportions of aliphatic hydrocarbon, oxygen-containing, and substituted benzene functional groups but in a quadratic polynomial relationship with the contact angle. The results of the research reported here clarify the gas adsorption characteristics for coals of different ranks frozen by LN2 and advance our understanding of LN2 fracturing technology.
•Mechanical changes of frozen coals under different liquid nitrogen treatments.•The freezing-thaw damage effect of LN2 and the principle of freezing enhancement.•The mechanics principle of liquid ...nitrogen changing the pore structure of coal.
Anhydrous liquid nitrogen cracking low-permeability coal seams can improve the efficiency of coalbed methane extraction. Under normal pressure, liquid nitrogen (-196℃) contacts the coal body, and the temperature stress and frost heave force generated will change the mechanical properties of the coal body, cause internal structural damage, and increase the seepage channel of coalbed methane. In order to study the influence of different liquid nitrogen freezing variables on the mechanical properties of coal, Brazilian splitting, uniaxial compression, and ultrasonic velocity measurement tests of saturated frozen coal were carried out. The results show that a single freezing of liquid nitrogen has a certain enhancement effect on the mechanical properties of coal: the tensile strength increases by 64.0%, the uniaxial compressive strength increases by 54.6%, but as the freezing time increases, the coal strength increases first and then decreases The strength of coal under the action of freeze–thaw cycles shows an exponential decline in varying degrees: tensile strength has dropped by 81.3%, uniaxial compressive strength has dropped by 68.9%, and the degree of decline is positively correlated with the number of freeze–thaw cycles. The change in mechanical strength changes the elastic stage. The freezing enhancement factor I and the freeze–thaw damage factor D are defined by the change of elastic modulus. I presents a quadratic function relationship that first increases and then decreases with freezing time, and D continues to decrease as the number of freeze–thaw cycles increases. The longitudinal wave velocity first increases and then decreases with the freezing times, and decreases with the increase of the number of freeze–thaw cycles. The porosity is negatively correlated with the wave velocity. The wave speed decreases, the porosity increases, and the internal pores of the coal are connected to form a fracture network, which accelerates the damage of the coal and reduces the mechanical properties of the coal. Through analysis, it is concluded that when liquid nitrogen acts on coal, the change in mechanical properties is the result of the combined action of multiple forces.
•A non-destructive pore characterization of coal in different scales during liquid nitrogen cold shocks.•Microscopic pore structure test and analysis by nuclear magnetic resonance ...technology.•Evolution of mesoscopic pore-fracture network by steromicroscope imaging and quantitative analysis.•Nondestructive characterization of macroscopic damage by ultrasonic wave velocities and dynamic parameters.•Combination of macroscopic, mesoscopic and microscopic pore structure characteristics.
Liquid nitrogen cyclic cold shock is a new waterless fracturing method suitable for coal seams in arid areas. The pore structure of coal after cold shock plays a key role in gas drainage, but there is currently a lack of study combining macroscopic-mesoscopic-microscopic scale. Multiscale pore characterization will be helpful to clarify the damage mechanism of liquid nitrogen cold shock to coal. In this paper, the same coal samples were tested for ultrasonic inspection, stereomicroscopic imaging and nuclear magnetic resonance (NMR) after different cold shock times. The macroscopic damage were studied by ultrasonic wave; the changes of mesoscopic pores and fractures (0.1 ~ 10 mm) were studied by stereoscopic microimaging and digital image processing; the changes of microscopic pores (2 nm ~ 0.1 mm) were studied by NMR. Finally, the macroscopic - mesoscopic - microscopic pore characteristic parameters were coupled. The main results are as follows: With the increase of cold shock times, the P/S wave velocities decreased, the dynamic parameters decreased linearly, and the macroscopic damage of coal samples was obvious. The number of mesoscopic pores, surface porosity, and probability entropy increased. As can be seen from the T2 spectra of coal samples, the microscopic pores expanded and increased with cold shock times. The effective porosity increased, among which the increase of micropores contributed the most. The macroscopic wave velocity and dynamic parameters of coal samples were negatively correlated with porosities. The calculation models of T2 cutoff value were established by multiple linear regression analyses, which were in good agreement with the experimental values.
Liquid nitrogen (LN2) fracturing, as an environmentally friendly waterless fracturing technology, is given more and more attention in coalbed methane (CBM) exploitation. In this paper, a series of ...microscopic tests and a thermal damage simulation were combined to evaluate the changes in the internal structure of the coal caused by the LN2 cooling, firstly. Then, Brazilian splitting tests were carried out on the bedding coal, and influences of cryogenic cooling fracturing and bedding orientation on mechanical properties and fracture morphology of the bedding coal were analyzed. The results show that the micro-structure of the coal is obviously damaged under the LN2 cooling due to the thermal tensile stress. The P-wave velocity, tensile strength, Brazilian splitting modulus, and brittleness index of the coal sample have significant anisotropic characteristics and their values are reduced after the LN2 cooling. The weakening degree in mechanical characteristics of the coal induced by the LN2 cooling is also closely related to the bedding direction of the coal. After the LN2 cooling treatment, the maximum and minimum reduction of the tensile strength is at the bedding angle 0° and 45°, respectively, with the reduction of 55.81% and 6.98%, respectively. The maximum and minimum changes of the Brazilian splitting modulus and brittleness index before and after the LN2 cooling locate at 0° and 60°, respectively. The LN2 cooling treatment can increase the ductility of the coal. The length and surface complexity of the induced fracture increase after the LN2 cooling. The increase in the fracture length is particularly evident at the high bedding angle, while the significant increase of the crack surface complexity is more likely to occur at the low bedding angle. The results contribute to the design of LN2 fracturing for coal beds.
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•Vacuum-mediated boiling point suppression of liquid nitrogen improves parahydrogen conversion.•Low-cost modification of existing liquid nitrogen cooled parahydrogen ...generators.•Improvement translates across hydrogen flow rates.•Of interest to researchers beginning parahydrogen experiments or those at institutions with limited research support.
The isomeric enrichment of parahydrogen (pH2) gas is readily accomplished by lowering the gas temperature in the presence of a catalyst. This enrichment is often pursued at two distinct temperatures: ∼51% pH2 is generated at liquid nitrogen temperatures (77 K), while nearly 100% pH2 can be produced at 20 K. While the liquid nitrogen cooled generator is attractive due to the low cost of entry, there are benefits to having access to greater than 51% pH2 for enhanced NMR applications. In this work, we introduce a low-cost modification to an existing laboratory-constructed liquid nitrogen cooled pH2 generator that provides ∼ 65% pH2. This modification takes advantage of vacuum-mediated boiling point suppression of liquid nitrogen, allowing the temperature of the liquid to be lowered from 77 K to nitrogen’s triple point of 63 K. The reduced temperature allowed for the generation of parahydrogen fractions of 63–67% at gas flow rates from 20 to 1000 standard cubic centimeters per minute. We compare this to equivalent experiments that did not utilize the temperature-lowering effects of pressure reduction; these controls generally maintained pH2 fractions of ∼ 50%. All results (experimental and control) agree with the theoretically expected parahydrogen generation at these temperatures. This straightforward modification to an existing pH2 generator may be of interest to a broad range of scientists involved with parahydrogen research by introducing a simple and low-cost entryway to increased pH2 fractions using a conventional liquid nitrogen cooled generator.
The aims of this research are to quantitatively evaluate the complexity of the pore structure in coal frozen with liquid nitrogen (LN2) and then study the influence of the modified pore system on ...coalbed methane (CBM) extraction. To do this, nuclear magnetic resonance (NMR) and fractal dimension theory were used to determine the properties of the coal's pore system after samples of low rank coal had been frozen and then thawed. The fractal dimensions of pores in frozen-thawed coal samples were divided into five types according to pore size and the state of the fluid in the coal pores. The results showed that the fractal dimension DA of adsorption pores was less than two, indicating that these pores did not exhibit fractal characteristics. The fractal dimensions Dir and DT representing closed pores and total pores presented low fitting precision, so the closed pores showed insignificant fractal characteristics. However, the fractal dimensions DF and DS representing open pores and seepage pores had high fitting precision, suggesting that open and gas seepage pores exhibited a favorable fractal characteristic. Correlation analysis revealed that DF and Ds were negatively correlated with LN2 freezing time and the number of freeze-thaw cycles. After being frozen and thawed, coal porosity and permeability showed a strong negative correlation with fractal dimension and this relationship allowed predictive models for permeability and fractal dimensions (DF and DS) to be constructed. The models showed that the smaller the fractal dimension, the more uniformly the pores were distributed and the higher their degree of connection. These properties favor the production of CBM. This study also showed that compared with single LN2 freezing events, repeated cyclic freezing with LN2 followed by thawing is more favorable for CBM production.
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•NMR and fractal dimension were used to determine the properties of coal pores.•Open pores and gas seepage pores exhibit significant fractal characteristics.•DF and Ds are negatively correlated with LN2 freeze-thaw time and cycles.•Cyclic freeze-thaw with LN2 is more favorable for CBM production.
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•The heat treatment increases the pore volume of the coal sample. When the heat treatment temperature exceeds a certain limit, the pore volume decreases.•The pore volume of ...heat-treated coal sample after liquid nitrogen increased significantly.•The pore volume of the water-saturated coal sample is obviously higher than that of the dry coal sample.•The fractal dimension D1 of the coal sample treated with heat treatment and liquid nitrogen freezing increased, and D2 decreased, indicating that the surface of the coal sample was rough after the treatment and the pore structure became single.
To explore the evolution of pores and fractures of coal under heating–freezing effects, a tube furnace was used to heat samples. The coal sample was heated to 100 °C and 300 °C in air, and liquid nitrogen was used to freeze the heated coal sample. Low-temperature nitrogen adsorption experiments were used to analyse the damage degree of the coal pore structure quantitatively, and the surface characteristics of the coal sample were qualitatively analysed using a scanning electron microscope. Experimental studies have shown that liquid nitrogen injection increases the pore volume of coal. By injecting liquid nitrogen into the heat-treated coal sample, the pore volume of the macropores of the water-saturated coal sample increased by 19.61%. When the heat treatment temperature reached 300 °C, the macroporosity of the dry coal sample increased by 38.75%. In addition, the fractal dimension of the pores increased after liquid nitrogen treatment, indicating that the freezing of liquid nitrogen caused the surface of the coal sample to become rough. The development of pores and fractures caused by liquid nitrogen injection results from the low-temperature fracturing characteristics of liquid nitrogen and the volume expansion caused by the freezing of pore water.
The propensity of residual oxidized coal to be reignited within enclosed fire areas in coal mines under liquid nitrogen (LN2) cold soaking remains unclear. Therefore, scanning electron microscopy ...(SEM), low-temperature nitrogen adsorption (LTNA), and thermogravimetric (TG) experiments were conducted to characterize the pore structure and secondary oxidation of oxidized cold-soaked coal. The results indicate that pre-oxidation or cold soaking alone can promote the generation and merging of pores and make the pore structure more complex, especially when pre-oxidized. However, pore evolution is suppressed and then improves when lignite is simultaneously subjected to pre-oxidation and cold soaking. In particular, the macropore volume and specific surface area decrease and then increase. The cryogenic shrinking of the LN2 results in the collapse of the loose coal matrix, which subsequently blocks the macropores. In contrast, vaporization expansion enhances pore connectivity with continuous LN2 injections, transforming mesopores into macropores, especially in the pore ranges of 2–5 nm and >50 nm. Additionally, pre-oxidation or cold soaking alone improves the oxidizing activity and combustion performance of raw coal. However, continuous cold soaking initially increases and subsequently decreases the characteristic temperatures and activation energy of oxidized coal. Oxidization at 200 °C with one cold soaking can further lower reignition propensity. The average activation energy of oxidized coal decreases by 22.9%–29 % when soaked more than three times. The improvement in combustion property is closely related to the increasing number of macropores because a larger pore volume and specific surface area are conducive to oxygen diffusion and adsorption. Therefore, pre-oxidation and cold soaking promote and inhibit the reignition of coal when performed simultaneously. These results provide theoretical guidance for rational liquid-nitrogen injection to control coal fires.
•Pore structure and combustion property of lignite affected by pre-oxidation and liquid nitrogen cold soaking were investigated.•Pre-oxidation significantly encouraged the formation of new pores, while liquid nitrogen cold soaking fostered the merging of mesopores.•The coupled effect of pre-oxidation and cold soaking first inhibited and the facilitated the macropore evolution and reignition risk of oxidized coal.•The oxidizing activity and combustion performance are mainly affected by the volume and specific surface area of macropore.
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