Shengqiang Yang

A Mesostructure-based Damage Model for Thermal Cracking Analysis and Application in Granite at Elevated Temperatures

Thermal stress within rock subjected to thermal load is induced due to the different expansion rates of mineral grains, resulting in the initiation of new inter-granular cracking and failure at elevated temperatures. The heterogeneity resulting from each constituent of rock should be taken into account in the study of rock thermal cracking, which may aid the better understanding of the thermal cracking mechanisms in rock. In this paper, a mesostructure-based numerical model for the analysis of rock thermal cracking is proposed on the basis of elastic damage mechanics and thermal–elastic theory. In the proposed model, digital image processing (DIP) techniques are employed to characterize the morphology of the minerals in the actual rock structure to build a numerical specimen for the rock. In addition, the damage accumulation induced by thermal (T) and mechanical (M) loads is considered to modify the elastic modulus, strength and thermal properties of individual elements with the intensity of damage. The proposed model is implemented in the well-established rock failure process analysis (RFPA) code, and a DIP-based RFPA for the analysis of thermally induced stress and cracking of rock (abbreviated as RFPA-DTM) is developed. The model is then validated by comparing the simulated results with the well-known analytical solutions. Finally, taking an image from a granite specimen as an example, the proposed model is used to study the thermal cracking process of the granite at elevated temperatures and the effects of temperature on the physical–mechanical behaviors of the granite are discussed. It is found that thermal cracks mostly initiate at the location of mineral grain boundaries and propagate along them to form locally closed polygons at the elevated temperatures. Moreover, the effects of temperature on the uniaxial compressive strength and elastic modulus of the granite are quite different. The uniaxial compressive strength decreases consistently with increasing temperature, but there exists a threshold temperature for elastic modulus which starts to decrease as the temperature increases after it exceeds the threshold.

Free Radical and Functional Group Reaction and Index Gas CO Emission during Coal Spontaneous Combustion

ABSTRACT To explore the reaction mechanism of the low-temperature oxidation of coal and the law of active groups producing CO through oxygenolysis, this article analyzed the reaction characteristics of free radicals and functional groups during low-temperature oxidation by using electron spin resonance and Fourier transform infrared technologies. Based on gas chromatograph analysis, the index gas CO produced by active groups during coal spontaneous combustion was analyzed by studying the reaction of free radicals and oxygen-containing functional groups. The experiments showed that, with rising oxidizing temperatures from 30°C to 230°C, the g-value increased first and then decreased, while the concentration of free radicals constantly rose by 48.3%. When the oxidation temperatures rose to 100°C, the maximum g-value occurred, and the concentration of free radicals changed from slowly increasing by 10.2% to dramatically rising by 31.7%. Moreover, the relative intensities of various oxygen-containing functional groups, including –OH, C=O, C–O, and –COOH, exhibited different change laws with rising temperatures. The –OH constantly declined by 67.0% and began to slowly decrease when the temperatures reached 100°C, while C=O declined first, then increased, and began a rapidly rising stage after the temperatures increased to 80°C. The C–O was nearly unchanged below 180°C, while the growth was accelerated after reaching 180°C, and –COOH decreased at first, then rapidly increased after 80°C. Based on the free radical theory of coal spontaneous combustion, it was revealed that carbonyl radicals are the important active groups to produce CO. It can be assumed that a C=O functional group can also produce CO by taking phenylacetaldehyde as an example. A C–O functional group can produce CO by generating carbonyl radicals, so carbonyl radicals are the direct active groups to produce CO during low-temperature oxidation. CO concentration sharply increased after the oxidizing temperatures reached 100°C, which is consistent with the change of concentration of free radicals. There is a significant inflection point of oxygen-containing functional groups at 80°C, so 80–100°C (especially 100°C) is the allowable maximum temperature of the low-temperature oxidation of coal mass.

The Relationship between Functional Groups and Gaseous Productions and Micropore Structures Development of Coal Oxidized at Low Temperature under Methane-Diluted Atmospheres

ABSTRACT As coal reservoirs containing high methane content are more prone to spontaneous combustion, disasters caused by the combined effects of methane and coal spontaneous combustion have been increasingly prominent. Given that air flows in different zones in a goaf contain methane with different contents, the mechanism governing the changes of micropores and functional groups of left-over coal subjected to spontaneous combustion in a goaf with air flows containing methane were investigated by sufficiently considering the diluted influence of methane. The change of microscopic functional groups and variation of gaseous products from coal oxidized at low-temperature (i.e., 70°C< it < 230°C) under different oxidizing atmospheres were separately acquired by employing Fourier transform infrared spectroscopy (FTIR) and gas chromatography. By using the Brunauer, Emmett and Teller (BET) method, a low-temperature nitrogen adsorption experiment was carried out on coal samples taken from the Shigang coal mine in Shanxi province, China under different methane-diluted oxidizing atmospheres. The results showed that the content of aliphatic hydrocarbons (methyl and methylene) of the coal oxidized at low temperature decreased with rising oxidizing temperature and reached a maximum at the condition of a 25% methane concentration. Meanwhile, other oxygen-containing functional groups and corresponding gaseous products were generated. The aforementioned results indicated that the initial temperature for the generation of CO and the amount generated both showed delayed effects in oxidizing atmospheres with different methane concentrations. By carrying out a low-temperature nitrogen adsorption experiment, it was determined that the average sizes of the pores in coal decreased in an oxidizing atmosphere (i.e., < 15% methane concentrations at a high temperature). However, the specific surface area (SSA) and accumulative total internal surface area of the pores increased. Additionally, the pore structure in the coal tended to be microscopic and complex in the aforementioned oxidizing atmosphere, as determined by using the fractal dimension D. The pore structure in the coal was increasingly complex after being oxidized in an oxidizing atmosphere containing methane, which increased the likelihood of the coal oxidation reaction and increased the occurrence probability of coal spontaneous combustion.

Analysis of the Talent Demand and Cultivation of Safety Engineering in the New Normal

In China, the main features of the new normal are the slowdown of economic growth, the release of market dynamics and the optimization and upgrading of the industrial structure. In this period, primary and secondary industry are facing problems such as massive overcapacity, large amounts of major hazards and severe safety production. With the increasing requirements and expectations for security, the scale and cultivation orientation of the safety engineering specialty cannot satisfy the demands for the development of various industries. Therefore, it is necessary to broaden and extend the cultivation orientation of the safety engineering specialty. This paper discusses the developing trends, the talent demand, and the construction and development of the safety engineering specialty. Then, it analyzes the talent demand and cultivation of safety engineers in the new normal. It is suggested that two types of safety engineering talent—academic and applied—should be cultivated. Concerted effort should be devoted to establishing a new mechanism of school-enterprise cooperation. All the above efforts may provide strong support for safety production in China.