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To judge the coal and gas outburst (hereinafter referred to as ‘outburst’) risk of working faces in coal roadways according to the index of drilling cuttings weight (DCW), the influence factors and critical value of DCW were investigated. The study was conducted by deducing the theoretical calculation formula for DCW and using a set of self-designed, self-machined measuring apparatus for DCW which can be used to simulate the working face in a real coal roadway. The theoretical calculation formula for DCW indicates that the DCW increases with the increase of in situ stress and gas pressure while it also increases with decreasing coal strength. Among the three factors, with the change of gas pressure, the change in DCW is the smallest. On this basis, so that the critical value of DCW for judging the outburst risk of coal masses can better indicate the outburst risk at a working face, the tests for measuring DCWs under different outburst risk levels were carried out by using a set of apparatus for outburst risk which can be used to simulate the working face in a coal roadway by taking gas pressure as a controllable factor. The test results show that, with increasing outburst risk in a simulated coal seam, the DCW linearly increases at first and then suddenly rises. It also implies that the gas pressure exerts significant influences on DCW only when the outburst risk of a coal mass increases to a certain degree. There is a linear relationship between DCW and initial expansion energy of released gas (IEERG). On this basis, according to the critical values of IEERG indicating weak outburst and strong outburst risks, the critical values of DCW for outburst risk are judged. By using discriminant analysis, it can be obtained that the critical value of DCW when a weak outburst occurred is 4.88 kg/m. The critical value can be used to distinguish data samples with measured DCWs having weak outburst risk, whose accuracy reaches 89.47%. The critical value of DCW when a strong outburst occurred is 7.55 kg/m, which can accurately (100%) distinguish between data samples from weak and strong outbursts based on the measured DCWs. The final judgement accuracy of outburst risk is 93.75%.

Changes in coal seam permeability induced by roadway excavation may lead to many engineering problems, such as low gas extraction concentration in in-seam boreholes, coal and gas outburst disasters, and gas emissions in the process of coal roadway excavation. To solve these problems, this paper proposed a new method to directly determine the distribution of coal seam permeability around roadways. For this purpose, method principle was analyzed, equipment was developed, and field experiments were carried out in Baiyangling coal mine. Finally, the engineering applications of the method were analyzed, and a field application effect experiment was conducted to verify the new method. The results showed that the coal seam permeability around roadways was divided into four stages. In stages I and II, permeability increased, in stage III, permeability decreased, and stage IV maintained the initial permeability of the coal seam. These test results will aid in increasing the gas extraction concentration of in-seam boreholes and will guide the design of gas control areas. Using the verification experiment, it was concluded that the new method is able to accurately determine the redistribution of coal seam permeability caused by roadway excavation and provide theoretical guidance in solving engineering problems.

Side-chain fluorination of conjugated polymers has been proved to be a highly effective approach for optimizing optical and electrical properties of the relative polymers. However, current studies on the fluorination are all based on symmetric molecular structures. In this work, two new D-A type photovoltaic polymers, namely PITPh-DTffBT and PITPhf-DTffBT, based on asymmetric indenothiophene (IT) donor units with alkoxyphenyl or fluoroalkoxyphenyl substituents were designed and synthesized. Effects of the fluorine substitution in the asymmetric IT donor units on the electronic structure, ordering structure, photovoltaic properties, and charge generation and recombination dynamics were investigated. It is found that side-chain fluorination in the asymmetric donor units of the D-A polymers endowed the relative polymer with a deeper HOMO level, higher and more balanced charge mobilites, increased charge dissociation efficiency and reduced bimolecular recombination. As a result, the bulk heterojunction solar cell based on the blend film of PITPhf-DTffBT and PC71BM demonstrated an efficiency of 7.03%, whereas the cell efficiency based on PITPh-DTffBT was only 5.68%. These results indicate that our design strategy by introducing a fluoroalkoxyphenyl unit as side chain to fabricate asymmetric IT-based polymer is efficient in improving the photovoltaic performance. We believe that the results provide new insights into the design of high-performance semiconducting photovoltaic polymers.

Viscosities of ternary mixtures of VOSO4 + H2SO4 with water were measured across the entire range of total ionic strength (IT = 1 to 11) mol·kg−1 and over a temperature range of (283.15–318.15) K. The viscosity B-coefficients of the binary aqueous were determined from Jones-Dole equation. Using the B-coefficients, the activation parameters for viscous flow of the solutes were further evaluated. On the basis of Eyring’s equation, a semi-empirical method to estimate the viscosity for concentrated and mixed electrolyte solutions was put forward. The estimated viscosity values of VOSO4 + H2SO4 + H2O are in quite good agreement with the corresponding experimental results with the average absolute relative deviation of 0.96%.

Coal and gas outbursts (hereinafter, collectively referred to as ‘outbursts’) often last for just a few tens of seconds and occur mostly in gas-bearing tectonically-deformed (TD) coal. It has been proved that, in comparison to primary-undeformed (PU) coal, TD coal is more susceptible to outbursts due to its lower mechanical strength, stronger gas-adsorption capability, and faster gas-desorption rate. At the same time, the energy required for the outburst process is mainly provided by the gas. However, the characteristics of the gas-desorption behavior and gas-release energy in TD and PU coal during the first dozen seconds or so have rarely been studied. In this study, gas desorption experiments were carried out on TD and PU coal at different gas pressures using self-assembled desorption equipment. The experiments focus on roughly the first 13 s. The results show that the drop rate of gas pressure is lower with TD coal compared to PU coal, while the gas desorption rate is higher for TD coal compared to PU coal. Most of the gas is released from the two types of coal in the first few seconds. For both TD and PU coal, the time taken to release 90% of the total gas expansion energy (TGEE) is shorter than that to release 90% of the total gas energy (TGE). At the different gas pressures used, the TGEE of the two types of coal accounts for about 14–16% of the TGE. The higher the gas pressure is, the greater the TGEE released by the coal samples gets, and thus the larger the TGEE: TGE ratio becomes. Although the TGEE released from the TD coal is about 3 times that released from the PU coal, it accounts for a slightly smaller proportion of the TGE compared to that in the PU coal samples.

Coal and gas outbursts (hereinafter referred to as ‘outbursts’) result in serious damage and often occur in tectonically-deformed coal that is rich in gas. It has been demonstrated that the level of outburst risk declines with increasing coal moisture content. Moreover, an outburst generally lasts for mere tens of seconds and the required energy is mainly provided by the gas expansion energy. However, the gas desorption and energy release characteristics of tectonically deformed coal with different moisture contents during the first dozen seconds have not often been studied. In this study, a set of self-designed gas desorption equipment was used to carry out gas adsorption-desorption experiments in the first dozen seconds (about 13 s) on coal with different moisture contents. The results show that the drop rate of the gas pressure increases with an increase in coal moisture content, and the total amount of gas desorption and the mass flow rate of gas desorption reduce correspondingly in the gas desorption process. Moreover, the gas velocity also slows with increasing moisture content. Under different gas pressures, the total gas expansion energy (TGEE) and the total gas energy (TGE) released from the coal decrease with the increasing moisture content. Correspondingly, it takes a shorter time for coal to release 90% of the TGEE and the TGE. For all of the coal samples, the time taken for releasing 90% of the TGEE is shorter than that for releasing 90% of the TGE. Meanwhile, the ratio of TGEE in the TGE increases with the increasing moisture content. The TGEE accounts for 14–16% of the TGE released from coal samples of different moisture contents under different gas pressures.

Coal and gas outburst accidents occurring in underground coal mines generally cause great casualties and economic losses, especially in construction areas of coal seams where the outburst risks are not accurately evaluated. Therefore, accurately identifying the outburst risks of coal seams is necessary and critical to prevent and control outbursts. In order to improve the prediction accuracy for outbursts in working faces of coal roadways, a self-designed drilling device that can simulate the in-situ outburst prediction of coal roadways was used in this study. We used the coal samples with different coal ranks to adsorb CO 2 and N 2 with different pressures in the laboratory, so as to simulate the coal seams with different outburst risks. Finally, boreholes for predicting the outburst risk were drilled in the simulated coal seams to study the correlation between common dynamic phenomena (e.g. gas and coal being ejected from the borehole (GCEB) and drill pipe being stuck by coal mass in the borehole (DPSC)) and outburst risks. The results show that the greater the outburst risks of coal seams, the more frequent the occurrence of GCEB and DPSC is. The GCEB phenomenon for outburst prediction is attributed to small-sized coal and gas outbursts in boreholes, while the occurrence of DPSC phenomenon does not affirm that there definitely are outburst risks on working faces. The DPSC phenomenon indicating outburst risks is generally accompanied with GCEB, which form a linkage system and are triggered successively.

Two new donor-acceptor (D-A) polymers, namely PNDT-QX and PNDT-QXS, with naphtho [1,2-b:5,6-b']dithiophenes (NDT) as donor unit and quinoxaline (QX) as acceptor unit, were designed and synthesized. The difference in the structures of the two polymers is that, in PNDT-QXS the side chains on the QX acceptor units are (2-ethylhexyl) thio thienyl groups, whereas in PNDT-QX the side chains are 2-ethylhexyl thienyl groups, which allow us to systematically compare the effects of alkylthio group on optical, electrochemical, molecular packing properties and PSCs performance of polymers. Our study demonstrated that, relative to alkyl group, alkylthio substituents tune down the HOMO and LUMO energy levels, red-shift and broaden the absorption, and enhance the interchain interaction, therefore significantly affect the BHJ morphology of the copolymer with PC71BM. As a result, the polymer solar cell based on the blend of PNDT-QXS and PC71BM got a highest PCE of 5.03% without using extra post-treatment steps, whereas the PNDT-QX cell only had a 1.93% highest PCE. The results prove for the first time that introducing alkylthio side chains into acceptor moiety would also be an effective strategy for improving the photovoltaic performance of the D-A copolymers.

Coal and gas outburst (hereinafter referred to as “outburst”) is a complicated dynamic disaster formed in the temporal and spatial evolution process. To further improve our understanding of outburst mechanism and help more effective outburst prevention and control measures to be devised, a finite number of outburst simulation tests were carried out in the study. The results show that the contribution of the gas pressure to the energy required for an outburst is hundreds of times that of the in situ stress during the outburst. With increasing mining depths, the higher the in situ stress, the gas pressure, and the temperature; however, the temperature shows the lowest influence on the outburst intensity among various parameters such as gas pressure, the thickness of soft coal, and the temperature. The initial expansion energy of releasing gas (IEERG) is an index reflecting the influences of in situ stress, gas pressure, and coal strength on outburst intensity. The test results indicate that the potential amount of coal ejected can be predicted by measuring the IEERG of coal masses and the thickness of soft coal in outburst-prone zones. According to the failure characteristics of coal masses after an outburst, the larger the outburst intensity is, the smaller the volume of residual coal area relative to the outburst holes. The shapes of extrusive coal-gas flows after an outburst show that the outburst process is a chain of events occurring over multiple cycles as follows: preparation, initiation, development, and termination. The recorded video footage of an outburst also shows this.

Coal and gas outburst is a serious coal and rock dynamic disaster, and effective prediction is essential to control. The initial expansion energy of released gas (IEERG) is a new method to predict coal and gas outburst, which depends primarily on the gas pressure and degree of coal fragmentation. The IEERG reflects the effects of the ground stress, the gas pressure, and coal strength on the coal and gas outburst. On the basis of the single factor on the IEERG, three factors have been studied by orthogonal experiments, including gas pressure, coal particle size, and moisture. The results show that the influence of gas pressure, coal particle size, and water content on the IEERG decreases in turn; that gas expansion energy increases significantly as gas pressure increases; and that the greater the gas pressure, the faster the increase. The index Wp of IEERG first increases and then decreases with an increase in coal particle size and decreases with an increase in moisture content. Through this work, the main influencing factors of gas expansion energy have been defined. This method offers an important theoretical and practical significance to predict coal and gas outbursts.

The amount of initial gas release after the destruction of coal by ground stress relates closely to the outburst danger. In this paper, the following method was proposed to accurately determine the initial gas desorption law of coal. First, real-time temperature and pressure data of sealed coal sample tank after the sudden exposure of coal that had reached adsorption equilibrium within the tank were collected first via a high-precision, high-frequency response sensor. Then, the mass flow of gas passing through the pneumatic component at different times was calculated according to flow rate characteristics of pneumatic component that communicated with the atmosphere on the tank. In this process, it is determined that a convergent nozzle with a larger contraction ratio should be used as the channel for gas desorption and release. Moreover, in light of the characteristics of the testing process, the existing parameters that characterize the flow characteristics of pneumatic component were optimized and measured. Finally, the initial gas desorption laws of two coal samples with different metamorphic degrees were measured and analyzed under two pressures using three different-sized convergent nozzles.

Coal and gas outbursts are a rapid and powerful energy-releasing process. The damage to coal and the release of gas in the initial stage play key roles in the occurrence of outbursts. Existing studies have mainly focused on the gas desorption processes in coal under specific conditions such as particle size, specified mass and exposure time of pulverized coal. However, these studies have focused less on the gas release during the initial stage, and the quantitative relationship between the outburst risk level and the test results. Seven coal samples with different metamorphic degrees were chosen for use in outburst simulation experiments to investigate trends in gas release from pulverized coal in the presence of N 2 and CO 2 . The results demonstrate a strong correlation between the gas release capacity during the first 10 s and natural desorption gas content of the onset of coal exposure during the first 120 min. In general, the higher the gas content, the larger the released gas volume, and thus the larger the limiting amount of desorptive gas. The gas release from an outburst coal sample is larger than that from a non-outburst coal sample within the first 10 s. There is a good linear relationship between the initial volume of released gas (in the first 10 s) and the initial expansion energy of released gas (IEERG), which can reflect the risk of coal outburst. Thus, according to the outburst critical value (42.98 mJ/g) of IEERG, the critical value of initial volume of released gas could be calculated to be 1.113 cm 3 /g. The results can provide a reference for further study of the effect of gas on outbursts and the investigation of coal and gas outbursts disasters.

The gas pressure of a coal seam is one of the important parameters for studying gas reserves, law of gas flows, gas emissions, predicting coal and gas outburst risks and developing gas-control measures. In this study, the effects of the depth of the borehole drilled in a coal seam on the time required to determine the gas pressure and its value were discussed. Accordingly, elastic-plastic mechanics were applied to analyze the variation in the stress around the borehole, and the effective flow radii expressions of the spherical and radial flow boreholes were derived. The COMSOL Multiphysics numerical simulation software, was used to simulate the gas-flow range of the boreholes drilled at different depths in a coal seam, which were exposed for approximately 2 h. Finally, the boreholes of different depths was used to determine the gas pressure of No. 二 2 coal seam in the Xuehu coal mine (Henan Province, China). The results show that the time required in measuring the gas pressure is longer for boreholes drilled to a greater depth in a coal seam. For the same exposure times, the determined gas-pressure value will be smaller. Furthermore, the confined water and fractures in the roof and floor of the coal seam can be sealed by injecting high-pressure slurry into the borehole, which provides a reference for determining the gas pressure of the coal seam quickly under different geological conditions in future studies.

Coal and gas outburst is a dynamic phenomenon with violent eruptions of coal and gas from the working coal seam, resulting in economic loss and casualties. It has been proved that rapid desorption within a short period is necessary for occurrence of an outburst. Due to limitation of the present test condition, gas desorption characterization for the first several seconds (0–10 s) hasn’t been researched sufficiently. In this study, initial desorption characterization of gas in coal is studied by experiments with methane and carbon dioxide. The most remarkable characteristic of the experimental setup is application of a self-developed real-time data acquisition system with time interval of about 1.6 ms, which achieves the goal of collecting enough pressure and temperature data for analysis and calculation. The data is used to study gas pressure variation and calculate initial amount of desorbed gas and initial expansion energy of released gas. From the experimental results, new proof has been found to verify that coal and carbon dioxide outburst is more dangerous than coal and methane outburst.

A number of additives have been proven to improve the performance of vanadium redox flow batteries (VRFBs). Polyacrylic acid (PAA) is considered to be one of the promising candidates because of its beneficial effects on the stability of batteries. This paper studied the influence of PAA on viscosity, which is one of the electrolyte transport properties. The viscosity of VOSO4 + PAA + H2O ternary solution was measured at different temperature, ionic strength, and ionic strength fractions in the temperature range of 283.15 K–318.15 K. Several thermodynamic parameters of the PAA aqueous solution were calculated, such as the viscosity B-coefficients, the apparent molar volume of infinite dilution and the free energy of activation. In addition, two different semi-empirical prediction equations were established for ternary solution viscosity on the basis of the Eyring's theory, and the better one was determined through comparison. The average relative deviation is 0.74% between the predicted and the experimental values, which indicates this function can be used to predict the viscosity of VOSO4 + PAA + H2O ternary solution.

Currently, there are many methods to predict outburst risk when uncovering coal in crosscuts. When used, these prediction methods produce inconsistent prediction results. Consequently, engineers at the mining site are often at a loss on how to proceed. In the present studies, in the hope of addressing this problem, 23 different types of coal samples from 12 mines in five coal-producing provinces in China were collected to conduct 46 outburst simulation experiments in the laboratory under carbon dioxide and nitrogen conditions. In addition, coal seams under conditions identical to the outburst simulation experiments were fabricated, and various methods to predict outburst risk when uncovering coal in crosscuts were comparatively investigated. Furthermore, the accuracy of various outburst prediction methods was determined. Finally, the cause for the erroneous predictions generated using each index was theoretically analyzed. The research results provide a basis for selecting methods to predict outburst risk when uncovering coal in crosscuts at mining sites.

The advantages and disadvantages of existing methods for predicting coal and gas outbursts in coal roadways were analyzed, and the primary factors influencing coal and gas outbursts, including the expansion energy of the initially released gases, were studied. To this end, a theoretical formula for calculating the amount of gas released from boreholes during drilling was deduced. In addition, it was determined that the gas emission per unit borehole length was related to the ground stress, gas pressure, and coal strength around the borehole during drilling. By collecting these continuously released gases from boreholes during drilling, that is, by adopting line prediction technology, the influence of those parameters (ground stress, gas pressure, coal strength) on the outburst process can be determined, and the outburst hazard can be predicted. To test this approach, a specially developed line prediction device was applied to the No. 3 coal seam of the Xinyuan coal mine, Shanxi Province, China. The test results demonstrate that the gas emissions per meter of borehole, QLm, measured by the line prediction technique differed between outburst and non-outburst samples. Additionally, compared with existing indices used for predicting coal and gas outbursts in coal roadways, including K1 and Smax, the line prediction technique was more sensitive and could reflect the outburst hazard in front of working faces more accurately.

Given the increasing exploitation depth, outburst under the condition of uncovering coal in crosscut is increasingly dangerous. To prevent the occurrence of outburst, a novel forecast indicator, which is the initial volume of gas emission from the borehole (IVGEB) was proposed to validate the outburst-prevention effect. A self-developed experimental device for measuring the initial rate of gas emission from the borehole (IRGEB) was developed correspondingly. The IVGEB value is the sum of the unit length of IRGEB. Experiments have been conducted under different gas injection rates and gas pressures. CO2 and N2 gases have been used as injection media. After 50 test runs, 48 sets of valid data have been collected. The results suggested that the IVGEB value is approximately proportional to the gas pressure: the higher the gas pressure, the larger the IVGEB value. Furthermore, based on the relationship of the initial expansion energy of released gas (IEERG), which is another forecast indicator and gas pressure, the weak outburst pressure P1 and intense outburst pressure P2 of each coal sample could be calculated because the critical values of IEERG were 42.98 and 103.8 mJ/g, respectively. Thus, the IVGEB critical value of weak outburst and intense outburst could be obtained by applying a relation formula between IVGEB and gas pressure. The obtained IVGEB critical value of weak outburst and intense outburst were 39.69 L and 93.32 L, respectively. This approach might provide a possible method to validate the outburst-prevention effect quickly in the process of uncovering coal in crosscuts within deep mines.

Two new D-A type photovoltaic polymers, namely PITPh-DfQx and PITPhf-DfQx, based on asymmetric indenothiophene (IT) donor units with alkoxyphenyl or fluoroalkoxyphenyl substitutes were designed and synthesized. Effects of the fluorine substitution in the asymmetric IT donor units on the electronic structure, ordering structure, photovoltaic properties, and charge generation and recombination dynamics were investigated. It is found that side-chain fluorination in the asymmetric donor units of the D-A polymers endowed the relative polymers with a deeper HOMO level, higher and more balanced charge mobilites, increased charge dissociation efficiency and reduced bimolecular recombination. As a result, the bulk heterojunction solar cell based on the blend film of PITPhf-DfQx and PC71BM demonstrated an efficiency of 6.10%, whereas the cell efficiency based on PITPh-DfQx was only 3.00%. In addition, a triblock copolymer, poly(styrene-block-butadiene-block-styrene) (SBS), was employed for the first time as a polymer additive into the active layers based on PITPh-DfQx/PC71BM and PITPhf-DfQx/PC71BM devices to promote donor crystallization and tune the extent of phase separation between the donor and acceptor. The presence of SBS obviously improved the molecule packing and induced the crystallization of the two polymers, giving rise to a better phase separation due to enhanced aggregation effect of photovoltaic polymers. Therefore, with a small addition of SBS, the optimal PCE was further increased from 6.10% to 6.60% for PITPhf-DfQx based device and from 3.00% to 5.50% for PITPh-DfQx based device. The positive effects of SBS additive on the performance of photovoltaic polymer/fullerene BHJ solar cells provide a new strategy for developing high performance polymer solar cells.

In this paper, a model for viscosity of high-concentrated additive (methanesulfonic acid) in vanadium electrolyte is reported. Methanesulfonic acid was evaluated as a possible stabilizing agent for the thermal precipitation of supersaturated V(V) solutions at elevated temperatures. Viscosity (η) of the ternary solution CH3SO3H + VOSO4 + H2O was measured between the temperatures 283.15 K and 318.15 K under atmospheric pressure. Based on the viscosity, viscosity B-coefficient, activation energy (Δμ0≠0) and Gibbs free energy of activation (ΔG≠) were further calculated. With the help of these paraments, a semi-empirical predicting function is established to estimate the viscosity of high-concentrated additive, i.e. methanesulfonic acid in vanadium electrolyte. AARD of this semi-empirical method is 0.22%, which demonstrates that viscosity estimation for high-concentrated ternary solution CH3SO3H + VOSO4 + H2O can be achieved.

Gas diffusion law for coal particles has been researched by many authors due to theoretical and practical significance to gas emission prediction and coal and gas outburst prevention. But its definition remains fuzzy for indeterminacy of the concentration boundary condition that mainly depends on the change rules of external gas pressure and temperature of the coal particles during the desorption process. In this paper, reliable data of the external gas pressure and temperature are obtained by using a self-developed experimental setup of which the most remarkable characteristic is the application of a specially-made real time data acquisition system capable of recording data at very short time intervals (several milliseconds). Experimental results showed that after the opening of pneumatic valve gas pressure decreased nonlinearly with increasing time, and gas pressure had exponent relation to time. Gas temperature decreased rapidly at first, then increased with a decreasing speed, and finally reached a constant. The final gas temperature did not rise to the initial equilibrium value. With increase of the equilibrium gas pressure, the lowest gas temperature decreased, and the temperature difference between the equilibrium gas temperature and the lowest gas temperature increased gradually. Based on the tested change rules of gas pressure and temperature, the concentration boundary condition is achieved.

Given the problem of the long time required for testing gas pressure, we propose a fast-test method in which we used a technique of fast borehole sealing and air replenishing. Based on the characteristics of gas emission from boreholes to be tested, we built a theoretical model for calculating parameters during the process of increasing natural pressure and aeration. Using this model, we investigated the effect of different aeration conditions on velocity of pressure tests. The result shows that: 1) aerating air into boreholes can speed up gas pressure tests and 2) the more similar the pressure of the aerated air to the original gas pressure, the smaller the gas volume absorbed by coal and the shorter the time needed in pressure test. A case study in the Lu'an mining area shows that the time needed for gas pressure test is only 4 h using our method of aeration and 29 h under conditions of increasing natural pressure, saving time by 86.2%. This case study also indicates that, by using the aeration method, only one hour is needed for gas pressure to reach a stable state, which breaks the record of the shortest time needed for gas pressure tests in China.

The determination of gas pressure before uncovering coal in cross-cuts and in shafts is one of the important steps in predicting coal and gas outbursts. However, the time spent for testing gas pressure is, at present, very long, seriously affecting the application of outburst prediction techniques in opening coal seams in cross-cuts and shafts. In order to reduce the time needed in gas pressure tests and to improve the accuracy of tests, we analyzed the process of gas pressure tests and examined the effect of the length of boreholes in coal seams in tests. The result shows that 1) the shorter the borehole, the easier the real pressure value of gas can be obtained and 2) the main factors affecting the time spent in gas pressure tests are the length of the borehole in coal seams, the gas emission time after the borehole has been formed and the quality of the borehole-sealing. The longer the length of the borehole, the longer the gas emission time and the larger the pressure-relief circle formed around the borehole, the longer the time needed for pressure tests. By controlling the length of the borehole in a test case in the Huainan mining area, and adopting a quick sealing technique using a sticky liquid method, the sealing quality was clearly improved and the gas emission time as well as the amount of gas discharged greatly decreased. Before the method described, the time required for the gas pressure to increase during the pressure test process, was more than 10 days. With our new method the required time is only 5 hours. In addition, the accuracy of the gas pressure test is greatly improved.

This paper suggests that the rapid increase in urbanization and human activities has important impacts on visibility in the Pearl River Delta (PRD) region. A long-term trend of visibility in Guangzhou (one of the largest cities in PRD) shows that between 1954 and 2006, there is a rapid change in visibility. Between 1954 and 1972, low visibility has the smallest occurrence (less than a few days per year). Between 1972 and 1980, the visibility condition is within a transition period, and the occurrence of low visibility increases rapidly. Between 1980 and 2006, the occurrence of low visibility remains very high (150 days year−1), suggesting that the city of Guangzhou is often under low-visibility conditions since the 1980s. In order to understand the causes of the low visibility in Guangzhou, we analyze physical and chemical characterizations of a low-visibility event occurring between 15 and 29 November 2005. The analysis suggests that high concentration of aerosol particles is a major cause for the low visibility. The threshold aerosol concentration that is corresponding to the low visibility (