Effect of CO2 injection pressure on enhanced coal seam gas extraction.

The CO2 displacement of coal seam gas can simultaneously promote gas extraction and CO2 sequestration, with gas injection pressure being a key factor influencing both efficiency and safety. This study examined the impact of varying CO2 injection pressure on gas extraction and sequestration. The findings indicated that higher CO2 injection pressures reduced the gas injection and extraction time costs, increased the equilibrium pressure and coal seam temperature, and decreased carbon sequestration efficiency. As the CO2 injection pressure increased, the CH4 cumulative desorption capacity and desorption rate rose by 4.98%, the time to reach the residual gas content critical value (TLV of 2 m3/t) shortened by 62.04 min, and the maximum CO2 storage per unit mass of coal increases by 3.6 m3/t. Increasing the injection pressure enhanced gas desorption and CO2 sequestration. A higher CO2 injection pressure resulted in a higher CH4 yield rate and a reduced CO2 injection rate in the later stages. Therefore, it is advisable to reduce the CO2 injection pressure during the low efficiency desorption stage.

Research on the technology of uniformly injecting nitrogen into the porous long pipes in the gob of the gob-side entry retaining mining mode with roof cutting and pressure relief.

The implementation of the Gob-Side Entry Retaining Mining Mode with Roof Cutting and Pressure Relief (GERRCPR) results in the gob connecting to the retaining roadway, creating an open space that causes significant air leakage and increases the risk of spontaneous combustion. A study was conducted during the implementation of the GERRCPR in the Xiaonan Coal Mine N1-1502 working face to investigate spontaneous combustion characteristics, along with fire prevention and extinguishing measures. To analyze gob airflow, Computational Fluid Dynamics (CFD) was employed to collect data on airflow conditions, O2 concentration, and temperature. Based on this, this study focuses on exploring the effects of nitrogen injection treatment under various rates and positions to optimize parameters for buried pipe nitrogen injection. Results indicated that within the GERRCPR, air leakage in the gob increased, leading to an increase in O2 concentration, expansion of the oxidation zone, and an elevated risk of spontaneous combustion. Air leakage primarily occurred from the retaining roadway and the working face near the intake-air roadway, peaking at a retaining roadway length of 500 m, with a flow rate of 226 m3/min. Following nitrogen injection treatment, the oxidation zone was significantly reduced, with optimal treatment achieved at a nitrogen injection depth of 70 m and a rate of 600 m3/h. Field monitoring data showed that the inertization measure of using porous long pipes, a nitrogen injection spacing of 30 m, and a nitrogen injection rate of 600 m3/h significantly decreased the O2 concentration within the gob. This reduction meets safety production requirements and outperforms the effectiveness of traditional buried-pipe nitrogen injection methods, thereby validating the simulation accuracy. Understanding the laws governing spontaneous coal combustion in the GERRCPR and enacting preventive measures for nitrogen injection can improve safety standards in mining operations. This proactive approach can effectively prevent spontaneous coal combustion accidents, resulting in substantial social benefits.

Head-neck local ventilation mode for long-narrow mine working face.

To improve the thermal health state of workers in mines facing heat hazards and enhance cooling capacity utilization efficiency of mine ventilation, this study proposes a suitable air distribution for mine workers' local cooling, taking into account the characteristics of long-narrow underground space and workers. The suggested air distribution involves harnessing underground cold air jets along with the mine's crossflow (mainstream ventilation) to create a localized safeguard airflow around the worker's head-neck, known as jet ventilation in crossflow (JVIC). The flow visualization experiment identified five flow patterns within a confined space. The study explores the impact of the velocity ratio (R) and confinement scale (C) on the evolution of JVIC flow patterns and presents a parametric description of the resulting flow field. Drawing on the microclimate control scope around mine workers' head-neck, the study defines the effective cooling zone and the ineffective cooling zone as the boundaries for controlling JVIC air distribution within confined mine spaces. This clarifies the applicability of the air distribution form and introduces an evaluation model for assessing the cooling effect and the efficiency of cooling capacity utilization to manage the non-uniform underground environment.

Experimental study on flow characteristics of jet ventilation in crossflow in confined mine spaces.

The increasing depth of mine excavation presents greater challenges in mine ventilation and in managing cooling energy consumption. Therefore, there is an urgent need for comprehensive research on jet ventilation influenced by low-speed crossflows. This study investigated the impact of flow velocity ratios (R) and jet exit diameters (d) on flow-field distribution and flow characteristics through velocity measurements and smoke flow visualization experiments. The results of the study revealed two distinct types of air lakes formed by jet ventilation in crossflow (JVIC), with one being wall-attached and the other suspended. Notably, a significant secondary flow phenomenon was observed in the near-field near the upper wall. Additionally, the deflection angle (θj) of JVIC decreases as R and d/D increase, leading to the formation and movement of a semi-confined point (SP) and a confined point (CP) in the -x direction. Moreover, the wall confinement effect diminishes the jet's diffusion and deflection ability in the -z direction, leading to increased penetration in the x direction. Before the formation of the SP, the deflection section of the jet lengthens, followed by a rapid shortening upon its formation. Finally, the study further developed empirical equations for the jet axial trajectory and diffusion width.

Study on Nitrogen Injection Fire Prevention and Extinguishing Technology in Spontaneous Combustion Gob Based on Gob-Side Entry Retaining.

Under the gob-side entry retaining mining mode with roof cutting and pressure relief (GERRC), the gob and retained roadway section are interconnected to create an open area. Owing to the increased airflow, the coal remnants in the gob are more prone to spontaneous combustion. This study aimed to investigate the distribution of oxygen concentration within a gob and identify optimal parameters for nitrogen injection. The engineering context was the "110 method" introduced in the 1201 working face of the South Five mining area at Daxing. Computational fluid dynamics simulation software was used to analyze the effects of various nitrogen injection treatment parameters on the overall performance of the gob, including their impact on oxygen distribution. The simulation results showed that air leakage within the gob primarily originates from the working face adjacent to the intake roadway, as well as gaps within the retained roadway. The increased air leakage causes the high O2 concentration range in the gob to expand, and the retained roadway section is connected to an area with a high concentration of oxygen near the working face, which increases the risk of residual coal spontaneous combustion. The results show that the optimal nitrogen injection conditions for inerting and reducing the risk of spontaneous combustion within the gob require an injection quantity of 500 m3/h, with the injection point located at a depth of 60 m. With these parameters, the range of the oxidation zone was significantly reduced. To monitor the O2 concentration and temperature change curves in the gob during the project implementation, a bundle tube monitoring system was used, considering the actual mining situation. By varying the nitrogen injection spacing and quantity, we found that injecting nitrogen at a spacing of 30 m and at a quantity of 500 m3/h effectively placed most areas of the gob in the suffocation zone, reducing the risk of spontaneous combustion of residual coal. The accuracy of the simulation was verified. The study offers valuable insights into improving safety in coal mines and reducing spontaneous combustion incidents, providing important reference significance for fire prevention and control.

Research on controlling gas overrun in a working face based on gob-side entry retaining by utilizing ventilation type "Y".

To determine the characteristics of air leakage concerning a "Y" type ventilation in gob-side entry retaining with roof cutting, pressure relief, and the law of a resulted gas accumulation (GA), research is conducted by employing the CFD simulation incorporated with the gauged parameters of working face (WF) mining to analyze the air leakage of "Y" type ventilation. For this purpose, the 1201 fully mechanized coal mining face in the south Wu mining location of the Daxing coal mine is taken as an illustrative example to study the air leakage in the "Y" type ventilation. So, the gas concentration (GC) issue surpassing the limit in the upper corner of the goaf was simulated. The results show that the goaf is formed into an open space when roof cutting and pressure relief technology along the goaf is implemented. The air pressure at the upper corner of the WF would be the lowest, which is only 1.12 Pa. The airflow of air leakage under a pressure difference would move from the gob-side entry retaining to the goaf. Moreover, the simulation of mine ventilation indicates that the volume of air leakage positively correlates with the length of gob-side entry retaining. When the WF is advanced 500 m ahead, the maximum volume of air leakage would reach 247 m3/min within the range of 500-1300 m, and then the rate of air leakage gradually would decrease. When the WF is advanced at 1300 m, the air leakage would become the smallest, which is 175 m3/min. When gas control is under consideration, the effect of gas extraction would be best with the buried pipe whose depth and diameter are set to 4.0 m and 400 mm, respectively. So, the GC in the upper corner would become 0.37%. After the high-level borehole with a 120 mm diameter is mined, the GC in the deep goaf decreased to 3.52%, and the GC at the upper corner became further reduced to 0.21%. While the high-level borehole gas is extracted by employing the extraction system of the high-concentration gas, the extraction system of low-concentration gas is utilized to extract the upper corner gas of the WF, thus, the problem of gas overrun was resolved satisfactorily. During the recovery period of the mining, the GC at each gauging point was less than 0.8%, which effectively guided the secure production in the Daxing coal mine and provided a theoretical foundation to control gas overrun during the mining process.

Influence of CO on Explosion Limits and Characteristics of the CH4/Air Mixture.

The study of gas explosion under the influence of CO generated by spontaneous combustion of coal has practical value for preventing and controlling such accidents. The explosion limit and the explosion characteristic parameters of the CO/CH4/air mixture were measured with a 20 L explosion tank. The changes in free radical concentration and temperature sensitivity in the process of mixture explosion reaction were analyzed using the GRI-mech 3.0 mechanism. The test results show that with the increase of the CO concentration in the mixture, both the lower explosion limit and the upper explosion limit of CH4 explosion decreased, the explosion limit range became wider, and the maximum explosion pressure of the mixture decreased. The time for the H•, O•, and •OH radical molar fractions to reach the peak value was found to be prolonged with the increase of the CO ratio in the mixture. Under oxygen-enriched conditions, the •OH and O• mole fractions were larger than those under oxygen-lean conditions, while the H• concentration was reversed. The higher the proportion of CO in the premixed gas, the higher the value of the temperature sensitivity coefficient. The reaction processes R155 CH3 + O2 ⇌ O• + CH3O and R158 2•CH3 (+M) ⇌ C2H6 (+M) had the greatest influence on the temperature of the reaction process. Explosion suppression techniques can be developed for similar explosive environments based on this study.

Experimental research on rapid removing characteristics of carbon monoxide generated during gas explosions.

A large amount of gas, such as CO, accumulates in a coal mine after an explosion, leading to CO poisoning. In this study, a self-developed platform was used to eliminate CO from coal mines and determine the mass of the rapidly eliminated CO and its concentration in the eliminated gases. Equations were derived to calculate the amount of CO eliminated and the removing rate. The results showed that a rapid removing reagent in the form of nonprecious metal catalysts is useful for removing CO. Removing agents with larger masses facilitated the activation, irrespective of the CO concentration. For removing reagent amounts of 10, 15, 20, 25, and 30 g, the amount of CO eliminated, the removing rate, and the time required to complete catalytic oxidation increased sequentially. The CO removing process could be divided into three stages (I, II, and III) based on the variations in the CO, CO2, and O2 concentrations during CO removing. The removing reagent first chemically adsorbs CO and O2, and then desorbs CO2. The final CO concentration tends to 0, the O2 concentration remains stable, and the CO2 concentration decreases. This shows that the ablation agent has an impact on the changes in the CO and CO2 concentrations.

Effect of Sn on the CO Catalytic Activity and Water Resistance of Cu-Mn Catalyst.

In view of the problem that excessive CO in underground coal mine space can easily lead to a large number of casualties, Cu-Mn-Sn water-resistant eliminators with different Sn contents were prepared by a co-precipitation method. The activity of the eliminators was analyzed by using an independently developed activity testing platform, N2 adsorption and desorption, XRD, SEM, XPS, and FTIR to characterize the activity factors and water resistance. The results showed that Cu-Mn-Sn-20 with 20% Sn content had the highest activity, which was 3.23 times that of Cu-Mn. The main reason for the increased activity is that Cu-Mn-Sn-20 doped with 20% Sn provides a larger specific surface area and more active sites and reduces the pore size, so that the crystallization degree of Cu1.4Mn1.5O4 is lower. The doping of 20% Sn reduces the absorption of lattice water and coordination water and improves the water resistance of Cu-Mn-Sn-type eliminators. The Cu-Mn-Sn-20 water-resistant eliminator is used to quickly eliminate CO in underground coal mines, which is of great significance for the rescue workers in underground coal mines after disasters.

Simulation of emergency evacuation from construction site of prefabricated buildings.

To ensure the safe construction of prefabricated buildings and improve the efficiency of the safe evacuation of construction personnel after a fire caused by improper operation during construction, this study used the PyroSim software to numerically simulate a fire situation based on the size and volume of a prefabricated building construction site. The variation rules of smoke visibility, CO concentration, and ambient temperature in the construction site of prefabricated buildings were analyzed and the available safe evacuation time was determined. Moreover, the Pathfinder software was used for simulation in combination with the physical attributes of personnel, evacuation speed, and personnel proportions. The time required for safe evacuation was determined and the factors influencing the evacuation time, such as the quantity and location of stacked prefabricated components, machinery, and appliances, and the number of on-site construction personnel, were analyzed. The data collected by the temperature sensor, CO concentration sensor, and visibility sensor reveal that the visibility and crash time are the key factors restricting the efficiency of personnel avoidance and evacuation. At 400 s, the visibility at the escape exit of the prefabricated apartment construction site was lower than 5 m. The crashing time of the building was 360 s, which is the critical point for casualties. The first emergency evacuation simulation took 398.7 s. The required safe evacuation time (TREST) > available safe evacuation time (TASET), and the original site layout cannot facilitate the safe evacuation of all construction workers. The evacuation time can be effectively reduced by re-planning the stacking positions of prefabricated construction site components, construction equipment, and other items, and reducing the number of personnel in the construction plane. The results of the second simulation reveal that the safe evacuation time (TREST) is 355.2 s. Because it is required that the safety evacuation time (TREST) < available safe evacuation time (TASET), the results are in line with the emergency evacuation requirements. The findings of this study can provide a theoretical basis for the rational planning of evacuation passages at the construction sites of prefabricated buildings and assist the management of construction site safety.

Influence of Volatile Content on the Explosion Characteristics of Coal Dust.

To study the influence of different volatile contents on the explosion characteristics of coal dust, the volatile content in coal dust was controlled under different final temperatures of pyrolysis. The maximum explosion pressure, maximum pressure rising rate, and explosion index were used to characterize the pressure behavior, the pressure ratio to characterize the explosibility, and the minimum ignition temperature of the coal dust cloud to characterize the sensitive characteristics. A 20 L of nearly spherical coal dust explosion parameter measuring device and a dust cloud minimum ignition temperature measuring device were used to study the influence of the explosion characteristics of dust with different volatile contents prepared under different pyrolysis temperature conditions. The results showed that the volatile matter content in lignite dust has little effect on the maximum explosion pressure, with an average change rate of 5.435%. When the volatile content was reduced from 45.4 to 2.45%, the maximum explosion pressure rise rate dropped by 65.976%. The explosion index of the experimental sample was in the range of 0-1.6, with weak explosion characteristics; the lower the volatile content, the weaker the explosion intensity. When the volatile content was only 2.45%, the pressure ratio was still greater than 2, that is, the dust was still explosive. When the volatile content in lignite was reduced from 45.4 to 18.21%, the lowest ignition temperature of the dust cloud was consistently 490 °C. At this stage, the contents of H2, CO, CH4, CO2, and other precipitated dases were low. When the volatile content was reduced from 18.21 to 2.45%, the precipitated volatile gas increased rapidly, the remaining precipitated gas content decreased, and the dust could not be easily ignited. The experimental results lay the foundation for studying the influence mechanism of volatile matter in coal dust on the explosion characteristics.

Experimental Research on Rapid Fire Zone Sealing and Explosion Venting Characteristics of an Explosion Venting Door Using a Large-Diameter Explosion Pipeline.

To study the law of influence of an explosion venting door on gas explosion characteristics and verify its venting effect and fast sealing performance, a large-sized explosion pipeline experimental system was used. The gas explosion tests were carried out under the conditions of 5.5, 7.5, 9.5, and 11.5% gas concentration. The gas explosion characteristic parameters were measured by a data acquisition system. The laws of change in characteristic parameters and the flame-proof effect were analyzed. The results showed that the pressure peak was attenuated by 42.25, 50.54, 53.27, and 52.88% under the aforementioned four working conditions. As the gas volume fraction increased, the peak explosion pressure decayed as a quadratic function, and the average closing time of the fire zone was 13 h. This showed that the explosion venting door had significant explosion venting characteristics and the function of quickly closing the fire zone. The law of temperature change was basically the same, no matter how the gas concentration changed, and the explosion venting door had no inhibitory effect on the gas explosion flame. Under the four operating conditions, the maximum average values of the flame propagation speed were 103.56, 105.73, 136.67, and 138.34 m/s. The results of the study provide theoretical support for explosion-proof technology and emergency rescue technology in coal mines.

Influence of air supply on coal spontaneous combustion during support withdrawal in fully mechanized coal mining and its prevention.

It is necessary to change the air supply rate of the working face during the withdrawal of fully mechanized mining, making it important to study the oxidation characteristics of coal samples under different air supply rates. Through a self-made temperature-programmed experimental device, our focus was on studying the change laws of indicator gases released during the low-temperature (303.15-473.15 K) oxidation stage when the air supply rates of the coal samples were 0.67, 1.33, 2, 2.67, and 3.33 mL/s. The experimental results showed that the air supply increased, the concentrations of CO, C2H6, C3H8, C2H4, and C2H2 generated by the coal sample at the same temperature decreased, and the oxidation process decelerated. The initial temperatures of the four hydrocarbon gases were delayed to varying degrees with the increase in the air volume, and C2H4 was found to be more suitable as a hydrocarbon gas for the early warning of coal spontaneous combustion. Surface fitting was applied to analyze the change law of the CO generation rate under the combined effect of temperature and air supply; the change was divided into three stages. The CO concentration model at the upper corner of the working face during the withdrawal period was deduced, and comprehensive safety measures were put forward to prevent coal spontaneous combustion during the withdrawal period.

Removal of CO Generated by a Gas Explosion Using a Cu-Mn Elimination Agent.

Addressing the issue of suffocation and casualties caused by a large amount of poisonous CO gas generated after a gas explosion, research involving an experimental system for the removal of CO using a Cu-Mn elimination agent was studied. The influence of O2 concentration, temperature, and CO concentration on the elimination performance of the agent after a gas explosion was studied. The quantitative relationship between the amount of CO eliminated, the elimination rate, the O2 concentration, and temperature was analyzed. Further analysis was completed regarding the influence of O2 concentration, temperature, and CO concentration on the thermal effect in the elimination process. The results showed that the elimination agent had a rapid effect on the removal of CO. When the ratio of CO concentration to O2 concentration was closer to the stoichiometric ratio, the elimination and reaction were more complete, the time to complete elimination was shorter, and the peak temperature was higher. As the temperature increased, the time to reach the elimination limit became longer, the elimination rate decreased, the reaction was slower, and the peak temperature was lower. As the CO concentration increased, it was observed that the higher the peak temperature, the longer it took to reach the peak time. The results of the study provide a theoretical support for the catalytic oxidation of CO using the Cu-Mn eliminator after a coal mine gas explosion.

Novel Reassortant Avian Influenza A(H9N2) Virus Isolate in Migratory Waterfowl in Hubei Province, China.

In December 2017, an influenza A(H9N2) virus (B51) was isolated from migratory waterfowl in Hubei Province, China. Phylogenetic analysis demonstrated that B51 is a novel reassortant influenza virus containing segments from human H7N4 virus and North American wild bird influenza viruses. This suggest that B51 has undergone multiple reassortment events.