Experimental Study on Influencing Factors and Thermal Effects of CO2 Adsorption by Coal.

The utilization of CO2 is extremely important to solve the environmental problems and coal spontaneous combustion in goaf. There are three kinds of CO2 utilization in goaf: adsorption, diffusion, and seepage. Since adsorption will consume CO2 in goaf, the optimization of CO2 injection amount is very critical. A self-developed adsorption experimental device was used to determine the CO2 adsorption capacity of three different particle sizes of lignite coal samples at 30-60 °C and 0.1-0.7 MPa. The factors affecting CO2 adsorption by coal and its thermal effect were studied. In the coal and CO2 system, the CO2 adsorption characteristic curve is not affected by temperature, but there are differences in that with different particle sizes. The adsorption capacity increases with the increase of pressure, while it decreases with the increase of temperature and particle size. Under atmospheric pressure, the adsorption capacity of coal is a logistic function relationship with temperature. Furthermore, the average adsorption heat of CO2 on lignite shows that the interaction force between CO2 molecules has a stronger effect on CO2 adsorption than the effect of heterogeneity and anisotropy on the coal surface. Finally, the existing gas injection equation is improved theoretically with CO2 dissipation, which provides a new idea for the work of CO2 prevention and fire suppression in goaf.

Characteristics and Safety of CO2 for the Fire Prevention Technology with Gob-Side Entry Retaining in Goaf.

The mining technology of gob-side entry retaining without a coal pillar is gradually becoming a mature and increasingly important mining technology. As it maintains the roadway near goaf, the air leakage should be greater than a U-type ventilation system in goaf, so it is prone to cause coal spontaneous combustion problems. CO2 is an inert gas, and it is usually used to prevent spontaneous combustion and extinguish coal fire. However, there is a lack of research on characteristics and safety of CO2 for the mining technology of gob-side entry retaining without the coal pillar. In this paper, the indexes of influencing factors were proposed on gas, pipelines, and mining technical parameters. Using a three-dimensional physical model of coal stope, the gas migration law of CO2, the relationship between gas injection rate and the oxidation zone area, and the safety of the CO2 inerting technology were analyzed. The results indicate that the O2 concentration is diluted between the working face and the injection port, especially in the air intake side. Furthermore, the CO2 injection rate is an important parameter to the fire prevention and extinguishing technology. When the gas injection rate ranges from 240 to 720 m3/h, the oxidation zone area varies from 7380 to 14 760 m2, and the gas injection rate grows exponentially with the area of the oxidation zone. Moreover, the redundant CO2 gas flows to the retaining roadway, and it reduces the O2 concentration, resulting in asphyxia accidents of miners. The research results are helpful to balance the relationship between inert gas injection and production safety and provide guidance for the practical application of the inert gas fire prevention technology.

Optimisation of multisource injection of carbon dioxide into goafs based on orthogonal test and fuzzy comprehensive theory.

Carbon dioxide is a common inert gas used for fire prevention and extinguishing fires in underground coalmines. However, the single pipeline used for carbon dioxide injection has the characteristics of a narrow inerting area, serious gas leakage in the field, and poor control. To address these risks, we herein propose a technique for multisource injection of carbon dioxide and for building up an index system of carbon dioxide injection for fire prevention and extinguishing in goafs by analysing the parameters of the gas injection pipe, mining technique, and physical qualities of the inert gas. In addition, a mathematical multi-pipeline and multisource model is built on the basis of the diffusion model of carbon dioxide injection with a single pipeline. To improve the insertion efficiency of carbon dioxide injection, a parameter optimisation method of multisource injection based on an orthogonal test and the fuzzy comprehensive theory is proposed. Using the orthogonal test and the FLUENT numerical simulation method, the relationship of the mining speed at the working face, gas injection position, height of the pipeline outlet, and gas flow are studied. Furthermore, the optimal injection parameters are calculated using fuzzy comprehensive theory. The application of this model at the No.7271 working face in the Yaoqiao Coalmine shows that the optimal injection parameters are calculated with the gas injection volume being 500 m3/h, the gas injection depth being 90 m, and the pipeline number being two. After adopting the optimal parameters of carbon dioxide injection, the concentration of oxygen demonstrates an obvious downward tendency, which is the same as the numerical simulation result. In conclusion, the technique for multisource injection of carbon dioxide can effectively reduce the width of the oxidation zone and prevent spontaneous combustion accidents in goafs.