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 m(3)/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.