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Investigation of Heatings and Related CO and H2 Gas Flow Patterns in Longwall Goafs

Underground » Detection and Prevention of Fires and Explosions

Published: April 09Project Number: C15022

Get ReportAuthor: Ting Ren, Rao Balusu | CSIRO Exploration and Mining

This report presents a summary of the research work carried out under the Australian Coal Association Research Program (ACARP) project C15022, entitled "Investigation of heatings and related CO and H2 gas flow patterns in longwall goafs". The main objective of the project was to develop advanced CFD models and conduct detailed investigations of the heatings and associated carbon monoxide (CO) and hydrogen (H2) gas flow patterns in longwall goafs under various heating/oxidation scenarios. This work has provided a greater insight into heating gas flow patterns in longwall goafs and improved mine operator's capability to evaluate heating conditions at an early stage and develop appropriate control measures.

Review of laboratory tests and field observations indicated that CO make and absolute CO and H2 concentrations are closely associated with the development of spontaneous heating in longwall goafs. Three heating incidents in Australia longwalls were reviewed in this study, and in all cases, the development of goaf heating was closely linked with the rise of spontaneous heating indicator gases such as CO and H2 gas at different locations of the goaf. CO and H2 gas levels up to 2000ppm and 1000ppm respectively had been recorded during the heating incidents. These heatings were eventually brought under control by goaf inertisation and face ventilation management. It was also noted that the trends of heating gas were heavily influenced by a number of factors and a good understanding of goaf gas flow dynamics was critical for accurately locating the heating and its control.

Three dimensional CFD models were developed to investigate heatings and related CO and H2 gas flow patterns in longwall goafs. Based upon previous studies of goaf gas flow and oxygen ingress patterns in the goaf, the CFD model incorporates a number of 'heating blocks' representing heatings in the most likely locations in the goaf. Rather than modelling the complexity of the oxidation process of coal, the evolution rate of CO and H2 gas was assumed to follow a defined value at different stages of the heating. Base CFD models were developed to model heatings on both sides of the longwall goaf, imitating the heating experiences at an Australian underground coal mine. Results from the base CFD models were calibrated and validated against field data.

The validated CFD models were used for extensive parametric studies to investigate the impact of a number of inherent and operational parameters on the distribution pattern of CO and H2, including goaf gas emission rate, goaf leakage, face ventilation and the impact of goaf inertisation strategies. Field data from three heating incidents was then analysed in detail to link the real heating scenarios with the outcomes of CFD modelling results.

CFD modelling results demonstrate that the behaviour of gaseous products from goaf heatings is influenced by a number of factors and the understanding of goaf gas flow patterns is an important prerequisite for the correct interpretation of CO and H2 monitoring data. The project studies indicate that the location of gas monitoring sensors or tubes is critical for the early detection of a goaf heating, and where possible additional gas monitoring points should be placed at strategic locations on both sides of the goaf to ensure that increasing CO and H2 gas levels can be detected before the heating escalates into an advanced stage. Modelling results also indicated that more effective goaf inertisation for heating suppression and gaseous products (such as CO and H2 gas) dilution can be achieved by in-bye injection of inert gas at a distance from the face, either via a cut-through or surface borehole(s), in combination with a foam 'Air Plug' to reduce air ingress.

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