Goaf gas drainage and its impact on coal oxidation behaviour: A conceptual model

Abstract

In gassy underground and spontaneous combustion prone coal mines, goaf management is a challenging task to reduce the risks of gas explosion and spontaneous combustion to acceptable levels. Goaf gas drainage via vertical surface goafholes is a primary control method to manage gas emissions in Australian coal mines. However, the operation of vertical surface goafholes maybe significantly impacted by insufficient understanding of goaf gas flow and coal self-heating behaviour, especially under different goaf gas drainage operating conditions. This paper focuses on analysing the field monitoring data collected from 161 goaf drainage holes at two coal mines in Australia. The data formed the basis to develop a conceptual model, which represents the changes of goaf atmosphere during the longwall retreat along with goaf drainage operations. The results indicate that the goaf drainage operating conditions may significantly affect goaf gas composition, which contributes to a different goaf atmosphere in comparison to existing models without goaf drainage. Within the studied longwall goaf, the ‘oxidation zone’ and ‘inert zone’ are identified with different carbon monoxide (CO) and oxygen (O2) trends, and the ‘oxidation zone’ is conceptually further divided into the ‘heat accumulation zone’, ‘elevated oxidation zone’ and ‘slow oxidation zone’ based on the variation of gas composition and potential degree of oxidation of residual coal. The coal oxidation potential risk and corresponding goaf gas drainage strategies at these conceptual zones are also evaluated in this paper. The results of this study are expected to act as a guideline for future goaf gas drainage operations.