Coal spontaneous combustion prediction in gob using chaos analysis on gas indicators from upper tunnel

• Coal spontaneous combustion can be predicted by gas indicators from upper tunnel.
• Gas indicator time series are persistent series that have fractal characteristic.
• Gas indexes from upper tunnel follow the same chaos rule.
• Fractal dimension and Hurst index can indicate self-heating risk of coal in gob.

While gas sampling through upper tunnel is available right now, only little research has been reported for the prediction of coal spontaneous combustion in working face gob by analyzing drained gas in upper tunnel.This study investigated the characteristic of air leakage of 401 working face gob in Tianchi mine under stereo gas extraction condition. The air that flows to the deep gob and is then drained to upper tunnel makes the prediction of coal spontaneous combustion in gob possible. Sample collector was employed to collect gas samples in the drainage pipeline plugged in upper tunnel and gas chromatograph was applied to analyze the indicator gases. A total of 195 gas samples were gathered including 44 groups that containing CO, O2 and O2/N2 fluctuated in a small range and dropped dramatically when the oxidation of coal in gob was accelerated, while N2 and the depletion ratio of O2 obeyed the reverse rule.R/S analysis (Rescaled Range Analysis) was used to investigate the fractal characteristic of different gas indicator time series. The Hurst indexes of different gas indicator time series follow the same rule that they would gradually become stable with the accumulating data and could deviate their chaos characteristic when CO was detected, which means the oxidation stage of coal in gob was accelerated. Changes of Hurst indexes and fractal dimensions controlled by effective delay time was researched. The fractal dimension (or Hurst index) of O2, N2, O2/N2 and depletion ratio of O2 from upper tunnel can be equally applied to indicate the combustion condition of residual coal in gob.Taken 70 days as the effective delay time, the risk of coal spontaneous combustion in 401 working face gob were divided into two stages: low-temperature oxidation stage (Hurst index >0.8776 or fractal dimension <1.2224) and dangerous stage of coal spontaneous combustion (Hurst index ≤0.8776 or fractal dimension ≥1.2224). The chaos characteristic analysis of gas index series in upper tunnel provides a novel and effective method to predict the risk of coal spontaneous combustion in working face gob.