Contamination – Ethylene from Timber Supports

This project investigated the potential, timeframes, and circumstances for timber to produce ethylene in an underground environment. The findings led to the required management of timber supports, to avoid ethylene contamination underground and to increase confidence in mine gas analysis systems.

Ethylene is a key spontaneous combustion indicator monitored in underground coal mine atmospheres. Ethylene is significant as it is not typically detected in gas evolution testing by micro gas chromatography until the coal has reached a temperature of approximately 80-100˚C. The detection of ethylene is, therefore, traditionally considered confirmation of a coal heating. It is commonly used as a trigger for coal mine worker evacuation in spontaneous combustion Trigger Action Response Plans (TARPs).

Trace amounts of ethylene have been detected, in routine gas samples, in the absence of elevated carbon monoxide and hydrogen. The absence of elevated carbon monoxide when ethylene is detected is uncharacteristic of a coal heating event which is increasing in intensity. It was found that ethylene could have originated from contamination by timber, Westthorp and Phillips (2017).

Ethylene produced from timber, used in underground coal mines, was investigated through:

• Gas evolution testing;
• Testing in an environment chamber; and
• Testing in a gas bag containing either instrument air or nitrogen.

Results show the presence and magnitude of ethylene produced was dependent on the time since harvest of the timber. The maximum time taken for ethylene to be produced from the timber for all tests was eight days following harvest. It should be noted that the ethylene concentrations detected were confined to a sample bag or test chamber. The ethylene concentrations measured were not affected by dilution, as would be the case for timber supports installed in an underground coal mine atmosphere.

As a conservative factor, it is recommended that a minimum of 21 days to have passed, following the harvest of the timber, before it is installed in an underground coal mine. In addition, if ethylene is detected in an underground atmosphere where the timber supports were not a live tree more than eight days prior, then any ethylene detected should not be attributed to the timber. Detection of ethylene should be treated as an indicator of a coal heating unless proven otherwise.