Pollution
Humain
Environnement
Economique

The biogas produced by the slug digesters in a municipal wastewater treatment plant was to be sent to the flare tower at around 2:50 a.m. This operation was rendered impossible as the flare tower’s supply valves had become frozen following a period of extreme cold (negative temperatures). The pressure began to increase in the digesters due to the biogas being unable to be discharged to the flare tower. This overpressure triggered the opening of the pressure relief valves. The operations crew alerted the on-call maintenance crew, who were able to unblock the valves at 11:50 a.m.: the pressure relief valves then closed. Over a period of 9 hours, 13,323 Nm³ of biogas containing 65% methane (i.e. 14.5 t) had been released into the atmosphere.

This was the first time that this malfunction had appeared as, up to the previous year, the biogas produced by the digesters had been continuously sent to the flare tower. However, for the last few months, biogas was being used locally by a cogeneration unit and the flare tower was only being used occasionally as a backup unit. The freezing of the valves was linked to the high moisture content in the biogas produced.

The operator continued to supply the flare tower continuously for 72 hours, then restarted cogeneration operations with the flare tower operating at minimum flow throughout the period of extreme cold. Finally, the operator installed a hot air blower which was able to warm up the valves and promoted moisture condensation as close as possible to the purge pots (25 m upstream from the flare tower) by decalorifying the biogas line at this level. A durable solution was sought with the supplier to ensure that the flare tower could operate at temperatures below -10 °C. A similar accident had already occurred at the site a few days earlier (ARIA 47805).