(Mercaptan-type) sulphur-containing products were released from a chemical plant for 20 min at around 13.00. Emergency services took eight employees who felt unwell (nausea, headache, vomiting) to hospital from a neighbouring undertaking. They were discharged soon afterwards. The operator did not trigger its emergency response plan.

The oxidiser (= incinerator) of the unit producing calcium phenates stopped automatically when it was detected that the flame, which had previously been unstable, had been extinguished. It had already stopped during the night at around 01.00 but without significant impact. The stopping of the oxidiser caused that of the phenates unit, the H2S and mercaptan-type sulphur-containing effluents of which are also treated by the oxidiser (after passing through a tank in which those with the highest H2S content are converted). If they are not oxidised, these sulphur-containing effluents are released to the stack untreated. A similar incident three weeks previously affected five people at the same neighbouring undertaking. The site was temporarily shut down as an administrative measure until the oxidiser was working reliably. The site risk assessment had not anticipated the accidental release of mercaptans. Production losses stood at several thousand euros. An expert’s report showed the oxidiser flame to be outside the flame detector field as a result of combustion instability. This instability was caused by excessive dilution of the fuel (natural gas) with surplus oxidant (surplus oxygen and nitrogen contained in the effluent to be treated arriving at a rate of 8 000 kg/h). The expert’s report made a number of technical recommendations to improve combustion (reduce and preheat the surplus oxidant, split the injection of fuel, etc.) and suggested that an activated charcoal filter be installed to trap mercaptans and H2S upstream of the stack should the oxidiser stop working. The operator made these improvements to the oxidiser.