In an ammonia (NH3) and fertiliser plant, 200 kg of nitrogen oxide (NOx) were released into the atmosphere via the chimney in the nitric acid (HNO3) workshop during restart and following a two-week shutdown for maintenance. The large cloud of red smoke generated caused various complaints from neighbours. Measurements by the operator at the site’s ground level did not exceed 2 ppm; therefore, the workshop was not stopped and the IOP not activated. A crisis centre was set up, but the release continued for 50 minutes while the operator tried to identify the causes of the incident: a malfunction of the measuring device for HNO3 at the denitrator outlet (before sending the HNO3 to storage); the sequence of events following overfilling of the denitrator; greater pressure drop in the upstream airstream; a lack of air in the oxygen concentrator, undermining the nitric oxide (NO) conversion into nitrogen dioxide (NO2); and the release of excess NO into the chimney and its transformation into NO2 (red fumes) when in contact with outside air. Upon observing these conditions, the initial measures taken by the operator consisted of forcing HNO3 into storage (after analysis of its concentration by a technician) and decongesting the oxygen concentrator. A press release was issued by the operator.

The Classified Facilities Inspectorate visited the site and noted that the threshold limit for high NOx emissions was not suitable for the start-up phases (saturation of the NOx analyser during unit restart and reported measurements not taken into account). Only the red smoke alerted plant technicians. The operator introduced a new device to monitor and warn of NOx emissions (including the restart phase). The operator needed to justify its decision to keep the workshop operating during the incident (50 minutes) without more precise knowledge of the characteristics of the gases released (simple visual assessment).