Pollution
Humain
Environnement
Economique

Nearly 1.7 tonnes of ammonia (NH3) were released into the atmosphere upon restart of the nitric acid production workshop at a fertiliser plant. Several neighbours upset by the foul smell alerted the police and plant.

The 1st phase of nitric acid manufacturing consisted of synthesising nitrogen monoxide (NO) by the catalytic oxidation of gaseous NH3. The burners used to trigger this reaction were lit by injecting hydrogen (H2) at the level of the catalyst meshes once the flow rate of NH3 mixed with primary air was optimal and stabilised. Start-up of the nitric acid production workshop catalyst systematically led to an atmospheric discharge of NH3 for 5 to 10 min (time necessary to stabilise gaseous flows), evaluated at between 350 to 700 kg. An H2 supply defect delayed burner ignition and extended the period of atmospheric discharge of the NH3 / primary air mix. H2 was supplied by a system composed of 3 bottles, an expansion valve and a solenoid valve. This system had been verified the day before, yet on start-up day the H2 pressure was insufficient to service the lighting ramps. It was therefore decided to replace the H2 bottles while keeping NH3 evaporation at a minimum. The H2 bottle pressure drop was most probably due to incomplete closure.

Several measures were adopted to both lessen the probability of repeat occurrence of this type of incident and reduce the installation’s stabilisation time, hence the time of atmospheric discharge, via an H2 supply system subsequently comprising 2 bottles in service and one held in reserve to compensate for an eventual insufficiency in H2 resources, a study conducted to improve NH3 evaporation, and lastly a revision to local authority alert systems and modes.