One morning, when restarting after a scheduled annual shutdown, the operating team of the nitric acid manufacturing unit at a fertiliser plant observed an orange plume exiting the workshop’s chimney. This cloud was due to emission of nitrogen oxides (NOx, NO, and NO2) generated by the unit. These compounds are treated by a DeNOx reactor, via a catalytic reduction process using ammonia (NH3). Upon startup, before the reactor reached the optimal nitrous gas treatment temperature, nitrous gases were channelled to a hydrogen peroxide (H2O2) absorption column. Initially, the operator suspected a DeNOx malfunction. It attempted different procedures on the facility (adding NH3, checking analysers) and the absorption column (adding H2O2) to reduce nitrogen oxide emission. As these procedures did not stop the discharges, the nitric acid unit was shut down at 10.30 a.m.

The peripheral sensors and air quality measuring stations did not detect high nitrogen dioxide (NO2) concentrations. Based on its modelling and weather conditions, the operator concluded that the discharges had no toxicological impact on the soil.

The hypothesis taken to explain these discharges is a tightness issue with an anti-surge valve on one of the unit’s NO compressors. This valve is closed during normal operation. It opens in the event of unit shutdown to protect the compressor. Its discharge is channelled to the unit’s chimney. This valve was serviced during the shutdown by a sub-contractor, which issued a compliant tightness test report. The operator disassembled the valve and sent it back to the sub-contractor for inspection. It observed a mechanical jam affecting the main seat. The sub-contractor admitted that it had not fully inspected the tightness of the valve during initial servicing. The valve was repaired and reinstalled in the unit. The operator lodged a complaint against the sub-contractor.

In the course of this event, the operator discovered that the technicians did not know enough about the analysers. They did not check one of the DeNOx outlet analysers, and therefore suspected one of these systems had failed. In addition, the NOx chimney discharge management procedure required that the unit be shut down in the event of major build-up of NOx emissions. The operating team initially blamed the DeNOx system before shutting down the workshop.

The operator put in place the following actions:

• raising the awareness of the sub-contractor regarding the impacts generated on the site by the leaky valve;
• raising the awareness of the operating teams and sharing feedback;
• modifying view on supervision;
• adding analyser names to the start-up guide;
• adding the offending valve as a critical system in procedures.