Approximately 300 ppm of nitric oxide (NO) was released from the stack outlet of a nitric oxide removal unit at a fertiliser plant. The event started when a large amount of water in a bleaching column that was being restarted prevented secondary air from being fed into the NOx absorption tower. The oxygen-depleted tower was unable to absorb all the nitric oxide, so the gas was fed to the removal unit. However, it was inadequate as well. The nitric oxide was therefore released into the atmosphere.

According to the operator, the water level of the bleaching column monitored in the control room did not reflect reality. The data sent back to the control room was a pressure delta measurement set for normal operating density with 62% acid. When the incident occurred, the column contained too much water, which changed the acid’s density. The water level relayed to the control room was therefore incorrect. The bleaching column is also outfitted with physical device that measures the water level, but its data was not sent back to the control room and the technicians did not go to see it.

The operator implemented the following actions:

• added checks on the water levels and secondary-air flow rate to the bleaching column startup guide;
• looked into installing a high-level safety sensor on the bleaching column, but this device could create more risks if it suddenly switches off the unit;
• changed the level feedback system so that density readings are not corrected;
• changed the mimic panels by adding an alarm to the secondary-air low level and a view of the physical water-measurement device on the bleaching column;
• technicians are now required to check the water level in the bleaching column whilst making their rounds;
• shared experience feedback from the event with the group’s other sites.