One of the fuel-oil boilers in an industrial slaughterhouse malfunctioned, causing a significant amount of soot to be released. As the company was experiencing an increase in activity at the time, it was decided to start the boilers on Sunday evening instead of Monday morning. The boiler room foreman, who had been with the company for more than 20 years, pressurised (vapour) boiler No. 1 at 9:35 p.m., without any problem whatsoever. The subsequent start-up of boiler No. 2 proved to be more difficult and it was taking too long to raise the temperature of the fuel (120 °C). The boiler was finally started after the burner’s settings were optimised. Difficulties were encountered when starting boiler No. 3 between 10:00-10:30 p.m., during which burner control problems were encountered, and smoke opacity could not be controlled (voluntarily deactivated during adjustment). The foreman pressurised the boiler despite these problems, and its start-up was put on standby in the event it was needed.

The release of soot was caused by sub-optimal combustion of boiler No. 3 during the adjustment phase. Due to the wind, the soot was deposited over a distance of 100 to 400 m. As the operations took place at night, and since the smoke opacity control system had been deactivated, the foreman had been unable to assess the impact of the malfunction.

The foreman reported the situation the following morning at 5 a.m. upon arriving at work. When the shift supervisor arrived at 7 a.m., the decision was made to shut down boiler No. 3. The maintenance department checked the burner on boiler No. 2 and confirmed that it was working correctly. The company in charge of boiler operations intervened at 2 p.m. It confirmed that boiler No. 2 was operating correctly and noted a malfunction on the modulating valve and the fuel solenoid valve on boiler No. 3. This equipment was replaced. The modulating valve on boiler No. 3 had already been replaced with new equipment on Nov. 11, 2016.