In the early afternoon, a flaming methane leak began at a chemical plant while a contractor was replacing the solenoid valve on the burner of a furnace. The contractor sustained minor burns. The four damaged burners were isolated and locked out. The site’s emergency services marked off the area and prohibited access to it.

As a result, production of carbon disulphide (CS2) in line A was reduced (damaged burners) and production in line B could not be increased to offset this because its controls were in the prohibited area. This generated a decrease in hydrogen sulfide production. To compensate, the operator lowered the rate of the two methanethiol (a toxic gas with a rotten-cabbage odour) production lines. The rapid drop in the production rates of both lines destabilised the process, activating lines A and B at the same time. This in turn caused the pressure in the two reaction lines and the two columns leading to the flare stack to drop.

At 3:00 p.m., 75,500 kg/h of gas was sent to the flare stack for 40 seconds. As the flare stack is designed to only handle a flue gas flow rate of 28,000 kg/h, a puff containing highly odorous unburnt sulphur product was released. This puff contained 250 kg of methanethiol, 161 kg of hydrogen disulphide, 100 kg of methanol, 19 kg of dimethyl sulphide (DMS, a toxic gas), and 1142 kg of sulphur dioxide (SO2). A foul odour was reported for 30 km around. Fearing a gas leak, several schools around the plant were contained.

The operator informed the surrounding town councils and issued a press release.

In order to physically limit the flow of gas to the flare stack, restriction orifices were installed on the two reaction lines and on one of the columns. These orifices restrict pressure drops and ensure that the flue gas flow rate always remains lower than the flare stack’s designed flow rate.