An explosion, followed by a fire, occurred around 4:15 p.m. on a 370 m³ tar extractor associated with a gas treatment facility in a coking plant. The extractors were actuated manually to limit the spread of fire, and the operator initiated the internal emergency plan (IEP) at around 4:25 p.m. The personnel were evacuated, and a safety perimeter was established. The fuel supply to the facility was interrupted, and the heating of the coke oven batteries was stopped. The fire was extinguished around 8 p.m. by nitrogen inerting. The structure had been weakened, putting it a risk of collapse. Around 8:30 p.m., the valves upstream and downstream from the tar extractors and the H2S scrubbers were closed, gas from the coking plant was returned, nitrogen was injected into the coke oven gas collector, and the structure was inspected. The operator installed blind flanges to isolate the tar extractors and the H2S scrubber.

According to the operator, no dangerous or polluting material was released into the atmosphere apart from smoke from the fire (limited duration). The maximum quantity of material having reacted in the explosion of the tar extractor was evaluated at an equivalent of 170 kg of TNT. It was indicated that the raw coke gas from coal distillation was burned by the flare stacks (torches above the oven batteries) for nearly 5 days, the time required to put the gas treatment facility back into service (the destroyed tar extractor and the H2S scrubber were isolated from the coke oven gas treatment and remained unavailable for several months). An estimated 3,776,600 Nm³ of coke oven gas was sent to the flare stacks.

Flaring of the coke oven gas continued for 90 days until the tar extractor was replaced.

The operator estimates that 36,790 t of coke oven gas was released to the flare stacks (then to emergency torches) to be burned off. A health assessment was conducted, and, according to the operator, SO2 is the only pollutant resulting from combustion and which could impact the population. It was concluded that the concentrations over the daily average limit value for protection of human health, the information and recommendation thresholds and the alert threshold were not exceeded.

Following the event, the degradation of the outlet valves and pipes was analysed. The reconstruction of the tar extractor took several months. Loss of property was estimated at €10M, and production losses at €9M.

A 25-mm hole and a crack had been identified on the steam system 8 months before the accident. These defects could not be repaired without shutting down the tar extractor. As the oxygen measurement on the outlet of the tar extractor did not show any discrepancy, the repair was scheduled for the normal shutdown 3 days after the day of the accident.

Several hypotheses as to the origin of the activation energy have been put forward: electrostatic energy caused by friction of the air/gas mixture (also composed of dust) on the metal plates, chemical energy due to the presence of pyrophoric materials or mechanical energy caused by a metal component falling inside the tar extractor. The third-party expert report concluded that the most probable hypothesis is that the mixture of coke oven gases came into contact with the oxygen in the air coming from the leak on the steam network and that ignition occurred due to the electrostatic energy generated by the friction of the gas on the metal walls.

An electrostatic tar extractor had already exploded at the site in July 2005 and at other sites of the group abroad in November 2017 and in 2019. The explosions always occurred while the tar extractor was restarted while supplying the electric fields, but never during the washing and dewatering phases where the electric fields were not energised (no ignition source). No feedback was provided, and the operator had not identified this risk.

Following this accident, the operator reviewed the risk analysis, took additional measures to revise the tar extractor washing procedure and changed its maintenance plan:

• washing and dewatering operations are now performed under nitrogen pressure to expel the coke oven gas and to avoid any influx of air;
• tar extractor maintenance includes an annual inspection of all the earthing systems and an annual inspection of the shell to check the fasteners of the various elements.

The operator has also been careful to share its experience with the group’s other sites.

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