Investigation of the explosion at a Kuopio fluting mill finished – Tukes’s investigating team proposes several measures to improve safety

Tukes’s accident investigation team has finished its report on the explosion at the Kuopio fluting mill on 4 November 2024. The immediate cause of the accident was determined to be an explosion of a hydrogen-ammonia gas mixture accumulated in a pulp storage. The investigating team proposes a number of recommendations to improve technical safety and the operation of companies in order to avoid similar accidents.

The detachment of the pulp storage tank roof was caused by the combustion of a hydrogen-ammonia gas mixture

The mill manufactures semi-chemical fluting media, and the production process is based on ammonia. A similar production process is not known to be used elsewhere in Finland. Bacterial strains producing hydrogen had formed in the white water and pulp storage tanks of the mill before the accident. The conditions in the pulp storage tank were favourable for them, and hydrogen was released into the air in the tank. The residual ammonia produced in the process was gasified under the conditions of the pulp storage tank, causing hydrogen and ammonia gas to form a flammable gas mixture. 

The gas mixture was most likely ignited by a spark from static electricity because no other ignition sources were identified in the pulp storage tank. The roof of the pulp storage tank was detached due to the pressure generated in the explosion, and it landed on to the roof of the adjacent mill building, which partially collapsed. The accident did not cause any personal injury. The accident caused material damage and fluting production was halted for around six weeks.

The production process of the mill, which has been operating since 1968, has recently undergone significant reforms. The upgraded production process was introduced in November 2023. Since the reform, there have been numerous shutdowns and downtimes in production, which contributed to the conditions behind the accident. The accident occurred when the production process was being shut down due to a disruption.

The investigating team found room for improvement in the organisation’s operations

The investigating team identified several factors related to the organisation’s operations that contributed to the accident. Based on the investigation, the investigating team issued several recommendations related to process safety (industrial handling and storage of hazardous chemicals) and explosion risk management for the company. These included:

• The reproduction of hydrogen-producing bacteria must be prevented or their growth conditions deteriorated.
• The generation of explosive gas mixtures and the accumulation of gas in the pulp storage must be prevented and controlled.
• The impacts of significant process changes must be identified and assessed to ensure safe implementation.
• There must be clear responsibilities and systematic procedures for investigating the causes and impacts of process disruptions and for taking corrective action.

The investigating team proposes safety-enhancing measures for industrial companies

Similar accidents have occurred in the past, for example in France and the United States. The presence of hydrogen-producing bacteria is also possible in other pulp and water treatment processes besides this case. Proper attention must also be paid to controlling the risk of explosion. 

In its report, the investigating team issues recommendations for preventing similar accidents and improving safety. The recommendations are intended for a wide range of industrial sectors. 

Recommendations to improve technical safety include:

• In pulp processing in the paper and board industry, the risks associated with microbial activity and the possibility of the occurrence of bacteria producing flammable gases (such as hydrogen) must be identified.
• In exceptional situations (such as a downtime), a situation-specific process safety and explosion risk assessment may be needed.

Recommendations to develop the management and operations of companies include:

• The chemical plant must have sufficient information on the chemicals that are essential for process safety and the reactions between them as well as microbiological conditions.
• Significant changes in the technical and operating methods of the chemicals plant must be handled through a systematic change management procedure. Disruptions occurring in the implementation of a change and their causes must be determined systematically.
• If it is possible for an explosive gas mixture to form in a space or a storage tank, preparation for the risk of explosion should be taken into consideration in the construction of the space or the container.

Tukes emphasises that chemical plants must be familiar with the properties of the chemicals they process and assess their combined effects and microbiological conditions.

The investigating team’s report presents in more detail the course of an accident at the Kuopio fluting mill, the identified causal factors and the investigating team’s recommendations for preventing similar accidents.

Appendix: Summary of the investigation report (pdf)

More information:
Director Kirsi Levä, tel. +358 295 052 162

Senior Officer Aatu Isotalo, tel. +358 295 052 243

email: [email protected]