Ammonia-hydrogen explosion in a pulp production plant

The incident occurred at a pulp mill plant producing fluting media.
The explosion took place at a pulp storage tank and was caused by the combustion of a hydrogen-ammonia gas mixture. 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.

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 in a gas form under the conditions of the pulp storage tank, so that 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 accident did not cause any personal injury, but material damage/ The production was halted for approximately six weeks.

The INITIATING CAUSE was the accidental generation of hydrogen by bacteria in the pulp storage.

Considering that (1) the possibility of hydrogen formation is known, and (2) that the plant had been modified and since then was affected by ROOT CAUSE could be identified in some (1) shortcoming in designing preventive and mitigating measures and (2) proper management of the challenges related to the introduction of a new process.

pulp storage tank

The accident occurred when the production process was being shut down due to a disruption.

The production process of the mill, which has been operating since 1968, had recently undergone significant modifications. The upgraded production process was introduced in November 2023. Since the modifications, there have been numerous shutdowns and downtimes in production, which contributed to the conditions behind the accident.

The plant was at more than 2 km from the centr of the nearest city.

DESCRIPTION OF THE PROCESS
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.
The process uses ammonium sulphite (NH₄)₂SO₃ or ammonium bisulphite (NH₄HSO₃) as chemical agent. The main advantage vs. other methods is a lower environmental impact by lowering sulphur emissions and facilitating easier chemical recovery. The final product has moderate fibre strength, often used in niche applications where environmental concerns are a priority.
(https://www.cnsunrisepaper.com/blog/the-semi-chemical-pulping-process.html#:~:text=3.,Green%20Liquor%20Semi%2DChemical%20Pulping)

No injury, storage tank and a building destroyed.

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 the specific process involved in this case (for example, see HIAD_886).

Proper attention must be paid in avoiding accidental hydrogen production, and whenever not possible, in avoiding the conditions bringing to formation of explosive mixture. A risk assessment should also inform the design of spaces and component where explosion could occur.

The investigating team issued recommendations meant for broad range of industrial applications.

On technical safety:
(1) 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 and managed.
(2) In exceptional situations (such as a downtime), a situation-specific process safety and explosion risk assessment may be needed.

On the management and operations:
(1) 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.
(2) 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 analysed and tackled systematically.
(3) 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.