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.

A full plant risk assessment had been performed and regularly reviewed, the last time
in connection with the renovation in 2023. The pulp tower has not been classified as an explosive atmosphere, as the gases were not expected to be explosive. According to the plant's latest risk assessment (HAZOP), the greatest risk to the operation of the pulp tower was over-pressure and under-pressure in connection with a blockage of the vent pipe.
The possibility of growth of anaerobic bacteria was though eliminated using pressurised air aeration of the water circulation system.

The ROOT CAUSES have the following components:
(1) The modifications brought to the plant one year before had caused several production disruptions. This caused longer storage times in the pulp tower than under normal production rates. The management was probably still learning by doing.
(2) The risk assessment performed after the modifications failed in considering formation of hydrogen by anaerobic digestion a credible scenarios.

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 production of cardboard (fluting media) is based on a semi-chemical ammonia-based “fiberisation method”.
Birch wood chips processed in the wood processing line are cooked in a pressurised cooker using ammonium sulphite (NH₄)₂SO₃ as the cooking chemical. 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)
After cooking, the pulp passes through a series of different treatment units, such washers and evaporators. At the end of these steps, the clean pulp is fed to a storage tank called a pulp tower, from where it goes to the cardboard machine. The explosion occurred in the pup tower.

DESCRIPTION OF THE PULP TOWER
The pulp tower is an insulated tank approximately 36 meters high and with a volume of approximately 5,000 m3. It is a pressureless and freely breathing tank, meaning that it is always at ambient air pressure. It is designed to withstand a maximum overpressure of 0.01 bar. The roof section weighs approximately 22 tons and has an area of approximately 190 m2.

Approximately 8–10% of the pulp in the pulp tower is birch fibre and the rest is water. Due to washing losses, the pulp still contains traces of ammonium compounds and sulphur. The residence time of the pulp in the tower is normally approximately 4–8 hours. The pulp temperature is approximately 60 °C and the pH is approximately 7.5.
Gases can form in the upper part of the pulp tower, to remove which there is a heat-treated exhaust pipe with a diameter of 0.6 meters on the roof of the pulp tower.

No injury, storage tank and a building destroyed.

An emergency call was made from the factory at 12:00.
The first alarm about the accident came to the emergency center as an explosion or collapse.
The rescue service, emergency medical units and police patrols arrived at the scene of the accident at 12:11. In total, seven rescue services and five emergency medical units arrived at the scene.

The situation was later clarified to be the collapse of the factory building's mezzanine floor caused by the pulp tower's roof coming loose and falling. When personnel in the area were contacted and everyone was found to be safe, the alarm was canceled and unnecessary alarm units were dispatched.

The accident area was isolated and the tank was photographed, and the area was also filmed using a drone. The Safety Investigation Authority (OTKES) was notified of the incident, which conducted an immediate scene investigation in the area and interviewed those present at the time of the accident. The last rescue units left the scene at 14:44.

1211

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.