Repair of a fertiliser prill tower
Fertiliser is often sold as small pellets known as ‘prills’, formed by pumping hot liquid fertiliser through spray heads at the top of a tall tower. As the fertiliser falls down the drop shaft, it cools and solidifies to form prills, which are directed by conveyor from a collection hopper at the tower base to the packing and despatch area. Fertilisers are frequently based on ammonium nitrate or urea. The same prilling technique can be used for either of these processes.
While urea and ammonium nitrate are valuable commodities as fertilisers, they present an aggressive environment to concrete. Protective design measures are required, together with ongoing maintenance to repair the affected areas.
One of the earliest operating ammonium nitrate prill towers still in existence is operated by Terra Nitrogen (UK) Ltd at Severnside, Hallen, near Bristol. Constructed in 1964, it consists of a 15.6m-diameter reinforced concrete cylinder, 178mm thick and 86m high, on concrete piled foundations. A five-storey reinforced concrete plant house, supported on the drop shaft, lift shaft and a single column, forms the upper section of the tower from 75m to 100m (see Figure 1).
Ammonium nitrate attack
The primary deterioration mechanism is the reaction of ammonium nitrate with the calcium hydroxide in the cement paste to form calcium nitrate. This leads to a reduction in the calcium hydroxide content, together with an associated increase in porosity and decrease in alkalinity. The calcium nitrate then reacts with hydrated calcium aluminate, present in cement, to form calcium nitroaluminate. The resultant volumetric increase associated with this reaction leads to an expansion of the cement matrix and subsequent bursting of contaminated layers.
Ammonium nitrate attack is generally regarded as a removal of surface laitance followed by a loss of aggregate from the weak cement matrix. In time, the concrete can be attacked throughout its full thickness. Where alternating wetting and drying conditions exist, the surface laitance may not be removed, leading to the bursting of an apparently sound concrete surface. This may be confused with the more familiar cracking and spalling that arise from reinforcement corrosion.
As little as 0.5% by weight of concrete can cause significant damage over time, and the degradation increases with ammonium nitrate concentration. Of more concern in ammonium nitrate environments is the potential for stress corrosion of the steel reinforcement. This is coupled with tensile stress, which causes brittleness and results in direct tensile failure of reinforcement. The likelihood of stress corrosion increases if the steel has surface defects. Only a short initial crack length is required for stress-corrosion cracking to propagate. Factors affecting the rate of deterioration include temperature, residual surface stresses, surface defects, roughness and topography, chemical composition, diameter and ultimate tensile strength. Stress cracking at Severnside was particularly evident in links that, when exposed to ammonium nitrate attack, were frequently found to have broken at positions where they had been bent and subjected to high residual stresses. The combination of concrete softening and stress causing the corroded steel to crack presents a significant structural concern because of the reduction in ductility of reinforced concrete sections and their ability to redistribute loads.
Repair history
Ammonium nitrate attack has been known in the fertiliser industry for some years, and protective measures were taken during the original design of the Severnside tower. The general approach to concrete repairs has been to remove degraded concrete and replace it with either epoxy or polymer-modified concrete. Such repairs tend to fail at the interface with the original concrete owing to the difficulty of removing all the contamination. Subsequent repair requires the removal of further degraded concrete until eventually it becomes necessary to carry out a full structural repair or element replacement. Examples of previous major repairs have included the reconstruction of the concrete roofs at the 86m and 100m levels and the spraying on of a reinforced silica fume concrete jacket to the outside of the drop shaft over its full height.
Plant house integrity project
Continued confidence in the nitrate market led Terra to specify an extended service life of 15 years from the tower. Their technical advisors, ABB Eutech, identified that major work was necessary and produced a repair strategy to avoid loss of production. Taylor Woodrow’s engineering and consultancy team were subsequently appointed to undertake the project management, from detailed investigation, testing and assessment, through design to supervision of remedial works.
The project team - comprising Terra, ABB Eutech, Taylor Woodrow and main contractors Bierrum - were able to draw on the combined strength and experience of all parties involved in tackling this project.