In their search for an FGD technology that would be suitable for an oil-fired peaking power station, Kjell Nolin and his colleagues at Sydkraft’s 3x340MWe Karlshamn (the only large fossil-fired condensing plant in Sweden) hit upon a novel idea.

Rather than employing the conventional spray tower system, they decided to carry out desulphurisation on a large sieve tray, which provides a very effective gas/liquid contact zone and achieves very high efficiency.

Development work started in 1992, with ABB coming on board the following year and subsequently acquiring the technology. Following the award of a contract to ABB in 1995, Karlshamn 3 was equipped with the sieve tray absorber system in 1996 and this has operated successfully since.

Following its acquisition of ABB’s power generation interests, the sieve tray absorber technology came into the ownership of Alstom, which is now marketing it under the name Flowpac.

The flue gas enters the absorber under the sieve tray, flows through the sieve tray holes and then rises through the limestone slurry with a high degree of turbulence.

This highly turbulent zone allows intimate contact between the gas and liquid, creating excellent conditions for SO2 absorption and natural oxidation.

Another unique feature of the technology is that the absorption medium is circulated by means of an airlift effect, generated by the difference in densities when oxidation air is introduced into the central reaction tank for forced oxidation. The need for slurry circulation by pumps is thus eliminated, reducing maintenance requirements and saving energy.

The central reaction tank is encircled by the sieve tray. When the absorption slurry is at the upper level of the tank, and when the oxidation air is dispersed at the bottom of the reaction tank, the absorption medium expands and overflows the sieve tray. The medium then flows to downcomers located at the circumference of the sieve tray, where the dispersed gas is released from the slurry, causing an increase in density. The density difference of 5-10 per cent between the slurry-with-dispersed-air in the reaction tank and the pure slurry in the down pipes generates the circulation of fresh slurry.

Owing to the pressure drop across the sieve tray, the distribution of flue gas flow over the entire absorption zone is uniform, minimising the gas sneakage common in some conventional systems.

The intimate contact between the gas and the absorbing liquid, due the turbulent zone, achieves very high liquid/gas ratios. This high liquid hold-up in the absorption zone is a unique chemical feature of the Flowpac system. It not only permits high SO2 collecting efficiency, but also almost full utilisation of the limestone, insensitivity to pH changes, as well as high limestone dissolution and sulphite oxidation in the absorption zone.

Other advantages include: high SO3 and dust collection efficiency; very high gypsum quality; very low liquid entrainment, which ensures problem-free droplet eliminator operation even with reduced flushing; lower auxiliary energy consumption than conventional spray scrubbers; and lower maintenance and supervision costs than conventional spray towers.

Over five years operation of the Flowpac FGD at Karlshamn 3, it has not proved necessary to increase the operating staff for the power plant (which has remained at four people) and maintenance costs have been low.

As well as the absence of slurry pumps the low overall height of absorber structure, allowing all the internals to be easily reached and inspected (without scaffolding), also helps reduce O&M costs, as does the overall simplicity of the design – with four agitators the only moving parts.

Basic data for the Karlshamn Flowpac FGD installation (440 MWe oil fired plant, 3.5 per cent sulphur)