Flue Gas Desulphurization Plant at 600 MW Unit 6 in the Šoštanj Thermal Power Plant

Construction of the largest Flue Gas Desulphurization (FGD) Plant in Slovenia is a reference project, opening the door to RUDIS for construction of this type of plants also in the future.

Šoštanj Thermal power plant is the largest power plant in Slovenian power plant system, generating almost 35 % of electrical energy in Slovenia. By constructing the new 600 MW Unit 6, Šoštanj Thermal Power Plant fulfilled Slovenia’s desire for its own national energy supply to the greatest possible extent, lowered the generation costs, and above all significantly reduced burden on the environment. One of the most important contractual ecological parameters of the new Unit 6 was the content of SO2.

Permitted amount of sulphur oxides emissions is 100 mg/Nm3, actual amount of SO2 outlet concentration from the plant amounts to 20 – 30 mg/Nm3 (dry, 6 % 02), which presents more than 99 % efficiency of flue gas treatment. This is the first plant of its kind in Europe.

Rudis is an established contractor for construction of flue gas desulphurization plants

Management of the flue gas desulphurization technology, excellent references in the field of management and implementation of demanding projects and reputation of a reliable partner enabled Rudis to be awarded the contract for construction of flue gas desulphurization plant in new Unit 6 at Šoštanj Thermal Power Plant together with consortium partners Esotech Velenje and Engineering Dobersek from Germany.

Flue gas desulphurization on large thermal power plants is Rudis’ target technology also for future projects, mainly on markets abroad.


Termoelektrarna Šoštanj
Completion date:
Scope of works:

Construction of the Flue Gas Desulphurization Plant of Unit 6 »on turnkey principle«

Technology description:

Flue gas desulphurization plant is based on wet calcite process and as such fulfils the requirements of BREF documents and complies with BAT technology (Best Available Technology).

Technology according to wet calcite process

The highest level of treatment of all acidic components (sulphur oxides, chlorides and fluorides) is reached with the referenced technology, by using accessible and price-wise favourable neutralizing agent – limestone. Gypsum is formed as a by-product of treatment, which is used by the client for commercial purposes or it is used for stabilization of ash from electrostatic precipitator.

The main physicochemical processes of wet calcite procedure comprise:

  1. transition of sulphur oxide and other acidic components from gas to suspension – absorption,
  2. dissolving limestone,
  3. neutralization of acidic components,
  4. oxidation of sulphite to sulphate,
  5. crystallization of gypsum.

Flue gas treatment takes place in absorber, which is roughly divided into three parts. In the bottom part of absorber is a recirculation vessel for preparation of limestone suspension and gypsum is formed here. The most intensive treatment takes place in the middle part of absorber, because untreated flue gas enters in this part; limestone suspension is falling in counter-flow via spraying levels falls limestone slurry, neutralizing acidic components. Mist eliminator is installed in the upper part of absorber. Mist eliminator eliminates water droplets from saturated wet treated flue gas.

Treated flue gas is then via flue gas duct and cooling tower released into air.

Description of works:

FGD6 plant of Unit 6 is connected after the boiler of Unit 6, which uses lignite as fuel. Flue gas from the boiler passes through the electrostatic precipitator, where solid particles are eliminated. Two ID fans are installed after electrostatic precipitator and fans assure adequate negative pressure and flue gas flow through the boiler, electrostatic precipitator and FGD 6. Inlet flue gas duct to FGD 6, including the compensator is connected to the flue gas duct after the ID fan. Flue gas desulphurization takes place in the absorber with the wet calcite process, in which adequately ground limestone is used as additive. Saturated treated flue gas leaves the absorber after desulphurization and passes through the flue gas duct to the cooling tower of Unit 6 and into the atmosphere together with the water vapour from the cooling tower.

Flue gas desulphurization plant FGD6 comprises:

  • Flue gas absorber,
  • Circulation and other pumps,
  • Oxidation air system,
  • Compressed air system,
  • Transport and preparation of ground limestone,
  • First stage suspension dewatering,
  • Drain system of thickened gypsum,
  • Discharge and drainage system,
  • Process water system,
  • Steel structures, platforms and facades.

Flue gas absorber

Absorber is a steel structure with appropriate inside and outside corrosion protection, it is insulated and cladded with façade galvanized sheet from the height of 13.1 m upwards.

Absorber is designed as single spray tower with forced oxidation system where the contact between the gaseous and liquid phase occurs in counter-flow.

Flue gas flow through the absorber is enabled by the induced draught fan.

Six platforms of spray pipelines with nozzles are installed in the absorption zone of the absorber, spraying the suspension in both directions, thus achieving a longer contact of the suspension with flue gas. Nozzles are arranged in levels; each level of spray nozzles is fed by one circulation pump.

Flue gas cooling and the prevailing part of treatment of sulphur dioxides, chlorides and fluorides from flue gas is carried out in the spray nozzle area. Maintenance of appropriate pH value (pH = 5.4 – 5.6), appropriate density (1,080-1,150 g/l) and temperature (60°C-63°C) in the absorber is very important for the effective elimination of sulphur dioxides from flue gas.

Ground limestone with adequate granulation and chemical composition is used as additive. The quantity of ground limestone is determined by weighing screw conveyor and it is regulated with the help of rotary valve.

Density in the absorber is determined by the ratio of crystalized gypsum in the suspension. Adequate density of gypsum suspension in the absorber is regulated by taking away the suspension with the help of frequency-regulated pump and number of operating hydrocyclones.

Forced flue gas cooling is assured with the system of spray nozzle installation in the area of flue gas entry into the duct in front of the absorber. By quenching, we lower the flue gas temperature from approximately 150°C to approximately 63°C – water evaporation.

Two phases of eliminators are installed in the upper part of the absorber to eliminate mist from flue gas. The task of the mist eliminator is to eliminate those droplets that remained in flue gas after spray levels.

Absorber is equipped with agitators that assure appropriate mixing of the suspension, thus preventing sedimentation of coarse gypsum particles, and oxidation air is simultaneously blown in the impeller area of the agitator.

The absorber has the connections for measurements and protection; it also includes the start-up and operation system of the agitators.

Circulation and other pumps

The number of circulation pumps assures economical operation even at partial loads. Each circulation pump supplies its own level of spray nozzles. They are made of corrosion and abrasion resistant materials.

Two pumps for gypsum discharge from absorber have the capacity 2 x 100 % (one in operation, second one on standby) at full load of flue gas absorber. Pumps enable maintenance of adequate density of gypsum suspension in the absorber with regulation of revolutions.

Oxidation air system

Oxidation air for the absorber is supplied from three OXI-compressors (2 in operation, 1 on standby), located in the process building. In front of the absorber, the common pipeline is divided in five branches and it is connected to the absorber above the agitators. The oxidation process is accelerated by direct supply in front of the agitator.

The number of compressors enables economical operation even in case of partial loads. Regulation of compressors is possible is the area from 45 to 100%.

Compressors are located in the same room and each compressor is soundproof.

Compressed air system

Compressors for instrumental air with the corresponding equipment are located in the main process building FGD6. Screw compressors are used. Compressed air is stored in pressure vessels. Control of compressors and air dryers is carried out via the control cabinet.

Screw compressor is used for pneumatic transport of ground limestone; simultaneous unloading of two tank lorries is enabled. Compressor is installed together with the air cooler in the common compressor room by electrostatic precipitators.


Transport and preparation of ground limestone

Ground limestone is delivered by tank lorries. Two connections to connect the tank lorries and to supply the adequately prepared compressed air for pneumatic transport of ground limestone to daily silo are carried out by the process building.

An additional central silo is carried out with the intention to enable the possibility of additional reserve of ground limestone storage and in this regard lowering the possibility of the flue gas desulphurization system blackout in case of problems with ground limestone supply.

Ground limestone is fed into the mixing vessel for preparation of fresh ground limestone suspension from daily silo with the help of rotary valve and two screw conveyors.

In the mixing vessel, ground limestone is mixed with the watery of the gypsum suspension from the hydrocyclone separator, with water from filtrate tank from belt filter and with process water and flows out through adequately executed pipe to the absorber. Mixing vessel is located above the level of suspension in the absorber.

First stage dewatering

A part of suspension from the absorber is pumped into the hydrocyclone separator via gypsum pumps. Separation of both phases occurs in the hydrocyclone due to different densities of the liquid and solid phase in the suspension.

Solid particles with a higher density fall into the lower part of the cyclone (content of solid particles in the solution amounts to approximately 55%) and through underflow flow into the product tank.

Water and lighter particles are raised in the upper part of the cyclone (content of solid particles in the solution amounts to approximately 1 - 3%).

Overflow from hydrocyclone mainly contains fine solid particles (fine gypsum particles and electrostatic precipitator ash, fresh limestone, insoluble limestone impurities) and is led by gravity into the mixing vessel for adding ground limestone. Overflow from the hydrocyclone creates a strong tangential flow in the mixing vessel into which fresh limestone is fed. The prepared mixture flows to the absorber by gravity.

Thickened gypsum discharge system

Volume of the product tank enables storage of thickened gypsum that forms in 24 hours at operation of the flue gas treatment plant at the rated power of the unit.

Two pumps for thickened gypsum discharge to transport of FGD products have 2 x 100 % capacity (one in operation, one on standby) and enable emptying the product tank in two shifts daily via the circulation line, from where we lead the gypsum suspension into the mixing station of products and/or to vacuum belt filter.

Thickened gypsum is mixed with ash and water in adequate ratio in the mixing station of products so that the formed product is stabilized with stabilized physical – chemical properties. This product is transported to intermediate stockpile with pipe conveyor belt, where it is loaded on transport vehicles and disposed to final stockpiles.

Dewatered gypsum falls on rubber belt conveyor that transports it to gypsum silo or on to the conveyor for ash transport to the stockpile.

Discharge and drainage system

Discharge tank is intended for intermediate storage of the absorber content, namely if the absorber should be discharged for revision purposes or due to a certain operation malfunction.

In principle, the drain pit is made of concrete and protected with corresponding material. All liquids that form during operation, shutdown, cleaning or possible leakage are collected in the drain pit. The liquid is then pumped into the absorber or into the discharge tank.

Process water system

The process water system is intended for supply of the entire FGD plant with fresh process water. This water is used to compensate for the loss that occurs due to rinsing flue gas in the absorber, evaporation of dewatered product suspension into the mixture and vacuum belt filter and all other losses.

Decarbonized water is used to flush and cool oxidation air.

Volume of the process water tank allows the storage of process water for the operation time of at least one hour at the rated power of the unit. The tank is filled in intervals with the help of level control.

Steel structures, platforms and facades

Bearing steel structure is formed by transverse frames and vertical bracing. Platforms are located at different heights for technology needs.

Steel structure is covered with roof and facade cladding; platforms are covered with tread gratings or tread sheet metal.

Sandwich insulation panels are foreseen for roofing and façade cladding.


Technical data:
  • Fuel: Lignite
  • Wet flue gas flow: 2,100,000 Nm3/h
  • Inlet SO2 concentration: 8,200 mg/Nm3
  • Outlet SO2 concentration: < 100 mg/Nm3
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