Emission Control: Seawater FGD debuts at Alba

Fisia Babcock Environment (FBE) has developed its own sea water FGD technology. Its first plant, located in Bahrain with a capacity of 150 MW, has been in operation for more than a year and may serve as a reference for coal fired plants in the EU considering the technology for emission reduction.

Over the past 20 years there have been significant advances in the design and operation of flue gas desulphurization (FGD) systems, driven by differences in fuels and economic pressures. Larger coal and oil fired power plants within the European Union (EU), for example, have to reduce their sulphur dioxide to at least 400 mg/Nm3 by January 1, 2008. This will require some 20-25 power stations in the EU, with more than 30 units of 300 to 600 MW, to undergo an FGD retrofit within the next four years.

Fisia Babcock Environment GmbH (FBE) ” which has about 50 000 MW of wet FGD systems in operation in Europe, Asia and North America ” is one company which has developed a wide range of experience in FGD technology. Notably it has developed its own sea water scrubbing technology. A first reference plant in Bahrain ” with a capacity of 150 MW firing bituminous coal ” has been in operation for more than a year and has demonstrated satisfying performance.

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The operation of the plant will be important to plant operators and owners within the EU. Since wet scrubbing is the best available technology for most of these plants, the traditional limestone/gypsum process will be installed. However, at some coastal sites, sea water scrubbing will be possible if the respective environmental agency grants approval.

Seawater FGD gives plant operators the flexibility of using high sulphur coal by offering the most economic method of reducing sulphur emissions.

Sea water FGD

Sea water contains sodium, magnesium bicarbonate, small quantities of calcium bicarbonate and is slightly alkaline. This makes it a suitable absorbent for SO2 removal. Through many years experience gained with conventional flue gas cleaning plants, FBE developed a sea water FGD process which uses sea water as absorbent for SO2 removal.

The main component of the system is an absorber which is designed as a spray tower. It essentially consists of three zones, the spray or contact zone, the gas zone and the oxidation zone. Outside of the absorber there is a mixing zone where used and fresh sea water are mixed.

Inside the absorber the flue gases come into intensive contact with sea water and are thus largely cleaned of the pollutants SO2, HCl and HF as well as partially of SO3. At the same time oxygen is absorbed from the flue gas and dissolved in the scrubbing liquid where it is then available as a reaction partner.

  • Spray zone (contact zone): The hot flue gases enter the absorber through a radial nozzle. The gases flow through the absorber in an upward direction. Spray banks fitted with spray nozzles are located in the upper area of this zone. All spray banks are fed with fresh sea water by absorber pumps. The sea water in the form of droplets of a defined diameter is distributed over the spray zone of the spray banks. This offers a large exchange surface for SO2 removal.
  • Gas zone: In the upper area of the absorber, the flue gases are led through a horizontally arranged droplet separator and freed from the entrained liquid droplets. The droplet separator is of the single-layer type designed for the same residual droplet content compared with a conventional FGD. The clean flue gas leaves the absorber saturated with vapour and is cooled down close to the sea water temperature.
  • Oxidation zone: The scrubbing liquid including the absorbed pollutants from the flue gas is collected in the absorber sump which is mainly used for the oxidation of hydrogen sulphite to sulphate. Therefore, additional oxidation air is supplied into the sump. Good oxidation rates are met at pH above 4.5. An optimum in terms of investment charges is thus reached by keeping the pH above 5.

The sea water emerging from the spray zone has a pH value of 3 to 4 and additional sea water must therefore be introduced into the absorber sump in order to keep the pH value above 5.

The oxidation air is introduced to the sump liquid with the help of an oxidation air compressor and distributed through lances equipped with nozzles in the form of fine bubbles. The close contact, combined with an adequate control of bicarbonate to the sump makes sure that most of the hydrogen sulphite is oxidized to sulphate. The oxidation rate of sulphite to sulphate is higher than 99 per cent.

  • Mixing zone: The sea water coming from the absorber has a lower pH value than is usually allowed for discharge to the sea. To raise the pH value above 6, untreated sea water is used. These two streams are united directly in the discharge pipe from the absorber to the sea shore. If the sea water quality is poor or there is limited availability of sea water, the pH of the discharge water can be controlled by adding sodium hydroxide (NaOH).

FGD at Alba

In April 2002, Alba (Aluminium Bahrain) placed an order with FBE for the turnkey retrofit of a complete sea water FGD system downstream of its existing aluminium calciner in Bahrain. The decision was taken to fulfil the legal emission limits of the Kingdom of Bahrain. Because high sulphur green coke is used at the plant, SO2 concentrations of up to 5000 mg/Nm3 could be expected in the raw gas.

Pre-engineering works on the new FGD system started in February 2002 in order to meet the extremely tight time schedule of 15 months. The project was, however, completed in just 10 months. The site was opened in June 2002, the first flue gas was taken over in the beginning of 2003 and commissioning and testing was finished in April 2003.

The plant itself consists of two identical calciner units each producing up to 250 000 Nm3/h wet flue gas. The flue gases are cooled down in steam boilers to a temperature of about 200-240à‚°C. The particulates consisting mainly of graphite dust are removed by fabric filters.

After de-dusting in the existing fabric filters, the flue gases of both units generated in the boilers of both units are combined into one common duct where a radial booster fan provides the necessary pressure increase. Downstream they pass a quench zone before they are fed into the absorber for absorptive removal of the acid gases SO2, HCl and HF. The quench is designed for an inlet temperature of about 250à‚°C. The absorber itself is an open-spray tower of proven design with no internals. This means plugging inside the absorber will not occur.

A special feature of this plant is that it uses either fresh sea water or brine water from a desalination process. Inside the absorber, SO2 is removed from the flue gas by the use of fresh sea water or brine (the effluent from a sea water desalination plant). Sulphur dioxide is absorbed and reacts with the hydrogen carbonates in the sea water. An integrated oxidation air system is installed within the absorber sump. It provides the necessary oxidation air for complete oxidation of SO3-ions to SO4-ions which are a natural constituent of sea water.

The amount of sea water or brine used at the plant was lower than originally specified. Therefore, a sodium hydroxide system was installed to cover peaks of high SO2 concentration and/or high flue gas volumetric flow rates. If there is a lack of sea water, sodium hydroxide can be added to increase the pH value of the effluent. Usually, the system is operated at nominal load and with an average SO2 concentration of about 4500 mg/Nm3 (dry) only using sea water.

The saturated flue gas leaves the absorber by passing a demister and is released through a wet stack mounted on top of the absorber.

The effluent from the absorber is released to the sea via an outfall ditch. No additional treatment is necessary.

For corrosion protection purposes the cylindrical part of the absorber, stack and piping are made of fiberglass, while the absorber sump is made of coated concrete.


The sea water scrubbing technology installed at Alba has shown excellent performance
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Operational results

The sea water operated absorber demonstrated up to 98 per cent SO2 removal efficiency over the whole load range of the two calciner units. This is much more than the required 90 per cent SO2 performance of the environmental authority of Bahrain.

As the Alba FGD is FBE’s first FGD to be operated with pure sea water, it was designed with sufficient margin to minimise the risk of unexpected side effects which might not have been detectable in the development phase. Against expectations, the sea water operated concurrent quench in the raw gas duct directly upstream of the absorber showed very good efficiency. The performance of the quench layer, which is designed for cooling the raw gas close to the saturation temperature, was almost as effective as a comparable spray bank integrated in the absorber.

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The effluent conditions of the environmental authorities of Bahrain for the used sea water flowing back to the sea are similar to European standards and have been met for all values. In particular, the high sulphite oxidation ratio of more than 99 per cent is remarkable. This excellent oxidation ratio is exhibited in the pH range between 5 and 5.6 where the oxidation in the sump area of the absorber is operated.

A simulation programme was developed to predict pH values, the necessary sea water amount and the SO2 removal efficiency. The sea water flow rate used for the process corresponded closely with the calculation results of the simulation programme

Novel technology

Alba is FBE’s first reference for sea water scrubbing. It shows excellent performance and is economically attractive due to the lack of external oxidation basins. Having analysed numerous measurements at this plant, FBE’s design tool has confirmed that this novel technology can be applied to much larger units.

Sea water scrubbing is useful to low and medium sulphur applications. Sulphur dioxide control efficiencies of more than 95 per cent can be achieved. Limiting factors are the alkalinity and availability of the sea water as well as the quality of the upstream dedusting device. Also, prior to implementation the permitting situation must carefully be investigated by the operators in close co-operation with the local environmental agency.

Reference

“Most Recent Developments and Optimisation Aspects for FGD Technologies” Presented by Dr. Wolfgang Schàƒ¼ttenhelm; Dr. Werner Braun, Helmut Dreuscher, Fisia Babcock Environment GmbH, Gummersbach, Germany at Power-Gen Europe, Barcelona, Spain, May 25-27, 2004.

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