Meeting European emissions requirements

A Romanian power plant implemented a new dry flue gas desulphurization system to meet the EU’s Industrial Emissions Directive, write Diana Baganz, Frank Oberheid and Peter Esser

The CET II power plant in Romania

Credit: Doosan Lentjes

The CET II power plant is located in the city of Iasi, approximately 400 km northeast of the Romanian capital Bucharest.

It consists of two pit coal and backup fuel fired steam boilers each with a capacity of 420 t/h which is equivalent to 285 MW.

Of these two boilers, only one is in operation, which generates 50 MW of electricity being exported to the national grid plus district heating for domestic and industrial applications.

Since Romania became a full member of the European Union in 2007, the country is required to continuously implement stringent European legislation and ensure full compliance with all applicable emissions reduction requirements with respect to power generation from fossil fuel types.

Against this background, the owner of the CET ll plant, the Municipality of Iasi, was required to retrofit its energy generation facility with an efficient and reliable FGD plant that reduces harmful pollutants released during the combustion process to a legally-approved minimum as per the Industrial Emissions Directive.

The IED replaced the Directive 2008/1/EC of the European Parliament and of the European Council on Integrated Pollution Prevention and Control (IPPC) and a range of further previous directives concerning high-emissions installations encompassing industrial plants, waste incineration facilities and power stations. Put into force in 2011, the IED increases efforts to minimize the negative environmental impact of solid and liquid fuel fired power generation stations with a capacity of more than 50 MW through an improved application of Best Available Techniques (BAT).

Defined and reviewed in so called Best Available Techniques Reference (BREF) documents, BAT set technology standards with respect to both environmental performance and efficiency. The derived emissions limit values depend on the thermal capacity and the fuel types used.

According to the directive, a coal-fired power plant such as CET ll with a thermal capacity of more than 280 MW is required to limit outlet sulphur dioxide emissions (SO2) to 250 mg/Nm3 on 6 per cent O2 dry basis (standard basis) while limiting solid particle emissions to 25 mg/Nm3 on standard basis.

In the continuous process of updating the BREF documents and adopting improved standards and technologies, emissions values for existing solid fuel fired power plants with thermal capacities from 100-300 MW are, in future, expected to become even stricter and will range at values not exceeding 220 mg/Nm3 SO2 and 22 mg/Nm3 dust, standard basis.

Against this background, in autumn 2014, the German energy technology provider Doosan Lentjes together with the Romania-based main contractor ELSACO Electronic SRL, was awarded the contract to deliver a new FGD plant.

Considering both economic and environmental requirements, it was decided to apply a Circocleanà‚® dry FGD system characterized by low capital investment costs and highest removal rates.

Defined as BAT in the BREF documents, the Circocleanà‚® dry FGD process is in line with all efficiency and environmental performance requirements.

Consisting of a Circocleanà‚® circulating fluidized bed reactor, a downstream fabric filter, a booster fan and auxiliary equipment, the plant is designed to capture a range of pollutants such as sulphur dioxide (SO2), sulphur trioxide (SO3), hydrogen chloride (HCl), hydrogen fluoride (HF) and solid particles from the flue gas.

The applied Circocleanà‚® dry FGD system operates on a simple recirculation principle. Entering the Circocleanà‚® reactor from the bottom, the raw gas flows through a circulating fluidized bed into which pulverized hydrated lime (Ca(OH)2) used as absorbent is injected.

For economic reasons, the Ca(OH)2 is hydrated on site from burnt lime (CaO). Pressurized water is sprayed into the reactor to lower the flue gas temperature and improve the reaction with pollutants to achieve maximum desulphurization efficiencies.

Downstream of the reactor, the chemically-cleaned gas enters a low pressure pulse jet (LPPJ) fabric filter which separates solid particles from the flue gas. Designed to handle large raw gas flows, the LPPJ fabric filter achieves optimum particle separation efficiencies.

Collected in a subjacent solid return vessel, the particles are partially recirculated into the Circocleanà‚® reactor to maximize utilization potentials of un-reacted reagents which has positive effects on consumption efficiencies.

The cleaned gas is released through the stack into the atmosphere while recirculating a certain amount into the Circocleanà‚® circulating fluidized bed reactor to balance different flue gas flows resulting from various boiler load conditions. Figure 2 provides a process overview.

Figure 2 The Circoclean dry FGD Process.

Source: Doosan Lentjes

The compact Circocleanà‚® dry FGD plant is accommodated on a small footprint, which simplified the retrofit at the Iasi power station with its given layout. Designed as a vertically-arranged cylindrical vessel, the reactor consists of a vertical inlet duct, a venturi nozzle section which is followed by a diffusor and the circulating fluidized bed in which the desulphurization process takes place.

The downstream LPPJ fabric filter consists of two parallel rows of four filter chambers, which can be independently separated from the process to allow online maintenance work in order to ensure high plant availability.

The hoppers of the filter are connected to large solid return vessels, one per row, which recirculate the product and unused absorbents to the Circocleanà‚® reactor and which allow compensating fluctuating boiler loads by buffering the material when needed. A key aspect for the design of the new FGD was the required flexibility to cope with changing boiler loads, which is also compensated with a clean gas recirculation duct for small boiler loads.

At the CET ll power station, the commissioning of the Circocleanà‚® dry FGD was executed from February 2016 to mid-April 2016 and completed by both a reliability and performance test.

During the latter, three different boiler load cases proved the performance of the FGD plant covering a range of load cases down to 60 per cent of boiler load with flue gas volume flows ranging from 623,500m3/h to 355,000m3/h.

During the performance test, the SO2 concentration was reduced from approx. 2,190 mg/Nmà‚³ on standard basis to less than 50 mg/Nmà‚³ on standard basis – far below the IED directive of 250 mg/Nm3 and equivalent to a SO2 removal efficiency of more than 97 per cent.

Tables 1 and 2: European emissions requirements.

Source: European Power Plant Suppliers Association, Brussels (EPPSA, 2015).

Using the LPPJ fabric filter, the solid content was reduced to less than 5 mg/Nmà‚³ on standard basis in the cleaned flue gas. After the scheduled summer outage of the CET II plant, the Circocleanà‚® dry FGD has been in successful operation since October 2016.

Designed to achieve SO2 emissions far below the legally required 250 mg /Nm3 on standard basis, the plant has the flexibility to comply with in future, even stricter emissions reduction directives expected to be derived from the continuous review and update of the BREF documents in the framework of the European IED directive – a critical success factor to ensure reliable power generation for Romanian citizens and their economy in the long-term.

Diana Baganz is Marketing Manager and Frank Oberheid is Product Director Air Pollution Control at Doosan Lentjes. Peter Esser is Project Manager for the CET II Iasi Flue Gas Desulphurization Project at Doosan Lentjes.

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