Edenderry starts up ahead of schedule

The 117 MWe net Edenderry peat-burning power station was officially opened on 27 November 2000. Located about 60 km west of Dublin, it has been a very fast track construction project and was completed 6 months ahead of schedule, in time to contribute to meeting the 2000/2001 winter peak in electricity demand. According to Fortum this was one of the fastest ever construction schedules for a solid fuel plant of this size, going from green field site to power generation in 21 months and requiring 800 000 man hours of effort from start of construction to start of the trial run.

One of the challenges to be addressed in the project, according to Fortum, was that it was carried out during a boom time for the Irish economy and its construction industry, causing a shortage of manpower in certain areas. Due to the tight schedule, several contractors of different trades were working in congested areas and special attention was needed to the organisation of working schedules. The project has also been truly multinational, with contractors from ten countries (Ireland, Finland, the UK, Germany, France, Sweden, the Czech Republic, Poland and Turkey) and equipment deliveries from a dozen countries.

The idea of building the plant originated with peat supplier, Bord na Mona, which was looking for a market for its peat following closure of some old peat burning stations (using pulverised fuel firing) in the East Midlands of Ireland. After Russia, Finland and Belarus, Ireland is the world’s fourth largest peat producer.

Bord na Mona undertook a major feasibility study, which showed that, by using state-of-the-art technology, eg fluidised bed, a peat power plant was economic. Edenderry is envisaged as the first of three new-generation peat fired plants.

At the beginning of 1997 the Department of Public Enterprise in Ireland invited independent power project developers to submit proposals for a peat-fired condensing power plant to be built at Clonbulloge county in Offaly, Ireland. Five companies were shortlisted in the spring of 1997 and eventually Fortum won the tender competition to build and operate the first ever independently built, owned and operated power station in Ireland, representing at the same time the first concrete evidence of free competition in the Irish power markets.

The turnkey responsibility was awarded to Fortum Engineering. The plant is owned and operated by Edenderry Power Ltd, a subsidiary of the Fortum group. The power purchaser is ESB, under a 15 year agreement.

At the heart of the plant is a 274 MWt bubbling fluidised bed boiler supplying steam to a two-cylinder reheating condensing steam turbine. Services provided by Fortum Engineering included: project development; all design; construction; procurement; installation; and commissioning. The boiler has been supplied by Kvaerner Pulping Oy (Power Division) of Finland. The steam turbine was supplied by Alstom. A Balcke Duerr cooling tower is used to condense steam after the turbine. Fortum Engineering also provided the plant automation system, which is based on Neles hardware. Project Management Ltd, of Tallaght, provided civil engineering design. PJ Walls and Ascon were responsible for the civil construction work. Analysers for CO, NOx, SOx, dust, O2, flow, moisture and temperature were supplied by Moeller Electric Oy of Finland. The project was assisted by the EU Infrastructure Operational Programme.

The £100 million power plant will have a significant influence on the peat industry in Ireland, as this is the first peat power plant to be built in the country for many years. The plant has been built, literally, in the middle of a peat bog and will burn about 1 million tonnes of peat per year. The pre-crushed peat is supplied using a narrow gauge railway system that already existed to serve previous plants. Magnetic separators remove metals before the peat is conveyed to the covered storage (capacity 12 000 m3). Two screw reclaimers under the storage area feed conveyors, which take the peat to two boiler silos (200 m3 each).

The overall plant efficiency for Edenderry will be 38.6 per cent. The effect of the plant on the nearby River Figile was a major concern during the planning stage and the decision was taken not to use that river for supplying the plant’s cooling towers. However, the river supplies the plant’s other water needs, through a 3 km long pipeline.

Bubbling bed boiler

The bubbling bed boiler is particularly suited to combusting the rain soaked peat that is unavoidable in the Irish climate. Even after a long rainy season the peat burns well and support fuel is not needed. Four fuel-oil burners are used for start up.

In furnace sizing, the key factors taken into account were the wide variation in moisture and heating value of the peat. In the design the peat was assumed to have the following characteristics (figures in parenthesis show range):

LHV (MJ/kg) 7.7 (5.5-11.2)

Moisture (%) 55 (40-65)

Density (kg/m3) 300 (150-400)

Ash (%) dry 5 (2-15)

Sulphur (%) dry 0.36 (0.2-0.5)

Nitrogen (%) dry 1.3 (0.5-1.7)

Chlorine (%) dry 0.05 (0.03-0.07)

Typical ash analysis (%) Ca 26.2, Silica 0.98, Mg 3.9, Fe 5.4, Al 1.42, S 2.7, P 0.63, Na 0.16, Mn 0.12

The boiler furnace cross section is dimensioned for high moisture peat. The active bed cross section can be adjusted for drier peat or lower loads. Primary or fluidising air is fed below the bed via water-cooled air beams. The secondary and tertiary air is fed form the front and rear walls using a semi-interlaced air pattern.

Design of the overfire air system for such a boiler is a very demanding job, requiring extensive use of computational fluid dynamics. Phoenics software was used to solve the governing equations of the heat transfer models.

The Edenderry BFB features a reheat superheater, which improves overall efficiency of the plant. The main process diagram is shown below.

HYBEXTM floor

In a bubbling fluidised bed the ash-forming material leaves the furnace as fly ash with the flue gas. However, coarser impurities drop to the bottom of the bed and dense particles over 2 mm in size do not fluidise, so they must be removed from the furnace. To increase the cross section area for removal of the coarse material Kvaerner has introduced what it calls the HYBEX floor. In this floor the fluidising air is introduced via water-cooled air beams, leaving over 30 per cent of the total area open for coarse material removal. Edenderry is the largest HYBEX floor installed to date.

In the Edenderry case the coarse material is collected via hoppers to two water-cooled screws. After the screws a drag chain conveyor takes the material to a light railway car for disposal. The automation system runs a sequence to remove the unfluidised material with a preset program to clean the whole area.

The boiler cross section has to be large enough to maintain bed temperature at the highest peat moisture level and loads. This means the bed is oversized when the fuel is dry and loads are low.

To overcome the problem Kvaerner has added a new feature to the HYBEX floor: a controllable bed area. This is accomplished by closing the air supply to the individual air beams. The bed above the closed beams slumps down and the amount of gas for fluidising the active bed area is decreased. With high loads the HYBEX control feature can be used to minimise flue gas recirculation. With low loads it can be used to achieve a wider turn-down ratio and to maintain air staging and thus low NOx emissions.

Emissions control

Air staging is the principal means of NOx control. Sulphur removal is accomplished by limestone injection into the furnace. Dust is removed by a one-chamber, three-field electrostatic precipitator. Dense phase pneumatic senders convey ash from the ESP to the ash silo. The ash is fed via a moisturising screw from the silo to light railway cars and disposed of by returning it to the peat production areas for reclamation.

Operating experience

The boiler has been in commercial operation since the beginning of 2001 and has achieved full capacity easily. The fuel characteristics have been generally in line with expectations.

It has however contained large amounts of stones, which have necessitated increased coarse material removal rate. Metals have also entered the boiler in spite of the magnetic separators. Nevertheless the HYBEX floor has proved fully capable of handling the impurities.