Type of CHP system: CFB boiler

Plant owner: Etelä-Savon Energia Oy is owned by the City of Mikkeli.

Commissioned: 1990

Status of plant: Commercial

Contractor details:

The main boiler of Pursiala plant was commissioned by Ahlström Pyroflow (present Foster Wheeler Oy) in 1990. Raumaster Oy and BMH Wood technology delievered the fuel conveyor and bin.

Power generation: Steam turbine

The main boiler is a circulating fluidised bed boiler with a maximum thermal capacity of 84 MWth. The plant is provided with an auxiliary water cooler (45 MWth max) to prime the power production.

When the plant was designed and constructed, special attention was paid to environmental protection. The authorities also set fairly strict limits for emissions. However, these limits were clearly passed with correct equipment and operation techniques.

The emissions from the power plant are very low.

Particle and nitrogen emissions are only about a half of rating values and sulphur emission even lower.

Main fuel:

Multifuel (more biofuels and/or peat)

Pursiala power plant was designed for wood and peat fuels. Also coal can be used. The main fuels are peat and by-products from sawmillst. In 2002  the total use of solid fuels was about 560 GWh.

Previously, the main fuel of the power plant was peat, while in 2000 about 60% of fuels were wood-derived ones. The use of wood has exceeded that of peat since 1998. The annual consumption of forest chips is about 100 000 loose-m3, its share being about 10%. The Forest Office of the city of Mikkeli delivers about half of forest chips, and the other half is delivered by local forestry associations.

The production of forest chips from the forests of Mikkeli and from forests of companies and private owners employs on average 40 workers. Forest chips are delivered from a distance of about 40 km around the power plant.

Nominal capacities & efficiencies:

Thermal input, biomass & waste: 93 MW

Total thermal input, incl. fossil fuels: 93 MW

Gross electrical output: 28,7 MWe

Heat output: 54,4 MJ/s

Electrical efficiency, gross: 31 %

Overall efficiency, gross: 89 %

Power to heat ratio: 0,52

Maximum district heat capacity is 54.4 MW, maximum electric output of generator is 28.7 MW.

How is plant controlled: Plant is controlled by the electricity or heat demand.

Heat backup: Heat generation is secured by peak and reserve heat stations located at different sites in the town.

In Pursiala there are also two solid fuel fired boilers (47 MW). Total capacity of the boilers excluding the Pursiala CHP plant is almost 175 MW.

Heat usage: Heating only

Energy usage:

Heat is used for district heating in the city of Mikkeli. In 2002 district heat production  was 334 GWh. Net electricity generation was about 139 GWh.

The number of district heat customers was about 1400 and the heated building stock more than 7 million cubic metres.

Some steam (280°C, 80 bar, 2 kg/s) is supplied for wood thermal treatment plant.

District heat coverage (% of total demand in DH network): 77 %

DH inlet temperature: 85 – 115 °C

DH return temperature: 45-55°C

Fuel storage:

Fuels are transported by lorries to the power plant. Only the wood residues of a sawmill located close to the plant are conveyed through a pipeline to the power plant. In cold winter days, about 45 lorry loads a day are received. There are two receiving stations at the plant. The first one is used mainly for receiving peat and the second one for receiving wood fuels. The fuels are conveyed by flight conveyors from the receiving stations to the storage bins of both the heating station and the power plant. The fuel blend is prepared in the bins with the aid of conveyors.

Pursiala power plant has a storage bin of 1500 m3 for solid fuels.

Ash discharge: Ash is used for landscaping of the old land area used for taking of soil. In addition  7% of ash was directly utilised in 2002.

Plant description:

Pursiala Heating Power Plant generates power and district heat for the distribution area of Etelä-Savon Energia Oy. Practically 100% of fuels used for energy production are of indigenous origin. All solid fuels are domestic, and more than half of that are wood-derived. The fuels comprise wood residues of mechanical wood-processing industry (sawdust and bark), wood-derived source separated crushed dry waste, peat, forest chips and oil. Oil is used only as starting fuel and in cases of malfunction.

The boiler output of the power plant is 150 MW, and the plant has been constructed in three stages. First, the peat heating station with a 20 MW grate boiler and a 20 MW oil-fired boiler was built. The heating station was commissioned in 1981.

In 1984, a 27 MW fluidised-bed boiler and an automation system were constructed, and the flue gas clean-up system was modified. In 1990, a heating power plant was built and commissioned in Pursiala. The power plant is highly automated, and hence, only 30 employees are needed for operating it.

Fuel feeding system:

In the boiler room there are two boiler bins of 2×100 m3. Fuel feed is controlled in such way that the boiler load regulates the rotation speed of the discharge screws in the bins.

Boiler: CFB boiler by Foster Wheeler.

Turbine: Back pressure turbine.  ABB Turbinen Nurnberg GmbH (G40)

Steam data: 33 kg/s, 535°C, 114 bar

Flue gas cleaning:

The flue gases are cleaned with a three-field electric precipatator delivered by ABB Oy. The efficiency ot the electrostatic precipatator is more than 99.9% (of particles separated).

Staff:

The number of operation personnel is 21. Two of them work only in day shift, one fills in for others. There are always 4 persons working in morning and evening shifts and 3 persons in night shift. Maintenance personnel is as follows: machinery 1 + 2, automation 1+1, electricians 1+1, alltogether 7 persons in maintenance.

Electricity sale info: Electricity is sold through to the clients of Etelä-Savon Energia Oy.

Total capital cost: € 43,000,000

Data collected and quality assured by: VTT Processes.

Contact person: Kati Veijonen