Type of CHP system: BFB boiler

Plant owner: Kokkolan Voima Oy

Commissioned: 2001

Status of plant: Commercial

Contractor details:

Bubbling fluidised bed boiler was delivered by Kvaerner Power Oy and the turbine unit by Siemens AG. The fuel handling system was supplied by Vesme Systems Oy.

Power generation: Steam turbine

The boiler is a conventional bubbling fluidised bed boiler with a maximum capacity of 70 MWth. Bled steam from two steps is used for heating of district heating water.  (See figure)

Main fuel: Multifuel (more biofuels and/or peat)

Kokkolan Voima power plant was designed for wood and peat fuels. The main fuels are by-products from sawmills and peat. In 2002 the total use of solid fuels was about 305 GWh from which about 66 % was milled peat and 34 % wood fuels.

Presently the share of wood fuels is about a half, but technically their use can be increased up to 80%. The wood raw material is collected from neighbouring areas, and this employs 30-40 workers in wood supply and transports.* During the autumn of 2003 a new “drying on pile” system for wood fuel was commissioned at the plant.

Nominal capacities & efficiencies:

Thermal input, biomass & waste: 80 MW

Total thermal input, incl. fossil fuels: 80 MW

Gross electrical output: 22 MWe

Net electrical output: 20 MWe

Heat output: 50 MJ/s

Electrical efficiency, gross: 27,5 %

Electrical efficiency, net: 25 %

Overall efficiency, gross: 87 %

Power to heat ratio: 0,4

Maximum heat output of the plant is 50 MJ/s. In addition 15 MJ/s is recovered from waste heat by heat exchangers located at Kemira Chemicals factory.

How is plant controlled: Plant is controlled by the heat demand.  The plant is provided with heat accumulator that balances short-term variations in heat production demand.

Heat backup:

Kokkolan Voima owns one heavy fuel oil fired back-up boiler of 12 MW which is also used for upgrading the waste heat for district heating.

There is a 3200 m3 heat storage at the power plant directly connected to the district heating network. City of Kokkola owns several heavy fuel oil fired district heating plants. These are only used in case of peak load or malfunction in the power plant.

Heat usage: Heating only

Energy usage:

Heat is used for district heating in the city of Kokkola. In 2002 district heat production  was 180 GWh and net electricity generation was about 65 GWh in this power plant.

A speciality of the plant is a 45 m high heat accumulator of 3 200 m3, pressurised by the steam bed. The accumulator balances short-term variations in heat demand. The nominal discharge capacity of the district heat accumulator is 50 MW. It is directly connected to the district heating system without any heat exchanger. The accumulator has proved to be an excellent system. Even in case of plant shut-down, district heat can be delivered without disturbances, within the limits of the capacity of the accumulator.

At daytime, when the price of electric power is high, the heat accumulator makes it possible to run the plant at full load, i.e., the back-pressure plant reaches the same efficiency as a condensed power plant. At night, heat is generated for the heat accumulator. This offers an optimum cost-effectiveness.

District heat coverage (% of total demand in DH network): 76 %

DH inlet temperature: 85 – 115 °C

DH return temperature: 45-55°C

Heat storage size: 3200 m3 m3

Hours of full load that the heat storage can meet: Theoretical maximum storage capacity is 150 MWh. The nominal discharge capacity of accumulator is 50 MW.

Fuel storage:

The biofuels are transported by trailer lorries to the plant. Most of the wood fuel is stored for drying on the outdoor fields and picked up with a bucket loader when the need arises. The unloading area of the receiving station is roofed and it is open on one wall only to prevent peat dust from spreading to the environment. Basically all peat is unloaded directly into the storage pockets of the receiving station. Almost all wood fuel is unloaded in the receiving station, from where it is transferred with a belt conveyor to drying piles. Receiving station is an aboveground building with four separate reception pockets. The useful volume of the storeroom is 4×200 m3. The fuel is unloaded immediately after weighing.

There are two big right circular boiler bins of 2×150 m3. Fuel feed is controlled in such way that the boiler load regulates the rotation speed of the discharge screws in the bins.

Waste water:

Cooling water is not released to river and lake system. Waste heat from process equipment is exploited for preheating the combustion air, for example, and the excess heat is released to air through air heat exchangers. Any waste water from processes is led to the city´s sewage system.

Ash discharge: Bottom ash is used for earth filling and the fly ash for landscaping closed landfills.

Plant description:

Kokkolan Voima Oy is a daughter company of Pohjolan Voima Oy (PVO). The new power plant fits well in with the PVO’s Bioenergy Programme, according to which PVO will increase the electric power generated from bioenergy by 409 MW and invest in total € 450 for this purpose. The wood energy consumption of PVO is estimated to increase to more than 6 TWh by the year 2005. The implementation of the power plant is based on the operation model of PVO, according to which PVO constructs power production capacity, and the shareowners, Kokkolan Energia in this case, obtain energy in proportion to their share series.

The power plant is located next to the sulphuric acid plant of Kemira Chemicals. Kokkolan voima has installed some heat exchangers at Kemira’s factory. This makes is possible to use the heat of hot sulphuric acid. The waste heat can be upgraded for DH by using the heat system “in series” or “in parallel”. With outside temperatures of 5 to 10 degrees celsius, heating system is connected “in parallel”, when temperature is lower, the systems runs “in series”. This makes it possible to exploit the major part of waste heat and to maximize the power production.

Sources: PVO and www.tekes.fi/opet/

Fuel feeding system:

Four different fuels can be fed from the pockets onto the scraper conveyor which conveys the fuel to screening. After separating metal with a magnetic separator, the fuel is crushed. The different fuels are mixed on the conveyor, in sieving and crushing to a homogeneous mixture. The fuel is screened with a disc screen. The system is highly automated. This means that the fuel fed is controlled by automatic regulation of the discharge rate of receiving stations according to the fuel mixture chosen by the operator.

Boiler: Conventional BFB boiler by Kvaerner Power.

Turbine: Back pressure turbine. Siemens AG. The steam turbine is a one-stage reaction turbine with bleeders for feed water tank, low pressure preheater and two district heaters.

Steam data: 27 kg/s, 482°C, 80 bar(g)

Flue gas cleaning: The boiler is equipped with a one-box and three-field electric precipitator.

Staff: The number of O&M personnel is 9.

Electricity sale info: Electricity and heat is sold by Pohjolan Voima to Kokkolan Energia .

Total capital cost: € 25,200,000

Total costs including the changes made after completing the plant and the building lot is about € 26-27 million.

Data collected and quality assured by: VTT Processes.

Contact person: Kati Veijonen