Finnish-based Wärtsilä Corporation established a biopower business unit and acquired the leading small-scale biomass technology, the BioGrate combustion system, almost two years ago.

The company’s biopower operation now concentrates on serial production of wood-based biomass-fuelled power plants in the range of 1-10MW electrical output as well as 3-20MW thermal energy plants. It is market leader in Scandinavia and has more than 70 power and thermal plants in seven countries.

In May Wärtsilä signed an agreement to supply two combined heat and power BioPower plants to Finnish timber products giant Finnforest Corporation for its Vilppula and Renko sawmills. These are the largest business units in Finnforest’s solid wood division and Vilppula is the largest sawmill in Europe. Their power plants will start up this year.

Co-operation between Wärtsilä and Finnforest goes back to 1994, when the first plant was delivered to Soinlahti. This was also the first bioenergy plant delivered with BioGrate technology. Today all Wärtsilä bioenergy plants use this patented combustion technology, which is particularly suitable for burning extremely wet wood residues, wood chips, bark and sawdust.

Kilning heat

Since Soinlahti, Wärtsilä has delivered five more bioenergy plants to Finnforest’s sawmills to supply heat for kilns.

At Vilppula, Wärtsilä is building a BioPower 5 plant producing electricity and heat, and a BioEnergy plant producing heat only. Together the plants will generate over 70% of the sawmill’s electricity and 100% of the heat needed for timber drying. In addition, they will supply all the heat needed in the town of Vilppula. The electrical output of the plants is 2.9MW and the total heat output is 22.5MW.

The facility at Renko, a BioPower 2, will be smaller but produce almost all the heat needed in drying processes and about 50% of the mill’s electricity. The electrical output will be 1.3MW and heat 8MW.

Both Vilppula and Renko power plants will be fuelled mainly with bark and sawdust. This will give best value for these co-products, lower energy bills, and allow accurate long-term energy cost predictions regardless of oil price fluctuations.

Stable combustion

The BioGrate solution is based on a rotating conical grate. In the primary combustion chamber, fuel is fed upwards on to the rotating grate bars through a channel in the centre of the chamber. Combustion is stable because fresh fuel does not disturb the surrounding burning fuel bed and the rotating grate bars control the thickness of the fuel layer.

As fresh fuel flows in a controlled way down to the combustion grates it is dried by heat radiation and the refractory surfaces lining the primary combustion chamber. The further the fuel rolls from the centre, the more combustion takes place; when it reaches the outer edge of the grate it has been completely combusted leaving only ash. This method – which requires no pre-drying – provides a very stable constant combustion process for fuels with moisture content of up to 65%. Secondary combustion time and temperature can also be very accurately adjusted and the technology can handle a variety of bio fuels.

The gases from the primary combustion chamber flow into the secondary chamber walled by a water tube boiler where gas and particles burn out completely. The combustion air inlet is three-phased to optimise combustion and minimise exhaust gas emissions, while an electrostatic precipitator is used for particle removal.

All main components in Wärtsilä BioPower plants are either manufactured by the company or subcontracted under strict quality controls. The result is operational reliability, high thermal efficiency, fuel flexibility and long equipment lifetime.

The biggest challenge in a small-scale biopower project is the relatively high investment cost. Wärtsilä plans to tackle this through serial production and modularisation of plants as we have done with traditional engine-driven power systems.