Boiler system for dry and moist biofuel
by J Yrjölä, Satakunta Polytechnic, Finland

A new boiler system for biofuels has been developed in Finland as a heat source suitable for large buildings, built-up areas or industrial plants. The technology was developed by Satakunta Polytechnic and Nakkilan Konepaja. It uses fuels such as wood chips and forestry residues including sawdust and crushed bark. Preliminary tests at a small-scale experimental plant were successful and a full-scale demonstration plant is now operating at Pori Institute of Forestry, where the boiler system produces heat for an experimental sawmill drying plant and other buildings.

The boiler system
Biofuels must be dried before they can be burned. The new boiler system dries the moist fuel using the thermal energy in its combustion gases. Outdoor air is heated in a heat exchanger and led to an evaporation silo where it is blown through the layer of moist biofuel, absorbing the moisture. If the fuel is already dry on arrival at the plant, the thermal energy in the combustion gases is used to pre-heat the combustion air instead. This system has the following advantages over a traditional system:

  • it can burn both wet and dry fuels;
  • combustion control is easier as the moisture content of the fuel fed to the boiler is more homogeneous;
  • the boiler is smaller, since the average temperatures in the combustion chamber and the convection part are higher than in traditional boilers.

Lab testing
The optimum dimensions for the drying silo were determined by laboratory testing at Satakunta Polytechnic. The drying curve and pressure drop of air blowing through the layer of chips or crushed bark were measured. The parameters were: the thickness of the layer (0.3­0.6 m), the size of particle (11­30 mm), the air velocity (0.3­0.6 m/s) and the air temperature (50­85°C). The goal for moisture content was 30%.

During the study, it was found that the most important issue in drying biofuel was the capacity of the air to hold water; the exhaust air was almost saturated at every value of each parameter. If the air velocity was increased from 0.3 m/s to 0.6 m/s, or drying temperature increased from 50°C to 80°C, the additional drying capacity was almost identical. The drying capacity varied between 14 and 43 kg/h/m2.

The pressure drop of air blown through the layer was also measured. No significant difference was found in the pressure drop for moist and dry chips and forestry residues. The pressure drop for moist crushed barks was 10­40% greater than that for dry crushed bark. The pressure drop increases as the particle size decreases.


Sketch of the full-scale plant at the Pori Institute of Forestry.

Small-scale test plant
A small test plant with a heat output of 40 kW was built at Nakkilan Konepaja Ltd. The boiler, stoker burner, air heater (the heat load of the system) and the stoker construction of the drying silo were all standard products. The heat exchanger, drying silo and all pipe, duct and electrical installations were constructed by Nakkilan Konepaja Ltd. Early in the testing, some changes to the drying silo were made to enable it to function properly. The drying capacity was sufficient; the initial moisture content was about 55% and the final content was below 30%. The temperature of the drying air was typically 70°C and the exhaust air was 20­30°C. The exhaust air was almost saturated.

Part ofthe wall of the combustion chamber was insulated to achieve a higher combustion temperature. The under pressure in the combustion chamber was 40­45 Pa. The carbon dioxide content of the combustion gases was 13­16% and the heating efficiency was above 80% (more than 80% of the energy of 60% moist biofuel was transferred into the heating system). The power consumption of the electrical devices was 1.5 kW when the heat output of the boiler was 42 kW.

Full-scale demonstration
A full-scale system with a total heat output of 1,000 kW has been functioning since January 1998 at the Pori Institute of Forestry. The boiler system produces heat for the experimental drying chamber of the sawmill and other buildings of the Institute. The heating capacity of the new boiler system is 500 kW and there is also a 500 kW boiler which uses light oil as fuel in the same system. This new system replaces the old light oil heating system.

The boiler is a standard steady-grate, warm water type. The only speciality is a control damper in the bypassing part of the convection area. The heat exchanger is an ordinary tube bundle type made by Nakkilan Konepaja Ltd. All fans have variable air volume controlled via an inverter.

Schematic of the new boiler system.

The test period will continue until spring 1999. The measuring programme includes:

  • the technical capacity of the main components;
  • the emissions in the combustion gases and the exhaust air;
  • the reliability of the system (a long-term manual follow-up);
  • the thermal efficiency of the system according to DIN1942.

Final reporting will take place in summer 1999. The aim of the demonstration project is to provide useful information to develop the system further.

For more information contact the author:
Mr Jukka Yrjölä
Satakunta Polytechnic, Technology Pori
Tekniikantie 2, FIN-28600 Pori
Tel: +358 2 620 3214
Fax: +358 2 620 3300

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.