Pellet power: a multi-purpose bioenergy plant
by K Tomic, Skellefteć Kraft, Sweden

A Swedish combined heat and power (CHP) plant, fuelled with unprocessed biomass residues, incorporates an integrated pellet-manufacturing process which produces around 30 tonnes/hour of biopellets for sale as fuel.

The CHP plant

Located at Hedensbyn in Skellefteć, this is the largest biomass-fuelled CHP facility operated by the Swedish company Skellefteć Kraft. Costing around SEK 310 million (where SEK is the Swedish krona) to build, at full power it produces 35.6 MWe and 62.9 MWth for the district heating system.

The circulating fluidised bed Pyroflow compact boiler is fuelled with unprocessed biomass residues, such as wood chips and bark. Steam at 140 bar and 540°C is fed to the VAX-type steam turbine which consists of two modules, one high-pressure and one low-pressure, with a two-stage district heating exhaust. By using two turbine modules, it is possible to run the turbine at different speeds (10,700 rpm and 6,950 rpm) to maximise efficiency. The modules drive a common generator in a single-string arrangement. When planning the CHP plant, the turbine was designed for steam extraction of up to 9 kg/second and 26 bar, enabling the process steam to be used for drying the biofuel used for pellet production.

The pellet plant

At a cost of SEK 216 million, the pellet-manufacturing plant comprises a biofuel handling system, steam dryer, steam recovery system, low-pressure turbine, cooling-water system, and pelletisation and ship-loading facilities with storage.

Sawdust is used for pellet production and this is provided at around 56 tonnes/hour with a moisture content of about 55%. The sawdust is sieved before it is fed into the steam dryer, which consists of two 1.6 m wide, 30 m high vertical condensers with 121 tubes, each 110 mm in diameter. The fuel is transported through the tubes by a large fan and stays in the drying zone for about 30 seconds to reduce the moisture content to 9%, before being fed to the pellet mills. The drying steam (270°C at 26 bar) from the CHP plant condenses outside the tubes and, therefore, does not mix with the steam which is produced from the wet fuel. In this way, the pure condensate from the dryer can be used again.

When planning the integrated CHP and pellet plant, emphasis was placed on using energy efficiently and increasing electricity production. For this reason, it was decided to install a process to recover steam from the dryer. However, the exhaust steam contains too many impurities to be used directly in a steam turbine. In addition, it cannot be used in the district heating system because it would decrease electrical production from the CHP plant. The solution was to feed the contaminated steam into a steam/steam heat-exchanger used to convert pure condensate from the dryer into saturated steam to drive a condensing steam turbine. Depending on the level of pellet production, this generates 1–5 MW more electricity.

Pelletisation

After the sawdust leaves the dryer, it is treated and then ground in a milling line to make a coarse powder with a maximum particle size of 3 mm. The powder is then converted into 8 mm pellets using four parallel machines.

Electricity consumption in the pellet plant is around 100 kWh/tonne of pellets produced. This means that, even at full production, the plant is more than self-sufficient in electricity because of the recovery process described above. Pellet production can reach around 30 tonnes/hour when the plant is operating at full capacity. The production level can be varied according to the demand for district heating, and consequently the supply of steam, as well as the demand for pellets. Annual production is expected to be around 130,000 tonnes. Table 1 (overleaf) compares the total output of the integrated facility with that of the CHP plant alone.

The market for biopellets

In recent years, the market for biofuels has been growing in the Nordic countries and it is anticipated that it will continue to do so. The bulk of the pellets produced by Skellefteć Kraft are sold to existing water-heating plants as a substitute for coal. A large proportion of the pellets are transported to customers by ship. In the nearby port of Skelleftehamn, a 45,000 m3 pellet store has been built and a 900 tonnes/hour ship-loading facility has been installed.

Skellefteć Kraft also builds small district heating systems with pellet-fuelled boilers and converts small oil-fired boilers to use pellets for heating houses and schools which are not connected to a district heating system. Although there is great potential for developing the demand for pellets in the domestic sector, it remains largely untapped. However, there are some pellet-burning units on the market for household heating, and pellet-fuelled stoves are also available. Compared with wood, pellets are much easier for the consumer to store and handle. When taxes and other charges are taken into consideration, pellets are cheaper than fossil fuels. They are also more economical to transport than unprocessed biofuel because of their higher energy content.

Alternative concepts

The integrated CHP and pellet plant was built in two stages and so certain possibilities for integration were not available to the designers.

If similar plant were to be built in a single stage, it would be possible to integrate the energy recovery function in different ways – two examples are given below.

The first would be to have the condensing turbine module driving the generator of the CHP turbine via its low-pressure module. This would save space and construction costs, and eliminate the need for a separate generator. This solution calls for the speeds of the condensing turbine and the low-pressure module of the CHP turbine to be equal, which is not the case in this project.

The other possibility, which Skellefteć Kraft is planning for a new CHP plant, is to tap steam from the dryer’s energy-recovery equipment into the CHP turbine. In order to be able to do this when the district heating requirement is low, surplus heat must be removed by installing a re-cooler. In this way, more steam could be passed through the turbine and more electricity produced. The investment in the turbine installation would be only marginally higher compared with a conventional CHP turbine. However, since the steam would then have a shorter expansion path in the CHP turbine than is normal in a condensing turbine, the electrical yield would be lower.

Another future possibility is to integrate the production of liquid biofuel with a CHP plant.

For more information contact Mats Johansson, KanEnergi Sweden AB, PO Box 41, S-532 21 Skara, Sweden. Tel: +46 511 347 873; Fax: +46 511 347 665; e-mail: mats.johansson@kanenergi.se

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.