Green electricity from waste wood
by K Kwant and W van Zanten, Novem, the Netherlands

A sawmilling company in Schijndel, the Netherlands, is producing electricity from the waste wood arising from its operation. A contract negotiated with a local utility allows the company to sell the electricity at a premium as 'green electricity'.

 

The power plant at Schijndel.

 

Delivery and mixing near the boiler.

Background
The Dutch government's Energy from Waste and Biomass Programme (EWAB) is to spend
NLG 8.5 million (where NLG is the Dutch guilder) to stimulate the use of biomass and waste as energy sources. The programme focuses on projects in the areas of fuel supply, technology development feasibility, demonstration and market introduction. Green electricity from wood in Schijndel is one project which has benefited from the programme.

Houtindustrie Schijndel BV specialises in drying, planing, sawing and varnishing all types of wood. A large quantity (about 15,600 tonnes) of clean waste wood arises from the drying, planing and sawing operations. The waste wood has an energy content of about 15 GJ/tonne, making it a valuable resource. The company uses the wood to fuel a steam boiler, to supply heat for the drying chambers, ovens and drying booths, and for space heating. However, the waste wood available is more than sufficient for the company's own heat requirements, so Houtindustrie Schijndel began to investigate the possibility of burning the waste wood to produce electricity. The local utility, PNEM, was willing to provide a contract to buy the electricity produced, which they will sell on to their customers as 'Green Energy'. The extra value 'green' electricity made the project viable.

The power plant
Clean wood residues are stored in four silos, from which two feed systems transport the wood to a dosage silo in the boiler house. If necessary, the two separate feed systems can be manipulated to achieve the correct mixture of fuel. A screw augur then feeds the fuel into the boiler.

A moving, water-cooled, graduated-grate boiler combusts the wood. The grate is divided into several zones so the combustion parameters in each zone can be regulated to achieve control over the combustion temperature. Another screw augur automatically removes ash from the bottom of the grate. Water is circulated through the entire grate to prolong its lifespan.

The combustion gases pass through a water tube screen in the boiler above the grate, to a superheater with horizontal fire tubes. The boiler generates steam at 28 bar and 420C. A five-stage vacuum condenser turbine expands the steam to 0.15 bar. The steam condenses and the condensate is returned to the boiler by a delivery pump. In a separate circuit, water from a cooling tower chills the condenser.

The installation runs primarily on pressure control. The boiler pressure controls the power of the turbine, which produces at maximum capacity. The generator supplies electricity directly to the mains. If a mains power failure occurs, the turbine automatically switches into a different mode of operation. The whole installation is controlled and operated by a PC.

 

Process flow at Houtindustrie Schijndel.

Results
Construction of the installation started in June 1996, and power generation began in April 1997. The plant's electrical output is 1.1 MW, of which 180 kW is used to supply the company's own electricity consumption. The remaining capacity supplies some 1,800 homes with 'green' electricity. Fuel consumption is about 1,400 kg/hour of clean wood residue.

The plant's emission limits are in line with Dutch regulations: carbon monoxide below 250 mg/Nm3; nitrogen oxides below 400 mg/Nm3; hydrocarbons below 50 mg/Nm3 and dust below 25 mg/Nm3. The designers planned the whole system to comply with Dutch steam regulations with a 24-hour unmanned operation.

Economics
The total investment was NLG 6.5 million, including buildings and silos, for an installation capable of operating for 7,000 hours/year. Thanks to the 'green energy' tariff and the use of recovered heat, it is possible to achieve payback in 7­10 years.

For more information on this project, contact the CADDET Dutch National Team in Sittard.
For information about the EWAB Programme, contact Kees Kwant, E-mail:
K.Kwant@novem.nl.

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.

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Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.