Energy-from-waste technology first in UK
by the CADDET UK National Team

A new £35 million (where £ is the UK pound) energy-from-waste (EfW) plant in Scotland will process 120,000 tonnes/year of municipal and commercial waste plus small amounts of non-hazardous clinical waste and liquid wastes. This is the first EfW facility in the UK to use bubbling fluidised bed technology for waste treatment. Around 8.3 MW of electricity will be sold to the grid.

Baldovie energy-from-waste plant, Dundee  Photograph: Balfour Beatty Major Projects

Background

The City of Dundee has had an incinerator to handle municipal solid waste since 1979, but that plant at Baldovie (which did not include energy recovery apart from low-grade heat recovery for in-house use) closed at the end of 1996 when stricter EU emissions limits came into effect. Four years before the December 1996 deadline, the City Council had the choice of upgrading the old plant to meet the new standards, building a new facility, or revising its waste management strategy and increasing its reliance on landfill. It opted to construct a new EfW facility and entered into a joint venture arrangement with the private sector using the UKÕs Private Finance Initiative (PFI) mechanism for funding. Dundee Energy Recycling Ltd (DERL) Š a joint venture between Dundee City Council and three private sector partners Š are responsible for operating the plant which opened this month. The new 2 hectare site on the Baldovie Industrial Estate is adjacent to the site of the old incinerator. The old plant will be demolished or destined for a change of use once the new plant is completed.

The process

The new facility encompasses two separate stages: fuel processing and combustion.

Fuel processing

To produce the fuel, incoming waste will be tipped onto a flat floor in the reception area before being fed into one of the two hammer mills, where it will be shredded coarsely. Each of the hammer mills can handle 30 tonnes/hour of waste, which is almost double the plantÕs throughput (15.6 tonnes/hour) Š the over-capacity allows for unplanned downtime. Magnets will remove ferrous materials and the waste will then be fed by conveyor through the roof of the fuel storage building, a 20 m high, A-framed building capable of holding enough fuel for two days of operation or around 800 tonnes of refuse-derived fuel (RDF). From here the coarse RDF will be automatically fed by belt conveyor into the two combustors. Just before the fuel is delivered to the combustors, an eddy current separator will remove non-ferrous metals. An overfeed system will ensure that there is always enough fuel available to the boilers, and will return any balance to the store.

RDF combustion

The RDF will be burned in the bubbling fluidised bed boiler; a bed of hot sand which is 'fluidised' by the upward flow of pressurised air through it. Recirculated flue gases will blow the fuel across the bed. Non-combustible material is continuously removed from the bed, which will have dolomite added to it to reduce boiler tube fouling and to control emissions of sulphur dioxide. Heat released during combustion will be used to produce steam, which in turn will drive a turbine. The process will generate enough electricity for the plantÕs in-house needs of around 2.2 MW and an estimated 8.3 MW surplus for sale.

The stand-alone flue gas cleaning system will use cyclone pre-collectors, dry lime injection and fabric filters with activated carbon. The plantÕs chosen dry sorbent system has the advantage of increasing the potential energy output by around 0.5 MW. A new twin flue stack, 70 m high, has been constructed.

Around 23,000 tonnes/year of residues will be generated at the plant: 5,000 tonnes/year of filter ash; 9,000 tonnes/year of ash from the cyclone; and 9,000 tonnes/year of bed ash. Dundee City Council is to negotiate a separate contract for the management (disposal or recycling) of this ash.

The RDF store during construction  Photograph: Balfour Beatty Major Projects

The Baldovie plant's contribution to the region's ferrous and non-ferrous materials waste diversion

Total waste generated in region

640,000 tonnes/year

Total waste landfilled

535,000 tonnes/year (includes 23,000 tonnes/year ash residues from the new EfW plant)

Total materials diverted

120,000 tonnes/year incinerated, less 23,000 tonnes/year residues, so total diversion is 97,000 tonnes/year. 5,000 tonnes/year of ferrous metals and 450 tonnes/year of non-ferrous metals are recovered

Financial issues

Even though the scheme qualified for a contract under the Scottish Renewables Obligation (SRO) Š the Scottish equivalent of the Non-Fossil Fuel Obligation (NFFO) in England and Wales Š which allows market access for renewables-generated electricity, the amount of borrowing needed to secure the project was beyond the Council. The formation of the joint venture and the use of the PFI, which allows public sector bodies to purchase services from the private sector, made the scheme possible. DERL will charge Dundee City Council a £29/tonne disposal fee. DERLÕs revenues will come from three long-term waste disposal contracts, the SRO contract, short-term commercial waste disposal contracts, sales of metals, and revenues from the recovery of packaging waste.

Funding for the project has also come from a number of different sources. As one of the investors in the scheme, with a 40% shareholding in DERL, Dundee City Council will retain some of the risk but will also share in the profits based on the success of the project. The Council has the further benefit of a 20-year waste management contract at fixed costs, making long-term planning possible.

Future development

The revenues from the recovery of packaging waste will provide support for the expansion of materials recycling schemes, diverting more recyclable waste from the plant. Diversion of recyclable materials, particularly those which are not combustible, will help to improve the plantÕs operation. In the long term, Dundee City Council has the option to take over the plant from DERL once repayments are completed.

For more information contact the CADDET UK National Team in Oxfordshire.

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