Burning ambition: the plan to burn biomass in a big way at Drax power station demands a substantial investment in plant.
That moment will mark the full commissioning of a 400MW biomass co-firing project at Drax, which is expected to deliver savings of more than 2.5 million tonnes of C[O.sub.2] a year. But to get there requires the completion of some significant infrastructure, namely an 18 million [pounds sterling] rail unloading and bulk storage system. This construction project, which is being undertaken by the engineering contractor C Spencer while Drax remains fully operational, represents a complex engineering challenge.
"We are building a subterranean unloading facility next to the railway that brings coal into a power station that generates 7% of national demand," says Andrew Brade, the engineer who is managing the biomass unloading and storage project. "There are other construction activities taking place, and there are people who need operational access all the time. So there are an awful lot of dimensions that might not normally be experienced."
The biomass, in the form of pellets, cakes or briquettes, will come into Drax in fully enclosed freight wagons to ensure that there is no water ingress. Each train will be carrying 1,100 tonnes of cargo, spread across 21 wagons, which will need to be unloaded in 30 minutes. From there, the biomass will be despatched via a mechanical handling system to two enormous 12,000[m.sup.3] storage silos.
Originally, engineers at Drax looked at modifying the existing coal unloading facility to accept biomass, but that was deemed impractical for a host of reasons. A decision was taken to construct an underground unloading plant, excavated to a depth of 9m with a sheet piled wall. As there wasn't room to build a dedicated railway line into the new facility, the unloading plant had to be built adjacent to the existing freight line which brings in the coal deliveries.
"We have had to do a lot of digging and a lot of mechanical plant installation," says Brade. "And as the new facility was so close to the coal delivery line, we have been surveying the track every day and recording its exact position to the nearest millimetre to make sure there is no movement."
The unloading system is a proprietary technology developed for Drax by C Spencer. While its exact mechanism remains protected, the system effectively discharges the biomass into a shallow hopper where it is fed onto a conveyor. The key to the technology is its ability to unload the hiomass and transport it away by conveyor at an extremely fast pace, eliminating the need for large, and costly, underground structures.
The biomass is carried upwards along a 50m conveyor before it is fed into a screening and sampling house. Here any large contaminants are filtered out, and samples of biomass are taken for analysis.
The biomass continues on its journey along a materials handling system which reaches a height of 40m, before it is fed into the top of the storage silos. Both 30m-diameter silos are fitted with huge screw reclaimer arms which travel in a circular motion, dragging the biomass into the centre of the silo before discharging it through a chute onto a conveyor that runs under the centre line. The conveyor then enters a separation facility where a magnetic system is used to remove any ferrous material, and an eddy current detector identifies any non-ferrous material for automatic removal. From then onwards, the biomass is delivered to a milling and preparation facility, before being blown into the boilers.
Brade says that the work on the rail unloading and storage system began three months ago and that it should be completed in nine months' time. "All the main mechanical plant such as the unloading devices, screw reclaimers, conveyor drives, screening/ sampling equipment and magnetic extraction has been contracted. The longest lead time was for the screw reclaimers. These are 15m-radius, cantilevered structures, so the bearings do not come off any shelf. They have had to be specially made," he says.
Brade says that, despite the complexity of the project, the installation work is going very well. "It's all on a scale that's never been done before with biomass" he says. "The unloading speed, the size of the silos, the reclaiming technology, the sampling--technically, it has all been very challenging."
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|Publication:||Professional Engineering Magazine|
|Date:||Jul 29, 2009|
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