Engineering news
Work on the final stage of a 400MW biomass co-firing project at the Drax Power station in North Yorkshire is nearing completion. But a dispute over the level of government support for biomass means there is still uncertainty as to when the new facility will become operational.
The construction of the £18 million biomass rail unloading system, developed for Drax by engineering firm C Spencer, will be finished on-time on 1 June. The subterranean facility will allow delivery trains to pass through without stopping, depositing biomass in the form of pellets, cakes or briquettes along the way.
Each freight train will carry 1,100 tonnes of cargo, spread across 21 wagons, which will be unloaded in 30 minutes. From this point, the biomass will be dispatched via a powerful mechanical handling system to two enormous 12,000m3 storage silos, where it will be housed before eventually being fed into the boilers.
The silos were constructed using a process known as slipforming, whereby a circular mould was raised slowly by jacks, with the concrete setting beneath it. The continuous casting process meant that the two silos, which are 15-metre tall, 30-metre diameter and have a wall thickness of 500mm, were each constructed in just three days.
Andrew Brade, the engineer who has managed the biomass unloading and storage project, said: “It’s been a challenging project that has gone remarkably well. By far the biggest achievement has been that the construction work has been undertaken with practically no disruption to our coal deliveries, particularly when you consider that it has entailed a ten metre deep excavation beneath the existing railway line.
“Throughout the project, we have worked closely with Spencer, three train operating companies and Network Rail to ensure that trains could continue to run safely.”
However, despite the success of the £80 million construction project, Drax is yet to confirm when it will start burning biomass at the plant. It remains unhappy that the government has in its view failed to encourage the use of biomass through its Renewables Obligation strategy.
Drax says that a cap imposed on the number of Renewables Obligations Certificates from co-firing means that biomass generators receive just one quarter of the support given to offshore wind. As a result, plans to use biomass co-firing as a means of helping the coal-fired plant to cut CO2 emissions by 17.5% by 2011 have been put on hold.
Drax said: “We will have the co-firing capability ready by June but government policy is standing in the way of us using it at full-load. Co-firing using some materials such as wood pellets just isn’t cost-effective when compared to burning coal.
“We are calling on government to re-think its treatment of biomass under the Renewables Obligation and have had assurances that matters are being reconsidered.”
Slipforming explained
The slipforming process used to construct the massive silos comprised a circular shutter with working platforms on either side. The platform and shutter were suspended from a ring of support tubes. At each suspension point a hydraulic jack was fitted, which allowed the shutter to be jacked up by about 100mm for each stroke of the jack. The whole shutter ascended at a slow rate, with rebar fixing and concrete pouring happening continuously, 24 hours a day. Each silo was constructed in three days using this technique.