Power trip
(continued)
The Drax power station in Yorkshire is one of the single most important elements in the whole of the UK’s electricity generation infrastructure. Its six generators have a maximum combined output of nearly 4,000MW, which means that by itself it supplies around seven per cent of the country’s electricity requirements. But the station is also coal-fired - it currently consumes around ten million tonnes of coal per year - something that inevitably puts a question mark over its future as governments worldwide commit themselves to the establishment of ‘low-carbon’ economies.
That last factor, though, is set to change. Right now a major project that will enable the plant to initiate the ‘co-firing’ of coal and bio-mass materials is at an advanced stage. The station is being retrofitted with a complete system due to go on-line in June 2010 that will facilitate the on-site reception, storage, transport and milling of 1.5 million tonnes of biomass materials every year, which will then be mixed with pulverised coal before being burned in one of the station’s generators.
Efficiency
The project cost is £50 million, though a further £35 million is being invested in associated off-site facilities - specifically a pelletising facility for biomass materials at Goole. But in addition another £100 million is going into a parallel turbine upgrade programme intended to increase the station’s thermal efficiency from 38 to 40 per cent.
According to Peter Emery, production director at Drax, the two projects together will deliver annual savings in CO2 emissions from the station of 3.5 million tonnes per year. This will represent a 17.5 per cent reduction in emission levels compared with those for 2006. It will also further reduce the site’s present emission level of 809kgCO2/MWh, which is already well below the UK average for the coal-fired sector of 950kgCO2/MWh.
The biomass project at Drax, though, will be the most high profile part of this whole campaign of CO2 reduction. Quite literally, since it involves the construction of a network of silos and conveyors with associated quality control systems that will be fed by regular deliveries of from four to six truckloads of biomass material every hour. The project is being carried out by Alstom Power under a deal signed in May last year. But, in fact, as Emery explains, its roots go back to 2004, when Drax first started experiments with the use of biomass materials as a fuel supplement.
At that point Drax commissioned a pilot project involving the addition of biomass materials to the coal used in just one of its generators. Significantly, though, that project involved a ‘through the mill’ approach in which biomass materials were mixed with coal before the pulverisation process. But the technique was found to be inefficient, not least because the existing equipment for pulverising coal could not simultaneously reduce the biomass to a satisfactory powdered form.
Approach
That first foray into the use of biomass materials was therefore quickly followed by a second the following year. This time, though, the approach adopted involved the optimal milling of the biomass materials in a discrete operation quite separate from the coal pulverisation process. Only then were the two materials mixed in an air stream for ‘direct injection’ into the generators and subsequent combustion.
This is the approach that forms the basis of the system now being installed at the plant. The facility at Goole, though, is far from being peripheral to the whole enterprise. It will reduce raw biomass materials including straw, forestry residue and specialised ‘energy crops’ to highly compact pellets. Importantly the pressures involved in the pelletising process, which can be up to 15bar, will reduce their moisture content and hence increase their calorific value.
Once delivered to Drax the biomass materials embark on a journey that starts with them being tipped into a reception silo from where a conveyor system raises them around two hundred feet into the air. They then fall under gravity though a system of quality control stations which remove both ferrous and non-ferrous contaminants and also allow for the removal of samples for off-line analysis. Further transport systems lead them finally to a series of silos, each with a capacity of 3,000m3, plus smaller back-up silos from where the materials can be fed into the combustion process.
The significance of the installation lies not so much in its construction or the technologies it utilises - they are entirely conventional - but in the extent to which it confirms the importance the mainstream power generation industry now attaches to low carbon processes. Indeed Peter Emery describes the adoption of low carbon processes as “critical” for the industry’s future.
Security
In fact the real issue that needs to be addressed in order to increase the use of carbon neutral materials is nothing to do with technologies or logistics, but instead is that of security of supply. That much is admitted, for example, by Stephen Burgin, president of Alstom UK. What has previously held many utility companies back from implementing schemes like that at Drax, he observes, is simply the fear “that they might not be able to get the fuel.”
In the case of Drax the issue has been addressed in two ways. The first is the versatility of the system itself, which can handle as many as 50 different biomass material types. The second is that some hard bargaining has taken place to ensure that the power station can be sure of its sources, though details of supply deals whether agreed or still in discussion remain confidential. Nevertheless what is officially billed as the largest CO2 saving initiative currently underway in the UK is currently on-budget and on-schedule. The expectation at Drax is that it will be equally efficient in practice.
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