How can we increase the flexibility of coal to fit in with the intermittency of renewables?
Renewable energy sources provide varying, often unpredictable, amounts of electrical power, so coal-fired power plants are becoming increasingly important in balancing power grids. In this role, the coal-fired plant has to be highly flexible so it can ramp up quickly, for example to cover for when the wind drops.
Unfortunately, coal-fired power plants in the past were generally designed to provide a fairly constant output. Rapid changes in output or frequent shut-down and start up operations were avoided as far as possible, as they increased the stresses on a plant, shortened component life, increased costs and reduced efficiency.
However, developments by the suppliers have resulted in features that give the flexibility required with much less adverse impact on the plant.
Dr Colin Henderson of the IEA Clean Coal Centre has reviewed how recent coal-fired plants and future advanced coal-fuelled technologies can be designed to achieve this (Increasing the flexibility of coal-fired power plants, Colin Henderson, IEA CCC, Sept 2014). Dr Henderson points out that the importance of having flexible fossil-fired plants on power grids is not widely recognised by people at large: “Increasing the amount of renewable power plants makes electricity grids more unstable, threatening blackouts. Keeping coal-fired units with fast responses on the grid is an essential part of ensuring reliable electricity supplies.”
There are means for increasing flexibility in all the main plant equipment areas: boiler firing, boiler pressure parts, emissions control systems, turbine and water-steam systems, control systems, auxiliary plant.
The flexibility aspects of the turbine and water-steam systems of CFBC (circulating fluidised bed combustion) plants are essentially similar, although the different boiler system means that startâ€up times can be longer.
Existing IGCC (integrated gasification combined cycle) systems were not designed specifically for high flexibility, but it could be improved by providing facilities for storage of syngas or oxygen. Polygeneration (multiâ€product) IGCC systems would allow greater load range. If IGCC plants start to be ordered in large numbers, designing for flexibility will become more important and new approaches to achieving it may emerge.
Manufacturers and utilities are working to achieve efficiencies of about 50% (LHV basis) by using steam conditions of 700°C and above in advanced ultraâ€supercritical (Aâ€USC) plants. Providing the capability to operate flexibly could be limited by properties of the nickel alloys that will be used in such plants.
If necessary, it may be possible to design plants incorporating CO2 capture to have similar flexibility to that of their non-capture equivalents, but cost considerations would influence the extent of flexibility realised in practice.
The report shows that potential damage mechanisms from plant cycling are known, and the technical means exist for conventional combustionâ€based plants to achieve the flexibility required without unacceptable effects on equipment life, thermal efficiency or costs. The importance of having flexible fossil-fired plants on power grids cannot be overemphasised in ensuring the reliability of electricity supplies.
The report, Increasing the flexibility of coal-fired power plants, CCC/242 by Dr Colin Henderson (57 pp, September 2014) is available for download from the IEA Clean Coal Centre Bookshop http://bookshop.iea-coal.org.uk/site/uk/clean-coal-technology-research-reports?LanguageId=0
Residents of member countries and employees of sponsoring organisations can download the report at no charge after a one-off registration.
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