Chimney sweep — how traditional power stations reduce emissions
Generating power has always been a polluting process, but new European Directives are set to clean up the energy industry. Laura Buckley, Corrosion Engineer from The Engineering Centre of Scottish and Southern Energy, Perth, UK, describes how traditional stations have to adapt their resources.
In January 2008, the Large Combustion Plant Directive (LCPD) came into force across Europe. The Directive sets limits for the emission of sulphur dioxides, nitrogen oxides and dust from power stations, petroleum refineries and other industrial processes powered by gas, liquid or solid fuel. To comply with the targets, UK coal-fired power stations have either opted out of the LCPD, meaning they have only 20,000 hours operation remaining from January 2008, or are currently using, or installing, a flue gas desulphurisation (FGD) system.
There are two main types of FGD, seawater scrubbing and the limestone–gypsum process. Seawater scrubbing uses the natural alkalinity of the ocean to reduce the acidity of flue gas, particularly as a result of sulfur dioxide (SO2). Raw flue gas is circulated through the sea water, then neutralised, re-heated and released through the chimney. This requires close proximity to a compatible body of water and is commonly installed in marine areas.
In the limestone–gypsum process flue gas is passed through a limestone slurry (CaCO3) and undergoes a chemical reaction producing gypsum (CaSO4). The gypsum can then be processed to produce goods such as plasterboard for the construction industry.
Scottish and Southern Energy (SSE), Perth, owns and operates two two-gigawatt coal and biomass fired power stations which are being fitted with FGD using the limestone process. Operation is planned for summer 2008. The new plant is expected to extend the life of the stations, but it must work with the original 40-year-old plant. Therefore, together with FGD construction, upgrades and integration works are essential. These include installing tall chimneys that vent the by-products and form a historic part of the landscape. The stacks have been in continuous operation since the stations were commissioned in the late 1960s.
At Fiddlers Ferry (FF) Power Station, Widnes, UK, there are four flues within a concrete wind shield. Each flue is seven metres in diameter at the base and six metres at the top, and emits gas from an individual generating unit, three of which are being abated by FGD. The flues consist of a double skin with an outer shell of reinforced concrete and an inner shell constructed from an acid-resistant brick separated by an insulating layer.
There are two chimneys at Ferrybridge (FB) Power Station, Knottingley, UK, each designed to accommodate the flue gas from two generators. The chimneys are free standing and without an outer windshield and are 16m in diameter at the base and nine metres at the top. Like the FF flues, they are constructed of an outer reinforced concrete shell with a Nori brick lining, approximately 200m high. Two generating units are being fitted with FGD, so only one of the chimneys requires updating, although general maintenance will support the other chimney of the two units not opted in the LCPD.
The inner brick linings are not constructed of a single lift. There are up to 21 lifts in each flue supported at intervals of 8-10m on concrete corbels. The upper sections of the lift are under less compressive force than the lower sections and are susceptible to settlement and thermal movement, which leads to cracking and deformation. At FB the upper and lower lift of the brick lining had to be removed entirely, the latter a result of thermal shock and movement caused by the flue gas, and the former to permit access and remedial work to the sulphated concrete behind.
The Nori bricks had resisted hot acidic gas for 40 years with relatively little degradation. However, the mortar had deteriorated due to thermal movement and cleaning. As a result the lining has been enhanced to ensure the flues meet further service life. The lining has to accommodate treated and untreated flue gas, chimney operational temperatures ranging from 80-200ºC, with flow rates of approximately 800m3/s per individual flue at FF and 1,600m3/s for the chimney at FB. A borosilicate glass block lining system overlaying the existing brickwork has been fixed by an adhesive membrane. A Viton seal prevents gas egress from degrading the external concrete shell.
Overlaying an existing brick wall with a light block using a chemically cured adhesive appears simple at ground level, but all of the lining has been applied at a height of over 15m, within an enclosed work space where internal access was limited to small apertures.
Repairs and maintenance
For repairs and maintenance a platform was installed in each flue to travel up and down eight high strength steel cables suspended from a cathead on the top of the chimney. The platform edge extends and retracts to accommodate the change in flue diameter. A gondola and bucket can then be winched through apertures cut in the false floor, allowing people and materials access to and from the platform.
The platform is load tested to three tonnes, but the normal working load is much less. Together with the size of the structure, this restricts the amount of workers and materials allowed on. Consider the surface preparation, crucial in the application of coatings. Grit blasting at ground level is a simple operation comprising an air hose and feed pot (containing blast material) attached to a compressor. When working 200m in the air on a small, suspended platform, it is not possible to take the compressor. Therefore, it has to be set on the ground and capable of delivering air via up to 200m of hosing. Similarly, large quantities of blast material cannot be left on the platform, nor can the used blast material be allowed to fall to the base of the chimney. It is all collected and transferred by the materials winch.
This limitation applies to any of the functions necessary to install the lining – working at height with a load limited support structure and a tight deadline requires careful logistical planning to ensure all materials, particularly the membrane which has a 30 minute pot life, are delivered and installed on time.
Following the grit blast, the internal face needs washing and any undulations and areas of cracking rendered to level the surface. At this time the upper, deformed courses of brickwork of each lift are replaced.
Because of access issues and requirements for the chimney to be in service, where possible the ‘fix and forget’ mentality has been employed. In reality this takes the form of components traditionally formed from steel such as the capping, stop bars and pins, being made from titanium at FF. Flue lining was completed at the end of summer 2007.
To view detailed images, graphs and diagrams referred to in this feature, please download the PDF of the original article using the link below.