Didcot A Coal Fired Power Station – and potential impact of large wind energy on maintenance costs of two shifting power stations

Note: Some notes have been added, via the comment field which originate from the Claverton emails, from authoritative persons, which inter alia indicate that the maintenance and operating costs for a plant such as this are slightly less than 0.4p/kWh.

Background and Location
 
Didcot A is was built as a coal fired steam power station over a ten year period with the last units coming on line in 1970.Didcot B , which is a gas fired CCGT was commissioned 14 years ago. Power stations are needed at this point in Britain to help supply the south of England (Oxford and Reading are nearby)
 
Didcot is a small town, south of Oxford about 120 km west of London. It is on the Great Western train route and is currently being supplied by coal trains from Bristol and elsewhere. The coal presently used is of the low sulphur type mainly from Russia. Apparently thought was given to building another coal plant at Didcot, but the railway line capacity is limited. Water is taken from the River Thames for cooling. 
 
Didcot A Station Characteristics
 
Didcot A consists of four 500 MW units. Inlet steam is at 568°C/165bar and the reheat is  568°C/39 bar. Condenser pressure is about  0.5 psi ( 0.035 bar/ 40°C). The station uses the usual hyperbolic cooling towers, using water drawn from the Thames. The water loss is 32000 gallons per day (145 cu m) and a similar amount of slightly warm water is returned to the Thames. There are four cooling towers on the plant itself  plus two others about half a mile away. I think that the designers hoped to build another two but permission was refused.
 
In contrast the cooling system used for the 1460 MW CCGT station at Didcot B are of the low level, forced draft type water tower in which the cooling water from the condensers is sprayed over slats. The cooled water is then returned back to the condensers in the power station.
 
Didcot A has on high pressure turbine, one reheat turbine  and three low pressure turbines. I think the HP turbine supplied about 27% of the electricity    
 
Station efficiency is 35% net, at base load, and this takes into account the ancillary power of about 100MW which is needed to run the coal mills and fans.
 
To cover black start conditions the station has four 27 MW Rolls Royce Avons. These can be at full  load in less than 5 min ( just over 2 minutes is possible). They can also be used for peak lopping. Fuel is kerosene.
 
A schematic picture showed that the burners fire from one side of the furnace  Start up is done with propane as a pilot flame, then fuel oil ,and final pulverised coal.
 
About 5% of biomass, as nuts or sawdust, can be incorporated into the coal. Oily nuts are  a problem since the oil sticks to the sides of the mills and increases the risk of fires.
 
The control room consists of four separate desks, each of which faces a large screen plus a set of PC type monitors, which are used to control each unit. There are three men on each desk, one of whom is the plant attendant. There are another two desks in the centre who are the engineering specialists and shift controller. Shift duration is 12 hours. 
 
Six men look after the coal heaps which contain 1.3 million  tonnes of coal  At a rate of 18000 tonnes of coal a day this gives a reserve of over 70 days. One of the main jobs is to compact the heaps to prevent ingress of air and degradation.
 
 
Two Shifting and Peak Load
 
At present, the station tends to be off line during the summer as the price of gas is so low. It will definitely be on in November 2010 when it will be used as a two shift unit. The station was given about 20000 hours  to run up to 2015 , but has so far only gone through about 7500 hours of this. Running tine is based on “chimney hours”, so if just one unit is running, the assumption is that the other three are also in operation. This obviously means that when Didcot has to run, the tendency is only to run if all four units can be put on line.  
 
For most of its life the station has been used for two shifting and sometimes for triple and quadruple shifting. In the early 2000s , Didcot A  had 720 starts in one year, and over 400 in another year. After an overnight shut down the station takes 3.5 to 4.0 hours to reach full load. From a cold start it is three days.
 
From a completely hot start, as might happen when triple or quadruple shifting, I gained the impression from a recent steam turbine book that Didcot A could be up to full load in about an hour…..I need to check on this.  
 
In terms of load following, the load change on each unit is 5MW/minute.The minimum reasonably stable load on each 500 MW unit is 265MW. 
 
Maintenance
 
A steam drum had had to be replaced on one unit after some years because of cracking
Superheaters have been replaced
Blade erosion was found early on but is now under control
Steam turbines are refurbished every four years  
 
 
   F Starr: Claverton

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