Clayton Enhances Efficiency
at Medway Power Station
Mitsui Babcock Energy Services Limited
have announced the completion of a project which has increased
efficiency and flexibility at one of the UK’s most advanced combined
cycle power stations.
The improvements have been brought about at the Medway Power Station on the Isle of Grain in Kent by the installation of a new Steam Generator, which provides an auxiliary supply of superheated steam to the plant.
The 688 MW Power Station operates with two GE Frame 9FA gas turbines and one GE reheat, condensing steam turbine. The waste heat in the exhaust from both gas turbines is utilised to produce steam in heat recovery steam generators (HRSG’s) which is used to power the single steam turbine.
The driving force
behind the project was the commitment of Medway Power Limited to
‘flexible operations‘ which involves the plant being shut down and
re-started, as required, in response to electricity demand and prices. A
new Clayton steam generator was installed as part of a package of
measures to meet this commitment and it allows the station to be brought
on-line and achieve base load significantly sooner after a shutdown than
was previously possible. The time saving is achieved because the new
steam supply can be made available to seal the glands on the steam
turbine long before steam from the HRSG’s is available. This seal
permits the vital vacuum conditions to be established on the steam
condenser and reduces the time to synchronization of the gas and steam
turbines.
The new steam supply is provided by the Clayton Steam Generator, which is capable of producing 7700 kg/hr of superheated steam at a pressure of 17 barg. Norman Bell who is a Plant Leader at the Medway Power Station said “To ensure the success of this project we needed a boiler which could start up and quickly produce steam of high quality and at the right terminal conditions from cold, basically at the flick of a switch.We also investigated taking the traditional approach of using duty and standby wet back type boilers and keeping one of them warm all the time so that it would be ready for operation when we needed steam. However, because of the flexibility we required as well as the space & budget restraints combined with efficiency and emissions considerations, the Clayton Steam Generator proved to be ideal for our purpose”.
The high efficiency and quick start-up demanded by this application are inherent in the design of the Clayton Steam Generator which operates on the principle of forced circulation of water through a single tube. This water tube is of a specially designed coil construction and has been developed to maximise heat transfer and minimise energy loss.
Water entering the steam generator is directed to the topmost layer of coil from where it spirals downwards through each level to the lower part of the boiler. In this lower section, the tube forms a cylindrical shield around the combustion chamber. One of the main advantages of this design is that the steam generator contains only a small volume of water and is therefore safe and can be started up very quickly from a cold condition. Boiler feedwater is forced through the coil by Clayton variable speed, positive displacement pumps. As the water passes through the steam generator it picks up heat, and steam is produced at the outlet from where the flow is directed to a pressurised separator vessel. The required superheat is then obtained by feeding this dry steam back to the steam generator where it passes through an integral superheat coil before entering the main steam system leading to the sealing glands of the steam turbine.
The Clayton Steam Generator was fitted with a special low NOx gas burner which had a predicted NOx level of less than 60 mg/Nm3, however levels of below 25 mg/Nm3 are being achieved in practice.
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