Coolkeeragh ESB: Combined Cycle Gas Turbine Electricity Generation

The plant at Coolkeeragh has switched from the outdated oil-burning technology of the old power plant to Combined Cycle Gas Turbine (CCGT) power generation of the new generating plant.

CCGT is considered to be the most efficient and environmentally friendly method of generating electricity, as it re-harnesses energy, steam and heat produced in the first stage generation process and uses this to generate electricity. As a result, for the same input of energy, CCGT generates over a third more electricity than a traditional thermal power plant.

How it Works

The new CCGT plant at Coolkeeragh operates using natural gas, which is delivered to the power plant via the newly constructed northwest gas pipeline. Connection within the Coolkeeragh power station is made via the natural gas compound on-site, consisting of a pressure regulating station, metering, and gas filters/separators to remove impurities. To ensure continuity of power generation in all circumstances, the station also operates on liquid fuel (low sulphur distillate) should the supply of gas be interrupted for any reason. Liquid fuel is delivered to the site in sea-going tankers via the existing jetty facilities.

The Main Process

Electricity is generated by CCGT in two stages.

The first stage of the process utilises a gas turbine which can be fuelled using natural gas or liquid fuel. The gas turbine is coupled to a generator which generates electricity as it is rotated by the turbine. After the fuel is burnt in the gas turbine, the hot gases enter a heat recovery steam generator which produces steam to power a steam turbine (the second stage). The rotational energy of the steam turbine is also used to generate electricity. (The two stages of power generation give rise to the term "combined cycle")

Steam, as it exhausts from the steam turbine, is condensed in a condenser using cooling water. The condensed steam is returned to the steam generator by pumps to complete a continuous process. It is replenished as required from a make up system to allow for system losses.

The water for cooling is provided at Coolkeeragh by a once-through system using water extracted from and returned to the Foyle Estuary adjacent to the plant site. The water is returned to the Estuary at a quality level and temperature limit that meet environmental legislation requirements.

The Back-Up Process

The gas turbine burns natural gas from the new northwest pipeline supply as the primary fuel in normal operation. In fact, the northwest pipeline was only made viable by the demand of Coolkeeragh as anchor customer for the supply.

It will also be possible to operate using liquid fuel (gas oil or distillate) if for any reason the gas supply is interrupted. This is a requirement of the electricity regulatory authorities. The required standby fuel is stored in the refurbished oil storage tanks.

Change over to liquid fuel is automatic when operating on natural gas, without the need to shut down the plant. If the gas supply is interrupted, the gas turbine fuel control system will sense falling gas pressure and immediately switch supply from gas to liquid fuel without a break in plant operation.

Change over back from liquid to gas is performed manually once gas supply has been restored.

Workings of a power plant.

 

Coolkeeragh ESB: The North-West Natural Gas Pipeline

Central to the construction of the CCGT power plant at Coolkeeragh is the availability of natural gas for the power plant, which was made possible by the North-West pipeline, connecting Coolkeeragh to the Scottish Northern Ireland Pipeline System (SNIPS).

Construction of the North-West pipeline between Belfast and Coolkeeragh began in March 2004 and was completed in October 2004 by Bord Gais Eireann.

The development of the North-West pipeline was itself only made possible by the construction of Coolkeeragh ESB as a CCGT power plant, and whose role as 'anchor' customer thereby made the rest of the pipeline project economically viable. As a result, the North-West pipeline will facilitate the extension of natural gas to many towns throughout Northern Ireland contributing further to an all-Ireland energy market.

Construction activities were carried out using the latest and least intrusive engineering methods and the pipelines are built, operated and maintained to the highest national and international safety standards.

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