Radley

The PFA is slurried into a 70% water and 30% PFA mixture and pumped along a pipeline. This system has to handle 3600 tonnes of PFA daily in a 12 hour period. The dispo'"’ area comprises four separate lakes which settle out the PFA from the slurry mixture. The surplus water from the lakes is returned to the River Thames under strictly-controlled conditions.

Figure 2.86 shows the general disposal arrangements and lake filling sequence.

22 Auxiliary boilers

The auxiliary boiler plant is required to provide auxiliary steam for the main generating plant and auxiliaries. Typical services include:

•Heating for the de-aerator on start-up.

•For atomising the fuel oil burners.

•For fuel oil pumping and heating services.

•Hot water and space heating services for station buildings, including the administration and welfare buildings.

For a 21)00 MW fossil-fired station the plant would typically comprise five identical oil-fired boilers sup­ plying superheated steam at 17.5 bar and 228°C: Normally four boilers are required for peak demand with one on standby. Typically the boilers would consist of the multi-tubular, wet back, three pass type, having twin furnaces each fitted with a single rotary cup burner (see Fig 2.87). The burners tire fitted with a voluvalve system which controls the volume of fuel oil passed either to the rotary cup atomiser or recirculated

Gas generation and storage

back to the pump suction. Air is supplied to the burner from a forced draught fan mounted adjacent to the burner. Some of this air is supplied to a primary fan mounted on the burner shaft. A fuel/air ratio motor controls the quantity of air and fuel required for combustion. This motor is operated by electrical signals from a position controller, which receives a desired value from the steam range pressure controller or from manually operated pushbuttons. Initial ignition of the fuel oil is from a flame of propane gas ignited by an electric spark. Each boiler is fitted with shell and superheater safety valves which discharge through silencers to atmosphere.

The boilers are normally located in a separate auxiliary boiler house as shown in Fig 2.88 and are arranged to each supply steam into a ring main distri­ bution system; condensate is returned via a separate system to recovery tanks adjacent to the auxiliary boilerhouse.

As the auxiliary boilerhouse serves several areas on the station site it is difficult to optimise its location. Generally it is positioned adjacent to the station main buildings. Other factors that can affect its position are whether or not the auxiliary boilerhouse and gas turbine house share the same chimney for visual rea­ sons. This is the case in the typical example shown.

Ideally the boilerhouse should be located centrally to the areas it serves, to reduce the extent of steam and condensate distribution main pipework.

23 Gas generation and storage

Gases are used on power stations as essential com­ ponents of the electrical power generation system and also as part of safety and protection systems. Some-

access and fire protection measures are of paramount importance. Open-to-atmosphere environments are chosen and consideration given to the separation of the different gas storage locations where leakages could combine to create hazardous situations.

23.1 Hydrogen

Hydrogen gas is needed for main electrical generator cooling on all modem CEGB power stations, for the production of methane gas on nuclear AGR stations and, amongst other things, for control of oxygen in the reactor coolant on nuclear PWR stations. This need for hydrogen gas is satisfied by the provision of a suitably "sized on-site hydrogen gas generator plant, housed in a simple building consisting basically of a roof on legs, i.e., a dutch barn type of construction. Depending on the site, local decisions may require the addition of

fully-louvred walls to the building, thus affording plant and personnel protection in exposed locations.

Two types of gas generator plant are currently utilised, electrolytic and methanol-cracking and are described as follows:

Electrolytic type gas generators consist of a series of cells containing electrodes suspended in an electrolyte solution of caustic potash, and separated by a mem­ brane (usually a woven type of asbestos mesh). The application of a DC current from a transformcr/rectifier source, produces hydrogen at the cathode and oxygen at the anode. Figure 2.89 shows the general features of the process.

On nuclear AGR stations both hydrogen and oxygen are utilised, the hydrogen firstly being used in conjunc­ tion with carbon dioxide to produce methane for inhi­ biting reactor core corrosion, and secondly for main electrical generator cooling. The oxygen is recombined into the reactor cooling circuit to re-establish the carbon dioxide from carbon monoxide produced by radiolysis-.

On nuclear PWR and conventional stations, the hydrogen only is utilised, the former requiring gas for

Fig. 2.88 Typical auxiliary boiler house

storage and generation Gas