oxygen control, etc., and main electrical generator cooling, the latter for generator cooling only.

Hydrogen gas produced by electrolysis is wet and contaminated, and therefore is subjected to filtration, deoxidation and drying stages before being stored at high pressure.

Methanol type gas generators consist of modules containing vapouriser, catalyser and diffuser sections. A methanol liquid feedstock is fed into the generator

two main components, hydrogen and carbon dioxide. The resulting gas mixture passes through a diffuser section which allows hydrogen gas to pass, but rejects carbon dioxide and other impurities. Figure 2.90 shows the general features of the process.

Hydrogen gas produced by methanol-cracking is extremely pure and dry and in the normal course of events does not require any further treatment before being stored at high pressure.

The methanol type system does require methanol feedstock and this must be stored adjacent to the hydrogen generation plant in suitable storage vessels.

In both systems described, the high pressure gas storage facility is served by compressors from the gas generators, and can take the form of large vessels or

Gas generation and storage

smaller cylinders, positioned adjacent to the generator plant and sized to accommodate two main electrical generator charges.

From the storage facility the gas passes through a control panel which regulates pressure and flow in two distribution pipelines to the electrical generator cooling systems. One pipeline allows flow of normal make-up

open environment position on the site. Risk factors taken into account are those presented by both the methanol store and hydrogen store as well as the need to conform to the guidance given in documents pro­ duced by the Health and Safety Executive (HSE) and the gas producing industry in terms of separation distances, hazardous zoning and general safety matters. Road facilities are provided for tanker access.and fire fighting appliances.

23.2 Carbon dioxide

Carbon dioxide (COj) is used as a purge gas for main electrical generator purging in both nuclear and con-

Station design and layout

ventional stations, and for the reactor cooling circuit and various purging requirements in nuclear AGR stations.

Storage is normally in bulk liquid form, large quan­ tities being required for the nuclear stations usually in the region of six 1001 vessels, whilst a typical fossilfired station requires far less — one 6 t vessel on average. The storage facility is normally made up of thermally-insulated vessels, each provided with a refrigeration unit to maintain the liquid state. Vapourising equipment is provided to enhance the gas flow rate at times of high demand. In both nuclear and conventional stations the refrigeration units are located adjacent to each storage vessel with conden­ sing coils mounted within the vessel vapour space. The vapourising equipment is a common installation and serves all vessels, being located in the discharge line from the storage facility. The distribution pipe­ work system is routed throughout the station to points of usage.

The whole plant arrangement is contained within an open environment adjacent to the main buildings together with the necessary road tanker access and unloading facilities.

23.3 Nitrogen

Nitrogen (N2) is used as a purge gas and sometimes for preservation purposes in steam and water spaces, although the latter examples are not common at the present time.

In nuclear AGR stations it is used for the secondary shutdown system, this being the need for an immediate supply of nitrogen gas in (he event of a demand for nitrogen injection into a reactor.

In nuclear PWR stations it is used for back-up, purge and cover gas systems, the former being a means of operating say, essential valves in the. event of loss of initial means of actuation. Cover gas relates to a blanketing function within tanks or vessels to avoid oxygen pick-up.

In conventional coal-fired stations nitrogen is used for mill loading, i.e., pressurising the grinding facility rams on the mills.

The storage facility normally takes the form of a vacuum insulated liquid nitrogen storage vessel com­ plete with road tanker delivery connections, followed by a liquid nitrogen pump discharging through a vapou­ rising section into a nitrogen gas store. The store is made up of large cylinders which discharge through the distribution pipework system to points of usage. The

.whole storage arrangement is contained within an open environment adjacent to the main buildings and pro­ vided with the necessary read tanker access and unloading facilities.

Chapter 2

23.4 Miscellaneous gases

Oxygen (O2)

The reactor coolant gas circuit on nuclear AGR sta­ tions is continuously producing carbon monoxide by radiolysis and oxygen is required to oxidise this back to carbon dioxide.

On AGR stations, oxygen is produced by means of the electrolytic hydrogen generation plant previously described, and stored by means of compression into cylinders. From the store, oxygen is discharged via a pipework system to a recombination unit in the reactor cooling circuit.

The storage arrangements, including the oxygen compression facility, are normally in an open environ­ ment adjacent to the hydrogen generation plant. Road access is provided for unloading facilities.

Propane (CjHr)

Propane is used for burner ignition purposes on both main and auxiliary boiler installations.

In nuclear stations, due to the relatively small demand from auxiliary boilers only, storage of propane may be in the form of cylinders arranged in banks. They are collectively connected up to manifolds dis­ charging via a pressure control facility and distribution pipework system to points of usage.

In conventional stations the demand for propane gas is created by both main and auxiliary boilers for burner ignition purposes. Storage takes the form of large vessels containing liquid propane which are supplied by road tanker delivery. If necessary, vapourisers may be fitted to each vessel to enhance gas flow at times of increased demand. The vapourisers are sited adjacent to the storage vessels, taking their supply from the liquid space and discharging into the gas space. Pro­ pane gas taken from the storage vessels passes through a pressure control facility before being distributed to points of usage through a pipework system.

The location of the propane store is chosen carefully. Due to the nature of propane, consideration must be given to hazardous zones, separation distances, etc., as laid down in HSE guidance documentation. Road faci­ lities are provided for tanker access and fire fighting appliances.

Methane (CH4)

Methane gas is used in nuclear AGR stations for injection into the reactor cooling gas circuit to inhibit corrosion of the graphite core. It is produced in an on­ site methane gas generator plant served by supplies of hydrogen and carbon dioxide, the former being pro­ duced by means of the electrolytic hydrogen generation plant described previously, the latter being supplied from the carbon dioxide system described in Section

23.2 of this chapter.