Methane is a colourless flammable gas and is the first member of the alkane series of hydrocarbons. It is soluble in alcohol and ether and slightly soluble in water. It is the major component of natural gas and is normally shipped as a compressed gas or occasion­ally in bulk as a cryogenic liquid. It is used in the production of ethanol, methyl chloride and methylene chloride. It is not considered a toxic gas but acts as an asphyxiant.

Oxygen is a colourless non-flammable gas essential for combustion, vigorously oxidising many elements directly at room temperature or aboe. it is slightly soluble in water and more soluble in organic solvents. It is used in many industries including iron blast fur­naces, glass manufacture, ТіСЬ production and in the direct oxidation of ethene to ethylene oxide. Oxygen is normally produced by the liquefaction-distillation of air and is supplied both as a gas and a liquid. Argon and oxygen are not completely separated by this process because of their relative boiling points. The argon under irradiation will transmute to the radioactive Ar-41 and hence oxygen for magnox and AGR use is produced elect го I у t icall у to minimise the argon concentration.

Carbon dioxide is used by many industries and’is supplied to the CEGB in liquid form, being delivered to each station in road tankers. It is stored in refri­gerated and insulated vessels at a pressure in excess of reactor operating pressure. The liquid is evaporated by passing the carbon dioxide through a steam jacketed heater and passed to the reactor through reducing akes.

The specification for the methane supplied to the

reactor is:

Maximum permitted concentration 20 ppm

Impurity Maximum permitted concentration

20 vpm 1 vpm 1 vpm (total) 10 vpm

Nil

I vpm 100 vpm Not specified Not specified Not specified

The methane used in an AGR can be supplied from two sources; either manufactured on site or delivered to site as a compressed gas or in liquid form. In both cases care has to be exercised in the transport of the methane to the main circuit due to its explo­sive properties when mixed with air. The flammability limits for methane are 5-15.4 Vo in air. This may invoke the safe routing of pipework, the use of double pipe tubing or its dilution with an inert gas (carbon dioxide) to below the lower explosive limit and the adequate ventilation of rooms through which the gas is transported.

The method of manufacture on site is by electrolysis of water using conventional electrolysis plant to form hydrogen and oxygen. The hydrogen is washed to remove alkali carryover and is mixed with either the stoichiometric or an excess quantity of carbon dioxide and passed over a two stage 0.5°!) ruthenium on an alumina support catalyst bed maintained at a tempera­ture of 350°C. Between the two stages is a cooler/ drier to remove moisture and to increase the effluent methane concentration from typically 49 Vo after the first stage to 60 ‘/о (dry volume) after the second stage (less than 5 Vo effluent hydrogen concentra­tion). The gas is then dried and compressed to above reactor pressure for transfer to the reactor gas cool­ant. The above processes follow* the following chemical reactions:

2H20 2H; + Cb and 4H2 + C02 CH4 + 2H2O.

The primary advantage of this process is that the requirements for methane and oxygen (excluding the small imbalance due to reactor leak rate and carbon monoxide production from graphite corrosion) is met from a readily available source, namely water, which is exactly balanced by the water removed in the drier system. The disadvantages of the process are the costs and difficulties associated with constructing and op — erating/maintaining a chemical plant compared to the much reduced initial costs of installing and maintain­ing bought-іп supplies of methane and oxygen.