rotary kiln 24 is also designed so that the low grade coal has a relatively long residence time therein and a relatively large surface area of high temperatures for ensuring combustion of very low concentration mine methane the apparatus 24 shown in fig2 and 3 can therefore be utilised to burn reject coal low grade coal and waste mine methane the ash from the burnt coal can be processed so that it is converted into a useful by product such as closed or open cell spherical nodules ideal for manufacture of lightweight building materials and gravel substitutes in this manner little or no fly ash or slag is produced from combustion of low grade coal with extremely high ash contents limestone can be added to convert any sulphur compound to calcium sulphate greenhouse benefits can be maximised by sizing the plant to use all the mine ventilation air and rejects this utilises carbonaceous waste that may eventually become carbon dioxide emissions by spontaneous combustion if stockpiled on the surface having separate flow paths provides the system with the flexibility to vary the proportion of ventilation air to gas turbine flow rate this is managed by setting the temperature at which the gas is exhausted from the high temperature heat exchanger fig4 shows the ratio of ventilation air to gas turbine flow rate.