The electricity demand of the process is mainly determined by the work required for pumping,
compression, air coolers, and solids-handling operations, but also includes the power needed for
instruments, lights, and other small users. The power required may be generated on site, but will
more usually be purchased from the local supply company. Some plants generate their own electricity
using a gas-turbine cogeneration plant with a heat recovery steam generator (waste-heat boiler)
to raise steam (Figure 3.1). The overall thermal efficiency of such systems can be in the range 70%
to 80%; compared with the 30% to 40% obtained from a conventional power station, where the