Owing to an abundance of coal reserves, about 92 percent of the electrical power produced in South Africa is generated in central power stations fired on cheaply priced coal. With a few power stations approaching the end of their design life, the question arises as to what to do with these outdated and inefficient plants. Retrofitting or repowering a station with gas turbines is one option being considered. As a case study, this paper investigates the technical and economic feasibility of repowering the Arnot power station to convert it to a combined cycle plant with increased capacity. This power station has six generating units, each of nominal capacity 350 MW and of average age 25 years. Four are in service, and the others are in reserve storage. Several repowering options were considered and the proposed re-design is parallel repowering, where additional steam for a steam turbine is generated in a gas turbine heat recovery steam generator to supplement the steam generated in a coal-fired boiler. Since natural gas, the preferred fuel for gas turbines, is not readily available in the country, kerosene was used as gas turbine fuel. Consequently, the performance of the chosen gas turbine had to be re-evaluated.

The output of each unit increased by 77 MW and the efficiency by 8 percentage points to 43 percent, after repowering. Repowering was feasible, technically. An economic analysis was required to determine the magnitude of the economic benefits of repowering, if any, and it turned out that the cost of electricity generated by the new technology was higher than that produced by the outgoing one. It was concluded, therefore, that repowering the steam turbine units with gas turbines fired on kerosene was uneconomical, for the performance level achieved.

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