A natural gas fired combined cycle power plant with indirectly-fired heating for additional work output is investigated in the current work. The mass flow rate of coal for the indirect firing mode in circulating fluidized bed combustor is estimated based on fixed natural gas input to the topping combustor. The effects of pressure ratio, gas turbine inlet temperature, inlet temperature to the topping combustor on the exergetic performance of the combined cycle configuration are analysed. The use of coal in indirect-firing mode reduces with increase in turbine inlet temperature due to increase in the use of natural gas. The exergetic efficiency increases with pressure ratio up to the optimum pressure and it also increase with gas turbine inlet temperature. The exergy destruction is highest for the circulating fluidized bed combustor (CFBC) followed by the topping combustor. The analyses show that the indirectly fired mode of the combined cycle offers better performance but with higher exergy destruction and the opportunity for additional net work output by using solid fuels (coal in this case) in existing natural gas based power plant is realized.

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