A unified approach to assess performance of different techniques for recovering exhaust heat from gas turbines

Exhaust heat from gas turbines can be recovered externally or internally to the cycle itself. Of the technology options for external recovery, the combined gas–steam power plant is by far the most effective and commonly used worldwide. For internal recovery conventional solutions are based on thermodynamic regeneration and steam injection, while innovative solutions rely on humid air regeneration and steam reforming of fuel. In this paper a unified approach for analysing different exhaust heat recovery techniques is proposed. It has been possible to define a characteristic internal heat recovery plane, based on a few meaningful parameters and to identify an innovative scheme for repowering existing combined cycles. The characteristic plane indicates directly the performance obtainable with the different recovery techniques, showing that performances close to combined cycle plants (external recovery) can only be achieved with combined recovery techniques (humid air regeneration, steam reforming of fuel). The innovative repowering scheme, which requires the addition of a gas turbine and one-pressure level HRSG to an existing combined gas–steam power plant, significantly increases power output with fairly high marginal efficiency.