Performance Improvement of a Dry Mode Natural Gas Fired Turbine Plant for Combined Cycle Operation

Abstract

This research considers the design of combined cycle (CC) operation for a dry mode natural gas fired turbine plant in southern Nigeria. It entails evaluation and utilization of the amount of waste heat energy exhausted by the Omoku gas turbine (GT) power plant by integrating a steam Rankine cycle retrofitted with a heat recovery steam generator (HRSG) for CC operation, with the focus to improving its performance and reducing waste heat intensity to the environment. Gathered data from the human machine interface (HMI) and log sheets were used for the analysis. Thermodynamic sensitivity analysis was implemented for the combined cycle system (CCS) using a developed model in the MATLAB platform. The outcome of energy balance of the HRSG having a heat load of 38.49 MW showed that for every kg of exhaust gas, 0.1164 kg of steam is generated at an optimum pressure of 40 bar and mass flow of 14.45 kg, with acceptable steam turbine exhaust moisture content of 10%. These revealed a quantified amount of 45.28 MW heat energy contained in the usually wasted exhaust gas of the dry mode GT which was thus recovered in the HRSG, producing additional 16.32 MW as the steam turbine (ST) power output with a feed pump heat load of 0.06 MW and a condenser heat load of 28.96 MW. Further analysis in terms of power outputs, energy efficiencies, and environmental impacts showed that the CCS achieved 41.32 MW, 49.26% and HRSG stack temperature of 170.25oC compared to the previously 25 MW, 26.60% and exhaust gas temperature (EGT) of 487°C respectively of the dry mode GT. These indicate that the CCS generates about 65.30% boost in the net power output, 85.20% improvement in overall efficiency and 65.10% reduction in waste heat intensity to the environment when compared with the dry mode GT operating in isolation. Thus, the work showed that for the design of a CCS with a single pressure level HRSG without supplementary firing, a recommended range for the power output of the steam bottoming plant falls within 34 – 40% of the total power output of the CCS while that of the gas topping plant falls within the range of 60 – 66% of the total power output of the CCS. This study therefore confirms the viability as well as demonstrates the application, of the combined cycle concept for the Omoku gas turbine and recommends for further research, the introduction of a multiple pressure level HRSG with supplementary firing to the combined cycle system for an improved efficiency and output.