Thermodynamic Assessment of Exhaust Gas Recirculation in High-Volume Hydrogen Gas Turbines in Combined Cycle Mode

To reach net-zero while ensuring grid reliability and resiliency, gas turbine (GT) technology has a place for years to come. However, shifting to low-carbon fuels, such as hydrogen, is the key to maintain positive returns in combined cycle (CC) power plants. By recirculating a fraction of the exhaust gas exiting the heat recovery steam generator (HRSG) back to the inlet of a natural gas (NG) and hydrogen cofired GT, the gas flow passing through the compressor and entering the combustor has a reduced oxygen concentration thus lowering flame temperature, hence NO_x formation. Hydrogen reactivity is then turned into a benefit since the exhaust gas recirculation (EGR) rate can be higher than that with NG, without facing flame stability issues. In light of this, a thermodynamic assessment of EGR effects on a 2 × 1 large-scale CC is presented considering GT with hydrogen capability up to 65%. The impact of partially replacing NG with hydrogen on GT behavior and overall CC performance was first evaluated at both full and part load, with no EGR. Then EGR was simulated for a rate up to 0.5 for different fuel mixtures, under the assumptions of GT inlet flow at low (ISO) and high (up to 47 °C) temperature. The analysis was again carried out at full and part load. In the latter case, EGR was exploited to improve CC efficiency at very low loads. For each scenario, CO₂ emission intensity was computed thus highlighting the environmental benefits of hydrogen-NG blends.