Prediction of de-NOx performance using monolithic SCR catalyst under load following operation of natural gas-fired combined cycle power plants

As penetration of variable renewable energy power sources (VREs) increases, dispatchable power plants are expected to contribute to grid stabilization. Natural gas-fired combined cycle (NGCC) power plants can provide convenient functions such as quick start-up, shut-down, and load following with partial load operation. Transient operations are known to significantly increase the NO2 content of NOx in the exhaust gas. However, the de-NOx performance using selective catalytic reduction (SCR) with ammonia (NH3) injection decreases with an increase in NO₂content. Additionally, SCR de-NOx performance depends on transient mechanisms such as the adsorption of NH₃onto the catalyst and surface redox reactions. As such, transient operation modes of NGCC affect NOx slip and the traditional SCR operation methodology should be updated. This study examines whether de-NOx performance can be maintained during the operational fluctuations projected for power grids with high VRE penetration. A numerical simulation including a modified de-NOx reaction scheme was developed and verified against various exhaust gas compositions of NGCC expected for both steady-state and transient operation using a commercial monolithic SCR catalyst. The results of the numerical calculations indicate that a sudden load change causes a decrease in the performance of the SCR catalyst due to changes in gas composition and temperature. It was found that changing the injection amount of NH₃a few minutes before load alteration was effective for maintaining the NOx removal performance.