Carbon Capture Efficiency in Natural Gas Combined Cycle Power Plants: Analyzing the Effects of Variable Load Operations

Abstract

Natural gas power generation retrofitted with carbon capture technology is poised to play a crucial role in ensuring energy reliability amidst the transition to variable renewable energy resources. While natural gas generation is used primarily for baseload power, it is expected to transition towards an intermittent power generator, serving as a load-following resource during periods of low renewable energy availability. It will be critical to understand how start-up, shutdown, and load-following behavior may impact system performance and influence future grid design.

This thesis performs a comprehensive literature review to establish context on various techniques of carbon capture technology. Post-combustion carbon capture, specifically absorption-based technology, remains the preferred candidate for retrofitting natural gas plants due to its technical maturity, scalability, relatively high capture efficiencies, and ease of retrofitting. The literature highlights that absorption-based carbon capture units exhibit degraded performance during non-steady-state operating conditions. Specifically, cold start-ups result in lower capture efficiencies and higher heat rates, although hot start-ups incur significantly less performance reduction.

The literature review findings are integrated into GenX, a grid optimization tool, to evaluate natural gas combined cycle power plants equipped with carbon capture technology. The modified optimization models are run using the ISO New England grid system, and results suggest that incorporating advanced start-up penalties for natural gas plants reduces operational flexibility in an emissions-constrained environment. As capture efficiencies decrease and heat rates increase during start-ups, utilizing natural gas plants becomes more expensive due to the additional emissions and reduced thermal efficiency. Comparing models with different levels of performance degradation during start-up suggests that installing less gas capacity could be optimal, with those units operating at higher capacity factors to mitigate start-up penalties. Under modest emissions constraints, natural gas units may be operated continuously even during periods of renewable energy surplus. Harsher start-up penalties applied to natural gas plants likely increase the incremental value of alternative energy technologies, although natural gas retains a critical role in the energy mix.