The need to reduce and mitigate the risk of global warming necessitates developing new methods or at least improving existing ones while maintaining a continuous, reliable and efficient supply of electricity. The increased share of power production from renewable sources like wind and solar brings challenges to grid stability due to their intermittent nature. Gas-fired power plants currently provide dispatchable generation; however, their continued reliance on fossil fuels presents a pressing environmental challenge. One potential solution to decarbonize gas turbines is the deployment of bioethanol, a renewable liquid fuel, that has the potential to have a closed carbon cycle.
Lean Premixed Pre-vaporized (LPP) technology has emerged as a promising solution, enabling the use of bioethanol in LPM systems by vaporizing the liquid fuel prior to injection. This study evaluates the use of pre-vaporized bioethanol in a combined cycle power plant by specifically examining the design and transient behavior of the fuel conditioning system required for bioethanol deployment. While existing research primarily focused on combustion behavior and steady-state performance, this study aims to fill the gap regarding the system integration, equipment design, transient operation, and the impact on the plant overall performance.
Initially, heat integration is conducted to optimize energy recovery within the combined cycle plant. Subsequently, preliminary design and sizing of major fuel conditioning equipment including a vaporizer, a superheater, and phase separator. The sizing of heat exchangers are performed using ASPEN Exchanger Design and Rating software. In the third phase, 1D dynamic models of these heat exchangers are developed using the finite volume (FV) method, chosen for its effectiveness in capturing complex transient thermal behaviors. Finally, these dynamic models are integrated with real combined cycle plant data to simulate startup.
Due to ethanol’s lower heating value compared to natural gas, a significantly higher mass flow rate was required to achieve the same heat input to the turbine. Intermediate-pressure (IP) steam was identified as the most suitable heating medium, minimizing heat exchanger size, cost, and installation challenges. A horizontally oriented shell-and-tube heat exchanger was selected for ethanol pre-vaporization due to its effectiveness in managing phase-change heat transfer. Results showed significant temperature gradients highlighting potential risks of localized heating, thermal stress and critical heat flux exceedance, necessitating further mechanical assessment.
Transient model results revealed that controlling heat input rates and managing the transition from external heating to IP steam are critical for maintaining stable fuel supply system operation during startup. Additionally, deployment of bioethanol required an external heating input for startup before IP steam became available. The use of IP steam as a heating medium introduced a measurable power penalty to the overall power output.
In conclusion, key recommendations are provided that include conducting detailed mechanical stress evaluations to address thermal stress concerns in the fuel conditioning system. Additionally, experiments on boiling heat transfer for ethanol under gas turbine operating conditions are essential to improve the accuracy of heat exchanger design. Furthermore, improving the transient model by incorporating real-time dynamic control strategies and relaxing simplifying assumptions such as accounting for pressure drop and void fraction will significantly improve the accuracy of transient predictions. Finally, conducting a pilot-scale test is recommended to validate the numerical model and confirm bioethanol’s feasibility, accelerating its deployment and marking an additional step toward decarbonized power generation.