Performance and Control of the Primary Heat Exchanger in a Closed-Loop sCO2 Brayton Cycle With Solid Fuel Combustion

Abstract

Abstract The supercritical carbon dioxide (sCO2) Brayton cycle has been studied for a variety of heat sources as a replacement for the steam-Rankine cycle with higher thermal efficiency in power plants. The lack of a phase change in this cycle enhances the energy efficiency but also increases the likelihood of overheating of the heat exchanger tubes, especially when using solid fuels. The rapid sensible heating of the working fluid requires careful management of the temperature profile of the heat exchanger tubes inside the boiler. It is necessary to identify the system response of the controlling parameters of the system on the temperature profile of the primary heat exchanger (PHX) which can be used in the development of a model predictive controller (MPC) to manage the PHX surface temperature. A 1500 kWth solid fuel furnace (L1500) was retrofitted with a PHX and turbomachinery to investigate the use of solid fuel combustion with the sCO2 Brayton cycle. The PHX design requires limiting the PHX to a maximum surface temperature of 1089 K (1500 °F) and a maximum rate of change of 100 K/h. CFD modeling and operational data of the L1500 system showed that the firing rate, sCO2 flowrate, and refractory wall temperature significantly impact the temperature profile of the PHX. It is also expected that the amount of excess air will be a critical controlling parameter. Operational data showed that handling measurement noise will be vital to developing a predictive model of the heat transfer to the PHX.