Session: 30-15 Thermal Component Performance

Paper Number: 126013

126013 - Performance and Control of the Primary Heat Exchanger in a Closed-Loop sCO2 Brayton Cycle With Solid Fuel Combustion 

The supercritical carbon dioxide (sCO₂) Brayton cycle can generate electricity more efficiently than the steam-based ultra-supercritical Rankine cycle. While the sCO₂ Brayton cycle has been studied while using heat from gaseous fuels, the literature does not show experimental work of the sCO₂ Brayton cycle using solid fuels. The primary objective of this work is to demonstrate the coupling of the closed-loop sCO₂ Brayton cycle with solid fuel combustion.

Using solid fuels in a sCO₂ Brayton cycle presents challenges not found when using gaseous fuels. Unlike the steam-based Rankine cycle, the working fluid in the sCO₂ Brayton cycle does not change phase in the boiler. The absence of a phase change allows for rapid sensible heating of the heat exchanger tubes and the working fluid. If the tube metal temperature and heat flux through the tube wall are not controlled, the tube integrity can weaken and eventually rupture, causing an uncontrolled release of highly pressurized CO₂. Solid fuel combustion results in a more complicated heat release profile than gaseous fuels due to the additional physics involved in the heat transfer of the gas-solid phases. Solid fuel particles undergo three stages in combustion:  demoisturization, devolatilization, and char oxidation. Combustion aerodynamics strongly influence the kinetics of solid fuel combustion. Changes in the rate of solid fuel combustion results in different heat release profiles in the boiler.

An existing pilot-scale 1.5 MW_th coal furnace (L1500) was retrofitted with a Primary Heat Exchanger (PHX) and a convective heat exchanger which are connected to a skid that provides CO₂ at conditions representative of a high-temperature sCO₂ Recompression Brayton cycle. The sCO₂ system includes a compressor, a recuperative heat exchanger, an expansion valve to simulate the effects of a turbine, a condenser, an active inventory control system, and integrated instrumentation and controls.

The design and performance evaluation of the PHX were critical aspects of this investigation. The PHX was designed using a coupling of Computational Fluid Dynamics (CFD) simulations and process modeling. These models were used to design the geometry of the PHX as well as provide input on how to operate the combustion system without damaging the PHX tubes. The CFD models showed that regions of the PHX are prone to receive unsafe levels of incident heat flux. Extending the length of the PHX closer to the burner decreased the peak incident heat flux of the PHX. Adjusting the combustion operating conditions in the CFD model also showed that the combustion stoichiometry greatly affects the peak incident heat flux. Feeding enough air into the system to have super-stoichiometric conditions was found to reduce the adiabatic flame temperature and reduce the peak incident flux to the PHX. Efforts to manipulate the combustion aerodynamics by staging combustion air or increasing the amount of swirl in the combustion air created regions of sub-stoichiometric conditions, resulting in a higher peak incident heat flux.

The L1500 – sCO2 Brayton cycle system has been successfully operated while combusting bituminous coal trimmed with natural gas. A mass and energy balance in the L1500 is used to determine the efficiency of the PHX in capturing heat from the radiant and convection sections of the L1500 during different operating conditions. Targeted CO₂ state points from the process modeling are compared against actual temperature and pressure measurements. The targeted state points for the CO₂ are 688 K and 20.4 MPa entering the PHX and are 878 K and 19.9 MPa exiting the L1500. Insight into the interactions of the combustion of solid fuels and the sCO₂ Brayton cycle equipment was gained.

Presenting Author: Brian Schooff Brigham Young University

Presenting Author Biography: Brian Schooff is a PhD candidate in the Chemical Engineering Department at Brigham Young University.

Authors:

Brian Schooff Brigham Young University
Rajarshi Roy Brigham Young University
Fletcher Smith Brigham Young University
Daniel Tree Brigham Young University
Brian Iverson Brigham Young University
Andrew Chiodo Reaction Engineering International
Timothy J. Held Echogen Power Systems
Jason Miller Echogen Power Systems
Brett Bowan Echogen Power Systems
Kyle Sedlacko Echogen Power Systems
Michael Johnson Babcock Power
Scott Montgomery San Rafael Energy Research Center
Andrew Fry Brigham Young University