Carbon capture and storage (CCS) technologies such as oxy-fuel combustion, chemical looping combustion, and post-combustion capture systems offer the most promising approaches for reducing CO2 emissions from fossil fuel power plants. Large-scale commercial deployment of CO2 capture technologies requires an understanding of how to scale laboratory designs of multiphase flow reactors to industrial sizes. However, the direct scale-up of such reactors is known to be unreliable, and the current approach requires building and testing physical systems at increasingly larger intermediate scales. The cost in both dollars and development time of having to build and extensively test systems at multiple intermediate scales is prohibitive. High-fidelity computational tools that use exascale computing power can be used to model emerging CCS technologies to enable the design and optimization of these systems, which are critical to controlling costs and reducing the risk of designs failing to meet performance standards.
Summary
The United States is adding renewable energy sources to its power grid at an increasing pace, but fossil fuels still generate about 60% of the grid’s power. Although that number is decreasing, the US Energy Information Administration projects that it will still be more than 40% in 2050. Carbon capture and storage technologies such as chemical looping reactors (CLRs) will be critical in reducing the impact of greenhouse gases released from burning fossil fuels. However, current designs are still in laboratories and must be scaled to industrial proportions. Building and testing large- and intermediate-scale reactors are expensive and time-consuming tasks, so researchers are using high-performance computers to simulate various designs and identify the most promising candidates. Using this approach, carbon capture technologies can be developed at less expense and with greater assurance that reactors will meet necessary safety and performance standards.
The Exascale Computing Project (ECP) MFIX-Exa application was built to model industrial reactors for carbon capture applications with the highest possible fidelity The application models the coupled motion of particles and fluids within traditional power plants and industrial processes based on fossil fuels. These models will enable chemical looping combustion and point source capture, two key technologies used to capture CO2 before it is released into the atmosphere. MFIX-Exa simulations accelerate the development of functional reactors that operate with low carbon emissions and minimal cost and energy penalties.
Modeling even small-scale reactors at a sufficient fidelity to track individual particles within a fluid flow requires enormous computational resources. Even a small 50-kilowatt thermal unit contains approximately 5 billion particles, and commercialized systems could easily contain trillions. High-fidelity physical simulations at these scales require billions of computational cells to represent fluid and particle dynamics, and this computational load is prohibitive without advanced software solutions to increase computational efficiency and access to the most powerful computers available.
The MFIX-Exa team enabled physically accurate CLR simulations on Frontier, the world’s fastest supercomputers. The team collaborated with other ECP development efforts to enable new simulation methods such as adaptive mesh refinement and block-structured numerical algorithms, which improve computational efficiency without sacrificing simulation fidelity in critical regions. Furthermore, the MFIX-Exa application boasts extended physical modeling capabilities such as heat and mass transfer and chemical reactions for improved predictive power. These innovations coupled with the power of exascale computation helped the team to successfully model the National Energy Technology Laboratory’s 50-kilowatt thermal CLR with almost a billion computational cells. This achievement marks the first simulation of a large-scale gas-solids chemical reactor with individual particle tracking.
Exascale computing with MFIX-Exa will be used to evaluate and improve emerging carbon capture technologies. These high-fidelity simulations will dramatically reduce development costs and will ensure that reactor designs scaled up from the lab will meet performance and safety targets. These accelerated technologies will be critical for reaching US decarbonization goals and will contribute to an energy sector free from carbon pollution in the decades to come.
Technical Discussion
This work specifically targets scale-up of gas-solids reactors like chemical looping reactors (CLRs) through the creation of MFIX-Exa, an exascale capable computational fluid dynamics–discrete element method (CFD-DEM) code, which represents the next generation of the highly successful NETL-based MFIX code. CFD-DEM is an approach that allows for tracking of individual particles (DEM portion) within a continuum fluid phase (CFD portion). Rather than starting the new code from a completely blank slate, MFIX-Exa is built within the AMReX framework, an ECP co-design center for adaptive mesh refinement and block structured numerical algorithms at exascale. The multi-lab team integrates expertise in HPC directly with expertise in multiphase flow modeling and will outperform the existing MFIX by orders of magnitude. MFIX-Exa will enable the development of gas-solids reactors for the removal of CO2 from point sources, such as fossil-fuel based power plants and industrial processes, or directly from the atmosphere, with minimum cost and energy penalty.
Figure 1: The 50kW Chemical Looping Reactor at NETL, (left) and details of particle flow possible in the riser section of the reactor (right).
The challenge problem for MFIX-Exa is a full-scale simulation of NETL’s 50kW CLR as depicted in Figure 1 and discussed by PI Jordan Musser in a Let’s Talk Exascale podcast.
A fully reacting simulation of the complete CLR was conducted at 1/10th the fluid spatial resolution and 1/500th of the particle count of the challenge problem. As shown in an animation of a 2D slice through the center of the reactor, the particle-in-cell (PIC) simulation contains all the necessary physical models of the challenge problem, including the complex geometry and the various gas- solids flow regimes occurring in the CLR (bubbling bed, riser, cyclone, standpipe, and L-valve), gas-solids chemical reactions, and interphase heat and mass transfer. A novel PIC-to-DEM ‘bootstrapping’ algorithm created by the MFIX-Exa team will be used to convert the result of this lower-fidelity simulation into a high-quality initial condition (starting point) for the challenge problem. This approach ensures that the MFIX-Exa demonstration run contains all the complex flow features found in a CLR simulation.
