COMPASS Code Research Articles

A numerical investigation into the environmental impact of underground coal gasification technology based on a coupled thermal-hydro-chemical model

This study aims to improve the understanding of the potential environmental impact of the underground coal gasification (UCG) process based on a numerical investigation of the gaseous and dissolved chemicals’ transport in the strata surrounding the UCG reactor. A coupled thermal-hydro-chemical (THC) model in the framework of COMPASS code is employed and further developed for this purpose. The model is compared against analytical solutions in the verification process and applied against a former UCG trial (Hoe Creek UCG programme) for validation purpose. The numerical model is then applied to investigate the potential environmental impact of UCG in three different geological materials (sandstone, coal, and shale) in two steps: the potential gas propagation during UCG process and the solute transport in UCG decommissioning stage. The results indicate that the gas propagation is limited to a near-cavity area in geological strata with low permeability and high air entry value such as shale, which is mainly attributed to the dry zone generated by the high temperature, while a larger gas plume is observed for materials with poor water retention characteristics, such as coal and sandstone. Besides, the gas propagation is retarded to some extent due to the reducing effect of high pressure on gas diffusion. The propagation of dissolved chemicals via diffusion is limited to near-cavity areas, i.e. 2 m, in three studied materials after 10 years. Based on the improved understanding of the various chemicals’ migration in different materials, by selecting the target coal seam surrounded by strata layers with high air entry value and low permeability, the gas losses from the reactor during the UCG reactor operation stage can be minimized and the potential negative environmental impact of ash leaching and solute propagation in UCG decommissioning stage can be avoided. This work also provides further insight and suggestions into the importance of investigating the adsorption capacities of different geological media to evaluate their performance as a natural cleaning system.

Coding fault-tolerant quantum computing

Quantum Computing Recent advances in creating quantum processors comprising several tens of qubits bring the era of quantum advantage tantalizingly close. Reaching that point, however, will require quantum processors that are scalable and fault tolerant. With the underlying architectures of these processors based on superconducting qubits laid out in a two-dimensional array, so-called surface codes are being developed that enable a robust error-correction capability due to the collective behavior of several qubits. Li et al. describe compass codes, a generalized version of surface codes, that can deal various kinds of noise affecting the processor. A compass code can be engineered to correct for specific types of noise and thus provides a general route to fault-tolerant quantum processors. Phys. Rev. X 9 , 021041 (2019).

Underground coal gasification – A numerical approach to study the formation of syngas and its reactive transport in the surrounding strata

In this work, a method to study the formation of syngas during the underground coal gasification (UCG) process and its reactive transport in the surrounding strata is proposed. It combines a thermodynamic equilibrium stoichiometric model of the cavity reactions with a coupled thermo-hydraulic-chemical–mechanical (THCM) framework of COMPASS code for the transport of UCG products away from the cavity. With the input information of coal properties obtained from the South Wales coalfield, gasification reagents (air and steam) and thermodynamic conditions (initial temperature and pressure), the thermodynamic equilibrium model developed provides the maximum yield of gasification products and temperature from a UCG system. Gasification results giving the syngas composition with the highest percentage of methane and carbon dioxide, are then used as the chemical (gas) and thermal boundary conditions for the coupled thermo-chemical model of the THCM framework to analyse the variations of temperature and gas concentrations, in strata surrounding the UCG reactor. For that purpose, a set of numerical simulations considering three porous media (coal, shale and sandstone) with different physico-chemical properties is conducted. The gasification results demonstrate that increasing the amount of steam injected in the UCG reactor decreases the temperature of the system as well as the concentration of carbon monoxide and nitrogen, while benefiting the production of hydrogen, methane and carbon dioxide. The numerical simulations performed using the THCM model indicate that multicomponent gas diffusion and advection are competing transport mechanisms in porous media with intrinsic permeability higher than 1 mD (sandstone), while the gas diffusion becomes a dominant transport process in porous media with an intrinsic permeability lower than 1 mD (coal and shale). Moreover, the simulation results of reactive transport of methane and carbon dioxide in different porous media demonstrate the significance of considering the adsorption effect in the gas transport in the overall UCG process. In particular, the retardation of the gas front due to gas sorption is the most pronounced in coal, followed by shale and then sandstone. In conclusion, the model presented in this study demonstrates its potential application in managing the environmental practices, reducing pollution risk and securing greater public and regulatory support for UCG technology.

ComPASS : a tool for distributed parallel finite volume discretizations on general unstructured polyhedral meshes

The objective of the ComPASS project is to develop a parallel multiphase Darcy flow simulator adapted to general unstructured polyhedral meshes (in a general sense with possibly non planar faces) and to the parallelization of advanced finite volume discretizations with various choices of the degrees of freedom such as cell centres, vertices, or face centres. The main targeted applications are the simulation of CO2 geological storage, nuclear waste repository and reservoir simulations. The CEMRACS 2012 summer school devoted to high performance computing has been an ideal framework to start this collaborative project. This paper describes what has been achieved during the four weeks of the CEMRACS project which has been focusing on the implementation of basic features of the code such as the distributed unstructured polyhedral mesh, the synchronization of the degrees of freedom, and the connection to scientific libraries including the partitioner METIS, the visualization tool PARAVIEW, and the parallel linear solver library PETSc. The parallel efficiency of this first version of the ComPASS code has been validated on a toy parabolic problem using the Vertex Approximate Gradient finite volume spacial discretization with both cell and vertex degrees of freedom, combined with an Euler implicit time integration.

Precise relative positioning using real tracking data from COMPASS GEO and IGSO satellites

China completed a basic COMPASS navigation network with three Geostationary and three Inclined Geosynchronous satellites in orbit in April 2011. The network has been able to provide preliminary positioning and navigation functions. We first present a quality analysis using 1-week COMPASS measurements collected in Wuhan. Satellite visibility and validity of measurements, carrier-to-noise density ratio and code noise are analyzed. The analysis of multipath combinations shows that the noise level of COMPASS code measurements is higher than that of GPS collected using the same receiver. Second, the results of positioning are presented and analyzed. For the standalone COMPASS solutions, an accuracy of 20 m can be achieved. An accuracy of 3.0 m for the vertical, 1.5 m for the North and about 0.6---0.8 m for the East component is obtained using dual-frequency code only measurements for a short baseline. More importantly, code and phase measurements of the short baseline are processed together to obtain precise relative positioning. Kinematic solutions are then compared with the ground truth. The precision of COMPASS only solutions is better than 2 cm for the North component and 4 cm for the vertical. The standard deviation of the East component is smaller than 1 cm, which is even better than that of the East component of GPS solutions. The accuracy of GPS/COMPASS combination solutions is at least 20 % better than that of GPS alone. Furthermore, the geometry-based residuals of double differenced phase and code measurements are analyzed. The analysis shows that the noise level of un-differenced phase measurements is about 2---4 mm on both B1 and B2 frequencies. For the code measurements, the noise level is less than 0.45 m for B1 CA and about 0.35 m for B2 P code. Many of the COMPASS results presented are very promising and have been obtained for the first time.

Development of a Computational Framework on Fluid-Solid Mixture Flow Simulations for the COMPASS Code

The COMPASS code is designed based on the moving particle semi-implicit method to simulate various complex mesoscale phenomena relevant to core disruptive accidents of sodium-cooled fast reactors. In this study, a computational framework for fluid-solid mixture flow simulations was developed for the COMPASS code. The passively moving solid model was used to simulate hydrodynamic interactions between fluid and solids. Mechanical interactions between solids were modeled by the distinct element method. A multi-time-step algorithm was introduced to couple these two calculations. The proposed computational framework for fluid-solid mixture flow simulations was verified by the comparison between experimental and numerical studies on the water-dam break with multiple solid rods.

Improvement of Basic Fluid Dynamics Models for the COMPASS Code

The COMPASS code is a new next generation safety analysis code to provide local information for various key phenomena in core disruptive accidents of sodium-cooled fast reactors, which is based on the moving particle semi-implicit (MPS) method. In this study, improvement of basic fluid dynamics models for the COMPASS code was carried out and verified with fundamental verification calculations. A fully implicit pressure solution algorithm was introduced to improve the numerical stability of MPS simulations. With a newly developed free surface model, numerical difficulty caused by poor pressure solutions is overcome by involving free surface particles in the pressure Poisson equation. In addition, applicability of the MPS method to interactions between fluid and multi-solid bodies was investigated in comparison with dam-break experiments with solid balls. It was found that the PISO algorithm and free surface model makes simulation with the passively moving solid model stable numerically. The characteristic behavior of solid balls was successfully reproduced by the present numerical simulations.