Computational fluid dynamics study of CO2 dispersion with phase change of water following the release of supercritical CO2 pipeline

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Abstract

Accidental release of pressurized CO2 pipeline in carbon capture and storage involves the interaction of phase change and heavy-gas dispersion. However, the effect of the phase change of water vapor in air on the performance of cold CO2 dispersion is usually neglected. In this study, a three-dimensional two-phase computational fluid dynamics (CFD) model is developed to evaluate the cold CO2 dispersion by considering the phase change of water. A phase-change model based on the homogeneous relaxation model is used to describe the evaporation and condensation of water. The effects of terrain roughness, atmospheric stability, and turbulence models on the dispersion are also considered. The numerical results show that the model that uses the k–ω turbulent equations is superior to the other models. The results in which the phase change of water is considered exhibit a better agreement with the data from the experiments than those that do not consider it. The model is subsequently used in urban areas, which results in over-predicted CO2 concentration in the near field and under-predicted CO2 concentration in the far field when the phase change of water vapor is considered than that when it is neglected. Therefore, we proposed that the phase change of water vapor in the atmosphere should not be overlooked in the more accurate modeling of cold CO2 dispersion.

Introduction

As a non-renewable energy source, fossil fuels have been playing an important role in the development of human society by providing most of the energy requirement of the world (Outlook, 2019). However, carbon dioxide (CO2), which is the main product of fossil-fuel combustion, is simultaneously regarded as the main cause of greenhouse effect. Carbon capture and storage (CCS) technology is regarded as one of the most potential industrial solutions to this problem, which has attracted increasing attention in recent years (Størset et al., 2019). In CCS, CO2 is usually captured at a large point-emission source (e.g., power plants) and transported via long pipelines to another spot for storage (Liu et al., 2019a). Some studies have shown that transporting large-scale CO2 through supercritical pipelines is currently the most economical method (Li et al., 2016). In the popularization and application of the CCS technology, extensive deployment of supercritical carbon dioxide pipelines is inevitable.

However, potential leakage can occur with the occurrence of pipeline corrosion and other outside forces such as construction defects, solid movement, and others. As a highly concentrated asphyxiant and heavier-than-air gas, large CO2 leaks would accumulate in low-lying land, which can pose a significant threat to public health and safety (Guo et al., 2016; Liu et al., 2016; Wang et al., 2021; Witlox et al., 2011). In addition, because of the large pressure difference between the inside and outside parts of a pipeline (the pressure inside a supercritical CO2 pipeline is usually higher than 10 MPa), the strong Joule–Thomson effect and sublimation of CO2 particles in the dispersing jet that causes the temperature of the dispersed-gas cloud to decrease rapidly to below −100 °C (Gant et al., 2014). Therefore, understanding the dispersion behavior of low-temperature carbon dioxide is important in determining the safe distance of pipelines and for risk assessment.

In recent years, many heavy-gas-dispersion models have been developed, such as the Gaussian model, SLAB, DEGADIS, PHAST, TRACE (Colenbrander, 1980; Ermak, 1990; Hanna et al., 2008; Havens, 1988; Witlox et al., 2009a). PHAST is a commercial software package developed by DNVGL that contains a wide range of models for consequence and risk assessment. The dispersion model (Witlox and Holt, 1999; Witlox and Harper, 2014) allows the use of a range of source terms (evaporation pool, instantaneous release, finite duration, and time-varying release). A major component of PHAST is the Unified Dispersion Model (UDM), which incorporates submodels for two-phase jets, heavy and passive dispersion, droplet rainout and pool spreading, and evaporation (Gant et al., 2018). The TRACE algorithm covers emissions of high or low density gases based on ground or high concentration releases (Systems, 1996). It can handle the dispersion of gas or vapor/aerosol streams. Compared to integral models such as DRIFT, CFD models allow taking into account the geometry of elements present in real scenarios, such as the presence of terrain and complex obstructions (Gant and Tucker, 2018; Schleder and Martins, 2016). These models provide many assumptions and simplifications on the physical process of heavy-gas dispersion and can quickly calculate the dispersion range; thus, they are still widely used in the industry. However, these models predictive ability in case of large-scale gas release is unclear(Liu et al., 2019a) and their cannot simulate complex dispersion situations such as heavy-gas dispersion involving phase change and dispersion over complex topography (Tauseef et al., 2011). Therefore, more accurate computational fluid dynamics (CFD) models are widely used (Gant et al., 2014; Hanna et al., 2009; Kim et al., 2012; Liu et al., 2014). Compared with the conventional heavy-gas-dispersion models, the CFD models can simulate a complicated physical process involving mass and heat transfer in complex computational domains. Therefore, CFD models are increasingly used in CO2 dispersion simulations even though they are more complex and time consuming than the analytical models. Mazzoldi et al. (2008) used CFD simulation software Fluidyn Panache to simulate and verify the Prairie Grass and Kit Fox experiments. They found that compared with the Gaussian model, the CFD model could better simulate gaseous CO2 dispersion. Xing et al. (2013) used different turbulence models (k–ε, RNG k–ε, and k–ω SST) to verify the CO2 heavy-gas-dispersion experiment, and their study demonstrated that the k–ε and k–ω SST models exhibited better prediction results while the RNG turbulence model exhibited a large deviation. Liu et al. (2016) simulated CO2 dispersion in complex terrains using the CFD approach. They assumed that CO2 dispersion mainly occurred in the following: (1) plain with axisymmetric hills and (2) in a town with buildings. The effect of the strength of the release source, wind speed, and height of the building on the dispersion range was studied. Tan et al. (2018) used ANSYS Fluent to simulate CO2 dispersion in street canyons, and the results showed that the numerical simulation results of the k-ω SST turbulence model were in good agreement with the experimental data from wind tunnel test. Tan et al. (2019) used CFD software ANSYS Fluent to simulate the dispersion of heavy gases in a regularly distributed building, and the results showed that the height of the buildings, aspect ratio, and roof shape significantly affected the dispersion of CO2 clouds. Scargiali et al. (2011) simulated the dispersion of heavy gases in regularly distributed buildings using CFD software package ANSYS CFX, and the results showed that the presence of buildings reduced the maximum CO2 concentration and enhanced the horizontal dispersion of clouds. The numerical model of the dynamic dispersion behavior of a gas that is heavier than air in the atmosphere usually does not consider the phase change of water involved. The results are basically consistent with the experiment; however, gaps still exist that can be further improved. Currently, many CFD models are applied to simulate the dispersion of CO2. However, because of the complexity of the coupling of various factors (such as different terrains, different buildings, and different turbulence-model selection) during the research process, implementation of these models has been limited. Therefore, the present study focuses on the influence of water-vapor phase change in the atmosphere on the dispersion characteristics of cold CO2 that is released from pipelines and comprehensively considers the influence of terrain roughness, atmospheric stability, and turbulence model.

The effect of the water-vapor phase change in air on the cold CO2 dispersion is unknown. It is well known that gaseous CO2 is colorless, but Joule-Thomson Cooling caused the temperature of CO2 fluid to decrease rapidly until the triple point and then the formation of solid CO2 particles in the atmosphere followed while water vapor in air condenses into a mass of fog and ice, which spreads out as the CO2 plume spreads (Martynov et al., 2014; Mocellin et al., 2015, 2016; Teng et al., 2016; Vianello et al., 2014; Woolley et al., 2014). Thus, the cold CO2 plume that follows a supercritical CO2 release is visible and the color of the jet flow is white (Teng et al., 2018). Initially, the water vapor becomes water droplets or even ice particles in a cold region. Water or ice particles may increase the visibility of clouds. With the development of the dispersion process, the condensed phase (fog and ice) evaporates into gas again. Consequently, the CO2 concentration evolution may be affected by the phase change of water because the mass and heat transfer during this process changes the density and buoyancy of the CO2 cloud. Further, it greatly affects the safe distance of the CO2 pipeline and risk assessment.

In the present study, a three-dimensional (3D) two-phase CFD model was developed to quantifiably evaluate the effects of water phase change in the surrounding air on the behavior of the cold CO2 cloud dispersion. In this model, the turbulent closure, wall function, atmospheric stability, and terrain roughness were considered. The physical properties of materials and boundary conditions were studied in detail. The equations were discrete and computed using commercial CFD software ANSYS Fluent 14.5. The phase change sub-model of water was implemented using user-defined function (UDF) programming. The heat- and mass-transfer process between phases was compiled by introducing mass and energy-source terms between the phases. The simulation results were compared with the experimental data from the BP CO2 release test. The results demonstrated that the model that used the k–ω turbulence equation was better than the alternative models. The result that considered the water phase change was more accurate than that that neglected the water phase change. Therefore, in the cold CO2 dispersion model, the phase change of water vapor in air should not be ignored. Finally, the model was utilized in simulating cold CO2 dispersion in urban buildings. The results demonstrated that the water-vapor phase change affected the CO2 dispersion in the near field.

Section snippets

Governing equations

The CO2 gas phase can be assumed to be an ideal gas in the atmosphere when cold CO2 disperses in the far field. Therefore, a 3D two-phase flow with a phase change can be predicted by solving the governing conservation equations. In this paper, the mixture model is applied for the multiphase flow model. The governing equations describe the conservation of mass, momentum, and energy, as expressed in Eqs. (1)–(3).ρmt+ρmv¯m=0ρmv¯mt+ρmv¯mv¯m=p+μmv¯m+v¯mt+ρmg+k=12αkρkv¯dr,kv¯dr,ktk

Test description of the BP CO2 dispersion experiments

Between 2006 and 2007, GL Noble Denton conducted a series of industrial-scale (up to 1.6 tons per single dispersion) liquid and supercritical CO2 release and dispersion experiments (pressure: 8.2–15.9 MPa; temperature: 5–147 °C) at the Spadeadam test field in northern England (Witlox, 2012; Witlox et al., 2014a). All experiments were carried out on cement flats with a horizontal direction of release. The CO2 concentration, temperature, leakage flow rate, and wind velocity were measured in real

Model description

Fig. 8 shows that the urban area is simply modeled using regular blocks to mimic buildings and streets. The size of the computation domain is 200 × 106 × 100 m, and the building is composed of a block type with length, width, and height of 10, 8, and 4.2 m, respectively. A vertical upward CO2 release point exists upstream in the wind direction 20 m from the first row of buildings. The hexahedral structured mesh is used to fine the grid near the terrain and surface of the building. To ensure a

Conclusions

A 3D CFD model was established to evaluate the cold CO2 dispersion, which considered the phase change of water. The effect of terrain roughness, atmospheric stability, and selection of turbulence model on the dispersion was also considered. The simulation results were compared with the data obtained from the BP large-scale CO2 release experiment. The result showed that compared with the turbulence models such as k–ε, RNG k–ε, and k–ω SST, the k–ω model was more suitable for simulating

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

The present work is supported by National Science Foundation of China (Grant No.51374231), Natural Science Foundation of Fujian (Grant No.2020J05098) and Natural Science Foundation of Chongqing (Grant No.CYY202010102001).

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      In the category of pipeline transport, several modeling studies have been conducted to predict the transient flow of multiphase CO2-rich mixtures in pipes, an extensive review of which until 2013 was presented by Aursand et al. [18]. For safety applications in the same category, several studies also took advantage of various modeling approaches (especially CFD) to study the potential atmospheric dispersion of CO2 leaked from CCUS transportation pipelines [19–24]. In the category of tanker-based (especially ship) transport of liquified CO2, there has been a considerable effort on the progress, design, and optimization of the re-liquefaction/refrigeration process units for CO2 boil-off gas (BOG) recovery purpose [6,25–33].

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