Large-scale validation of a numerical model of accidental releases from buried CO2 Pipelines

The work presented in this paper concerns a number of experiments and simulations performed as part of National Grid's COOLTRANS research programme, initiated in order to address knowledge gaps relating to the safe design and operation of onshore pipelines for transporting dense phase carbon dioxide (CO2) from industrial emitters in the UK to storage sites offshore. Such pipelines are considered to be the most likely method for the transportation of captured CO2. The research presented here describes further developments of a state-of-the-art multi-phase heterogeneous discharge and dispersion model capable of predicting fluid dynamic and phase phenomena in releases from high pressure pipelines of CO2 into air. Model validation is included against a number of field-scale experiments considering various vertical releases of CO2 into free air. The model is also used to simulate a puncture release in a buried pipeline and the results near the crater edge are compared to field-scale experimental data. Model predictions are found to describe the experimental observations very well, with a high level of agreement between the two. The study demonstrates the advantages of using a model for addressing accidental releases of CO2 that includes shock-capturing methods and complete three-phase formulations. Such models are required to predict the physical and thermodynamic properties of CO2 in order to accurately predict the details of the discharge and dispersion phenomena of interest in risk assessments. Carbon capture and storage refers to a set of technologies designed to reduce carbon dioxide emissions from large point sources of emission such as coal-fired power stations, in order to mitigate greenhouse gas release. CCS technology, or sequestration, involves capturing CO2 and then storing it in a suitable storage facility, instead of allowing its release to the atmosphere, where it contributes to climate change. Necessary transportation can be achieved in different ways, but it is commonly acknowledged that high pressure pipelines transporting liquid CO2 will be the most reliable and cost effective choice. Their safe operation is of paramount importance as the inventory would likely be several thousand of tonnes, and CO2 poses a number of issues upon release due to its physical properties; it is a colourless, odourless asphyxiant which sinks in air and has a tendency to solid formation upon release with subsequent sublimation. It is directly toxic in inhaled air at concentrations around 5% and is likely to be fatal at concentrations around 10%. Predicting the correct fluid phase during the discharge process in the near-field is of particular importance given the very different hazard profiles of CO2 in the gas and solid states. A state-of-the-art multi-phase heterogeneous discharge and dispersion model capable of predicting both the near- and far-field fluid dynamic and phase phenomena in such CO2 releases was presented at ESCAPE 22. Predictions are based on solution of the density-weighted forms of the transport equations for mass, momentum and total energy. Closure of this equation set is achieved using a compressibility-corrected k- turbulence model. Solutions are obtained of the time-dependent forms of the descriptive equations and the integration of the equations is performed by a shock-capturing conservative, upwind second-order accurate Godunov numerical scheme. The fully-explicit time-accurate cell-centred finite-volume Godunov method is a predictor-corrector procedure, where the predictor stage is spatially first-order, and used to provide an intermediate solution at the half-time between time-steps. This solution is then subsequently used at the corrector stage for the calculation of second-order fluxes that lead to a second-order accurate cell-centred solution. A Harten, Lax, van Leer Riemann solver is employed to calculate fluxes at cell boundaries. A non-ideal equation of state with additional formulations to accurately predict solid phase properties is implemented to describe the physical and thermodynamic characteristics of CO2. The paper will describe further developments of this model in terms of its ability to accurately predict both gaseous and solid phases, and its validation against field scale experimental data from a vertical release of CO2 into free air. The model has also been used to simulate a puncture release in a buried pipeline and the results near the crater edge will again be compared to field scale data. This work forms part of the case studies performed in the National Grid COOLTRANS Research Programme. National Grid has initiated this research programme to address knowledge gaps relating to the safe design and operation of onshore pipelines for transporting dense phase CO2 from industrial emitters in the UK to storage sites offshore.