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.