Prediction Of Transport Properties Of H₂O/CO₂ Mixtures In The Near-Critical Regions Of Water

Due to the abundance of coal and less oil and gas,the coal-based energy structure in China will not change in the short term.Today,most coal is consumed by burning directly in the air,and inevitably produces pollutants such as NO_X,SO_X,PM2.5,etc.,causing serious environmental problems.The negative impact of coal utilization on the environment and the associated costs of maintaining environmental safety have inhibited the growth of coal consumption,which has stimulated research into clean and efficient coal utilization methods.Among them,supercritical water coal gasification technology is an important way for efficient and clean utilization of coal.In the process of supercritical water coal gasification,organic matter in coal is mainly converted into H₂ and CO₂ and some inorganic matter is precipitated,and the H₂O/CO₂ supercritical mixed working fluid produced by this technology can enter a steam turbine to generate electricity.Therefore,obtaining the viscosity,thermal conductivity,and self-diffusion coefficient of the H₂O/CO₂ binary mixed working fluid in the vicinity of water and supercritical conditions has a significant effect on the design of pipelines,heat exchangers,and steam turbines in supercritical water gasification power generation systems.However,there is still no corresponding research on the viscosity,thermal conductivity,and self-diffusion coefficient of H₂O/CO₂ binary mixed working fluids in the vicinity of water supercritical conditions.The main work and conclusions of this article are as follows:（1）The molecular dynamics method was used to study the viscosity,thermal conductivity,and self-diffusion coefficient of pure working fluids using a variety of water and carbon dioxide molecular force field models.And the simulation results are compared with NIST values and theoretical formula values.（2）Through molecular dynamics simulation calculation and comparison,the molecular force field model of pure water and carbon dioxide near the critical point of water is optimized.The preferred conclusions are as follows:for the viscosity,when the temperature is lower than 653K,the TIP4P/2005 model is selected;when the temperature is higher than 653K,the SPC/Fw model is selected;for carbon dioxide,the Cygan model is selected;for the self-diffusion coefficient,the TIP3P/Fw model is selected when the temperature is lower than 653K.When the temperature is higher than 653K,the SPC model is selected;for carbon dioxide,the Cygan model is selected;for thermal conductivity,the SPC/Fw model is selected when the temperature is lower than 653K;when the temperature is higher than 653K,the SPC model is selected.（3）H₂O/CO₂ binary mixed working fluid model was established.And the viscosity,thermal conductivity and self-diffusion coefficient of the H₂O/CO₂ binary mixture at different temperatures and different component ratios were calculated using the optimized molecular force field model of pure water and carbon dioxide.（4）The radial distribution functions of H₂O,CO₂,and H₂O/CO₂ binary mixtures are analyzed by molecular dynamics methods,and the variations of the number of hydrogen bonds in H₂O and H₂O/CO₂ binary mixtures with temperature and ratio are studied.Results show that the number of hydrogen bonds in H₂O and H₂O/CO₂ binary mixtures gradually decrease with increasing temperature,but hydrogen bonds still exist in supercritical water,As the proportion of carbon dioxide increases,the number of hydrogen bonds in the mixture decreases.