A Simulation And Theoretical Study Of Heat Capacities For Supercritical H₂O/CO₂ And CO₂/H₂ Mixtures

Coal as a primary energy source plays an important role in the global energy structure.The development of efficient and clean coal utilization technologies is of great significance for solving the problems of sustainable energy development and environmental pollution.Supercritical water coal gasification technology,as a novel clean coal technology,can convert hydrogen and carbon elements in coal into H₂ and CO₂,and decompose part of the water into hydrogen,so as to achieve efficient and clean utilization of coal.H₂O/CO₂and CO₂/H₂ mixtures are an important working fluid in the thermal cycle of the system.The supercritical H₂O/CO₂ mixture can enter the steam turbine to perform work to drive the generator to generate electricity.The CO₂/H₂mixture can be separated to obtain H₂with high heating value.Therefore,studying the thermodynamic properties of mixture is of great significance for the design and application of the development of supercritical water gasification hydrogen production technology.This paper uses simulation and theoretical calculations to predict the heat capacity properties of pure working fluids,and CO₂/H₂,H₂O/CO₂ binary mixtures,the prediction results were compared with the NIST database.The main work of this article is as follows:(1)The heat capacities of multiple pure force field models are simulated by molecular dynamics methods,and compared the simulation results with the NIST database.By comparing the simulation results of various molecular force field models of pure H₂O and CO₂,the force field model with the smallest error is preferred,in which water molecules are TIP4P model and carbon dioxide molecules are TraPPE-flex model.For hydrogen use common Two-site models.(2)The system model of the mixtures is established according to the optimized pure working force field model,and properties of isobaric heat capacity,isochoric heat capacity,volume expansivity,isothermal compressibility of the H₂O/CO₂ and CO₂/H₂binary mixtures are simulated by molecular dynamic.At the same time,the Peng-Robinson(PR)cubic state equation are used for the theory Calculation.Comparing the simulation and theoretical calculation results with the NIST database,it is found that the average error between the simulation results and the theoretical calculation results of the H₂O/CO₂ binary mixed working fluid is less than 5%.However,in near-critical region for pure supercritical H₂O and at lower mole fractions of CO₂,the molecular dynamics simulation method is more accurate than the PR state equation.In addition,the prediction results of the CO₂/H₂ binary mixed working fluid show that their average absolute relative errors are within 2%.(3)The microscopic properties of mixtures are further analyzed by molecular dynamics methods,including size effects,radial distribution functions,and hydrogen bonding in H₂O/CO₂ mixtures.The analysis shows that hydrogen bonding has a certain effect on the prediction of the heat capacity of the H₂O/CO₂ mixed working fluid.When the large number of hydrogen bonds in the system,molecular dynamics simulation is more accurate than the theoretical calculation method using the PR equation of state.