Research progress of coal utilization technology based on supercritical water gasification for hydrogen production

Coal will continue to play an important role as energy source in the near future in China. Traditional com⁃ bustion utilization of coal inevitably produces large amounts of SOx， NOx， dust and greenhouse gas， which are main causes of environmental problems such as haze and global warming. A coal⁃to⁃hydrogen power generation multi⁃generation process based on supercritical water gasification was originally proposed by the team of Liejin Guo at the State Key Laboratory of Multiphase Flow in Power Engineering， Xi’an Jiaotong University， China. Coal is com⁃ pletely gasified in a supercritical water homogeneous environment to produce hydrogen⁃rich mixtures. The hydrogen is completely oxidized to form supercritical H2O / CO2 mixed working fluid which is then sent into the turbine for pow⁃ er generation. The exhaust gas from the turbine is separated and the carbon dioxide is further captured. The develop⁃ ment status of the key technologies in the schemes were reviewed， especially the research progress in the last decade by the State Key Laboratory of Multiphase Flow in Power Engineering， Xi’an Jiaotong University. In terms of strength⁃ ening the gasification process of coal in supercritical water， it was revealed that the ring⁃opening reaction of polycyclic aromatic hydrocarbon is the rate⁃limiting step of coal gasification. The gasification model of porous coke particles in su⁃ percritical water was established and the gasification mechanism was obtained. The gasification rate of coal was in⁃ creased by about 17% by circulating the fluid with high free radical concentration. The low⁃temperature and low⁃veloc⁃ ity side reaction zone was reduced by adjusting the nozzle jet angle. In⁃situ coupling matching of endothermic⁃exother⁃ mic reactions was achieved by adding catalysts. In terms of multiphase flow and heat transfer in a supercritical water fluidized bed， a new energy minimum multi⁃scale (EMMS) drag model was proposed based on the experimental re⁃ sults of flow characteristics of mesoscale structures. The characteristics of fluidized bed internal flow pattern evolution and the corresponding calculation criteria were obtained based on the pressure difference experimental data. The dy⁃ namic characteristics of bubbles in the fluidized bed were revealed and the calculation criteria of bubble chord length and rising velocity were presented. The bed⁃wall heat transfer correlation and the heat transfer model based on the par⁃ ticle renewal theory in the homogeneous expansion were established. In terms of kinetics of hydrogen oxidation in su⁃ percritical water， the reaction pathways and kinetic parameters of hydrogen oxidation were obtained from the atomic level sing the reactive force field (ReaxFF) method. The global reaction rate expression of hydrogen oxidation was es⁃ tablished based on the experimental data. An elementary model of hydrogen oxidation in supercritical water was estab⁃ lished， and the reaction path analysis and sensitivity analysis were conducted. The above research results provide a guidance for the regulation and strengthening of supercritical water coal gasification reaction， and the design of gasi⁃ fication reactor and oxidation reactor.