Proceedings of the 9th World Congress on Momentum, Heat and Mass Transfer (MHMT 2024)
London, United Kingdom – April 11 – 13, 2024
Paper No. ICMFHT 145
DOI: 10.11159/icmfht24.145

Dynamics of Reactive Particles in Supercritical Water Turbulence
Bofeng Bai, Kunpeng Zhao, Shuaiqi Zhao, Rui Zhang, Han Huang
State Key Laboratory of Multiphase Flow in Power Engineering
Xi’an Jiaotong University, Xi’an, China
bfbai@mail.xjtu.edu.cn

Extended Abstract

Coal consumption yields over 15 billion tons of CO2 annually, which is the largest source of CO2 emissions in the world.
Supercritical water gasification (SCWG) is a promising technology to convert coal into hydrogen cleanly, and it could capture
the pure CO2 as well as the inorganic salts synchronously. The large-scale industrial application of SCWG urgently needs toimprove the reaction rate of hydrogen production in the reactor, in which the time and space scales of multiphase flow, heattransfer, and chemical reaction are multiple and extremely mismatched. However, the theoretical guidance and basic models
are lacking.
In the present research, we established a multi-scale kinetic theory to explain the physiochemical processes in the reactorof hydrogen production, including the heterogeneous and homogeneous reactions inside the micro-scale particles, theinteraction between reactive particles, as well as the macro-scale fluidization in the multiphase multicomponent supercriticalfluid. The main conclusions are as follows.
1) For the reaction of individual particles, we experimentally found that the reaction mainly occurs in the pores ofparticles in SCWG[4], which has been unable to be explained by previous researches which ignore the homogeneous reaction
and the multi-species diffusion inside the reactive particles. A three-dimensional model coupling reaction and diffusion isthereby proposed to describe the multiple physical processes and chemical fields inside the non-spherical porous particles.
We observe that the intrapore reaction inhibits the gasification of reactive particles by slowing down the fluid diffusion andretarding the fixed-carbon steam reforming heterogeneous reaction. Increasing gasification temperature enhances theinhibition by particle intrapore reaction while increasing the mole fraction of H2O weakens the inhibition.
2) For the motion of reactive particles, we found that the aggregation of coal particles significantly reduces the contact
area between particles and supercritical water, which is the other key factor to impede the reaction. We proposed a four-way
coupled model to analyze the interaction between particles, as well as between particles and supercritical fluids[1]. We foundthat the flocculation of particles proceeds most rapidly when the fluid and particle time scales are balanced[2]. The
Kolmogorov scale of turbulence effectively inhibits the growth of floc. Adjusting the Kolmogorov scale to be close to theparticle size can greatly limit the flocculation of coals and improve the reaction rate[3].
3) For the macro-scale fluidization, an optimal slurry feed rate and a temperature control strategy are proposed to achieve
the highest reaction rate by balancing the transport rate of fluid species and the aggregation rate of reactive particles. The
research found the role of natural convective vortices in the reactor in promoting the fluid-solid reaction. It is clarified that thehydrogen production rate at the reactor outlet increased with the increase of the supercritical water flow rate, and the optimal
interval of the ratio of the supercritical water flow rate to the coal slurry feed flow rate was given.

References[1] K. Zhao, B. Vowinckel, T.-J. Hsu, T. Köllner, B. Bai, E. Meiburg, "An efficient cellular flow model for cohesive particle
flocculation in turbulence" Journal of Fluid Mechanics, vol. 889, no. R3, 2020.
[2] K. Zhao, F. Pomes, B. Vowinckel, T.-J. Hsu, B. Bai, E. Meiburg, "Flocculation of suspended cohesive particles inhomogeneous isotropic turbulence" Journal of Fluid Mechanics, vol. 921, no. A17, 2021.
[3] K. Zhao, B. Vowinckel, T.-J. Hsu, B. Bai, E. Meiburg, "Cohesive sediment: intermediate shear produces maximum
aggregate size" Journal of Fluid Mechanics, vol. 965, no. A5, 2023.
[4] R. Zhang, S. Zhao, C. Sun, H. Jin, K. Zhao, B. Bai, "Particle morphology evolution and its enhancement for lignite
gasification in supercritical water" Industrial & Engineering Chemistry Research, vol. 62, pp: 16268−16279, 2023.

ICMFHT 145-1