Experimental study of wall-to-bed heat transfer in a supercritical water fluidized bed

Supercritical water (SCW) fluidized bed is a new reactor concept for hydrogen production from biomass or coal gasification. This paper presents an experimental study of wall-to-bed heat transfer characteristics for Geldart-B particles in SCW fluidized bed with the conditions of pressure ranges from 22.3 MPa to 25.7 MPa and temperature ranges from 651 K to 730 K. The effects of velocity, temperature and pressure on wall-to-bed heat transfer coefficient in SCW fluidized bed were discussed. The wall-to-bed heat transfer coefficient was found to be positively correlated with velocity and pressure, while negatively correlated with temperature within present experimental conditions. A dimensionless analysis was conducted using the Buckingham-theorem to derive the general form of an empirical wall-to-bed heat-transfer correlation in supercritical water fluidized bed for the Nusselt number, which was finalized based on the experimental data. What's more, the sold phase contributes more to the wall-to-bed heat transfer in SCW fluidized bed and the presence of solid phase enhances the heat transfer of SCW phase, which can be concluded from comparison of heat transfer between SCW fluidized bed and single phase of SCW. The experimental results and the proposed correlation in present work are useful for the design of SCW fluidized bed reactor.