Numerical simulations and experiments on heat transfer around a probe in the downer reactor for coal gasification

The heat transfer performance of a downer reactor has great significance for coal gasification processes due to the very short residence time of coal. In this study both experimental and numerical works were carried out to study heat transfer around a heating probe in a downer. Both experimental results and numerical results revealed that average heat transfer coefficients decreased with increasing superficial air velocity or decreasing solid mass flux, which could be attributed to decreasing solid holdup, as particle–particle convection was dominant in the heat transfer mechanism for the current cases. Numerical simulations also revealed that heat transfer may deteriorate with increasing particle size. Finally it was found that the heat transfer performance with constant temperature boundary condition around the probe was much better than that with constant heat flux boundary condition. These results may help us better understand heat transfer in downers for improving the design of downers with higher efficiencies.

Distribution of temperature around probe under constant heat flux in the downer reactor for coal gasification. The solid holdup on the upper surface of the probe was much higher than that on the lower surface. Most of the sand particles flowing from the upstream region of the downer would deposit on the upper surface of the probe.Figure optionsDownload as PowerPoint slideHighlights
► Heat transfer mechanism between gas–solid flow and inserted surface was revealed.
► Heat transfer coefficients increased with increasing solids holdup in downer.
► Developing region had better heat transfer performance than developed region.
► Constant surface temperature condition led to better heat transfer performance.