Numerical modelling of multiphase flow in raceway of ironmaking blast furnace

摘要

Pulverized coal injection (PCI) technology is widely used in ironmaking blast furnaces due to its various benefits. Therefore it is natural to seek further improvements to maximise its benefits. An extensive study has discovered that PCI technology can be further improved by co-firing coal with coal blends, charcoal and other injectants. Effects of these injectants have been experimentally studied to reveal the complex in-furnace physicochemical phenomenon. However these studies are insufficient in describing the key governing phenomena. Hence numerical studies is preferred to overcome the limitations. In the first half of this thesis, the combustion behaviours of coal blends and charcoal are investigated in a pilot-scale test-rig using a three-dimensional computational fluid dynamics (3D CFD) model. The results show that the combustion efficiency of ternary blends is higher than binary blends by 4-8% for the same fraction of highest volatile coal due to the higher synergistic effect. The combustion efficiency of charcoal is found to be higher than coal by 10% for the same volatile matter content and sizing due to the faster char combustion. A raceway is a cavity formed from the injection of a high speed jet into the packed coke bed. Blast furnace operation is heavily dependent on the supply of heat and gas from the raceway to the surrounding coke bed for the smelting purpose. Therefore when the PCI is used, the combustion of coal inside the raceway is important for determining the heat and gas distributions. The majority of past studies have focused on the effect of PCI technology on the raceway formations. Therefore in the latter half of this thesis, the effects of raceway shape and size on the PCI performances are investigated. The PCI in the raceway is simulated in a full-scale 3D CFD model. The results indicate that the combustion efficiency of coal can be improved with an enlarged recirculation zone and/or shortened jet and their extension. Increasing the raceway size is also found to improve the combustion efficiency as the particle travelling time is extended. The findings in this thesis are helpful to understand and optimize PCI operations in practice.