PhD Thesis Defence: Ms. Smita Kamble (13/03/23)
Study of gas-fine flow in a packed bed with an application to the ironmaking blast furnace
Prof. Govind S Gupta
13th March, 2023 (Monday), 11:00 AM (India Standard Time)
KPA Auditorium, Department of Materials Engineering
The iron and steelmaking industry uses coal as the primary reducing agent. The carbon that is the major component of coal is finally released into the environment as carbon dioxide. Therefore, energy consumption in this industry is equivalent to carbon dioxide emission, and various efforts, to improve energy efficiency, have been made. One of the effective ways to reduce the coke consumption and thus the reduction in greenhouse gases is to introduce coal or other carbonaceous materials through the tuyere. However, this can be done only up to some extent as injection of these materials reduces the bed permeability which in turn affects the operation and productivity of the blast furnace (BF).Previous studies have shown that at a higher pulverised coal injection (PCI) rate and depending on the operating conditions of the furnace, some amount of coal remains unburnt and consequently the ashes and coal particles, in the form of powders, may be entrained in the gas stream or be deposited into the lower zones of the BF. Degradation of coke in this region also introduces coke fines which increases more on the introduction of pulverised coal (PC). The powder holdup reduces the permeability of the coke layer and in some cases results in detrimental effects on the gas flow and liquid drainage. This situation becomes more complex in presence of a cohesive zone. These phenomena lead to lower production efficiency and unstable operations. However, the physics of the pulverised coal within the furnace is still not well understood, especially in presence of raceway and tuyere protrusion. Raceway is very important in iron-making BF as its size and shape determines the aerodynamics of BF and thus the heat and mass transfer. This is the reason that the productivity of a BF has been related to its raceway. The literature reveals that the raceway formed in the stationary packed bed under the decreasing gas velocity condition is more relevant to the blast furnace or moving bed condition. In decreasing gas velocity conditions, the gas velocity is raised above the desired velocity and then is reduced to the desired velocity. No study on two-phase flow is available under the decreasing gas velocity condition. Also, no study is available on two-phase (gas-fine) flow in the presence of raceway and tuyere (nozzle) protrusion inside the packed bed. Therefore, the study of fine flow in a packed bed is a necessary precursor to understand the above mentioned processes/phenomena.
For the validation purpose, two-dimensional experimental studies were performed at room temperature on the various packed bed such as a rectangular bed with and without a cohesive zone and a cylindrical bed without a cohesive zone. Both the packing and fine particles were made of glass material. Steady state of experiment with fines was achieved based on the recently developed accurate mass balance method which dictates that fines into the system must be equal to fines out of the system. All similarity criteria with respect to the BF have been observed reasonably except thermal and chemical. All the experiments carried out in the current study are under the decreasing gas velocity condition which is more relevant to the BF study at room temperature in the laboratory.
Most research neglect vital characteristics of BF, such as lateral inflow, presence of tuyere protrusion, the raceway’s shape and size, and cohesive zone leading to a significant incongruence with the BF being modelled. The present work incorporates these features into a 2D numerical study of a gas–fines–solid system. The mathematical modelling considers the gas and fines as an Eulerian–Eulerian system with the constant voidage model for the solid phase representing the packing particles. In presence of a cohesive zone, the cohesive blocks were assigned zero porosity. Well-established theoretical relations and correlations are used to determine the inter-phase forces and fines accumulation regions. Particular emphasis is placed on the accurate representation of the raceway formed at the tuyere exit, and three approaches are considered, viz., its absence, a correlation-based prediction, and an iso-stress-based model. The effect of various parameters such as gas inlet velocity, packing particle material, packing particle size, fines flux through the tuyere opening, and fines size is simulated and analysed in detail with a marked interest in the raceway shape and size and accumulation profiles of the fines which are important parameters in the overall flow characterization. An insight into the physics based on the interaction forces between the phases, which is difficult to obtain experimentally, is offered to explain the change in the shape and size of the raceway and the accumulation profile observed as the parameters mentioned above are varied. Additionally, structural parameters such as cohesive zone configuration, cohesive block porosity, and tuyere protrusion are also varied, and their effect on the raceway and static holdup profiles are characterised. The sensitivity analysis shows that the raceway shape and size play a vital role in the flow of gas-fines and influence the accumulation of fines. While most of the parameters varied have a noticeable effect, certain factors such as fines feed rate and size exhibit negligible consequences.
The model predicts the static holdup profile through a correlation based on experimental data. Developed model is able to predict the pressure loss reasonably well against experimental and published data for both clean gas and gas–fine flow in a packed bed with a lateral and bottom injection of gas and fine. Similarly, the model is able to predict correctly the raceway shape & size, pressure profiles in both vertical and horizontal directions as well as fines static holdup both in the presence and absence of cohesive blocks. All the results indicate an excellent agreement with the experimental data and lend credibility to the model and solver.