Blast furnace operation
Blast furnaces are widely used in the iron-making industry to produce iron and other base metals. Using blast furnaces, raw feed materials such as fuel, ore, and limestone are continuously charged onto the top of a blast furnace, while air is blown into the lower section of the furnace through the tuyere, so that the chemical reactions take place throughout the furnace as the feed materials move downward.
This process produces molten metal and slag phases that are extracted from the bottom of the blast furnace. It is a typical multi-phase counter-current exchange process involving the downward flow of the solid feed materials and melted metals in the liquid phase, and an up flow of hot, carbon monoxide-rich combustion gases.
The running of blast furnaces consumes 60 per cent energy of the whole steel industry and is responsible for 90 per cent CO2 emission to the environment from a typical steel plant. In order to reduce the energy consumption and CO2 emissions, the development of a thorough scientific understanding of the operation of blast furnaces and of science-based predictive tools for optimizing the operation of blast furnaces are urgently needed. Since blast furnaces are operated at elevated temperatures (1100 Celsius) and at large scales (4000m3, up to 400 tonnes per hour), conventional visualisation and monitoring techniques are incapable for such a challenging process. In particular, it is difficult, even impossible, to experimentally explore what is happening inside. Due to the complex physical process obtaining empirical solutions is also a non-trivial task.
The blast furnace operation is depicted in figure one below. The blast furnace is powered by hot air and oxygen that enters the blast furnace through the tuyeres to form a combustion zone that heats the layers of coke and iron ore. The heated iron ore softens, melts and drips under gravity to collect at the bottom of the blast furnace for further processing. The blast furnace is continuously charged with coke and iron ore by a hopper at the top of the blast furnace.
Blast furnace charging
The permeability of burden distribution is influenced by the charging process and the geometric properties of the feed particles. Therefore, a thorough understanding of the behaviors of burden formation and descent is critical in developing appropriate strategies for energy-efficient blast furnace operations.
The furnace should be charged in such a way that the coarse and fine particles are distributed appropriately on the burden surface to control the burden distribution, so that the gas utilisation of the furnace can reach the optimal and the furnace can be operated in a more energy-efficient manner. Ideally, in order to increase gas utilisation, the sinter of small sizes should be discharged in the periphery area for protecting the lining and preventing excessive heat losses, and particles of large sizes in the central area to form a strong central gas flow.
In addition, the mean size of the sinter should gradually increase from the periphery to the center in order to attain an increasing gas distribution from the periphery area to the central area (a weak gas flow at the wall area and a strong gas flow in the central line of furnace). Creating a thin iron-bearing material layer at the periphery with a small quantity of small size sinters leads to a thin cohesive zone at the lower part of furnace and a reduced pressure drop of whole furnace.
Although the benefit of the controlled burden distribution (i.e. burden of a controlled microstructure achieved with an optimal distribution of feed materials) has been anticipated, it is a challenging task to realise it in practice. Hence, only the height of the burden layer is currently controlled in the blast furnace through the control of the mass flow rate in the discharging system. Therefore, it is of great scientific and commercial interest to study the burden distributions and packing structure accurately.
Figure two below is a graph that depicts the various charging systems for blast furnaces, we are considering in this project.
Of the first charging systems developed was the single bell systems that later developed into the double bell systems as depicted in figure two (a). This was followed by the bell-less top charging systems with parallel hoppers as depicted in figure two (b). This allows for near continuous blast furnace charging that was lately further refined into gimbal charging systems by Siemens-VAI as depicted in figure two (c).