Mathematical modelling of the flow and combustion of pulverized coal injected in ironmaking blast furnace

Pulverized coal injection (PCI) technology is widely practised in blast furnace ironmaking due to economic, operational and environmental benefits. High burnout of pulverized coal in the tuyere and raceway is required for high PCI rate operation. A comprehensive review reveals that although there have been a variety of PCI models, there is still an evident need for a more realistic model for PCI operation in blast furnace.Aiming to build a comprehensive PCI model of a full-scale blast furnace, this thesis presents a series of three-dimensional mathematical models, in terms of model development, validation and application, in a sequence from a pilot-scale to a full-scale, from a simple to complicated geometry, from a coal only system to a coupled coal/coke system. Firstly a three-dimensional model of pulverized coal combustion is developed and applied to a pilot-scale PCI test rig. This model is validated against the measurements from two pilot-scale test rigs in terms of gas species composition and coal burnout. The gas-solid flow and coal combustion are simulated and analysed. The results indicate that the model is able to describe the evolutions of coal particles and provide detailed gas species distributions. It is also sensitive to various parameters and hence robust in examining various blast furnace operations. This model is then extended to examine the combustion of coal blends. The coal blend model is also validated against the experimental results for a range of coal blends conditions. The overall performance of a coal blend and the individual behaviours of its component coals are analysed. More importantly, the synergistic effect of coal blending on overall burnout is examined and the underlying mechanisms are explored. It is indicated that such synergistic effect can be optimized by adjusting the blending fraction, so as to compensate for the decreased burnout under high coal rate operation. The model provides an effective tool for the optimum design of coal blends.As a scale-up phase, the coal combustion model is applied to the blowpipe-tuyereraceway region of a full-scale blast furnace, where the raceway is simplified as a tube with a slight expansion. The in-furnace phenomena are simulated and analysed, focusing on the main coal plume. The effect of cooling gas conditions on combustion behaviours is investigated. Among the three types of cooling gas (methane, air, and oxygen), oxygen gives the highest coal burnout.Finally, a three-dimensional integrated mathematical model of pulverized coaVcoke combustion is developed. The model is applied to the blowpipe-tuyere-raceway-coke bed region of a full-scale blast furnace, which features a complicated raceway geometry and coke bed properties. The model is validated against the measurements in terms of coal burnout from a test rig and gas composition from a blast furnace, respectively. The model gives a comprehensive full-scale picture of the flow and thermo-chemical characteristics of PCI process. The typical operational parameters are then examined in terms of coal burnout and gas composition. It is indicated that the final burnout along the tuyere axis is insensitive to some operational parameters. The average burnout over the raceway surface can better represent the amount of unburnt coal particles entering the surrounding coke bed and it is also found to be more sensitive to the changes of most parameters. In addition, the underlying mechanisms of coal combustion are obtained. The coal burnout strongly depends on both oxygen availability and residence time. The existence of recirculation region gives a more realistic coal particle residence time and burnout. Compared with the fore-mentioned two models, this model is considered as a more comprehensive model of PCI operation for understanding the infurnace behaviours and provides more reliable information for the design of operational parameters

Three-Dimensional Simulation of Combustion Behaviour of Pulverised Coal Injection in A Blast Furnace

Pulverized coal injection technology is widely used in blast furnace ironmaking due to economic, operational and environmental benefits. High burnout within the raceway is required for high coal injection rate operation. In order to analyze the flow and combustion in the tuyere and raceway, a three-dimensional model of coal combustion has been developed, considering details such as an inclined coaxial lance, three streams of different gases and various heterogeneous reactions of char particles. The model is first validated in connection with our previous study. The effects of coal properties and other typical operational parameters on the coal combustion efficiency are then investigated. The results indicate: (1) typical combustion and flow phenomena can be simulated successfully; (2) devolatilization plays a dominant role in coal combustion efficiency; (3) various measures for burnout improvements have been investigated and a linear dependence of overall burnout can be found for particle size distribution, volatile content and oxygen enrichment into the blast. Notable improvements in combustion efficiency are also obtained for coal with more fine particles and high volatile matter, and by increasing blast temperature and oxygen enrichment. The model offers a cost-effective method to examine the effects of variables related to different BF operations, which is useful for blast furnace design and control

Application of a Coal Combustion Model in the Design of Blast Parameters for an Ironmaking Blast Furnace

With significant economic drivers to reduce consumption of expensive coking coal, Pulverized Coal Injection (PCI) commenced at BlueScope Steel in 2002, at injection rates ranging between 100 and 150 kg-coal/tonne of liquid iron. The key limitation to injection rates is associated with the reduction in packed bed permeability via additional char load into the furnace. The coal is injected via a simple co-axial lance, consisting of an inner pipe (for coal and carrier gas) and an outer annulus (for cooling gas to protect the lance from the high furnace temperatures). The cooling gas can be compressed air, natural gas or pure oxygen. Depending on the choice of cooling gas, the oxygen-to-carbon ratio of the system will change. In this paper, the application of a validated three-dimensional numerical model of the blowpipe/tuyere/raceway is described. The model is used for various plant-specific investigations of blast parameters such as oxygen enrichment, blast temperature and atomic oxygen-to-carbon ratio. The model results show the sensitivity of coal burnout to different operating parameters and confirm that burnouts higher than 80% are difficult to obtain due to the short residence times of the coal

Three-dimensional modelling of coal combustion in blast furnace

Pulverized coal injection technology is widely used in blast furnace ironmaking due to economic, operational and environmental benefits. High burnout within the tuyere and raceway is required for high coal injection rate operation. In order to analyze the flow and combustion in the tuyere and raceway more accurately and reliably, a three-dimensional model of coal combustion is developed. This model is validated against the measurements from two pilot scale test rigs in terms of gas species composition and coal burnout. The gas-solid flow and coal combustion are simulated and analysed. The results indicate that compared to our previous model, the present model is able to provide more detailed gas species distributions and better describe the evolutions of coal particles. It is more sensitive to various parameters and hence more robust in examining various blast furnace operations