Ironmaking blast furnace industry involves many complex thermochemical processes including, for example, multiphase flows, heat and mass transfers and chemical reactions. It is essential to understand, visualize and optimize the in-furnace phenomena for competitiveness and sustainability under increasing economic and environmental demanding conditions. Mathematical modelling, facilitated by physical modelling, provides a cost-effective way of achieving this goal. In particular, multiscale modelling is widely used in academia and industry for their proven effectiveness. This paper will review our recent CFD- and DEM-based process models and discuss their roles in the development of new technology for sustainable ironmaking. Some recent examples are used: 3D modelling of blast furnace with raceway, 3D modelling of pulverised coal injection, and 3D modelling of cokemaking. It is demonstrated that mathematical modelling indeed plays a significant role in process understanding and optimisation, vital to sustainable modern ironmaking.
Hydrogen production by coal gasification in supercritical water is a promising way of coal utilization due to its clean, efficient and low-carbon characteristics. How to further lower the temperature for complete conversion of carbon in supercritical water is a hot topic. Extensive theoretical and experimental investigations were conducted in State Key Laboratory of Multiphase Flow in Power Engineering since 1997.
(1) The rate-determining steps for the supercritical water gasification of coal particles were obtained at different spatial scales. Experimental device such as supercritical water visual platform that combining Raman spectrometer was established to reveal the microscopic kinetic mechanisms. Density functional theory (DFT) and reactive empirical force fields (ReaxFF) were combined to investigate the rate-determining steps and its reaction enhancement strategy. The directional gasification characteristics can be obtained by coordination regulation and control method of temperature and pressure for chemical/transport properties of supercritical water.
(2) A fluidized reactor operating in supercritical water condition was invented. A series of theories of multiphase flow, heat/mass transfer, and chemical reaction gasification in supercritical water fluidized bed reactor were established. By the approach of the coordinated matching of mass flow and energy flow, the primary reactions were enhanced while the side reactions were suppressed.
(3) A demonstration plant was constructed to verify the above-mentioned theories. The carbon content in coal is completely gasified below 670 °C. The C, H, O content in typical coals can be converted to H2 and CO2, while other impurities are flushed off in clean, non-toxic and inert ashes. The demonstration plant operation has lasted continuously and stably for more than ten thousand hours, which lays a foundation for large scale industrialization and widespread application, so as to provide a new way for the solution of energy shortage problems, haze controlling and the realization of a clean and efficient utilization of coal.
Non-oxidative dehydroaromatization can convert methane into important aromatic products, such as benzene toluene and naphthalene, and produce large amounts of hydrogen simultaneously. A pilot-scale methane dehydroaromatization—H2 regeneration fluidized bed system (MDARS) has been developed. The catalyst circulates between two fluidized beds which are methane dehydroaromatization reactor and catalyst H2 regeneration reactor. The fluidization and energy transportation including mechanism of catalytic reactions are modelled in the Eulerian-Lagrangian Multi-Phase Particle-in-Cell (MP-PIC) methodology, also called Computational Particle Fluid Dynamics (CPFD). MP-PIC model uses a stochastic particle method and an Eulerian method for the fluid phase to solve equations for dense particle flow.
The mechanisms of methane dehydroaromatization reaction and catalyst deactivation reaction were investigated by fixed bed reactor and agreed with the experiment data. The whole system with two reactors and one cyclone was simulated in one model so that the interaction between two fluidized beds and catalyst deactivation can be investigated. The simulation results indicated that the catalytic activity remain stable, and the optimal regeneration-reaction ratio is 8 which consistent with the experiment results. The influence of catalyst particle size and catalyst loading were also investigated. The results showed that the small catalyst particle size can increase the methane conversion and products formation rate. The suitable catalyst loading is between 20~30 gram in consideration of the conversion and formation rate per catalyst mass. More operating conditions can be varied over a wide range to optimize and scale-up the methane dehydroaromatization—H2 regeneration fluidized bed system.
The downer reactor, in which gas and solids move downward in a co-current way, has attracted many attentions in the past two decades due to its unique features such as shorter residence time, narrower residence time distribution, less solids back-mixing and lower pressure drop since gravity acts in the same direction with the flow direction of gas and solids when compared to the flow behaviors in riser. How to increase the solids holds-up in a downer is still an important issue. In this talk, solids volumetric flux (Vs), which is expected to replace the solids mass flux (Gs) as the key factor influencing solids holdup in circulating fluidized beds, was proposed to investigate the solids holdup variation and predict the extreme operation conditions in the downers. It is considered that using Vs to replace Gs to define high-density operation in the downer could be more suitable in this study To increase the solids-up in the downer, the flow behaviors in a series of novel gas-solids co-current downflow conical fluidized bed which is expected to realize a high-density solids holdup along a downer-type pyrolyzer to strengthen the heat transfer efficiency, were systematically investigated by means of a numerical approach. In addition, A Eulerian–Eulerian model incorporating the kinetic theory of granular flow was adopted to simulate the gas-solids flow behaviors in a dense downer below a conventional downer, which could be used for the further pyrolysis of coal and/or decomposition of tar on the generated char before the char and tar are completely separated in a triple-bed combined circulating fluidized bed (TBCFB) system.
(Acknowledgments:This work is supported by the JSPS KAKENHI Grant-in-Aid for Scientific Research B (Kiban B, 17H03451), Japan and the National Natural Science Foundation of China (U1710101)
Biomass is a carbon-neutral fuel and has the potential to replace coal in ironmaking blast furnaces (BFs) under the carbon-constrained environmental policy conditions. However, the flow and thermochemical in-furnace phenomena related to biomass combustion are not clear at industrial scale BF conditions yet. In this study, a three-dimensional (3D) industrial scale computational fluid dynamics (CFD) model is developed for describing the flow and thermo-chemical behaviours related to charcoal injection into the lower part of a BF under the real BF conditions. The computational domain includes lance, blowpipe, tuyere, raceway and the surrounding coke bed regions. The model features characteristics of the industrial scale BF and charcoal materials, including the real dimensions, operating conditions, bird's nest within the raceway, coke bed around the raceway, non-spherical particle shape of charcoal particles, modified sub-models of charcoal chemical reactions. The simulation results show that the charcoal combustion process can be classified into 5 stages based on the evolutions of the gas species, burnout, fuel gas and gas temperature along the particle plume. The behaviour of different charcoal particle size groups varies considerably within the raceway. The combustion profiles of the charcoal and two typical PCI coals are then compared. It is indicated the burnout profiles comparable qualitatively, confirming the potential of charcoal utilization in PCI technology, whereas the temperature and gas species profiles are different from typical PCI operation quantitatively, indicating the charcoal injection and its control strategy should be redesigned in BF practice, for example, the use of finer mean particle size. This industrial scale model is useful for understanding the combustion behaviour of pulverized charcoal and optimising biomass injection operations.