Hydrogen direct reduction ironmaking process for zero co2 emission: a multiscale study of the non-catalytic gas-solid reactions.

Author(s)

Patrícia Metolina

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
• Defense date: 2024-09-13

Examining board members

Roberto Guardani; José Adilson de Castro; Gabriela Cantarelli Lopes; José Luis de Paiva; Tiago Ramos Ribeiro

• Advisor: Roberto Guardani

Abstract

The direct reduction of iron ore with hydrogen (H-DR) is a promising technology for achieving carbon neutrality in the steel industry, providing an immediate solution to replace the traditional carbon-intensive iron production process. Advancing this technology requires a comprehensive understanding of reaction and transport mechanisms across multiple scales, as the performance of the process at the industrial macroscopic scale is significantly affected by solid transformations driven by non-catalytic gas-solid reactions at the microstructural scale of the pellets. This study investigates the H-DR process through an experimental approach and the development of a detailed mathematical model, encompassing both pellet and reactor scales. At the pellet scale, a multivariate experimental design of direct reduction of industrial hematite pellets using pure hydrogen was investigated to evaluate the effect of pellet diameters (10.516.5 mm), porosity (0.360.44), and temperature (6001200°C) on the reduction rate of the process. A strong interactive effect between temperature and pellet size was observed, indicating that these variables cannot be analyzed independently. Increasing the temperature and decreasing the pellet size significantly favor the reduction rate, while the effect of porosity within the studied range was not relevant. Changes in pellet size during the reduction were negligible, except at temperatures above 1000°C due to crack formation. Microscopic analysis of complete and partial reduction of iron oxide pellets at temperatures from 600 to 900°C revealed the significant impact of temperature on microstructural changes during reduction. Furthermore, isothermal and non-isothermal mathematical models for porous pellets were developed based on the finite element method to describe the reaction and transport mechanisms of the H-DR process. The models considered the grain model, including the three reduction reactions and changes in porosity, as well as solid transformations along the pellet radius during reduction. Simulation results under different conditions showed good agreement with experimental data, and a significant improvement in model prediction was observed when changes in pellet tortuosity were considered, particularly at higher temperatures. Additionally, simulations of solid and gas concentrations and temperature profiles along the pellet radius during the reduction process highlighted the transient behavior of the process. At the reactor scale, a multiscale moving-bed reactor model was developed to simulate non-catalytic gas-solid reactions and evaluate parameters at the pellet scale that hierarchically impact the performance at the reactor scale. The reactor and solid particles were treated as two distinct coupled physical domains, enabling the computation of solid transformations and gas flow distribution along both scales. Since no commercial H-DR data was available, the model was successfully validated using the MIDREX direct reduction process. Predictions for the H-DR process highlight the substantial impact of H2 molar flow on reactor performance. A sensitivity multivariate analysis revealed the significant effect of pellet structural characteristics and gas inlet operational conditions, such as temperature, molar flow rate, and gas concentration. Increased pellet porosity offers flexibility for process optimization, enabling the use of lower H2 percentages and temperatures. Additionally, 2D simulations of the multi-scale reactor showed that the H-DR process exhibits greater heterogeneity in gas concentration and temperature distribution compared to existing industrial direct reduction processes. These findings offer critical insights for system engineering, enabling the optimization of operational conditions and the design of pellets with ideal characteristics for the H-DR process, thereby advancing the implementation of CO2-free technologies. (AU)

• FAPESP's process: 19/05840-3 - Mathematical modeling and computational simulation of gas-solid reactions in moving bed: application to the direct reduction process of iron ore
• Grantee: Patrícia Metolina
• Support Opportunities: Scholarships in Brazil - Doctorate