Chemical-Looping Gasification Process of Torrefied Woodchips based on Experimental and Numerical Studies

Abstract

Over the last 100 years, the average temperatures on the Earth’s surface, water, and atmosphere have risen by 0.75 °C. The temperature increase is mainly caused by human activities such as the burning of fossil fuels releasing a large amount of carbon dioxide into the air, which in turn results in an environmental concern over the world. Currently, the mitigation of carbon dioxide emissions is one of the greatest global challenges. Additionally, the energy demand worldwide has increased in the last few decades because of rapid industrialization and the ensuing improved standard of living. Biomass is abundant in the world and represents one of the most auspicious renewable energy resources to replace fossil fuels. Demand for a renewable source of hydrogen in the world leads to growing research activities in the field of biomass conversion processes. Chemical looping gasification is a novel technology to produce efficiently and sustainably valuable products such as heat, power, and hydrogen-enriched gas from biomass combined with carbon capture. This thesis demonstrates the feasibility of chemical looping gasification of biomass in a pilot-scale bubbling fluidized bed reactor. In a first step, torrefied woodchips as biomass fuel were characterized before further investigations. Non-isothermal experiments were carried out in a thermogravimetric analysis instrument to determine the kinetics of gasification of torrefied woodchips char under steam and CO₂ atmospheres. According to the experimental results, two kinetic models combined with four conversion models were implemented to determine the best fitting kinetic model for biomass char gasification. Johnson model combined with Langmuir-Hinshelwood kinetic model is the best agreement to the experimental data with more than 90 % of the coefficient of determination, R², among the combinations. Afterward, an experimental study of biomass gasification was performed in a pilot-scale bubbling fluidized bed reactor to assess the feasibility of biomass gasification under various operating conditions. It was found that hydrogen fraction in the product gas can reach 49 vol.%, while carbon conversion efficiency achieves around 77 % at high gasification temperature and steam-to-biomass ratio. Furthermore, the results showed that the presence of oxygen in the gasifier could cause a significant decrease in hydrogen production due to oxidation reaction, but the carbon conversion efficiency increases significantly, reaching approximately 90 %. Based on the figure obtained from the study of biomass gasification, an experimental investigation of chemical looping gasification of biomass was conducted to analyze the influences of operating parameters on the process performance. It was observed that the maximum fraction of hydrogen and the carbon conversion efficiency obtained from the experimental results are approximately 43 vol.% and 90 % for both oxygen carriers, respectively. The two iron-based oxygen carriers show good performance in the study, their reactivity with different gaseous fuels decreases in the following order: H₂ > CO > CH₄. Iron-based oxygen carriers perform their capability of hydrogen-enriched gas production from chemical looping gasification of biomass along with a reduction of CO₂ emissions. The evaluation of this study can provide good knowledge of the phenomena of the chemical looping gasification process and the behavior of ilmenite and iron ore during the gasification process. Finally, a comprehensive process simulation model was developed in the Aspen Plus flowsheet environment based on the experimental data to simulate biomass gasification in a bubbling fluidized bed reactor. Hydrodynamics and kinetics were implemented simultaneously in external FORTRAN codes. It was found that the model predictions are in good agreement with the experimental investigations with the mean errors ranging between 0.027 and 0.289. The validated model can simulate biomass gasification in a bubbling fluidized bed reactor and provide a good basis for large-scale applications.