Events at Department of Energy Technology

PhD defence by Tianbao Gu


31.08.2022 kl. 13.00 - 16.00


Tianbao Gu, AAU Energy, will defend the thesis "Advanced Modeling of Biomass Pyrolysis and Combustion"


Advanced Modeling of Biomass Pyrolysis and Combustion


Tianbao Gu


Associate Professor Chungen Yin


Assistant Professor Torsten Berning


Assistant Professor Vincenzo Liso


Associate Professor Matthias Mandø, Aalborg University, Denmark (Chairman)
Professor Peter Glarborg, Technical University of Denmark
Senior Research Scientist Nils Erland L. Haugen, SINTEF Energy Research, Norway


As an abundant carbon-neutral resource, biomass has great potential to yield renewable energy and valuable chemicals. The utilization of biomass is significant for achieving carbon neutrality. Thermochemical conversion technologies such as pyrolysis, gasification, and combustion, play a vital role in converting biomass to bioenergy or other by-products. Among them, pyrolysis that can produce high-value bioproducts from the decomposition of solids in the absence of oxygen, is recognized as a promising technology for biomass efficient utilization. Combustion technology, oxidizing fuel into gaseous products with sufficient oxygen and producing heat, is widely applied to various types of biomass for recycling energy. In addition, pyrolysis is an inseparable sub-process of combustion and gasification, i.e., devolatilization of the volatiles. Bearing this in mind, this Ph.D. project aims to investigate pyrolysis and combustion technologies to promote the efficient and clean utilization of biomass.
For biomass pyrolysis, kinetic modeling, which connects the academic research and engineering applications of solid fuel conversion, is of great significance in exploring the underlying mechanism and optimizing the reaction process. Whereas, the complexity of existing kinetic models and the diversity of the evaluated kinetics in previous biomass pyrolysis studies largely compromise the effectiveness of kinetic modeling. In this thesis, a universal description of solid fuel decomposition is derived for all common reaction mechanisms, which shows statistical characteristics. For example, the expression for first-order reactions is consistent with the standardised general extreme value distribution. Accordingly, four applications are demonstrated: conversion peak identification, reaction mechanism determination, conversion rate prediction, and kinetics evaluation. Moreover, a simplified kinetic model with only one kinetic parameter is developed to predict the conversion rate and validated using experimental data. The model has been employed to analyze poplar wood pyrolysis, which exhibited superior performance in terms of accuracy, stability, and simplicity compared to the conventional Arrhenius-type model.
For biomass combustion, municipal solid waste (MSW), as one of the dominant biomass resources, is selected as the incinerating feedstock in this project, for which grate-firing technology is widely employed. Due to the limitations and difficulties of experimental study, simulation investigation is regarded as powerful and cost-efficient for understanding and optimizing large-scale MSW combustion. In this thesis, a moving-grate boiler incinerating 750 tons of MSW per day was simulated based on Computational Fluid Dynamics (CFD). The coupling strategy: model the fuel bed and freeboard separately and then couple them together, is implemented. An in-house bed model including NOx formation is developed to describe the solid conversion processes (i.e., drying, pyrolysis, combustion, and gasification) on the moving grate. The freeboard simulation is performed in ANSYS Fluent to solve the turbulent reacting flow, containing the bed model results, namely, profiles of temperature, velocity, and gas species. The simulation results are verified by comparing with the operation data inside the boiler. Subsequently, two simulation-based investigations on this boiler are conducted: the impacts of feedstock change caused by waste classification and co-combustion of MSW and sewage sludge. The former research reveals the potential issues for incinerating the new MSW, e.g., ineffective oxygen utilization, low mixing, and non-uniform temperature in the freeboard, when maintaining the current operating conditions. Subsequently, adjustments in the air supply and thermal input are proposed and probed in the simulation, which addresses these potential issues and shows improvements in operation stability and energy recovery. The latter investigation numerically tests the feasibility of MSW co-firing with sewage sludge and industrial solid waste in the existing boiler, in which the industrial waste serves as the supporting fuel. It explores suitable blends and operation guidelines that promote the combustion stability and NOx reduction up to 11.54%.
In summary, the contributions of this Ph.D. project are twofold: kinetic modeling of biomass pyrolysis and CFD simulation of MSW combustion, developing two original models for each part. Applications and investigations based on the two models are conducted, resulting in valuable findings and demonstrating their potential for biomass utilization via thermochemical conversion technologies. 

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THE DEFENCE will be IN ENGLISH - all are welcome.


AAU Energy