Detailed knowledge of devolatilization behavior of solid fuels is required for CFD simulations of burners and boilers as the pyrolysis step affects a lot of subsequent processes like ignition, char burnout and pollutant formation. The experimental determi-nation of pyrolysis kinetics can be achieved with different reactors, while thermogravimetric analyzers are the most common devices. In this study, the impact of the experimental setup and the related boundary conditions is evaluated for two setups: a standard thermogravimetric analyzer (TGA) and a small scale fluidized bed reactor (FBR) designed for the determination of kinetic parameters of gas-solid reactions. During TGA, particle heating rate is low and precisely controlled. In the FBR the heating rate is several orders of magnitude higher which leads to almost isothermal reaction conditions. Both setups have in common that they can capture the entire pyrolysis process and are not limited in residence time. The Chinese Zhundong lignite with a particle size range of 60-70 µm is used as reference fuel. For each setup the range of operating conditions which ensure kinetic control is identified and kinetic parameters evaluated for two different empirical devolatilization models. Afterwards, the model performances outside their calibration ranges are investigated for three test cases approximating reaction conditions in thermogravimetric analyzer, fluidized bed and entrained flow reactor. Additionally, the results are compared with predictions of the sophisticated Chemical Percolation and Devolatilization (CPD) network model. The results reveal that kinetic extrapolations of the empirical models outside their calibration ranges can generate significantly different release characteristics. The fundamental-based CPD model instead is capable to capture the relevant time-scales at all boundary conditions but lacks in the prediction of final volatile yield
Comparability and validity range of pyrolysis kinetics from Chinese coals obtained with different experimental setups
2019
Abstract
Detailed knowledge of devolatilization behavior of solid fuels is required for CFD simulations of burners and boilers as the pyrolysis step affects a lot of subsequent processes like ignition, char burnout and pollutant formation. The experimental determi-nation of pyrolysis kinetics can be achieved with different reactors, while thermogravimetric analyzers are the most common devices. In this study, the impact of the experimental setup and the related boundary conditions is evaluated for two setups: a standard thermogravimetric analyzer (TGA) and a small scale fluidized bed reactor (FBR) designed for the determination of kinetic parameters of gas-solid reactions. During TGA, particle heating rate is low and precisely controlled. In the FBR the heating rate is several orders of magnitude higher which leads to almost isothermal reaction conditions. Both setups have in common that they can capture the entire pyrolysis process and are not limited in residence time. The Chinese Zhundong lignite with a particle size range of 60-70 µm is used as reference fuel. For each setup the range of operating conditions which ensure kinetic control is identified and kinetic parameters evaluated for two different empirical devolatilization models. Afterwards, the model performances outside their calibration ranges are investigated for three test cases approximating reaction conditions in thermogravimetric analyzer, fluidized bed and entrained flow reactor. Additionally, the results are compared with predictions of the sophisticated Chemical Percolation and Devolatilization (CPD) network model. The results reveal that kinetic extrapolations of the empirical models outside their calibration ranges can generate significantly different release characteristics. The fundamental-based CPD model instead is capable to capture the relevant time-scales at all boundary conditions but lacks in the prediction of final volatile yieldI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
