• Energy Research

Abstract In the present study, thermally induced changes in microstructure and CO2-gasification rates of fuel particles from Rhenish lignite and torrefied poplar wood are experimentally investigated. A literature review reveals a distinct gap in the field of quantitative analysis of thermal deactivation influencing low-rank and biogenic fuels reacting under gasification conditions. Therefore, the reactivity of Rhenish lignite and torrefied poplar wood towards carbon dioxide is examined after the process of heat treatment. Experiments are conducted in a small scale fluidized bed reactor. Assessment of thermal deactivation is achieved by varying heat treatment temperature between 1023–1173 K and heat treatment time between 0–1800 s. Flue gases are analyzed using a Fourier-transform infrared spectrometer to deduce a reaction rate from the temporal evolution of the product gas. Experimental results are reproduced by applying an nth-order power law and a model with distributed activation energies to propose parameters for implementation in comprehensive gasification/combustion codes and for comparison of these two model types: Both models approximate the experimental results almost equally well. In parallel to that, heat treated particles are analyzed by high resolution transmission electron microscopy. Thereby, a progressive transformation of the carbon from an amorphous microstructure to turbostratic arrangements is revealed for both types of fuel. In case of Rhenish lignite, formations of wrinkled carbon layers were detected, indicating a faster progress of transformation for lignite compared to biomass. This finding is in accordance with the kinetic study conducted, where the determined deactivation rate of lignite exceeds the one of biomass. In this work it is successfully shown that both reactivity and microstructure are affected by heat treatment on equal time scales.

Abstract In the present study, thermally induced changes in microstructure and CO2-gasification rates of fuel particles from Rhenish lignite and torrefied poplar wood are experimentally investigated. A literature review reveals a distinct gap in the field of quantitative analysis of thermal deactivation influencing low-rank and biogenic fuels reacting under gasification conditions. Therefore, the reactivity of Rhenish lignite and torrefied poplar wood towards carbon dioxide is examined after the process of heat treatment. Experiments are conducted in a small scale fluidized bed reactor. Assessment of thermal deactivation is achieved by varying heat treatment temperature between 1023–1173 K and heat treatment time between 0–1800 s. Flue gases are analyzed using a Fourier-transform infrared spectrometer to deduce a reaction rate from the temporal evolution of the product gas. Experimental results are reproduced by applying an nth-order power law and a model with distributed activation energies to propose parameters for implementation in comprehensive gasification/combustion codes and for comparison of these two model types: Both models approximate the experimental results almost equally well. In parallel to that, heat treated particles are analyzed by high resolution transmission electron microscopy. Thereby, a progressive transformation of the carbon from an amorphous microstructure to turbostratic arrangements is revealed for both types of fuel. In case of Rhenish lignite, formations of wrinkled carbon layers were detected, indicating a faster progress of transformation for lignite compared to biomass. This finding is in accordance with the kinetic study conducted, where the determined deactivation rate of lignite exceeds the one of biomass. In this work it is successfully shown that both reactivity and microstructure are affected by heat treatment on equal time scales.

In this work, the reaction kinetics of biomass during pyrolysis is investigated experimentally and described by modelling. A laboratory-scale fluidised bed reactor coupled with a Fourier-transform infrared spectrometer is used for the experimental investigations. To determine the intrinsic reaction rate, individual samples of pulverised fuel are batch-wise dosed into the reactor at different reactor temperatures (573–1473 K) and the volume fractions of the gaseous pyrolysis products are measured ex situ, quantitatively, and time-resolved in the FTIR gas cell. In addition to different real biomasses, extracted biomass structural components (cellulose, hemicellulose, lignin) are investigated as important model fuels while fossil fuels serve as references. The experimental data obtained are used to calibrate two empirical models of different complexity. Furthermore, the phenomenologically based chemical percolation devolatilisation model (CPD) is optimised, modified for biogenic fuels, and adapted for the experimental boundary conditions of the fluidised bed reactor by implementing suitable balance equations.Using Rhenish lignite as a reference fuel, the model adaptations of the CPD model can be successfully validated. The subsequent analysis of the parameter sets of the bio-CPD model available in the literature provides a combination to describe the time-dependent reaction behaviour for cellulose and hemicellulose in the fluidised bed reactor. For lignin, an improved parameter set is derived via a sensitivity analysis. The superposition of the individual components to describe the pyrolysis behaviour of real biomasses shows that trends (especially for the integral products) can be well expressed. However, the kinetic process itself depends strongly on the interaction of the components. This finding applies equally to the empirical models as well as to other reactor concepts. Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2022). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022 Published by RWTH Aachen University, Aachen

In this work, the reaction kinetics of biomass during pyrolysis is investigated experimentally and described by modelling. A laboratory-scale fluidised bed reactor coupled with a Fourier-transform infrared spectrometer is used for the experimental investigations. To determine the intrinsic reaction rate, individual samples of pulverised fuel are batch-wise dosed into the reactor at different reactor temperatures (573–1473 K) and the volume fractions of the gaseous pyrolysis products are measured ex situ, quantitatively, and time-resolved in the FTIR gas cell. In addition to different real biomasses, extracted biomass structural components (cellulose, hemicellulose, lignin) are investigated as important model fuels while fossil fuels serve as references. The experimental data obtained are used to calibrate two empirical models of different complexity. Furthermore, the phenomenologically based chemical percolation devolatilisation model (CPD) is optimised, modified for biogenic fuels, and adapted for the experimental boundary conditions of the fluidised bed reactor by implementing suitable balance equations.Using Rhenish lignite as a reference fuel, the model adaptations of the CPD model can be successfully validated. The subsequent analysis of the parameter sets of the bio-CPD model available in the literature provides a combination to describe the time-dependent reaction behaviour for cellulose and hemicellulose in the fluidised bed reactor. For lignin, an improved parameter set is derived via a sensitivity analysis. The superposition of the individual components to describe the pyrolysis behaviour of real biomasses shows that trends (especially for the integral products) can be well expressed. However, the kinetic process itself depends strongly on the interaction of the components. This finding applies equally to the empirical models as well as to other reactor concepts. Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2022). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022 Published by RWTH Aachen University, Aachen

Abstract The development of a rapidly responding lab-scale fluidized bed reactor (FBR) optimized for the investigation into combustion reactions of solid fuels is presented. The experimental setup represents a novelty in FBR systems, as it quantitatively captures reactions with an apparent 90% carbon conversion time t 90 of Original and improved design are modeled with a 1-D finite volume approach to gain insights into the ideal operating conditions and operating limits. In a second step, the theoretical reaction rate limits are compared with experimental combustion experiments. Reaction rate measurements with chars from two pulverized Colombian coals (Calenturitas, Mina Norte) and one pulverized biomass (beech wood) are undertaken in N2/O2 atmosphere at different temperatures ranging from 823 to 1273 K. It is found that results of the 1-D simulation are a good guidance for actual experiments.

Abstract The development of a rapidly responding lab-scale fluidized bed reactor (FBR) optimized for the investigation into combustion reactions of solid fuels is presented. The experimental setup represents a novelty in FBR systems, as it quantitatively captures reactions with an apparent 90% carbon conversion time t 90 of Original and improved design are modeled with a 1-D finite volume approach to gain insights into the ideal operating conditions and operating limits. In a second step, the theoretical reaction rate limits are compared with experimental combustion experiments. Reaction rate measurements with chars from two pulverized Colombian coals (Calenturitas, Mina Norte) and one pulverized biomass (beech wood) are undertaken in N2/O2 atmosphere at different temperatures ranging from 823 to 1273 K. It is found that results of the 1-D simulation are a good guidance for actual experiments.

Abstract This work presents detailed information on pyrolysis and char oxidation for a high-volatile Colombian bituminous coal. The investigation includes experiments at low and high particle heating rates, performed in a thermogravimetric analyzer (TGA), a drop-tube reactor (DTR), a flat-flame burner (FFB) and a fluidized-bed reactor (FBR). The TGA and DTR data were used when developing and calibrating the kinetic model for the conversion of coal in air and oxy-fuel atmospheres, while the FFB and FBR data were used to validate the resulting mechanism. The proposed model is an updated version of the CRECK-S-C model from the Politecnico di Milano (PoliMi), consisting of a fuel characterization step, coupled with a multi-step kinetic mechanism based on reference coals. Both the devolatilization and heterogeneous char reactions are accounted for and interconnected seamlessly. Key reactions were introduced and the existing reactions were calibrated to account for the particularities of this fuel and the effects of the abundant CO2 concentration in the reactors. The importance of successive gas-phase reactions was observed and a gas-phase kinetic model was coupled to properly simulate such conditions. The resulting model is applied to simulate and systematically evaluate the experimental findings, highlighting the model’s features and limitations.

Abstract This work presents detailed information on pyrolysis and char oxidation for a high-volatile Colombian bituminous coal. The investigation includes experiments at low and high particle heating rates, performed in a thermogravimetric analyzer (TGA), a drop-tube reactor (DTR), a flat-flame burner (FFB) and a fluidized-bed reactor (FBR). The TGA and DTR data were used when developing and calibrating the kinetic model for the conversion of coal in air and oxy-fuel atmospheres, while the FFB and FBR data were used to validate the resulting mechanism. The proposed model is an updated version of the CRECK-S-C model from the Politecnico di Milano (PoliMi), consisting of a fuel characterization step, coupled with a multi-step kinetic mechanism based on reference coals. Both the devolatilization and heterogeneous char reactions are accounted for and interconnected seamlessly. Key reactions were introduced and the existing reactions were calibrated to account for the particularities of this fuel and the effects of the abundant CO2 concentration in the reactors. The importance of successive gas-phase reactions was observed and a gas-phase kinetic model was coupled to properly simulate such conditions. The resulting model is applied to simulate and systematically evaluate the experimental findings, highlighting the model’s features and limitations.

Biomass is a complex material mainly composed of the three lignocellulosic components: cellulose, hemicellulose and lignin. The different molecular structures of the individual components result in various decomposition mechanisms during the pyrolysis process. To understand the underlying reactions in more detail, the individual components can be extracted from the biomass and can then be investigated separately. In this work, the pyrolysis kinetics of extracted and purified cellulose, hemicellulose and lignin are examined experimentally in a small-scale fluidized bed reactor (FBR) under N2 pyrolysis conditions. The FBR provides high particle heating rates (approx. 104 K/s) at medium temperatures (573–973 K) with unlimited reaction time and thus complements typically used thermogravimetric analyzers (TGA, low heating rate) and drop tube reactors (high temperature and heating rate). Based on the time-dependent gas concentrations of 22 species, the release rates of these species as well as the overall rate of volatiles released are calculated. A single first-order (SFOR) reaction model and a 2-step model combined with Arrhenius kinetics are calibrated for all three components individually. Considering FBR and additional TGA experiments, different reaction regimes with different activation energies could be identified. By using dimensionless pyrolysis numbers, limits due to reaction kinetics and heat transfer could be determined. The evaluation of the overall model performance revealed model predictions within the ±2σ standard deviation band for cellulose and hemicellulose. For lignin, only the 2-step model gave satisfying results. Modifications to the SFOR model (yield restriction to primary pyrolysis peak or the assumption of distributed reactivity) were found to be promising approaches for the description of flash pyrolysis behavior, which will be further investigated in the future.