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Item type:Publication, Access status: with files , Diskrete-Elemente-Modellierung der pyrolytischen Zersetzung und Fragmentierung von Holzpellets im Chemical-Looping Vergasungsprozess(2024); To enable a more accurate prediction of breakage and attrition for the chemical looping gasification of biomass, a numerical model for the behavior of devolatilized wood pellets was developed. Experimental investigations revealed the generation of a macro-porous structure during devolatilized. Under mechanical loads, this results in complex failure behavior. Based on these findings the bonded-particle model has been extended to predict the thermochemical conversion and the induced structural changes. Subsequent mechanical simulations show good agreement with the experiments and demonstrate the possibility of modeling the breakage of the pellets in a gasification reactor.Publicationtype: Doctoral Thesis43 335 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Characterizing devolatilized wood pellets for fluidized bed applicationsWe investigated devolatilized wood pellets to characterize their mechanical behavior and their microstructure. The work’s aim is to increase the understanding and modeling capabilities for the application in fluidized bed gasification as a sustainable alternative to generate synthesis gas. Our experiments showed that devolatilized wood pellets are a stable but highly porous and fragile structure. Computed tomographic images of the same pellets before and after devolatilization showed that the existing pore network in raw conditions characterizes the final structure. Along with the pores, the reaction rate likely increases and the pores massively enlarge, and internal cavities are formed. The resulting pore network dominates the mechanical behavior and leads to micro fragmentations already at low static loads or slow dynamic impacts. This results in the creation of fines or breakage already at low impact velocities. For fluidized bed devolatilization, the large-scale open pore network of the biochar pellets allows the penetration of bed material into the pellet leading to an estimated increase in the pellet’s mass of up to 45%. However, an increase in pore size caused by the penetration was not apparent. Due to the pellet’s porous structure, breakage and attrition induced by mechanical stresses are likely to be as or even more important than primary fragmentation caused by the devolatilization process itself in a reactor.Publicationtype: Journal ArticleScopus© Citations 5 194 346 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Mixing evolution behavior of raw and gasified biomass pellets in a fluidized bed reactor(2022-12-31); ; ; ; Due to a large particle size and a small specific surface, a homogeneous mixing of biomass pellets with bed materials during gasification plays a critical role in the devolatilization and carbon conversion. In this work, the mixing evolution behavior of biomass pellets at different gasification stages is investigated for the first time. Two bubbling fluidized beds are established to perform the preparation of biomass samples undergoing different gasification times and visualized mixing experiments, respectively. An image processing technique is introduced for the determination of the real-time distribution of biomass pellets. The vertical and lateral migration paths of biomass pellets at different gasification stages are revealed. The improvement of binary mixing by adjusting the operating conditions as well as the adaptability to different biomass loadings are discussed. A convolutional neural network is developed to validate the influence of fluidization velocity on the resulting flow and classify the fluidization behavior.Publicationtype: Journal Article43 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , MP-PIC simulation of biomass steam gasification using ilmenite as an oxygen carrier(Multidisciplinary Digital Publishing Institute (MDPI), 2022-06-22); ; ; ; Biomass chemical looping gasification (BCLG) is a complex process for the conversion of biomass using an oxygen carrier, which is influenced by various operating parameters. For a better understanding of this process, biomass steam gasification using ilmenite as an oxygen carrier is numerically investigated in this work using the multiphase particle-in-cell (MP-PIC) method, which is a modified Euler-Lagrange approach. As a first step, a reduced reaction network for biomass gasification is investigated in a spouted bed. As a second step, the reaction network is coupled with oxygen carrier kinetics of ilmenite for the simulation of BCLG in a lab-scale fluidized bed. For both steps, the influence of the main operating parameters, such as reactor temperature, steam-to-biomass ratio, and oxidation degree of the oxygen carrier, are investigated and compared with experimental data from the literature. In general, the simulations show satisfying results and the predicted syngas compositions with varied operating parameters are in good agreement with the experimental data. Furthermore, the main trends for the syngas composition are predicted correctly and the oxidation degree of the oxygen carrier has a significant influence on the resulting syngas composition confirming the experimental results.Publicationtype: Journal ArticleScopus© Citations 2 95 1379 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Modeling the devolatilization and fragmentation of biomass pellets with the bonded particle method for fluidized bed applicationsBy extending the bonded particle method, the major structural changes during the devolatilization of a wood pellet in a fluidized bed and the resulting mechanical behavior have been successfully reproduced. A comparison with experiments from the literature showed that the implemented particle-based pyrolysis model enables predicting the entire pellet’s kinetics with a high agreement. The developed shrinkage model for the particles and bonds further allowed to emulate the reported formation of a large-scale pore network inside the pellet. The simulation of a radial compression test with the predicted structure showed good agreement with experimental data and could confirm the importance of the pores for the mechanical behavior. The results demonstrated that the large pores cause the fragmentation of agglomerates already at low mechanical loads which could promote attrition. In general, the results have shown that the developed extension of the bonded particle method allows studying and predicting the behavior of a single pellet during conversion inside a fluidized bed gasification reactor in more detail.Publicationtype: Journal ArticleScopus© Citations 2 38 169 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Mechanical strength evolution of biomass pellet during chemical looping gasification in fluidized bedDue to a large particle size and a small specific surface, biomass pellet fragmentation during Chemical looping gasification (CLG) process plays a critical role in the internal devolatilization rate and its conversion. To investigate the mechanical strength evolution of biomass pellet during CLG process, a gasification reactor of bubbling fluidized bed capable of controlling the gasification time arbitrarily is established. Sawdust and rice-husk pellets with different ash content are selected as fuels. More than 720 biomass samples undergoing different gasification time ranging from 15 s to 180 s are collected at different conditions. A porous and brittle morphology of char samples is revealed to be a gradual evolutionary process from the surface to the internal structure during CLG process. Uni-axial compression test shows that the reduction of the peak compressive force for crushing the samples mainly depends on the consumption and destruction of the overall carbon structure including internal skeleton and external epidermis. The penetration of oxygen carrier through pores and/or cracks and the internal overpressure because of rapid volatiles release are the remarkable boosts to the breakage and attrition of the internal carbon skeleton. A corresponding thermal-damage model is developed to predict the mechanical strength of pellet during CLG process.Publicationtype: Journal Article67 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Linearization-based methods for the calibration of bonded-particle models© 2020, The Author(s). In the work at hand, two methods for the calibration of the elastic material parameters of bonded-particle models (BPMs) are proposed. These methods are based on concepts of classical mechanics and enable a faster calibration compared to the conventional trial and error strategy. Moreover, they can be used to counter-check the consistency of the BPM. In the first method, the mathematical model is linearized and solved in a classical matrix-vector formulation similar to a finite element scheme. Further linearization in the second approach enables the direct calculation of the elastic stiffness tensor, reducing computational costs massively. To analyze the capabilities and limitations of both methods, they have been applied in three different case studies. Obtained results have shown that the new strategy allows us to significantly reduce the calculation time.Publicationtype: Journal ArticleScopus© Citations 8 203 213 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Increasing the efficiency of chemical looping combustion of biomass by a dual-stage fuel reactor design to reduce carbon capture costsThis paper analyzes the capabilities of a pilot-scale chemical looping combustion plant firing wood biomass in two stages to efficiently achieve negative carbon dioxide emissions. The utilized in situ gasification-chemical looping combustion (iG-CLC) process isolates the oxygen supply via air from the fuel conversion itself with the help of two separate fluidized bed reactors and an oxygen carrier to supply the necessary oxygen for the combustion. As a result, a relatively pure stream of carbon dioxide and steam is generated. Thus, the process makes capturing carbon emissions more feasible since it eliminates the need for the cost- and energy-intensive separation of the produced gases. A major issue when using biomass in a chemical looping plant is the high amount of the volatiles exiting unconverted. This problem was mitigated by using a two-stage fuel reactor system. Two bubbling fluidized beds were arranged one upon the other. The lower stage, where the fuel is introduced, is used to release the volatiles and partly convert them. The remaining volatiles rise up into the second stage and are further converted to a high degree. A series of experiments were carried out with a 25-kWth pilot plant located at the Hamburg University of Technology. Gas concentrations were continuously measured after both stages of the fuel reactor to see the gradual conversion of the fuel gases. Additionally, carbon slip at the exhaust was measured to show the effectiveness. The experiments with the reactor concept showed promising results since already at a reactor temperature of 850 °C, the total oxygen demand needed to oxidize the combustible component in the exhaust gas was well below 2%. The carbon dioxide (CO2) capture efficiency when using German hardwood slightly decreased to 93–96% compared to 97% for German lignite. In the future, the reactor design must prove that it scales and that the efficiency can be further increased. Nevertheless, firing biomass with a two-stage iG-CLC process might allow a cost-efficient negative carbon dioxide emission while generating heat with relatively high efficiency. Therefore, it might be a sustainable alternative to generate heat in the future.Publicationtype: Journal Article143 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Modelling of mechanical behavior of biopolymer alginate aerogels using the bonded-particle model(Multidisciplinary Digital Publishing Institute (MDPI), 2019-07-12); ; A novel mesoscale modelling approach for the investigation of mechanical properties of alginate aerogels is proposed. This method is based on the discrete element method and bonded-particle model. The nanostructure of aerogel is not directly considered, instead the highly porous structure of aerogels is represented on the mesoscale as a set of solid particles connected by solid bonds. To describe the rheological material behavior, a new elastic-plastic functional model for the solids bonds has been developed. This model has been derived based on the self-similarity principle for the material behavior on the macro and mesoscales. To analyze the effectiveness of the proposed method, the behavior of alginate aerogels with different crosslinking degrees (calcium content) was analyzed. The comparison between experimental and numerical results has shown that the proposed approach can be effectively used to predict the mechanical behavior of aerogels on the macroscale.Publicationtype: Journal ArticleScopus© Citations 17 629 368