Multi-stage model for the release of potassium in single particle biomass combustion

The release of potassium during biomass combustion leads to several problems as the emissions of particle matter or formation of deposits. K release is mainly described in literature in a qualitative way and this work aims to develop a simplified model to quantitatively describe it at different stages. The proposed model has 4 reactions and 5 solid species, describing K release in 3 steps; during pyrolysis, KCl evaporation and carbonate dissociation. This release model is coupled into a single particle model and successfully validated with experiments conducted in a single particle reactor with spruce, straw and Miscanthus pellets at different temperatures. The model employs same kinetic parameters for the reactions in all cases, while different product compositions of the reactions are employed for each fuel, which is attributed to differences in composition. The proposed model correctly predicts the online release at different stages during conversion as well as the final release for each case

Multi-scale modelling of fluidized bed biomass gasification using a 1D particle model coupled to CFD

For many fluidized bed applications, the particle movement inside the reactor is accompanied by reactions at the particle scale. The current study presents for the first time in literature a multi-scale modelling approach coupling a one-dimensional volumetric particle model with the dense discrete phase model (DDPM) of ANSYS Fluent via user defined functions. To validate the developed modelling approach, the current study uses experimental data of pressure drop, temperature and gas composition obtained with a lab-scale bubbling fluidized bed biomass gasifier. Therefore, a particle model developed previously for pyrolysis was modified implementing a heat transfer model valid for fluidized bed conditions as well as kinetics for char gasification taken from literature. The kinetic theory of granular flow is used to describe particle¿particle interactions allowing for feasible calculation times at the reactor level whereas an optimized solver is employed to guarantee a fast solution at the particle level. A newly developed initialization routine uses an initial bed of reacting particles at different states of conversion calculated previously with a standalone version of the particle model. This allows to start the simulation at conditions very close to stable operation of the reactor. A coupled multi-scale simulation of over 30 s of process time employing 300.000 inert bed parcels and about 25.000 reacting fuel parcels showed good agreement with experimental data at a feasible calculation time. Furthermore, the developed approach allows for an in-depth analysis of the processes inside the reactor allowing to track individual reacting particles while resolving gradients inside the particle.This project has received funding from European Union's Horizon 2020 Research and Innovation Programme under grant agreement number 731101 (BRISK II). Furthermore, the financial support of the COMET Module project BIO-LOOP (Austrian Research Promotion Agency - FFG - Project Number 872189) funded by the federal government of Austria and the federal province Styria is gratefully acknowledged. The authors want to thank Mario Blehrmühlhuber for conducting cold-flow simulations and evaluating the applicability of the DDPM for the developed model. We further want to thank Markus Braun for his helpful hints when using the DDPM and Simon Schneiderbauer for his advice regarding the coupling strategy.Publicad

Evaluation of heat transfer models at various fluidization velocities for biomass pyrolysis conducted in a bubbling fluidized bed

Four different models for heat transfer to the particles immersed in a fluidized bed were evaluated and implemented into an existing single particle model. Pyrolysis experiments have been conducted using a fluidized bed installed on a balance at different temperatures and fluidization velocities using softwood pellets. Using a heat transfer model applicable for fluidized beds, the single particle model was able to predict the experimental results of mass loss obtained in this study as well as experimental data from literature with a reasonable accuracy. A good agreement between experimental and modeling results was found for different reactor temperatures and configurations as well as different biomass types, particle sizes ¿ in the typical range of pellets - and fluidization velocities when they were higher than U/Umf=1.5. However, significant deviations were found for fluidization velocities close to minimum fluidization. Heat transfer models which consider the influence of fluidization velocity show a better agreement in this case although differences are still present.This project has received funding from European Union's Horizon 2020 Research and Innovation Programmeunder grant agreement number 731101 (BRISK II)

Effect of bed material density on the performance of steam gasification of biomass in bubbling fluidized beds

Steam gasification of lignocellulosic biomass in a bubbling fluidized bed reactor was analyzed by means of the composition of the producer gas, including tars, and temperature distribution in the reactor. The catalytic and sorbent effect of sepiolite particles was studied by comparison of the tars generated with those produced in a bed of olivine, widely used in biomass gasification applications. Sepiolite has a lower particle density, which influences the forces acting on fuel and char particles and leads to a more homogeneous distribution of them in the dense bed during the gasification process. Fluidized beds of sepiolite particles contribute to increase the heating value of the producer gas and its hydrogen content compared to gasification under the same operating conditions in olivine beds. Furthermore, the tar yield is around 25% lower when gasifying in sepiolite beds, reducing the requirement of secondary methods for tars removal. Long-term gasification tests were also conducted in a sepiolite bed to evaluate the mitigation of the sorbent/catalytic effect of sepiolite with time.This project has received funding from European Union’s Horizon 2020 Research and Innovation Programme under grant agreement number 731101 (BRISK II)

Validation of a biomass conversion mechanism by Eulerian modelling of a fixed-bed system under low primary air conditions

This work presents a three-dimensional Computational Fluid Dynamics study of a small-scale biomass combustion system operating with low primary air ratios. The Eulerian Biomass Thermal Conversion Model (EBiTCoM) was adapted to incorporate a pyrolysis mechanism based on the detailed Ranzi-Anca-Couce (RAC) scheme. Two scenarios were simulated using woodchips with 8% and 30% moisture content, and the results were validated against experimental data, including in-flame and bed measurements. The model accurately predicted bed temperature profiles and the influence of fuel moisture content on the pyrolysis and drying fronts, as well as on the distribution of volatiles and temperatures above the solid fuel bed. For the 8% moisture content case, the average gas temperature above the bed is approximately 700 °C, while for the 30% case, it drops to around 400 °C. The lower temperatures hinder the tar cracking reaction, resulting in a 25% higher tar content in the producer gas for the 30% moisture content fuel. The lower part of the bed consists of a thick layer of char undergoing reduction reactions, similar to that of an updraft gasifier. The developed model can accurately simulate biomass combustion systems with solid fuel beds consisting of numerous particles, while maintaining low computational requirements.Agencia Estatal de Investigación | Ref. PID2021-126569OB-I00Agencia Estatal de Investigación | Ref. PRE2019-090110Universidade de Vigo/CISU

Biomass pyrolysis TGA assessment with an international round robin

The large variations found in literature for the activation energy values of main biomass compounds (cellulose, hemicellulose and lignin) in pyrolysis TGA raise concerns regarding the reliability of both the experimental and the modelling side of the performed works. In this work, an international round robin has been conducted by 7 partners who performed TGA pyrolysis experiments of pure cellulose and beech wood at several heating rates. Deviations of around 20 – 30 kJ/mol were obtained in the activation energies of cellulose, hemicellulose and conversions up to 0.9 with beech wood when considering all experiments. The following method was employed to derive reliable kinetics: to first ensure that pure cellulose pyrolysis experiments from literature can be accurately reproduced, and then to conduct experiments at different heating rates and evaluate them with isoconversional methods to detect experiments that are outliers and to validate the reliability of the derived kinetics and employed reaction models with a fitting routine. The deviations in the activation energy values for the cases that followed this method, after disregarding other cases, were of 10 kJ/mol or lower, except for lignin and very high conversions. This method is therefore proposed in order to improve the consistency of data acquisition and kinetic analysis of TGA for biomass pyrolysis in literature, reducing the reported variability

Optimization of an integrated biomass gasifier-fuel cell system: An experimental study on the cell response to process variations

Coupling biomass gasification with high temperature Solid Oxide Fuel Cells (SOFCs) is a promising solution to increase the share of renewables and raise energy efficiency. Optimal system integration and high durability of its components are both required for this purpose. Moreover, system optimization may ask for a compromise between optimal SOFC operating temperature and system thermal integration. Nevertheless, the producer gas quality and operating temperature have a noticeable impact on SOFC performance, which has to be considered. In order to address the challenges mentioned, this study focuses on experimental investigation of commercial SOFC single cells of industrial size fuelled with different gas mixtures. Namely, typical producer gas compositions from downdraft fixed bed gasification with air (with and without drying) and fluidized bed gasification with steam are considered. In addition to that, the effect of temperature variation is analysed. The results show that, although a higher cell temperature (800°C) and a dry product gas composition lead to higher efficiencies, it is feasible to run the SOFC on wet producer gas at lower cell temperatures (750°C). Despite a decreased efficiency, this would optimize the gasifier-SOFC coupling. Moreover, the presence of water in the producer gas (when drying is not included) reduces the possibility of carbon deposition, thus increasing the cell durability. Therefore, cases with low cell temperatures and presence of water in the gas should be considered as viable alternatives for the system coupling and possible SOFC degradation in such cases is going to be extensively analysed