Extension of the layer particle model for volumetric conversion reactions during char gasification

The so-called 'layer model' or 'interface-based model' is a simplified single particle model, originally developed for shorter computation time during computational fluid dynamics (CFD) simulations. A reactive biomass particle is assumed to consist of successive layers, in which drying, pyrolysis and char conversion occur sequentially. The interfaces between these layers are the reaction fronts. The model has already been validated for drying, pyrolysis and char oxidation. Layer models in the literature have commonly employed surface reactions at the reaction front to describe char conversion. In this work, the suitability of this surface reaction concept is assessed when gasifying biochar. It is shown that a particular layer model, already available, which originally employed surface reactions, was unable to adequately describe the mass loss during gasification of a biochar. In order to overcome this incapability, the model was extended to consider volumetric reactions in the char layer. The influence of intraparticle diffusion was considered through an effectiveness factor. The model is easily adaptable for different gas-solid kinetic rate laws, while still allowing for comparably fast solutions of the model equations. The extended model was validated using theoretical calculations and experimental measurements from literature. It was demonstrated that intraparticle diffusion can significantly slow down the biochar gasification process. A general guideline for when to employ volumetric reactions, rather than surface reactions, and when to consider intraparticle diffusion is provided based on the Thiele modulus as the criterion.Acknowledgments. The COMET Module BIO-LOOP (Austrian Research Promotion Agency Project Number 872189) is funded within COMET - Competence Centers for Excellent Technologies - by the Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology and the Federal Ministry for Digital and Economic Affairs as well as the co-financing federal province Styria. The COMET programme is managed by FFG (Austrian Research Promotion Agency, www.ffg.at/comet). The funding is gratefully acknowledged.Publicad

Prediction accuracy in modelling beech wood pyrolysis at different temperatures using a comprehensive, CFD-based single particle pyrolysis model

CFD modelling is a novel approach to overcome problems in predicting the pyrolysis outcome in a reliable and repeatable way. It allows through real-time, model-based investigation the assessment of parameters that are impossible to be analysed experimentally. The aim of this study was to establish a comprehensive 2D single particle model of beech wood pyrolysis, which would be a reliable tool for process optimization with respect to the properties of the resulting liquid and solid pyrolysis products (biochar). The model comprised the primary biomass degradation according to the RAC kinetic scheme (48 compounds). Modelled wood cylinders were in a dry state and had a size of Ø8 mm x 10mm, with 660 kg/m3 bulk density and 1430 kg/m3 true density. The model was validated with experimental data of pyrolysis of dry beech wood cylinders, conducted in a single-particle reactor at 5 different temperatures (300, 400, 500, 700, 900 °C). The validation dataset consisted of the evolution of particle’s center and surface temperatures, mass loss, and composition of 14 evolved volatiles (CO2, CO, H2O, CH4, C2H4, formaldehyde, acetic acid and furfural, among others). Prediction of the particle’s temperature and mass loss evolution were deemed accurate. For all products, up to 500 °C, the predictions differed from the experimental data by a few %. For the temperature range between 700 °C and 900 °C, the model however, strongly over-predicted the yield in bio-oil at the expense of pyrolysis gases. The implemented primary kinetic scheme showed satisfactory results in the investigated temperature range. However, the model did not reflect very well the pyrolysis products evolution at higher temperatures (thermal tar cracking and gasification range), so implementation of accurate secondary kinetic schemes is deemed necessary

Study of the effects of thermally thin and thermally thick particle approaches on the Eulerian modeling of a biomass combustor operating with wood chips

Two particle treatments, thermally thin and thick, are applied to Eulerian combustion modeling for biomass packed beds and tested through the simulation of an experimental plant. The paper shows the efficiency of the Eulerian approach for large packed beds and tests the behavior of both particle treatments, tested with in-bed and flame temperatures and released volatiles measurements at different locations, which is not common in the literature for a full size boiler. Both approaches are implemented in a model with a comprehensive framework that includes several submodels for the thermal conversion kinetics, bed motion, heat and mass transfer with the gas phase, and gas flow and reaction. Two experiments are performed with wood chips fuels with different moisture contents. The simulations of the two cases result in reasonably good predictions for both particle treatments. The results are similar for higher moisture content and, for the low-moisture test, the bed temperature distribution and reaction fronts are slightly different due to the different predictions of the drying and devolatilization fronts. The volatile measurements show that the T. Thin model results in slightly more accurate predictions than the T. Thick, possibly because the wood chips have a more thermally thin behavior.Acknowledgments. This research was financially supported by the project PID2021-126569OB-I00 of the Ministry of Science, Innovation and Universities (Spain). Funding for open access charge: Universidade de Vigo/CISUG

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)

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 degrees C, while for the 30% case, it drops to around 400 degrees 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.This research was funded by the project PID2021-126569OB-I00 of the Ministry of Science and Innovation (Spain). The work of César Álvarez-Bermúdez has been supported by the grant PRE2019-090110 of the Ministry of Science and Innovation (Spain). Funding for open access charge: Universidade de Vigo/CISUG.Publicad

Combined influence of inorganics and transport limitations on the pyrolytic behaviour of woody biomass

A deeper understanding and quantification on the influence of inorganic species on the pyrolysis process, combined with the presence of heterogeneous secondary reactions, is pursued in this study. Both chemical controlled and transport limited regimes are considered. The former is achieved in a thermogravimetric analyser (TGA) with fine milled biomass in the mg range, while the latter is investigated in a particle level reactor with spherical particles of different sizes. To account for the influence of inorganics, wood particles were washed and doped with KCl aqueous solutions, resulting in K concentrations in the final wood of around 0.5% and 5% on dry basis. Gas species and condensable volatiles were measured online with Fourier transform infrared (FTIR) spectroscopy and a non-dispersive infrared (NDIR) gas analyzer. The removal of inorganic species delayed the pyrolysis reaction to higher temperatures and lowered char yields. The addition of inorganics (K) shifted the devolatilization process to lower temperatures, increased char and water yields, and reduced CO production among others. Higher heating rates and temperatures resulted in lower char, water, and light condensable yields, but significantly higher CH4 and other light hydrocarbons, as well as CO. The increase in these yields can be attributed, at least in part, to the gas phase cracking reactions of the produced volatiles. Larger particle size increased the formation of char, CH4 and other light hydrocarbons, and light condensables for low and high pyrolysis temperatures, while reduced the release of CO2 and H2O. This novel data set allows to quantify the influence of each parameter and can be used as basis for the development of detailed pyrolysis models which can include both the influence of inorganics and transport limitations when coupled into particle models.EC/H2020/731101/EU/Biofuels Research Infrastructure for Sharing Knowledge II/BRISK I

Surface properties and chemical composition of corncob and miscanthus biochars: effects of production temperature and method

Biochar properties vary, and characterization of biochars is necessary for assessing their potential to sequester carbon and improve soil functions. This study aimed at assessing key surface properties of agronomic relevance for products from slow pyrolysis at 250-800 °C, hydrothermal carbonization (HTC), and flash carbonization. The study further aimed at relating surface properties to current characterization indicators. The results suggest that biochar chemical composition can be inferred from volatile matter (VM) and is consistent for corncob and miscanthus feedstocks and for the three tested production methods. High surface area was reached within a narrow temperature range around 600 °C, whereas cation exchange capacity (CEC) peaked at lower temperatures. CEC and pH values of HTC chars differed from those of slow pyrolysis biochars. Neither CEC nor surface area correlated well with VM or atomic ratios. These results suggest that VM and atomic ratios H/C and O/C are good indicators of the degree of carbonization but poor predictors of the agronomic properties of biochar

Production and characterization of bio-oil from fluidized bed pyrolysis of olive stones, pinewood, and torrefied feedstock

Advancements in fluidized bed pyrolysis mechanisms and analytical methodologies are critical for progress in the  biorefinery sector in general and the aviation fuel sector in particular. The statistical modelling of pyrolysis  product yields and composition allowed us to observe advantages of operating temperature and feedstock selections over the torrefaction process and catalyst addition in a fluidized bed reactor. Results suggest that the  chemical composition and physical properties of bio-oil from pyrolysis of olive stones at 600◦C and pinewood  pellets at 500◦C are the most suitable for use as fuels. This work suggests that only combined use of selected gas  chromatography mass spectroscopy, UV fluorescence, nuclear magnetic resonance spectroscopy, and rheology  can provide comprehensive information on pyrolysis bio-oil composition. Importantly from a technological point  of view, bio-oil was characterized i) by a viscosity similar to that of fossil-based oil; ii) by a low oxygen and water  content; and iii) by a balanced composition of aliphatic and aromatic species. These factors indicate that bio-oil  from fluidized bed pyrolysis of biomasses is a promising material for use in the aviation industry and energy  production.  </p

Surface Properties and Chemical Composition of Corncob and Miscanthus Biochars: Effects of Production Temperature and Method

Biochar properties vary, and characterization of biochars is necessary for assessing their potential to sequester carbon and improve soil functions. This study aimed at assessing key surface properties of agronomic relevance for products from slow pyrolysis at 250–800 °C, hydrothermal carbonization (HTC), and flash carbonization. The study further aimed at relating surface properties to current characterization indicators. The results suggest that biochar chemical composition can be inferred from volatile matter (VM) and is consistent for corncob and miscanthus feedstocks and for the three tested production methods. High surface area was reached within a narrow temperature range around 600 °C, whereas cation exchange capacity (CEC) peaked at lower temperatures. CEC and pH values of HTC chars differed from those of slow pyrolysis biochars. Neither CEC nor surface area correlated well with VM or atomic ratios. These results suggest that VM and atomic ratios H/C and O/C are good indicators of the degree of carbonization but poor predictors of the agronomic properties of biochar