Challenges in Kinetic Parameter Determination for Wheat Straw Pyrolysis

Wheat straw is a renewable agricultural by-product that is currently underutilized in the production of bioenergy and bioproducts due to its high ash content, as well as high transport costs due to its low volumetric energy density. The thermogravimetric analysis of this material produces derivative curves with a single broad peak, making it difficult to identify the three conventional pseudo-components (cellulose, hemicellulose, and lignin), which is resolved using the second derivative to determine inflection points. Model-fitting methods and isoconversional methods were applied to determine the degradation kinetics of wheat straw at two different particle sizes, as well as that of a reference feedstock (beech wood), and the obtained values were used to divide the degradation curves to be compared to the experimental data. Seven different pyrolysis reaction networks from the literature were given a similar treatment to determine which provides the best estimation of the actual pyrolysis process for the case of the feedstocks under study. The impact of the potassium content in the feedstock was considered by comparing the original pathway with a modification dependent on the experimental potassium content and an estimated optimum value

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

Pyrogenic carbon capture and storage

The growth of biomass is considered the most efficient method currently available to extract carbon dioxide from the atmosphere. However, biomass carbon is easily degraded by microorganisms releasing it in the form of greenhouse gases back to the atmosphere. If biomass is pyrolyzed, the organic carbon is converted into solid (biochar), liquid (bio-oil), and gaseous (permanent pyrogas) carbonaceous products. During the last decade, biochar has been discussed as a promising option to improve soil fertility and sequester carbon, although the carbon efficiency of the thermal conversion of biomass into biochar is in the range of 30%–50% only. So far, the liquid and gaseous pyrolysis products were mainly considered for combustion, though they can equally be processed into recalcitrant forms suitable for carbon sequestration. In this review, we show that pyrolytic carbon capture and storage (PyCCS) can aspire for carbon sequestration efficiencies of >70%, which is shown to be an important threshold to allow PyCCS to become a relevant negative emission technology. Prolonged residence times of pyrogenic carbon can be generated (a) within the terrestrial biosphere including the agricultural use of biochar; (b) within advanced bio-based materials as long as they are not oxidized (biochar, bio-oil); and (c) within suitable geological deposits (bio-oil and CO 2 from permanent pyrogas oxidation). While pathway (c) would need major carbon taxes or similar governmental incentives to become a realistic option, pathways (a) and (b) create added economic value and could at least partly be implemented without other financial incentives. Pyrolysis technology is already well established, biochar sequestration and bio-oil sequestration in soils, respectively biomaterials, do not present ecological hazards, and global scale-up appears feasible within a time frame of 10–30 years. Thus, PyCCS could evolve into a decisive tool for global carbon governance, serving climate change mitigation and the sustainable development goals simultaneously. © 2018 John Wiley & Sons Lt

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)

Towards Biochar and Hydrochar Engineering—Influence of Process Conditions on Surface Physical and Chemical Properties, Thermal Stability, Nutrient Availability, Toxicity and Wettability

The impact of conversion process parameters in pyrolysis (maximum temperature, inert gas flow rate) and hydrothermal carbonization (maximum temperature, residence time and post-washing) on biochar and hydrochar properties is investigated. Pine wood (PW) and corn digestate (CD), with low and high inorganic species content respectively, are used as feedstock. CD biochars show lower H/C ratios, thermal recalcitrance and total specific surface area than PW biochars, but higher mesoporosity. CD and PW biochars present higher naphthalene and phenanthrene contents, respectively, which may indicate different reaction pathways. High temperatures (>500 °C) lead to lower PAH (polycyclic aromatic hydrocarbons) content (<12 mg/kg) and higher specific surface area. With increasing process severity the biochars carbon content is also enhanced, as well as the thermal stability. High inert gas flow rates increase the microporosity and wettability of biochars. In hydrochars the high inorganic content favor decarboxylation over dehydration reactions. Hydrochars show mainly mesoporosity, with a higher pore volume but generally lower specific surface area than biochars. Biochars present negligible availability of NO −3 and NH +4 , irrespective of the nitrogen content of the feedstock. For hydrochars, a potential increase in availability of NO −3 , NH +4 , PO 3−4 , and K + with respect to the feedstock is possible. The results from this work can be applied to “engineer” appropriate biochars with respect to soil demands and certification requirements

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

Understanding the torrefaction of woody and agricultural biomasses through their extracted macromolecular components. Part 2: Torrefaction model

A new torrefaction mode! was proposed for predicting solid mass loss in torrefaction as a function of biomass main macromolecular composition and type, as well as on the operating conditions. To do this, solid degradation kinetics were modelled following a 2-successive reaction scheme for each macro­compound and the additive modelling approach through biomass macromolecular component behavior in torrefaction proposed by Nocquet et al. (2014). The use of extracted fractions from different woody and agricultural biomass species (ash-wood, beech, miscanthus, pine and wheat straw) instead of commercial compounds increased the accuracy of the prediction of solid kinetics in biomass torrefaction. The validation of the proposed mode! with 9 raw biomasses in torrefaction showed an accurate pre­diction for woods, white the prediction for agricultural biomasses was acceptable

Ein Mehrskalenmodell zur Beschreibung thermochemischer Wandlungsprozesse von Biomasse in Festbetten

Das Ziel dieser Arbeit ist die Zusammenführung von Modellen für Prozesse, die auf unterschiedlichen Zeit- und Längenskalen im Zuge der thermochemischen Wandlung von Biomasse stattfinden. Es soll also eine Beschreibung dieser Prozesse im Rahmen einer Mehrskalenmodellierung erfolgen. Der Fokus liegt dabei auf der Festbettpyrolyse. Die damit verbundenen Prozesse werden auf Molekular-, Einzelpartikel- und Reaktorebene beschrieben. Auf der molekularen Ebene werden verschiedene Netzwerke chemischer Reaktionen diskutiert sowie deren Kinetik bestimmt. Mit Hilfe einer Thermowaage werden die Kinetiken des Schwelbrands - inklusive Pyrolyse von Holz und Oxidation der entstehenden Holzkohle - ermittelt. Auf der Einzelpartikelebene müssen neben den Kinetiken auch die Transportprozesse berücksichtigt werden. Ein Einzelpartikelmodell zur Beschreibung der Pyrolyse eines Holzpartikels sowie eine experimentelle Anordnung für dessen Validierung werden vorgestellt. Der wesentliche Nachteil der Integration eines Partikelmodells in ein Reaktormodell ist der enorme rechentechnische Aufwand zur numerischen Lösung eines solchen Mehrskalenmodells. Um diesen Rechenaufwand zu reduzieren wird ein neuer iterativer numerischer Algorithmus zur Lösung des Einzelpartikelmodells vorgestellt, welcher nach Analyse der charakteristischen Zeitskalen der beteiligten Prozesse entwickelt wurde. Außerdem wird anhand von Laser-Induzierter Fluoreszenz gezeigt, dass heterogenes sekundäres Teercracking bereits innerhalb eines pyrolysierenden Einzelpartikels geschehen kann. Auf der Reaktorebene muss neben den Partikeln auch die Gasphase zwischen den Partikeln berücksichtigt werden. Die Integration des Einzelpartikelmodells in das Reaktormodell geschieht auf Basis des Repräsentativen Partikelmodells (RPM). Dieser Ansatz ermöglicht eine vertretbare Rechenzeit für die Lösung des Mehrskalenmodells auch für einen Reaktor in technischer Größenordnung. Dabei wird lediglich ein Partikelmodell für jedes finite Volumenelement des Reaktors gelöst. Die RPM-Methode wird angewendet auf die Erwärmung sowie die Pyrolyse einer Holzpartikelschüttung. Die Simulationsergebnisse werden mit verfügbaren Daten aus der Literatur verglichen. Letztere werden vom Modell gut wieder gegeben, wobei die Gradienten innerhalb der Partikel in wesentlich kürzerer Rechenzeit beschrieben werden können als in einem Diskreten Partikelmodell (DPM).The goal of this thesis is to converge the models of different time and length scales that are present in thermo-chemical processes of biomass in order to describe them from first principles in a multi-scale approach. The focus will be on fixed-bed pyrolysis and the molecular, particle and reactor level will be described. At the molecular level the reaction schemes that should be applied to describe the processes are discussed and the kinetics of these reactions are calculated. Kinetics of biomass smouldering, including biomass pyrolysis and char oxidation, were calculated by thermo-gravimetric analysis. At the particle level transport phenomena should be taken into account in combination with kinetics derived from the molecular level. A particle model describing pyrolysis of a single biomass particle was developed together with an experimental set-up. The main disadvantage of incorporating a particle model in a reactor model is the high computational time needed for the numerical solution of such a multi-scale model. To decrease this computational time a novel iterative solution method for solving a particle model based on the analysis of characteristic times was developed. Also, the presence of secondary heterogeneous tar cracking reactions in single particle slow pyrolysis was shown by laser induced fluorescence. At the reactor level the single biomass particles should be considered together with the interstitial gas phase. The introduction of a particle model in a reactor model was done with the representative particle model (RPM) approach, which numerically solves the problem in a feasible computational time for a technical scale reactor. In the RPM approach an intra-particle model is solved for each finite volume element of the reactor. The RPM framework was applied to fixed-bed heating up and pyrolysis and compared to experimental results available in the literature. It was able to predict the experimental results, describing intra-particle gradients in a much more feasible computational time than the discrete particle model (DPM)

Performance of the CFD model of the pyrolysis of a single cylindrical and spherical maple wood particles in a thermally thick regime over a range of temperatures and sizes

Predicting pyrolytic conversion of larger particles of wood (thermally thick regime) is a complex task due to heat transfer limitations, which detailed influence is still not fully known. The CFD modelling allows for the real-time investigation of parameters that are impossible to be analysed experimentally. That can provide new insights into the conversion mechanism and help to fill the knowledge gaps. The aim of this study was to establish a comprehensive single particle model of wood pyrolysis in the thick regime, valid over a wide range of parameters, to serve as a reliable tool for process optimisation and product tailoring. The investigation was based on the 2D-axisymmetrical models of the particles (without surroundings), developed in the commercial software COMSOLTM v5.3. The particles were modelled as porous matter with solid and fluid phases. As the kinetic was used the RAC scheme. Models included heat transfer (conduction, convection, and radiation), the vapours flow (Darcy’s) and mass diffusion (Fick’s), but not shrinking (software’s limitations). The thermochemical properties and the boundary conditions were based on the experimental data of Attreya et al. [1], also used for the validation. The experimental data consisted results of the maple wood particles with cylindrical shape (D/H [mm/mm] = 10/20, 15/20 and 20/20) and spherical shape (D [mm] = 10, 15 and 20), pyrolysed in temperatures 500 °C, 610 °C, 720 °C and 840 °C (in total 24 scenarios). All models were checked for grid independence to avoid numerical errors in the results of the simulations. The fit of the predicted profiles of temperature at the centre and mass loss to experimental data had more than satisfactory considering a broad range of conditions. The absolute error between experimental and predicted char yield was 1.3±0.7 wt. % and 1.8±1.5 wt. % for the spheres and cylinders, respectively. The yields of products per initial mass did not change linearly with the particle’s size for the same shape, and such an outcome was assigned to the influence of the heat transfer limitations. The temperature distribution and the release profiles of vapours indicated the existence of the outer thermal layer (c.a. 2.5 mm thickness), which had a significantly higher heating rate than the inner areas of the particle. The developed model is suitable for the detailed investigation of the pyrolysis process. The particle shrinking and the adjustment of the heat transfer have to be implemented to increase the model’s accuracy. The existence of the layer has to be experimentally proven considering the numerical foundation of the observation. If confirmed, the relation between, e.g., the thermal conductivity of the wood and the outer layer thickness should be investigated. [1] Atreya A, Olszewski P, Chen Y, Baum HR. The effect of size, shape and pyrolysis conditions on the thermal decomposition of wood particles and firebrands. International Journal of Heat and Mass Transfer. 2017;107:319-28