Publikationen

Peer reviewed papers | 2024

Chemical looping of synthetic ilmenite, Part I: Addressing challenges of kinetic TGA measurements with H2

Steiner T, Schulze K, Kienzl N, Pauritsch M, Hacker V, Bock S, Abad A, Scharler R, Anca-Couce A. Chemical looping of synthetic ilmenite, Part I: Addressing challenges of kinetic TGA measurements with H2. Fuel. 15 July 2024. 368:131528.

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Reliable experimental data and models are required to better understand and design chemical looping processes with oxygen carrier materials like ilmenite. A dubious variability of suggested kinetics for similar oxygen carrier materials has been presented in the literature. Part I of this work focuses on thermogravimetric analysis (TGA) of gas–solid kinetics and addresses several of its challenges, which are possible reasons behind such deviations. The reduction of synthetic ilmenite (60 mass% Fe2O3 +40 mass% TiO2) powder with H2 in a TGA system was investigated for this purpose.
Multiple steps were necessary to overcome mass transfer limitations during the measurements: (i) small sample masses down to 1.6 mg, (ii) high gas flow rates, (iii) a suitable sample carrier and (iv) proper sample dispersion on the sample carrier. Three types of sample carriers (crucible, basket and plate) were tested; the plate showed the best performance overall. It was alarming that an exemplary increase in sample mass from 1.6 to 3 mg, which was still significantly lower than all other studies reviewed, already introduced a noticeable influence of diffusion. Isothermal (650–950 °C, 17–50 vol% H2) and nonisothermal parameter studies were conducted and yielded vastly different isoconversional activation energies. A computational fluid dynamics (CFD) study of the TGA system suggested considerable axial dispersion of H2 influencing the initial conversion period.
These findings help to assess the reliability of kinetic studies and guide towards diffusion-free, kinetic measurements. The results will be used for model development in part II.

Peer reviewed papers | 2024

Comparison of single particle models: simplified layer model and detailed volumetric model for biomass, chemical looping and metal oxide conversion processes

Steiner T, Schulze K, Scharler R, Anca-Couce A. Comparison of single particle models: simplified layer model and detailed volumetric model for biomass, chemical looping and metal oxide conversion processes. Chemical Engineering Journal. 15 May 2024. 488:150993

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Various single particle models to describe the conversion of porous solids with gaseous reactants are available in the literature. It is, therefore, not obvious which models should be selected for specific problems and applications. This work focuses on two popular types of particle models: the volumetric model (VM) and the layer model (LM). Different variations of the layer model were considered: the standard layer model, which is similar to common shrinking core models, and an extended layer model, which solves inherent problems of the shrinking core approach by replacing surface reactions with volumetric reactions. For the first time, all these models were benchmarked together regarding prediction quality and computational cost for relevant applications: gasification and oxidation of biochar, reduction of nickel oxide and oxidation of magnetite. These cases covered a wide range of Thiele moduli and Biot numbers. The volumetric model reliably predicted the conversion for all cases considered. Its computational effort was, however, significantly higher than for the layer models. Suitable reactions kinetics in combination with heat of reaction and pore diameters were integral to prediction accuracy. Char oxidation, having a high Thiele modulus, could be described suitably by the standard layer model and the extended layer model, when it accounted for the residual ash layer. Char gasification and nickel oxide reduction had moderate Thiele moduli, rendering the standard layer model unsuitable in the general case. The extended layer model overcomes these limitations due to its volumetric reaction approach. All layer models showed inferior temperature predictions for biochar gasification and magnetite oxidation owing to their lower spatial resolution compared to the volumetric model. Additional, possible problems for layer models were addressed.

Peer reviewed papers | 2024

Release of N-containing compounds during pyrolysis of milk/dairy processing sludge – Experimental results and comparison of measurement techniques

Kwapinska M, Sommersacher P, Kienzl N, Retschitzegger S, Lagler J, Horvat A, Leahy JJ, Release of N-containing compounds during pyrolysis of milk/dairy processing sludge – Experimental results and comparison of measurement techniques. Journal of Analytical and Applied Pyrolysis.2024_178:10639. 10.1016/j.jaap.2024.106391

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A dried dairy processing sludge (sludge from wastewater treatment of an effluent from a milk processing plant) was pyrolysed in a single-particle reactor at different temperatures from 400 °C to 900 °C. NH3 and HCN were measured online and offline by means of FTIR as well as by cumulative sampling in impinger bottles (in 0.05 M H2SO4 and 1 M NaOH, respectively) and analysed by photometric method. NO and NO2 were measured online using a nitric oxide analyser while N2O was measured by FTIR. Nitrogen (N) in the sludge and in the remaining char, char-N, was determined. Moreover, tar content in pyrolysis gas was measured and tar-N was determined. The results with respect to N mass balance closure are discussed. The different measurements techniques are compared. For pyrolysis at 520 ℃ and 700 ℃ nitrogen in the gas phase was mainly contained as N2 (36 % and 40 % respectively), followed by NH3 (15 % and 18 %), tar-N (10 % and 9 %), HCN (1 % and 3 %), NO (1 %) and NO2 (0.2 %). The dairy processing sludge has very specific properties with organic-N present predominantly as proteins and a high content of inherent Ca. These characteristics affected the distribution of N. The amount of char-N was higher while the amount of tar-N lower than for sewage sludge from literature, at comparable pyrolysis temperature.

Other Publications | 2023

BIOPOLYCOMP - Biochar for Polymer Composites

Martinie S, Kienzl N, Sarsteiner J. BIOPOLYCOMP - Biochar for Polymer Composites. BEST Center Day. 28 June 2023

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Char originating from biomass can be used as a sustainable carbon additive in the production of polymer compounds with enhanced characteristics.

Other Publications | 2023

Chemical Looping for efficient biomass utilization

Schulze K, Kienzl N, Steiner T, Martini S, Priscak J. Chemical Looping for efficient biomass utilization. BEST Center Day. June 2023

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With respect to the climate objectives Chemical Looping (CL) processes constitute a promising alternative to traditional thermochemical conversion routes. Through the application of solid materials, so-called oxygen carriers (OC), instead of air as oxygen supply, CO2 can be easily separated from the flue gas. By this, biomass can be used for hydrogen production (Chemical Looping Hydrogen, CLH) or it can be burnt without CO2 emissions (Chemical Looping Combustion, CLC).

Peer reviewed papers | 2023

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

Steiner T, Schulze R, Scharler R, Anca-Couce A. Extension of the layer particle model for volumetric conversion reactions during char gasification. Combustion and Flame. 2023:256,112940.

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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.

Other Publications | 2023

Further development of gas-fermentation towards syngas utilization and electro-fermentation

Ludwig K, Hiebl C, Marzynski M, Deutsch M, Poms U, Schulze K, Neubauer M, Knoll L, Rachbauer L, Gölles M, Fuchs W, Drosg B. Further development of gas-fermentation towards syngas utilization and electro-fermentation. BEST center day. 28 June 2023.

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Gas-fermentation is the conversion of gaseous feedstocks (e.g. CO2-rich off gases, CO, H2) into
valuable products such as organic acids and alcohols by microorganisms such as clostridia.
By supplying electrical energy (an alternative source of reducing/oxidizing energy), the fermentation
environment can be further optimized, resulting in products with higher purity, a broader product
spectrum and higher cell densities.

Other Publications | 2023

Green Carbon as reducing agent in iron and steel production via the blast furnaces

Deutsch R, Krammer G, Kienzl N, Strasser C. Green Carbon as reducing agent in iron and steel production via the blast furnaces. BEST Center Day. 28 June 2023

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Iron production via blast furnace utilizes coal and coke to reduce iron oxides resulting in high greenhouse gas emissions. This important issue for the iron and steel industry may be mitigated by application of biomass-based reducing agents (bioreducer).

Peer reviewed papers | 2023

Multi-scale modelling of a fluidized bed biomass gasifier of industrial size (1 MW) using a detailed particle model coupled to CFD: Proof of feasibility and advantages over simplified approaches

von Berg L, Anca-Couce A, Hochenauer C, Scharler R. Multi-scale modelling of a fluidized bed biomass gasifier of industrial size (1 MW) using a detailed particle model coupled to CFD: Proof of feasibility and advantages over simplified approaches. Energy Conversion and Management. 15 June 2023.286:117070

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Fluidized bed biomass gasification is a complex process whereby gas source terms are released by reactions at the particle level during the movement of fuel particles throughout the reactor. The current study presents for the first time the application of a multi-scale modelling approach for a fluidized bed biomass gasifier of industrial size, coupling a detailed one-dimensional particle model based on the progressive conversion model (PCM) with a commercial CFD software. Results of particle movement and gas source terms are compared with results of an additional simulation employing the simplified uniform conversion model (UCM) which is commonly used in literature. Validation at the particle level showed that the UCM leads to a massive underprediction of the time needed for pyrolysis whereas the PCM is in good agreement with experimental data. This heavily influences the gas sources released during pyrolysis of the biomass particles in the coupled reactor simulations. Volatiles are much more concentrated to the close proximity of the fuel feed when using the UCM whereas the PCM leads to a more homogeneous distribution over the reactor cross-section. The calculation time analysis of the coupled simulations showed that despite the increased complexity, the PCM shows only an increase of 20% in calculation time when compared to the UCM, whereas it is much better suited for these conditions. The coupled multi-scale simulations using the PCM showed the numerical feasibility of the modelling approach for 1,200,000 bed parcels and about 80,000 reacting fuel parcels and furthermore highlighted the importance of a comprehensive description of the particle level.

Other Publications | 2023

Numerical investigation of reaction mechanisms on NOx emissions from biomass combustion with enhanced reduction

Eßl M, Schulze K. Numerical investigation of reaction mechanisms on NOx emissions from biomass combustion with enhanced reduction. BEST Center Day. 28 June 2023

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With the increasing demand for lower emissions and innovative combustion technologies, it is necessary to have a reaction mechanisms that is accurate as well as computationally affordable for geometry and process optimization using computational fluid dynamics (CFD). The objective of this work is to explore the applicability of several reaction mechanisms in predicting NOx emissions from various combustion systems. This work focuses on the selection of suitable mechanisms from literature (see Table 1) in a full scale 3D model for the prediction of NOX especially for furnaces with low oxygen concentration in the fuel bed and enhanced reduction zones.

Peer reviewed papers | 2022

Assessment of measurement methods to characterize the producer gas from biomass gasification with steam in a fluidized bed

Anca-Couce A, von Berg L, Pongratz G, Scharler R, Hochenauer C, Geusebroek M, Kuipers J, Vilela CM, Kraia T, Panopoulos K, Funcia I, Dieguez-Alonso A, Almuina-Villar H, Tsiotsias T, Kienzl N, Martini S. Assessment of measurement methods to characterize the producer gas from biomass gasification with steam in a fluidized bed. Biomass and Bioenergy 2022.163:106527

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Measuring the producer gas from biomass gasification is very challenging and the use of several methods is required to achieve a complete characterization. Various techniques are available for these measurements, offering very different affordability or time demand requirements and the reliability of these techniques is often unknown. In this work an assessment of commonly employed measuring methods is conducted with a round robin. The main permanent gases, light hydrocarbons, tars, sulfur and nitrogen compounds were measured by several partners employing a producer gas obtained from fluidized bed gasification of wood and miscanthus with steam. Online and offline methods were used for this purpose and their accuracy, repeatability and reproducibility are here discussed. The results demonstrate the reliability of gas chromatography for measuring the main permanent gases, light hydrocarbons, benzene and H2S, validating the obtained results with other methods. An online method could also measure NH3 with a reasonable accuracy, but deviations were present for compounds at even lower concentrations. Regarding tar sampling and analysis, the main source of variability in the results was the analysis of the liquid samples, especially for heavier compounds. The presented work pointed out the need for a complementary use of several techniques to achieve a complete characterization of the producer gas from biomass gasification, and the suitability of certain online techniques as well as their limitations.

Conference presentations and posters | 2022

BEST-Day

Sustainable biorefineries and digitalization

Schwabl M, Wopienka E, Drosg B, Kuba M, Weber G, Eßl M, Gölles M, Kaiermayer V, Liedte P, Fuhrmann M. BEST-Day: Sustainable biorefineries and digitalization. 7th Central European Biomass Conference CEBC 2023. 18. January 2023. Graz. Oral Presentation.

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List of presentations:

Biorefineries

  • Learnings from biomass combustion towards future bioenergy applications (M. Schwabl)
  • Green Carbon perspectives for regional sourcing and decarbonization (E. Wopienka)
  • Bioconversion processes for renewable energy and/or biological carbon capture and utilisation (B. Drosg)
  • Second generation biomass gasification: The Syngas Platform Vienna – current status and outlook (M. Kuba)
  • Utilization of syngas for the production of fuel and chemicals – recent developments and outlook (G. Weber)

Digital methods, tools and sustainability

  • Evaluation of different numerical models for the prediction of NOx emissions of small-scale biomass boilers (M. Eßl)
  • Digitalization as the basis for the efficient and flexible operation of renewable energy technologies (M. Gölles)
  • Smart Control for Coupled District Heating Networks (V. Kaisermayer)
  • Integrated energy solutions for a decentral energy future - challenges and solutions (P. Liedtke)
  • Wood-Value-Tool: Techno-economic assessment of the forest-based sector in Austria (M. Fuhrmann)
Conference presentations and posters | 2022

MATHEMATICAL MODEL FOR MODEL-BASED CONTROL OF ABSORPTION HEAT PUMPING SYSTEMS

Zlabinger S, Unterberger V, Gölles M, Horn M, Wernhart M, Rieberer R. MATHEMATICAL MODEL FOR MODEL-BASED CONTROL OF ABSORPTION HEAT PUMPING SYSTEMS. 2nd International Sustainable Energy Conference – ISEC 2022. October 2022.

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Peer reviewed papers | 2022

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

von Berg L, Anca-Couce A, Hochenauer C, Scharler R. Multi-scale modelling of fluidized bed biomass gasification using a 1D particle model coupled to CFD. Fuel. 15 September 2022.324:124677

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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.

Peer reviewed papers | 2022

Solid oxide fuel cell operation with biomass gasification product gases: Performance- and carbon deposition risk evaluation via a CFD modelling approach

Pongratz G, Subotić V, Hochenauer C, Scharler R, Anca-Couce A. Solid oxide fuel cell operation with biomass gasification product gases: Performance- and carbon deposition risk evaluation via a CFD modelling approach. 1 April 2022. 244.

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Solid oxide fuel cell (SOFC) models used in the past for biomass-to-power plant simulations are limited in their predictability of the carbon deposition risk. In this work, industrial-relevant cell designs were modeled in 2D-CFD considering detailed reaction kinetics which allowed more accurate performance simulations and carbon deposition risk assessments. Via a parametric study, the influence of varying cell operating conditions on the cell performance and carbon deposition risk was quantified when utilizing product gases from steam- and air gasification with varying steam addition. Considering the results from this parameter study and carbon deposition risk assessment, recommendations for promising gasifier-SOFC configurations and cell operating points for stable long-term operation are presented. For smaller-scale biomass-to-power systems, the utilization of product gas from air gasification in anode supported cells with Ni/zirconia-based anode can be recommended, with only moderate steam dilution of the product gas at 750°C cell operating temperature. For larger scales, steam gasification might be meaningful, offering a generally higher electrical efficiency and power output in fuel cells than air gasification. However, a higher risk for carbon deposition could be determined in comparison to air gasification. Hence, a cell temperature of 850°C besides the use of cells with Ni/ceria-based anodes is recommended.

Conference presentations and posters | 2021

A platform for energy management in communities

Derflinger N, Zellinger M. A platform for energy management in communities. ComForEn 2021 11. Symposium Communications for Energy Systems. 23 November 2021.

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Conference presentations and posters | 2021

Advances in biomass gasification for the production of Bioheat, bioelectricity and biofuels

Anca-Couce A, Archan G, Von Berg L, Pongratz G, Martini S, Buchmayr M, Rakos C, Hochenauer C, Scharler R. Advances in biomass gasification for the production of Bioheat, bioelectricity and biofuels. 29th European Biomass Conference and Exhibition, EUBCE 2021, 26-29 April 2021. 2021.

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Current barriers to increase the use of bioenergy for different applications are first discussed. Then, recent advances are presented on gasification-based technologies to overcome these barriers that have been reached at TU Graz together with several partners. Gasification-based fuel bed concepts integrated in biomass combustion can significantly reduce emissions for bioheat production. Advances are presented for modern biomass boilers, significantly reducing nitrogen oxides and particle matter emissions as well as increasing the feedstock flexibility; and micro-gasifiers for traditional biomass utilization, significantly reducing the emissions of unburnt products. Gasification-based processes have as well the possibility to score high electrical efficiencies and to synthetize several products as second-generation biofuels. Advances are presented on measures for reducing the presence of contaminants as tars, including the catalytic use of char for tar cracking; and in applications of the producer gas, including gas cleaning and direct coupling with a solid oxide fuel cell to maximize electricity production. © 2021, ETA-Florence Renewable Energies.

Peer reviewed papers | 2021

Analysis of H2S-related short-term degradation and regeneration of anode- and electrolyte supported solid oxide fuel cells fueled with biomass steam gasifier product gas

Pongratz G, Subotić V, Schroettner H, Hochenauer C, Skrzypkiewicz M, Kupecki J, Anca-Couce A, Scharler R. Analysis of H2S-related short-term degradation and regeneration of anode- and electrolyte supported solid oxide fuel cells fueled with biomass steam gasifier product gas. Energy.2021.218:119556.

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Using solid oxide fuel cells in biomass gasification based combined heat and power production is a promising option to increase electrical efficiency of the system. For an economically viable design of gas cleaning units, fuel cell modules and further development of suitable degradation detection methods, information about the behavior of commercially available cell designs during short-term poisoning with H2S can be crucial. This work presents short-term degradation and regeneration analyses of industrial-relevant cell designs with different anode structure and sulfur tolerance fueled with synthetic product gas from wood steam gasification containing 1 to 10 ppmv of H2S at 750°C and 800°C. Full performance regeneration of both cell types was achieved in all operating points. The high H2O content and avoided fuel depletion may have contributed to a lower performance degradation and better regeneration of the cells. A strong influence of the catalytically active anode volume on poisoning and regeneration behavior was quantified, thereby outlining the importance of considering the anode structure besides the sulfur tolerance of the anode material. Hence, cells with less sulfur tolerant anode material but larger anode volume might outperform cells less sensitive to sulfur in the case of an early detection of a gas cleaning malfunction.

Peer reviewed papers | 2021

Bioenergy technologies, uses, market and future trends with Austria as a case study

Anca-Couce A, Hochenauer C, Scharler R. Bioenergy technologies, uses, market and future trends with Austria as a case study. Renewable and Sustainable Energy Reviews.2021;135:110237.

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The current bioenergy uses and conversion technologies as well as future trends for the production of heat, power, fuels and chemicals from biomass are reviewed. The focus is placed in Austria, which is selected due to its high bioenergy utilization, providing 18.4% of the gross energy final consumption in 2017, and its strong industrial and scientific position in the field. The most common bioenergy application in Austria is bioheat with 170 PJ in 2017 mainly obtained from woody biomass combustion, followed by biofuels with 21 PJ and bioelectricity with 17 PJ. Bioheat has a stable market, where Austrian manufacturers of boilers and stoves have a strong position exporting most of their production. Future developments in bioheat production should go in the line of further reducing emissions, increasing feedstock flexibility and coupling with other renewables. For bioelectricity and biofuels, the current framework does not promote the growth of the current main technologies, i.e. combined heat and power (CHP) based on biomass combustion or biogas and first generation biofuels. However, an increase in all bioenergy uses is required to achieve the Austrian plan to be climate neutral in 2040. The current initiatives and future possibilities to achieve this increase are presented and discussed, e.g. mandatory substitution of old oil boilers, production of biomethane and early commercialization of CHP with a high efficiency or demonstration of advanced biofuels production based on gasification.

Peer reviewed papers | 2021

Correlations between tar content and permanent gases as well as reactor temperature in a lab-scale fluidized bed biomass gasifier applying different feedstock and operating conditions

von Berg L, Pongratz G, Pilatov A, Almuina-Villar H, Scharler R, Anca-Couce A. Correlations between tar content and permanent gases as well as reactor temperature in a lab-scale fluidized bed biomass gasifier applying different feedstock and operating conditions.Fuel.2021.305:121531

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The major problem of fluidized bed biomass gasification is the high tar contamination of the producer gas which is associated with the complex and time-consuming sampling and analysis of these tars. Therefore, correlations to predict the tar content are a helpful tool for the development and operation of biomass gasifiers. Correlations between tars and gas composition as well as reactor temperature derived for a steam-blown lab-scale bubbling fluidized bed gasifier are investigated in this study to assess their applicability. A comprehensive data set containing over 80 experimental points was obtained for various operation conditions, including variations in temperature from 700 to 800 °C, feedstock, amount of steam for fluidization, as well as the addition of oxygen. Linear correlations between tar and permanent gases show good accuracy for H2 and CH4 when using pure steam. However, experiments conducted with steam-oxygen mixtures show high deviations for the CH4-based correlation and smaller but still significant deviations for the H2-based correlation. No relation between tar and CO or CO2 was found. The correlation between tar and temperature shows highest accuracy, including good agreement with the steam-oxygen experiments. All tar correlations showed useful results over a broad operating range. However, significant deviations can be obtained when considering just one gas compound. Therefore, a combination of different correlations considering gas components and temperature seems to be the best method of tar prediction. This leads to a powerful tool for fast online tar monitoring for a broad range of operating conditions, once a calibration measurement was conducted.

Peer reviewed papers | 2021

Emission minimization of a top-lit updraft gasifier cookstove based on experiments and detailed CFD analyses

Scharler R, Archan G, Rakos C, von Berg L, Lello D, Hochenauer C, Anca-Couce A. Emission minimization of a top-lit updraft gasifier cookstove based on experiments and detailed CFD analyses. Energy Conversion and Management. 2021.247:114755.

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Around 2.7 billion people worldwide have no access to clean cooking equipment, which leads to major health problems due to high emissions of unburned products (VOC, CO and soot). A top-lit updraft gasifier cookstove with forced draft was identified as the technology with the highest potential for reducing harmful emissions from incomplete combustion in simple cookstoves. The basic variant of the stove was equipped with a fan for efficient mixing of product gas with air and fired with pellets to increase the energy density of low-grade residues. The development was conducted based on water boiling test experiments for wood and rice hull pellets and targeted CFD simulations of flow, heat transfer and gas phase combustion with a comprehensive description of the reaction kinetics, which were validated by the experiments. Emphasis was put on the reduction of CO emissions as an indicator for the burnout quality of the flue gas. The optimisation was carried out in several steps, the main improvements being the design of a sufficiently large post-combustion chamber and a supply of an appropriate amount of primary air for a more stable fuel gasification. The experiments showed CO emissions <0.2 g/MJdel for wood and rice hull pellets, which corresponds to a reduction by a factor of about 15 to 20 compared to the basic forced draft stove concept. Furthermore, these values are between 5 and 10 times lower than published water boiling test results of the best available cookstove technologies and are already close to the range of automatic pellet furnaces for domestic heating, which are considered to be the benchmark for the best possible reduction of CO emissions.

Peer reviewed papers | 2021

Modelling fuel flexibility in fixed-bed biomass conversion with a low primary air ratio in an updraft configuration

Anca-Couce A, Archan G, Buchmayr M, Essl M, Hochenauer C, Scharler R. Modelling fuel flexibility in fixed-bed biomass conversion with a low primary air ratio in an updraft configuration. Fuel. 2021.296:120687.

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Fixed-bed biomass conversion with a low primary air ratio and a counter-current configuration has a high feedstock flexibility, as it resembles updraft gasification, and the potential to reduce emissions when integrated in biomass combustion systems. A 1D bed model was validated with experimental results from a biomass combustion boiler with such a bed conversion system, predicting with a good accuracy the temperatures in the reactor and producer gas composition. The model was applied for different cases to investigate the fuel flexibility of this combustion system, including the influence of moisture content and the maximum temperatures achieved in the bed. It was shown that with variations in fuel moisture content from 8 to 30% mass w.b. the producer gas composition, char reduction to CO or maximum temperatures at the grate were not affected due to the separation of the char conversion and pyrolysis/drying zones. Flue gas recirculation was the only possible measure with the tested configuration to reduce the maximum temperatures close to the grate, which is beneficial e.g. to avoid slagging with complicated fuels. A higher tar content was obtained than in conventional updraft gasifiers, which is attributed to the absence of tar condensation in the bed due to the limited height of the reactor and the integration in the combustion chamber. The presented model can support the development of such combustion technologies and is a relevant basis for detailed CFD simulations of the bed or gas phase conversion.

Other papers | 2020

Advanced modular process analysis tool for biomass-based Chemical Looping systems

Steiner T, Schulze K, Scharler R. Advanced modular process analysis tool for biomass-based Chemical Looping systems. 3RD DOCTORAL COLLOQUIUM BIOENERGY. 2020.

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In order to limit global warming to 1.5 °C compared to the pre-industrial temperature level, zero net CO2 emissions are needed on a global scale until 2050. A Chemical Looping (CL) process represents a technological system which is CO2-negative when using biomass as fuel and thus can substantially contribute to this target. In principle, the process uses a metal oxide as oxygen carrier material (OC) which is cyclically oxidized by air or steam and reduced by the fuel. Without air as the direct oxygen source for fuel conversion, high calorific product gases or pure carbon dioxide in case of combustion are obtained after the condensation of water vapor, which can then be stored or further utilized.
Within the funded project ”BIO-LOOP”, different Chemical Looping processes (for example combustion, gasification, hydrogen production) and reactors (fixed bed, fluidized bed) are investigated numerically and experimentally. An advanced process analysis tool based on mass and energy balances of the system considered will be presented. It provides data about the specific internal and external streams, process conditions and efficiencies. Within the analysis tool, various independent modular units describe individual process steps, e.g. mixing, chemical reaction or splitting. These components can be adjusted, combined and interconnected according to the flow chart of the system. The process model represents the first step towards a flexible Chemical Looping reactor simulation toolbox to analyze various process scenarios. Emphasis is put on the flexibility regarding the fuels and oxygen carriers, their conversion and possible process variations. The tool developed will support upcoming CFD modeling and further economic considerations.

Peer reviewed papers | 2020

Biomass pyrolysis TGA assessment with an international round robin

Anca-Couce A, Tsekos C, Retschitzegger S, Zimbardi F, Funke A, Banks S, Kraia T, Marques P, Scharler R, de Jong W, Kienzl N. Biomass pyrolysis TGA assessment with an international round robin.Fuel.2020;276:118002.https://doi.org/10.1016/j.fuel.2020.118002

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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.

Peer reviewed papers | 2020

Correction to: Investigation of solid oxide fuel cell operation with synthetic biomass gasification product gases as a basis for enhancing its performance

Pongratz G, Subotić V, Schroettner H, Stoeckl B, Hochenauer C, Anca-Couce A, Scharler R. Correction to: Investigation of solid oxide fuel cell operation with synthetic biomass gasification product gases as a basis for enhancing its performance. Biomass Conversion and Biorefinery. 2020

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The authors want to acknowledge, that during the production of the final version of the publication the image for Figure 9 has been replaced with the image for Figure 12, however without changing the content of the paper. This issue is resolved in the current version of the publication.

Peer reviewed papers | 2020

Detailed experimental investigation of the spatially distributed gas release and bed temperatures in fixed-bed biomass combustion with low oxygen concentration

Archan G, Anca-Couce A, Gregorc J, Buchmayr M, Hochenauer C, Gruber J, Scharler R. Detailed experimental investigation of the spatially distributed gas release and bed temperatures in fixed-bed biomass combustion with low oxygen concentration. Biomass and Bioenergy. 2020;141:105725

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This publication focuses on the experimental investigation of a novel small-scale fuel flexible biomass combustion technology with a fixed-bed employing a low oxygen concentration. It was obtained through a low primary air ratio and the additional supply of recirculated flue gas. The plant was operated with spruce wood chips, which contained three different mass fractions of water, and miscanthus pellets. All relevant components of the released gas above the fixed-bed were measured, as well as the 3D bed temperature distribution. The balances confirmed a high experimental data consistency. Therefore, it was possible to determine the location of the four different conversion zones inside the fixed-bed: drying, pyrolysis, char gasification and char oxidation. The reduction of CO2 to CO in the char reduction zone worked efficiently across the entire grate area. Furthermore, the results showed that the water mass fraction of the fuel did not influence the dry product gas composition, but significantly affected the location for the release of pyrolysis products such as tars. It was found that the low oxygen concentration in the fixed-bed combined with flue gas recirculation was an effective method to reduce bed temperatures and therefore its inorganic emissions while significantly increasing feedstock flexibility. The investigations provided fundamental findings on the conversion and release behavior of the new technology under real operating conditions and are very useful for further experimental work and CFD simulations targeting the reduction of PM and NOX emissions.

Peer reviewed papers | 2020

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

von Berg L, Soria-Verdugo A, Hochenauer C, Scharler R, Anca-Couce A. Evaluation of heat transfer models at various fluidization velocities for biomass pyrolysis conducted in a bubbling fluidized bed. International Journal of Heat and Mass Transfer. 2020;160:120175

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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 . 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.

Peer reviewed papers | 2020

Investigation of solid oxide fuel cell operation with synthetic biomass gasification product gases as a basis for enhancing its performance

Pongratz G, Subotić V, Schroettner H, Stoeckl B, Hochenauer C, Anca-Couce A, Scharler R. Investigation of solid oxide fuel cell operation with synthetic biomass gasification product gases as a basis for enhancing its performance. Biomass Conversion and Biorefinery.2020.

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Solid oxide fuel cells represent a promising technology to increase the electrical efficiency of biomass-based combined-heat-power systems in comparison to state-of-the-art gas engines, additionally providing high temperature heat. To identify favorable fuel gas compositions for an efficient coupling with gasifiers at low degradation risk is of major importance to ensure stability, reliability, and durability of the systems used, thus increasing attractiveness of electricity production from biomass. Therefore, this study presents a comprehensive analysis on the influence of main gas components from biomass gasification on the performance and efficiency of a cell relevant for real application. An industrial-size electrolyte supported single cell with nickel/gadolinium-doped ceria anode was selected showing high potential for gasifier-solid oxide fuel cell systems. Beneficial gas component ratios enhancing the power output and electric efficiency are proposed based on the experimental study performed. Furthermore, the degradation stability of a SOFC fueled with a synthetic product gas representing steam gasification of woody biomass was investigated. After 500 h of operation under load at a steam-to-carbon ratio of 2.25 in the fuel gas, no performance or anode degradation could be detected.

Other papers | 2020

Numerical simulation of fuel nitrogen conversion and NOx emissions in biomass boilers with advanced air staging technology

Essl M, Schulze K, Scharler R. Numerical simulation of fuel nitrogen conversion and NOx emissions in biomass boilers with advanced air staging technology. 3RD DOCTORAL COLLOQUIUM BIOENERGY. 2020

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The increased biomass utilization leads to the need of an efficient and flexible usage of available sources. Therefore, it is necessary to combust low-cost biogenic residues, which inherently have higher nitrogen contents that lead to increased NOx emissions. In order to tackle this issue a new combustion technology with double air staging and flue gas recirculation is under development. The technology also features an increased fuel bed height and very low oxygen concentrations in the fuel bed to reduce fuel bed temperatures. This work focuses on the CFD simulation of the formation and reduction of NOx emissions of in a small scale boiler (35 kWth). Compared to previously applied models, major modification concerning the heat and mass transfer in the fuel bed as well as the subsequent conversion in the freeboard were made. The fuel bed is modelled via representative fuel particles with a Lagrangian approach and a thermally thick particle model considering intra-particle
gradients. Due to the increased fuel bed height and the relatively low oxygen concentration the formation and cracking of tars has to be considered in the simulation. This heavily influences the formation and reduction of NOx and its precursors. The fuel bound nitrogen is released via the particle model in the form of NO during char burnout and via a lumped tar species during pyrolysis. The cracking of the lumped tar species is modelled via two global gas phase reactions that releases the NOx precursors NH3 and HCN. The cracking reactions are added to a skeletal reaction mechanism with 28 species and 102 reactions that includes the fate of the N species. The simulation results are compared to experimental data from test runs with spruce wood chips and Miscanthus pellets as fuels. The comparison showed good agreement for the test runs with wood chips, where the temperature distribution inside the fuel bed and the released species above the fuel bed were predicted well. The test runs with Miscanthus showed a greater deviation between the measured and simulated values. For both fuels the NOx reduction that was experimentally observed in the secondary combustion zone could not be predicted with reasonable agreement. Therefore, it is necessary to further investigate the cracking of the tars and the subsequent formation of the NOx precursors. The presented work forms the basis for further improvements of the numerical models and subsequently the optimization of the new technology.

Peer reviewed papers | 2020

Transient CFD simulation of wood log combustion in stoves

Scharler R, Gruber T, Ehrenhöfer A, Kelz J, Mehrabian Bardar R, Bauer T, Hochenauer C, Anca-Couce A. Transient CFD simulation of wood log combustion in stoves. Renewable Energy 2020.145:651-662

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Wood log stoves are a common residential heating technology that produce comparably high pollutant emissions. Within this work, a detailed CFD model for transient wood log combustion in stoves was developed, as a basis for its optimization. A single particle conversion model previously developed by the authors for the combustion of thermally thick biomass particles, i.e. wood logs, was linked with CFD models for flow and turbulence, heat transfer and gas combustion. The sub-models were selected based on a sensitivity analysis and combined into an overall stove model, which was then validated by simulations of experiments with a typical wood log stove, including emission measurements. The comparison with experimental results shows a good accuracy regarding flue gas temperature as well as CO2 and O2 flue gas concentrations. Moreover, the characteristic behavior of CO emissions could be described, with higher emissions during the ignition and burnout phases. A reasonable accuracy is obtained for CO emissions except for the ignition phase, which can be attributed to model simplifications and the stochastic nature of stove operation. Concluding, the CFD model allows a transient simulation of a stove batch for the first time and hence, is a valuable tool for process optimization.

Conference presentations and posters | 2019

A CFD-method for the analysis and optimization of the fixed bed conversion in biomass grate furnaces

Singer M, Gruber T, Mehrabian R, Scharler R. A CFD-method for the analysis and optimization of the fixed bed conversion in biomass grate furnaces. 27th European Biomass Conference & Exhibition (Poster). 2019.

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To optimize the combustion of biomass grate furnaces a sensitivity analysis is carried out by means of CFD simulation. The methodical procedure consists of a 3D packed bed biomass combustion model, which describes the most essential characteristics of the thermal conversion of biomass particles, such as the detailed consideration of drying, pyrolysis and char oxidation in parallel processes. Within the sensitivity analysis the following parameters have been investigated: distribution of false air, residence time of fuel on the grate and distribution of recirculated flue gas and primary air below the grate. To evaluate the influence of the varied parameters on the combustion process the focus lied on the position of the thermal conversion of the biomass and the CO at the outlet of the simulation domain. The results of the sensitivity analysis show a shift of the thermal conversion towards the grate end for increased false air as well as for reduced momentum of primary air/recirculated flue gas mixture. An increase of the fuel residence time leads to a shift of the thermal conversion towards the fuel inlet. Consequently a large region of the primary combustion zone is not used due to earlier release of CO inside the fuel bed.

Peer reviewed papers | 2019

Applicability of the SOFC technology for coupling with biomass-gasifier systems: Short- and long-term experimental study on SOFC performance and degradation behaviour

Subotić V, Baldinelli A, Barelli L, Scharler R, Pongratz G, Hochenauer C, Anca-Couce A. Applicability of the SOFC technology for coupling with biomass-gasifier systems: Short- and long-term experimental study on SOFC performance and degradation behaviour. Applied Energy. 2019.256:113904

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Coupling biomass gasification with high temperature Solid Oxide Fuel Cells (SOFCs) is a promising solution to increase the share of renewables and reduce emissions. The quality of the producer gas used can, however, significantly impact the SOFC durability and reliability. The great challenge is to ensure undisturbed operation of such system and to find a trade-off between optimal SOFC operating temperature and system thermal integration, which may limit the overall efficiency. Thus, this study focuses on experimental investigation of commercial SOFC single cells of industrial size fueled with different representative producer gas compositions of industrial relevance at two relevant operating temperatures. The extensive experimental and numerical analyses performed showed that feeding SOFC with a producer gas from a downdraft gasifier, with hot gas cleaning, at an operating temperature of 750 °C represents the most favorable setting, considering system integration and the highest fuel utilization. Additionally, a 120 h long-term test was carried out, showing that a long-term operation is possible under stated operating conditions. Local degradation took place, which can be detected at an early stage using appropriate online-monitoring tools.

Conference presentations and posters | 2019

Biomassevergasung als künftige Säule der Gasversorgung in Österreich?

Strasser C. Biomassevergasung als künftige Säule der Gasversorgung in Österreich?. 10. Internationale Anwenderkonferenz Biomassevergasung. December 2019

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Peer reviewed papers | 2019

Experimental investigation on biomass shrinking and swelling behaviour: Particles pyrolysis and wood logs combustion

Caposciutti G, Almuina-Villar H, Dieguez-Alonso A, Gruber T, Kelz J, Desideri U, Hochenauer C, Scharler R, Anca-Couce A. Experimental investigation on biomass shrinking and swelling behaviour: Particles pyrolysis and wood logs combustion. Biomass and Bioenergy 2019;123:1-13.

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Biomass is a suitable energy source to reduce the carbon footprint and increase the use of renewable energy. However, the biomass exploitation is still slowed by many technical issues. In most practical applications, such as gasification or combustion devices, it is important to predict the fuel physical behavior in order to determine the emissions and heat release profile as well as for modeling and design purposes. Within this paper, the study of the dimensional evolution of a biomass fuel (beech wood) in pyrolysis and combustion processes were carried out with the use of the image analysis tool. Sizes from 15 mm to 300 mm characteristic length range were employed in the experiments and the collected data were related to the mass loss and temperature evolution of the biomass particle. It was found that for all the fuel sizes employed a similar volume reduction (60%–66%) was obtained at the end of pyrolysis. However, for the small particles with minor intra-particle gradients shrinkage took place mainly at the end of conversion, while for bigger particles the size variation patter was more linear. Furthermore, swelling was detected in the pyrolysis experiments, and it was higher for a bigger particle size, while cracking and fragmentation phenomena was observed for large wood logs combustion in the stove.

Peer reviewed papers | 2019

Single large wood log conversion in a stove: Experiments and modelling

Anca-Couce A, Caposciutti G, Gruber T, Kelz J, Bauer T, Hochenauer C, Scharler R. Single large wood log conversion in a stove: Experiments and modelling. Renewable Energy 2019.143:890-897.

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Natural draft wood log stoves for residential bioheat production are very popular due to the low fuel costs, the ecological aspect of a renewable energy source and the visual appeal of the flame. However, they have rather high pollutant emissions, specially of unburnt products. The description of large wood logs conversion in stoves needs to be improved to allow a process optimization which can reduce these emissions. The transient conversion of a single wood log in a stove is experimentally investigated with test runs quenching the log after defined time intervals and measuring the flue gas composition and temperatures in the log and stove. The experiments have been described with a volumetric single particle model, which predicts with good accuracy the log conversion until a time of around 30 min, when pyrolysis is almost ending. At that point, log fragmentation takes place and smaller fragments are detached from the log falling onto the bed of embers. Despite the increase in external surface area, char oxidation takes place at a moderate rate. This last stage of wood log conversion in a stove is the most challenging to model. Finally, preliminary recommendations are provided for reducing CO emissions in wood log stoves.

Peer reviewed papers | 2018

Experiments and modelling of NOx precursors release (NH3 and HCN) in fixed-bed biomass combustion conditions

Anca-Couce A, Sommersacher P, Evic N, Mehrabian R, Scharler R. Experiments and modelling of NOx precursors release (NH3 and HCN) in fixed-bed biomass combustion conditions. Fuel. 2018, 222: 529-537.

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There is a need to reduce NOx emissions, which can only be achieved through a detailed understanding of the mechanisms for their formation and reduction. In this work the release of the NOx precursors, NH3 and HCN, for different fuels is experimentally analysed and modelled in typical fixed-bed combustion conditions. It is shown that NH3 and HCN are released during the main devolatilization phase and the NH3/HCN ratio increases for fuels with a higher nitrogen content. A simplified two-steps model for their release is presented. The model can predict with a reasonable accuracy the release for fuels with a low nitrogen content, however deviations are present for fuels with a high nitrogen content, which probably arise due to a reduction of NH3 and HCN taking place already in the bed.

Peer reviewed papers | 2017

A Generalization of Ackermann’s Formula for the Design of Continuous and Discontinuous Observers

Anca-Couce A, Sommersacher P, Scharler R. Online experiments and modelling with a detailed reaction scheme of single particle biomass pyrolysis. Journal of Analytical and Applied Pyrolysis. Available online 17 July 2017

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Detailed reaction schemes and experimental data for the online release of pyrolysis volatiles are required to gain a more fundamental understanding of biomass pyrolysis, which would in turn allow the process to be controlled in a more precise way and the development of more targeted applications. A detailed online characterisation of pyrolysis products has been conducted in single particle experiments with spruce pellets at different temperatures, obtaining a good closure of the elemental mass balances. The yields and online release of CO, CO2, H2O, CH4, other light hydrocarbons and total organic condensable species, as well as char yield and composition, can be predicted with a reasonable accuracy with the application of a single particle model, coupled with a detailed pyrolysis scheme, and a simple one-step scheme for tar cracking. In order to achieve it, improvements have been conducted in the pyrolysis scheme, mainly concerning the release of light hydrocarbons and char yield and composition. Deviations are still present in the different groups in which organic condensable species can be classified.

Other papers | 2017

A Theoretical and Experimental Study of the Formation of Aromatic Hydrocarbons (BTX/PAH) as Soot Precursors from Biomass Pyrolysis Products

Mehrabian R, Shiehnejadhesar A, Bahramian H, Anca-Couce A, Sommersacher P, Hochenauer C, Scharler R. A Theoretical and Experimental Study of the Formation of Aromatic Hydrocarbons (BTX/PAH) as Soot Precursors from Biomass Pyrolysis Products. 25th European Biomass Conference & Exhibition (oral presentation). May 2017, Stockholm, Sweden.

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In this work a novel reaction mechanism for gas phase reactions has been developed to predict the formation of aromatic compounds from the pyrolysis products of woody biomass particles. The aromatic compounds are important for being main soot precursors as well as their toxic properties. The developed gas phase mechanism is validated with experimental data from literature as well as experimental data performed with a single particle reactor for three different pyrolysis temperatures, namely 550, 800 and 1000°C. A good agreement is achieved between model results and experimental data for the total yield of each main family of aromatic hydrocarbons, i.e. phenolics, BTXs and PAHs.

Peer reviewed papers | 2017

CO/CO2 Ratio in biomass char oxidation

Anca-Couce A, Sommersacher P, Shiehnejadhesar A, Mehrabian R, Hochenauer C, Scharler R. CO/CO2 Ratio in biomass char oxidation. INFUB 2017, 11th European Conference on Industrial Furnace and Boilers. 18-21 April 2017, Albufeira, Portugal.

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The CO/CO2 release ratio obtained during char combustion of single biomass particles has been analysed in this work experimentally and by modelling. Experiments have been conducted with spruce, straw and Miscanthus pellets at different temperatures. Furthermore, these experiments have been modelled with a single particle model coupled with a CFD model of the single particle reactor. The results show that the CO/CO2 ratio strongly depends on the feedstock, being lower for spruce than for straw or Miscanthus. Furthermore, the most commonly employed correlations for this ratio in literature are not adequate, as they either under- or over-predict it.

Other papers | 2017

Innovative flexible grate solutions for future biomass combustion appliances

Feldmeier, S., Wopienka, E., Schwarz, M., Mehrabian Bardar, R.: Innovative flexible grate solutions for future biomass combustion appliances. (European Biomass Conference and Exhibition 2017, Stockholm).

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The energetic utilization of alternative fuels (short rotation coppice, miscanthus), agricultural by-products (straw, corn cobs) or biomass residues (nut shells, coffee grounds) becomes of increasing interest. Due to variations in fuel properties – and the ash content in particular – biomass fuels considerably influence the conditions in the combustion zone and especially in the fuel bed. Usually, state-of-the-art combustion appliances are optimized for a particular fuel quality and typically approved only for utilization of standardized wood pellets or wood chips. Research activities within the GrateAdvance project focus on fuel flexible grate technologies being capable of adapting conditions in the combustion zone by a systematic and targeted adjustment of grate parameters in order to minimize emissions and slagging problems, thus setting the basis for a new generation of biomass technologies. Moreover, a novel control concept will ensure optimal combustion conditions for any biomass fuel, and specifically adjust to relevant fuel properties.

Peer reviewed papers | 2017

Modelling heat of reaction in biomass pyrolysis with detailed reaction schemes

Anca-Couce A, Scharler R. Modelling heat of reaction in biomass pyrolysis with detailed reaction schemes. Fuel. 15 October 2017;206: 572-579.

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Primary devolatilization and the exothermic heterogeneous secondary charring of the primary volatiles need to be described in a consistent manner in order to correctly predict the heat of reaction of biomass pyrolysis. Detailed reaction schemes can currently predict mass loss and product composition of biomass pyrolysis with good accuracy, but have a weakness in the description of the heat of reaction. In this work it is shown for the first time that including secondary charring reactions a detailed reaction scheme can predict the evolution of the heat of pyrolysis for different conditions. The enthalpy of reaction is calculated for each reaction as the difference between the net calorific value of reactants and products. The presented model is able to describe the heat evolution in micro-TGA-DSC experiments conducted without a lid, where pyrolysis is endothermic, and with a lid, where secondary reactions are enhanced and the global heat of reaction shifts to exothermic. Furthermore, when it is coupled to a particle model, it correctly describes single particle pyrolysis experiments conducted with beech spheres where there is a remarkably exothermic peak in the centre temperature.

Peer reviewed papers | 2016

Dominating high temperature corrosion mechanism in low alloy steels in wood chips fired boilers

Gruber T, Retschitzegger S, Scharler R, Obernberger I. Dominating high temperature corrosion mechanism in low alloy steels in wood chips fired boilers. Energy and Fuels. 17 March 2016;30(3): 2385-2394.

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Ash related problems such as slagging, fouling, and high temperature corrosion in biomass fired boilers are still insufficiently explored due to the complexity of the underlying processes. High temperature corrosion of low alloy steels like 13CrMo4-5 has already been investigated in plants firing chemically untreated wood chips. In this earlier work it has been suggested that the oxidation of the steel is the dominating mechanism in the material temperature range between 450 and 550 °C. Unfortunately the exponential dependence of the material degradation on the flue gas temperature also found within this work cannot be explained with the proposed corrosion mechanism. To determine the dominating corrosion mechanism, additionally test runs have been carried out in a specially designed drop tube reactor. To investigate the time-dependent corrosion behavior of 13CrMo4-5, a newly developed mass loss probe was applied under several constant parameter setups. In addition to these measurements, the time-dependent oxidation of 13CrMo4-5 under air was investigated in a muffle furnace. To gain relevant information regarding the corrosion mechanism prevailing, the deposits as well as the corrosion products have been examined subsequently to the test runs by means of scanning electron microscopy and energy dispersive X-ray analyses. With the experimental data gained it could be shown that the dominating corrosion mechanism strongly depends on the conditions prevailing (e.g., steel temperature, flue gas temperature, and velocity) and can either be the oxidation of the steel by gaseous O2 and H2O or a combination of oxidation and active Cl-induced oxidation.

Peer reviewed papers | 2016

Reaction mechanisms and multi-scale modelling of lignocellulosic biomass pyrolysis

Anca-Couce, A. Reaction mechanisms and multi-scale modelling of lignocellulosic biomass pyrolysis. Progress in Energy and Combustion Science. Volume 53, March 2016, Pages 41–79.

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In this work about pyrolysis of lignocellulosic biomass, the individual reaction mechanisms of cellulose, hemicellulose and lignin are initially described. The recent advances in the understanding of the fundamental reaction pathways are described, including quantum-mechanical calculations, and the description of pyrolysis as a two-step process, i.e., primary pyrolysis and secondary charring, the effect of the presence of an intermediate liquid compound, and the influence of inorganic species are discussed.

The need to describe biomass pyrolysis as the sum of the contributions of its individual components is then emphasised. The process of determining biomass mass loss kinetics is analysed, and the product composition and heat of reaction that are experimentally obtained during pyrolysis are presented, along with detailed schemes that can be used to predict them.

Finally, it is demonstrated that a multi-scale consideration of pyrolysis on multiple levels – specifically, on molecular, particle and reaction levels – is required to accurately describe biomass pyrolysis. Intra-particle phenomena and particle models are discussed and the reactor level is analysed with a focus placed on fixed bed and fluidised bed pyrolysis. In summary, a list of 10 research focal points that will be important in the future is presented.

Peer reviewed papers | 2016

The Virtual Biomass Grate Furnace - An Overall CFD Model for Biomass Combustion Plants

Ali Shiehnejadhesar, Ramin Mehrabian, Robert Scharler, Christoph Hochenauer. The Virtual Biomass Grate Furnace - An Overall CFD Model for Biomass Combustion Plants. 24rd European Biomass Conference & Exhibition (poster).

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This paper presents the virtual biomass grate furnace, which comprises of comprehensive CFD models of all relevant processes for the simulation of biomass grate furnaces. The models consist of a 3D packed bed model, a gas phase combustion model for laminar to highly turbulent flows and a model to account for the influence of the flue gas streaks arising from the fuel bed in the freeboard. The simulation results of a 20 kW underfeed stoker furnace show that the overall CFD model is able to provide valuable insight on the processes occurring in the packed bed and freeboard and their interactions.

Peer reviewed papers | 2015

Application of an empirical model in CFD simulations to predict the local high temperature corrosion potential in biomass fired boilers.

Gruber T, Scharler R, Obernberger I. Application of an empirical model in CFD simulations to predict the local high temperature corrosion potential in biomass fired boilers. Biomass and Bioenergy. Volume 79, August 2015, Pages 145-154.

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To gain reliable data for the development of an empirical model for the prediction of the local high temperature corrosion potential in biomass fired boilers, online corrosion probe measurements have been carried out. The measurements have been performed in a specially designed fixed bed/drop tube reactor in order to simulate a superheater boiler tube under well-controlled conditions. The investigated boiler steel 13CrMo4-5 is commonly used as steel for superheater tube bundles in biomass fired boilers. Within the test runs the flue gas temperature at the corrosion probe has been varied between 625 °C and 880 °C, while the steel temperature has been varied between 450 °C and 550 °C to simulate typical current and future live steam temperatures of biomass fired steam boilers. To investigate the dependence on the flue gas velocity, variations from 2 m·s−1 to 8 m·s−1 have been considered. The empirical model developed fits the measured data sufficiently well. Therefore, the model has been applied within a Computational Fluid Dynamics (CFD) simulation of flue gas flow and heat transfer to estimate the local corrosion potential of a wood chips fired 38 MW steam boiler. Additionally to the actual state analysis two further simulations have been carried out to investigate the influence of enhanced steam temperatures and a change of the flow direction of the final superheater tube bundle from parallel to counter-flow on the local corrosion potential.

Conference presentations and posters | 2015

Application of numerical modelling to biomass grate furnaces

Mehrabian R, Shiehnejadhesar A., Scharler R. Application of numerical modelling to biomass grate furnaces. Internation conference on advances in mechanical engineering, Istanbul 2015.

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The direct combustion of the biomass is the most advanced and mature technology in the field of energetic biomass utilisation. The legislations on the amount of emitted pollutants and the plant efficiency of biomass combustion systems are continually being restricted. Therefore constant improvement of the plant efficiency and emission reduction is required Numerical modelling is gaining increasing importance for the development of biomass combustion technologies. In this paper an overview about the numerical modelling efforts deal with the most relevant phenomena in biomass grate firing systems is given. The numerical modelling results in a deeper understanding of the underlying processes in biomass combustion plants. Therefore, it leads to a faster and safer procedure of development of a new technology.

Peer reviewed papers | 2015

Development and validation of CFD models for gas phase reactions in biomass grate furnaces considering gas streak formation above the packed bed

Shiehnejadhesar A, Mehrabian R, Scharler R, Obernberger I. Development and validation of CFD models for gas phase reactions in biomass grate furnaces considering gas streak formation above the packed bed. Fuel Processing Technology. Volume 139, November 2015, Pages 142–158.

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State-of-the-art packed bed models supply continuous concentration profiles as boundary conditions for subsequent CFD simulations of gas phase, leading to pre-mixed combustion conditions. However, in reality the “porous” nature of the packed bed leads to streak formation influencing gas mixing and combustion. Therefore, in the present work, in order to account for the influence of the streaks on gas phase combustion, a gas streak model based on a correlation between the local gas residence time and a mixing time has been developed based on numerical simulations. Finally, the streak model was linked with an in-housed developed hybrid gas phase combustion model suitable for laminar to highly turbulent flow conditions and applied for an under-feed pellet stoker furnace (20 kWth) concerning the simulation of gas phase combustion and NOx formation. The results in comparison with a simulation without the streak formation model show that the flue gas species prediction can be improved with the proposed streak formation model. Especially, in the region above the fuel bed (in the primary combustion chamber), this is of special importance for NOx reduction by primary measures.

Other papers | 2015

Numerical modelling of biomass grate furnaces with a particle based model

Mehrabian R, Shiehnejadhesar A, Scharler R, Obernberger I. Numerical modelling of biomass grate furnaces with a particle based model, INFUB 10th European Conference on Industrial Furnace and Boilers 2015, 7th-10th of April 2015, Porto, Portugal.

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Peer reviewed papers | 2014

Multi-physics modelling of packed bed biomass combustion

Mehrabian R, Shiehnejadhesar A, Scharler R, Obernberger I. Multi-physics modelling of packed bed biomass combustion. Fuel. 2014;122:164-78.

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A transient 3D model for two main zones, namely the fuel bed and the freeboard, of biomass packed bed combustion systems was developed. It integrates the models for the biomass conversion sub-processes and solves the governing equations for the gas and solid phase and their interactions. The intra-particle gradients are included by considering the biomass particles as thermally thick particles. The shrinkage of the packed bed and the variations of the bed porosity due to the uneven consumption of the fuel are taken into account. Detailed kinetic mechanisms are used for the simulation of homogeneous gas phase reactions. To verify the model and to increase the understanding of packed bed combustion, laboratory-scale fixed-bed batch experiments have been performed in a reactor with 9.5 cm diameter and 10 cm length. The model performance was extensively validated with gas phase measurements (CO, CO2, CH4, H2, H2O and O2) above the fuel bed, temperatures at different heights in the bed and in the freeboard, and the propagation rate of reaction front. The simulation results are in a good agreement with the measured values. © 2014 Elsevier Ltd. All rights reserved.

Peer reviewed papers | 2012

A CFD model for thermal conversion of thermally thick biomass particles

Mehrabian R, Zahirovic S, Scharler R, Obernberger I, Kleditzsch S, Wirtz S, et al. A CFD model for thermal conversion of thermally thick biomass particles. Fuel Process Technol. 2012;95:96-108.

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A one-dimensional model for the thermal conversion of thermally thick biomass particles is developed for the simulation of the fuel bed of biomass grate furnaces. The model can be applied for cylindrical and spherical particles. The particle is divided into four layers corresponding to the main stages of biomass thermal conversion. The energy and mass conservation equations are solved for each layer. The reactions are assigned to the boundaries. The model can predict the intra-particle temperature gradient, the particle mass loss rate as well as the time-dependent variations of particle size and density, as the most essential features of particle thermal conversion. When simulating the fuel bed of a biomass grate furnace, the particle model has to be numerically efficient. By reducing the number of variables and considering the lowest possible number of grid points inside the particle, a reasonable calculation time of less than 1 min for each particle is achieved. Comparisons between the results predicted by the model and by the measurements have been performed for different particle sizes, shapes and moisture contents during the pyrolysis and combustion in a single-particle reactor. The results of the model are in good agreement with experimental data which implies that the simplifications do not impair the model accuracy.

Peer reviewed papers | 2012

Effects of pyrolysis conditions on the heating rate in biomass particles and applicability of TGA kinetic parameters in particle thermal conversion modelling.

Mehrabian R, Scharler R, Obernberger I. Effects of pyrolysis conditions on the heating rate in biomass particles and applicability of TGA kinetic parameters in particle thermal conversion modelling. Fuel. 2012;93:567-75.

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A one-dimensional single particle model is utilised to investigate the effects of radiation temperature, moisture content, particle size and biomass physical properties on the heating rate in biomass particles during pyrolysis. The model divides the particle into four layers - drying, pyrolysis, char and ash layer - corresponding to the four main stages of biomass thermal conversion. The average of the time derivative of the pyrolysis layer centre temperature weighted by the pyrolysis rate is introduced as an appropriate indicator for the heating rate in the particle during pyrolysis. The influencing parameters on the heating rate are summarised in the Biot number and the thermal time constant, to make the investigation of their effects easier. The heating rate is inversely proportional to the thermal time constant. The effect of a variation of the Biot number on the heating rate is negligible in comparison to the thermal time constant. Therefore, the thermal time constant can be sufficiently used to specify the heating rate regimes during pyrolysis. It is found that for thermal time constants of more than 50 s, pyrolysis takes place in a low heating rate regime, i.e. less than 50 K/min. Additionally, the heating rate during pyrolysis of various biomass types under a wide range of thermal conversion conditions has been examined, in order to classify the heating rate regime of pyrolysis in state-of-the-are combustion/gasification plants. The pyrolysis of wood dust and wood pellets is found to happen always in high heating rate regimes. Therefore, the kinetic parameters obtained by conventional TGA systems (typically with heating rates lower than 50 K/min) are not applicable for them. On the contrary, the pyrolysis of wood logs always happens in low heating rate regimes, which indicates that kinetic parameters obtained by conventional TGA systems can be applied. However, pyrolysis of wood chips can undergo low or high heating rate regimes depending on their particle size. Concerning the moisture content, it can be stated that it does not strongly influence the heating rate regime of certain biomass particles. © 2011 Elsevier Ltd. All rights reserved.

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