04 Fakultät Energie-, Verfahrens- und Biotechnik

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Institute: search.filters.UBSInstitut.Institut für Feuerungs- und KraftwerkstechnikSubject: search.filters.subject.333.7

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    Lab-scale investigation of palm shell char as tar reforming catalyst
    (2020) Chen, Yen-Hau; Schmid, Max; Chang, Chia-Chi; Chang, Ching-Yuan; Scheffknecht, Günter
    This research investigated the application of palm shell char as a catalyst for the catalytic steam reforming of tar after the sorption enhanced gasification (SEG) process. The catalytic activities of palm shell char and metal-supported palm shell char were tested in a simulated SEG derived syngas with tar model compounds (i.e., toluene and naphthalene) at a concentration of 10 g m-3 NTP. The results indicated that palm shell char had an experimentally excellent catalytic activity for tar reforming with toluene and naphthalene conversions of 0.8 in a short residence time of 0.17 s at 900 °C. A theoretical residence time to reach the complete naphthalene conversion was 1.2 s at 900 °C for palm shell char, demonstrating a promising activity similar to wood char and straw char, but better than CaO. It was also found that potassium and iron-loaded palm shell chars exhibited much better catalytic activity than palm shell char, while the parallel reaction of gasification of K-loaded palm shell char influenced the conversion with its drastic mass loss. Moreover, contrary to CaO, palm shell char presented relatively low selectivity to benzene, and its spontaneous gasification generated extra syngas. In summary, the present study demonstrated that the low-cost material, palm shell char, can successfully be used as the tar-reforming catalyst after SEG process.
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    Steam-oxygen fluidized bed gasification of sewage sludge
    (2023) Schmid, Max; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
    Sewage sludge is a residue that is generated unavoidably by the population. On a first sight, sewage sludge may be a hazardous waste that requires safe disposal. By looking closer, it is recognized as secondary resource. The mineral fraction contains valuable elements such as phosphorous, which can be retrieved as secondary raw material. This thesis focuses on the organic fraction, which is a renewable fuel and carbon source and can be used to substitute fossil carbon in fuels and chemicals. The first step in converting sewage sludge to renewable goods is syngas production via gasification. The experimental work of this thesis demonstrated the feasibility of synthesis gas production from sewage sludge by steam-oxygen fluidized bed gasification. It was shown that the process works reliably in the investigated 20 kW scale and that the syngas contains high H2 and CO concentrations and is thus suitable for synthesis of fuels and chemicals. The impurities NH3, H2S, COS and tar species, including heterocyclic species such as pyridine, were measured in considerable concentrations in the syngas. Small amounts of limestone bed additive enabled cracking of heavy tars and partial capture of H2S and COS. It was further found that the cold gas efficiency increases with rising gasification temperature due to improved tar and char conversion at higher temperatures. The typical operation temperature 850 °C requires an oxygen ratio of 0.33, obtaining a cold gas efficiency of 63 %. Moreover, the H2/CO-ratio could be controlled efficiently by altering the steam to carbon ratio, as steam promotes the water gas shift reaction in the gasifier to achieve the desired stoichiometry for synthesis, however, resulting in higher energy demand for steam provision. The experimental results can be utilized for process design, e.g., for a TRL 7-demonstrator. Furthermore, a gasifier model was developed and an integrated process chain was simulated to assess the conversion of sewage sludge to synthetic natural gas (SNG) with and without inclusion of power-to-gas through electrolysis. The total efficiency of the conversion including own consumption for the case without electrolysis was 51 % with a carbon utilization of 33 %. These values could be enhanced by inclusion of power-to-gas. It was predicted that the produced SNG has a CH4-concentration of between 0.81 m3 m 3 and 0.84 m3 m 3 and nitrogen concentrations of up to 0.16 m3 m 3 originating from fuel-bound nitrogen. The simulations on process integration showed that up to 20% of the sewage sludge feed can be dried by heat integration. This implies that also external heat sources have to be used for drying. Overall, the steam-oxygen gasification proved to be an efficient and technically feasible process for sewage sludge treatment and can be considered as an alternative to fluidized bed incineration for future mono-treatment plants.
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    Measuring circularity in cities : a review of the scholarly and grey literature in search of evidence-based, measurable and actionable indicators
    (2023) Kapoor, Kartik; Amydala, Nikhil Sayi; Ambooken, Anubhav; Scheinberg, Anne
    Circularity in cities is key to Earth’s sustainable and resource-efficient future. In contrast to the broad framework of circular economy, circularity is a technical concept associated with avoiding disposal and prolonging the useful life of products and materials, and thereby extracting fewer resources. In search of metrics and indicators to measure the impacts of circular processes in cities in real time, the authors reviewed the literature on the circular economy and circularity, in search of evidence-based circularity indicators suitable for cities to use to benchmark the environmental and climate benefits of six waste prevention cascades. This paper reports on a systematic literature review using the PRISMA protocol to screen, evaluate, and review published and grey literature sources. From more than 15,000 papers screened, after application of criteria, fewer than 25 papers were found that presented evidence-based, measurable, and actionable indicators or indicator sets for benchmarking the performance of circular processes in cities. The authors concluded that the practical commitment to evidence-based tracking of circularity (in cities) is weak. Practical progress towards a circular economy and physical and economic circularity will require stakeholders to strengthen and test the very small number of indicators and indicator sets that are relevant and useful for cities and regions to use for measuring their progress towards becoming more circular, and increase evidence-based monitoring for circularity and the circular economy.
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    Leaching of fly ash particulate matter in MEA solutions and its relevance to the CO2 capture process with flue gas of coal-fired power plants
    (2020) Schallert, Bernd; Scheffknecht, Günter (Prof. Dr.)
    This study underlines the relevance of leaching of fly ash particulate matter to carbon capture plants and strives for a better understanding of the solubility of various elements and heavy metals, especially Fe, in MEA solutions and of relevant leaching parameters.
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    CO2-Abtrennung aus Synthesegasen mit Hydrotalciten unter Hochtemperatur-Hochdruckbedingungen
    (2017) Bublinski, Martin; Seifert, Helmut (Prof. Dr.-Ing.)
    Membranverfahren trennen Gasgemische kontinuierlich, selektiv und energieeffizient. Damit stellt dieses Trennverfahren eine interessante Alternative zu herkömmlichen, energieintensiven Gasreinigungsverfahren mittels Wäschern dar. Allerdings steigen insbesondere für Hochtempera-turanwendungen die material- und prozesstechnischen Anforderungen an das Membransystem, wodurch die Materialauswahl stark eingegrenzt wird. Bisher existiert im kommerziellen Maßstab noch kein hochtemperaturtaugliches, kontinuierliches Abscheideverfahren für die selektive Abscheidung von CO2 aus Synthesegas. Der Einsatz von Membranen aus Hydrotalcit stellt für die CO2-Abscheidung aus vorgereinigten Synthesegasen einen vielversprechenden Lösungsansatz dar. In dieser Arbeit wird die systematische Entwicklung von anorganischen mehrschichtigen Hydrotalcit-Membranen vorgestellt, mit denen CO2 selektiv aus dem Gasstrom unter Hochtemperatur- und Hochdruckbedingungen (T > 350 °C, p ≤ 80 bar) abgeschiedenen werden kann. Die Prozesse CO2-Sorption bzw. CO2-Desorption auf der Membranoberfläche sowie die Diffusionseigenschaf-ten der Membran wurden dabei getrennt voneinander untersucht. Zuerst wurden an reinen und mit Kaliumcarbonat dotierten Hydrotalciten mit einem Sorptions-/Druckreaktor CO2-Sorptionsgleichgewichtsdaten im Temperatur- und Druckbereich zwischen 200-500 °C bzw. 20-80 bar aufgenommen. Dabei wurden maximale CO2-Kapazitäten von 1,2 mol/kg für reines Hydrotalcit und 2,0 mol/kg für dotiertes Hydrotalcit mit trockenem, bzw. 1,95 mol/kg für reines Hydrotalcit und 5,70 mol/kg für K-dotiertes Hydrotalcit mit feuchtem Gas ermittelt. Die Desorptionseigenschaften wurden mittels zyklischen CO2-Sorptionsexperimenten bestimmt. Dabei stellte sich sowohl für reines als auch für K-dotiertes Hydrotalcit für trockenes und feuchtes Gas eine konstante Arbeitskapazität nach mehreren Sorptionszyklen auf dem Niveau von zwei Drittel der ursprünglichen CO2-Sorptionskapazität ein. Die Hydrotalcit-Membransynthese erfolgte auf Al2O3-Substraten mit einem Harnstoff-Hydrolyse-Verfahren. Dadurch konnte ein Wachstum der Hydrotalcit-Kristalle direkt auf der Substratoberfläche erreicht werden und eine homogene Hydrotalcit-Membran synthetisiert werden. Durch eine weitere Hydrotalcit-Schicht gelang es, die Defekte der Membran weiter zu verringern. Die CO2-Permeanzen wurden mit einem Hochtemperatur-Membranmodul bei 200 °C auf 3,03·10-7 mol/(m2·s·Pa) und 500 °C auf 1,06·10-6 mol/(m2·s·Pa) bestimmt. Neben der Knudsen-Diffusion wurde die Lösungs-Diffusion von CO2 als weiterer Transportmechanismus durch die Hydrotalcit-Membran identifiziert. Die idealen CO2-Selektivitäten bezüglich N2, H2 und CO lagen demnach meist leicht über den Knudsen-Selektivitäten. Gegenüber N2 konnte bei Tempera-turen von 350 °C eine Selektivität von 1,31 ermittelt und folglich eine partielle CO2-selektive Trennung mit den synthetisierten Hydrotalcit-Membranen erzielt werden.
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    Experimentelle Untersuchung des Calcium-Looping-Verfahrens im Pilotmaßstab
    (2022) Dieter, Heiko; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
    In dieser Arbeit wird durch experimentelle Untersuchungen an einer 200 kWth-Pilotanlage mit realitätsnahen Prozessbedingungen eine Datengrundlage für das Calcium-Looping (CaL)-Verfahren geschaffen. Die experimentellen Untersuchungen umfassen die Felder Karbonator- und Regeneratorbetrieb sowie Erkenntnisse zum Sorbensverhalten. Zum erreichten Ziel eines Karbonatorabscheidegrads deutlich über 90% CO2 tragen vielschichtige Faktoren bei, welche im einzelnen untersucht und charakteristische Betriebsfenster abgeleitet werden. Mittels simulativer Berechnung von Betriebsfenstern wird das Verfahren anhand seiner charakteristischen Parameter optimiert und Effizienzpotentiale anhand des Nettowirkungsgradverlusts durch CO2-Abscheidung aufgezeigt. Abschließend wird eine Methode zur simulationsgestützten Verfahrensauslegung und -skalierung entwickelt. Auf dieser Grundlage kann eine CaL-Anlage bestehend aus gekoppelten Wirbelschichten ausgelegt, skaliert und betrieben werden.
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    Extended theoretical and experimental studies of the calcium looping process for carbon dioxide capture
    (2017) Duelli, Glykeria; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
    The world is at a critical juncture in its efforts to combat climate change. Since the first Conference of the Parties (COP) in 1995, greenhouse-gas (GHG) emissions have risen by more than one-quarter and the atmospheric concentration of these gases has increased steadily to 435 parts per million carbon-dioxide equivalent (ppm CO2-eq) in 2012 [1]. The international commitment to keep the increase in long-term average temperatures below two degrees Centigrade, compared to pre-industrial levels, requires substantial and sustained reductions in global emissions. Given the dominant role that fossil fuels continue to play in primary energy consumption followed by the continuously increasing global energy demand, the deployment of carbon capture and storage technologies (CCS) is imperative [1]. The individual component technologies required for CO2 capture, transport and storage are generally well-understood and, in some cases, technologically mature. The largest challenge for CCS deployment is the integration of component technologies into large-scale (demonstration) projects. In this direction simulation and modeling works allow a cost effective investigation of the feasibility and the applicability of the prototype technology as well as its development and optimization. In addition, complete process approach allows determination of the impact that integration of the CO2 capture plant imposes on the power plant. However, a reliable assessment of the process performance requires the process models to be validated with experimental data. In this work, one of the major CCS technologies, the calcium looping process is realized, investigated and evaluated at a 10 kWth dual fluidized bed (DFB) continuously operating facility at the University of Stuttgart. The performance of the process in terms of CO2 capture in the carbonator and sorbent calcination in the regenerator is studied. Natural limestones were used. The process was realised in presence of water vapor in both carbonator and regenerator reactor. The calcination took place in high CO2 concentration representative of the oxy-fuel combustion in the regenerator. Synthetic flue gas was used while both reactors were electrically heated with supplementary CH4 combustion in the regenerator when necessary. The Ca flow circulating between the reactors as well as the Ca mass in the reactors were varied. The regenerator and the carbonator temperatures were varied. The sorbent CO2 capture ability was studied through thermogravimetric analysis of the samples taken during experimentation. Attrition phenomena were studied by measuring the particle size distribution and weighting the material collected from the cyclones of the DFB facility. The experimentation was successfully performed with reliable data and the trends observed are in good agreement with previous works. It was shown that CO2 capture efficiencies of more than 90% can be achieved at conditions closer to the industrial ones. The CO2 capture efficiency was improving by increasing bed inventory and looping ratio. The sorbent calcination degree is a decreasing function of the carbonate content of the incoming solid flow and an increasing function of the particle residence time and reactor temperature. In presence of water vapor, CO2 capture efficiencies of more than 90% and complete sorbent calcination were achieved for looping ratios of around 8. The temperatures were for the regenerator not more than 1193K and for the carbonator around 903K. The sorbent carbonation conversion was retained at about 0.2 molCaCO3/molCaO, constant for many hours of operation. The material loss was measured to be around 4.5%wt/h based on the total system inventory while the mean particle size of the sorbent decreased to around 400 µm and remained constant for many hours of operation. Simplified semi-empirical models were successfully implemented in the experimental results. Kinetic and attrition constants were calculated and a good agreement between the predicted and the actual data is shown. Design parameter of active space time was found to be 30s for the carbonator and 0.11h for the regenerator with efficiencies of more than 90% in both reactors.
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    Enhancing the quality of syngas from biomass gasification via methane conversion
    (2024) Kertthong, Thiansiri; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
    One promising way of utilizing renewable energy is to gasify biomass into syngas and convert it to methanol, an essential and versatile chemical and fuel. High-quality syngas required for methanol synthesis can be produced by sorption-enhanced gasification (SEG) of biomass. In recent years, the SEG process based on a dual fluidized bed system has received much attention due to its potential for carbon capture, utilization, and storage (CCUS). Using a CO2-active sorbent as bed material enables in-situ CO2 capture during gasification, resulting in hydrogen-rich and tailored syngas suitable for chemical and fuel production. An attractive option is the SEG process with oxy-fuel combustion (Oxy-SEG), which produces a flue gas with a high CO2 concentration. A challenging area in the field of methanol from biomass is the remaining hydrocarbons in syngas, including gaseous hydrocarbons, which are unusable in methanol synthesis, and condensable hydrocarbons (tar), which foul downstream equipment and deactivate reforming or methanol synthesis catalysts. This research thus examined how to improve the syngas quality to make it more suitable for methanol synthesis by converting hydrocarbons into usable gases (H2 and CO) if possible, focusing on methane conversion, the most abundant hydrocarbon in syngas. Experiments were conducted on the non-catalytic partial oxidation method (POX) in a non-premixed burner system and the catalytic steam reforming method (CSR) in a fixed bed reactor at atmospheric pressure under various conditions, including SEG-derived syngas and purge gas exiting the methanol synthesis. Based on experimental data, process simulation models were developed and simulated in Aspen Plus® to evaluate the impact of methane conversion concepts on the overall process. Biochar and CaO, which have promising catalytic activity for tar reforming, were found to be ineffective as methane reforming catalysts in experiments. As a result of its high activity, a commercial Ni-based catalyst was investigated further. The findings of this research demonstrated the significance of syngas composition (mainly H2 and H2O) on reforming processes. Increasing hydrogen and decreasing steam concentrations were suggested for a high CO yield. In POX, highly reactive hydrogen allowed syngas with high steam concentrations of up to 0.60 m3 m−3 to ignite at a preheating temperature of approximately 670 °C. This temperature is within the range of gasification temperatures. Unfortunately, high steam concentration had almost no effect on CH4 conversion, but it was useful for adjusting product distribution (H2/CO ratio and CO/CO2 ratio). In CSR, hydrogen inhibited methane reforming, whereas steam promoted it. The high steam content of SEG-derived syngas enabled reaching the optimum CH4 conversion and slowing catalyst deactivation. In addition to hydrogen and steam, this research found that the oxygen-to-fuel ratio 𝑛O2 is critical for POX. However, a trade-off existed between high CH4 conversion at high 𝑛O2 and high H2/CO ratio at low 𝑛O2. Although the preheating temperature had a negligible effect on methane, it increased the conversion of more reactive hydrocarbons such as C2H4 and CO yield. Unfortunately, the conversion of the desired hydrogen could not be avoided under methane oxidation in excess hydrogen. Nevertheless, desirable CO could be produced at high steam concentrations (0.50 m3 m−3 to 0.60 m3 m−3) and low 𝑛O2 (up to 0.43). As a result of this study, it is suggested to use POX to provide the heat required for CSR (known as autothermal reforming or ATR). Furthermore, this research highlighted the importance of simultaneously investigating gaseous and condensable hydrocarbons in syngas reforming. When tar model compounds were introduced into the feed at 600 °C, CH4 conversion dropped by about 30%, and catalyst deactivation due to carbon formation was observed. The deactivation reduced tar conversion over time, while CH4 conversion stayed nearly constant up to 240 min. The presence of naphthalene, a heavier tar model compound, inhibited toluene and methane reforming. A high temperature above 800 °C is recommended to maximize CH4 conversion and CO yield while minimizing catalyst deactivation due to carbon formation. The simulation findings also revealed that the reformer benefits the gasification and methanol synthesis process chains. The utilization of purge gas exiting the methanol synthesis is interesting but inefficient. Although methanol production per biomass input could be slightly improved, this route requires a larger reactor and two recycle loops, complicating the process in practice. Direct conversion of hydrocarbons in the syngas is more appealing and was investigated using CSR and ATR in optimized cases and compared with the reference case using a rapeseed methyl ester (RME) scrubber. Regarding carbon conversion, the optimized cases clearly outperformed the reference case, which simply discharged hydrocarbons from the process. The optimized case had nearly twice the biomass-to-methanol conversion efficiency as the reference case (0.53 and 0.31, respectively). At the same outlet temperature, CSR appeared to be a better option for producing usable gases, whereas ATR was more efficient in terms of energy. Although both methods had a trade-off, the results are auspicious.
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    To prevent or promote grid expansion? : analyzing the future role of power transmission in the European energy system
    (2021) Cao, Karl-Kiên; Pregger, Thomas; Haas, Jannik; Lens, Hendrik
    Future energy supply systems must become more flexible than they are today to accommodate the significant contributions expected from intermittent renewable power sources. Although numerous studies on planning flexibility options have emerged over the last few years, the uncertainties related to model-based studies have left the literature lacking a proper understanding of the investment strategy needed to ensure robust power grid expansion. To address this issue, we focus herein on two important aspects of these uncertainties: the first is the relevance of various social preferences for the use of certain technologies, and the second is how the available approaches affect the flexibility options for power transmission in energy system models. To address these uncertainties, we analyze a host of scenarios. We use an energy system optimization model to plan the transition of Europe’s energy system. In addition to interacting with the heating and transport sectors, the model integrates power flows in three different ways: as a transport model, as a direct current power flow model, and as a linearized alternating current power flow model based on profiles of power transfer distribution factors. The results show that deploying transmission systems contribute significantly to system adequacy. If investments in new power transmission infrastructure are restricted - for example, because of social opposition - additional power generation and storage technologies are an alternative option to reach the necessary level of adequacy at 2% greater system costs. The share of power transmission in total system costs remains widely stable around 1.5%, even if cost assumptions or the approaches for modeling power flows are varied. Thus, the results indicate the importance of promoting investments in infrastructure projects that support pan-European power transmission. However, a wide range of possibilities exists to put this strategy into practice.
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    Bridging granularity gaps to decarbonize large‐scale energy systems : the case of power system planning
    (2021) Cao, Karl‐Kiên; Haas, Jannik; Sperber, Evelyn; Sasanpour, Shima; Sarfarazi, Seyedfarzad; Pregger, Thomas; Alaya, Oussama; Lens, Hendrik; Drauz, Simon R.; Kneiske, Tanja M.
    The comprehensive evaluation of strategies for decarbonizing large‐scale energy systems requires insights from many different perspectives. In energy systems analysis, optimization models are widely used for this purpose. However, they are limited in incorporating all crucial aspects of such a complex system to be sustainably transformed. Hence, they differ in terms of their spatial, temporal, technological, and economic perspective and either have a narrow focus with high resolution or a broad scope with little detail. Against this background, we introduce the so‐called granularity gaps and discuss two possibilities to address them: increasing the resolutions of the established optimization models, and the different kinds of model coupling. After laying out open challenges, we propose a novel framework to design power systems in particular. Our exemplary concept exploits the capabilities of power system optimization, transmission network simulation, distribution grid planning, and agent‐based simulation. This integrated framework can serve to study the energy transition with greater comprehensibility and may be a blueprint for similar multimodel analyses.