04 Fakultät Energie-, Verfahrens- und Biotechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5
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Item Open Access Behavior of sulfur oxides in air and oxy-fuel combustion(2019) Spörl, Reinhold; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)This thesis evaluates the behavior of sulfur oxides in pulverized fuel (PF) fired air and oxy-fuel systems. Sulfur oxides are responsible for certain operational problems and considerable gas cleaning requirements in air as well as oxy-fuel firing. A better understanding of the related issues will allow for a technical and economical optimization of the oxy-fuel combustion technology. A range of experimental investigations studying the stability and retention of sulfur oxides in ashes and deposits, acid gas (SO2, SO3, and HCl) control in air and oxy-fuel combustion by dry sorbent injection, and SO3 formation were conducted. The experimental work is in parts supported by theoretical considerations and thermodynamic equilibrium simulation. Studies for different coals and lignites showed that in practically relevant oxy-fuel configurations the exclusion of airborne N2 from combustion leads to an increase of the SO2 concentrations in oxy-fuel, compared to air firing, by a factor of about 3.4 to 4.2, referring to dry, and of about 2.9 to 3.5, when referring to wet flue gas conditions. The increased SO2 levels in oxy-fuel combustion are responsible for an increased stability of sulfates in oxy-fuel power boiler systems so that for example the decomposition temperature CaSO4 rises by about 50 to 80 °C, depending on flue gas atmospheres. The enhanced stability of sulfates in deposits at high temperatures when operating with increased SO2 levels was experimentally demonstrated. Compared to air firing, a considerable increase of the sulfur retention in the ash by 10 to 12 percentage points has been observed for oxy-fuel recycle combustion of Lusatian lignites. This leads to lower SO2 emissions and higher SO3 levels in process ashes and deposits. The results indicate that for fuels, such as the used lignites, the temperature level at which fouling by sulfatic deposits is problematic may be shifted to higher temperatures in oxy-fuel combustion and that the sintering of deposits by sulfation may be more pronounced. In contrast, in air and oxy-fuel combustion experiments with a hard coal with a low sulfur retention potential differences in the SO3 contents and degrees of sulfation of ashes and deposits were small. Besides higher SO3 contents and sulfation degrees, no other significant changes between the deposit samples from air and oxy-fuel combustion were identified. Experiments on dry sorbent injection in air and oxy-fuel mode showed that an increase of the average flue gas residence time in the furnace by flue gas recirculation and, to a lesser extent, the higher sulfate stability enhance the desulfurization efficiency in oxy-fuel recycle combustion considerably. SO2 capture efficiencies in oxy-fuel recycle combustion of 50 % to more than 80 % at moderate molar sulfur to calcium ratios between 1.7 and 2.9 were reached, when injecting CaCO3 and Ca(OH)2 together with the fuel or directly to the furnace. Under comparable injection conditions, the oxy-fuel performance was by as much as 29 percentage points higher than in air firing. Also an efficient SO3 and HCl control by DSI could be demonstrated. Experiments on formation of SO3 show that higher SO2 levels in oxy-fuel firing are the most important parameter responsible for the observed increase of the SO3 concentrations.Item Open Access Abschaltung von dezentralen Erzeugungsanlagen infolge von Unterspannungen im Kontext von LFSM-O(2022) Schöll, Christian; Lehner, Joachim; Weidner, Johannes; Lens, HendrikBei einer Überfrequenz jenseits von 50,2 Hz müssen sämtliche Erzeugungsanlagen ihre in das Netz eingespeiste Wirkleistung reduzieren. In den zumeist resistiv geprägten Verteilungsnetzen der Mittel- und Niederspannung führt diese überfrequenzbedingte Leistungsreduktion zu einer lokalen Absenkung des Spannungsprofils. Unterschreiten die Spannungen die Vorgaben aus den Netzanschlussbedingungen, unter denen dezentrale Erzeugungsanlagen die Verbindung zum Netz aufrechterhalten müssen, kann es zu unterspannungsbedingten Schutzabschaltungen kommen. Theoretisch könnten diese bei großflächigem Auftreten zu einer Umkehr des ursprünglichen Überfrequenzereignisses in ein Unterfrequenzereignis führen. Anhand von simulationsbasierten Untersuchungen wird in diesem Beitrag jedoch aufgezeigt, dass unter realitätsnahen Annahmen und unter Berücksichtigung der nationalen Richtlinien sowie der zugehörigen netzplanerischen Leitplanken kein besonderes Risiko hinsichtlich der Anregung der beschriebenen Wirkungskette vorliegt.Item Open Access Brennstoffzellensysteme auf Basis regenerativer Energieträger(2010) Grüner, Heike; Scheffknecht, G. (Prof. Dr. techn.)Der Einsatz von regenerativen Energieträgern in Brennstoffzellensystemen ermöglicht eine beachtliche Reduzierung von Treibhausgasemissionen im Vergleich zur konventionellen Strom- und Wärmegewinnung. Im Rahmen dieser Arbeit wurde der am besten geeignete, nicht-leitungsgebundene regenerative Energieträger für ein stationäres PEM-Brennstoffzellen-BHKW identifiziert und ein ursprünglich auf Erdgas basierendes Gesamtsystem mit diesem Energieträger charakterisiert. Als leicht speicherbare Energieträger kommen dabei Methanol, Ethanol und Dimethylether (DME) sowie Pflanzenöl, Biodiesel und flüssige synthetische Kohlenwasserstoffe in Frage. Aus diesen wurden Methanol, Ethanol und DME vorselektiert und hinsichtlich der Brennstoffherstellung (Well-to-Tank-Wirkungsgrad) und -reformierung sowie der simulierten Gesamtsystemeffizienz und -komplexität bewertet. Bei vergleichbaren Werten in der Brennstoffherstellung sowie in der Systemeffizienz lässt sich dabei die Aussage treffen, dass grundsätzlich alle drei genannten Brennstoffe für ein stationäres PEM-Brennstoffzellen-BHKW geeignet sind. Die geringe Systemkomplexität sowie die einfache Dampfreformierung von DME führten zu dessen Auswahl als dem am besten geeigneten, nicht-leitungsgebundenen regenerativen Energieträger für ein stationäres Brennstoffzellensystem. Anschließend erfolgten die Charakterisierung des Gesamtsystems im DME-Betrieb sowie der Vergleich des Systems im Betrieb mit Erdgas. Erdgas diente dabei als Ersatz für biogenes Erdgassubstitut (SNG), welches stellvertretend für den Einsatz leitungsgebundener regenerativer Energieträger steht.Item Open Access Lab-scale investigation of palm shell char as tar reforming catalyst(2020) Chen, Yen-Hau; Schmid, Max; Chang, Chia-Chi; Chang, Ching-Yuan; Scheffknecht, GünterThis 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.Item Open Access 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.Item Open Access 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, AnneCircularity 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.Item Open Access Modellierung laminarer und turbulenter Flüssig-Sauerstoff/Wasserstoff-Sprayflammen unter kryogenen Hochdruckbedingungen(2001) Schlotz, Dominik; Gutheil, Eva (Prof. Dr. rer. nat.)Im Hinblick auf die technische Anwendung der Sprayverbrennung in Raketentriebwerken wird in dieser Dissertation die laminare und turbulente Verbrennung von Flüssig-Sauerstoff/Wasserstoff-Flammen unter kryogenen Hochdruckbedingungen behandelt. In Raketentriebwerken findet die Verbrennung im Flamelet-Bereich turbulenter Reaktionszonen statt. D.h., die Diffusionsflammenfronten können als ein Ensemble laminarer, gestreckter Flämmchen aufgefaßt werden, deren Struktur der der Gegenstromkonfiguration sehr ähnlich ist. Deshalb wird nach einer Einführung und einer Beschreibung der Grundgleichungen für reaktive Zwei-Phasen-Strömungen die Struktur laminarer Gas- und Sprayflammen in der Gegenstromkonfiguration mit numerischer Methoden untersucht. In dem zugrunde liegenden Modell werden detaillierte Chemie, detaillierter Transport, die Transporteigenschaften bei kryogenen Eintrittstemperaturen sowie die Druck- und Temperaturabhängigkeit des Phasengleichgewichts berücksichtigt. Die Ergebnisse werden in Flammenbibliotheken niedergelegt. Das Modell ermöglicht die Bestimmung von Verlöschbedingungen, die für die Simulation turbulenter Spraydiffusionsflammen mittels eines Flamelet-Ansatzes grundlegend sind. Für die Berechnung turbulenter Flüssig-Sauerstoff/Wasserstoff-Sprayflammen wird ein System gemittelter zweidimensionaler Gasphasen-Erhaltungsgleichungen gelöst, welche Quellterme des Phasenaustauschs beinhalten und durch das k-e-Turbulenzmodell geschlossen werden. Die Phasenaustauschterme werden mit Spraygleichungen berechnet. Gas- und Flüssig-Phase sind durch einen Euler-Lagrange Formalismus gekoppelt. Die in dieser Arbeit berechnete Gasflammenbibliothek wird bei der Bestimmung mittlerer Massenbrüche verwendet. Von der DLR Lampoldshausen durchgeführte Experimente an einer Mikrobrennkammer liefern Startwerte für die Berechnung und werden für einen Vergleich mit numerischen Ergebnisse herangezogen.Item Open Access Heterogeneously catalyzed reactions over newly developed SCR DeNOx catalysts(2022) Schwämmle, Tobias; Scheffknecht, Günter (Prof. Dr. techn.)Catalysts for selective catalytic reduction (SCR) can, in addition to the reduction of NOx, also contribute to the oxidation of elemental mercury (Hg0) as well as to the undesired conversion of SO2 to SO3. By placing the catalysts in the high-dust configuration, oxidized mercury (Hg2+) can then be separated in downstream wet flue gas desulfurization units, allowing mercury to be removed efficiently from the flue gas. The aim of this work is to show how mercury oxidation can be increased by newly developed SCR catalysts, which influences there are on Hg oxidation, and which mechanisms lie behind the three reactions over the SCR catalysts. The research with parameters derived from the experiences and conditions in power plant operation is carried out in synthetic flue gas in laboratory micro-reactors as well as in a lab-scale firing system under real flue gas conditions. The research forms a comprehensive examination of all reactions relevant in power plant operation with conventional as well as newly developed SCR catalysts. As a benchmark of the catalysts regarding all reactions, the performance indicator P3 is introduced. The research is mainly conducted with standard SCR catalysts as reference and newly developed, modified honeycomb SCR catalysts, and supplemented by tests on plate-type SCR catalysts. In this research, modifications in the active component (V, Cu, Fe, Mn, Ce), of the promoters (W, Mo) as well as modifications of the base materials are studied. Through the dedicated application of the promoter molybdenum and modifications of the base material, a significant and clear increase in catalyst performance (high values of P3) can be achieved. An increased wall thickness of the catalyst also leads to an increase in Hg oxidation; however, the SO2/SO3 conversion is increased in parallel. Examinations on the influences of the flue gas on the oxidation of Hg show a strong effect of the halogen content (HCl, HBr) in the flue gas. Likewise, the sulfation of the catalysts has a positive effect on the reactions over the catalysts. A parallel DeNOx reaction in the catalyst with the addition of NH3 and the presence of CO in flue gas inhibits Hg oxidation as well as SO2/SO3 conversion. The oxidation of Hg over SCR catalysts seems to proceed according to an Eley-Rideal or Mars-Maessen mechanism: mercury adsorbs on the SCR catalyst and reacts with weakly adsorbed hydrogen halide or hydrogen halide species from the gas phase. The Hg adsorption and release can be correlated with the catalyst composition. The Deacon reaction might bring an additional contribution, but does not seem to be exclusively responsible for the measured effects.Item Open Access CO2-Abscheidung aus Kraftwerksrauchgasen mit wässriger MEA-Lösung - Waschmitteldegradation und Aufbereitungsverfahren(2016) Rieder, Alexander; Scheffknecht, Günter (Prof. Dr. techn.)In der Arbeit wird die Aminwäsche zur Abscheidung von CO2 aus Rauchgasen eines Steinkohlekraftwerks untersucht. Das Verfahren stellt eine vielversprechende Möglichkeit zur Reduktion der CO2-Emissionen im Kraftwerksbereich dar. Als Waschmittel wird dafür eine 30 Gew. %-ige wässrige MEA-Lösung eingesetzt. Die Ergebnisse aus drei MEA-Versuchskampagnen an der CO2-Testanlage Heilbronn in den Jahren 2011, 2013 und 2014 zeigen, dass bereits nach relativ kurzen Betriebszeiten erhöhte Gehalte an Degradationsprodukten und Metallen im Waschmittel vorliegen. Mit typischen Sauerstoffkonzentrationen zwischen 5 und 6 Vol. % (tr.) in den Kraftwerksrauchgasen dominiert die oxidative Degradation der wässrigen MEA-Lösung. Um einen langfristigen, stabilen und wirtschaftlichen Betrieb einer CO2-Abscheideanlage zu gewährleisten, sind Maßnahmen zur Minderung unerwünschter Degradationsprodukte im Waschmittel erforderlich. Neben der detaillierten Analyse der MEA-Degradation unter realen Betriebsbedingungen ist es Ziel der Arbeit, eine experimentelle Untersuchung geeigneter Waschmittelaufbereitungsverfahren durchzuführen. Dazu werden die konventionelle und bipolare Elektro-Dialyse sowie die Umkehrosmose und Nanofiltration herangezogen. Aufbauend auf den gewonnenen Erkenntnissen werden Empfehlungen zur Waschmittelaufbereitung und Minderung von primären Oxidationsprodukten gegeben.Item Open Access Dynamic monitoring of Frequency Containment Reserve activation(2021) Maucher, Philipp; Lens, HendrikFrequency Containment Reserve (FCR), also known as primary control reserve, is the fastest active power reserve (APR) used for system balancing. In the past, FCR was provided by a limited number of large units and the technical requirements on FCR activation could be monitored by a manual procedure. Due to the fact that FCR increasingly is provided by a large number of smaller units, manual procedures are no viable option for the future. For this reason, this article describes an automated concept for monitoring the quality of the activation of FCR during operation. Before FCR monitoring can be carried out, however, the measured active power has to be apportioned among the scheduled power and the different APR products, as they may be provided simultaneously. This article presents a method for the separation of different APR products based on dynamic models. In the proposed monitoring concept, a tolerance channel is created based on the FCR setpoint, defining the admissible range for FCR activation. This tolerance channel is established by means of a dynamic model corresponding to the slowest reserve provider (RP) whose FCR activation is still considered to be compliant. For the purpose of evaluation for longer time periods, the actual FCR activation is normalized relative to the tolerance channel. Finally, the article discusses results obtained with the proposed monitoring concept for different kinds of non-compliant FCR activation such as limited or delayed activation.