Browsing by Author "Scheffknecht, Günter (Univ.-Prof. Dr. techn.)"

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Open Access
Abgaswärmenutzung in der Stahlerzeugung mittels calciumbasierter thermochemischer Hochtemperaturenergiespeicherung
(2022) Hartfuß, Georg; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
Die vorliegende Arbeit befasst sich mit der Entwicklung eines Konzeptes für die Abgaswärmenutzung für die Elektrostahlerzeugung. Die Abgaswärmerückgewinnung in Elektrostahlwerken ist derzeit nicht der etablierte Stand der Technik in der Bundesrepublik Deutschland und so gehen bis zu 30 % des Energieeinsatzes des Lichtbogenofens verloren. Die korrosiven und erosiven Eigenschaften des Abgases erschweren den Einsatz von Abgaswärmeübertragern. Die batchweise Produktion erfordert zudem die Integration eines Wärmespeichers. Es ist aus diesen Gründen kein System zur wirtschaftlichen Rückverstromung der Abgaswärme verfügbar. In dieser Arbeit wird daher ein neuartiges Konzept zur Abgaswärmeauskopplung und -speicherung erarbeitet. Zur Vermeidung des Einsatzes von Abgaswärmeübertragern wird die thermochemische Dehydrierung von Ca(OH)2-Partikeln im Flugstrom genutzt. Die Dehydrierung von Ca(OH)2 erfolgt endotherm, wobei Kalk und Wasserdampf gebildet werden. Die Auskopplung der Abgaswärme soll über das Einbringen des Ca(OH)2 in die Nachbrennkammer des Lichtbogenofens erfolgen. Das bestehende Abgassystem wird als Reaktionsstrecke für die Dehydrierung im Flugstrom genutzt. Die anschließende Abscheidung der heißen Kalkpartikel soll mit einem Zyklonabscheider realisiert werden. Ein Schüttgutbehälter dient zur Wärmespeicherung und dem Ausgleich von Schwankungen im Abgaswärmestrom. Zur exothermen Kalkhydrierung und Erzeugung von überhitztem Dampf zum Antrieb einer Entnahmekondensationsturbine wird eine blasenbildende Wirbelschicht verwendet. Die neugebildeten Ca(OH)2-Partikel werden anschließend gespeichert, bevor ein erneutes Einbringen in die Nachbrennkammer erfolgt. Der Schwerpunkt der Arbeit liegt auf der Auslegung der Anlagen des Flugstromkonzeptes und deren Integration in das Referenzstahlwerk. Die thermochemische Zersetzung von Ca(OH)2 im Flugstrom ist bisher noch nicht untersucht worden, weshalb Versuche zur Bestimmung der Umsatzraten durchgeführt und mit anderen Arbeiten verglichen worden sind. Zum Vergleich des Flugstromkonzeptes mit anderen Konzepten zur Abgaswärmenutzung in der Elektrostahlherstellung und der Beurteilung des ökonomischen Potentials wird eine Wirtschaftlichkeitsbetrachtung durchgeführt. Es wird bei einer Förderung der Anlage mit einer Amortisationszeit von 6,1 Jahren gerechnet. Jedoch sind noch viele Forschungsfragen bis zur Marktreife des Prozesses unbeantwortet, welche in weiteren Projekten untersucht werden müssen.
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.
Open Access
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.
Open Access
Erzeugung eines wasserstoffreichen Produktgases aus Biomasse mittels Wasserdampfvergasung und anschließender CO2-Abscheidung
(2018) Rothermel, Nina; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
Open Access
Experimental analysis of Calcium Looping CO2 capture in coal-fired power plants
(2023) Moreno, Joseba; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
The Calcium Looping (CaL) technology has recently emerged as a viable option for efficiently decarbonizing power plant flue gases. The process is based on the sequential calcination and carbonation of a calcium-based sorbent, usually limestone. Although CaO-based sorbents offer many advantages, they typically suffer from a rapid decline in CO2 capture during cyclic operation. This latter aspect has remained an urgent issue to be addressed for CaL power plant application. Besides, coal-fired power stations are expected to operate in a load-following mode due to the growing share of renewable energy. Moreover, the usage of alternative fuels in existing coal-fired power units is envisaged to ensure sustainable energy generation and to avoid an overshoot of CO2 emissions into the atmosphere. This thesis aims at addressing each of the previously anticipated challenges individually. A range of experimental investigations studying the cycling conversion of three originally distinct limestones – Rheinkalk, Riyadh, and Saabar – were conducted by thermogravimetric analysis (TGA). To this end, several carbonation routines were employed, including SO2 and steam. The Saabar metamorphosed limestone showed to be negatively influenced by the presence of steam, while limited sulfation positively affected its CO2 capture performance. This unusual behavior can be ascribed to pore blocking during carbonation. Besides, Rheinkalk and Riyadh behaved similarly, resembling the typical behavior of common unmetamorphosed limestones. The decay in Rheinkalk conversion upon cycling was further explored at a 20 kWth CaL facility setting different carbonation conditions. A mathematical expression is proposed to compare the results obtained in both facilities, correlating them with adequate accuracy. In the following, an alternative reactor concept based on a bubbling fluidized bed (BFB) carbonator was employed for flexible load operation. Within the first phase of the tests, a parametric study was conducted at the 20 kWth CaL facility to evaluate the influence of temperature, CO2 loading, and steam concentration upon the BFB carbonator performance. Hereafter, investigations at a 200 kWth semi-industrial CaL plant were conducted to evaluate the flexible behavior of the suggested carbonator setting. It was demonstrated that the BFB carbonator can be operated stably with gas superficial velocities ranging from 0.8 to 2.0 m/s without affecting the solid circulation between reactors. The latter range corresponds to a maximum reduction in the flue gas load of 60 % with respect to the nominal operation case. A simple carbon material balance was applied for preliminary validation of the carbonator performance. In addition, a carbonator model approach based on the active space time (𝜏active) was proposed for a more detailed result interpretation. According to the results, an active space time value of 41 s was identified as sufficient to achieve an equilibrium normalized capture efficiency (Enorm) of 90 %. Moreover, the circulating fluidized bed (CFB) calciner operation appeared independent from the flue gas load set in the carbonator. The reactor could be successfully operated with recirculation rates as low as 27 %, reaching inlet dry oxygen concentrations as high as 0.55 m3/m3. Within the next phase of the experiments, the impact of fuel selection in the calciner was evaluated. Oxy-combustion of hard coal, wheat straw, and solid recovered fuel (SRF) was demonstrated during more than 43 h of continuous operation. A range of experiments was conducted to address the influence of fuel blending and inlet oxygen concentration on pollutant formation (i.e., NOx, SO2, HCl) and hydrodynamic behavior. The calciner inlet O2 concentration appeared to barely affect the pollutant formation process. In contrast, biomass substitution influenced gaseous emissions by modifying the fuel mixture's nitrogen and chlorine content. Concurrently, specific HCl emissions were significantly reduced by the presence of Ca-species in the calciner solid inventory, yielding chlorine retention rates above 0.90 mol/mol with at least 30 % biomass substitution. Besides, ash accumulation led to elevated pressure drops over the CFB riser when operating with alternative fuels. Although this was not a limiting aspect in this work, results anticipate that ash accumulation might constitute a key challenge to be addressed in fluidized beds employing combustion of low-grade quality fuels.
Open Access
Experimentelle und simulative Untersuchung der Wasserdampfvergasung von Klärschlamm und weiteren biogenen Brennstoffen
(2018) Schweitzer, Daniel; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
Die allotherme Zweibett-Wirbelschichtwasserdampfvergasung ist ein thermochemisches Konversionsverfahren zur Umwandlung eines festen Brennstoffes in ein wasserstoffreiches und hochqualitatives Produktgas. In dieser Arbeit wird mittels experimenteller und simulativer Methoden der Einfluss verschiedener kostengünstiger biogener Brennstoffe wie Klärschlamm, Rinder- und Schweinegülle sowie Holzpellets auf den Vergasungsprozess untersucht. Die experimentellen Untersuchungen haben gezeigt, dass die Wasserdampfvergasung dieser biogenen Brennstoffe möglich ist: Die Untersuchungen zeigten, dass eine hohe Produktgasausbeute erreicht werden kann. Ebenso zeigte sich, dass sich die Zusammensetzung des Produktgases zwischen den verschiedenen Brennstoffen nur leicht unterscheidet. Zu berücksichtigen ist allerdings eine deutlich erhöhte Konzentration an Begleitgasen wie Teere, NH3, H2S oder HCl im Produktgas bei der Vergasung von biogenen Brennstoffen. Basierend auf den experimentellen Ergebnissen wurde ein Prozessmodell der Wasserdampfvergasung von Klärschlamm erstellt. Mittels der Prozesssimulation konnten Kaltgaswirkungsgrade von nahezu 70 % berechnet werden.
Open Access
Experimentelle Untersuchung der sorptionsunterstützten Reformierung
(2016) Poboß, Norman; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
In dieser Arbeit wird mittels experimentellen Untersuchungen eine Datengrundlage für die sorptionsunterstützte Reformierung geschaffen. Auf dieser Basis kann eine Vergasungsanlage nach dem gekoppelten Wirbelschichtprinzip ausgelegt bzw. betrieben werden. Als Ausgangspunkt für die Versuchsplanung werden Bewertungsgrößen und Einflussgrößen definiert und vor dem Hintergrund des aktuellen Forschungsstandes erörtert. Die Einflussgrößen: Biomasse, Vergasungstemperatur, Bettmaterialumlaufrate und Bettaustauschzeit, Raumgeschwindigkeit und Wasserdampf-zu-Brennstoffverhältnis werden in das Versuchsprogramm aufgenommen und deren Auswirkung auf die Bewertungsgrößen in verschiedenartigen Wirbelschichtversuchs- und Demonstrationsanlagen untersucht.
Open Access
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.
Open Access
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.
Open Access
Investigation into the behavior of mercury in wet flue gas desulfurization systems
(2022) Masoomi, Ida; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
Open Access
Metal mesh filter systems for small biomass furnaces
(2022) Baumgarten, Björn; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
Open Access
Pilot testing, simulation, and scaling of an oxyfuel burner for cement kilns
(2021) Carrasco Maldonado, Francisco; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
In the context of the present work the combustion concept oxyfuel, which entails the combustion to take place in an O2/CO2 atmosphere instead of air is investigated with reference to its application in cement kilns. Pilot testing at a 500 kWth once-through furnace and numerical simulations were employed to investigate the influence of high CO2 atmosphere and burner configuration on the combustion behavior of selected fuels. To investigate oxyfuel combustion in relevant cement production conditions, a modern kiln burner was scaled down and used as a prototype in the test rig. Conducted investigations in pilot-scale show that oxygen concentration in oxyfuel mode plays an important role in achieving an air-firing-like temperature profile. Flame length is also influenced under oxyfuel operation. As the oxygen concentration increases in combus-tion gas, the flame length shortens and the concentration of CO rises in the near burner region due to the influence of heterogeneous and homogenous reactions with CO2. Further, the operation in oxyfuel mode opens the possibility to redistribute the oxygen mass flow in the primary and secondary gas stream to optimize combustion behavior. According to the tests conducted, when the oxygen concentration is increased in pri-mary gas (with a corresponding decrease in secondary gas) the generation of CO in the near burner field decreases significantly. Finally, full-scale CFD simulations of a cement kiln show that the results in pilot scale can be applied to replicate similar tem-perature profile as in air-blown combustion with a slight increase in radiative heat fluxes to wall. According to these results, the prototype burner can be used to retrofit a cement kiln for operation in oxyfuel mode.
Open Access
Plasma-assisted ignition and combustion in pulverised fuel burners
(2022) Youssefi, Reyhane; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
This work focuses on the application of plasma-assisted ignition and combustion in pulverised fuel burners. Plasma-assisted ignition of pulverised fuel as an alternative technology for the start-up of pulverised fuel furnaces is investigated in a 400 kWpilot-scale facility using various qualities of lignite as well as woody biomass with distinct ignition characteristics. Short-time plasma-supported ignition under the cold furnace condition was demonstrated with parametric studies regarding the required power of plasma torch, air swirling, torch positioning, particle size distribution, etc. to identify the influence of different burner design parameters.
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.
Open Access
Use of additive to mitigate deposit and corrosion problems in pulverized biomass-fired boilers
(2021) Paneru, Manoj; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
When replacing coal with biomass fuel in existing boilers which are originally designed for coal, ash related deposition and corrosion problems are of critical concern. The problem is inherent to the composition and properties of inorganic matter, namely ash, present in the biomass fuel original feedstock source. The interactive chemistry of ash inorganic species along the combustion process forms low melting ash components, like K-silicate, and corro-sive ash components, like KCl. Such components have a direct implication to the boiler dep-osition and corrosion risk. Therefore, they are considered to be problematic ash species. One of mitigative option is to modify the ash formation chemistry to hinder the formation path-way of problematic ash species. Aluminosilicate minerals from the kaolin group, kaolinite and halloysite, are well known additives suitable to enforce such a mitigative chemistry. In this work, the combustion behavior of various biomass fuels, woody and herbaceous, were investigated in relation to boiler deposition and corrosion severity associated with ash. The combustion tests include cases of biomass alone without an additive and biomass with an additive. The additive was supplied together with the fuel into the combustion chamber. The employed combustion test facility simulates a scenario of a pulverized fuel firing combus-tion system. The deposit and ash sampling location represents an environment, i.e. tempera-ture and residence time, comparable to the super heater zone of a power plant boiler. Online deposition sensors (ODS) are employed to quantitatively measure the outer deposit growth that is bulk ash accumulation. Further, the morphology, chemical composition, and fusion behavior of outer deposit ash bulk was characterized. Temperature controlled (cooled) deposit probe are implemented to investigate inner deposit growth and to evaluate the morphological and chemical composition of inner deposit ash and its implication to boiler tube material corrosion. According to chemical composition the deposit ash is discussed as silicatic deposit and salt deposit. Silicatic deposit represents the ash particles dominated by silicon while the salt de-posit represents the ash/aerosol particles rich in sulfur or chlorine. The mitigative effect of additive was demonstrated by lower deposition propensity, im-proved morphology (less sintering), and overall reduction of molten ash components and salt species, especially KCl, in deposit ash. The absence of KCl apparently explains the low-er corrosion activity in cases with additives compared to corresponding cases with biomass alone. The result shows that for both, silicatic and salt deposits, the potassium species are of uttermost concern. The mitigative chemistry is fundamentally related to potassium capture reactions driven by aluminosilicate (derive from additive mineral). In presence of alumino-silicate in the system, the potassium prefers to form refractory (high melting), stable (irre-versible capture), and non-corrosive K-aluminosilicate instead of low melting K-silicate or corrosive KCl. The amount of additives required for a certain biomass fuel is influenced by ash system chemistry of fired biomass fuel and the transformation level of reactive mineral, kaolinite or halloysite, in the given reaction system. The chemistry is essentially gas-solid capture reac-tion, K-species and aluminosilicate (kaolinite derive), respectively. The reaction basically proceeds outside the burning fuel/char particles. The potassium capture reactions are ther-modynamically favored and not kinetically limited in high temperature combustion zone. The biomass ash system chemistry governs the amount of gaseous K-species primarily re-leased in the combustion zone. The transformation state of kaolinite, meta-kaolin or mullite, governs the effectiveness of additive particles to adsorb the gaseous species available in the system and later the chemical incorporation of potassium within the aluminosilicatic matrix of additive mineral. KCl prefers to remain as gas in the high temperature combustion zone. Therefore, chemically it is the last K-species to be consumed by the additive mineral. In the process, chlorine escapes to the flue gas as HCl. This study investigates the applicability of HCl concentration in the flue gas measured with-in appropriate temperature boundaries as a suitable control parameter to evaluate and opti-mize the fuel specific additive amount. Further, the HCl concentration is a feasible parame-ter for a benchmark comparison across various commercially available aluminosilicate based mineral additives with regard to the effectiveness of capture chemistry in combustion application scenario.
Open Access
Weiterentwicklung und Erprobung eines semikontinuierlichen Online-Verfahrens zur Bestimmung von Teerkonzentrationen in Biomassevergasungsprozessen
(2018) Gredinger, Andreas; Scheffknecht, Günter (Univ.-Prof. Dr. techn.)
Die sogenannten Teere zählen nach wie vor zu einem der Hauptprobleme bei der Vergasung von Biomassen. Vor allem ihre aufwändige messtechnische Bestimmung mit heute gängigen Adsorptionsverfahren, die nur eine zeitlich verzögerte Angabe eines Mittelwerts über einen bestimmten Zeitraum zulassen, ist ein Grund für das Fehlen der routinemäßigen Bestimmung ihrer Konzentration in Vergasungsprozessen. Folglich gilt auch der Mangel an entsprechend verfügbarer instrumenteller Analytik noch heute als eine Ursache dafür, dass (industrielle) Vergasungsprojekte scheitern. Im Rahmen der Bemühungen, diese Lücke in der Gasmesstechnik zu schließen und eine Alternative zu den konventionellen Adsorptionsverfahren zu schaffen, wurde in der Vergangenheit am Institut für Feuerungs- und Kraftwerkstechnik der Universität Stuttgart ein Prototyp eines Online-Teermessgeräts entworfen, welches in dieser Arbeit bis hin zur Marktreife weiterentwickelt und umfassend erprobt wird. Es basiert auf einem Differenzmessverfahren, in dem die Teere auf einem entsprechenden Kondensatabscheider von den permanent gasförmigen Kohlenwasserstoffen des Produktgases getrennt werden. Als Kohlenwasserstoffdetektor wird ein FID eingesetzt. Zuerst werden die gegenüber der Vorgängerversion des Prototyps realisierten notwendigen technischen Änderungen des Messgeräts und die Weiterentwicklungen seiner zugrundeliegenden Analytik beschrieben. Die durchgeführten Untersuchungen der Genauigkeit des verwendeten, neu entwickelten Detektors und der Wiederholbarkeit der Ergebnisse einzelner Messzyklen zeigen in einer beispielhaften Messreihe Abweichungen vom Mittelwert der Messwerte von unter 1 %. Die Nachweisgrenze der Teerkonzentrationen liegt, abhängig vom ausgewählten Messbereich, bei 50 mgC/m³i.N., 250 mgC/m³i.N. oder 500 mgC/m³i.N.. Sie ist dabei aber jeweils unabhängig vom Kohlenwasserstoffgehalt des zu untersuchenden Gases. Die Einflüsse verschiedener Messgerätebetriebsparameter auf die Detektorempfindlichkeit werden ebenfalls gezeigt und optimale Betriebseinstellungen angegeben. Bei der Ermittlung eines optimalen Teerkondensatabscheiders für die Trennung der Kohlenwasserstoffe an der Grenze der Teerdefinition (Differenzmessung) zeigt sich, dass inerte Kondensatfallen nicht ausreichen, um leichte Kohlenwasserstoffe, wie bspw. BTX, vom restlichen Produktgas zu trennen. Als Lösung wird auf adsorptive Materialien zurückgegriffen. Als vielversprechendste Materialien erweisen sich dabei natürlich vorkommende (Schicht)silikate. Sie adsorbieren in einem großen Konzentrationsbereich alle Teersubstanzen mit Ausnahme von Toluol komplett, lassen aber permanent gasförmige KWS vollständig passieren. Bei Toluol und Benzol findet dagegen jeweils eine Teiladsorption statt. Bei bestimmten Konzentrationsverhältnissen zeigte sich aber, dass sich die Teiladsorption der beiden Substanzen gegenseitig ausgleicht und somit die Angabe einer Teerkonzentration an der als Teer definierten Kohlenwasserstoffgrenze (zwischen Benzol und Toluol) grundsätzlich möglich ist. Die Durchführung von Vergleichsmessungen mit dem standardisierten, konventionellen Tar Protocol bestätigen die zuvor in Laborumgebung gewonnenen Erkenntnisse. So liegen die Abweichungen der Ergebnisse der Teermessungen bzw. Teeranalysen des Online-Teermessgeräts und der GC-FID-Analysen des Tar Protocol sowohl in Produktgasen einer Luftvergasung als auch (mit Korrekturen um den Wasserstoff- und Wasserdampfeinfluss auf das Messverfahren) einer Wasserdampfvergasung in einem Band von +/- 20 %. Die absoluten Abweichungen liegen dabei immer unter der Marke von 2,5 gC/m³i.N.. Die Messzykluszeit von 60-90 Sekunden und die direkte Gasanalyse sowie Ausgabe der Messwerte erlaubt im Gegensatz zu konventionellen Teermessmethoden die unmittelbare Überwachung der Gesamtteerkonzentration eines Vergasungsprozesses. Im Falle von auftretenden Teerspitzen oder ähnlichen Ereignissen ist dann ein schnelles Eingreifen in die Prozessführung möglich. Der demonstrative Einsatz des weiterentwickelten Messgeräts an einem semiindustriellen Vergaser zur Ermittlung optimierter Vergaserbetriebsparameter mit dem Ziel einer minimierten Teerproduktion bestätigt abschließend die im Verlauf der Arbeit gewonnenen Erkenntnisse und getätigten Aussagen hinsichtlich eines instrumentellen Ersatzes konventioneller Teermessverfahren.