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Experimental and modelling evaluation of an ammonia-fuelled microchannel reactor for hydrogen generation / Steven ChiutaChiuta, Steven January 2015 (has links)
In this thesis, ammonia (NH3) decomposition was assessed as a fuel processing
technology for producing on-demand hydrogen (H2) for portable and distributed fuel cell
applications. This study was motivated by the present lack of infrastructure to generate H2 for
proton exchange membrane (PEM) fuel cells. An overview of past and recent worldwide
research activities in the development of reactor technologies for portable and distributed
hydrogen generation via NH3 decomposition was presented in Chapter 2. The objective was to
uncover the principal challenges relating to the state-of-the-art in reactor technology and obtain
a basis for future improvements. Several important aspects such as reactor design, operability,
power generation capacity and efficiency (conversion and energy) were appraised for innovative
reactor technologies vis-à-vis microreactors, monolithic reactors, membrane reactors, and
electrochemical reactors (electrolyzers). It was observed that substantial research effort is
required to progress the innovative reactors to commercialization on a wide basis. The use of
integrated experimental-mathematical modelling approach (useful in attaining accurately
optimized designs) was notably non-existent for all reactors throughout the surveyed openliterature.
Microchannel reactors were however identified as a transformative reactor technology
for producing on-demand H2 for PEM cell applications.
Against this background, miniaturized H2 production in a stand-alone ammonia-fuelled
microchannel reactor (reformer) washcoated with a commercial Ni-Pt/Al2O3 catalyst (ActiSorb®
O6) was demonstrated successfully in Chapter 3. The reformer performance was evaluated by
investigating the effect of reaction temperature (450–700 °C) and gas-hourly-space-velocity
(6 520–32 600 Nml gcat
-1 h-1) on key performance parameters including NH3 conversion, residual
NH3 concentration, H2 production rate, and pressure drop. Particular attention was devoted to
defining operating conditions that minimised residual NH3 in reformate gas, while producing H2
at a satisfactory rate. The reformer operated in a daily start-up and shut-down (DSS)-like mode for a total 750 h comprising of 125 cycles, all to mimic frequent intermittent operation envisaged
for fuel cell systems. The reformer exhibited remarkable operation demonstrating 98.7% NH3
conversion at 32 600 Nml gcat
-1 h-1 and 700 °C to generate an estimated fuel cell power output of
5.7 We and power density of 16 kWe L-1 (based on effective reactor volume). At the same time,
reformer operation yielded low pressure drop (<10 Pa mm-1) for all conditions considered.
Overall, the microchannel reformer performed sufficiently exceptional to warrant serious
consideration in supplying H2 to low-power fuel cell systems.
In Chapter 4, hydrogen production from the Ni-Pt-washcoated ammonia-fuelled
microchannel reactor was mathematically simulated in a three-dimensional (3D) CFD model
implemented via Comsol Multiphysics™. The objective was to obtain an understanding of
reaction-coupled transport phenomena as well as a fundamental explanation of the observed
microchannel reactor performance. The transport processes and reactor performance were
elucidated in terms of velocity, temperature, and species concentration distributions, as well as
local reaction rate and NH3 conversion profiles. The baseline case was first investigated to
comprehend the behavior of the microchannel reactor, then microstructural design and
operating parameters were methodically altered around the baseline conditions to explore the
optimum values (case-study optimization).
The modelling results revealed that an optimum NH3 space velocity (GHSV) of 65.2 Nl
gcat
-1 h-1 yields 99.1% NH3 conversion and a power density of 32 kWe L-1 at the highest operating
temperature of 973 K. It was also shown that a 40-μm-thick porous washcoat was most
desirable at these conditions. Finally, a low channel hydraulic diameter (225 μm) was observed
to contribute to high NH3 conversion. Most importantly, mass transport limitations in the porouswashcoat
and gas-phase were found to be negligible as depicted by the Damköhler and Fourier
numbers, respectively. The experimental microchannel reactor produced 98.2% NH3 conversion
and a power density of 30.8 kWe L-1 when tested at the optimum operating conditions established by the model. Good agreement with experimental data was observed, so the
integrated experimental-modeling approach used here may well provide an incisive step toward
the efficient design of ammonia-fuelled microchannel reformers.
In Chapter 5, the prospect of producing H2 via ammonia (NH3) decomposition was
evaluated in an experimental stand-alone microchannel reactor wash-coated with a commercial
Cs-promoted Ru/Al2O3 catalyst (ACTA Hypermec 10010). The reactor performance was
investigated under atmospheric pressure as a function of reaction temperature (723–873 K) and
gas-hourly-space-velocity (65.2–326.1 Nl gcat
-1 h-1). Ammonia conversion of 99.8% was
demonstrated at 326.1 Nl gcat
-1 h-1 and 873 K. The H2 produced at this operating condition was
sufficient to yield an estimated fuel cell power output of 60 We and power density of 164 kWe L-1.
Overall, the Ru-based microchannel reactor outperformed other NH3 microstructured reformers
reported in literature including the Ni-based system used in Chapter 3. Furthermore, the
microchannel reactor showed a superior performance against a fixed-bed tubular microreactor
with the same Ru-based catalyst. Overall, the high H2 throughput exhibited may promote
widespread use of the Ru-based micro-reaction system in high-power applications.
Four peer-reviewed journal publications and six conference publications resulted from
this work. / PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2015
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Samo-rotirajući impeleri u airlift reaktoru sa spoljašnjom recirkulacijom / Self-agitated impellers in an external-loop airliftreactorLukić Nataša 18 February 2017 (has links)
<p>Cilj doktorske disertacije je poboljšanje hidrodinamičkih i<br />masenoprenosnih karakteristika airlift reaktora sa<br />spoljašnjom recirkulacijom umetanjem samo-rotirajućih<br />impelera u uzlaznu cev. Uticaj ugradnje samo-rotirajućih<br />impelera na osnovne hidrodinamičke i masenoprenosne<br />osobine ispitivan je pri radu sa različitim tečnostima i<br />distributorima gasa. Rezultati su tumačeni poređenjem<br />vrednosti za sadržaj gasa u uzlaznoj cevi, za brzinu<br />tečnosti u silaznoj cevi, i za zapreminski koeficijent<br />prenosa mase, dobijenih za dve konfiguracije reaktora (sa<br />i bez impelera).<br />Rezultati disertacije ukazuju da ugradnja samo-rotirajućih<br />impelera dovodi do znatnog razbijanja mehurova i<br />smanjenja srednjeg prečnika mehurova gasa, naročito u<br />viskoznim rastvorima karboksimetilceluloze. Prividna<br />brzina gasa, vrsta tečne faze i tip distributora gasa u<br />velikoj meri utiču na efikasnost samo-rotirajućih impelera.<br />Dobijene vrednosti sadržaja gasa u uzlaznoj cevi su do<br />47% veće u konfiguraciji sa impelerima u odnosu na<br />konfiguraciju bez impelera. Iako samo-rotirajući impeleri<br />predstavljaju dodatni otpor proticanju tečnosti, njihovom<br />ugradnjom je u svim ispitivanim sistemima postignuto<br />relativno malo smanjenje brzine tečnosti (oko 10%).<br />Zahvaljujući impelerima, zapreminski koeficijent prenosa<br />mase uvećan je do 82% pri manjim protocima gasa. Pri<br />većim protocima, koji odgovaraju uslovima rada koji se<br />sreću u većini fermentacionih procesa, postignute su oko<br />20-30% veće vrednosti zapreminskog koeficijenta prenosa<br />mase.<br />Pored empirijskih modela, u ovom radu uspešno su<br />razvijeni i modeli veštačkih neuronskih mreža kojim se<br />predviđaju sadržaj gasa u uzlaznoj cevi, brzina tečnosti i<br />zapreminski koeficijent prenosa mase za obe konfiguracije<br />airlift reaktora sa spoljašnjom recirkulacijom.</p> / <p>The aim of this study was intensification of<br />hydrodynamic and mass transfer properties of<br />external-loop airlift reactor by means of self-agitated<br />impellers mounted in the riser section. The influence<br />of impellers’ insertion on the main hydrodynamic<br />and mass transfer characteristics was investigated for<br />various liquid phases and sparger types. The values<br />of riser gas holdup, downcomer liquid velocity and<br />volumetric mass transfer coefficient obtained in<br />configurations with and without impellers were<br />compared.<br />Results showed that the installment of self-agitated<br />impellers led to a significant bubble breakage and<br />decrease of bubble size, especially in viscous<br />carboxymethylcellulose solutions. The efficiency of<br />self-agitated impellers was immensely influenced by<br />superficial gas velocity, liquid phase, and distributor<br />type. Obtained riser gas holdup values were up to<br />47% higher in the configuration with impellers, in<br />comparison to the configuration wthout impellers.<br />Despite the fact that self-agitated impellers<br />represented an obstacle to liquid flow, relatively low<br />reduction of downcomer liquid velocity was attained<br />(about 10%). Furthermore, the insertion of impellers<br />induced up to 82% higher values of volumetric mass<br />transfer coefficient at lower superficial gas velocities.<br />At higher superficial gas velocities, i.e. conditions<br />mainly operated in various fermentation processes,<br />about 20-30% higher values of volumetric mass<br />transfer coefficient were achieved with impellers.<br />In addition to empirical correlations, artificial neural<br />network models were sucessfully developed to<br />predict riser gas holdup, downcomer liquid velocity<br />and volumetric mass transfer coefficient in both<br />external-loop airlift configurations.</p>
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Experimentelle und numerische Untersuchungen zur stabilen Entsorgung von Schwachgasen in porösen VerbrennungsreaktorenEndisch, Matthias 15 November 2013 (has links) (PDF)
Deponierte Abfälle emittieren auch Dekaden nach dem Ende der Einlagerung am Standort eine Vielzahl von Schadstoffen. Durch verantwortungsvollen Aufbau und Betrieb oder eine nachträgliche Sanierung kann das Gewicht von unkontrollierten Emissionen hin zu kontrollierten verschoben werden, die wiederum einer entsprechenden Entsorgung zugeführt werden können. Neben dem Deponiesickerwasser spielt das Deponiegas mit den Hauptbestandteilen Methan und Kohlendioxid auf Grund des lokalen und globalen (anthropogener Treibhauseffekt) Gefährdungspotentials eine wichtige Rolle. Sowohl die Gasmenge als auch der Gehalt an brennbaren Methan verringern sich mit zunehmender Standzeit der Deponie. Für Stark- und Mittelgase (ca. 25-60 Vol.-% Methan) ist eine entsprechende Nutzungs- und Entsorgungstechnik etabliert. Für den Schwachgasbereich ist die Entsorgung durch Verbrennung auf Grund der schlechten Verbrennungseigenschaften problematisch. Es existieren einige Lösungsansätze, von denen sich jedoch angesichts verschiedener technologiebedingter Nachteile noch keiner für die Schwachgasentsorgung über den gesamten relevanten Bereich an Methangehalten etablieren konnte.
Ziel der vorliegenden Arbeit war es, einen neuen Ansatz für eine einfache, kostengünstige und robuste Technologie zu erarbeiten, die eine vollständige Eliminierung des Methans und weiterer schädlicher Bestandteile gewährleistet. Im Weiteren sollen Methangehalte im Deponiegas von 5-11 Vol.-% bzw. Modellgemische mit einem Äquivalenzverhältnis von 0,3-0,5 betrachtet werden. Dieser Bereich erweist sich als besonders problematisch hinsichtlich einer stabilen Entsorgung. Bei Grubengasen und methanbeladener Abluft sind unter Umständen ähnliche Fragestellungen anzutreffen. Neben experimentellen Arbeiten in Labor und Technikum liefern vor allem numerische Simulationen einen wesentlichen Beitrag zur wissenschaftlichen Durchdringung der ablaufenden Prozesse und der Vorausberechnung technischen Reaktoren und Brennern. Für hier behandelte Problemstellung bietet sich die Software Ansys Fluent® an.
Für die Entsorgung von Schwachgasen bieten sich mehrere grundsätzliche Herangehensweisen an. Für diese Arbeit wurde sich auf die Oxidation des Methans in porösen Medien, z.B. Füllkörperschüttungen oder offenzellige SiC-Schaumkeramiken, konzentriert.
In einem ersten Ansatz wurde eine katalytische Funktionalisierung von SiC-Schaumkeramik mit Manganoxid dotierter Calciumaluminat-Beschichtung durchgeführt, um so die hervorragenden thermischen Eigenschaften des Siliciumcarbids mit der Oxidationsaktivität eines Katalysators zu kombinieren. Nach der Evaluierung einer optimalen hochtemperaturfesten Katalysatorkombination und experimentellen Erarbeitung kinetischer Parameter bilden Modellrechnungen im einfach durchströmten Monolith (1D) die Basis für eine Beurteilung dieser Variante.
Der zweite Ansatz verfolgt eine nicht-katalytische, rein thermische Umsetzung des Methans. Eine Verbesserung der Verbrennungseigenschaften durch Vorwärmung des Brenngases
erfolgt über interne Rekuperation ohne externen Abgaswärmeübertrager. Auf Basis von theoretischen Überlegungen und Simulationsrechnungen wurde eine entsprechende Technikumsanlage mit einer aus Alumina-Raschig-Ringen bestehenden porösen Matrix entworfen und aufgebaut. Ausgewählte Versuchsreihen dienten zur Demonstration der Funktionalität und zur Validierung des CFD-Modells. Mit Hilfe des Modells sind eine Variation der Eigenschaften der porösen Matrix und ein Upscaling auf einen technischen Maßstab möglich. Die Berechnung des Einsatzbereiches erfolgt beispielhaft mit einem Vorschlag für eine technisch relevante Größe an einem typischen Deponie-Schwachgas. Für die Modellierung von Oxidationsprozessen in porösen Strukturen ist die Implementierung des Wärmetransports ein wesentlicher Baustein. Ein entscheidender Parameter hierin ist die effektive Wärmeleitfähigkeit der porösen Matrix. Insbesondere für keramische Schäume ist die Anzahl der publizierten Arbeiten gering. Durch Vermessung der SiC-Keramikschäume mit Hilfe der Hot-Disk-Methode (Raumtemperatur) und mit dem Plattenmessverfahren (300-1000 °C) konnte auch an dieser Stelle ein Beitrag erbracht werden.
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Thorium–based fuel cycles : saving uranium in a 200 MWth pebble bed high temperature reactor / S.K. GintnerGintner, Stephan Konrad January 2010 (has links)
The predominant nuclear fuel used globally at present is uranium which is a finite resource. Thorium has been identified as an alternative nuclear fuel source that can be utilized in almost all existing uranium–based reactors and can significantly help in conserving limited uranium reserves. Furthermore, the elimination of proliferation risks associated with thorium–based fuel cycles is a key reason for re–evaluating the possible utilization of thorium in high temperature reactors. In addition to the many advantages that thorium–based fuel has over uranium–based fuel, there are vast thorium resources in the earth's crust that up until the present have not been exploited optimally.
This study focuses on determining the amount of uranium ore that can be saved using thorium as a nuclear fuel in HTR's. Four identical 200 MWth high temperature reactors are considered which make use of different fuel cycles. These fuel cycles range from the conventional uranium fuel cycle to a thorium–based fuel cycle in which no U–238 is present and have been simulated using the VSOP–A system of computer codes. This study also considers the effect that protactinium, an isotope that occurs in thorium–based fuel cycles, will have on the decay heat production in the case of a depressurized loss of coolant (DLOFC) accident. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.
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Thorium–based fuel cycles : saving uranium in a 200 MWth pebble bed high temperature reactor / S.K. GintnerGintner, Stephan Konrad January 2010 (has links)
The predominant nuclear fuel used globally at present is uranium which is a finite resource. Thorium has been identified as an alternative nuclear fuel source that can be utilized in almost all existing uranium–based reactors and can significantly help in conserving limited uranium reserves. Furthermore, the elimination of proliferation risks associated with thorium–based fuel cycles is a key reason for re–evaluating the possible utilization of thorium in high temperature reactors. In addition to the many advantages that thorium–based fuel has over uranium–based fuel, there are vast thorium resources in the earth's crust that up until the present have not been exploited optimally.
This study focuses on determining the amount of uranium ore that can be saved using thorium as a nuclear fuel in HTR's. Four identical 200 MWth high temperature reactors are considered which make use of different fuel cycles. These fuel cycles range from the conventional uranium fuel cycle to a thorium–based fuel cycle in which no U–238 is present and have been simulated using the VSOP–A system of computer codes. This study also considers the effect that protactinium, an isotope that occurs in thorium–based fuel cycles, will have on the decay heat production in the case of a depressurized loss of coolant (DLOFC) accident. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.
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Plazmová modifikace práškových materiálů / Plasma modification of powder materialsČERNÝ, Pavel January 2011 (has links)
Thesis is focused on plasma modification of materials. The work has the character of a search of available literature, especially the papers presented in impacted journals. The aim is to provide a broader overview of the subject. The first section describes the most commonly modified powdered materials. The second part is focused on the plasma modification, the reasons for its use, capabilities and benefits respectively and disadvantages of these processes. The following sections are devoted to plasma discharges, plasma reactors, aspects of plasma modification and companies engaged in production, or modification of powders. Each chapter is intended to provide an overview of the modified powder materials, processes within the plasma discharges, construction and use of plasma reactors, plasma modification of aspects and the market situation in the context of powdered materials and companies dealing with the plasma modification.
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Plynné výpusti 14C z ETE / Gasseous effluents of 14C from NPP TemelínJANOVSKÝ, Daniel January 2007 (has links)
Within the presented thesis there were collected data of effluents of 14C chemical forms from ventilation stacks of the Unit 1, the Unit 2 and the Auxiliary Building of the Temelin NPP for the period from 2001 to 2006. These data are compared to power of both reactors and concentration of ammonium ions in coolant of the primary circuit of the Unit 1 and Unit 2.
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Využití reaktoru s kalovým mrakem a externím separátorem biomasy pro výrobu metanu ze substrátu pro BPS / The use of the reactor with sludge cloud and external biomass separator for methane production from the substrate for Biogast PlantŽIVNÝ, Jakub January 2013 (has links)
The aim of the study was to compere the reactor with sludge cloud and external separator biomass in different modes. This typ of reactor is principally used in wastewater treatment and in this work was designed for production of biogas from biosmass, commonly used in agricultural biogas plants. The reactor assembly was prepared in the laboratory of the available glass flasks, connected by rubber tubing and mixing pump. The entire device was immersed in a water bath heated thermostat. For the purpose of this work was used samples from biogas Týnec near Dobrovice at Mladoboleslavko. The reaktor was injectable liquid portion, formed after adjusment method IFBB (Integrated Generation of Solid Fuel and Biogas from Biomass). The principle of this method consists in separating the liquid and solid components of biomass. Operation of the reactor was carried out in three stirring models: without stirring, medium stirring, intensive stirring. Further, the operation was compared at 40°C and 53 °C in a mode without stirring. This monitoring is carried abut for five hours. The specimens was taken every hour and after drying to COD (determine chemical oxygen demand), which expresses the degree of degradation of organic matter. The results show that the laboratory devie model stirring does not show. However, the temperature change from 40°C to 53°C showed an increased degradation of organic matter.
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Porovnání výpustí českých a světových JE / Comparison of the gaseous and liquid releases of the Czech and world nuclear power plantsDOBEŠ, Petr January 2007 (has links)
In this work, which deals with problematics of releases from nuclear power plants, I tried to make an overview of various types of nuclear power plants and radioizotopes which are released through liquid and gasseous effluents. As a part of this comparison evaluation of czech and world nuclear power plants gaseous and liquid releases was made. Introductory part of this work contains information about different types of nuclear power plants and radioizotopes, which are produced in their reactors. It continues with today{\crq}s legislative and information about releasing levels and methods and systems used for measurement of radioactive gaseous and liquid effluents from nuclear power plants. Second part of this work describes the aim of this work and hypothesis. Third part explains the methods, which were used for completing of this work. Fourth part contains results in the form of tables and graphs. Fifth part represents discussion of the results. Last part is a summarization of the results.
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Využití reaktoru s kalovým mrakem a externím separátorem biomasy pro výrobu metanu ze substrátu pro BPS Chotýčany, zpracovaném technologií IFBB (Integrated Generation of Solid Fuel and Biogas from Biomass) / The use of the reactor with sludge cloud and external biomass separator for methane production from the substrate for BPS Chotýčany, processed by IFBB technology (Integrated Generation of Solid Fuel and Biogas from Biomass)BROŽEK, Jiří January 2011 (has links)
There were two points of research in this graduation work. The first was IFBB technology (Integrated generation of Solid Fuel and Biogas from Biomass) used for preparation of the substrate for fermentation. The second point was to test reactor with sludge cloud (completed with external biomass separator). This reactor was filled by substrate (prepared by IFBB technology) from agricultural biogas energy plant located in Chotýčany. After IFBB preparation substrate was filled in laboratory reactor´s group. Then it was inoculated by digestate. Digestate is a substance, which is in the end of fermentation process. That substance was first taken in biogas station Chotýčany, then, for comparsion, from station Věžovatá Pláně. Results are that IFBB technology is good for substrate preparation and folowing fermentation is better. Reactor with sludge cloud is good for intensive fermentation. Efficiency of the reactor was measured by CHSKCr. The best fermentation was found after using inoculum from Věžovatá Pláně. The best temperature for fermentation was 50 degree Celsius for both inoculums.
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