1,720,986 research outputs found
Carbon capture with pure monoethanolamine aerosols: Method development for assessing safety hazards and identifying a safe operating window using oxygen-containing flue gas streams
No full textRecent research proved the potential of aerosol reactors for post-combustion capture of CO2 with highly concentrated monoethanolamine (MEA). However, aerosol flammability in a continuous aerosol reactor has not been studied before. Safety cannot be guaranteed when processing oxygen-containing flue gases. The goal of this work is to develop a continuous aerosol safety testing method to investigate aerosol flammability. The proposed method is validated by determining the safe operating window of MEA aerosols by changing parameters on the aerosol and gas side. The relation between aerosol properties and flammability is investigated via high-speed camera. Safe operating conditions in air were found for MEA solutions up to of 70 wt%, suggesting that these solutions are safe to use with any flue gas. However, 80 wt% MEA and higher were flammable. 80 wt% MEA was only flammable at low liquid flow rates. This can be attributed to the increased droplet velocity at higher liquid flow rates, which results in shorter evaporation times. Therefore, the lower flammability limit (LFL) cannot be reached. Overall, MEA concentration and liquid flow rate appear to be the most influential parameters on aerosol flammability. Furthermore, the limiting oxygen concentration (LOC) was investigated. Maximum LOCs were identified as 13.5% and 15% for 100 and 90 wt% MEA, respectively. This information is crucial for matching appropriate flue gases with compatible aerosol reactor configurations. By developing and validating the method for MEA aerosols, this work narrows the gap between aerosol reactors for carbon capture and safely processing oxygen-containing flue gas streams.The authors acknowledge funding from VLAIO-Catalisti Moonshot projects “Intensification of CO2 Capture Processes” (HBC.2019.0109 – CAPTIN and HBC.2021.0254 – CAPTIN2)
Influence of Process Conditions on the Auto-Ignition Temperature of Gas Mixtures
Get PDFVele chemische processen maken gebruik van brandbare gassen en dampen bij verhoogde drukken en temperaturen. Om het zelfontstekingsrisico te kunnen inschatten en om de veilige en optimale werking van deze processen te verzekeren, is het belangrijk om de laagst mogelijke temperatuur te kennen waarbij spontane ontsteking kan optreden. De zelfontstekingstemperaturen (AIT's) die in de literatuur beschikbaar zijn, zijn meestal bepaald volgens gestandaardiseerde methodes in kleine volumes en bij atmosferische druk. Aangezien de zelfontstekingstemperatuur niet constant is maar daalt bij toenemende drukken en toenemende volumes zijn deze AIT's niet rechtstreeks toepasbaar voor industriële condities. Het gebrek aan zelfontstekingsdata bij verhoogde drukken en grote volumes en het gebrek aan uitgebreide modellen van de zelfontsteking waren de drijfveren van deze studie. Daarom bestaat deze studie uit een experimenteel en een numeriek gedeelte.De experimentele studie bestaat uit de bepaling van de zelfontstekingsgrenzen van methaan, propaan en butaan mengsels bij verhoogde drukken tot 30 bar en voor verschillende concentraties. Het is aangetoond dat de zelfontstekingstemperaturen significant dalen bij verhoogde drukken. De alkaanmengsels die aanleiding geven tot de laagste zelfontstekingstemperaturen hebben een rijke brandstof/lucht verhouding, die eveneens afhangt van de initiële druk. De zelfontstekingsgrenzen van propaan/butaan mengsels komen goed overeen met de zelfontstekingsgrenzen van de component met de laagste zelfontstekingstemperatuur, namelijk n-butaan. De ligging van de zelfontstekingsgebieden kon eveneens het verloop van de bovenste explosiegrenzen bij verhoogde drukken en temperaturen van propaan en n-butaan mengsels verklaren.De numerieke studie concentreert zich op de modellering van de zelfontsteking van methaan/lucht mengsels bij verhoogde drukken. Eerst werd een nuldimensionale aanpak toegepast, die gebaseerd is op het model van Semenov. De chemie van de ontsteking is gemodelleerd door middel van gedetailleerde reactiemechanismen. Een methaan reactiemechanisme van de British Gas Corporation toonde de beste overeenkomst met de experimentele data. Om de thermische en massa diffusie en de natuurlijke convectie in rekening te brengen, werd een tweedimensionaal model opgebouwd met inbegrip van het reactiemechanisme. De warmteoverdracht en de natuurlijke convectie binnenin het gesloten volume worden gemodelleerd door middel van het CFD programma. De koppeling van de reactiekinetica met de stromingsmodellering resulteert in een nauwkeurige voorspelling van de zelfontstekingsgrenzen van methaan/lucht mengsels bij verhoogde drukken. Dit model is eveneens aangewend om de volumeafhankelijkheid van de zelfontstekingstemperatuur voor sferische en cilindrische vaten te onderzoeken.status: Publishe
Zelf-ontstekingskarakteristieken van poederkool bij oxyfuel verbrandingsatmosferen
No full textDust accumulation including dust volume and dust layer often occurs in coal mills and power plants. Once in contact with the oxidant and a mild heat source, the accumulated dust may self-heat to initiate smouldering fires, which can further generate hot spots to trigger dust explosion. However, little is known about the combined effect of and CO2 diluent gas on self-ignition characteristics and subsequent smouldering combustion behaviour.
In the first phase of this dissertation, experimental materials including three different coal dust samples and gas atmospheres (both air and O2/CO2 mixtures) are presented. The standardized experimental apparatuses including hot-oven test (European norm EN 15188) and hot-plate test (European norm EN 50281-2-1) are elaborately introduced to determine the self-ignition temperature (SIT) of coal dust and the minimum ignition temperature of coal dust layer (MITL), respectively. In addition, a modified hot-oven test is used to look into the characteristics of self-heating ignition and the subsequent smouldering of a bituminous coal dust in oxy-fuel atmospheres with an emphasis on mass loss rate and gas emissions.
In the second phase of the dissertation, the focus is on the determination of SIT and MITL of coal dust in oxy-fuel combustion atmospheres via standard apparatuses introduced in the first phase. Both the self-ignition risk and dust explosion risk have been found to increase significantly with increasing O2 mole fraction. Moreover, the self-ignition risk of coal dust accumulation slightly decreases when replacing N2 with CO2 under the same conditions. The modified hot-oven test shows that a higher O2 mole fraction leads to a shorter ignition delay, a shorter burn-out duration, a faster O2 chemisorption at low temperature as well as a higher peak smouldering temperature. The emission-gas measurements show that the CO to CO2 ratio increases significantly after self-ignition, and CH4 counts for 1~8% of the total carbon emission.
The third part of the dissertation develops a 2D numerical model, using the commercial software COMSOL Multiphysics, to investigate self-ignition behaviour of coal dusts for both hot-oven test and hot-plate test with a main emphasis on the roles of oxygen, diluent gas and characteristic length. A one-step global 2nd-order reaction kinetic model considering both coal density and oxygen density is used to estimate the reaction rate of the self-heating process until ignition. Based on the hot-oven test results, the kinetic parameters are further calibrated for hot-oven tests are used for the simulation of both dust volume and dust layer. The accuracy of the numerical simulations has been discussed via varying the diameter of grid, time step and calculation termination time.
In the last part of the dissertation, the developed model has been validated to predict the transient temperature and concentration profiles of South African coal dusts until ignition. Both the computed critical ignition temperature and the ignition delay time related to the mole fraction of oxygen and the dust volume show a good agreement with experimental results. The ignition delay time and the most sensitive ignition position (MSIP) of dust layer has been found to vary with varying ambient gas conditions, hot plate temperatures and ambient temperatures using the verified model. Parameter analysis shows that the heating value has a comparatively pronounced effect on both SIT and MITL, and the heat transfer coefficient have a stronger effect on MITL. This is the first time that a physics-based model of ignition of coal dust layers has been developed in the oxy-fuel atmospheres, thus helping to understand the fire risk in this promising combustion technology.status: Publishe
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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