Auto-ignition and upper explosion limit of rich propane-air mixtures at elevated pressures

Department of Mechanical Engineering, University of Leuven, Louvain, Flemish, Belgium
Journal of Hazardous Materials (Impact Factor: 4.53). 10/2006; 137(2):666-71. DOI: 10.1016/j.jhazmat.2006.03.018
Source: PubMed


The auto-ignition limits of propane-air mixtures at elevated pressures up to 15 bar and for concentrations from 10 mol% up to 70 mol% are investigated. The experiments are performed in a closed spherical vessel with a volume of 8 dm3. The auto-ignition temperatures decrease from 300 degrees C to 250 degrees C when increasing the pressure from 1 bar to 14.5 bar. It is shown that the fuel concentration most sensitive to auto-ignition depends on initial pressure. A second series of experiments investigates the upper flammability limit of propane-air mixtures at initial temperatures up to 250 degrees C and pressures up to 30 bar near the auto-ignition area. Finally the propane auto-oxidation is modelled using several detailed kinetic reaction mechanisms and these numerical calculations are compared with the experimental results.

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    • "Many experimental data have been recorded for individual fuel species, such as hydrogen [5], methane [6][7][8][9][10][11], ethane[6][12], propane [7][13], n-butane [14][15], or blends that are representative of natural gas [7] or LPG [11]. Usually, the lowest auto-ignition temperature is not reached under the stoichiometric conditions, but for richer mixtures i.e. between the stoichiometric concentration and the upper flammability limit [16]. "
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    ABSTRACT: Gas turbines burn a large variety of gaseous fuels under elevated pressure and temperature conditions. During transient operations, variable gas/air mixtures are involved in the gas piping system. In order to predict the risk of auto-ignition events and ensure a safe operation of gas turbines, it is of the essence to know the lowest temperature at which spontaneous ignition of fuels may happen. Experimental auto-ignition data of hydrocarbon-air mixtures at elevated pressures are scarce and often not applicable in specific industrial conditions. AIT data correspond to temperature ranges in which fuels display an incipient reactivity, with time scales amounting in seconds or even in minutes instead of milliseconds in flames. In these conditions, the critical reactions are most often different from the ones governing the reactivity in a flame or in high temperature ignition. Some of the critical paths for AIT are similar to those encountered in slow oxidation. Therefore, the main available kinetic models that have been developed for fast combustion, are unfortunately unable to represent properly these low temperature processes. A numerical approach addressing the influence of process conditions on the minimum AIT of
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    ABSTRACT: Vele 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. Many chemical processes use combustible gases and vapours at elevated pressures and high temperatures. In order to evaluate the auto-ignition hazard involved and to ensure the safe and optimal operation of these processes, it is important to know the lowest possible temperature at which spontaneous ignition of these gases and vapours takes place. The auto-ignition temperatures (AIT's) found in literature usually are determined applying standardised test methods in small vessels and at atmospheric pressure. However, since the AIT is not constant but decreases with increasing pressures and increasing volumes, these AIT values are often not applicable in industrial environments. The lack of auto-ignition data at elevated pressures and the lack of comprehensive auto-ignition models were the motivations for this study. Therefore the present study consists of an experimental and a numerical part. The experimental study consists of the determination of the auto-ignition limits of methane, propane and butane mixtures at elevated pressures up to 3 MPa for a wide range of concentrations. It is shown that the auto-ignition limits decrease significantly with increasing pressure. The concentrations most sensitive to auto-ignition are high concentrations and depend on the initial pressure. The auto-ignition limits of the propane/butane mixtures correspond well with the auto-ignition limits of the component with the lowest auto-ignition temperature, which is n-butane. The location of the auto-ignition areas could explain the observations of the upper flammability limits at elevated temperatures and pressures of propane and n-butane mixtures. The numerical study focuses on the modelling of the auto-ignition process of methane/air mixtures at elevated pressures. First a zero-dimensional approach is adopted, based upon the model of Semenov. The chemistry is modelled by means of a detailed reaction mechanism. A methane reaction mechanism of the British Gas Corporation shows the best agreement with the experimental results. To take thermal and mass diffusion and the natural convection inside the vessel into account a two-dimensional model is built including the kinetic mechanism. A CFD-model is used to compute the heat transfer and the buoyant flows inside the vessel. The coupling of the reaction mechanism to this model results in an accurate prediction of the auto-ignition conditions at elevated pressures. This model is also used to investigate the volume dependency of the auto-ignition temperature for both spherical and cylindrical vessels. Doctor in de ingenieurswetenschappen Afd. Toeg. Mechanica & Energieconversie Departement Werktuigkunde Faculteit Ingenieurswetenschappen Doctoral thesis Doctoraatsthesis
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