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LAMINAR BURNING VELOCITY OF METHANE, HYDROGEN, AND THEIR MIXTURES AT EXTREMELY LOW TEMPERATURE CONDITIONS Gianmaria Pio, and Ernesto Salzano Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01796 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018
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Energy & Fuels
LAMINAR BURNING VELOCITY OF METHANE, HYDROGEN AND THEIR MIXTURES AT EXTREMELY LOW TEMPERATURE CONDITIONS
Gianmaria Pio, Ernesto Salzano* Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali, Università di Bologna, via Terracini 28, 40131 Bologna, Italy *Author to whom correspondence should be addressed:
[email protected] Abstract Methane consumption is strongly increasing due to its abundancy from natural gas, thus leading to reduced costs, and due to the reduced environmental impact in the case of combustion, a lower carbon content with respect to traditional fuels. These advantages encourage its utilisation in several industrial applications, such as methane steam reforming for the production of hydrogen and for automotive applications. The necessity of transporting the gases and large-scale distribution systems is however one of the main issue. Innovative processes, such as cryogenic storage, cryo-compression and liquefaction, require detailed information on the thermal and chemical properties of methane-hydrogen mixture at low and ultra-low temperatures. In this framework, detailed kinetic models for the total and partial oxidation of methane, hydrogen and methane-hydrogen mixtures in air at low (273 K > T > 200 K) and ultra-low temperatures (T < 200 K) must be developed and validated. In this work, the laminar burning velocity of these gases has been simulated and compared with the few available experimental data retrieved from the literature. Hence, simplified correlations for the burning velocity with respect to the initial composition and temperature have been adopted and further developed. The simplified approach proposed in this work reduces the number of degrees of freedom required for the application 1 ACS Paragon Plus Environment
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of the modified Gülder equation. Moreover, it is suitable for the description of the combined effect of initial temperature and gaseous composition. The performed analysis of the concentration and temperature profiles with respect to burner head distance indicates, as a possible explanation for the methane-dominated regime, the presence of a limitation in the hydrogen concentration hindering its production. A sensitivity analysis was performed to evaluate the effect of hydrogen addition and initial temperature on the methane kinetic mechanism in the presence of air. The results show that although the hydrogen production rate does not change, the reaction mechanism is strongly affected by the studied parameters.
Keywords: Cryogenic, Cryo-compressed gas, Extreme conditions, Low temperature, Laminar burning velocity, LNG, LH2
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1.
Energy & Fuels
Introduction
In recent years, research interest into the potential applications of methane-hydrogen mixtures as low carbon content fuels has considerably grown, as a short-term solution towards the development of hydrogen-based technologies that reduce pollutant emissions and widen the flammable region with respect to pure methane combustion. Cryogenic storage systems could be preferred for both fuels to maximise the energy density, making the direct combustion of cryogenic fuels in a hybrid engine convenient, as proved by several ongoing projects reported in a recent study (1) The safety parameters and mixture reactivity have been already studied for hydrogen-enriched methane at medium and high temperatures and pressures in terms of flame extinction, explosive limits and laminar burning velocity (2). Even though significant effort has been devoted in recent decades to increasing our knowledge of physical phenomena occurring at low and ultra-low temperatures and enhancing the quality of the model describing the release of boiling liquid in an atmosphere, additional studies are still required to guarantee an effective description of cryogenic vapour behaviour. In addition, the chemical behaviour of compounds at low temperature is still unclear, although additional data regarding these aspects could be beneficial for the development of new processes involving methane-hydrogen mixtures, as demonstrated by the growing interest in non-thermal plasma applications. More specifically, recent studies on hydrogen production by means of catalytic plasma-assisted partial oxidation of methane at low temperature were performed (3,4), indicating the capability of this technology to produce hydrogen at operating conditions that reduce fuel consumption, coke formation and other undesirable side reactions. On this basis, the laminar burning velocity (SU) represents an effective parameter to evaluate the reactivity, physical and chemical properties and safety parameters of different mixtures. 3 ACS Paragon Plus Environment
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For these reasons, this study has been devoted to the analysis of the additivity effect on SU at room temperature, the analysis of the effect of lower temperatures on ternary mixtures and the investigation of the reaction system variations for all the studied conditions. During the first step, several correlations developed to estimate the additivity effect on fuel behaviour in an oxidative environment were analysed. Among them, the modified Gülder correlation was selected, being suitable for the estimation of SU of the studied mixtures (5). However, the addition of an elevated content of hydrogen reduces the estimation quality of empirical rules, making the use of a detailed kinetic model necessary to increase the correspondence with experimental data. Both approaches were applied and the results compared. The power law was applied to evaluate the effect of temperature on the SU, which is often considered the most convenient correlation to estimate the dependence of the SU on the initial temperature above ambient conditions due to the elevated accuracy and simplicity (6). In the second section, the applicability of this correlation was tested in cryogenic conditions, since no information is available in the literature in this regard. An essential part of the analysis of chemical systems is often represented by the estimation of the profile of temperature and the main combustion products (7) and the sensitivity analysis (8). The combination of these tools provides further understanding on the chemical aspects involving the studied systems.
2.
Methodology
The numerical simulations to estimate the SU and the profiles of products in premixed gas combustion were performed by using a one-dimensional, adiabatic flame code and the detailed kinetic mechanism developed by the combination of the University of California, San Diego (9), and Lawrence Livermore National Laboratory (10) for
