Process Safety and Environmental Protection 1 0 2 ( 2 0 1 6 ) 513–522 Contents lists available at ScienceDirect Process Safety and Environmental Protection journal h om ep age: www.elsevier.com/locate/ps ep Influence of inert gas addition on propagation indices of methane–air deflagrations Maria Mitu a , Maria Prodan b,c , Venera Giurcan a , Domnina Razus a,* , Dumitru Oancea b a Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, 060021 Bucharest, Romania b University of Bucharest, Department of Physical Chemistry, Bd. Regina Elisabeta 4-12, 030018 Bucharest, Romania c INSEMEX (National Institute for Research and Development in the Mine Safety and Protection to Explosion), Str. Gral. Vasile Milea 33-34, Petrosani, Romania a r t i c l e i n f o Article history: Received 3 December 2015 Received in revised form 28 April 2016 Accepted 5 May 2016 Available online 12 May 2016 Keywords: Methane Inerting Spherical vessel Propagation Explosion Safety. a b s t r a c t The work examines the characteristic indices of laminar deflagrations propagating in methane–air gaseous mixtures diluted by several inert gases: He, Ar, N 2 or CO 2 . Experi- ments were performed in two spherical vessels of different volumes with central ignition, at ambient initial conditions. Mixtures with variable methane concentrations (6–12 vol%) and variable inert concentrations (5–40 vol%) were studied, in order to outline the inert influence on the most important and accessible safety-related parameters: the peak explosion pres- sure, the maximum rate of pressure rise (or the related property, i.e. the deflagration index) and the explosion time (the time necessary to reach the peak explosion pressure). Among the studied inert additives, CO 2 is the most efficient, followed by N 2 , Ar and He. Inert gas addition to any flammable CH 4 –air mixture determined the decrease of both experimental and adiabatic explosion pressure and of the maximum rate of pressure rise, along with the increase of the explosion time. Using an equation that describes the heat balance of the isochoric combustion of a fuel–air mixture under non-adiabatic conditions, a correlation between the peak explosion pressure and the mole fraction of inert gas was derived and validated for CH 4 –air– inert mixtures. © 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. 1. Introduction The flammability properties of methane–air mixtures were frequently measured and summarized in publications, as methane is the main constituent of natural gas, mine gas or biogas. Its recent uses as an alternative fuel for engines, in the form of LNG (Liquefied Natural Gas) or CNG (Compressed Natural Gas), explains the revival of interest for its explosive combustion. Reduction of the hazard associated to explosions of methane–air mixtures is made by dilution with an inert gas, most frequently nitrogen, carbon dioxide, water (vapor) ∗ Corresponding author. E-mail addresses: maria mitu@icf.ro (M. Mitu), mariaprodan@insemex.ro (M. Prodan), venerab@icf.ro (V. Giurcan), drazus@icf.ro, drazus@yahoo.com (D. Razus), doan@gw-chimie.math.unibuc.ro (D. Oancea). or even exhaust gas (i.e. a mixture of carbon dioxide, water vapor and nitrogen). Numerous studies were undertaken on explosion propagation in methane–air diluted by inert gases, driven by the need to measure MIC, the minimum inert con- centration necessary for suppressing the explosion. However, in many cases, the specific conditions for operating a reactor or a plant require the addition of an inert gas with a con- centration lower than MIC. In this case, the explosion still occurs but its characteristic parameters in confined conditions (the peak explosion pressure, the maximum rate of pressure rise, the time to peak explosion pressure, the normal burning http://dx.doi.org/10.1016/j.psep.2016.05.007 0957-5820/© 2016 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.