Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Thermodynamic analysis of methane dry reforming: Effect of the catalyst particle size on carbon formation Nicolas Abdel Karim Aramouni a,b , Joseph Zeaiter a, ⁎ , Witold Kwapinski b , Mohammad N. Ahmad a a Department of Chemical and Petroleum Engineering, American University of Beirut, Lebanon b Department of Chemical Sciences, Faculty of Science and Engineering, Bernal Institute, University of Limerick, Ireland ARTICLE INFO Keywords: Dry reforming Methane CO 2 Syngas Ni catalyst ABSTRACT The effect of catalyst particle size on thermodynamic equilibrium of methane dry reforming and carbon for- mation has been studied through the Gibbs free energy minimization method taking into account the deviation of carbon formed from graphite Gibbs energy and its dependence on catalyst particle size. Methane and CO 2 conversions are maximized at low pressure and high temperature, and a molar H 2 /CO ratio of 1 is obtained at 1100–1200 K and 5–10 bar. Carbon formation was found to increase with particle diameter, and carbon presence was noticed at conditions of high pressure/low temperature and high temperature/low pressure. Optimal op- erating conditions were found to be close to carbon limits, highlighting the need for active metal particle size to be less than 5–6 nm to minimize coking. CO was identified as the precursor for carbon at low temperature, while CH 4 was found to be the main precursor at high temperature. 1. Introduction The ongoing depletion of fossil fuels is driving the world to search for alternative, sustainable and renewable sources of energy [1] to meet its ever increasing energy demand [2]. Furthermore, global warming and environmental consequences from the continuously growing economies, in addition to environmental regulations, require new en- ergy sources to produce less pollutants such as SO x , NO x and green- house gases. In parallel, the population growth has caused waste to be generated at a higher rate [3] and frequently mismanaged [4,5]. Landfill is a widely used method of municipal solid waste disposal, especially in developing countries such as Lebanon [5], and large amounts of greenhouse gases, especially carbon dioxide and methane, are produced in landfills. Hydrogen is a promising alternative fuel due to its high efficiency and clean combustion [6]. It is also attractive as a renewable energy source since it can be produced from biomass, water and solar energy [7]. Steam reforming has been the most common process to produce hydrogen from hydrocarbon feedstock, especially methane from natural gas, through further processing of the syngas product, essentially composed of hydrogen, carbon monoxide, CO 2 and water. Tradition- ally, ammonia synthesis has been the primary consumer of syngas [8]. However, more recent developments in the field of hydrocarbon re- forming allowed for a better control of syngas composition, making it a very desirable feedstock for gas to liquid applications. Recently, it has been of interest to use biogas from landfills or anaerobic digestors as a feedstock for syngas production through the dry reforming process, both due to the expected syngas H 2 /CO ratio being close to unity, which is suitable for gas-to-liquid applications and Fischer-Tropsch synthesis [9,10], and due to the possibility of recycling two greenhouse gases, methane and CO 2 , into useful products. The main reactions governing the process are as follows: + = + =+ CH CO 2CO 2H (ΔH 247 kJ/mol) (main reaction) 4 2 2 298K + = + =− − CO H O CO H (ΔH 41.5 kJ/mol) (water gas shift) 2 2 2 298K + = + = CH H O CO 3H (ΔH 206 kJ/mol) (steam reforming) 4 2 2 298K In addition to the carbon formation reactions = + =− 2CO CO C(ΔH 171 kJ/mol) (Boudouard reaction) 2 298K = + = CH C 2H (ΔH 75 kJ/mol) (Methane cracking) 4 2 298K For more comprehensive set of possible reaction see the work of Nikoo and Amin [11]. The industrial application of the dry reforming process, however, has been heavily hindered by the performance of catalysts [12–16]. Traditional, nickel-based catalysts used in reforming technologies are cheap and active, but are prone to deactivation by sintering and carbon deposition [17]. This problem is more severe in the case of dry re- forming, where the endothermic nature of the reaction requires high http://dx.doi.org/10.1016/j.enconman.2017.08.056 Received 27 March 2017; Received in revised form 25 July 2017; Accepted 19 August 2017 ⁎ Corresponding author. E-mail address: jz08@aub.edu.lb (J. Zeaiter). Energy Conversion and Management 150 (2017) 614–622 0196-8904/ © 2017 Elsevier Ltd. All rights reserved. MARK