Thermodynamic Analysis of Hydrogen Production by a Thermochemical Cycle Based on Magnesium-Chlorine Ahmed Bensenouci * , Mohamed Teggar, Ahmed Medjelled, Ahmed Benchatti Laboratory of Mechanics, Amar Télidji University of Laghouat, B.P. 37G Laghouat 03000, Algeria Corresponding Author Email: a.bensenouci@lagh-univ.dz https://doi.org/10.18280/ijht.390222 ABSTRACT Received: 12 July 2019 Accepted: 26 December 2020 Most thermochemical cycles require complex thermal processes at very high temperatures, which restrict the production and the use of hydrogen on a large scale. Recently, thermochemical cycles producing hydrogen at relatively low temperatures have been developed in order to be competitive with other kinds of energies, especially those of fossil origin. The low temperatures required by those cycles allow them to work with heats recovered by thermal, nuclear and solar power plants. In this work, a new thermochemical cycle is proposed. This cycle uses the chemical elements Magnesium-Chlorine (Mg-Cl) to dissociate the water molecule. The configuration consists of three chemical reactions or three physical steps and uses mainly thermal energy to achieve its objectives. The highest temperature of the process is that of the production of hydrochloric acid, HCl, estimated between 350-450℃. A thermodynamic analysis was performed according to the first and second laws by using Engineering Equation Solver (EES) software and the efficiency of the proposed cycle was found to be 12.7%. In order to improve the efficiency of this cycle and make it more competitive, an electro-thermochemical version should be studied. Keywords: exergy analysis, hydrogen production, magnesium-chlorine cycle, thermochemical cycle, water splitting 1. INTRODUCTION The rapid rising world energy demand and climate change due to greenhouse gas emissions, have motivated the public and private sectors, to seek an alternative to conventional energies and limited ecological damage. The solution to those problems is the use of renewable energies. Currently, the world consumes about 85 million barrels of oil and 2.94 billion cubic meters of natural gas a day [1], releasing greenhouse effect gases that cause global warming. Unlike fossil fuels, hydrogen is a clean and sustainable energy carrier, which is widely considered the fuel of the next generation. The production of hydrogen from the dissociation of water molecule constitutes an interesting process of energy production which has the potential to be sustainable development. Hydrogen demand is expected to increase significantly over the next decades. The hydrogen market worldwide is currently growing by 10% per year, reaching between 20 and 40% by the year 2020 [2]. The prevailing process for hydrogen production at large scale is the steam- reforming of methane. This technology emits huge amounts of carbon dioxide as the primary greenhouse effect gas. On the other hand, the production of hydrogen based on thermochemical cycles does not emit harmful gases. The thermal energy required for the operation of these systems can be supplied in abundance for large-scale capacities either by heat recovered from thermal and nuclear power plants or directly by solar power plant [3, 4]. The thermochemical cycles of hydrogen production have received more attention because current estimates indicate that the production costs of H2 by these methods could be as low as 60% of those from room temperature electrolysis [5, 6]. Thermochemical cycles are more profitable than electrolysis on the hydrogen generation as they avoid the intermediate process of generating electricity before hydrogen is produced. Since the beginning of the development of thermochemical cycles four decades ago, a very long list of 280 thermochemical cycles [7] of water decomposition has been discovered, but few of them have undergone exhaustive studies and have promising results. Hydrogen can be produced using fossil fuels, water and biomass [8]. Water is one of the most promising resources for hydrogen production because of its abundance and equitable accessibility on the globe as well as its price which is almost symbolic compared to other sources of energy. Water can be broken down into hydrogen and oxygen by several technologies. High and low temperature electrolysis, photochemical and radiochemical systems, pure and hybrid thermochemical separation cycles are prospective technologies for hydrogen production. Several types of thermochemical processes exist for the production of hydrogen. The sulfur-iodine-based cycle (S-I) was proposed by General Atomics of the United States of America in the mid-1970s. These are the three chemical reactions that produce the dissociation of water molecule, the highest temperature of the cycle is estimated at 850℃. This cycle has a thermal efficiency of approximately 47% and potentially reaches 60% efficiency with cogeneration of hydrogen and electricity [9, 10]. The UT-3 cycle is a thermochemical cycle of hydrogen production initially developed at the University of Tokyo. This cycle is based essentially on the chemical elements which are bromine, calcium and iron (Br-Ca-Fe). The cycle consists of four steps that include only solid and gaseous components, the maximum temperature is about 750℃. The predicted thermal efficiency of the UT-3 cycle varies between 35-50%, depending on the efficiency of the membrane separators, and whether the International Journal of Heat and Technology Vol. 39, No. 2, April, 2021, pp. 521-530 Journal homepage: http://iieta.org/journals/ijht 521