Energy and exergy analyses of the fluidized bed of a copper–chlorine cycle for nuclear-based hydrogen production via thermochemical water decomposition
Introduction
Energy resources and their distribution and use significantly influence the well-being of human society and international relations. An important technical development in energy technology that may make possible the stable advancement of human society is the use of hydrogen produced from water using clean sources of primary energy, such as nuclear energy. Improved nuclear systems can provide the energy required for various activities, including hydrogen production and desalination of water. Adoption of hydrogen technology can decrease reliance on fossil fuels and the effects on the environment of burning them, and preserve fossil fuels for non-energy applications.
Hydrogen produced from water using renewable or nuclear resources and technologies is a renewable or environmentally benign fuel. Many studies of ways to supply ecologically clean energy to a growing human population suggest that a solution to this global problem involves the development and adoption of large-scale production of electricity, heat and hydrogen from nuclear power. These commodities can be used for the diverse needs of humanity.
Hydrogen is widely believed to be world's next-generation fuel, because of its lower environmental impact and greenhouse gas emissions, compared to fossil fuels. Hydrogen demand is expected to increase rapidly over the next decade. It has been suggested that hydrogen should replace petroleum products for fueling all forms of transportation (road, marine, air and rail) to reduce CO2 emissions, limit dependence on imported petroleum, and prepare for the time when oil reserves become overly expensive to recover or exhausted. Interest in hydrogen as a fuel is also being shown by the nuclear, chemical, automobile, aerospace and manufacturing industries. For instance, applications of hydrogen are envisioned as portable sources of power for such devices as mobiles phones, computers and other technologies. The use of hydrogen in all these industrial sectors is motivated by both the depletion of fossil fuel resources and the need to drastically reduce carbon emissions that affect the climate.
Several energy sources can be used for hydrogen production, including nuclear energy, renewables and fossil fuels. One option is to use electricity and/or heat from a nuclear power plant to break the chemical molecules of water, yielding hydrogen as a product. Using nuclear energy as the primary energy source for hydrogen production rather than other alternatives is advantageous for two main reasons. First, nuclear reactors do not emit greenhouse gases such as CO2. A large-scale hydrogen infrastructure can provide its environmental benefits only if hydrogen is produced by methods that are non-greenhouse gases emitting, such as nuclear or renewable energy processes. The second advantage is that nuclear energy can contribute to large-scale hydrogen production. Given the fast growing demand for energy in all sectors in the world, large-scale and clean hydrogen production will likely be essential. Nuclear energy is important because renewable energy resources do not appear to be able to meet global hydrogen needs. The limited contributions of renewables to total energy supply are due to some of their challenging characteristics, including low-density, intermittent and high cost.
One thermochemical cycle for hydrogen production that has been proposed for integration with nuclear reactors is the copper–chlorine (Cu–Cl) cycle. The cycle consists of a series of steps, involving different chemical reactions, which result in water being split into hydrogen and oxygen. The objectives of this study are to define the first step (the fluidized bed) of Cu–Cl cycle for hydrogen production as well as its operational and environmental conditions, to perform a comprehensive thermodynamic analysis of the Cu–Cl fluidized bed, including relevant chemical reactions, to evaluate its performance through energy and exergy efficiencies, and to perform various parametric studies on energetic and exergetic aspects with variable reaction and reference-environment temperatures.
Section snippets
Background
Many combinations of chemical reactions have been studied, where heat and electricity split water into hydrogen and oxygen in a closed cycle. A fission reactor as a primary energy source for hydrogen production was assessed by Torjman and Shaaban (1998). A complete nuclear-electro-hydrogen energy system was proposed for a medium-size city (population of 500,000), and the entire energy requirement was assessed including residential, industrial and transportation needs. A preliminary economic and
Approach and methodology
Exergy is used as a common currency to assess and compare reservoirs of theoretically extractable work, which we often call energy resources (Bonnet et al., 2005, Camdali et al., 2004, Cao and Zheng, 2006, Ertesvag, 2007, Gao et al., 2004, Gharagheizi and Mehrpooya, 2007, Kwak et al., 2003, Oladiran and Meyer, 2007, Ostrovski and Zhang, 2005, Prins and Ptasinski, 2005, Rivero and Garfias, 2006, Rosen, 1995, Rosen and Dincer, 2004, Utlu et al., 2006). Resources consist of matter or energy with
System studied
A conceptual layout of a Cu–Cl pilot plant is illustrated in Fig. 1. In this figure, the circled numbers are labels referring to the associated stream. Thermochemical water decomposition, potentially driven by nuclear heat with a copper–chlorine cycle, splits water into hydrogen and oxygen through intermediate copper and chlorine compounds. This cycle consists of three main thermally driven reactions and one electrochemical reaction. The cycle involves five steps (Lewis et al., 2003): (1)
Analysis
During the analysis, we consider 1 kmol of hydrogen produced per cycle, so all quantities are provided in terms of per kmol of hydrogen produced. Also, we assume that:
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The reference environment temperature (T0) and pressure (P0) are 25 °C and 1 atm, respectively.
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In the chemical reaction steps, reactants and products are at the reaction temperature and a pressure of 1 atm.
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The process occurs at steady state.
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The process is adiabatic (only reaction heat enters with reactants).
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The process proceeds to
Results and discussion
The first results presented are property data, which are needed in the subsequent analysis. The Gibbs free energy and the standard chemical exergy of selected compounds, which are calculated with Eqs. (15), (16), respectively, are given in Table 5. These values are based on a reference temperature and pressure of 25 °C and 1 atm, respectively. As illustrated in Table 5, the Gibbs free energy of elements that are stable at this temperature and pressure is zero.
The variation of the reaction heat
Conclusions
A comprehensive thermodynamic analysis incorporating energy and exergy analyses of the fluidized bed step of a Cu–Cl thermochemical cycle for hydrogen production, including the relevant chemical reactions, has been performed and the step and its operational and environmental conditions defined. The energy and exergy analyses of the fluidized bed step allow several conclusions to be drawn. This information should assist efforts to understand the thermodynamic losses and efficiencies of the
Acknowledgement
The authors acknowledge the support provided by the Ontario Research Excellence Fund.
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