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Method for Evaluation of Thermochemical and Hybrid Water-Splitting Cycles

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Method for Evaluation of Thermochemical and Hybrid Water-Splitting Cycles Ind. Eng. Chem. Res. 2009, 48, 8985–8998 8985 Method for Evaluation of Thermochemical and Hybrid Water-Splitting Cycles Miguel Bagajewicz,* Thang Cao, Robbie Crosier, Scott Mullin, Jacob Tarver, and DuyQuang Nguyen Department of Chemical, Biological and Materials Engineering, The UniVersity of Oklahoma, Norman, Oklahoma This paper presents a methodology for the preliminary evaluation of thermochemical and hybrid water-splitting cycles based on efficiency. Because the method does not incorporate sufficient flowsheet details, the efficiencies are upper bounds of the real efficiencies. Nonetheless, they provide sufficient information to warrant comparison with existing well-studied cycles, supporting the decision to disregard them when the bound is too low or continuing their study. In addition, we add features not present in previous works: equilibrium conversions as well as excess reactants are considered. The degrees of freedom of each cycle (temperatures, pressures, and excess reactants) were also considered, and these values were varied to optimize the cycle efficiency. Ten cycles are used to illustrate the method. I. Introduction species entering the process is water, and the only products of the process are hydrogen and oxygen. Heat and work are also Declining volumes of fossil fuel reserves and an increase in transferred across the system boundaries for the heating and their demand has caused a recent rise in the cost of energy. cooling of process streams, the separation of reactive species, Similarly, nations across the globe are aspirating to become less and to drive the chemical reactions. dependent on foreign resources for the fulfillment of energy requirements. Furthermore, a steady increase in greenhouse gas Many studies have been performed in previous years to emissions over the past decades has brought about the reality evaluate water-splitting cycles as a means to produce hydrogen. 5 of global warming. The need for a more environmentally Funk provided a brief literature review of water-splitting cycles 6 friendly, renewable source of energy is evident and imminent. up to the year 2001. Holiastos and Manousiouthakis explore With the expected change to a hydrogen-based economy, the generation of reaction clusters, of which a thermochemical research efforts toward the development of hydrogen producing cycle is just one case. They prove a few interesting properties processes are on the rise. regarding the choice of temperature. In turn, Fishtik and Datta4 In the United States, roughly 11 million tons of hydrogen provide certain means to generate new cycles and several are currently produced annually, the vast majority of which published works focused on one or a limited number of cycles (∼95%) is produced using steam reforming of fossil fuel based only. For example, Goldstein et al.2 studied the efficiency of feedstock. Most of this hydrogen is used in industrial settings, the sulfur iodine (SI) cycle, Lemort et al.3 studied the UT-3 such as the production of fertilizers, hydrocracking, and cycle together with two other cycles. In turn, there are only a hydrotreating (among other processes) in crude oil refineries, few published works that attempted to evaluate a good number etc. However, in a hydrogen-based economy, the US hydrogen of cycles to identify promising cycles, the objective of this work. production and usage will increase significantly. The projected Brown et al.7 screened and evaluated 100 cycles on a semi- use of hydrogen for transportation needs is expected to be 200 quantitative basis. The purpose of that study was to determine million tons per year, while over 440 million tons per year would the potential for efficient, economical, large-scale production 1 be required to fulfill the needs for all nonelectric energy. of hydrogen. They focused on cycles that would use heat from Publication Date (Web): September 18, 2009 | doi: 10.1021/ie801218b Electrolysis uses electricity to drive the decomposition of a nuclear reactor heat source. In a separate study, Lewis8 water to produce hydrogen. However, these processes have evaluated water-splitting cycles based on the efficiency of each Downloaded by UNIV OF OKLAHOMA on October 27, 2009 | http://pubs.acs.org relatively low efficiencies (roughly 25% for low-temperature cycle. The efficiency was defined on the basis of the standard 1 reactors and 37% for high-temperature reactors. In turn, steam enthalpy of formation of water at 298 K. The terms associated reforming of methane involves combining methane with steam with the calculation of the efficiency were the energy of the - ° at high temperature (700 900 C) to produce carbon monoxide heat and work requirements assigned to each cycle. The work - and hydrogen, which followed by water gas shift reactors term included energy contributions of both electrical work and renders carbon dioxide and hydrogen. An obvious downside to the work associated with separation. Furthermore, an efficiency this method is that it is dependent on fossil fuel resources. Also, of 50% was assumed for all work requirements calculated as the process results in the production of large quantities of reversible separation work of each cycle. Although the work greenhouse CO gas. 2 makes progress in introducing quantitative evaluation means, Aside from direct electrolysis, methods that produce hydrogen the assumption of 50% efficiency is too general. To ameliorate from an input of only water are known as “water-splitting this shortcoming, the authors propose a detailed flowsheet cycles”, which use a series of chemical reactions. The net simulation, which allows a better assessment of the power and chemical reaction is the decomposition of water to form heating needs. Although this process leads to accurate efficien- hydrogen and oxygen. After each reaction, chemical intermedi- cies, it is rather time-consuming. Another issue not explored in ates are separated and recycled to other reactions within the sufficient detail is the degree of freedom stemming from system, making the process cyclic in nature. Overall, the only equilibrium conversions that can be manipulated using excess * To whom correspondence should be addressed. E-mail: bagajewicz@ reactants. Finally, an optimization of temperatures, pressures, ou.edu. and excess reactants, when appropriate, was not performed. 10.1021/ie801218b CCC: $40.75 2009 American Chemical Society Published on Web 09/18/2009 8986 Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009 In this work, we propose a method that provides an upper ∆H° (298 K) H2O bound on efficiency. The upper bound on efficiency is based η )- (10) Q + W on the following: (i) A simplified process flowsheet is generated for each cycle, where ∆H° (298 K) is the standard heat of formation of water upon which the required amount of heat and real work is H2O at 298 K (25 °C), which is 68.3 kcal/mol. On addition, Q and calculated. W are the total heat and work required to process 1 mole of (ii) Consideration of reaction equilibrium where it applies, water by the water-splitting cycle. The heat term Q includes which adds another dimension for optimization by varying the the heat utilities and the heat of separation; the work term W amount of excess reactants. includes the separation work and the electrical work in (iii) Process configuration (recycle structure) and operating electrolysis. Note that the real work is considered with separa- conditions that are optimized to achieve the maximum efficiency. tion efficiency η ) 0.5 and electrical work efficiency η ) 0.9. The usefulness of our methodology is that it allows a quick s e Lewis8 used the overall efficiency of 0.5 for the work term. assessment of upper bounds on efficiency and, therefore, it eneables some decision making, like disregarding a cycle when In principle, we assume that a correct and realistic account its upper bound is too low. is made for the amount of heat and work needed. We recognize the following issues: II. Water-Splitting Cycles • Thermal issues: Water needs to be heated from room temperature to the corresponding reaction temperature. Likewise, In this work, information on the cycles, including the the products of the cycle (oxygen and hydrogen) need to be reactions’ description and the reference/nominal reaction tem- cooled to room temperature. In addition, products from one peratures, was obtained from Brown et al.’s GA-A24326 report.7 reaction need to have their temperature conditioned for the next. There are at least two chemical reactions in a water-splitting Finally, reactions that are endothermic need heat and heat from cycle with a net result of water molecules decomposing to their exothermic reactions can be utilized. As Lewis8 proposed, this individual components of oxygen and hydrogen. Cycles are can be dealt with using pinch analysis.9 composed of either only thermochemical reactions or a com- • Reaction conversion: Except in electrolysis reactions where bination of reactions which includes thermochemical and complete conversion takes place, chemical reaction equilibrium electrolysis. The first type of cycle is called the thermochemical is considered to determine the reaction conversion (assuming cycle while the combination is known as a “hybrid” cycle. all reactions in cycles reach equilibrium). Excess reactants can Products that leave a reactor go through a separation process also be used to manipulate the composition at equilibrium of and a heating or cooling process at which point they become reaction mixtures. the inlets for other reactors. Only oxygen and hydrogen may • Excess reactants:
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