Background and Challenges

Hydrogen produced from water splitting and clean energy sources is predicted by many to be a clean fuel that will serve as a substitute for conventional fuels because its oxidation does not emit greenhouse gases. Numerous thermochemical water splitting cycles have been proposed for clean hydrogen production. The copper-chlorine (Cu-Cl) cycle has a relatively low temperature requirement compared with other cycles and therefore is viewed as a promising method.

The cycle consists of three chemical reactions, as shown in Table 1:

Table 1.

The Cu-Cl cycle has been demonstrated in proof-of-principle tests, and a key issue is whether the cycle can be scaled up to a larger engineering and commercial scale. The challenges for scale-up must be overcome, and solutions must be obtained. This article will examine the scale-up feasibility, particularly by enlarging by 1,000 times the cycle from proof-of-principle tests (equivalently 3 g H₂ per day) to a large engineering scale (equivalently 3,000 g H₂ per day). Since the Cu-Cl cycle consists of one electrolytic and two thermal reactions, this article will focus on the scale-up of thermal reactions at the Clean Energy Research Laboratory (CERL) at the University of Ontario Institute of Technology (UOIT).

Scale-Up Methodology

For the two endothermic reactions—hydrolysis and oxygen production—the scale-up can be achieved by adopting the following strategy:

1. Construct low-temperature units to study the necessary safety improvements and flow characteristics. In these units, no chemical reactions occur but the processing rate is equivalent to 10,000 times the hydrogen production scale of proof-of-principle tests. The fixed bed equipment and experimental loop are illustrated in Figure 1.

Figure 1:  Fixed bed low-temperature reactor (10,000 times the processing scale of proof-of-principle tests)

2. Design and build reactors to study the actual chemical reactions at high temperatures. Both the hydrolysis and the hydrogen production reactors were designed and operated at 1,000 times larger scales than proof-of-principle tests. The equipment and the experimental loop for the oxygen production step are shown in Figure 2.

Figure 2:  High-temperature oxygen production reactor (1,000 times the processing scale of the proof-of-principle tests)

3. The scale-up was conducted with the goal of system integration of unit operations in terms of chemical stream composition and quantification, reaction thermodynamics and kinetics, and improvements in the energy efficiency of the whole cycle.

Results and Discussion

Figure 3 shows the friction factor results obtained from the experimental fixed bed unit at various flow conditions at 10,000 times the processing scale of the proof-of-principle tests. The results provide valuable scale-up data to improve packed bed designs and safety by limiting the adverse impact of undesirable pressure drops during operation at a larger engineering scale.

Figure 3: Measured and predicted friction factor for the packed bed at various operating conditions (1,000 times the scale of the proof-of-principle tests)

Table 2 shows the experimental results of the reverse reaction of oxygen production at various operating conditions. At an engineering hydrogen production scale, the reverse and undesirable side reactions of the oxygen production step can be minimized or avoided.

Table 2.

Figure 4 illustrates the estimated heat transfer to a hydrolysis reactor for 100 metric tons of hydrogen production (a large commercial-scale plant). When the ratio of water to cupric chloride exceeds 1.5—i.e., 3 times the stoichiometric requirement—the evaporation heat requirement of water will exceed the reaction enthalpy.  As a consequence, the hydrolysis reactor will function like a steam generator. Further research is needed to determine whether the excess steam requirement of hydrolysis is caused by thermodynamic or engineering limits. However, it is desirable for the ratio of water to cupric chloride to be below the transitional point of 1.5 so that the heat transfer rate to the hydrolysis reactor is directed primarily to the reaction enthalpy. Another option is to utilize a separate steam generator. This can control the ratio of water to cupric chloride more readily to be closer to the stoichiometric value and provide more flexibility for the selection of reactor types because other intake forms of CuCl₂, such as solid powder and slurry, can be introduced into the hydrolysis reactor. In addition, external water such as make-up water of the Cu-Cl cycle rather than water from the electrolytic cell also can be utilized. It is crucial to reduce the water requirement of the hydrolysis input stream to improve the overall thermal efficiency of the Cu-Cl cycle.

Conclusions

This article shows that the Cu-Cl cycle can be scaled up to a large engineering scale, and it has been scaled up 1,000 times from proof-of-principle tests. Experimental results were obtained from both the low-temperature and high-temperature units for the flow characteristics, safety improvements, reaction thermodynamics, kinetics, and chemical stream quantification. Future research is recommended to achieve system integration and improvement of the system’s thermal energy efficiency. Also, the economics of the Cu-Cl cycle at large industrial scales should be examined further.

Figure 4: Comparative roles of reaction enthalpy and water evaporation in the hydrolysis reactor.

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