Abstract

Over 200 thermochemical cycles have been identified to produce hydrogen by thermochemical water decomposition [1]. The Cu-Cl cycle was first proposed by the Institute of Gas Technology (IGT) [2]. Due to the lower temperature heat (550șC) required, the hybrid Cu-Cl cycle is one of the most promising thermochemical cycles for hydrogen production using nuclear or solar heat.  The cycle can be coupled with several types of nuclear reactors, e.g., the supercritical water reactor being developed in Canada, CANDU Mark 2, the sodium cooled reactor, or the high temperature gas reactor with the high temperature heat being used for electricity generation.  Several types of solar devices can also be used, e.g., a power tower with molten salt as a heat storage medium or a concentrator.  The lower temperature should mitigate some of the demands on the materials of construction. 

The copper-chlorine cycle consists of the three major reactions shown in Table 1. The electrolysis reaction (1) in which cupric chloride (CuCl₂) is produced at the anode and H₂ at the cathode is carried out electrochemically. The CuCl₂(a) from (1) is hydrolyzed to copper oxychloride (Cu₂OCl₂) according to the hydrolysis reaction (2). Molten cuprous chloride (CuCl) is then produced from the decomposition reaction (3).

Table 1. Reactions in the Copper-Chlorine Cycle

(1) Electrolysis/Hydrogen formation: CuCl(a) + HCl(a) + 2H₂O → CuCl₂·2H₂O(a) + œ H₂(g)  100șC, 24 bar

(2) Hydrolysis/HCl formation: 2 CuCl₂(s) + H₂O(g) → Cu₂OCl₂(s) + 2HCl(g)                            400șC, 1 bar

All reactions have been experimentally demonstrated in proof-of-concept work. The electrolytic reaction (1) was demonstrated at the Atomic Energy of Canada, Ltd. (AECL) recently. Meeting performance (500 mA/cm² at 0.5V) and cost target ($2500/m²) is the primary challenge for the electrolysis reaction. The two thermal reactions, the hydrolysis of CuCl₂ (2), and the decomposition of Cu₂OCl₂ (3) are being investigated in more detail to facilitate an engineering lab-scale demonstration.

We consider the hydrolysis reaction to be the most challenging reaction because of two factors: (i) a competing reaction of CuCl₂ and (ii) the need for excess water.

The competing reaction is the thermal decomposition of CuCl₂:

2CuCl₂ (s) → 2CuCl (s) + Cl₂ (g)                                                                     (4)

 

Because CuCl is a product of the subsequent reaction, this competing reaction is not a showstopper, provided the chlorine can be scavenged and the amount of chlorine formed is minimal. We believe that this competing reaction can be minimized by the choice of operating conditions and the reactor design.

Early work has shown that fixed bed reactor designs are not optimal. Inefficiencies in heat and mass transfer inhibited the desired reaction. Unreacted CuCl₂ was observed and this material tended to be agglomerated in the middle of the fixed bed. Under these conditions, the decomposition of CuCl₂ to form CuCl (4) was enhanced.

To improve mass and heat transfer and inhibit the decomposition reaction, a spray reactor was designed.  In this concept, a solution of CuCl₂ is injected into the reactor into a hot zone at 375-400șC using a nebulizer. This new reactor design should provide better mass transfer since CuCl₂ and water are injected together. The nebulizer produces a very fine mist of CuCl₂ droplets when an inert gas with a flow rate of 100-300 ml/min is used. The engineering lab-scale reactor has an ID of 4.75 in and is held in a 36-in long heated zone. The solid products of the reaction are collected at the bottom of the reactor which is maintained at a lower temperature (around 100-200șC) to prevent further reaction. Superheated steam is added to improve heat transfer either at the top (co-current flow) or at the bottom of the reactor (counter-current flow). The preliminary test results are encouraging. Very fine black particles were found at the bottom of the reactor. X-ray diffraction was used to identify the phases.  In most patterns, Cu₂OCl₂ was the predominant phase.  However, some unreacted CuCl₂ and possibly CuCl were also identified.  The effect of various operating parameters, e.g., the inert gas flowrate in the nebulizer and in the superheated steam line, the CuCl₂ solution flowrate, and the current flow designs are being investigated with respect to Cu₂OCl₂ and CuCl formation.

1.      McQuillan, B.W., Brown, L.C., Besenbruch, G.E., Tolman, R. Cramer, T., Russ, B.E, Vermillion, B.A., Earl, B., Hsieh, H.-T., Chen, Y., Kwan, K., Diver, R. Siegal, N., Weimer, A., Perkins, C., Lewandowski, A., High Effiiency Generation of Hydrogen Fuels using Solar Thermochemical Splitting of Water, Annual Report, GA-A24972, General Atomics, San Diego, CA (2005).

2.      Carty, R.H., Mazumder, M.M., Schreiber, J.D., and Pangborn, J.B., Thermochemical Hydrogen Production, Report GRI-80/0023.4, Institute of Gas Technology, Chicago, IL (1981).