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AN OVERVIEW OF R&D ACTIVITIES FOR THE Cu-Cl CYCLE WITH EMPHASIS ON THE HYDROLYSIS REACTION
Introduction
The Cu-Cl thermochemical cycle has been under development for several years. The goal is to achieve
a commercially viable method for producing hydrogen at a moderate temperature (∼550°C). This
chemical process, if successfully developed, could be coupled with several types of heat sources,
e.g. the supercritical water reactor, the Na-cooled fast reactor or a solar heat source such as the solar
power tower with molten salt heat storage. The use of lower temperature processes is expected to
place less demand on materials of construction compared to higher (∼850°C) temperature processes.
Besides the lower temperature requirement, other positive features of the cycle are the following:
• Proof-of-concept experiments have shown chemical viability and no showstoppers.
• No catalysts are required for the thermal reactions or for the anode reaction in the electrolysis
step.
• The preliminary conceptual process design and the corresponding flow sheet have shown an
efficiency of 39-41% (LHV). These values depend on assumptions regarding the operability of
the electrolyser and the crystalliser, which separates components in the spent anolyte and
catholyte.
The three reactions in the cycle are given in Table 1. This representation of the cycle is simplistic
as 100% yields and stoichiometric quantities of reagents are assumed for all reactions. The challenge
in developing any thermochemical cycle is to determine how to obtain these yields, while reducing
the amount of any excess reagent used to drive the reaction as far to the right as possible. This paper
will focus on laboratory and reactor design work for the hydrolysis reaction and will also describe the
conceptual process design, with special emphasis on the hydrolysis reaction.
Table 1: Reactions in the Cu-Cl thermochemical cycle
Reaction Temperature, °C
Hydrolysis 2CuCl 2(s) + H 2O(g) → Cu 2OCl 2 (s) + 2HCl(g) 340-400
Decomposition Cu 2OCl 2 (s) → ½ O 2 (g) + 2CuCl(s) 450-530
Electrolysis 2CuCl(s) + 2HCl(g) → 2CuCl 2 + H 2 (g) 100
The hydrolysis reaction, CuCl 2 + H 2 O(g) ⇔ Cu 2 OCl 2 + 2HCl(g), requires excess steam to obtain high
yields. Theoretically, the steam to CuCl 2 molar ratio (S/Cu) required for ∼100% yield is temperature and
pressure dependent. At 375°C, the S/Cu is about 17 at atmospheric pressure. Most of our early fixed
bed experiments confirmed that the S/Cu of 15-20 was needed to obtain 60-80% conversion to Cu 2 OCl 2 .
However, the solid products contained up to 25 wt.% CuCl and, in some cases, large amounts of
unreacted CuCl 2 , found agglomerated in the middle of the fixed bed. It was concluded that fixed bed
reactor designs provided poor heat and mass transfer. A spray reactor, which offers better mass and
heat transfer, was therefore designed, built and tested. The conceptual process design has incorporated
a spray reactor design and also includes engineering solutions that mitigate the costs associated with
the need for a large excess of steam. These are described below.
Experimental
A schematic of the apparatus is shown in Figure 1. The dimensions of the glass reactor are 12.1 cm
(4.75 in) OD, 3 mm wall thickness and 137.2 cm (54 in) long. Two types of injectors were tested: i) a
“pneumatic” quartz nebuliser, typically used for Inductively Coupled Plasma-Mass Spectrometry (ICP-MS)
analyses (Glass Expansion); ii) an ultrasonic nozzle (Sono-Tek). The pneumatic one requires a sweep
gas to create a fine mist of CuCl 2 solution while the ultrasonic nozzle uses mechanical vibrations to
atomise the solution. The heated zone of the furnace is 91.4 cm (36 in) long. Superheated steam was
used to assist heat transfer and was injected either co-currently or counter-currently with the CuCl 2
solution. The carrier gas was 99.999% Ar and its flow rate was controlled by mass flow controllers. The
liquid flows were controlled by syringe pumps. The CuCl 2 solution was made by dissolving 5 g of
99.99% purity CuCl 2 •2H 2 O from Aldrich into 10 g of deionised water.
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