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Cycles for the thermochemical decomposition of water belonging to the FeCl family have been considered potentially attractive. An analysis of the thermal efficiency, investments costs and development potential of this family is presented, where electrolysis of water serves as a reference process. It is concluded that the thermal efficiency of these cycles is probably considerably below the values obtained with direct electrolysis whereas investment costs are presumably higher by a factor of more than two. Main problem areas are situated in the hydrolysis of FeCl2 as well as in the thermal decomposition of FeCl3. Both problems have to be solved simultaneously; finding a solution to only one will not be sufficient to build an attractive process. A number of possible alternative solutions to these problems are discussed.

The reaction of SO 2 , Br 2 and H 2 O, forming HBr and H 2 SO 4 , is one of the reactions of the hybrid cycle for thermochemical decomposition of water, Mark-13. Experimental work on this reaction is described in this paper. Equilibrium measurements show that high sulfuric acid concentrations (higher than 80 wt%) are attainable. At low sulfuric acid concentrations only traces of Br 2 and/or SO 2 remain in the gaseous phase. Besides the equilibrium determinations, development work by dynamic experiments has also been carried out. A simple mathematical model for the reaction rate in packed columns has been developed which satisfactorily fits the available experimental data.

The experimental work in the field of the development of cycles of the Fe/Cl family is described. It appeared that application of a homogeneous support material for the gas-solids reaction was not feasible due to unexpected side reactions. The hydrolysis of ferrous chloride has been found to give good results and the development and operation of a continuously operating bench-scale reactor for this reaction has been described. The chlorination of the resulting magnetite could not be performed according to the originally proposed scheme. A satisfactory alternative has been found, i.e. chlorination at 150–200C with only hydrogen chloride and introduction of the reverse Deacon reaction as a fourth reaction in the cycle.

The Mark-13 cycle, invented by Schutz and Fiebelmann in 1974, consists of the following three reactions: SO 2 + Br 2 +2 H 2 O → H 2 SO 4 +2 HBr , (320–370 K) 2 HBr → H 2 + Br 2 , (electrochemical) H 2 SO 4 → H 2 O + SO 2 +0.50 2 . (1000–1100 K) In the present paper the state of the art of the experimental development work carried out at JRC-Ispra is given. The results obtained have been considered a sound basis for the design and construction of a complete laboratory-scale cycle. The expected net hydrogen production rate of this plant is 4.0 gmol/h, i.e. about 100 l/h S.T.P. The aim of the work is the construction of a reliable small scale model of the process which should yield indispensable information for further scale-up. Therefore we applied design and/or simulation rules in the design calculations for the process. It is believed that this installation is the first demonstration of a complete thermochemical hydrogen production process.

The bench-scale plant for thermochemical hydrogen production by the Mark-13 process has been successfully operated since May 1978. The nominal hydrogen production rate of the plant working at atmospheric pressure is 100 STP 1. h −1 . A concise description of the process flow scheme is given. Operation of the plant demonstrated that thermochemical water decomposition is a viable process and a valid alternative to other processes for hydrogen production from water. The availability of the plant for experimental work was very high and no special start-up procedure was required. All reactions and product separation units were working within their specifications and no by-product formation was observed.