(631d) Efficient Hydrogen Production with the Reformer Sponge Iron Cycle (RESC)

The reformer sponge iron cycle (RESC) is a process, based on a thermochemical cycle operated at high temperatures. It is designed to convert hydrocarbons from renewable resources to hydrogen with a quality that exceeds the requirements of fuel cells. The RESC process consists of a conventional catalytic steam reformer unit where natural gas is split into a gas mixture containing H2, CO, CO2 and H2O. In the subsequent sponge iron reaction (SIR) process the reducing components of the produced synthesis gas are converted into pure hydrogen. In a first step, CO and H2 are used to reduce iron oxide to metallic iron (Eq.1+2). During the following oxidation with steam pure hydrogen is released (Eq. 3).

Reduction step:

Fe3O4 + 4H2 → 3Fe + 4H2O (Eq. 1)

Fe3O4 + 4CO → 3Fe + 4CO2 (Eq. 2)

Oxidation step:

3Fe + 4H2O → Fe3O4 + 4H2 (Eq. 3)

The SIR process hence replaces the shift conversion as well as the final purification of the hydrogen rich gas. These steps conventionally consist of a combination of units for carbon dioxide removal, methanation, molecular sieves, purifiers, pressure swing adsorption (PSA) or liquid nitrogen scrubbing.

In order to accomplish a semi continuous work of the process more reactors have to be operated in parallel whereby the number of the reactors depends on the ratio of needed time for reduction to oxidation. Reformer unit and SIR operate at similar temperatures (approximately 800°C), which eases the system integration of both units. To achieve a higher efficiency 50% of the lean gas in the reduction step is recycled to the inlet of the reformer. Corresponding to the thermodynamic equilibrium in contact with the solid phase the lean gas contains CO2, H2O as well as non-reacted CO and H2. With the recycle no or only a slight amount of additional steam is necessary for the reforming process.Depending on the fuel source up to 80% efficiency can be achieved.

The limitations of the SIR process are given by thermodynamic equilibria. Choosing iron as contact mass for the SIR has several advantages: iron is highly available, it has a relatively high energy density, it is cheap, non toxic and it can be recycled in the steel industry after consumption.

However, iron is deactivated rapidly after few reduction-oxidation cycles due to sintering of the metal species. The sintering causes a dramatic decrease of active surface area and the densification of the solid which leads to diffusion limitation of the gas-solid reactions. The addition of proper foreign metal oxides to the iron oxide enhances the cycle stability considerably.

The research activities of the Institute of Chemical Engineering and Environmental Technology concentrate on the improvement of the process efficiency and optimization of the contact mass. For this purpose studies on contact mass deactivation and lifetime improvement were conducted and a prototype test rig with two SIR reactors was installed.