Hydrogen reactor offers pure product streams

Materials World magazine
1 Jul 2019

Pure streams of hydrogen and carbon dioxide can be made in one operation in a reactor, thanks to a material with variable oxygen concentrations.

Scientists have developed a thermodynamically reversible chemical reactor that can produce hydrogen as a pure product stream. The design promises to take up less space and be a more efficient way to produce the gas. The reactor uses a solid-state oxygen reservoir made with lanthanum strontium ferrite, and reacts carbon monoxide and water to make carbon dioxide and hydrogen. The material in the reservoir’s chemical structure has a flexible number of oxygen atoms that can be accommodated in the crystalline lattice. Due to this, the reservoir can attract or release oxygen atoms, depending on the chosen function of the device.

This ability also means carbon monoxide and hydrogen can be produced by using one unit in the reactor, not two, as the reservoir is never depleted or saturated with oxygen atoms. Instead, the desired gas transfers from one side of the reactor to the other, as a single stream. Through this, much more of the reactant gases are used and converted into the chosen end-products. The result is that hydrogen is produced as a pure product stream, removing the need for costly separation due to less efficient methods that produce partially contaminated gas.

Experts from Newcastle University, The University of Edinburgh, and Durham University, UK, and the European Synchrotron Radiation Facility, France, worked on the project, funded by the Engineering and Physical Sciences Research Council (EPSRC). The findings were presented in the paper, Overcoming chemical equilibrium limitations using a thermodynamically reversible chemical reactor, published in Nature Chemistry in May 2019, and was led by Newcastle University Professor of Chemical Engineering, Ian Metcalfe.

Simplifying the process

Metcalfe said in making hydrogen, a hydrocarbon needs to be pulled apart and made into a mix of carbon monoxide, carbon dioxide, hydrogen and water. Three steps take place, the first two of which
are a water-gas shift at a high temperature, then at a low temperature to help take some of the carbon monoxide and some of the water out of the gas mixtures, making more hydrogen and carbon dioxide. The third is separation.

‘Alternatively, oxygen reservoirs can be used in what is called a chemical looping process. However, these have not performed very efficiently until now. Once the materials used in these reservoirs are exhausted of oxygen, they change state. This leads to water in the hydrogen product and carbon monoxide in the carbon dioxide product,’ he said. ‘We have successfully combined those three steps – the high temperature shift, low temperature shift and separation – into one unit.’

Metcalfe said the key lies in the thermodynamically reversible reactor’s sponge-like material that takes oxygen from water and in turn, merges those three processes into one.

‘In the first phase, we pass water across the reservoir, which absorbs oxygen to produce hydrogen. In the second phase, we pass carbon monoxide across the reservoir, releasing oxygen from the reservoir to produce CO₂. This is called chemical looping because often, instead of having a material that sits fixed in a reactor, a material was looped between two reactors, so it actually moved and one reactor always had water coming in and one always had carbon monoxide coming in, and that’s where the name comes from. Here, we don’t move the material.’

Metcalfe said the material’s non-stoichiometric formula, La(Sr)FeO₃−δ, gave it the properties. ‘We don’t change the crystal lattice dramatically when we do that, and that means depending upon what atmosphere it is been exposed to, it will have different levels of δ. Different amounts of oxygen will be removed and that’s the way it transmits information between the water half cycle and the carbon monoxide half cycle.

‘You can have an infinite number of values of δ within a particular range, if you like, so it can transmit a lot of information from one
half cycle of the reactor operation to the other. That’s because of its non-stoichiometry.’

Metcalfe said the size of this machine would be smaller than a traditional reactor as only one unit was required to produce the products, not two. The team is working with multiple stakeholders keen to help develop the technology for industrial applications, including undisclosed firms from sectors such as gas distributors, chemical gas suppliers, planned construction, and oil and gas.