The fuel reformation

Researchers from the Universities of Leeds and Bath, along with a team from ImperialCollege, are working on a method which should make it easier to produce hydrogen from methane.

Stripping the carbon away from methane to leave molecular hydrogen is a well-known process, known as reforming. If the methane is brought into contact with steam at a high temperature, three reactions can occur.

Methane can be converted into carbon monoxide and hydrogen, with the CO itself reacting with the steam to produce carbon dioxide and water; and the methane can be converted directly to carbon dioxide and hydrogen.

However, all three reactions are reversible. This means that in a system containing all five chemical compounds — methane, water, carbon dioxide, carbon monoxide and hydrogen — the constituents will continually break up and recombine, but the proportion of each will be more or less constant and will depend on the temperature of the system.

This is known as an equilibrium, and it’s bad news for chemical engineers, because in this system, the reactions that produce hydrogen absorb energy, so the proportion of hydrogen will be low unless the temperature is extremely high.

There are several ways of tipping the reforming equilibrium so that hydrogen production increases. The most fruitful is to remove one of the product gases from the system as it is produced, as the system tries to maintain the equilibrium by consuming more of the reactants. There has been much research focusing on removing hydrogen using selective membranes, but the team, led by Yulong Ding at Leeds, is taking a different tack — removing the carbon dioxide.

‘Hydrogen removal gives you a very pure hydrogen product, but it’s very slow,’ explained Bath researcher Alexei Lapkin. ‘Removing the CO2 shifts the whole equilibrium, because it also forces the CO to be converted into more CO2. It gives you a much higher throughput, and although the hydrogen isn’t 100 per cent pure, it’s much purer than a standard reformer.’

Ding’s Leeds team is working on a pneumatic flow reactor to carry out the conversion. A variant of a fluidised bed reactor, this has a bed of static catalyst granules through which the reactant gases flow, carrying with them a stream of 150µm-diameter particles of a material which preferentially absorbs CO2.

The standard reforming catalyst, provided by Johnson Matthey, promotes the hydrogen-forming reaction, and as the CO2 is produced, it is swept out of the reactor on the adsorbing particles. Once outside the reactor, the particles are separated from the flow of product gases and heated to remove the CO2; they are then recycled into the reactor for another round of adsorption.

Lapkin’s research is focusing on the adsorbents, looking at the durability of different materials and the number of cycles of adsorption and regeneration it can withstand.

‘We’re currently using materials like hydrotalcides,’ he said. Once the pilot plant has proved the concept, the teams will start looking at refining the catalyst to boost hydrogen production further.

One advantage of adsorption systems is that they are truly continuous — other processes use multiple tanks packed with adsorbent, which are ‘filled’ and regenerated in turn in a ‘semi-batch’ process.

Continuous processes are more efficient, and easier to control and scale up, making them a more attractive option for industrial application. ‘We’re scheduled to complete this stage of the research in November,’ said Lapkin. ‘After that, we can start looking for commercial partners to develop it further.’