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'Methane cracking' offers economical route to carbon emissions reduction

20 November 2015

Producing energy from natural gas without generating carbon dioxide emissions could fast become a reality, thanks to new technology developed in Germany.

Solid black carbon is a by-product of methane cracking (photo: KIT)

In a joint project initiated by Nobel Laureate and former IASS Scientific Director, Professor Carlo Rubbia, researchers at the Institute for Advanced Sustainability Studies (IASS) in Potsdam and the Karlsruhe Institute of Technology (KIT) have been researching an innovative technique to extract hydrogen from methane in a clean and efficient way.

Instead of burning methane (CH4), its molecular components, hydrogen (H2) and carbon (C), can be separated in a process called ‘methane cracking’. This reaction occurs at high temperatures (750°C and above) and does not release any harmful emissions.

While hydrogen is the main output of methane cracking, its by-product, solid black carbon, is also an increasingly important industrial commodity. Moreover, black carbon can be stored away, using procedures that are much simpler, safer and cheaper than carbon dioxide storage.

Methane cracking itself is not an entirely new idea: in the last two decades, many experiments in different institutions have been carried out that have proven its technical feasibility. But these past attempts were limited by issues such as carbon clogging and low conversion rates.

The IASS and KIT have decided to build on this knowledge base and go one step further, setting up an experimental reactor that could demonstrate the potential of methane cracking and overcome previous obstacles. The starting point is a novel reactor design, as proposed by Carlo Rubbia and based on liquid metal technology.

Fine methane bubbles are injected at the bottom of a column filled with molten tin. The cracking reaction happens when these bubbles rise to the surface of the liquid metal. Carbon separates on the surface of the bubbles and is deposited as a powder at the top end of the reactor when they disintegrate.

This idea was put to the test during a series of experimental campaigns that ran from late 2012 to the spring of 2015 in KIT’s KALLA (KArlsruhe Liquid Metal LAboratory). Researchers were able to evaluate different parameters and options, such as temperature, construction materials and residence time. The final design is a 1.2-metre-high device made of a combination of quartz and stainless steel, which uses both pure tin and a packed bed structure consisting of pieces of quartz.

“In the most recent experiments in April 2015, our reactor operated without interruptions for two weeks, producing hydrogen with a 78 percent conversion rate at temperatures of 1,200°C," says Professor Thomas Wetzel, head of the KALLA laboratory at KIT.

The reactor is resistant to corrosion, and clogging is avoided because the microgranular carbon powder produced can be easily separated.

While these remain laboratory-scale experiments, researchers can extrapolate from them to gain insights into how methane cracking could be integrated into the energy system and, more specifically, what its contribution to sustainability could be. To this end, the IASS is collaborating with RWTH Aachen University to conduct a life cycle assessment (LCA) of a hypothetical commercial methane cracking device based on a scaling-up of the prototype.

Hydrogen production technology comparisons were made with steam methane reforming (SMR) and water electrolysis coupled with renewable electricity. With respect to emissions of carbon dioxide equivalent per unit of hydrogen, the LCA showed that methane cracking is comparable to water electrolysis and more than 50 percent cleaner than SMR.

IASS researchers have also analysed the economic aspects of methane cracking. At this stage, cost estimates are uncertain, since methane cracking is not yet a fully mature technology. However, preliminary calculations show that it could achieve costs of 1.9 to 3.3 euro per kilogram of hydrogen at German natural gas prices, and without taking the market value of carbon into consideration.

The next stage will involve the IASS and KIT focusing on optimising some aspects of the reactor design, such as the carbon removal process, and progressively scaling it up to accommodate higher flow rates.

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