Following two years of intensive experimentation, a team of researchers in Germany has developed an experimental reactor designed to cleanly and efficiently extract hydrogen from methane.
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The team is made up of researchers from the Institute for Advanced Sustainability Studies (IASS), Potsdam, Germany; and the Karlsruhe Institute for Technology (KIT), Karlsruhe, Germany.
Their work focuses on a methane cracking process that converts methane into carbon and hydrogen.
With its high energy density per unit mass and clean combustion, hydrogen is viewed by many as an important component of future energy systems — as well as being an important industrial commodity already in processes such as ammonia production. The researchers point out that steam methane reforming (SMR), one of the main manufacturing routes of industrial hydrogen, uses natural gas as feedstock but releases significant amounts of carbon dioxide in the process.
Meanwhile, the other product of methane cracking, solid black carbon, is an increasingly important commodity in the manufacture of steel, carbon fibers and many carbon-based structural materials. The researchers say storage of their carbon byproduct presents a simpler, safer and cheaper method of storing carbon dioxide gas, too.
Methane cracking itself is not entirely new, but recent attempts have been dogged by problems including carbon clogging and low conversion rates. So the starting point for IASS and KIT was a novel reactor design based on liquid metal technology, as proposed by former IASS scientific director and Nobel Laureate Carlo Rubbia.
Here, 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.
The first test run was completed in November 2013; further tests were carried out in 2014 to confirm qualitative results for hydrogen conversion rates. In the second half of 2014, the team made significant breakthroughs when experiments proved that high-quality carbon (i.e., suitable for industrial use) could be efficiently produced at temperatures above 800°C. On that basis, the researchers identified a final setup that relies on tin as the liquid metal of choice and on relatively inexpensive and easy-to-handle materials for the reactor.
The final design is a 1.2-m-high device made of a combination of quartz and stainless steel, using both pure tin and a packed bed structure consisting of pieces of quartz.
“In the most recent experiments our reactor operated without interruption for two weeks, producing hydrogen with a 78% conversion rate at temperatures of 1,200°C. In particular, the continuous operation is a decisive component of the kind of reliability that would be needed for an industrial-scale reactor,” says professor Thomas Wetzel, head of the Karlsruhe liquid metal laboratory (KALLA) at KIT.
The reactor proved resistant to corrosion. It also avoids clogging; the microgranular carbon powder produced can be separated easily. The reactor thus guarantees the technical preconditions needed for continuous operation of an industrial-scale reactor, notes Wetzel.
To fully understand the sustainability of the new process, IASS is working with RWTH Aachen University, Aachen, Germany, to carry out a lifecycle assessment (LCA) of a hypothetical commercial methane cracking device based on scale-up of the existing prototype.
Assuming that some of the produced hydrogen from the methane cracker is used to generate process heat, the LCA found that the process is comparable to the water electrolysis route to hydrogen manufacture and is more than 50% cleaner than SMR — based on emissions of carbon dioxide/unit of hydrogen.
IASS researchers also analyzed the economics of the process. While IASS describes current estimates as uncertain, preliminary calculations show it could achieve costs of €1.90–3.30 ($2.00–3.50) per kilogram of hydrogen — without taking the market value of carbon into consideration.
“This could be a gap-bridging technology, making it possible to tap into the energy potential of natural gas while safeguarding the climate and facilitating the integration of a clean energy carrier like hydrogen,” believes Rubbia.
For now, the researchers are focused on optimizing reactor design by, for example, improving the carbon removal process. They also are progressively scaling-up the process to handle higher flow rates.
