Methane Cracking Generates Energy Without CO2 Emissions
Engineering360 News Desk | November 26, 2015Researchers at the Institute for Advanced Sustainability Studies (IASS) and the Karlsruhe Institute of Technology (KIT) have demonstrated the feasibility of methane cracking to produce energy from natural gas without generating carbon dioxide emissions.
Rather than burning methane—the main component of natural gas—"cracking" involves separating it into its molecular components, hydrogen and carbon. This reaction occurs at temperatures of 750°C and above and does not release any harmful emissions.
The first product, hydrogen, is a fuel known for its clean combustion and high energy density per unit mass, researchers say. A by-product of the process—solid black carbon—is a useful industrial commodity used in the production of steel, carbon fibers and other carbon-based structural materials.
Solid black carbon is a by-product of methane cracking. Image credit: KIT.Methane cracking itself is not a new idea: in the last two decades, many experiments at different institutions have been carried out that have proven its technical feasibility. But the success of these attempts was limited by challenges related to carbon clogging and low conversion rates, the researchers say.
To improve on past experiments, IASS and KIT built a reactor based on liquid metal technology. The 1.2-meter-high device is made of quartz and stainless steel that uses pure tin and a packed bed structure consisting of pieces of quartz.
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.
In a series of experiments that ran from late 2012 to the spring of 2015, researchers say they were able to evaluate different parameters and options, such as temperature, construction materials and residence time.
“In the most recent experiments in April 2015, our reactor operated without interruption for two weeks, producing hydrogen with a 78% conversion rate at temperatures of 1,200°C, says Professor Thomas Wetzel, head of the KALLA laboratory at KIT. "The continuous operation is a decisive component of the kind of reliability that would be needed for an industrial-scale reactor.”
The reactor is corrosion resistant, and researchers say that clogging is avoided because the microgranular carbon powder produced can be easily separated.
While these are laboratory-scale experiments, researchers can extrapolate from them to gain insights into how methane cracking could be integrated into an energy system, as well as its contribution to sustainability. To that end, IASS collaborated 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.
For this exercise, it was assumed that a portion of the hydrogen produced is used to generate the required process heat. The compared hydrogen production technologies were steam methane reforming (SMR) and water electrolysis coupled with electricity. With regard 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% cleaner than SMR.
IASS researchers also analyzed the economic aspects of methane cracking. Preliminary calculations show that it could cost between 1.9 and 3.3 euro per kilogram of hydrogen produced, at current German natural gas prices, without taking the market value of carbon into consideration.
“Our experimental results as well as the environmental and economic assessments all point to methane cracking as a clear candidate option in our portfolio of measures to transform the energy system,” says former IASS Scientific Director Professor Carlo Rubbia.
In the next phase of the process, IASS and KIT will focus on optimizing several aspects of the reactor design, including the carbon removal process, and scaling it up to accommodate higher flow rates.