Breakthrough Cambridge reactor recycles 99% of gas to make hydrogen fuel and carbon

Researchers at Cambridge University have achieved the simultaneous production of carbon nanotubes (CNT) and clean hydrogen fuel, without producing carbon dioxide as a byproduct, by looping methane gas through the process.

The approach improves carbon nanotube production by over eightfold while also generating large amounts of clean hydrogen fuel.

Hydrogen gas is a highly sought-after fuel source because it burns completely, producing no carbon emissions. As countries look to move away from fossil fuels, demand for hydrogen is on the rise.

However, current hydrogen production methods, such as steam methane reforming, produce carbon monoxide as a byproduct.

Researchers at Cambridge looked at another reaction involving methane, which produces a small amount of hydrogen as a byproduct. Called methane pyrolysis, the reaction is designed to produce carbon nanotubes that are useful as conductive additives in lithium-ion batteries, another area where demand is currently surging.

Cambridge researchers wondered if the pyrolysis reaction could be further improved to produce large amounts of hydrogen gas, a byproduct.

The current process for CNT production typically uses a fluidized-bed or packed-bed reactor.

Over the years, attempts have been made to switch to a gas-phase catalyst, which produces higher-quality, longer CNTs that deliver improved performance in electrodes.

The process called floating catalyst chemical vapor deposition (FCCVD) uses methane, but also requires it to be diluted with hydrogen to prevent soot formation.

Scaling up FCCVD is challenging because it requires large amounts of hydrogen gas from the outset. In the end, the reaction produces a little more hydrogen than was put in.

FCCVD uses a single-pass setup, where the methane gas is let into the reactor once before being released. This leads to significant waste. Cambridge researchers decided to loop the gas flow until all the methane was consumed to make CNTs. This eliminated the need for additional hydrogen input.

Currently, there is only one FCCVD system operating at pilot scale that uses a single-pass configuration. To demonstrate their multi-pass system, the researchers built a lab-scale reactor.

After the gas stream passed through the pyrolysis reactor at 2372°F (1300°C), 1 percent of the gas was removed to remove hydrogen, while CNTs rolled out onto a mat. The gas also contains other hydrocarbons and hydrogen sulfide, but these do not affect CNT production.

Using a closed system significantly reduces the waste generated by the reactor otherwise. Compared to a single-pass reactor, the research team reported an “8.7-fold improvement in carbon yield and 446-fold improvement in molar process efficiency” in a research paper.

The molar process efficiency is a measure of how efficiently the system uses every gas molecule.

To determine how the system would operate in the real world, the team ran a computer model using data from a commercial plant to achieve an accurate simulation. They found that the multi-pass reactor design converts 75 percent of the gas in the system to CNTs and hydrogen in a 3:1 ratio.

The lab reactor also worked with a gas feed consisting of methane and carbon dioxide, an attempt to simulate output from a biogas plant. If this can be scaled, scientists can produce high-quality CNTs and clean fuel.