The boom in natural gas production has been essential to the drop in carbon emissions in the US, as methane, the primary component of natural gas, releases more energy for each carbon atom when burned than other fossil fuels. But there's still a carbon atom in each molecule of methane, so switching to natural gas will eventually lead to diminishing returns when it comes to emissions reductions. To keep our climate moderate, we'll eventually need to move off natural gas, as well.
But two new papers out this week suggest we could use natural gas without burning it. They detail efficient methods of converting methane to hydrogen in ways that let us capture much or all of the carbon left over. The hydrogen could then be burned or converted to electricity in a fuel cell—including mobile fuel cells that power cars. The supply obtained from methane could also be integrated with hydrogen from other sources.
The tech involved is also pretty cool in its own right, involving things like catalysts dissolved in liquid metal and solid materials that allow current to travel through them as protons, rather than as electrons.
Option 1: proton conduction
Methane is simply a carbon atom linked to four hydrogens (CH4). Those carbon-hydrogen bonds are intermediate in stability between combustible hydrogen-hydrogen bonds and the extremely stable molecules that form from its burning, carbon dioxide and water. So rearranging its bonds to liberate hydrogen requires some carefully thought-out thermodynamics.
One of the better options involves mixing methane and steam at high temperatures. Under these conditions, you break up water, which is extremely energetically unfavorable. But you form carbon dioxide, which is favorable, and you get hydrogen out of both the methane and the water. The problem is that you end up with a mixture of two gases, both of them at low pressure, which makes the output less useful as fuel without extensive processing.
To solve this issue, a Spanish-Norwegian team turned to a technology that I didn't even know existed: a solid proton-conducting electrolyte. Electrolytes are simply materials that allow charged ions to transit through them; while they're mostly liquids, a few solid electrolytes have been developed for batteries. In this case, the electrolyte is specific for protons, the ionized form of a hydrogen atom. Another way of viewing this is as a conductive material that allows charges to travel across it in the form of protons, rather than electrons.
The key feature for this application is that, if you apply a voltage difference across this material, the hydrogen ions produced by the methane-steam reaction will transit to the other side, where it can form hydrogen molecules. To improve matters further, the authors coated one side of the material with nickel, which catalyzes the methane-steam reaction.
When this material is used to make the walls of a reaction chamber, you end up producing compressed hydrogen with less than four parts-per-million impurities on the outside of the chamber. That's ready to burn or send to a fuel cell. Inside, you end up with concentrated carbon dioxide mixed with a bit of steam. The carbon from burning methane has essentially been captured, and the resulting material is ready for storage, meaning this could be an extremely low-emissions technology.
The authors also note that this technology scales down well. So they envision a small reactor hooked up to a natural gas line in a house, feeding hydrogen to a fuel-cell vehicle overnight. The waste heat from the reactor's 800 degree Celsius operating conditions could be used to heat the house or provide hot water. The downside is that the carbon dioxide produced in this use case would almost certainly end up released to the atmosphere.
Option 2: reactions in liquid metal
We recently discussed a case where researchers had developed chemical reactions that run on the surface of liquid metal, which provides rather distinctive reaction conditions. Now, a US-Indian team has found that it's possible to strip methane of its hydrogen using a reaction that runs inside a liquid metal.
The goal is to solve a problem for a different type of methane reaction, one that directly removes the hydrogen while leaving solid carbon behind. There are a variety of metals that can catalyze this reaction, but they all suffer a major problem: the carbon forms on their surface, eventually clogging the catalyst up so that the methane can't reach it. That problem is solved by running the reaction in liquid metal.
The idea is to take a catalytic metal and dissolve it as an alloy in a metal that's liquid under the reaction conditions. And those reaction conditions are hot, at more than 1,000 degrees Celsius, so you're not just limited to metals that are liquid near room temperature. Methane is then bubbled through the catalytic alloy, with the bubbles converted to nearly pure hydrogen by the time they reach the surface. The carbon that would normally foul the catalyst can't coat the single atoms within the alloy, and so it ends up pooling up and floating to the top.
The authors tested a variety of solvents (indium, bismuth, lead, silver, and more) as well as nickel, copper, and platinum catalysts. They found a combination of nickel in bismuth was the most effective catalyst. At 1,065 degrees Celsius, a one-meter column of liquid metal was enough to convert 95 percent of the methane input to hydrogen, and yields seemed to be rising further with longer columns. Methane is also not an issue for most fuel cells, so this is not a major contamination problem.
Running the column for 170 hours showed it worked as planned—there was no drop off in catalytic activity. And, as expected, the carbon simply floated to the top and could be skimmed off. It did pick up a small spattering of metal from bubbles bursting to the surface, but dunking the carbon back into the metal briefly was sufficient to remove this. While the carbon could be sequestered by simply dumping it anywhere convenient, it was primarily in the form of graphite and could be used to make things like capacitors or pencils.
Both of these approaches obviously are going to limit the amount of energy available from methane compared to simply burning it, since they require high temperatures to get the reactions to work. But there will almost certainly come a time where the carbon emissions of natural gas are considered a problem, and they do seem to provide a solution to that, allowing us to continue to use methane long after we try to phase out fossil fuels.
Nature Energy, 2017. DOI: 10.1038/s41560-017-0029-4
Science, 2017. DOI: 10.1126/science.aao5023 (About DOIs).
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