Turning landfill gases into hydrogen power isn't a distant dream. It happens already, particularly in South Carolina, where landfill gas-to-hydrogen conversion projects have advanced beyond pilot or experimental stages into functional energy systems used by large-scale manufacturing outfits, providing heat and/or power.
Landfill gas boosters want the same principle to scale up to use in vehicles, and that's presented some technological barriers—one of which is the carbon byproduct of the methane-to-hydrogen process. Carbon tends to build up on the conversion catalyst in the form of a gross black tar, slowing the reaction and eventually shutting it down if the element doesn't get manually scraped free of the carbon buildup. This build-up is hardly unique to biogas-to-syngas production and in old-school terms it's known as "coking."
New work being presented this week at the 248th National Meeting of the American Chemical Society by a Brazilian research team describes a way around the problem, using a similar catalytic material to that found in the anti-pollution devices used in cars and trucks. The material is what's called a "perovskite-type oxide," which catalyzes a new version of the reaction that's highly stable and relatively free of coke buildup.
Biogas is fundamentally pretty foul, ungreen stuff, a mixture of mostly methane and carbon dioxide, with less significant bonuses of nitrogen gas, oxygen, and hydrogen sulfide. The latter compound is especially gnarly, reeking of rotten eggs while being extremely poisonous, not to mention flammable and explosive.
All of this biogas pouring out of human civilization's many, many piles of rotting organic matter (sewage, municipal solid waste) is a clear contributor to climate change and said civilization would be well advised to, you know, do something about it. Hydrogen gas production is an excellent and not particularly new answer, a way to eliminate actual GHG emissions from landfills while providing a fuel that, when combusted in an engine, merely releases water vapor.
The process of converting methane to hydrogen is called methane reforming. A version of it called steam reforming is how much of our current hydrogen (and ammonia) needs are met. Basically, methane or another similar biogas is piped through tubes with steam while those tubes are heated to very high temperatures, upwards of 1,000 degrees Fahrenheit. The resulting reaction delivers hydrogen and carbon monoxide as its byproducts.
It's possible, however, to replace the steam with carbon dioxide in a variation of the reforming scheme that produces the same hydrogen results, called dry reforming.
A metallic catalyst is need to make any type of reforming work, though. Otherwise, the reaction would move so slowly it'd hardly be worth paying attention to. Nickel is typically used now, and in the carbon dioxide-less, steam-based process we obviously don't have to worry about carbon stuff building up on the catalyst.
"Therefore, the development of a catalyst resistant to carbon deposition during CO2 reforming of [methane] is one of the main issues of this technology," the presentation's abstract reads. "One approach to minimize or inhibit carbon formation is to control metal particle size by the appropriated selection of catalyst preparation method."
This is where the perovskite-type material comes in. While "perovskite" refers to a specific mineral first discovered in the Ural Mountains some 200 years ago, a whole class of compounds take the name for their similarities, particularly with regard to their rather special electrical properties: superconductivity, ionic conductivity, and magnetoresistance. As part of a catalyst, perovskite-like material has just the right sort of structure to keep the metallic particles that do the actual catalyzing at just the right size to prevent carbon from building up.
The nickle can do its work, but the perovskite its married to keeps the metal from attracting too much coke. Yay, chemistry.
The Brazilian team is next planning on taking their catalyst to a local landfill for more testing. If the new material continues working as expected, landfill gas could be poised for a new era of cheap, clean hydrogen production.