Oil fields often contain a lot of natural gas, but it can be uneconomical to transport this gas from remote production sites to where it can be used. Instead, the gas is simply vented or flared, which exacts an enormous toll on the environment.
On November 2, during the COP26 climate summit in Glasgow, more than 100 countries pledged to reduce global methane emissions by 30% from 2020 levels by 2030. At the same time, the US Environmental Protection Agency issued a proposal to curb methane emissions from the oil and gas industry, including measures to eliminate venting and reduce flaring.
Researchers at the University of Maryland have now developed a lab-scale reactor that demonstrates how methane could instead be converted into more valuable compounds without generating CO2 (Adv. Energy Mater. 2021, DOI: 10.1002/aenm.202102782). “It’s a way of tackling a major waste of resources,” says Eric Wachsman, who led the work with his colleague Dongxia Liu. Converting the gas to a liquid, like benzene, would mean “it becomes economical to put it in a barrel or a pipeline and take it away,” Wachsman says.
The World Bank’s Global Gas Flaring Reduction Partnership reports that more than 140 billion cubic meters of natural gas was flared in 2020, causing about 400 million metric tons of CO2-equivalent emissions every year, or roughly 1% of human greenhouse gas emissions. Venting the gas is even worse, because methane has a higher global warming potential that CO2.
To tackle this, researchers and companies are exploring various technologies that upgrade methane into liquid or solid hydrocarbons. For example, methane can be turned into syngas, a mixture of CO and H2, for conversion into heavier hydrocarbons by the Fischer-Tropsch process. But this involves multiple energy-intensive steps that require costly infrastructure and generate CO2. Methane can also be upgraded to ethylene in a simpler, one-step oxidative coupling reaction, but this also releases CO2.
To avoid these emissions, Wachsman and Liu’s new reactor uses a process called direct nonoxidative methane conversion (DNMC). Developing the catalytic process meant overcoming three major challenges. First, breaking the first carbon-hydrogen bond in methane takes a lot of energy, so any DNMC reactor needs a hefty supply of heat. Even then, the equilibrium between methane and products offers poor conversion rates. Finally, the reaction generates sooty carbon deposits that quickly deactivate the catalyst.
The new reactor solves these problems by uniting known materials — a catalyst and a ceramic membrane — in a cunning way. It contains a hollow tube of porous strontium cerium zirconium oxide, covered with a 25 µm thick membrane of a similar material doped with europium ions. The tube is packed with an iron-silica catalyst that breaks a C-H bond in methane, forming methyl groups that combine to make ethylene, benzene, naphthalene, and other molecules. Meanwhile, the hydrogen freed from the reaction passes through the membrane as protons and electrons, which meet a stream of air and react with oxygen to create water and heat.
Drawing hydrogen through the membrane shifts the reaction equilibrium so that more methane is converted into products. Meanwhile, the union of hydrogen and oxygen provides enough heat to drive methane splitting. And conveniently, a little oxygen can permeate through the membrane into the reaction tube, where it burns off any carbon deposited on the catalyst, producing some CO but no CO2.
Preliminary tests on a 17 cm long reactor at 1030 °C offered a methane conversion rate of about 18% over 50 hours, and Liu says they have recently made some adjustments to reach 30% conversion. Until now, the best performance for this kind of membrane reactor has been “only about 15% methane conversion, with very quick catalyst deactivation,” she says.
Crucially, almost 97% of the carbon atoms involved in the DNMC reaction were incorporated into products rather than waste gases. In principle, unreacted methane could be piped to other reactors, Wachsman says. He and Liu have founded a company, Alchemity, to scale up and commercialize the reactor.
Sebastian Wohlrab of the Leibniz Institute for Catalysis, who has worked on catalytic methane conversion and was not involved in the research, says that the reactor’s “methane conversion looks very good,” and that using an oxygen-permeable membrane is an “elegant” way to solve the catalyst deactivation problem. However, Wohlrab points out that hydrogen combustion can be difficult to control, one of several factors that could make it challenging to scale up the reactor.
This story was updated on Nov. 19, 2021, to correct a comment attributed to Dongxia Liu. She said that about 15% methane conversion was previously the best performance for the type of membrane reactor studied, not for the oxidative coupling of methane.