How Methane-Sourced Polymers Could Save the World
Craig Criddle is on his office phone, and he doesn’t appear pleased. The Stanford professor of civil and environmental engineering responds to his interlocutor on the other end of the line in terse, clipped phrases before finally hanging up.
“You know,” he says with some weariness after the call, “if you want something from somebody, it’s best not to harangue them. That’s not going to get you far.”
Criddle can be forgiven some irritation with overly importunate callers. A good many people want a piece of his time these days, and he has a plentitude of irons in the fire. Among his current projects are a process that converts nitrogen waste into nitrous oxide that can “turbocharge” engines at wastewater treatment plants; a microbial battery that converts dissolved organics into electrical energy; and a facility where technologies for wastewater treatment can be tested at the pilot scale.
But perhaps the most exciting project engaging Criddle involves methane – the organic gaseous compound that’s produced in voluminous quantities by wetlands, estuaries, landfills and wastewater treatment plants. It’s useful stuff, in that it’s the major component of natural gas. But it’s also problematic. It is a potent greenhouse gas, manifesting 21 times the heat retention properties of the much-reviled carbon dioxide.
A significant amount of money and effort have been put into harnessing methane – a major component of the “biogas” produced at landfills and wastewater treatment plants for energy production. Typically, the compound is combusted in a gas turbine, creating mechanical power that is used by a generator to produce electricity; the heat produced by this process can also be salvaged to run a steam turbine, which yields additional power.
That’s good as far as it goes, says Criddle, but biogas has some major drawbacks as an energy source.
“The main problem is that the gas produced in landfills and treatment plants is often contaminated with other compounds that damage the mechanical components,” he observes. “That’s why more often than not, the gas is just flared off at the site. In many cases, it’s not cost-effective to use it as a fuel for electricity production.”
Criddle has a better idea for biogas: plastics. More specifically, he and a team of Stanford faculty colleagues, students and post-docs advocate using biogas as a feedstock for the production of valuable polymers.
The technique relies on a specific group of bacteria known as “methanotrophs”– microorganisms that consume methane. Criddle and his colleagues have been able to enrich for methanotrophs that produce a useful polymer – a polyester-class compound known as polyhydroxybutyrate (PHB). The bacteria are cultured in aerobic bioreactors and fed copious amounts of methane; the PHB is produced as minute granules. The “right” bugs can yield up to 60 percent of their mass as PHB.
“That is a really impressive conversion efficiency,” says Criddle. “For every three to five kilos of methane, we’re getting one kilo of PHB.”
Contaminants in the biogas? That may be a big issue for methane power-generation systems, but the methanotrophs appear to handle dirty methane with nary a blip in polymer production, observes Criddle. And PHB is a highly versatile, useful polymer; with tweaking, it can be turned into a wide array of different plastics.
“From a business standpoint, it makes far more sense to use methane as a polymer feedstock than to burn it for power production,” continues Criddle. “PHB sells for $3 to $4 a kilogram on today’s market, while methane burned for electricity production would return from 40 to 80 cents a kilo.”
Methane-based polymers provide other benefits to a planet with limited natural resources and a rapidly changing climate. Currently, most polymers are derived from petroleum – a resource that is expensive to extract, dwindles daily and produces a big carbon footprint. Methane, a readily available waste product, also exacerbates global warming; by converting it into PHB, observes Criddle, the carbon is sequestered – quite a lot of carbon, up to two kilos of “CO2 equivalence” for every kilo of polymer produced.
“So when we capture methane and use it to make bioplastic, we’re helping climate change. As long as that carbon is in the plastic, it’s not going into the atmosphere,” Criddle observes.
But what happens when the products made from that plastic wear out? As the plastic degrades, doesn’t its carbon migrate into the atmosphere? Not necessarily, says Criddle: once again, amenable microorganisms come into play.
“Just as we’ve found aerobic bacteria that convert methane to PHB, there are anaerobic microorganisms that convert PHB back to methane,” he says. “We can create a cradle-to-cradle cycle – some microbes produce polymer from methane, and some produce methane from used and discarded products made from PHB. So the carbon is recycled at a molecular scale.”
The challenge at this point is taking the process from the lab to the commercial sector. Criddle acknowledges this is going to require some effort – but not because there are any significant technical kinks.
“This will take some major investment and a fairly long timeline to bring to fruition,” he observes, “and venture capitalists tend to think in the shorter term – they typically want a quick turnaround on investment.”
Still, the first moves toward commercial applications are occurring – thanks to Stanford alumni. Molly Morse (MS, PhD, civil and environmental engineering, Stanford), has worked with Criddle and is the CEO of Mango Materials, a start-up founded on the production of methane-based polymers. Morse recently won the $630,000 Postcode Lottery Green Challenge, an international competition devoted to products and services that mitigate global warming. Her company has entered into a partnership with Redwood City in the San Francisco Bay Area to operate an experimental PHB production facility at the municipality’s wastewater treatment plant.
“We’re still focusing on research, so we aren’t actively soliciting venture capital at this point, but we know this is a game-changing technology,” says Morse. “In northern California alone, there are 20 to 30 landfills and wastewater treatment plants that could each produce up to 2.5 million pounds of polymer a year.”
Word is starting to get out about the technology, and Morse is preparing to scale up.
“We’re getting random emails from all over the world – cities are asking us to start projects at their landfills and wastewater treatment plants. We hope to build our first commercial plant within two years.”
