This Is CDR is an ongoing series of online events to explore the range of carbon dioxide removal solutions that are currently in development. This week, we look at an elegant proposal from Dr. Ning Zeng of the University of Maryland. Dr. Zeng researches wood harvesting and storage, or WHS, a concept that’s about as straightforward as carbon removal gets. Plants absorb carbon from the air as they grow. They release it back into the air as they rot – unless we bury them, and that’s what Dr. Zeng proposes to do.
It almost sounds too simple to work, but Dr. Zeng’s team has conducted a series of experiments with very promising results. In 2013, they dug a 25-meter-long ditch in Quebec and buried enough waste wood – fallen wood that would normally be burned by forestry teams – to store 35 tons of CO2, capping it with clay from the ditch site to ensure the wood was insulated from air and bacteria.
Last November, they returned to find their samples were almost entirely preserved, with carbon dioxide unreleased, while wood they left on the surface as a control had rotted substantially. The emissions cost of digging the trench and burying the wood was only 2% of the emissions negated by the burial, and the dollar cost was around $30 per ton of CO2 sequestered, which makes it about 95% cheaper than current direct air capture technology. And as far as permanence, measuring the amount of decay on the buried samples, Dr. Zeng estimates that 97 percent of CO2 would stay sequestered for a century. (In fact, while initially digging the trench, the team unearthed a barely decayed red cedar that had been buried there for 3775 years!)
The next step is a larger prototype study burying a kiloton of CO2’s worth of wood in the Eastern U.S. The main constraint is finding a site with large natural amounts of a red clay called ultisol, which provides a reliable cap. With the right clay, the wood only needs to be buried about a meter deep, while sandier soil requires deeper burial. Luckily, ultisol deposits are very common in the east and southeast of the country.
That experiment will take place in the next couple of years, after which several more (scaled still larger) will follow. Dr. Zeng suggests that the eventual “real deal” deployment of the technology could take the form of mounds around the size of two soccer fields, half above-ground and half below, capped with clay and then topsoil so that grass could regrow. The land above the buried wood could then be used for parks, agriculture, or solar farming. And while this might sound like it would take a lot of land, Dr. Zeng points out that we could reach gigaton scales of CO2 removal in the US with an order of magnitude less land than we currently use for landfills.
All in all, it’s a thought-provoking idea and surprisingly robust in execution. Please check out Dr. Zeng’s excellent presentation, and check back next week for more This Is CDR. For more, you can watch the whole This Is CDR series on our resources page.