In a roundabout way, coal is solar-powered. Millions of years ago, swamp plants soaked up the sun’s energy, eating carbon dioxide in the process. They died, accumulated, and transformed over geologic time into energy-dense rock. This solar-powered fuel, of course, is far from renewable, unlike solar panels: Burning coal has returned that carbon to the atmosphere, driving rapid climate change.
But what if humans could reverse that process, creating their own version of coal from plant waste and burying it underground? That’s the idea behind a growing number of carbon projects: Using special heating chambers, engineers can transform agricultural and other waste biomass into solid, concentrated carbon. Like those ancient plants captured CO2 and then turned into coal, this is carbon naturally sequestered from the atmosphere, then locked away for (ideally) thousands of years.
To be abundantly clear: Such “carbon removal” techniques are in no way a substitute for reducing emissions and keeping that extra carbon out of the atmosphere in the first place. But at the annual COP28 conference last month, carbon removal was a hotter topic than ever before. For years, the Intergovernmental Panel on Climate Change has insisted that to keep warming below 1.5 degrees Celsius above pre-industrial temperatures, we’re going to need carbon removal in one form or another, preferably a bunch of techniques working in concert.
If scaled up in the coming years, biomass carbon removal and storage could be one of those techniques. To start, you gather up waste biomass like corn stalks and cook it in a high-heat, low-oxygen environment in a special reactor, a process known as pyrolysis. It’s not burning the material with fire, per se, but blasting it with heat to remove the water content and turn it into concentrated carbon. (Note that this differs from bioenergy with carbon capture and storage, in which you grow crops specifically to burn to produce electricity, capturing the emissions from the power plant.)
“It’s basically like heating it in a pizza oven without oxygen,” says Andrew Jones, CEO and cofounder of Carba, which is using the process to bury carbon. “The optimal place is actually an abandoned coal mine, kind of putting it right back where it came from. We’re basically reverse coal mining.”
The challenge is that microbes love chewing on dead plant material, releasing carbon dioxide as a byproduct, as well methane, an even more potent greenhouse gas. This is an especially acute problem in the Arctic, where permafrost is thawing, releasing ancient plant material for microbes to eat. But it’s also a problem much closer to major human populations: Agricultural waste, landscaping waste from yards, the biomass you’d get from thinning forests (to lessen the amount of combustible material and reduce wildfire risk), such matter is often left to rot, burping up its carbon, or is burned, which releases both carbon and aerosols that are terrible for air quality and human health.
Because the reactor removes the carbohydrates that microbes love, creating charcoal, the carbon that goes into the ground doesn’t become food, so it persists. “If you’re just burying carbohydrates, you always have this risk that you don’t have it in the right conditions,” says Paul Dauenhauer, senior adviser and cofounder of Carba and a chemical engineer at the University of Minnesota, Twin Cities. “And so if 10, 20, or 30 percent of the material that you bury ends up degrading, that’s a loss of a lot of credibility.”
You don’t even need an abandoned coal mine to get rid of the processed biomass—Carba is burying it in landfills, too—so the technique could be used pretty much anywhere. “Every municipality has wastepaper waste, tree clippings, and grasses, all that kind of stuff,” says Dauenhauer. “But also, you can imagine packaging centers, where they have all the waste cardboard. That’s all carbohydrate and cellulose also.”
When applied to agricultural fields, this sort of carbon is known as biochar, which also improves soils. Biochar can boost crop yields in some cases, says Sanjai Parikh, who created the Biochar Database, an open-access tool at UC Davis for those that make and use biochar. “It’s sequestering carbon still, even though it’s at the surface,” Parikh adds. “That biochar, some of it will degrade, but we’re talking stability of hundreds to thousands of years.”
The material also helps retain water in sandy soils, for instance, which tend to drain quickly otherwise. “Biochar is a highly, highly absorbent material,” says Wendy Lu Maxwell-Barton, executive director of the International Biochar Initiative. “This is why biochar is such an extraordinary soil amendment…it makes it more resilient to both drought conditions as well as flooding.”
Biochar is also quantifiable, Maxwell-Barton says: With a certain amount of biomass, you create a certain amount of carbon to store in soils or underground. Indeed, biochar accounts for 90 percent of the carbon removal market, in which companies pay to offset their greenhouse gas emissions.
Alternatively, it’s harder to quantify exactly how much carbon you’re sequestering by restoring a complex forest ecosystem. Not that humans shouldn’t also protect these habitats—such “nature-based solutions” sequester carbon, bolster species, reduce flooding, and boost tourism industries. The unfortunate risk, though, is that a wildfire might destroy a protected forest, returning the carbon to the atmosphere. Burying carbon as charcoal theoretically protects it better in the long run.
In addition to burying solid carbon or sprinkling it on fields, researchers are also turning waste biomass into liquid carbon—oil, essentially, that they pump back into the ground instead of pumping the fossil variety up. “What we do at the highest level is we make barbecue sauce—or liquid smoke for barbecue sauce—and then we inject it into old oil wells,” says Peter Reinhardt, CEO and cofounder of the carbon removal company Charm.
They also do this with pyrolysis, which spits out solid char for agriculture, but also liquid oil. That’s shipped to abandoned wells and pumped underground, where it solidifies. “There’s about 2 to 3 million abandoned, end-of-life oil and gas wells across the United States,” says Reinhardt. “It’s quite a problem, actually—a lot of them are methane emitters or improperly sealed, with fluid leaking up to the surface.” By pumping its biomass oil underground at these sites, Charm both sequesters carbon and seals up wells that have been leaking greenhouse gases.
Whatever the end product, biomass removal cleverly exploits nature’s own photosynthesis to sequester and then bury carbon. “The genius in this business model, in many ways, is letting nature do most of the work,” says climate economist Gernot Wagner of the Columbia Business School. “This is a natural process that’s been perfected over millions of years, so why not take advantage of it?”
In reality, though, things are more complicated, Wagner says. When fossil fuel companies remove coal or oil from the earth, they’re tapping into huge deposits that are relatively easy to exploit on the cheap, hence the prices of those fuels remain low. But there’s only so much biomass waste available above ground, and it’s distributed across the planet. (Though this is a potential strength of this kind of carbon removal, in that each municipality could process its own biomass waste for storage.) “The more demand there is for biochar, or for this kind of carbon removal technology, the more startups are out there clamoring for the same food waste, corn husk waste, and so on,” says Wagner. “Suddenly, the prices increase, rather than decrease.”
The other potential issue, Wagner says, is the “moral hazard”: If humanity is able to delete carbon from the atmosphere, that’s less incentive to slash emissions. There’s still so much money to be made in fossil fuels, and indeed, oil companies like Occidental Petroleum are investing heavily in carbon removal technologies like direct air capture, in which machines scrub the air of CO2. That way, they can keep on drilling. “There is always this moral hazard aspect,” says Wagner. “The big, big topic in the background behind any of these carbon removal conversations is: OK, well, we could—or should, frankly—be doing more to reduce emissions in the first place, as opposed to let’s suck it back out after the fact.”
Reinhardt, of Charm, says the carbon removal industry is catering to companies that are indeed reducing their emissions and are trying to do more. “If you look at who’s buying removals, it’s companies that are already doing a lot on the reduction side and are trying to zero-out the remainder,” says Reinhardt. “Every single startup in the carbon removal space is singing that same tune of: Have you done everything you can to reduce? OK, if you have, then great. Let’s talk about how we get you to net zero.”
In the end, the science is very clear that in addition to reducing emissions, humanity has to figure out how to pull more carbon out of the sky. It’s not just going to be about relying on forests to capture carbon, or on enhanced rock weathering that reacts with atmospheric CO2, or on buried biomass, but ideally some combination of the best of the best techniques, both natural and technological. “We can have lots of different strategies, and they can be highly engineered, or they can be very simple,” says Parikh. “We just need to create all of these tools so that for each location and goal, we can use something to make a difference.”