This story was originally published by WIRED and is reproduced here as part of the Climate desk cooperation.
Coal is powered by solar power on a revolution. Millions of years ago, swamp plants absorbed the sun’s energy and ate carbon dioxide in the process. They died, accumulated, and transformed into energy-dense rock over geologic time. Of course, this solar-powered fuel is far from renewable, unlike solar panels: Burning coal returned that carbon to the atmosphere, driving rapid climate change.
But what if people could reverse that process, create their own version of coal from plant waste and bury it underground? That’s the idea behind a growing number of carbon projects: Using special heating chambers, engineers can turn agricultural and other waste biomass into solid, concentrated carbon. Like those ancient plants that captured CO2 and then turned into coal, which is carbon naturally sequestered from the atmosphere, then locked away for (ideally) thousands of years.
To be very 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. The Intergovernmental Panel on Climate Change has been insisting for years that we will need some form of carbon removal to keep warming below 1.5 degrees Celsius above pre-industrial temperatures, preferably a bunch of techniques working together.
If scaled up in the coming years, biomass carbon removal and storage could be one of those techniques. To start, you collect 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 does not burn the material with fire per se, but blasts it with heat to remove the water content and turn it into concentrated carbon. (Note that this is different from bioenergy with carbon capture and storage(in which you grow crops specifically to burn to produce electricity, which captures the emissions from the power plant.)
“It’s basically like heating it in a pizza oven without oxygen,” said Andrew Jones, CEO and co-founder of Carba, which uses the process to bury carbon. “The optimal location is actually an abandoned coal mine, which kind of puts it back where it came from. We are basically reverse coal mining.”
The challenge is that microbes like to chew on dead plant material, releasing carbon dioxide as a byproduct, as well as methane, a even more powerful greenhouse gas. It is a particularly 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 large human populations: Agricultural waste, landscape waste from yards, the biomass you’ll get from thinning forests (to reduce the amount of combustible material and reduce the risk of wildfires), such matter is often left to rot, breaking down its carbon, or is burned, releasing both carbon and aerosols that terrible for air quality and human health.
Because the reactor removes the carbohydrates that microbes love, and creates charcoal, the carbon that goes into the soil does not become food, so it continues. “If you just bury carbohydrates, you always run the risk of not having them in the right conditions,” says Paul Dauenhauer, senior adviser and co-founder of Carba and a chemical engineer at the University of Minnesota, Twin Cities. “And so if 10, 20 or 30 percent of the material you bury ends up humiliating, that’s a loss of a lot of credibility.”
You don’t even need an abandoned coal mine to dispose of the processed biomass—Carba also buries it in landfills—so the technique can be used virtually anywhere. “Every municipality has waste paper waste, tree clippings and grasses, all that kind of stuff,” says Dauenhauer. “But you can also imagine packaging centers, where they have all the waste cardboard. It’s also all carbohydrates and cellulose.”
When applied to agricultural fields, this type of carbon is known as biochar, which also improves soil. Biochar can increase crop yields in some cases, says Sanjai Parikh, who created the Biochar Database, an open-access tool at UC Davis for those making and using biochar. “It’s still sequestering carbon, even though it’s on the surface,” adds Parikh. “That biochar, some of it will break down, but we’re talking stability of hundreds to thousands of years.”
The material also helps to retain water in, for example, sandy soils, which otherwise tend to drain quickly. “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 resistant to both drought conditions as well as floods.”
Biochar is also quantifiable, says Maxwell-Barton: With a certain amount of biomass, you create a certain amount of carbon to store in soil or underground. Indeed, biochar is responsible for 90 percent of the carbon removal marketin which companies pay to settle their greenhouse gas emissions.
Alternatively, it is more difficult to quantify exactly how much carbon you sequester by restoring a complex forest ecosystem. Not that humans shouldn’t also protect these habitats – such “nature-based solutions” sequester carbon, strengthen species, reduce flooding and promote tourism industries. However, the unfortunate risk is that a wildfire could destroy a protected forest and release the carbon back into the atmosphere. Burying carbon as charcoal theoretically protects it better in the long run.
In addition to burying fixed 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 up the fossil variety. “What we do at the highest level is we make barbecue sauce—or liquid smoke for barbecue sauce—and then we spray it into old oil wells,” says Peter Reinhardt, CEO and co-founder of carbon removal company Charm.
They also do this with pyrolysis, which spits out solid charcoal for agriculture, but also liquid oil. It is shipped to abandoned pits and pumped underground, where it solidifies. “There are approximately 2 to 3 million abandoned oil and gas wells across the United States,” says Reinhardt. “It’s actually quite a problem – many of them are methane emitters or improperly sealed, with liquid leaking to the surface.” By pumping its biomass oil underground at these sites, Charm both sequesters carbon and seals wells that have leaked 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 to let nature do most of the work,” says climate economist Gernot Wagner of the Columbia Business School. “It’s a natural process perfected over millions of years, so why not use it?”
In reality, however, things are more complicated, says Wagner. When fossil fuel companies remove coal or oil from the earth, they mine large deposits that are relatively easy to mine cheaply, and therefore the prices of those fuels remain low. But there is only so much biomass waste available above ground, and it is spread across the planet. (Though it is a potential strength of this type of carbon removal, in that each municipality can process its own biomass waste for storage.) “The more demand there is for biochar, or for this type of carbon removal technology, the more startups are out there calling for the same food. waste, corn husk waste, and so on,” says Wagner. “Suddenly the prices are going up, rather than going down.”
The other potential problem, Wagner says, is the “moral hazard”: If humanity is able to remove carbon from the atmosphere, there is less incentive to reduce emissions. There is still so much money to be made in fossil fuels, and indeed, oil companies like Occidental Petroleum are invest heavily in carbon removal technologies such as direct air capture, in which machines scrub the air of CO2. That way they can keep drilling. “There’s always this moral hazard aspect,” says Wagner. “The big, big theme in the background behind any of these carbon removal conversations is: OK, well, we can — or shouldfrankly – do more to reduce emissions in the first place, as opposed to let’s suck them back in afterwards.”
Charm’s Reinhardt says the carbon removal industry caters to companies that do reduce their emissions and are trying to do more. “If you look at who is buying relocations, these are companies that are already doing a lot on the reduction and trying to eliminate the rest,” says Reinhardt. “Every single startup in the carbon removal space is singing the 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 needs to figure out how to pull more carbon out of the air. It’s not just about relying on forests to capture carbon, or on improved rock weathering which reacts with atmospheric CO2, or on buried biomass, but ideally a combination of the best of the best techniques, both natural and technological. “We can have very different strategies, and they can be highly engineered, or they can be very simple,” says Parikh. “We just have to create all these tools so that for every location and purpose we can use something to make a difference.”
