Pushing the needle back on billowing carbon dioxide emissions may be necessary to avoid catastrophic warming. But for those who are squeamish about drastically engineering the climate by seeding algae blooms or spraying aerosols to form clouds, scientists are exploring the concept of negative emissions.
The physical science section of the Intergovernmental Panel on Climate Change's fifth assessment report, released last September, suggested that bioenergy with carbon capture and sequestration (BECCS) could effectively remove greenhouse gases from the atmosphere. The assessment also notes that bioenergy that produces char could also take a big bite out of greenhouse gases.
Over the course of 100 years, the report said, BECCS could pull 125 billion metric tons of carbon dioxide from the sky, while biochar energy systems could draw down 130 billion metric tons of the gas. For reference, the world churned out just less than 40 billion metric tons of carbon dioxide in 2013, according to the Tyndall Centre for Climate Change Research at the University of East Anglia.
However, "potentials for BECCS and biochar are highly speculative," the report acknowledged. "BECCS technology has not been tested at industrial scale, but is commonly included in Integrated Assessment Models and future scenarios that aim to achieve low CO2 concentrations."
The logic behind it is that plants breathe in carbon dioxide from the air as they grow, turning it into sugar for energy and structures like cellulose for rigidity. When plants die, their carbon compounds decay into carbon dioxide and methane, which other plants then use, thereby completing the carbon cycle.
Advocates for fuels like corn-based ethanol and power systems like wood-pellet-burning boilers argue that this makes biomass carbon-neutral, so that it has negligible effects on the global climate. Coaxing some of this carbon into the ground instead of the sky makes the process carbon-negative. This is almost the opposite of fossil fuel combustion, in which carbon stored underground for millions of years is burned and spews into the air, increasing atmospheric greenhouse gases.
The carbon-negative idea hinges on a lot of "could," "might" and "may." The bold predictions depend on aligning many variables, like supply chains, economics and technology, and in such a young field, it's too early to tell whether carbon-negative bioenergy will work in the real world. But with carbon dioxide concentrations in the atmosphere racing past 400 parts per million, some scientists and engineers are pushing on this front, arguing that carbon-neutral isn't enough.
Bold predictions, but little testing
One of the main problems is the dearth of practical research. Bioenergy, for both fuels and for electricity, is still a fledgling field on its own, while utilities are struggling to run viable commercial carbon capture and sequestration systems for fossil fuels.
"Our primary focus has been, for the last five to 10 years, on large point-source CO2 capture," said Jared Ciferno, director of the Office of Coal and Power Research and Development at the Department of Energy's Office of Fossil Energy.
The group conducted life-cycle assessments of bioenergy in the context of blending biomass with existing coal feedstocks, though the group did little in terms of practical experiments. "Our interest was 'What is the maximum amount of biomass you can feed in with coal?'" Ciferno said. "Mathematically, you can get carbon-negative."
However, in order to get negative emissions, the wood chips, corn stover or switch grass that feed generators have to come from nearby, because transportation fuel emissions eat into their "clean" credibility. Greenhouse gases from fertilizing and harvesting biomass also factor in, along with changes in land use.
In addition, processing biomass requires additional hardware, like dryers and grinders, which are very energy-intensive to run, according to Timothy Skone, a senior environmental engineer at the National Energy Technology Laboratory. "It's technically feasible, but then you have the logistics surrounding feedstock supply," he said.
BECCS revives past debates over whether energy from biomass is really neutral and renewable. Proponents note that past studies overestimated the environmental fallout from approaches like corn-based ethanol.
"One of the biggest concerns was indirect land-use change," said William Hohenstein, director of the Climate Change Program Office at the Department of Agriculture. "What we're finding in reality is that farmers are very innovative and are meeting demands by a number of ways we haven't anticipated."
Selecting the feedstock is another important consideration. Pulp from a massive tree that lived for a century will likely never get to carbon-neutral within a meaningful time frame, while a fast-growing crop like switch grass could balance out in less than a year.
Storing CO2 in char
Challenges still loom over the carbon capture and sequestration side of the equation. In a coal plant, a CSS system can downgrade its output by as much as 40 percent and can double the cost of the energy produced. Because biomass has a lower energy density than coal, the numbers look much worse.
Making biofuels also releases carbon dioxide, but in smaller streams at different steps of the fermentation process, so CCS installations are more tedious.
As a result, fossil fuels will be the proving ground for CCS systems before bioenergy can benefit from it. "Almost surely, capture and storage of CO2 from industrial sources (including coal and natural gas power plants) will [precede] the deployment at large scale of the various negative carbon strategies now being discussed," Robert Socolow, director of the Climate and Energy Challenge at the Princeton Environmental Institute, said in an email.
That's why producing char from biomass is so appealing. In pyrolysis, operators cook organic material to temperatures around 300 degrees Celsius, which releases hydrogen, methane, methanol and carbon monoxide, leaving behind char as a byproduct. Crank the temperature up to 700 C and take out the oxygen, and you have gasification, which also produces fuel and char.
Because some of it stays behind in the char, not all of the carbon from biomass oxidizes into carbon dioxide. This leads to a net reduction in greenhouse gases in the atmosphere without costly carbon dioxide absorbers. Farmers commonly use char to enrich soil, so blending it with earth or burying it effectively sequesters this carbon and helps more biomass grow, further driving emissions into negative territory.
"That material you generate is more persistent in the soil in the environment than the original biomass it's produced from," said Johannes Lehmann, a professor of soil science at Cornell University. He noted that char itself is a fuel and, if burned, makes pyrolysis or gasification carbon-neutral instead of carbon-negative, which is still a benefit to the extent it displaces fossil fuels.
However, it's difficult to keep track of where the carbon is going in these situations so that energy producers can claim carbon credits or tax incentives. "From a market point of view, there is at the moment no broadly agreed carbon methodology to trade and account for greenhouse gas emissions and reductions using biochar," Lehmann said.
BECCS and biochar approaches do have their critics, who argue that carbon-neutral energy should be a priority over carbon-negative.
"First of all, with regard to CCS, bioenergy plants typically emit 50 percent more CO2 than coal per megawatt produced," said Mary Booth, director of the Partnership for Policy Integrity, a think tank that studies energy trade-offs. "You would increase costs by 50 percent at no great gain."
"Char is unburned fuel. Why would any facility want to leave half their fuel unburned?" she added. "I think these claims about biochar are incredibly far-fetched."
Nonetheless, there are companies working to commercialize these systems, at least at small scales. All Power Labs, a Berkeley art collective turned energy startup, has sold more than 500 small-scale gasifiers.
"We got started in this because we were a bunch of artists that had our power turned off," said Tom Price, director of strategic initiatives at All Power Labs.
Can gasifiers substitute for diesel oil?
The devices produce 20 kilowatts, cost less than $30,000 and fit on the back of a Toyota pickup truck. "It was designed from the ground up for the hardest-to-reach places," Price said.
It is in these hard-to-reach places where biomass gasification stands to make the biggest dent in emissions. Trucking in diesel or coal is prohibitively expensive in regions with rough roads and sparse infrastructure. Price observed that in parts of West Africa, charging a cellphone costs the equivalent of $45 in local market rates.
Trees, shrubs, grasses and crops, by contrast, are ubiquitous. "Everywhere you find humans, you find biomass," Price said. Gasifiers burn biomass more cleanly than open-pit fires and stoves, so they provide short-term health benefits and negative greenhouse gas emissions, all while producing cheap local electricity. Users can also modify the gasifiers to provide shaft power to drive machinery or run chillers.
"These things can pay for themselves in less than a year," he added. The goal now is to make them simpler and easier to operate.
"At the small scale, most people believe the benefits can be much, much higher than the drawbacks," said Helena Chum, a research fellow at the National Renewable Energy Laboratory and the lead author of the bioenergy chapter in the IPCC's special report on renewable energy and climate change.
Though most of the individual steps in bioenergy processing -- like growing, harvesting and converting biomass into useful fuels and energy -- are mature in terms of technology, the economics are still a challenge, but increasingly less so.
"Every part of this chain has to make a profit," Chum explained. "I think in five to 10 years, we'll have several production facilities going."
As global energy demands grow and the climate changes, bioenergy will eventually have to step up at larger scales. "There are portions of our energy economy that are really hard to decarbonize, aviation being one of them," said Nathanael Greene, director of renewable energy policy at the Natural Resources Defense Council. "For that we need to find a low-carbon, high-density liquid fuel, and the only one anyone talks about is some kind of biofuel.
"The trick would be to make sure you're getting your biomass in a really sustainable way, then capturing as much of the carbon as you possibly could," he added.