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Charcoal takes some heat off global warming

Biochar can offset 1.8 billion metric tons of carbon emissions annually

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August 10, 2010 Share This!

  • Biochar made from dairy manure pellets at the Department of Agriculture’s Agricultural Research Station in Prosser, Wash. Photo courtesy of USDA-ARS, Prosser, Wash

  • Carbon emissions can be sustainably offset by producing biochar from waste and residue biomass, as demonstrated in this schematic of the pyrolysis process to create biochar.

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Biochar is made by decomposing biomass like plants, wood and other organic materials at high temperature in a process called slow pyrolysis. Normally, biomass breaks down and releases its carbon into the atmosphere within a decade or two. But biochar is more stable and can hold onto its carbon for hundreds or even thousands of years, keeping greenhouse gases like carbon dioxide out of the air longer. Other biochar benefits include: improving soils by increasing their ability to retain water and nutrients; decreasing nitrous oxide and methane emissions from the soil into which it is tilled; and, during the slow pyrolysis process, producing some bio-based gas and oil that can offset emissions from fossil fuels.

Making biochar sustainably requires heating mostly residual biomass with modern technologies that recover energy created during biochar's production and eliminate the emissions of methane and nitrous oxide, the study also noted.

Crunching numbers and biomass

For their study, the researchers looked to the world's sources of biomass that aren't already being used by humans as food. For example, they considered the world's supply of corn leaves and stalks, rice husks, livestock manure and yard trimmings, to name a few. The researchers then calculated the carbon content of that biomass and how much of each source could realistically be used for biochar production.



But to achieve any of these offsets is no small task, Amonette noted.


Experiencing the full benefits of biochar will take time. The researchers' model shows it will take several decades to ramp up biochar production to its maximum possible level. Greenhouse gas offsets would continue past the century mark, but Amonette and colleagues just calculated for the first 100 years.

Biochar and bioenergy work together


However, Amonette and his co-authors wrote that a flexible approach including the production of biochar in some areas and bioenergy in others would create optimal greenhouse gas offsets. Their study showed that biochar would be most beneficial if it were tilled into the planet's poorest soils, such as those in the tropics and the Southeastern United States.



Plantations need not apply

The study also shows how sustainable practices can make the biochar that creates these offsets.


The authors' estimates of avoided emissions were developed by assuming no agricultural or previously unmanaged lands will be converted for biomass crop production. Other sustainability criteria included leaving enough biomass residue on the soil to prevent erosion, not using crop residues currently eaten by livestock, not adding biochar made from treated building materials to agricultural soils and requiring that only modern pyrolysis technologies — those that fully recover energy released during the process and eliminate soot, methane and nitrous oxide emissions — be used for biochar production.

"Roughly half of biochar's climate-mitigation potential is due to its carbon storage abilities," Amonette said. "The rest depends on the efficient recovery of the energy created during pyrolysis and the positive feedback achieved when biochar is added to soil. All of these are needed for biochar to reach its full sustainable potential."



Tags: Energy, Environment, Fundamental Science, Biomass, Emissions, Climate Science


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