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Energy

The Case for Burying Charcoal

Research shows that pyrolysis is the most climate-friendly way to consume biomass.

Several states in this country and a number of Scandinavian countries are trying to supplant some coal-burning by burning biomass such as wood pellets and agricultural residue. Unlike coal, biomass is carbon-neutral, releasing only the carbon dioxide that the plants had absorbed in the first place.

Carbon capture: Heating biomass such as wood pellets (right) in an oxygen-free environment produces char (left) and byproducts such as methane that can be burned. Research shows that turning biomass into char and burying the char is a good way to avoid releasing greenhouse gases into the atmosphere.

But a new research paper published online in the journal Biomass and Bioenergy argues that the battle against global warming may be better served by instead heating the biomass in an oxygen-starved process called pyrolysis, extracting methane, hydrogen, and other byproducts for combustion, and burying the resulting carbon-rich char.

Even if this approach would mean burning more coal–which emits more carbon dioxide than other fossil-fuel sources–it would yield a net reduction in carbon emissions, according to the analysis by Malcolm Fowles, a professor of technology management at the Open University, in the United Kingdom. Burning one ton of wood pellets emits 357 kilograms less carbon than burning coal with the same energy content. But turning those wood pellets into char would save 372 kilograms of carbon emissions. That is because 300 kilograms of carbon could be buried as char, and the burning of byproducts would produce 72 kilograms less carbon emissions than burning an equivalent amount of coal.

Such an approach could carry an extra benefit. Burying char–known as black-carbon sequestration–enhances soils, helping future crops and trees grow even faster, thus absorbing more carbon dioxide in the future. Researchers believe that the char, an inert and highly porous material, plays a key role in helping soil retain water and nutrients, and in sustaining microorganisms that maintain soil fertility.

Johannes Lehmann, an associate professor of crops and soil sciences at Cornell University and an expert on char sequestration, agrees in principle with Fowles’s analysis but believes that much more research in this relatively new area of study is needed. “It heads in the right direction,” he says.

Interest in the approach is gathering momentum. On April 29, more than 100 corporate and academic researchers will gather in New South Wales, Australia, to attend the first international conference on black-carbon sequestration and the role pyrolysis can play to offset greenhouse-gas emissions.

Lehmann estimates that as much as 9.5 billion tons of carbon–more than currently emitted globally through the burning of fossil fuels–could be sequestered annually by the end of this century through the sequestration of char. “Bioenergy through pyrolysis in combination with biochar sequestration is a technology to obtain energy and improve the environment in multiple ways at the same time,” writes Lehmann in a research paper to be published soon in Frontiers in Ecology and the Environment.

Fowles says that there would be an incentive for farmers, logging communities, and small towns to convert their own dedicated crops, agricultural and forest residues, and municipal biowaste into char if a high enough price emerged for the sale of carbon offsets. “Every community at any scale could pyrolyse its biowaste … motivated by doing their bit against global warming,” he says.

Fowles believes that storing black carbon in soil carries less risk, would be quicker to implement, and could be done at much lower cost than burying carbon dioxide in old oil fields or aquifers. And he says the secondary benefits to agriculture could be substantial: “Biochar reduces the soil’s requirement for irrigation and fertilizer, both of which emit carbon.” Fowles adds that it has also been shown to reduce emissions of greenhouse gases from decay processes in soil. This would include nitrous oxide, a potent greenhouse gas. “Biochar has been observed to reduce nitrous-oxide emissions from cultivated soil by 40 percent.”

David Layzell, an expert on bioenergy and plant sciences at Queen’s University, in Kingston, Ontario, says that finding the right balance between energy generation from biomass and sequestration of its char is a major area of research with global implications. “The issue of how much you should burn and how much should go back to the land is partly an economic issue and partly a sustainability issue. We don’t have the full answers to this, but that’s the kind of research we need.”

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