Biochar soil sequestration and pyrolysis most climate-friendly way to use biomass for energy

The ancient technique of burying charcoal into agricultural soils has gained attention over the years as a way to sequester carbon dioxide and fight climate change. Earlier we referred to scientists who are studying this tradition as it existed in the Amazon rainforest, where human-made and very fertile soils were discovered filled with char ("terra preta", "dark earth" - earlier post).

Many biomass researchers are now looking into the 'biochar' or 'agrichar' technique to use it in combination with modern biofuels. Called 'geosequestration of biochar' or 'black-carbon sequestration', the technique is different from carbon capture and storage (CCS), in that the first carbon sequestration concept involves burying the carbon in soils that can be used to grow crops, whereas the latter technique merely involves storing CO2 underground in geological formations like saline aquifers or depleted oil and gas fields.

So biomass allows for the design of two types of carbon-negative energy systems: (1) Bio-Energy with Carbon Storage (BECS) involves burning biomass/biogas in power plants, capturing the carbon, and storing it in dedicated sites (earlier post and here); (2) Growing crops to use part of their biomass as a fuel source, while the rest of the crop is turned into charcoal that is not used for energy, but that is sequestered into the soil, a process that enhances soil fertility, making the biofuel crops grow even better. In principle, a combination of the two techniques can be imagined.

For the time being, several countries in the EU are trying to supplant some coal-burning by burning biomass such as wood pellets and agricultural residues. Unlike coal, biomass is carbon-neutral, releasing only the carbon dioxide that the plants had absorbed in the first place. Eventually, BECS could be applied to such systems.

But a new research paper [*abstract] published online in the journal Biomass and Bioenergy argues that the biochar technique may be an even better route in the fight against global warming. An optimal system would consist of heating the biomass in an oxygen-starved process called pyrolysis, extracting methane, hydrogen, and other byproducts for combustion and energy, while burying the resulting carbon-rich char that is another byproduct from biomass pyrolysis.

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:
bioenergy :: biofuels :: energy :: sustainability :: climate change :: biomass :: pyrolisis :: biochar :: soil :: sequestration :: terra preta ::

Such an approach could carry an extra benefit. Burying char 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.

Image: 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. Credit: U.S. Department of Energy

More information:

Malcolm Fowles, "Black carbon sequestration as an alternative to bioenergy" [*.abstract], Biomass & Bioenergy, Volume 31, Issue 6, June 2007, Pages 426-432 (available online, 6 March 2007) doi:10.1016/j.biombioe.2007.01.012

Johannes Lehman, John Gaunt, Marco Rondon, "Bio-char sequestration in terrestrial ecosystems - A review" [*.pdf], Mitigation and Adaptation Strategies for Global Change (2006) 11: 403–427

Johannes Lehman's site: Bio-char or Agri-char: the new frontier, Cornell University.
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